The Lipid-State Transduction Hypothesis: A Falsifiable Framework for Lipid-State Causality in Extracellular Lipid-Containing Particles

1. Introduction

The biological functions of lipids have long been organized around three conceptual roles. Lipids build biological membranes, store and transport energy, and act as single-molecule mediators that trigger specific receptors or pathways. [1,2] These three mechanisms are experimentally well grounded, yet they do not fully account for the behavior of extracellular lipid-containing particles in complex tissue environments. Lipids in cellular membranes are not isolated molecules dispersed in a passive background. Lipid classes, fatty-acid chain length and saturation, cholesterol and sphingolipid abundance, oxidative modification, phase behavior, membrane curvature, packing density, surface charge, topological exposure and lateral microdomain organization jointly shape membrane interfaces. [3,4,5] These interfaces, in turn, influence receptor clustering, membrane trafficking, fusion, phagocytic recognition and cell fate. For many lipid-mediated effects, the biologically relevant unit is therefore not a single molecule, but a lipid state generated by the joint organization of composition, modification and supramolecular structure. [6,7,8]

This raises a problem that has not been sufficiently conceptualized. When the lipid state of a source membrane enters the extracellular space through membrane turnover, vesicle release, membrane-fragment externalization, lipid transport, tissue injury or cell death, does it collapse completely into unrelated individual lipid molecules, or can it persist in a form that is partially retained, presented and sampled by other cells? The key question is not whether extracellular lipids exist. It is whether the lipid state of a source cell or tissue, after leaving its original membrane system, can still preserve source-related traces, interfacial features and functional consequences.

Existing research provides several entry points into this question. Extracellular vesicles are often viewed as delivery vehicles for RNA, proteins and metabolites, [9] whereas lipids are frequently treated as background materials needed to maintain vesicle structure. [10] However, vesicle budding, membrane sculpting, particle stability, target-cell contact and uptake all depend on lipid state. Ceramide, cholesterol, sphingolipids, membrane microdomains, lipid asymmetry and membrane curvature influence vesicle biogenesis and interfacial organization. The metabolic status, inflammatory activation, oxidative stress and mode of death of the source cell can also alter the lipid composition and surface properties of the released particles. [10,11] Thus, in some settings, extracellular vesicles may be more than cargo containers. They may represent an interfacial form through which the lipid state of a source membrane is externalized and presented.

The biological relevance of lipid state is also evident because such states can be read by cells and immune systems. Phosphatidylserine (PS) provides the clearest example. PS as a molecule does not automatically constitute a phagocytic signal. It becomes an eat-me interface for apoptotic cells and related particles only when it is translocated from the inner leaflet to the outer leaflet and exposed in a manner accessible to receptors or bridging molecules. [6] Similarly, oxidized phospholipids generated during oxidative stress, ferroptosis and necrotic injury do not act solely because of their molecular identity. They affect immune recognition through the combination of oxidative modification, surface presentation and the context of cell death. [12,13] Biological systems often read not the total abundance of a lipid molecule, but the combination of molecular identity, topological position and interfacial background. [14]

Carrier-dependent lipid signaling further supports this logic. The same lipid mediator may have different tissue distributions, receptor biases, signaling durations and functional outcomes in different carrier environments. For example, sphingosine-1-phosphate can influence endothelial barrier function and circulatory homeostasis differently when bound to albumin, associated with HDL/apolipoprotein M or presented in other carrier contexts. [15,16] This indicates that lipid signaling is not determined solely by molecular identity and total concentration. It also depends strongly on carrier state, local mode of presentation and the pattern of contact with target-cell membranes or receptors. [15,17]

Once extracellular lipid states enter biological fluids, they may acquire a new biological identity. Extracellular vesicles, lipoprotein-like complexes, lipid nanoparticles and other lipid-containing particles can adsorb albumin, apolipoproteins, complement factors, immunoglobulins, coagulation proteins, clusterin and other molecules after contact with plasma, interstitial fluid or inflammatory exudates. This generates a dynamic protein corona, or more broadly a biomolecular corona. [18,19] The corona is not a simple contaminant. It functions as an identity layer that influences circulation time, tissue distribution, cellular uptake, immune recognition and clearance route. [20,21] The curvature, charge, hydrophobic exposure, oxidation status, phase behavior and topological features of a particle's lipid interface may influence which fluid-phase proteins it recruits. The acquired corona then helps determine which tissues, cells or clearance systems sample the particle. [22,23,24]

Traditional classification systems have inherent limitations in this regard. Extracellular lipid-containing particles are usually assigned to three separate frameworks: extracellular vesicles, lipoproteins and single-molecule lipid mediators. This classification is useful for experimental workflow and disciplinary specialization, but it does not necessarily define functional boundaries. [25,26] Plasma extracellular vesicles and lipoproteins overlap substantially in size, density, lipid composition and isolation behavior. The discovery of nanoscale components with low levels of canonical vesicle markers also suggests that EV-centered marker frameworks do not cover all extracellular nano-entities. [27,28,29] Conversely, the author’s previous studies show that lipid particles generated by artificial reconstitution of source lipidomes (i.e., reconstituted lipid nanoparticles) can retain source-related biological biases in some models, implying that complex lipid combinations may preserve partial source-associated function even after separation from their original cells. [30,31]

These observations do not prove that a new particle class already exists, nor do they imply that all extracellular lipid-containing particles possess a special informational function. Instead, they point to a stricter and testable proposition: can part of the biological activity of extracellular lipid-containing particles depend on a lipid state jointly constituted by lipid composition, modification state, supramolecular organization, interfacial presentation and fluid-phase identity layer? If so, lipids should not be regarded only as particle cargo, structural membrane background or single-molecule mediators. They should also be considered state variables that can be externalized through particulate processes, reinterpreted by biological fluids and sampled by target cells. [11,19,22]

On this basis, this article proposes the lipid-state transduction hypothesis (LST). The hypothesis states that, in specific contexts, a source-cell or tissue state can be materially embodied in lipid composition and interfacial organization, externalized through particulate or carrier-associated presentation, remodeled by biological fluids, and converted through target-cell or clearance-system sampling into a functional state input in local or systemic homeostasis. Here, transduction denotes material state conversion and biological sampling; it does not imply deliberate encoding, a dedicated receptor, or a canonical receptor-centered signaling cascade. LST does not deny the importance of RNA, proteins, individual lipid mediators, classical extracellular vesicles or lipoproteins. It proposes an additional causal layer: when the function of an extracellular lipid-containing particle depends on an intact lipid state and cannot be sufficiently explained by conventional cargo or conventional particle identity, lipid state should be treated as part of the mechanism by which source-associated state is converted into biological consequence.

2. Minimal Theoretical Grounds for LST

For LST to become a testable hypothesis, it cannot rest only on the fact that lipids exist outside cells. The mere presence of extracellular lipids is insufficient to show that they participate in state-dependent transduction. Similarly, the ability of lipid-containing particles to affect target cells does not necessarily mean that their effects are dominated by lipid state. A stricter starting point is that extracellular lipid states can qualify as functional inputs only if at least three conditions are met. First, complex lipid states from source membranes must have a physical opportunity to be partially retained and presented after leaving the cell. Second, living systems must continuously generate state-associated flows of externalized lipids, rather than forming random aggregates only through accidental injury or technical preparation. Third, biological fluids, target cells and clearance systems must be able to distinguish these interfacial differences and convert them into differential biological processing.

2.1. Physical Carriage: Partial Retention of Complex Lipid States in Aqueous Environments

The extracellular milieu is predominantly aqueous, whereas biologically derived lipids include many amphipathic or hydrophobic molecules, including phospholipids, sphingolipids, cholesterol, neutral lipids, oxidized lipids and diverse metabolites. Such molecules are unlikely to retain complex compositional relationships for long as homogeneous, fully free and structurally unrelated species. Hydrophobic effects, the geometric constraints of amphipathic molecules and the free energy of water-hydrophobe interfaces together imply that complex lipid mixtures entering aqueous environments tend to reduce interfacial energy through self-assembly, membrane fragmentation, protein binding, lipoprotein-like complex formation, mixed micelles or particle formation. [32,33,34,35,36]

This physical tendency provides LST with the possibility of material continuity, not proof that a lipid state is preserved intact. Once source-membrane lipid states enter biological fluids, they are reshaped by lipid exchange, phospholipases, lipid-transfer proteins, albumin or lipoprotein takeover, redox reactions, protein adsorption, dilution and clearance by the liver, spleen and phagocytic systems. [37,38,39] Therefore, LST does not require an externalized lipid state to remain stable for long periods or to replicate the source membrane microdomain structure completely. A more reasonable premise is that, within defined time windows and environments, part of the source lipid combination, modification pattern and interfacial feature set may be partially retained in particulate or carrier-associated form while being remodeled, thereby remaining available for subsequent fluid-phase editing and biological sampling.

Such partial retention may be meaningful because the interfacial properties of lipid assemblies are not a linear sum of molecular lists. Differences in lipid ratio, fatty-acid chain length and saturation, cholesterol and sphingolipid content, degree of oxidation, charged group distribution and phospholipid topological exposure can jointly shape particle phase behavior, membrane fluidity, curvature, surface charge, packing density, hydrophobic exposure and interfacial tension. [3,5] Cholesterol- and sphingolipid-rich membrane regions, damaged membrane fragments enriched in oxidized phospholipids, particle surfaces exposing PS, and lipid domains containing ceramide or lysophospholipids are unlikely to form equivalent interfacial objects once they enter an aqueous phase. [4]

Thus, particulate or carrier-associated conversion should not be understood as passive packaging. It is a state transition: lipid combinations and interfacial features in the source membrane are reorganized after leaving the cell into extracellularly accessible interfaces. This process may preserve part of the source state and rewrite part of it at the same time. These two possibilities are not contradictory. The central issue for LST is not whether lipid state remains completely unchanged outside the cell, but whether source-associated differences retain sufficient material continuity through extracellular transformation to influence later routing, sampling and function.

2.2. Biological Origin: Membrane Turnover, Stress Remodeling and Cell Death Generate State-Associated Lipid Externalization Flows

Physical carriage explains how lipid states may exist in the extracellular space. LST also requires a second premise: real biological systems must generate extracellular lipid flows associated with source states. If extracellular lipid states were only products of accidental damage, sample processing or nonspecific aggregation, they would be unlikely to constitute reproducible inputs for transduction. By contrast, if physiological membrane turnover, stress remodeling and cell death continuously release lipid combinations bearing source traces into extracellular space, LST has a biological source basis.

The plasma membrane is not a static barrier. It undergoes continual renewal, flipping, repair, endocytosis, exocytosis, budding, transport and degradation. [40] Most membrane lipids are indeed recycled within cells through the endoplasmic reticulum, lysosomes, membrane contact sites and lipid-transfer systems. It would therefore be incorrect to assume that membrane-lipid turnover is automatically converted into extracellular release. Nevertheless, membrane homeostasis cannot be completely isolated from the extracellular environment. [41] Endocytosis-exocytosis cycles, extracellular vesicle release, microvesicle shedding, membrane-damage repair, renewal of cell-surface microstructures, lipid-transfer-protein-mediated exchange, lipoprotein-associated transport and cell death all allow part of the membrane lipid composition and interfacial state to enter extracellular space. [42,43]

This externalized flow is not necessarily random. Cellular state can be written into the lipid layer through multiple routes. [5] Metabolic reprogramming changes fatty-acid chain length, saturation and glycerophospholipid composition. [44] Inflammatory activation promotes arachidonic-acid metabolism, lysophospholipid generation and oxidized lipid accumulation. [38,45] Oxidative stress increases peroxidized phospholipids, oxysterols and other reactive lipids. [46] Endoplasmic-reticulum stress and mitochondrial dysfunction reshape phospholipid, cardiolipin and sphingolipid metabolism. [47] Mechanical stress and changes in membrane tension affect membrane curvature, packing density and microdomain organization. [3,48] Cellular state is not an abstract variable outside the lipid layer; it can leave nonrandom traces in lipid composition, modification and membrane-interface organization.

Cell death provides an amplified window for observing state-associated lipid externalization. Apoptosis is accompanied by PS externalization, membrane blebbing and the formation of apoptosis-associated membrane particles. Necrosis involves loss of membrane integrity and leakage of intracellular lipids. Ferroptosis is centered on polyunsaturated fatty-acid peroxidation, oxidized phospholipid accumulation and membrane injury. Pyroptosis alters membrane architecture through gasdermin-mediated pore formation and inflammatory content release. These death modalities do not merely release different soluble damage signals. They also alter lipid topology, oxidation state and particulate externalization in distinct ways. [49,50,51]

At the same time, death-associated lipid release is particularly prone to confounding. Large dying-cell fragments, apoptotic bodies, lipid-droplet leakage, protein-lipid precipitates, intracellular contamination and isolation-induced aggregates can all be mistaken for state-associated particles. The value of the death window is therefore not that all death-derived material should be treated as LST-relevant components. Its value lies in providing a high-intensity model to test whether different source states generate distinguishable, characterizable and functionally verifiable lipid interfacial objects.

2.3. Functional Sampleability: Lipid-Interface Differences Can Be Distinguished by Biological Fluids and Target Cells

Physical carriage and biological origin together indicate that state-associated lipid mixtures may enter the extracellular space and form accessible interfaces. The remaining question is whether these interfacial differences have biological consequences. If lipid particles are merely indistinguishable material residues, LST remains a description of externalization rather than a transduction mechanism. Lipid state becomes eligible as a transduced functional input only when biological fluids, target cells or clearance systems process these interfaces differently.

This argument does not require the assumption that cells actively encode a specialized lipid language. A stricter logic is sufficient. If source state can nonrandomly alter lipid composition and organization; if lipid composition and organization can nonrandomly alter particle interfaces; and if particle interfaces can nonrandomly influence fluid-phase protein recruitment and target-cell contact, then lipid particles satisfy the conditions for biological sampling. Here, information does not mean a deliberately designed symbolic code. It means that differences in source state are partly retained as material structural differences and that these structural differences alter subsequent interactions.

A first layer of sampleability arises from coupling between the lipid interface and the fluid-phase identity layer. Plasma, lymph, interstitial fluid and inflammatory exudates contain many proteins that interact with particle surfaces, including albumin, apolipoproteins, complement components, immunoglobulins, coagulation factors, clusterin and fibrinogen. These proteins do not form identical coronas on all particle surfaces. Particle size, curvature, surface charge, hydrophobic exposure, oxidative modification, lipid packing and topological exposure can all influence protein adsorption, exchange and stable retention. Different identity layers can in turn produce different circulation, clearance, tissue distribution and cellular uptake routes. [23,52]

A second layer of sampleability arises from interface-recognition mechanisms in target cells and clearance systems. Cell surfaces contain several systems capable of sensing lipid state or particle interfaces, including scavenger receptors, lipid-transport receptors, PS receptors and their bridging molecules, complement receptors, Toll-like receptor-associated recognition axes, as well as mechanisms of membrane contact, endocytosis, fusion and phagocytosis. A particle that exposes PS, a particle enriched in oxidized phospholipids, a particle bearing a specific apolipoprotein or complement corona, and a particle with a particular distribution of curvature and charge are unlikely to be treated as identical by target cells. [13,53]

Functional sampleability should not be equated with the claim that all differences in particle uptake carry state information. Particle size, charge, aggregation, protein contamination, endotoxin, nonspecific complement deposition and differences in the extent of cell death can also produce divergent cellular uptake or inflammatory readouts. Thus, LST does not require a simple demonstration that different lipid particles produce different responses. It requires evidence that source-state-associated lipid-interface differences survive extracellular transformation sufficiently to alter downstream processing, and that these effects cannot be fully explained by nonspecific particle properties or conventional cargo. [25,54]

Carbon dioxide offers a useful analogy. Carbon dioxide was not originally synthesized as a dedicated long-distance signaling molecule; it is a product of cellular metabolism. Yet because its production, accumulation, diffusion and clearance are stably coupled to metabolic intensity, ventilation status and acid-base balance, the organism can use it in respiratory regulation and homeostatic monitoring. [55] Lipid-state particles are not physically or regulatory equivalent to carbon dioxide. They are more heterogeneous, more local and more dependent on interface and fluid background. The analogy emphasizes only a general principle: living systems can use material traces generated by metabolism, membrane maintenance, stress injury or cell death and incorporate them into state sensing and homeostatic regulation. [56,57]

3. Theoretical Framework: Defining Lipid-State Causality

To make the lipid-state transduction hypothesis (LST) testable, lipid state must be treated neither as a vague descriptor of lipid composition nor as a universal explanation for any effect of lipid-containing particles. LST applies only when a source-state-associated lipid configuration that includes molecular composition, modification state, interfacial organization, particulate presentation and the fluid-phase identity layer contributes causally to a chain from source-state writing to extracellular transformation, biological sampling and functional output, and when this function is not sufficiently explained by conventional RNA or protein cargo, individual lipid mediators, DAMPs, apolipoproteins, endotoxin, cell debris or established particle identity.

3.1. LST Is a State-Transduction Process, Not a New Particle Category

LST describes a state-transduction process rather than a particle category. Its minimal causal chain is source state → lipid-state writing → extracellular presentation → fluid-phase editing and routing → biological sampling → functional state input. A lipid state generated in a source cell or tissue may enter extracellular space through membrane turnover, vesicle release, microvesicle shedding, membrane-fragment externalization, lipid transport, membrane repair, tissue injury or cell death. After release, this state is not simply transported unchanged; it is partially retained, reorganized and displayed through particulate, carrier-associated or complexed interfaces. In biological fluids, this interface may acquire an identity layer composed of albumin, apolipoproteins, complement factors, immunoglobulins, coagulation proteins, clusterin and other fluid-phase molecules. The resulting object can then be sampled by target cells through receptor recognition, membrane contact, endocytosis, fusion, phagocytosis, metabolic processing or clearance pathways. LST therefore requires neither unchanged transmission nor a dedicated signaling cascade; it requires sufficient continuity for source-associated material differences to survive transformation and influence downstream biological processing.

This formulation is intentionally orthogonal to conventional particle classification. Classical EVs, lipoproteins, apoptotic bodies, death-associated membrane particles, exomeres, supermeres and other extracellular nano-entities retain their own biogenetic routes, marker systems and technical definitions. LST does not ask whether a particle should be renamed. It asks whether, in a defined context, the particle participates causally in the transduction of a source-associated lipid state into a biological output. An EV may participate in LST; a lipoprotein may participate in LST; a death-associated particle may participate in LST. Conversely, if a particle’s core effect is fully explained by RNA, protein, DAMPs, apolipoproteins, free lipid mediators, endotoxin contamination or conventional particle identity, it is not an LST functional unit.

3.2. Lipid State, Particulate Presentation and Identity Layer

The central variable of LST is lipid state. It is broader than a lipidomic profile but narrower than an unrestricted description of all lipid-associated features. Lipid state refers to the source-associated molecular and interfacial configuration that becomes extracellularly presented under a specific particulate context and can be transformed during fluid exposure. It has two coupled dimensions.

The first is the lipid molecular state: lipid class, fatty-acid chain length and saturation, cholesterol and sphingolipid abundance, ceramide, lysophospholipids, PS, oxidized phospholipids, oxysterols and other metabolic or oxidative modifications. The second is the lipid interfacial state: phase behavior, membrane fluidity, curvature, surface charge, packing density, hydrophobic exposure, topological exposure, membrane-fragment orientation, interfacial tension, microdomain organization and mode of particle formation. The molecular state provides chemical content; the interfacial state determines how that content is displayed, contacted and interpreted.

Particulate presentation is the conversion of lipid state into a transportable and sampleable extracellular interface. Such presentation may occur through EV-like structures, lipoprotein-like complexes, membrane fragments, mixed micelles, protein-lipid complexes, death-associated lipid particles or other particle-like assemblies. Particle formation alone does not prove LST. Its importance is that it gives a lipid state a mode of display—a surface, curvature, topology, carrier context and fluid-facing architecture—through which the state can be further edited, routed and sampled.

The identity layer is the routing and interpretive layer acquired after the lipid interface enters biological fluids. It is not identical to lipid state, but it is coupled to it and constitutes an extracellular transformation step in LST. Different lipid interfaces recruit different fluid-phase molecules, and different coronas or biomolecular identity layers can assign particles distinct circulation times, tissue distributions, uptake routes, complement interactions, phagocytic fates and immune consequences. In this sense, LST is not a simple lipid-composition hypothesis. It is a hypothesis about how lipid composition, interface, particulate presentation and fluid-phase identity together convert source-associated material state into extracellular state causality.

3.3. Minimal Criteria for Lipid-State Causality

An extracellular lipid-containing particle should be considered to possess lipid-state causality only when four criteria are met.

First, source-state association must be demonstrated. The particle’s lipid molecular or interfacial state should change reproducibly with the metabolic, inflammatory, oxidative, differentiation, repair, senescence or death state of the source cell or tissue. Differences fully explained by particle number, size, total protein, culture background, extent of death or isolation method do not support LST.

Second, state dependence must be demonstrated. The particle’s core function should depend on lipid state rather than on a single RNA, protein, DAMP, apolipoprotein or free lipid mediator. Supportive evidence would include directional weakening, switching or rerouting of function after lipid-enzyme treatment, oxidation-state modulation, cholesterol or sphingolipid perturbation, membrane-phase alteration, masking of topological exposure or disruption of the particle interface.

Third, presentation dependence must be demonstrated. Similar lipid compositions should produce different routing, uptake or biological outputs when placed in different particulate forms, carrier states, topological orientations or identity layers. This criterion separates lipid-state causality from simple lipid-abundance effects.

Fourth, perturbation and reconstitution must be possible. Disrupting, transferring, reconstituting or restoring the lipid state should alter particle function in the predicted direction. Reconstitution does not need to recreate the original particle completely, because native particles may contain proteins, nucleic acids and other molecular layers. However, if a reconstituted source lipidome or reconstructed lipid interface partially reproduces a directional functional bias after conventional cargo has been controlled, lipid state gains causal weight.

3.4. Layered Versions of LST

LST has three nested versions. Weak LST is the most conservative. It proposes that classical EVs and lipoproteins can already mediate lipid-state transduction when source-associated lipid composition, modification and interfacial organization contribute causally to downstream routing or function beyond a purely structural role.

Intermediate LST adds that transduction depends not only on lipid molecular composition but also on particulate form, carrier state, interfacial presentation and the fluid-phase identity layer. In this version, extracellular presentation is transformative rather than neutral: similar source-associated lipid states can be routed or interpreted differently depending on whether they are displayed as an EV-like membrane, a lipoprotein-like complex, a membrane fragment, a lipid nanoparticle or a protein-coronated particle.

Strong LST is the highest-risk prediction. It proposes that, after classical EVs, classical lipoproteins, free lipid mediators, conventional RNA/protein cargo, DAMPs, cell fragments and technical artifacts have been rigorously controlled, some extracellular particle-like components may still remain whose function is source-state associated, lipid-state dominated and dependent on an intact lipid interface. These candidate components are referred to here as non-classical lipid-state transduction particles, or non-classical LSTPs, because they are predicted to participate materially in the transduction chain rather than merely to carry an unexplained lipid signature.

This layered structure gives LST both flexibility and falsifiability. Evidence that lipid state within known EVs or lipoproteins contributes to conversion of source-associated state into downstream biological processing supports weak LST. Evidence that carrier form and identity layer transform routing or interpretation supports intermediate LST. Evidence for a lipid-dominant functional residual beyond established particle classes supports strong LST. Conversely, if all observed effects are sufficiently explained by RNA, proteins, apolipoproteins, DAMPs, free lipid mediators or known particle classes, LST should contract accordingly.

4. Non-Classical LSTPs: A Candidate Layer Predicted by Strong LST

The highest-risk prediction of strong LST is that classical extracellular vesicles, classical lipoproteins and free lipid mediators may not exhaust all biologically relevant routes through which source-associated lipid states can be externalized, transformed and sampled. This prediction should not be derived from taxonomy alone. The overlap among EVs, lipoproteins, exomeres, supermeres and other nanoscale entities shows that current classifications are operationally incomplete, but a classification gap is not sufficient evidence for a new functional layer. The stronger argument is physiological: cells and tissues continuously generate lipid-state burdens that must be externalized, stabilized, routed, cleared or sampled, whereas known lipid-handling routes may be too specialized, centralized or molecularly simplified to preserve and process all such composite states.

Non-classical lipid-state transduction particles, or non-classical LSTPs, are therefore proposed as a candidate residual predicted by strong LST. They are not a new name for poorly purified particles, nor a substitute for EVs, lipoproteins or lipid mediators. They refer to extracellular lipid-dominant particle-like or complex-like components that are locally generated, retain partial source-state information, undergo fluid-phase identity-layer formation, and are predicted to mediate lipid-state transduction through functions dependent on intact lipid state after classical EVs, classical lipoproteins, soluble lipid mediators, conventional molecular cargo, cell fragments and technical artifacts have been controlled.

The necessity of this candidate layer comes from a mismatch between biological demand and known transport capacity. EVs are information-rich but biogenetically organized and relatively costly. Classical lipoproteins are powerful systemic transport particles but are structured around hepatic, intestinal and intravascular lipid metabolism, not immediate organ-autonomous export of every local membrane state. Free lipid mediators are chemically precise but too structurally simplified to preserve source-membrane architecture. If extracellular lipid state carries biological meaning beyond lipid mass and single-molecule signaling, an intermediate layer is expected: less canonical than EVs, more locally generable than lipoproteins and more state-preserving than free lipid mediators.

4.1. Physiological Necessity: Lipid-State Export Demand Exceeds Classical Extracellular Lipid Routes

The argument for non-classical LSTPs begins after, rather than before, the general premise of lipid-state transduction has been established. If source-cell lipid states can be written, externalized, partially retained, transformed and sampled, the next question is whether known extracellular lipid routes can accommodate the full range of local lipid-state burdens generated by living tissues. This is a capacity problem, not merely a classification problem. [10] Tissue membranes are continuously remodeled by metabolism, mechanical stress, inflammation, oxidative injury, repair, senescence and regulated or accidental cell death. Under these conditions, cells may need to externalize lipid interfaces enriched in oxidized phospholipids, exposed PS, ceramide-rich domains, lysolipid-containing regions, packing defects, abnormal curvature or hydrophobic exposure. [5] Such outputs are not simply lipid mass awaiting transport. They are composite membrane states that may require extracellular stabilization, transformation, routing, clearance or sampling before being degraded or diluted. [56]

This demand is unlikely to be uniform across organs. The brain operates behind restricted lipid-exchange barriers; the heart and endothelium face continuous mechanical and oxidative stress; renal, pulmonary and intestinal epithelia undergo high turnover and injury exposure; immune cells rapidly remodel membranes during activation, phagocytosis and death; and tumors generate heterogeneous lipid states under hypoxia, acidosis and therapy-induced stress. In these settings, lipid-state outputs may be local, time-sensitive and source-specific. [58] The critical issue is therefore whether canonical EV biogenesis, liver–intestine-centered lipoprotein metabolism and soluble lipid mediators can fully handle this diversity. Strong LST predicts that they cannot, and that an intermediate, locally generable and lipid-dominant mode of extracellular lipid-state handling should exist. [41,59]

Classical EVs provide an important but incomplete solution. EVs are suited for regulated intercellular communication, protected molecular delivery and structured membrane presentation, and they can certainly participate in LST. [41] However, canonical EV production is biologically organized. Exosome generation requires endosomal sorting, multivesicular-body formation, trafficking and membrane fusion. [60] Microvesicle release requires plasma-membrane remodeling, cytoskeletal reorganization, curvature generation and scission. These processes provide selectivity and control, but they are unlikely to serve as the universal outlet for every local lipid-state disturbance. [61]

This distinction is clearest under acute stress. Ferroptosis rapidly damages polyunsaturated phospholipids. Pyroptosis alters membrane integrity through gasdermin pores. Necrosis exposes disordered lipid material. Mechanical injury or ischemia-reperfusion can generate membrane repair and lipid oxidation faster than classical vesicle biogenesis can package the altered lipid burden into canonical EVs. [43,49,62] Under such conditions, a cell may need a lower-cost route for exporting lipid interfaces, such as partial membrane shedding, repair-associated extrusion, lipid-protein complex formation, corona-stabilized membrane fragments or other lipid-dominant assemblies. These outputs may be reproducible and functional without satisfying the full marker, cargo or biogenesis criteria of classical EVs.

Classical lipoproteins solve another major lipid problem, but they are not a universal route for preserving and transducing local lipid states. HDL, LDL, VLDL, chylomicrons and their remnants organize systemic lipid transport, cholesterol distribution, triglyceride metabolism and reverse cholesterol transport. [33,63] Yet these particles are centered on hepatic, intestinal and intravascular programs. Peripheral tissues can transfer cholesterol and phospholipids to HDL-like acceptors, but this is not equivalent to producing, on demand, an organ-autonomous apolipoprotein-defined particle that preserves local membrane topology, oxidation pattern, PS exposure, curvature, sphingolipid organization or death-modality-associated interface. In LST terms, lipoproteins are excellent carriers of lipid quantity and selected lipid species; they may participate in lipid-state transduction, but they are not necessarily sufficient carriers of local lipid-state architecture. [64,65]

Free lipid mediators and albumin-bound lipids solve a third problem: the transport of defined fatty acids, eicosanoids, lysophospholipids, sphingolipid mediators and related molecular signals. [66,67] Their strength is chemical precision. Their limitation is loss of architecture. A lipid state is not reducible to one molecule; its biological meaning may reside in the relation among lipid classes, oxidation clusters, acyl-chain composition, PS topology, cholesterol-sphingolipid packing, curvature, phase behavior, hydrophobic exposure and corona-forming propensity. Once a membrane state is fully dismantled into soluble or albumin-bound molecules, much of this interfacial information is lost. [19,40]

In the author’s previous works, whole lipids extracted from cells or tissues could be readily reassembled into uniform reconstituted lipid nanoparticles (rLNPs) upon entering the aqueous medium, with high reproducibility without the aid of additional proteins. [30,31] These rLNPs are not natural non-classical LSTPs. However, they provide an important physical clue: complex source-derived lipid mixtures, after removal from their original membrane and after substantial depletion of conventional protein and nucleic-acid cargo, can still form stable lipid-dominant particle-like interfaces with biological functionality. This supports the plausibility that endogenous extracellular lipid mixtures, when released into biological fluids, may also form transient, hybrid or low-marker lipid-state assemblies rather than being reduced only to free lipid molecules, canonical EVs or classical lipoproteins. This reasoning predicts several features. Non-classical LSTPs should be heterogeneous rather than a single uniform species. They may arise through membrane shedding, repair-associated extrusion, stress-induced lipid clustering, death-associated fragmentation, lipid-protein complex formation, extracellular lipid exchange, state transformation or corona-stabilized remodeling. They should be enriched when lipid-state pressure is high, such as oxidative stress, ferroptosis, pyroptosis, ischemia-reperfusion, tissue injury, chronic inflammation, defective clearance and tumor stress. They may be low in canonical EV markers and classical apolipoproteins because their formation logic is not primarily EV biogenesis or lipoprotein metabolism. Their defining property, however, should not be marker absence, but lipid-state-dependent function.

4.2. Positive Causal Residual Rather Than Taxonomic Gap

Physiological plausibility does not prove existence. A new candidate layer becomes meaningful only when there is a positive causal residual: a reproducible functional component that remains insufficiently explained by classical EVs, classical lipoproteins, soluble lipid mediators, conventional RNA/protein cargo, DAMPs, cell fragments, lipid droplets, protein-lipid aggregates, immune complexes, endotoxin or technical artifacts.

This residual is not a residue of purification, but a residue of explanation. A lipid-rich fraction is not a non-classical LSTP merely because it appears after EV isolation. A marker-poor nanoscale component is not a non-classical LSTP merely because it lacks CD9, CD63, CD81, ApoA1 or ApoB. A death-derived lipid particle is not a non-classical LSTP merely because it is released during injury. The residual required by strong LST must be extracellular and particle-like or complex-like; show reproducible association with a source-cell or tissue state; possess a characterizable lipid molecular and interfacial state; and exert a function that depends on that lipid state in a manner consistent with the LST chain.

This formulation avoids two opposite errors. The first is premature dismissal, in which all lipid-rich, low-marker extracellular components are treated as contaminants because they do not fit classical EV or lipoprotein definitions. The second is premature naming, in which any unexplained lipid-containing component is called a non-classical LSTP. Strong LST requires the stricter middle position: candidate non-classical LSTPs are worth pursuing when conventional categories do not explain the functional output, but they become scientifically credible only when lipid-state causality is demonstrated.

The most persuasive residual would have five features: persistence after rigorous reduction of classical EVs, lipoproteins, soluble factors, large debris and artifacts; lipid enrichment and particle-like or complex-like behavior under orthogonal isolation methods; reproducible lipid-state shifts with source condition; functional alteration after lipid-state perturbation, such as lipid-enzyme treatment, oxidation-state modulation, PS masking, cholesterol/sphingolipid perturbation, phase-behavior disruption or corona exchange; and partial restoration or reproduction of functional direction by reconstituting the relevant lipid state. Only this kind of positive residual supports non-classical LSTPs as a candidate layer of strong LST.

4.3. Operational Definition and Minimal Criteria

Candidate non-classical LSTPs can be operationally defined as extracellular lipid-dominant particle-like or complex-like components that retain source-state association, a characterizable lipid state and lipid-state-dependent function after rigorous control of classical EVs, classical lipoproteins, soluble lipid mediators, conventional molecular cargo, death fragments and technical artifacts, and whose properties are consistent with participation in the LST sequence from source-state representation to biological sampling.

This definition has three tiers. The first is screening. Candidate fractions may show relative lipid enrichment, low canonical EV-marker abundance, low classical apolipoprotein abundance, weak dependence on conventional RNA or protein cargo, and particle-like or complex-like behavior. These features identify a search space, not an entity.

The second tier is exclusion. Candidate non-classical LSTPs should not be sufficiently explained by classical EVs, low-marker EV subpopulations, HDL, LDL, VLDL, lipoprotein remnants, exomeres, supermeres, apoptotic bodies, large death fragments, lipid droplets, immune complexes, protein-lipid precipitates, culture-medium contamination, endotoxin or isolation artifacts. These exclusions must be tested by orthogonal methods rather than assumed from a single marker panel or isolation protocol.

The third tier is positive causality. Candidate non-classical LSTPs must show source-state association, lipid-state characterizability and lipid-state-dependent function. Source-state association means that the lipid molecular or interfacial state changes reproducibly with the condition of the source cell or tissue. Lipid-state characterizability means that the candidate can be described by lipid class, chain features, oxidation status, PS exposure, cholesterol/sphingolipid relationship, ceramide or lysolipid enrichment, size, density, charge, curvature, hydrophobic exposure, membrane fluidity, protein-corona profile or related interfacial properties. Lipid-state-dependent function means that the biological output is weakened, redirected or restored by perturbing or reconstituting the lipid state.

This definition intentionally avoids a single marker. A marker-based definition would be premature because the predicted layer is heterogeneous and may arise from several routes of lipid-state externalization. The central criterion is functional causality. A candidate component becomes relevant to strong LST only when its intact lipid state is necessary for its effect and conventional explanations remain insufficient.

4.4. Boundaries with Known Extracellular Particle Systems

Non-classical LSTPs should not be invoked when known extracellular particle systems provide sufficient explanations. Their purpose is not to expand nomenclature, but to identify lipid-state-dominant functional components that escape existing explanatory frameworks.

The boundary with EVs is critical. Low EV-marker abundance does not automatically define a non-classical LSTP. A particle that fits an EV subtype in size, density, membrane topology, protein/RNA cargo, biogenesis route or functional dependence should remain within EV biology. In that case, lipid state may still be important, but strong LST is not required.

The boundary with lipoproteins is equally strict. Lipid richness does not define a non-classical LSTP. If a candidate effect is explained by HDL, LDL, VLDL, chylomicron remnants, ApoA1, ApoB, ApoE, ApoM, lipoprotein lipases, lipid-transfer proteins or classical lipoprotein receptors, the non-classical attribution fails. Such a system may support LST within lipoprotein biology, but not a separate lipid-state-dominant residual.

The boundary with debris and artifacts must also be enforced. Large death fragments may carry DNA, histones, mitochondria, ER membranes, cytoskeletal components or organelle markers. Apoptotic bodies may enclose organelles and apoptotic cargo. Lipid droplets contain neutral lipid cores and droplet-associated proteins. Protein-lipid aggregates may be induced by freeze-thaw cycles, salt concentration, centrifugal force, precipitation reagents or sample processing. Immune complexes and endotoxin can generate strong functional readouts unrelated to lipid-state transduction. A candidate that cannot be reproduced by orthogonal isolation, retain a stable lipid-state signature, or survive contamination and aggregation controls should not be assigned to non-classical LSTPs.

These boundaries strengthen rather than weaken the concept. Non-classical LSTPs should be considered only when the evidence points simultaneously away from known particle systems and toward lipid-state-dependent causality.

4.5. Death-Associated Lipid Particles as the Priority Stress-Test

Cell death provides the most suitable initial stress-test because it amplifies lipid-state externalization and creates clear functional readouts. Death-derived material should not be presumed non-classical. Rather, death generates the conditions under which non-classical lipid-state particles are most likely to emerge: abnormal topology, PS exposure, lipid peroxidation, membrane fragmentation, disrupted packing, hydrophobic exposure, inflammatory fluid contact and urgent clearance demand. [6,68]

Different death modalities provide natural lipid-state perturbations. Apoptosis generates PS-exposed, blebbing-associated clearance interfaces. Necrosis releases ruptured membrane material and intracellular lipid mixtures in a disordered context. Ferroptosis produces polyunsaturated phospholipid peroxidation, oxidized phospholipid accumulation and membrane damage. Pyroptosis reshapes membranes through gasdermin-mediated pore formation and inflammatory release. These modalities differ not only in DAMP production but also in lipid-interface state, making them useful for testing whether lipid-state-dominant functional components remain after conventional death products are controlled. [51,69]

A rational workflow would induce defined death modalities in the same cell source while controlling for time window, degree of death, culture background and extent of lysis. Large debris, organelles, typical apoptotic bodies and lipid droplets should be removed. Candidate fractions should then be obtained by orthogonal size and density fractionation, EV-marker depletion, apolipoprotein depletion and contamination control. Characterization should include lipidomics, oxidized-lipid profiling, PS topology, size, density, charge, curvature, hydrophobic exposure, membrane fluidity, protein-corona composition and marker profiling.

Functional testing may examine macrophage phagocytosis, inflammatory mediator production, immune-tolerance bias, endothelial response, complement deposition, tissue routing or clearance behavior. Interpretation must remain strict. If function follows DNA, histones, mitochondrial components, soluble cytokines, DAMPs, endotoxin, classical EV markers or apolipoprotein-defined particles, non-classical attribution fails. If the function persists in a lipid-dominant low-marker component, changes with lipid-state perturbation and can be partially reproduced by reconstituting the relevant lipid state, death-associated particles become a powerful entry point for strong LST.

Death-associated models are therefore useful not because they are clean, but because they are demanding. They generate many confounders, yet they also generate strong lipid-state pressure and clear functional consequences. A candidate mechanism that survives this stress-test would not prove that all tissues use non-classical LSTPs, but it would establish that lipid-state transduction can extend beyond conventional EV, lipoprotein and free-mediator frameworks.

4.6. Summary: Why Non-Classical LSTPs Are the Central Prediction of Strong LST

Non-classical LSTPs are introduced because known extracellular lipid-handling routes leave a physiological and explanatory gap. Cells and tissues continuously generate complex lipid states that may need to be removed, displayed, routed or sampled. EVs, lipoproteins and free lipid mediators solve major parts of this problem, but each does so through a specialized logic that may not preserve local membrane-state architecture.

If extracellular lipid state is biologically meaningful, an intermediate layer is expected: lipid-dominant particle-like or complex-like interfaces that are locally generated, partially state-preserving, fluid-edited and functionally sampleable. Their existence is not assumed. It must be demonstrated as a positive causal residual after rigorous exclusion of known systems. If candidate effects are explained by EVs, lipoproteins, free mediators, DAMPs, debris or artifacts, strong LST contracts. If they retain source-state association, undergo biologically relevant extracellular transformation, and depend on intact lipid state after these alternatives are controlled, non-classical LSTPs become the central empirical prediction of strong LST.

5. Testable Predictions

LST is useful only if it makes predictions beyond established facts such as EV release, lipoprotein transport, lipid mediator signaling or DAMP biology. Its core prediction is a causal sequence: source states write reproducible lipid-state differences; these differences are externalized and presented through extracellular interfaces; biological fluids edit their identity and routing; target cells or clearance systems sample them; and this sampling generates a lipid-state-dependent functional output. This transduction chain is stratified and falsifiable.

5.1. State Writing Generates Reproducible Particle Lipid-State Fingerprints

LST predicts that defined source states generate reproducible lipid-state fingerprints in released lipid-containing particles. These fingerprints are not simple differences in total lipid abundance, particle number, size or canonical markers. They are combined molecular-interfacial patterns involving lipid class, chain saturation, cholesterol/sphingolipid ratio, ceramide, lysophospholipids, oxidized lipids, PS exposure, membrane fluidity, phase behavior, charge, curvature, hydrophobic exposure and propensity for identity-layer formation. [70]

Testing should compare basal, inflammatory, oxidative, metabolic, sublethal-injury and death-modality states under controlled conditions. Particles should be isolated by orthogonal methods while controlling for particle number, size, total protein, culture-medium background, extent of death, EV markers, apolipoproteins and major RNA/protein cargo. Support for the state-writing layer of LST requires blinded or cross-validated discrimination of source state by lipid-state variables that cannot be fully reduced to conventional particle parameters. [25]

5.2. Extracellular Retention and Presentation Preserve Lipid-State Dependence

The next prediction is that source-associated lipid-state differences remain sufficiently organized after externalization to influence particle function, rather than being reduced entirely to unrelated molecular abundance. If one RNA, protein, DAMP, apolipoprotein or free lipid mediator sufficiently reproduces the effect, LST is unnecessary. If perturbing lipid composition, oxidation status, phase behavior, curvature, topological exposure or interfacial organization changes function, and add-back of conventional cargo fails to restore it, the retained and presented lipid state has causal importance. [71]

Relevant assays may include macrophage phagocytosis, inflammatory mediator release, endothelial barrier regulation, metabolic response, tissue repair and clearance routing. Perturbations may include RNase, protease, lipid enzymes, oxidation-state modulation, cholesterol/sphingolipid perturbation, membrane-phase alteration, PS masking, corona exchange and carrier exchange. Particle integrity, aggregation, toxicity, endotoxin and nonspecific precipitation must be monitored in parallel. [25]

5.3. Fluid-Phase Editing and Identity Layers Transform Lipid-State Function

Intermediate LST predicts that extracellular presentation is not a neutral transport step: the same lipid composition will not necessarily be biologically equivalent across different carriers, interfaces or identity layers. If lipid function were determined only by composition and concentration, the same lipid mixture should behave similarly regardless of curvature, topology, corona or carrier. LST predicts instead that fluid-phase editing and particulate context can transform routing, sampling and function. [15]

This can be tested by reconstituting the same source lipidome into lipoprotein-like particles, EV-like liposomes, membrane-fragment-like particles or lipid nanoparticles, then comparing corona formation, cellular uptake and biological output. Alternatively, the same particle population can be exposed to albumin-rich fluid, HDL/ApoM-rich fluid, complement-activated plasma, inflammatory interstitial fluid or complement-depleted plasma. Supportive results would show that the identity layer changes uptake, complement deposition, tissue routing, phagocytic clearance or functional output. [22]

5.4. Target Systems Sample Death-Modality-Specific Lipid States

LST predicts that apoptosis, necrosis, ferroptosis and pyroptosis generate not only different soluble danger signals but also different death-associated lipid-state particles that can be differentially sampled by biological fluids, target cells and clearance systems. These particles should differ in lipid oxidation, PS exposure, membrane topology, phase behavior, curvature, charge, protein corona and immune outcome. [50]

This prediction extends danger-signal theory. DAMPs indicate that injury has occurred, but they may not fully represent membrane state, oxidation intensity, death modality or clearance requirement. If death-modality-derived lipid particles still predict or alter phagocytic, inflammatory, tolerogenic or repair responses after controlling for LDH, ATP, HMGB1, DNA, histones, cytokines, total death burden and large debris, LST gains incremental explanatory value. [56]

5.5. Strong LST Predicts Residual Transduction Components Beyond Classical EVs and Lipoproteins

The defining prediction of strong LST is that, in some high-perturbation contexts, a lipid-state-dependent functional residual remains after classical EVs, lipoproteins, soluble mediators, cell fragments, lipid droplets and obvious artifacts are removed. These residual components should be lipid enriched, low in conventional markers, source-state associated, sensitive to lipid-state perturbation, and functionally positioned within the LST chain rather than being unexplained purification residues. [54]

Suitable models include ferroptosis, severe oxidative stress, tissue injury, inflammatory microenvironments and death-modality comparisons. Required controls include orthogonal isolation, EV-marker immunodepletion, apolipoprotein removal, debris clearance, lipid-droplet exclusion, endotoxin control and culture-medium background control. Strong LST is supported only if function persists after these exclusions and is altered by lipid-enzyme treatment, oxidation-state modulation, PS masking, phase perturbation, corona exchange or lipid-state reconstitution. [25]

5.6. Lipid-State Transduction Transfers Partial Source-State Bias

The highest-order prediction is not that lipid-state particles reproduce an entire disease phenotype or copy a source state intact. A more restrained prediction is that, in defined recipient models, source-state-associated lipid particles transduce a limited, measurable and reversible functional bias: altered macrophage phagocytic threshold, endothelial barrier response, liver-spleen clearance routing, local inflammatory-resolution trajectory or metabolic-cell response tendency.

Support requires dose-related, reproducible and source-state-aligned effects after controlling for soluble factors, cytokines, DAMPs, classical EV RNA/protein cargo, lipoproteins, complement background, endotoxin and particle number. The transduced bias should be weakened by lipid-state disruption and partially reproduced by reconstituted source lipid-state particles. [25]

6. Failure Conditions and Theoretical Boundaries

LST must be able to contract. If every lipid-associated particle effect is assigned to lipid state, the hypothesis becomes unfalsifiable. Negative results should therefore weaken specific layers of LST rather than being ignored or treated as a simple binary refutation.

6.1. Preconditions for Interpretable Negative Results

A negative result is meaningful only when the experimental system is adequately controlled. The source-state model must be independently validated. Particle isolation should use orthogonal methods rather than a single ultracentrifugation, precipitation, size-exclusion or marker-based approach. Classical EVs, lipoproteins, exomeres, supermeres, apoptotic bodies, death fragments, lipid droplets, DNA/histones, protein aggregates, endotoxin, culture-medium background and isolation artifacts should be addressed systematically.

Lipid-state measurement should include both molecular composition and interfacial state. Total lipid mass or a small panel of lipid species is insufficient. Functional readouts should include positive controls, negative controls, toxicity controls and particle-integrity controls. Perturbation experiments require particular caution because lipid enzymes, cholesterol extraction, redox interventions or phase-state modifiers may rupture particles, induce aggregation, precipitate proteins or cause nonspecific toxicity. Only when dose, integrity, toxicity and contamination are controlled can changes in function be interpreted as evidence for or against lipid-state causality.

6.2. Stratified Failure Conditions

If qualified models repeatedly fail to detect source-state-associated lipid-state fingerprints, the state-writing layer of LST is weakened.

If particle function is fully explained by RNA, proteins, DAMPs, apolipoproteins or free lipid mediators, and lipid-state perturbation or reconstitution does not change function, lipid-state causality is not required in that system.

If carrier state, interfacial presentation and identity layer do not transform routing, sampling or function, intermediate LST should contract, although lipid molecular composition may still remain relevant.

If death-modality-specific responses are fully explained by DAMPs, extent of death, large debris or classical EV cargo, death-associated LST loses incremental explanatory value relative to danger-signal theory.

If all EV-low, Apo-low and lipid-enriched candidate components can be assigned to known particle systems or artifacts, the non-classical LSTP prediction fails. LST should then contract to lipid-state transduction within classical EVs, lipoproteins or known extracellular particle systems.

If source-state lipid particles cannot generate any lipid-state-dependent functional bias in defined recipient models, LST should not be extended to systemic homeostatic sampling, although local or particle-specific LST may remain viable.

If several of these layers fail in well-controlled systems, especially if lipid state neither records source state nor survives extracellular transformation sufficiently to affect downstream processing and conventional cargo explains all candidate effects, LST should be downgraded to a lipid-associated concomitant phenomenon rather than retained as an independent hypothesis.

7. Explanatory Increment and Scope of Application

LST does not replace EV biology, lipoprotein metabolism, DAMP theory, cytokine networks, the senescence-associated secretory phenotype, trained immunity, protein-corona theory, pharmacology or toxicology. Its value is to introduce an intermediate causal process that these frameworks do not systematically address: source-cell or tissue state may be materially represented in lipid state, transformed during extracellular presentation and fluid exposure, and converted through biological sampling into a functional input.

7.1. Death-Modality Recognition and Danger-State Interpretation

Death recognition is the most direct application of LST. DAMP frameworks explain how the immune system detects injury and initiates inflammation, clearance or repair. ATP, HMGB1, heat-shock proteins, mitochondrial DNA and related molecules indicate that damage has occurred. However, they do not fully explain the membrane state in which death occurred, whether dying material should be silently cleared or inflammatorily processed, or why different death modalities produce distinct immune outcomes. [72] LST extends death recognition from released molecules to death-associated lipid states, including apoptosis, ferroptosis and pyroptosis. These lipid states may inform phagocytes and immune systems about membrane integrity, oxidation burden, death modality and clearance requirement.

The minimal claim is not that death changes lipids. It is that death-modality-specific lipid states still predict or alter phagocytic, inflammatory, tolerogenic or repair outcomes after controlling for death burden, DAMPs, cytokines, large debris and classical EV/RNA/protein cargo. If this condition holds, lipid state functions as a danger-state input beyond DAMPs.

7.2. Tissue Injury, Remote Effects and Chronic Inflammatory Contexts

Local tissue injury can produce systemic effects, including endothelial responses, altered immune thresholds, remote organ dysfunction and inflammatory amplification. Conventional explanations include cytokines, complement activation, neuroendocrine responses, circulating immune-cell reprogramming, EV cargo and altered lipoprotein metabolism. These mechanisms remain primary. [73]

LST adds a tissue-context variable. Injury changes membrane lipid composition, oxidation status, death modality and fragmentation pattern in source cells. These altered lipid states may enter blood, lymph or interstitial fluid as EVs, lipoprotein-like complexes, membrane fragments, death-associated particles or other lipid-containing assemblies. After acquiring identity layers, they may be transformed in routing and sampleability while still retaining composite traces of source tissue, injury type, oxidative intensity, membrane integrity and clearance requirement. [41]

Chronic inflammation and inflammaging represent low-intensity extensions of this logic. Persistent oxidative pressure, defective cholesterol handling, sphingolipid remodeling, lipid-droplet dysregulation, mitochondrial dysfunction and impaired dead-cell clearance may continuously alter extracellular lipid-state backgrounds. LST adds value only if these lipid-state inputs alter phagocytic, endothelial or metabolic-inflammatory thresholds after conventional cytokines, DAMPs, trained immunity and EV cargo have been controlled. [74]

7.3. Exogenous Hydrophobic Molecules and Membrane Toxicity

LST primarily concerns endogenous lipid states, but exogenous hydrophobic or amphipathic molecules may perturb extracellular lipid-state networks. Many drugs, toxicants and environmental compounds can insert into membranes, bind lipoproteins, adsorb onto EVs or alter lipid nanoparticle interfaces. [75] Conventional pharmacology explains target binding; conventional toxicology explains metabolites, organ injury and direct membrane toxicity. LST adds a boundary-level question: can exogenous molecules alter immune recognition, routing or secondary tissue responses by changing extracellular lipid-particle interfaces?

Such perturbation may occur in three ways. Exogenous molecules may insert into EVs, lipoproteins, membrane fragments or lipid nanoparticles and alter phase behavior, curvature, charge, hydrophobic exposure or corona formation. They may alter intracellular lipid metabolism, lysosomal processing, mitochondrial membranes or oxidized lipid accumulation, thereby changing particles released by cells. They may also induce sublethal membrane injury that externalizes abnormal lipid mixtures and generates injury-associated lipid-state inputs. LST is needed only when these effects cannot be explained by target binding, metabolites, direct toxicity or classical immune stimulation.

7.4. When LST Is Needed and When It Is Not

LST is needed when source state, lipid interface, extracellular transformation, biological sampling and functional output are stably coupled in a way that conventional mechanisms cannot sufficiently explain. It is not needed when soluble cytokines, DAMPs, complement, EV RNA/protein cargo, lipoprotein metabolism, single-molecule lipid mediators, drug targets or direct toxicity provide adequate explanations.

Its scope should therefore remain conditional and contractible. The core application is death-modality recognition and danger-state interpretation. The intermediate application is tissue injury, remote responses and chronic inflammatory background signaling. The boundary application is perturbation of extracellular lipid-state networks by exogenous hydrophobic molecules. LST should not become a universal lipid theory. It should remain a delimited causal framework for testing whether source-associated lipid state is written, extracellularly presented and transformed, biologically sampled, and converted into functional consequence beyond conventional explanatory layers.

8. Discussion and Outlook

The main contribution of LST is to shift extracellular lipid-particle research from particle identity and cargo attribution toward extracellular state causality. Conventional frameworks ask what particle is present and which RNA, protein, metabolite or single lipid mediator explains its function. These questions remain essential, but they may not exhaust mechanism. LST asks a different question: what source-associated state is materially represented in lipid organization, how is that state transformed during extracellular presentation and fluid exposure, and can the resulting interface be biologically sampled in a way that changes function? In this view, extracellular lipid-containing particles are not only vehicles or residues; in defined contexts, they may serve as material transduction interfaces through which source-cell or tissue state is converted into biological consequence.

8.1. From Particle Identity to Lipid-State Causality

LST asks five practical questions. Does source state write a reproducible lipid molecular or interfacial pattern? Does that pattern remain sufficiently organized after externalization to be presented as an extracellular interface? Do carrier form and fluid-phase identity transform its routing or accessibility? Do target cells or clearance systems differentially sample the resulting interface? Do perturbation and reconstitution of lipid state change the downstream functional direction after conventional cargo is controlled?

Only affirmative answers move LST from description to mechanism. This approach also prevents overextension. Lipids are not assumed to be causal because they are present, nor is every extracellular response labeled transduction. They become part of an LST mechanism only when source-state association, extracellular presentation and transformation, biological sampling, functional perturbation and partial reconstitution converge.

8.2. Technical Challenges

Testing LST is difficult because extracellular lipid-containing particles are heterogeneous. EVs, lipoproteins, remnants, exomeres, supermeres, protein-lipid complexes, death fragments, lipid droplets and artifacts overlap in size, density, composition and isolation behavior. No single isolation method can define an LST unit. Ultracentrifugation, size exclusion, density gradients, immunodepletion, apolipoprotein removal and contamination control should be combined.

Lipid state is also multidimensional. Lipidomics defines molecular composition but not curvature, topology, phase behavior, hydrophobic exposure or corona. Particle biophysics defines interface but not molecular origin. Protein-corona profiling defines identity layer but not lipid causality. LST therefore requires integrated analysis: lipidomics, oxidized-lipid detection, topological-exposure assays, particle size and morphology, charge, membrane fluidity, corona profiling and functional readouts.

Causality is the central challenge. Correlation between lipid state and function is insufficient. Perturbation must change function without nonspecific rupture, aggregation, precipitation, toxicity or contamination. Reconstitution should partially reproduce the functional direction. Strong LST requires even stricter evidence: candidate EV-low/Apo-low/lipid-enriched components must retain lipid-state-dependent function after orthogonal isolation, EV-marker depletion, apolipoprotein removal, fragment exclusion, lipid-droplet exclusion and contamination control.

8.3. Minimal Validation Path

The most rational validation path begins with death-modality models rather than immediate systemic claims. Apoptosis, ferroptosis and pyroptosis can be induced in the same cell source while controlling for death extent, time window, culture background and lysis. Candidate particles can then be isolated after removing large debris, organelles, typical apoptotic bodies and lipid droplets.

The first step is to establish lipid-state fingerprints through lipidomics, oxidized-lipid detection, PS-topology analysis, size, density, charge, membrane fluidity and corona profiling. The second step is to test lipid-state causality in macrophage phagocytosis, inflammatory output, immune-tolerance bias or endothelial response, with controls for RNase, protease, DAMPs, apolipoproteins and lipid-state perturbation. The third step is to test presentation and identity layer by reconstituting similar lipid states into different carriers or exposing the same particles to different biological-fluid backgrounds.

Only after these steps should strong LST be tested directly. High-perturbation models should be used to search for EV-low/Apo-low/lipid-enriched functional residuals after depletion of classical EVs, lipoproteins, large debris, lipid droplets and artifacts. Candidate components must satisfy source-state association, lipid-state characterizability, extracellular persistence or transformation, lipid-state-dependent function and insufficiency of conventional cargo explanations. Only then should limited recipient models test whether source-state lipid particles transduce a measurable, reversible functional bias.

8.4. Conclusion

Lipids in living systems are not merely structural membrane materials, energy substrates or single-molecule mediators. Their composition, modification, topology, supramolecular organization, particulate presentation and fluid-phase identity layer can jointly determine how source-associated material state is externalized, transformed, routed and sampled in extracellular space. LST proposes that, in defined contexts, a source-cell or tissue state may be materially represented in lipid state, partially retained and reorganized through extracellular lipid-containing particles, and converted through biological sampling into functions that cannot be fully substituted by conventional cargo.

The hypothesis is layered and contractible. Weak LST asks whether known EVs and lipoproteins can mediate lipid-state transduction. Intermediate LST asks whether carrier form, interfacial presentation and fluid-phase identity actively transform routing and interpretation. Strong LST asks whether lipid-state-dominant functional residuals exist beyond established extracellular particle classes and cargo-centered mechanisms. The central empirical prediction of strong LST is the existence of non-classical LSTPs: extracellular EV-low, Apo-low and lipid-dominant particle-like or complex-like components predicted to participate in lipid-state transduction and whose functional identity is determined by lipid-state causality rather than by marker absence, purification residuals or taxonomic gaps.

The central experimental question is therefore simple: can source-associated lipid-state differences survive extracellular transformation sufficiently to be presented, fluid-edited, routed and sampled as state-bearing interfaces that alter biological function when conventional particle-, cargo- and artifact-based explanations are insufficient? If yes, extracellular-particle biology must include lipid-state transduction as a causal process. If no, LST should contract to local lipid regulation within known particle systems. Either outcome is useful, because the hypothesis converts a diffuse conceptual possibility into a definable, testable and falsifiable scientific problem.

At a broader conceptual level, LST reframes extracellular lipid biology from a question of particle identity and molecular cargo to a question of extracellular state transduction. Its distinctive proposition is not that a lipid message is copied intact, but that source-associated material differences can be written into lipid state, transformed during extracellular presentation, and converted through biological sampling into downstream functional bias. This perspective suggests that tissue stress, injury, death modality and homeostatic demand may be represented not only by soluble molecules or canonical vesicular cargo, but also by organized lipid states whose extracellular fate and function emerge from the coupled sequence of presentation, fluid-phase editing, routing and sampling.