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Intrinsic and Extrinsic Factors for Natural Killer Cells and Their Involvement in Behcet Disease

A peer-reviewed version of this preprint was published in:
Rheumato 2026, 6(2), 11. https://doi.org/10.3390/rheumato6020011

Submitted:

09 March 2026

Posted:

10 March 2026

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Abstract
Behcet disease (BD) is an inflammatory disorder with manifestation in mucosal tissues. Unlike autoimmune diseases that generate autoantibodies, BD is believed to be an autoinflammatory disease triggered by innate immune cells rather than adaptive cells. Hyperactivation of neutrophils causes vasculitis and thrombosis, and they migrate into cutaneous and ocular lesions. Dominance of M1 macrophages promotes the differentiation of Th1 cells. Moreover, the cross-reaction of bacterial heat shock proteins induces production of cytokines such as IL-4 and IFN-γ, in γδT cells, which alters the balance between Th1 and Th2 phenotypes. Nevertheless, natural killer (NK) cells play more critical roles in BD pathogenesis than other innate immune cells because not only their activity is precisely controlled by the interaction between ligands and receptors but NK1 shift also elicits Th1 dominance. The genetic factors associated with BD are HLA-B51 and major histocompatibility complex class I-related chain A (MICA), which stimulate NK receptors as ligands. Improperly processed peptides dysregulate their interaction with NK receptors, triggering the inflammatory response. NK1 and NK2 subsets represent cytokine production in relapse and remission periods; however, the cytotoxicity of NK cells in relapse is lower than that in remission periods. It still remains unclear how NK cells are activated recurrently and expand cytokine production. This review highlights the regulation of gene expression encoding NK receptors, tissue-resident NK cells, and adaptive NK cells to discuss their potential for relapsing. Splicing variants and readthrough genes encoding NK receptors easily alter cytokine production. Moreover, tissue-resident NK cells in mucosal tissues and adaptive NK cells that memorize the virus infection have the potential to trigger hyperactivation in relapse.
Keywords: 
Behcet disease
;  
NK cells
;  
NK1/NK2
;  
IFN-γ
;  
splicing
;  
readthrough
;  
tissue-resident NK cells
;  
memory-like NK cells
Subject: 
Biology and Life Sciences  -   Immunology and Microbiology

1. Introduction

Behcet disease (BD) is a chronic and recurrent inflammatory disorder. Due to its high incidence in regions along the Silk Road, environmental factors and genetic association have been considered as a cause of the disease. Although no specific bacterium or virus that triggers the onset of BD has not been identified, some microorganisms that associate with abnormal immune responses have been listed [1]. For instance, the frequency of Streptococcus sanguinis in oral flora correlates with the activation of neutrophils [2]. Regarding genetic factors, a higher prevalence of human leukocyte antigen (HLA) -B51 in patients with BD than healthy controls (HCs) was reported in Japan from a serological analysis [3] followed by genotyping studies including disequilibrium with MICA [4,5,6,7]. Inflammatory diseases such as ankylosing spondylitis, colitis, and psoriasis are strongly associated with HLA class1 alleles and are known as MHC-I-opathies [8]. In this concept, although autoimmune diseases exhibit common pathogenesis, they are attributed to the contacts of HLA class 1 molecules with NK cells and CD8-positive T cells.
The primarily manifestations of BD are cutaneous lesions, oral aphthous ulcers, uveitis, genital ulcers, and vascular and neurological complications. BD affects both sexes; however, the mortality rate in men is higher than in women. Ocular, vascular, and neurological manifestations are more frequently observed in men, whereas oral and genital ulcers, skin lesions, and arthritis occur more frequently in women [9]. A genetic association study comparing men with women demonstrated that association with HLA-B and MICA in men is higher than in women [10].
Although autoimmune and autoinflammatory diseases with similar phenotypes in the same tissues have a complicated diagnosis, network modularity analyses have identified 10 diseases [11] in which BD is different from systemic lupus erythematosus (SLE) and rheumatoid arthritis. The International Diagnostic Criteria for Behcet’s Disease (ICBD) has established a guide for the accurate diagnosis and classification of BD [12]. Nonetheless, no single definitive laboratory test has been available till date. A diagnostic model for vascular BD based on an increase in the level of inflammatory, hematological, and thrombosis parameters in vascular BD compared with those in nonvascular BD was recently proposed [13].
Lack of evidences concerning autoantibodies in BD has directed interests in the pathogenesis of innate immune cells. Previous research has shown that serum isolated from patients with BD improved the adherence of neutrophils to human umbilical vein endothelial cells monolayer in vitro [14]. Another study showed that activated neutrophils migrate into cutaneous lesions in pathergy tests [15]. Moreover, they form neutrophil extracellular traps (NETs) to eliminate pathogens and activate other immune cells, releasing inflammatory cytokines. NETs damage endothelial cells and contribute to thrombosis via the production of reactive oxygen species [16]. In uveitis, the feedback loop between NETs and IL-17A maintains the hyperactivation and infiltration of neutrophils [17].
In general, M1 macrophages play a proinflammatory role and eliminate pathogens via the release of IL-12, TNF-α, IL-β, and IL-6, whereas M2 macrophages play an anti-inflammatory role for wound repair. In BD, macrophages are polarized due to a shift toward the proinflammatory M1 phenotype and impairment of the M2 phenotype [18]. Decrease in IL-10 secretion along with the dominance of proinflammatory cytokine production causes M1 polarization [5,19], which consequently induces differentiation of Th1 cells.
Despite a quite low frequency of cell numbers among the entire T cell population, a higher portion of γδT cells was detected in mucocutaneous lesions [20]. These cells respond to antigens directly unlike CD8-positive T cells that recognize antigens presented by HLA class 1 molecules. Bacterial infection induces cross-reaction of heat shock proteins in γδT cells, triggering proliferation [21]. Moreover, γδT cells produce IFN-γ and promote the differentiation of Th1 cells along with producing IL-4 for Th2 cells, thereby altering the balance between Th1 and Th2 phenotypes [22]. TNF-α secreted from γδT cells increases IL-8 secretion and recruits additional neutrophils to the lesions [23].
The role of natural killer (NK) cells in BD pathogenesis has not been explored in detail because of limited information regarding their migration. As HLA class 1 molecules control the function of NK cells through their receptors, their dysregulation could cause hyperactivation. Immature NK cells generally develop into cytokine-producing cells (CD56bright) and finally acquire cytolytic functions (CD56dim) [24], which comprise the majority of peripheral NK cells. CD56bright NK cells secret IFN-γ by recycling endosomes [25]. NK cells undergo a licensing process in which they correctly recognize self-HLA through killer cell immunoglobulin-like receptor (KIR) [26,27]. Then, they acquire cytotoxicity and eliminate only unnecessary cells that do not possess self-HLA without damaging own cells [28]. Perforin forms pores in the membranes that deliver granzyme into target cells, resulting in cell death. Secretory lysosomes store perforin and granzyme, but LAMP1 (CD107a) is expressed on the cell surface and it reduces the binding of perforin to the NK cell’s own membrane [29,30].
This review summarizes the role of NK cells in BD pathogenesis where cytokine production and cytotoxicity are dysregulated by the interaction between ligands and receptors due to genetic polymorphism is summarized. Next, the potential pathogenesis of BD with transcriptional control of NK receptors, tissue-resident NK cells, and adaptive NK cells is discussed. Altered cell surface expression of NK receptors could easily modify the responses to ligands. Furthermore, some tissue-resident NK cells in mucosal tissues and adaptive NK cells are focused as latent populations to be involved in relapse.

2. Search Methods

A literature review was performed using the following terms: “Behcet disease”, “NK cells” (retrieved 107 articles with the term “Behcet disease”), “KLRK1” (8 articles), “KLRC4” (16 articles), “splicing”, “KLRK1” (retrieved 24 articles with the term “splicing”), “memory”, “NK cells”, IL-12”, “IL-15”, “IL-18” (retrieved 53 articles with “memory”), and “KLRC2” (36 articles). Relevant original and review articles were collected through the PubMed and MEDLINE between January and February 2026.

3. NK Cells in BD

3.1. Numbers, Cytokine Production, and Cytotoxicity

Table 1 summarizes the studies on the population and capability of peripheral NK cells in BD. Conflicting data exists regarding NK subsets. Increases in the numbers of NK cells (CD16+CD56+) were detected in peripheral blood collected from BD patients compared with that in HCs [20,31]. Conversely, a decrease in the numbers of NK cells (CD3−CD56+) was found in pulmonary patients [32]. Another study demonstrated no significant differences in numbers of NK cells (CD16+CD56+) between HCs and patients with BD despite increases in the number of CD94+ cells [33]. The numbers of NK cells (CD3−CD56+) in patients with BD are comparable to that in HCs [34]. CD16 and CD56 discriminate cytolytic cells and cytokine-producing cells; however, no significant differences were found between HCs and patients with BD [35].
As BD is a recurrent disorder, NK subsets in relapse and remission were analyzed, which revealed that although patients showed an increase in the number of terminally differentiated NK cells (CD16+CD57+) during relapse compared with that during remission, the cytotoxicity of peripheral NK cells against K562 cells during relapse was lower than that during remission [36]. Moreover, NK cells (CD16+CD56+) exhibited decreased cytotoxicity during relapse [37]. Both of CD56bright and CD56dim subsets were depleted in peripheral blood collected from patients with BD [38]. Nevertheless, CD56bright NK cells in patients with BD produce higher levels of IFN-γ than HCs, whereas CD56dim NK cells in patients with BD release comparable levels of perforin and granzyme to those in HCs [38].
NK1 and NK2 subsets were originally investigated to determine whether they produce IFN-γ or not. IFN-γ non-secreting NK cells produce IL-4, IL-5, and IL-13 [39]. NK cells produce proinflammatory cytokines, such as IFN-γ, with their NK1 profile during relapse period and promoting Th1 inflammation. Conversely, NK cells produce IL-4 and IL-10 with their NK2 profile during the remission period, which suppress Th1 response. An increased NK1/NK2 ratio was detected in patients with uveitis [40] and mucocutaneous BD [35]. Remarkably, one study classified NK cells based on the data of scRNA-seq and cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) [41]. The NK1 subset includes cytotoxic cells (CD56dim), whereas NK2 subset represents cytokine-producing cells (CD56bright). As a distinct population from peripheral NK cells, NK3 cells include memory-like NK cells that express KLRC2.
Cytotoxicity against K562 cells is also estimated by means of LAMP1 expression; however, it does not conform with earlier studies (Table 2). The expression level of LAMP1 in NK cells (CD16+) isolated from BD cultured with K562 cells is comparable to HCs [35], whereas NK cells (CD3−CD56+) isolated from patients with BD exhibited higher expression of LAMP1 than those obtained from HCs [34]. The expression level of LAMP1 in NK cells (CD3−CD16+) during relapse period is higher than that during remission [42]. Therefore, studies concerning the number of NK cells and their capability indicate that the activities of NK cells are not proportionate to number of NK cells. Further functional assays are required to understand the dysregulation of NK cells.

3.2. NK Cell Receptors and Their Ligands

3.2.1. HLA-B and KIR

HLA-B51 is the highest genetic risk factor for BD. Furthermore, HLA-B15 and HLA-B27 are associated with HLA-B51-positive BD [7]. Patients with HLA-A02, HLA-A24, and HLA-B57 are at risk for developing BD, whereas HLA-A03, HLA-B35, and HLA-B58 are protective [43].
NK cells recognize their own cells through the interaction of HLA-B with KIR3DL1, by which normal cells are protected from cytolysis [44]. HLA-B variants are determined by specific residues at 77-83. Bw4 recognizes KIR3DL1, which possesses two immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in the cytoplasmic region, and inhibits cytolytic signals, whereas Bw6 does not. Positions at 82 and 83 were identified as essential resides using HLA-B78 (HLA-B*1513), HLA-B75 (HLA-B*1502), and nonfunctional Bw6 motif [45,46].
The presence of neither inhibitory KIR3DL1 nor activating KIR3DS1 alleles increases susceptibility even if patients with BD carry Bw4 [47]. The frequency of KIR3DL1*004 encoding a misfolded protein [48] is lower in patients with BD than HCs in both HLA-B51-positive and -negative individuals [49]. The other component that influences the binding of HLA-B to KIR3DL1 is HLA class 1-bound peptides. The peptides bind to HLA-B51, but they exhibit weak affinity [50]. Insufficient licensing of NK cells may cause hyperactivation.
Next, HLA-B27 can be expressed as normal HLA-B27 with β-microglobulin (β2m) or as homodimers including free heavy chain forms (B272) [51]. Normal HLA-B27 binds to KIR3DL1; however, B272 binds to KIR3DL2 more strongly than other HLA class 1 molecules. Binding of B272 to KIR3DL2 expands Th17 proliferation and increases cytokine production in patients with ankylosing spondylitis [52]. A weak association between HLA-B27 and BD indicates that maintaining Th17 by IL-23 is more critical than B272.
Finally, KIR2DL4 possesses unique features because it contains a charged arginine in the transmembranes [53] and couples with an FcRγ adaptor [54]. KIR2DL4 induces IFN-γ production in NK cells, whereas CD16 and 2B4 induces cytolysis [55]. Although earlier studies found that HLA-G binds to KIR2DL4 and activates NK cells in the maternal vasculature during the initial period of pregnancy [56,57], KIR2DL4 was later identified as a novel susceptible gene in patients with severe uveitis [58].

3.2.2. MICA and KLRK1(NKG2D)

MICA is an MHC class 1 chain-related protein that does not require β2m to form a surface structure. KLRK1 homodimers bind monomeric MICA in solution [59]. MICA acts as a ligand of KLRK1 and causes cytolysis [60]. Pairs of KLRK1 and the adaptor protein HCST (DAP10) activate signaling molecules such as phosphatidylinositol 3-kinase (PI3K), GRB2, and VAV1 [61,62]. MICA gene has diverse polymorphisms, and its variance is associated with BD. In Slovak, Spanish, and other Eurasian populations, MICA*008 exhibited the highest frequency [63,64]. MICA exhibits strong linkage disequilibrium with HLA-B51 due to its proximity on the genome. MICA*009 is associated with HLA-B51 in Caucasian, Spanish, and Turkey patients [65,66,67]. MICA-TM A6 (MICA exon 5), which is a microsatellite polymorphism coding transmembrane region, detected in Greece patients [68] (Yabuki K et al., 1999) and MICA*006 found in Brazilian patients [69] (Marin ML et al., 2004) are associated with HLA-B51. The concentration of sMICA in serum patients with BD was comparable to that in HD despite its increases in patients with SLE [70].

3.2.3. HLA-E and KLRD1/KLRC1 (CD94/NKG2A)

HLA-E is a nonclassical MHC class 1 molecule that loads nonapeptides processed from other classical HLA class 1 molecules. Heterodimers of KLRD1 with KLRC1 recognize HLA-E and inhibit cytolysis [71,72]. No single polymorphism of HLA-E was associated with BD; however, specific combinations of SNPs demonstrate susceptibility. Individuals with HLA-E*0101 show a decreased risk while individuals without HLA-E*0101, NKG2A c.-4258*G/*G, and c.338-90*G show an increased risk of developing BD [73,74].
Nonapeptides presented by HLA-E were classified into 10 groups based on sequence, and their effect on genetic association was investigated. The frequency of N2: VMAPRTLVL and N7: VTAPRTVLL was found to be higher in patients with BD than in controls, indicating that these nonapeptides confer inaccurate presentation and BD risk [75].

3.3. Expression of NK Receptors

In the past decade, there has been increasing research on the expression of cell surface markers. Genome editing has become available for knock-out experiments using human cells. For instance, knockout of CD56 does not affect cytotoxicity against cell lines [76]; however, loss of KLRC1 improves cytolytic function against myeloma and solid tumors [77,78].
The interaction of ligands with receptors is a trigger for an immediate response. KIR3DL1 is an inhibitory NK receptor. NKB1 antibody detects variant surface expression of KIR3DL1 among patients with BD [79]. Conversely, KLRK1 (NKG2D), NCR1 (NKp46), NCR2 (NKp44), and NCR3 (NKp30) activate NK receptors and induce cytolysis. No significant differences were detected in cell surface expression in NK cells (CD16+CD56dim) obtained from patients with BD and HCs [80]; however, increases in KLRK1 expression were detected in NK cells (CD3−CD56+, CD3−CD16+) obtained from patients with BD [34,42] (Table 2). An approach using genome editing to eliminate or improve the recognition of KLRK1 was proposed [81]. Exploring the role of NK receptors with variance in surface expression through knockout experiments might provide a link to the production of substantial IFN-γ in relapse.

3.4. Other Genetic Factors

ERAP1 trims precursor peptides in ER to the correct length for binding to HLA class 1 molecules [82]. GWAS demonstrated association at ERAP1 rs17482078 (R725Q) in patients with uveitis from a Turkish population [83]. ERAP1 rs17482078 fails to trim peptides due to decreased stability and enzymatic activity, suggesting that unprocessed longer peptides interfere with the binding of HLA class 1 molecules and KIR.
IL-23 maintains Th17 cells and induces the production of IL-17 in patients with uveitis [84]. The feedback loop between NETs and Th17 cells underlies the activation of neutrophils and infiltration [17]. Moreover, the expression of IL-12β2 mRNA in remission is lower than that in relapse. The shift to NK2 decreases IFN-γ production in NK cells [85]. However, IL23R/IL12Rβ2 rs924080 is a polymorphism in the noncoding region [5]. It remains unclear how these receptors mediate ligands signaling in BD pathogenesis.

4. Regulation of NK Receptor Genes

4.1. Genomic Organization and Gene Expression of the NK Complex

NKG2 receptors are expressed on NK cells and possess killer cell lectin-like domains. Liganded with MICA or HLA-E, they regulate cytokine production and cytotoxic function. They are located on human chromosome 12 and consist of KLRD1, KLRK1, and KLRC1-4 (Figure 1A). KLRD1 is a common counterpart for KLRC1, KLRC2, and KLRC3 to form heterodimers, whereas KLRK1 forms homodimers. KLRC1 with the ITIM motif in its cytoplasmic region works as an inhibitory receptor, whereas KLRC2 pairs with TYROBP (DAP12) to activate receptors [86,87]. KLRC3 pairs with TYROBP; however, they are retained in the endoplasmic reticulum rather than being expressed on the cell surface [88]. KLRC4 does not pair with KLRD1 but can pair with TYROBP, suggesting that the cytoplasmic distribution of KLRC4 suppresses the activating signal by depriving KLRC2 and KLRC3 of TYROBP instead of forming extracellular structures that are stimulated by ligands [89].
The amino acid sequences of KLRC4 are conserved between humans and other primates but not between rodents [88,90,91] (Figure 1A). Tree analysis of amino acid sequence clearly separates rodents Klrc1-3 from primates KLRC1-4 (Figure 1B). Judging from the similarity of KLRC1-4 amino acids and the proximate location on the genome, errors during DNA replication and recombination with transposable elements would have produced paralogs, and accumulation of mutations provides a functional difference.

4.2. Splicing of KLRK1 (NKG2D) and Its Function

Mice Klrk1 has short (NKG2D-S) and long (NKG2D-L) isoforms. NKG2D-S is only expressed only in activated NK cells pairing with HCST (DAP10) and TYROBP (DAP12), whereas NKG2D-L is found in both resting and activating NK cells pairing with HCST [92,93]. TYROBP includes the ITAM motif in its cytoplasmic region and activates SYK and ZAP70 [94].
In humans, NK cells possess full-length of KLRK1 and it does not interact with TYROBP but HCST [95,96]. In contrast, TYROBP mediates KIR2DS2 stimulation and activates downstream tyrosine kinases, such as SYK and ZAP70 (Lanier LL et al., 1998 nature, Wu J et al., 2000). KLRK1 activates PI3K, GRB2, and VAV1 and initiates tyrosine-phosphorylation events [61,62]. Truncated KLRK1 (KLRK1TR) is a human isoform that does not form an extracellular structure unlike mouse NKG2D-S. Splicing failure leaves intron four that provides a stop codon, generating KLRK1TR. KLRK1TR is bound to HCST, thereby interfering with the pairing of full length of KLRK1 (KLRK1Full) and HCST [98]. Splicing isoforms of KLRK1 are not conserved among species.

4.3. KLRK1-KLRC4 Readthrough Gene

Human KLRK1 and KLRC4 are located next to each other on chromosome 12. According to the NCBI reference sequence, they transcribe NM_007360.4 and NM_013431.2, respectively. In addition, they also form a readthrough gene, KLRC4–KLRK1 (NM_001199805.1), which entirely covers both of KLRC4 and KLRK1. Readthrough transcription is an event where transcription machinery fails to find the termination site in the transcription process under stress [99]. Therefore, KLRC4–KLRK1 could encode a hybrid protein. Alternatively, it is possible for KLRC4–KLRK1 can produce several variants using different promoters and start codons. However, only a translation product (NP_001186734.1), which is equivalent to KLRK1 translation (NP_031386.2), has been registered in the data base (Figure 2). KLRC4 protein is detectable using antibody because it has a shorter C-type lectin-like domain than KLRC1, KLRC2 and KLRC3. Nonetheless, it will be hard to identify each isoform using antibodies that recognize the structure unique to KLRC4 if KLRC4–KLRK1 uses multiple start codons and produce multiple isoforms. CAGE-seq analysis that covers isoforms more efficiently than conventional RNA-seq analysis will be useful to explore the transcription of KLRK1-KLRC4 thoroughly. Moreover, identification of transcription initiation sites could help predict translation.
Transcripts of these genes along with their translation are depicted.

4.4. Dysregulation of KLRC4 in BD

KLRC4 rs2617170 (N104S) was identified in patients with BD as a nonsynonymous variants (NSV) [83,100]. KLRC4 is less likely to form an extracellular structure [89]; however, it remains unclear how the lack of fucosylation affects the partnering of KLRC4 N104S with KLRD1. Gene Chip data obtained from the peripheral blood mononuclear cells (PBMCs) of patients with BD patients (mucocutaneous manifestations, ocular involvement, and large vein thrombosis) categorized genes into the following four groups: negative regulators of inflammation, neutrophil granule proteins, antigen processing and presentation proteins, and regulators of immune response. Regarding the role of KLRC4, it is a regulator of immune response [101]. Interestingly, the effect of polymorphisms on KLRC4 mRNA expression and cytokine production was analyzed, which revealed that the mRNA expression of KLRC4 rs2617170 from CC carriers was significantly higher than that in CT and TT individuals. Furthermore, PBMCs collected from patients with TT and stimulated with LPS secrete more IL-8 than PBMCs collected from CC and CT carriers [102].

4.5. Fucosylation of CD16a

FUT2 plays critical roles in the gut epithelium. FUT2 dysregulation impairs mucosal fucosylation. The weak barrier function increases gut inflammation. FUT2 variants are associated with inflammatory bowel disease (IBD) and Crohn’s disease [103]. In mice, chronic stress causes gut dysfunction and decreases of Fut2 expression and fucosylation on the intestine epithelium that tethers microbiota [104]. SNP array analysis found FUT2 variants rs601338 (W143X) and rs602662 (G258S) [105]. Fucose-deficient IgG1 bound more strongly to CD16a than fucosylated IgG1 and elevated antibody-dependent cell-mediated cytotoxicity (ADCC) [106].

5. Distinct NK Subsets

5.1. Circulating and Tissue Resident NK Cells

It has been believed that circulating and resident NK cells are fundamentally separated during embryonic stages fundamentally. CD16−CD56bright is a major population in tissue resident NK cells, whereas peripheral NK cells predominantly include CD16+CD56dim predominantly [107]. Nonetheless, the frequency of CD16−CD56bright NK cells in patients with BD is higher than that in HCs [31]. Moreover, NK cells in the intestines decrease effector function and exhibit adaptation in their environment [107]. Despite these relevant observations, there has been limited evidence showing that tissue-resident NK cells are activated in the relapse period.
The other possibility is that some peripheral NK cells in patients with BD acquire tissue residency, although this assumption has not been supported. CD4+ T cells infiltrate predominantly; however, NK cells are infrequent in patients with uveitis [108]. Nevertheless, several studies have recently referred to the plasticity of peripheral NK cells. First, circulating NK cells acquire tissue residency when encountering acute infection [109]. Second, murine cytomegalovirus infection recruits NK cells to the salivary gland and forms a long-lived, tissue-resident, memory-like population (NKRM cells) [110]. NK cells appear to possess the ability to change their fate through virus infection and adapt to the tissue environment.

5.2. NK Cell Memory

Immediate immune response without prior sensitization is the capability of NK cells, although they can be sensitized. Some studies using anti-CD16a/CD30 bispecific antibodies, cancer cells, LPS, and cytokines have demonstrated the presence of memory-like NK cells [111]. Both mice and human NK cells cultured with IL-12, IL-15, and IL-18 produce substantial IFN-γ and persist for long time [112,113]. Moreover, these cytokine-induced memory-like (CIML) NK cells acquire cytotoxicity against myeloid leukemia [114]. Despite decreased expression of CD16 in CIML NK cells, they maintain the capacity to trigger antibody dependent cell-mediated cytotoxicity (ADCC) [115].
The other type of NK cells is memory/adaptive NK cells that have been infected by HCMV [116]. Human HCMV infection expands KLRC2-positive cells with increased cytokine production. These cells produce higher levels of IFN-γ in recipients with HCMV infection than in recipients without HCMV infection. KLRC2-positive NK cells maintain memory when they are transplanted to the recipient with antigen [117]. Furthermore, NK cells with a previous HMCV infection are FcRγ deficient and have distinct memory features. These cells persist for long term and produce IFN-γ against either HCMV or HSV-1 [118]. Memory/adaptive NK cells lack the expression of FcRγ and SYK due to epigenetic regulation and expand in an antibody-dependent manner [118,119].
Nevertheless, there are some ambiguous evidences when discussing the potential involvement of NK cell memory in BD. Firstly, HCMV IgG levels in patients with BD are lower than those in HCs [120]. which does not ensure that patients with BD have undergone the infection and generate memory. Secondly, periods of relapse alternate with periods of remission from weeks to years. NK cells in relapse exhibit lower cytotoxicity than those in remission [36,37], consistent with the dominance of CD56bright in relapse. Substantial IFN-γ production is common in both peripheral NK cells in BD and memory-like NK cells; however, there are some differences in population. Although peripheral NK cells in patients with BD are enriched CD56bright NK cells, CIML NK cells include both of CD56bright and CD56dim, and memory/adaptive NK cells predominantly have CD56dim. Seeking local environment where cytokine cocktails develop CIML NK cells or tracking cells infected with HCMV for identifying memory/adaptive NK cells among peripheral blood is difficult. Therefore, further studies using scRNA-seq data on CIML NK cells or memory/adaptive NK cells as reference will be effective in identifying original cells with memory that can be hyperactivated by specific stimulation and developing therapy to target those cells.

6. Conclusions

BD is a chronic autoinflammatory disorder characterized by unpredictable relapse. Innate immune cells contribute to its pathogenesis. The numbers of peripheral NK cells are variant among patients with BD and the cytotoxicity of peripheral NK cells tends to be lower during relapse than during remission. NK1/NK2 subsets clearly depict cytokine production in relapse and remission. Moreover, the strongest association with HLA-B51 indicates that the interaction with NK receptors is critical to BD pathogenesis. Polymorphisms and unprocessed peptides impair the accuracy of recognition and cause the hyperactivation of NK cells.
NK cells exhibit immediate response, including cytokine production and cytolysis, without prior sensitization, and their life is short. However, recent studies have demonstrated that NK cells can able to acquire memory through exposure to cytokine cocktails or HCMV infection and produce IFN-γ against specific stimulation. Although neither CIML NK cells nor memory/adaptive NK cells have been identified from patients with BD, hyperactivation in relapse is similar to the response of these adaptive NK cells. Alternatively, activation of tissue-resident NK cells and migration of peripheral NK cells to mucosal tissues are anticipated episodes. Moreover, transcriptional regulation of activating receptors, such as KLRK1 and KLRC4 could easily modify the responses to ligands and contribute to hyperactivation. Further studies focusing on adaptive NK cells among skewed innate NK cells in BD will clarify the mechanism underlying chronic and recurrent manifestations.

Author Contributions

Y.O. conceived and wrote the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. NKC and phylogenetic relationship between primate and rodent NKC genes. (A) Genomic organization of humans and mice is shown. (B) Phylogenetic relationship of amino acid sequences from humans, chimpanzees, mice, and rats is analyzed. Sequences were obtained from NCBI.
Figure 1. NKC and phylogenetic relationship between primate and rodent NKC genes. (A) Genomic organization of humans and mice is shown. (B) Phylogenetic relationship of amino acid sequences from humans, chimpanzees, mice, and rats is analyzed. Sequences were obtained from NCBI.
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Figure 2. Gene structure of human KLRK1, KLRC4, and KLRC4-KLRK1.
Figure 2. Gene structure of human KLRK1, KLRC4, and KLRC4-KLRK1.
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Table 1. NK subsets in BD pathogenesis.
Table 1. NK subsets in BD pathogenesis.
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Table 2. The expression of marker genes.
Table 2. The expression of marker genes.
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