UBR5 in the Tumor Microenvironment: Exploring Mechanisms of Immune Regulation and Possible Therapeutic Implications in MPNST

1. Ubiquitin Proteasomal System and UBR5 Structure

Ubiquitin proteasomal system (UPS), a fundamental post-translational modification in eukaryotes, attaches ubiquitin (Ub) to protein substrates thereby regulating various cellular processes such as protein degradation, signal transduction, apoptosis, immune response, and DNA repair[24]. While ubiquitination is commonly linked with protein degradation via the proteasome, ubiquitination also regulates protein levels, interactions, and localization[38,39]. UPS orchestrates this regulation, through a cascade of three enzymes: Ub-activating enzyme E1, Ub-conjugating E2, and E3 ligases[38,39]. E3 enzymes, such as UBR5, interact directly with substrates hence imparting substrate specificity to ubiquitination[40,41].

UBR5, a single polypeptide chain homologous to E6AP C terminus, belongs to the HECT-type E3 ubiquitin ligase family and is vital for mammalian embryonic development[24]. UBR5 is one of several E3 ligases encoded in the human genome. E3 ligases are classified into Ring between ring (RBR), the really interesting new gene (RING), and the homologous to E6AP C-terminus (HECT) family[24,26]. The modular structure of UBR5 comprises multiple domains, including a ubiquitin-associated (UBA) domain, a zinc-fingerlike ubiquitin-recognin (UBR) box, and a MLLE/ PABP-interacting motif 2 (PAM2) binding domain adjacent to the HECT domain[26,42]. Structural studies outline the HECT domain to be crucial for catalyzing ubiquitin transfer onto substrates. Notably, UBR5 is unique among N-end rule pathway E3 ligases for containing both an N-degron recognition UBR box and a HECT domain[26]. The N-degron recognition UBR box recognizes and degrades proteins based on the nature of their N-terminal amino acid residue[31]. Unlike other UBR-box containing E3 ligases, UBR5 is the only one containing a HECT domain which regulates proteins by forming a thio-ester linkage between Ub and the conserved catalytic cysteine residue within the HECT domain[31]. Consequently, transferring Ub from E3 ligase to target proteins[26,31,42].

Ubiquitin chain topology, including length and linkage type, dictates protein fate. For instance, branched ubiquitin chains, such as K11/K48, serve as potent signals for proteasomal degradation[38,39,43]. UBR5 plays a key role in assembling K11-K48 linked branched Ub chains, regulating the degradation of various substrates[24,40,41]. Structural analysis reveals that UBR5 specifically assembles K11-K48-linked branched Ub chains, including by branching chains preformed at K11- or K63-linked chains[45]. UBR5 might collaborate with other E3 ligases to synthesize branched K48 linkages off preexisting ubiquitin chains. One such instance is UBR5, along with UBR4, assemble K11/K48 branched ubiquitin chains, facilitating the rapid degradation of aggregation-prone proteins like 73Q-huntingtin (HTT) and C9orf7[38,45].

2. Functional Roles of UBR5

UBR5, a homologue of hyperplastic discs (HYD), plays critical tumor suppressor roles in Drosophila melanogaster, regulating cell proliferation during development[46]. Cells harboring mutated HYD failed to control proliferation, leading to tumorigenesis. Mutations in HYD/UBR5 have been shown to lead to developmental abnormalities, including adult sterility secondary to germ cell defects[46].

In diseases like mantle cell lymphoma and Huntington’s disease, UBR5 plays crucial roles in regulating normal cellular functions. For example, in mantle cell lymphoma, mutations that disrupt the HECT domain of UBR5 impair normal B cell maturation and differentiation, potentially contributing to the transformation of cells into malignancies [43,47]. Similarly, in Huntington’s disease, although the mutant huntingtin protein (HTT) is typically overexpressed, studies using induced pluripotent stem cells (iPSCs) have shown that elevated levels of UBR5 can promote the degradation of the mutant HTT protein. This suggests that UBR5 might help to reduce the toxic effects of mutant HTT in disease models, offering potential insights into therapeutic strategies [43,45,46].

3. Role of UBR5 in Immune Modulation

Although Chr8 has been reported in MPNST, the precise role played by the genes housed within this chromosome, including UBR5, is not fully understood. UBR5 has been implicated in various other cancers, underscoring its key role in tumorigenesis. Emerging evidence indicates that the tumorigenic activities of UBR5 are, in part, dependent on its modulation of the immune system. This is not surprising as components of the UPS such as E3 ligases and deubiquitinases have been discussed as modulators of both cellular and non-cellular components in the tumor microenvironment [69,70,71].

In a mouse model OC, the loss of Ubr5 was found to enhance chemotherapy response and augment immune response to anti-PD-1 and CAR T cell therapy [35]. This model demonstrated that Ubr5 facilitated OC growth and metastasis by maintaining an immunosuppressive tumor microenvironment (TME). Ubr5 drove growth and immunosuppression through the recruitment and activation of Tumor-Associated Macrophages (TAMs) via paracrine signaling involving CCL2 and CSF-1. UBR5 mediated recruitment of TAMs contributed to an immunosuppressive TME that supports tumor growth and metastasis. Additionally, Ubr5 was essential for sustaining β-catenin signaling, which promoted cancer stemness and distant metastasis independently of TAMs, pointing to a tumor autonomous process. The role of Ubr5 in modulating immune response is highlighted by the observed success of immune checkpoint blockade (ICB) and CAR T cell therapy in this OC mouse model following the loss of Ubr5. This success is particularly noteworthy given the historically limited efficacy of immunotherapy in OC[72,73], underscoring the critical interplay between Ubr5 and the immune system in regulating tumor growth and Immunotherapy response.

In a murine model of Triple-Negative Breast Cancer (TNBC), Ubr5 also enhanced tumor growth in a manner dependent on the immune system. In Ubr5-deficient tumors, there was a notable increase in antigen presentation, the percentage of CD8+ T cells and interferon-gamma (IFN-γ) producing CD4+ T cells in the spleen, as well as an increased infiltration of polymorphonuclear leukocytes in UBR5 knockout tumors. Conversely, Ubr5 wild-type (WT) tumors exhibited a higher number of regulatory T cells (Tregs) in the spleen and TDLN and a greater infiltration of mononuclear leukocytes was observed[34]. The proportion of tumor-infiltrating CD4 + and CD8 + T cells was increased in Ubr5−/- tumor bearing mice compared to control tumors. Furthermore, infiltrating CD8 + T cells exhibited enhanced granzyme B expression, indicating a more active cytolytic state of the CD8+ T cells in Ubr5-deleted tumors [35]. The infiltration of polymorphonuclear leukocytes in Ubr5-/- tumors was associated with reduced angiogenesis, suggesting a potential mechanism by which Ubr5 promotes tumor growth through immune modulation. Additionally, Ubr5 depletion in tumor-bearing mice reduced surface levels of PD-L1 on TAMs suggesting a role for Ubr5 in immune evasion[35].

The molecular mechanisms by which UBR5 regulates immune activation warrants further investigation. It is hypothesized that UBR5 may interact with key signaling pathways and transcription factors involved in immune cell function and tumor-immune interactions. For instance, the role of UBR5 in β-catenin signaling suggests a possible link to the Wnt/β-catenin pathway, which is known to play a role in immune evasion and cancer stemness [35,74]. Additionally, the involvement of UBR5 in CCL2/CSF-1 signaling as reported in the TNBC model [35] indicates a broader impact of UBR5 on the cytokine milieu within the TME, influencing the recruitment and activation of various immune cell populations.

In another murine model of TNBC, UBR5 was found to regulate IFN-γ-mediated PD-L1 expression to promote tumor growth[33]. Tumor cells commonly exploit the PD-L1/PD-1 interaction to evade immune surveillance, a phenomenon associated with poor prognosis across various cancers[75]. PD-L1 expression can be induced by several cytokines, including IFN-γ, tumor necrosis factor-α (TNF-α), interleukins (ILs), and epidermal growth factor (EGF), via multiple signaling pathways such as JAK/STAT1/IRF1, NF-κB, PI3K/AKT/mTOR, and JAK/STAT3 [76].

In this TNBC model [33], UBR5 regulated IFN-γ-induced PD-L1 transcription in an E3 ubiquitin ligase independent manner. Through its polyadenylate binding (PABC) domain, UBR5 enhanced PD-L1 transactivation by upregulating the transcription of EIF2AK2 gene that encodes protein kinase RNA-activated (PKR). The consequence was increased transcription of effectors such as, STAT1 and IRF1, downstream of PKR [33]. Further demonstrated in this same TNBC model, dual hairpin targeting of UBR5 and PD-L1 (CD274) resulted in dramatically reduced tumor growth and lung metastasis and significantly extended survival. Along the same lines, UBR5 has been shown to stabilize the WNT/β-catenin signaling, a key driver of cancer stemness [35,61,74].

In melanoma, β-catenin signaling has been shown to mediate resistance to ICB therapy [74,77]. Underscoring the significant role of WNT/β-catenin signaling in immune evasion. Takeuchi et al. reported that NSCLCs that lacked immune cell infiltration into the TME despite high tumor mutational burden (TMB), a marker high immunogenicity, preferentially up-regulated the WNT/β-catenin pathway. In high TMB NSCLC which are highly immunogenic, TMB does not always correlate with PD-L1 expression. In this NSCLC model, a PD-L1 independent immunosuppressive TME that is driven by WNT/β-catenin signaling was observed [78]. These additional pieces of evidence, underscore the role of UBR5 in regulating oncogenic pathways such as WNT/β-catenin to drive tumor immune evasion.

While the role of UBR5 in sarcomas, including MPNST, is understudied, the oncogenic pathways discussed above, through which UBR5 regulates immune escape, have been implicated in MPNST pathogenesis. For instance, signaling downstream of EGFR is known to induce PD-L1 expression and contribute to immune evasion. Similarly, the PI3K-AKT-mTOR and WNT/β-catenin pathways play roles in PD-L1 induction and immune evasion [76]. These signaling pathways are not only crucial drivers of tumorigenesis in MPNST but are also therapeutic targets in MPNST clinical trials, albeit with limited clinical success[6,8,79]. MAPK-ERK2, a downstream target of the Ras signaling implicated in NF1-MPNST pathogenesis, is reported to regulate UBR5 function in cellular signaling through phosphorylation[80,81]. As such, exploring the interplay between UBR5 and these key oncogenic pathways driving MPNST, could uncover mechanisms to enhance therapeutic response, in part, through augmenting anti-tumor immune response.

4. UBR5 in MPNST: Therapeutic Potential

In our lab, we have extensively studied eight NF1-MPNST parental tumors and their PDX counterparts. Our research highlighted pronounced aneuploidy including chr8 gain, in NF1-MPNST compared to the generally diploid PNFs. Using bulk RNA-seq we identified UBR5 among chr8 genes with the highest expression in MPNST[36]. Sarcomas typically exhibit more copy-number aberrations than other cancers, with MPNST showing particularly high CNAs[37,82]. This is a critical discovery, as DNA aneuploidy is recognized as an independent risk factor, for decreased metastasis-free survival in sarcomas, regardless of histologic grade or lymphovascular invasion [37,83,84].

In dissecting MPNST immune profile, Holand et al reported DNA copy number gains including 8q gain which also correlated with immune deficiency in MPNST tumors[37]. Chr8q gain has been reported in Ewing sarcoma and other pediatric soft-tissue sarcomas , suggesting this may be a critical event in multiple sarcoma types, including NF1-MPNST[36]. Moreover, our data linked Chr8 gain with poor OS in soft-tissue sarcoma datasets from TCGA. Chr8q contains numerous cancer-critical genes, including MYC (8q24), which is well-studied across various cancers, including MPNST where upregulation of MYC and or its targets was correlated with tumor recurrence and immune deficiency [37,85]. Additionally, 8q houses other significant cancer-related genes including UBR5 whose role in posttranslational regulation of Myc and Myc mediated apoptosis-priming has been reported [29,61]. Underscoring the significance of Chr8q gain in MPNST, Maren Holand et al classified MPNSTs into two transcriptomic subtypes defined primarily by immune signatures and proliferative processes. “Immune deficient” MPNSTs were more aggressive, characterized by proliferative signatures, and high genomic complexity (CNAs) including 8q gain.

Based on our study, among the significantly upregulated genes within chr8, genetic knockdown of UBR5 significantly impacted MPNST cell survival. Preliminary unpublished findings from our lab revealed that knockdown of UBR5 in MPNST cells led to reduced proliferative capacity, increased apoptotic cell death, and decreased tumor growth in vivo. These results coupled with established role of UBR5 in the pathogenesis of cancers such as ovarian, lung, and breast cancer, highlights potential involvement in MPNST progression.

5. MPNST Immune Architecture: Mechanisms of Immune Evasion

Although immune-based treatment strategies such as ICB, have revolutionized cancer care in malignancies like melanoma and non-small cell lung cancer (NSCLC) [86,87] progress in applying these strategies to MPNST have been notably limited[17,88]. [NF1 patients exhibit germline loss of a single NF1 allele, resulting in NF1 haploinsufficiency (NF1+/−) and hyperactive Ras signaling across all cells in the body including immune cells. Somatic loss of the second NF1 allele in schwann cell precursors triggers plexiform neurofibroma formation. The NF1+/− MPNST TME conditions plays a pivotal role in PN development and malignant transformation through the secretion of growth factors, chemokines, and proinflammatory mediators[14,15,89].

Mast cells are one such immune cell type in the neurofibroma microenvironment, that play a critical role in NF1-associated tumorigenesis. Nf1-/- Schwann cells (SCs) hypersecrete stem cell factor (SCF), the ligand for c-kit, which is essential for mast cell development and survival [13,15]. Nf1 haploinsufficient (Nf1+/− ), mast cells show enhanced migration, proliferation, survival, and degranulation in response to SCF in a Ras/PI3K-dependent manner [14,15]. Among the effector proteins hypersecreted by Nf1+/- mast cells is TGF-beta (TGFβ), promoting increased proliferation and collagen production by Nf1+/- fibroblasts a hallmark of PNF formation[15]. Additionally, mast cells hypersecrete various pro-angiogenic growth factors, including VEGF and metalloproteinases (MMPs), potentially contributing to PNF tumorigenesis [86,87,88]. This tumorigenic TME characteristic has motivated ongoing clinical trials investigating KIT/CSF1R inhibitors, targeting neurofibromas associated with mast cell infiltration[1,14,15]. Similarly, Hirbe et al. and Dodd et al. demonstrated that the Nf1 haploinsufficient TME accelerated MPNST onset in genetically engineered mouse models (GEMMs) of MPNST[11,89]. Unsurprisingly, an enriched myeloid cell population, particularly elevated mast cells, were observed in these tumors. Consistent with these preclinical findings, a phase I clinical trial evaluating a combination strategy targeting mast cells and TAMs in MPNST yielded promising results. The combination of pexidartinib (KIT, CSF1R, and FLT3 inhibitor) and sirolimus, an mTOR inhibitor (NCT02584647), demonstrated efficacy in 12 out of 18 patients with advanced sarcoma, including 5 with MPNST. Median progression-free survival (PFS) and OS in the MPNST cohort were 18.6 weeks and 145.1 weeks, respectively[90].

Despite these advances, other immune strategies such as ICB, ICB combined with targeted therapy, CAR-T cell therapy, and oncolytic viruses are still in their infancy in MPNST [88,91]. Nonetheless, the promotion of tumor formation and growth by mast cell-induced inflammation highlights the involvement of the MPNST TME in tumor pathogenesis and highlights the potential to restore anti-tumor immune function therapeutically. A summary of ongoing clinical evaluating these immune strategies is provided in Table 1.

Emerging clinical data highlight the potential of PD-1/PD-L1 checkpoint blockade in MPNST, particularly in tumors expressing high levels of PD-L1. In clinical case reports, PD-L1 expression was observed in 70-100% of tumor cells in three patients and 2% in one patient, as determined by IHC staining [20,21,22,88,92]. Case reports of these four MPNST patients, who were initially refractory to conventional therapies but achieved complete responses with pembrolizumab or nivolumab, further underscore the potential of ICB in MPNST management.

Discrepancies between the success of these individual responses and broader ICB related clinical outcomes MPNST may be attributed to various factors. Highlighted in these reports, is the need for robust biomarkers to identify which MPNST patients are likely to derive clinical benefit from ICB. Additional factors such as inter- and intra-tumoral heterogeneity, initial treatment prior to ICB, and variable PD-L1 expression levels among patient cohorts may also contribute to this discrepancy [93]. In a study of MPNST transcriptomes pooled from Gene Expression Omnibus (GEO), 30% of MPNST samples had scores predictive of ICB response. This study used a validated predictive score model based on T-cell exclusion and dysfunction signatures to assess the extent of predicted response to ICB. Concomitant with existing evidence of heterogeneity, considerable variability in the behaviors of immune cells even within TME of MPNST tumors with the same CTL score was observed[94].

PD-L1 expression has been considered the best available predictive biomarker to ICB response. However, it has notable limitations, as not all patients with elevated PD-L1 levels exhibit positive response to ICB therapy[95,96]. As such, efforts to identify predictive and prognostic biomarkers aim to enhance our understanding of the complex interplay between cancer and T cell response [93]. In MPNST, potential biomarkers for immunotherapy have not yet been evaluated in large-scale standardized prospective trials. Nonetheless preclinical evidence demonstrates the extent of immune evasion in MPNST. For instance, immune genes such as HLA which encode MHC class I and II, the transcription factor MHC II transactivator (MHC2TA), the transporter associated with antigen processing (TAP1), and the related chaperone CD74, show reduced expression in NF1-associated tumors compared to normal human Schwann cells highlighting downregulation of or impaired antigen presentation processes in MPNST [97].

Biomarkers for future trials in MPNST will need to highlight evidence or lack thereof, of immune activation and intact antigen presentation. Some relevant candidate biomarkers in MPNST include PD-L1copy number gain and expression [97], TILs [88,93], and TMB [88]. Typically, MPNSTs are characterized by low PD-L1 expression, however, reports on PD-L1 expression in MPNST are inconsistent, with some studies reporting high expression and others reporting low PD-L1 expression [17]. Elizabeth Shurell et al. reported 13% of MPNST tissue microarray lesions had at least 5% PD-L1 staining. In these samples, MPNST was characterized by absent PD-1 expression, low PD-L1 expression although higher than benign neurofibroma, and significant CD8+ TIL presence[98]. Similarly, transcriptomic analysis of T cell signatures revealed that a majority of human MPNST (62 out of 73 samples profiled) exhibited an immunologically “cold” phenotype characterized by reduced cytotoxic T cell infiltrates and increased tumor immune dysfunction and exclusion (TIDE) scores[94]. These findings highlight the lack of robust immune cell infiltration in MPNST, that is necessary to elicit robust anti-tumor immune response, emphasizing the challenges to effective application of immunotherapy approaches in this malignancy.

On such basis, MPNST has indeed been described as non-inflamed/immune cold [93,94,98]. Spatial gene expression profiling of human PNSTs across the neurofibroma-MPNST continuum revealed that ANNUBPs exhibited enhanced signatures of antigen presentation and T-cell infiltration, while MPNST was characterized by immune-excluded TME with an increased expression of genes associated with immune exhaustion [37,99]. Interestingly, “benign” neurofibromas contiguous to MPNST, similarly harbored distinct gene expression profiles characterized by signatures of impaired antigen presentation. Of particular interest in this study, is the potential to utilize signatures of impaired antigen presentation to identify precursor lesions histopathologically classified as “benign”, at high risk of undergoing malignant transformation [37,99].

An intriguing question is what oncogenic mechanisms underly this immunosuppressive environment that appears indispensable for the malignant transformation of ANNUBP to MPNST. Changes in immune composition resulting from the loss of the PRC2 complex—a histone-modifying complex primarily responsible for maintaining transcriptional silencing through H3K27 trimethylation (H3K27me3)—represent a plausible hypothesis. Indeed, evidence suggests that loss of the PRC2 complex, a frequent occurrence in MPNST, is linked to reduced antigen presentation and altered immune signatures [4,10,37]. PRC2 loss is also associated with an aggressive disease course, proliferative signatures, and frequent gains of chr8q and triploid genomes in MPNST [4,37]. In a recent study classifying MPNSTs into two transcriptomic subtypes defined primarily by immune signatures and proliferative processes, 8q was reported among the most frequently affected by copy number gain among the “immune deficient” tumors (69% and 66%, respectively). Importantly, identified possible target genes included UBR5, among others on chr8q.

Similarly, PRC2 inactivation, resulted in ICB resistance through the reprograming of chromatin landscape, disrupting chemokine production, impairing antigen presentation and T-cell priming [10]. Consistent with these findings, Cortes-Ciriano et al reported by whole genome sequencing, coupled with transcriptomic and methylation profiling of 95 NF1-related tumors, a significant association between H3K27 trimethylation (H3K27me3) status and immune-phenotype[4]. H3K27me3 loss was strongly correlated with decreased infiltration of immune cells into the TME, downregulation of granzyme expression, and decreased activation of adaptive immunity. H3K27me3 retention on the other hand, was associated with an immune-cell rich phenotype [4].

Tumors with a high tumor mutational burden (TMB) are likely to harbor more neoantigens. As such, albeit some exceptions, higher TMB correlate positively with response to ICB in multiple cancers [18] with a definition of high TMB being >10 mutations per megabase [100]. Soft tissue sarcomas, in general, are reported to have a low TMB of 1–2.5 mutations/Mb [101]. However, in an analysis of 100,000 different cancers, 8.2% of MPNSTs had mutations of more than 20 mutations/Mb [102]. In a separate analysis of whole-exome sequencing data of 12 MPNST patient samples, somatic coding variants per tumor ranged from 7 to 472 with a median value of 63 [103].

While immunotherapy, particularly ICB therapy, has revolutionized the treatment of several malignancies, its efficacy in MPNST remains limited. The immunosuppressive tumor microenvironment driven by NF1 haploinsufficiency, and other pro-tumorigenic factors such as PRC2 loss, WNT/β-catenin, and Ras/PI3K signaling, pose significant challenges to effective immune-based treatments. Promising clinical trials targeting mast cells and TAMs, [(pexidartinib :KIT, CSF1R, and FLT3 inhibitor + sirolimus, an mTOR inhibitor) (NCT02584647)], underscore the potential for combination strategies, yet the immunosuppressive TME and variable response to PD-1/PD-L1 blockade, highlight the need for predictive biomarkers and strategies to augment MPNST immunogenicity. Deeper understanding of the genetic and molecular mechanisms underlying MPNST, including the role of Chr8q gain and UBR5 amplification in shaping the immune architecture and antitumor immune response will be crucial for overcoming immune evasion and improving outcomes in MPNST.

6. Attempts to Enhance MPNST Immunogenicity: Prospects for ICB

Curative treatment for MPNST typically involves surgical resection with negative margins, but this approach is often constrained by factors such as tumor size and proximity to nerves (Ref). For patients with recurrent, unresectable, or metastatic disease, treatment options are limited and outcomes are poor [6,7]. ICB therapy has shown promise in the treatment of inoperable and undruggable malignancies, suggesting potential benefits for MPNST patients [6,7]. As discussed previously, the success of ICB is dependent on an immune-rich TME. Unfortunately, evidence highlights MPNST to be non-inflamed “immune cold” malignancy, characterized by low immune cell infiltration, posing a significant challenge to the use of ICB therapy. Nonetheless, reports of anti-PD-1/PD-L1 success, albeit few, have prompted efforts to increase intratumoral T cell density to enhance MPNST immunogenicity.

A recent study harnessed the cGAS/STING/IFN pathway to augment the innate immune response [17]. The cytosolic DNA–sensing enzyme cGAS binds to dsDNA and initiates a cascade of events leading to the production of type I IFNs and proinflammatory cytokines and chemokines. This cytokine milieu presumably recruits immune cells into the TME. Using a GEMM of MPNST (cisNf1+/– p53+/–), STING agonist reprogramed the TME to enhance T cell infiltration and sensitized tumors to ICB therapy. Treatment of MPNSTs with a STING agonist caused activation of the STING pathway, upregulation of cytokines and chemokines, and infiltration of immune cells, including T cells, into the tumor. Consequently, enhanced tumor growth delay and significant apoptosis was observed upon STING activation followed by ICB therapy compared to STING agonist alone[17].

Other strategies have employed the use of oncolytic viruses. Recent reports have highlighted the potential of viral treatments to enhance immune infiltration in MPNSTs. Ghonime et al. demonstrated that a multimodal oncolytic virus engineered to express EphrinA2—an antigen found in various tumor types—can elicit a strong immune therapeutic response in immune-competent mouse models of both glioma and MPNST [104]. Additionally, a more recent study showed that intratumoral delivery of an inactivated modified vaccinia virus Ankara (MVA) can transform the immune desert environment in MPNSTs, promoting immune infiltration hence rendering tumors more responsive to ICB therapy[105]. In addition to these reports, further preclinical studies evaluating oncolytic virus therapy in MPNST are reviewed here [88].

A 2023 study demonstrated that dual targeting of Ras effector kinases, CDK4/6, and MEK, sensitized MPNST to ICB therapy. Dual inhibition of CDK4/6 and MEK demonstrated synergistic antitumor activity in patient-derived xenografts (PDX) and de novo tumors in immunocompetent mouse models of MPNST. PD-L1 blockade alone delayed tumor growth but caused little tumor regression compared to combination CDK4/6-MEK and PD-L1 inhibition. Resistant tumors were characterized by an immunosuppressive TME with elevated MHC II-low macrophages and increased tumor cell PD-L1 expression. On the other hand, in responsive tumors, CDK4/6-MEK inhibition led to an increase in plasma cells and cytotoxic T cell infiltration compared to resistant tumors[106]. These findings are particularly noteworthy given that tumor infiltrating plasma cells and B cells (TIL-Bs), along with the antibodies they produce are emerging as key components of the immune response against cancer[107]. Notably, plasma cell tumor infiltration has not been reported with the CDK4/6-MEK drug combination or the single agents in any tumor model including MPNST [106] . Emerging evidence suggests that intratumoral plasma cells may serve as a better predictor of ICB response in cancer patients[107,108]. TIL-Bs are believed to use a distinct mode of antigen presentation that fosters the creation and maintenance of immunologically ‘hot’ tumor microenvironments involving T cells, myeloid and natural killer cells. Additionally, by attenuating self-tolerance mechanisms, TIL-Bs minimize the extent of immune editing, a process tumors adopt to evade immune surveillance[109,110,111].

These preclinical studies among others (Table 2) highlight potential avenues to reprogram MPNST immune desert into a more immunologically active TME. The role of UBR5 in immune modulation as already discussed, presents another promising avenue for further research. Evidence from other cancers indicates that UBR5 promotes tumor growth by maintaining an immunosuppressive tumor microenvironment and facilitating immune evasion mediated by WNT/β-catenin, and PD-L1 among other pathways. Targeting UBR5 could enhance the immunogenicity of MPNSTs, improving their response to immune checkpoint blockade. Exploring the interaction between UBR5 and the tumor microenvironment in MPNST will provide valuable insights for developing effective immunotherapeutic strategies for this therapeutically challenging malignancy.