gp100/pmel 17 is a murine tumor rejection antigen: induction of "self"-reactive, tumoricidal T cells using high-affinity, altered peptide ligand

LRIG1 engages ligand VISTA and impairs tumor-specific CD8+ T cell responses

Immune checkpoint blockade is a promising approach to activate antitumor immunity and improve the survival of patients with cancer. V-domain immunoglobulin suppressor of T cell activation (VISTA) is an immune checkpoint target; however, the downstream signaling mechanisms are elusive. Here, we identify leucine-rich repeats and immunoglobulin-like domains 1 (LRIG1) as a VISTA binding partner, which acts as an inhibitory receptor by engaging VISTA and suppressing T cell receptor signaling pathways. Mice with T cell-specific LRIG1 deletion developed superior antitumor responses because of expansion of tumor-specific cytotoxic T lymphocytes (CTLs) with increased effector function and survival. Sustained tumor control was associated with a reduction of quiescent CTLs (TCF1+ CD62Lhi PD-1low) and a reciprocal increase in progenitor and memory-like CTLs (TCF1+ PD-1+). In patients with melanoma, elevated LRIG1 expression on tumor-infiltrating CD8+ CTLs correlated with resistance to immunotherapies. These results delineate the role of LRIG1 as an inhibitory immune checkpoint receptor and propose a rationale for targeting the VISTA/LRIG1 axis for cancer immunotherapy.

Clinical drug screening reveals clofazimine potentiates the efficacy while reducing the toxicity of anti-PD-1 and CTLA-4 immunotherapy

Emerging as the most potent and durable combinational immunotherapy, dual anti-PD-1 and CTLA-4 immune checkpoint blockade (ICB) therapy notoriously increases grade 3-5 immune-related adverse events (irAEs) in patients. Accordingly, attempts to improve the antitumor potency of anti-PD-1+CTLA-4 ICB by including additional therapeutics have been largely discouraged due to concerns of further increasing fatal toxicity. Here, we screened ∼3,000 Food and Drug Administration (FDA)-approved drugs and identified clofazimine as a potential third agent to optimize anti-PD-1+CTLA-4 ICB. Remarkably, clofazimine outperforms ICB dose reduction or steroid treatment in reversing lethality of irAEs, but unlike the detrimental effect of steroids on antitumor efficacy, clofazimine potentiates curative responses in anti-PD-1+CTLA-4 ICB. Mechanistically, clofazimine promotes E2F1 activation in CD8+ T cells to overcome resistance and counteracts pathogenic Th17 cells to abolish irAEs. Collectively, clofazimine potentiates the antitumor efficacy of anti-PD-1+CTLA-4 ICB, curbs intractable irAEs, and may fill a desperate clinical need to improve patient survival.

From mice to men: An assessment of preclinical model systems for the study of vitiligo

Vitiligo is an autoimmune skin disease of multiple etiology, for which there is no complete cure. This chronic depigmentation is characterized by epidermal melanocyte loss, and causes disfigurement and significant psychosocial distress. Mouse models have been extensively employed to further our understanding of complex disease mechanisms in vitiligo, as well as to provide a preclinical platform for clinical interventional research on potential treatment strategies in humans. The current mouse models can be categorized into three groups: spontaneous mouse models, induced mouse models, and transgenic mice. Despite their limitations, these models allow us to understand the pathology processes of vitiligo at molecule, cell, tissue, organ, and system levels, and have been used to test prospective drugs. In this review, we comprehensively evaluate existing murine systems of vitiligo and elucidate their respective characteristics, aiming to offer a panorama for researchers to select the appropriate mouse models for their study.

Design of Cytotoxic T Cell Epitopes by Machine Learning of Human Degrons

Antigen processing is critical for therapeutic vaccines to generate epitopes for priming cytotoxic T cell responses against cancer and pathogens, but insufficient processing often limits the quantity of epitopes released. We address this challenge using machine learning to ascribe a proteasomal degradation score to epitope sequences. Epitopes with varying scores were translocated into cells using nontoxic anthrax proteins. Epitopes with a low score show pronounced immunogenicity due to antigen processing, but epitopes with a high score show limited immunogenicity. This work sheds light on the sequence-activity relationships between proteasomal degradation and epitope immunogenicity. We anticipate that future efforts to incorporate proteasomal degradation signals into vaccine designs will lead to enhanced cytotoxic T cell priming by these vaccines in clinical settings.

Design and Evaluation of Synthetic Delivery Formulations for Peptide-Based Cancer Vaccines

With the recent advances in neoantigen identification, peptide-based cancer vaccines offer substantial potential in the field of immunotherapy. However, rapid clearance, low immunogenicity, and insufficient antigen-presenting cell (APC) uptake limit the efficacy of peptide-based cancer vaccines. This review explores the barriers hindering vaccine efficiency, highlights recent advancements in synthetic delivery systems, and features strategies for the key delivery steps of lymph node (LN) drainage, APC delivery, cross-presentation strategies, and adjuvant incorporation. This paper also discusses the design of preclinical studies evaluating vaccine efficiency, including vaccine administration routes and murine tumor models.

Circulating cancer-specific CD8 T cell frequency is associated with response to PD-1 blockade in Merkel cell carcinoma

Understanding cancer immunobiology has been hampered by difficulty identifying cancer-specific T cells. Merkel cell polyomavirus (MCPyV) causes most Merkel cell carcinomas (MCCs). All patients with virus-driven MCC express MCPyV oncoproteins, facilitating identification of virus (cancer)-specific T cells. We studied MCPyV-specific T cells from 27 patients with MCC using MCPyV peptide-HLA-I multimers, 26-color flow cytometry, single-cell transcriptomics, and T cell receptor (TCR) sequencing. In a prospective clinical trial, higher circulating MCPyV-specific CD8 T cell frequency before anti-PD-1 treatment was strongly associated with 2-year recurrence-free survival (75% if detectable, 0% if undetectable, p = 0.0018; ClinicalTrial.gov: NCT02488759). Intratumorally, such T cells were typically present, but their frequency did not significantly associate with response. Circulating MCPyV-specific CD8 T cells had increased stem/memory and decreased exhaustion signatures relative to their intratumoral counterparts. These results suggest that cancer-specific CD8 T cells in the blood may play a role in anti-PD-1 responses. Thus, strategies that augment their number or mobilize them into tumors could improve outcomes.

CD3 downregulation identifies high-avidity human CD8 T cells

CD8 T cells recognize infected and cancerous cells via their T-cell receptor (TCR), which binds peptide-MHC complexes on the target cell. The affinity of the interaction between the TCR and peptide-MHC contributes to the antigen sensitivity, or functional avidity, of the CD8 T cell. In response to peptide-MHC stimulation, the TCR-CD3 complex and CD8 co-receptor are downmodulated. We quantified CD3 and CD8 downmodulation following stimulation of human CD8 T cells with CMV, EBV, and HIV peptides spanning eight MHC restrictions, observing a strong correlation between the levels of CD3 and CD8 downmodulation and functional avidity, regardless of peptide viral origin. In TCR-transduced T cells targeting a tumor-associated antigen, changes in TCR-peptide affinity were sufficient to modify CD3 and CD8 downmodulation. Correlation analysis and generalized linear modeling indicated that CD3 downmodulation was the stronger correlate of avidity. CD3 downmodulation, simply measured using flow cytometry, can be used to identify high-avidity CD8 T cells in a clinical context.

SOX10 Loss Sensitizes Melanoma Cells to Cytokine-Mediated Inflammatory Cell Death

The transcription factor, SOX10, plays an important role in the differentiation of neural crest precursors to the melanocytic lineage. Malignant transformation of melanocytes leads to the development of melanoma, and SOX10 promotes melanoma cell proliferation and tumor formation. SOX10 expression in melanomas is heterogeneous, and loss of SOX10 causes a phenotypic switch toward an invasive, mesenchymal-like cell state and therapy resistance; hence, strategies to target SOX10-deficient cells are an active area of investigation. The impact of cell state and SOX10 expression on antitumor immunity is not well understood but will likely have important implications for immunotherapeutic interventions. To this end, we tested whether SOX10 status affects the response to CD8+ T cell-mediated killing and T cell-secreted cytokines, TNFα and IFNγ, which are critical effectors in the cytotoxic killing of cancer cells. We observed that genetic ablation of SOX10 rendered melanoma cells more sensitive to CD8+ T cell-mediated killing and cell death induction by either TNFα or IFNγ. Cytokine-mediated cell death in SOX10-deficient cells was associated with features of caspase-dependent pyroptosis, an inflammatory form of cell death that has the potential to increase immune responses.

IMPLICATIONS

These data support a role for SOX10 expression altering the response to T cell-mediated cell death and contribute to a broader understanding of the interaction between immune cells and melanoma cells.

Infection of tumor cells with Salmonella typhimurium mimics immunogenic cell death and elicits tumor-specific immune responses

Some properties of Salmonella-infected cells overlap with immunogenic cell death. In this study, we demonstrated that intracellular infection of melanoma with Salmonella typhimurium induced high immunogenicity in melanoma cells, leading to antitumor effects with melanoma-antigen-specific T-cell responses. Murine B16F10 melanoma cells were infected with tdTomato-expressing attenuated S. typhimurium (VNP20009; VNP-tdT), triggering massive cell vacuolization. VNP-tdT-infected B16F10 cells were phagocytosed efficiently, which induced the activation of antigen-presenting cells with CD86 expression in vitro. Subcutaneous coimplantation of uninfected and VNP-tdT-infected B16F10 cells into C57BL/6 mice significantly suppressed tumor growth compared with the implantation of uninfected B16F10 cells alone. Inoculation of mice with VNP-tdT-infected B16F10 cells elicited the proliferation of melanoma-antigen (gp100)-specific T cells, and it protected the mice from the second tumor challenge of uninfected B16F10 cells. These results suggest that Salmonella-infected tumor cells acquire effective adjuvanticity, leading to ideal antitumor immune responses.

APR-246 increases tumor antigenicity independent of p53

We previously reported that activation of p53 by APR-246 reprograms tumor-associated macrophages to overcome immune checkpoint blockade resistance. Here, we demonstrate that APR-246 and its active moiety, methylene quinuclidinone (MQ) can enhance the immunogenicity of tumor cells directly. MQ treatment of murine B16F10 melanoma cells promoted activation of melanoma-specific CD8+ T cells and increased the efficacy of a tumor cell vaccine using MQ-treated cells even when the B16F10 cells lacked p53. We then designed a novel combination of APR-246 with the TLR-4 agonist, monophosphoryl lipid A, and a CD40 agonist to further enhance these immunogenic effects and demonstrated a significant antitumor response. We propose that the immunogenic effect of MQ can be linked to its thiol-reactive alkylating ability as we observed similar immunogenic effects with the broad-spectrum cysteine-reactive compound, iodoacetamide. Our results thus indicate that combination of APR-246 with immunomodulatory agents may elicit effective antitumor immune response irrespective of the tumor's p53 mutation status.

Design of Cytotoxic T Cell Epitopes by Machine Learning of Human Degrons

Antigen processing is critical for producing epitope peptides that are presented by HLA molecules for T cell recognition. Therapeutic vaccines aim to harness these epitopes for priming cytotoxic T cell responses against cancer and pathogens, but insufficient processing often reduces vaccine efficacy through limiting the quantity of epitopes released. Here, we set out to improve antigen processing by harnessing protein degradation signals called degrons from the ubiquitin-proteasome system. We used machine learning to generate a computational model that ascribes a proteasomal degradation score between 0 and 100. Epitope peptides with varying degron activities were synthesized and translocated into cells using nontoxic anthrax proteins: protective antigen (PA) and the N-terminus of lethal factor (LFN). Immunogenicity studies revealed epitope sequences with a low score (<25) show pronounced T-cell activation but epitope sequences with a higher score (>75) provide limited activation. This work sheds light on the sequence-activity relationships between proteasomal degradation and epitope immunogenicity, through conserving the epitope region but varying the flanking sequence. We anticipate that future efforts to incorporate proteasomal degradation signals into vaccine designs will lead to enhanced cytotoxic T cell priming by vaccine therapeutics in clinical settings.

Intralesional administration of VAX014 facilitates in situ immunization and potentiates immune checkpoint blockade in immunologically cold tumors

BACKGROUND

Immunologically cold tumors with an 'immune desert' phenotype lack tumor-infiltrating lymphocytes (TILs) and are typically impervious to systemic immune checkpoint blockade (ICB). Intratumoral treatment of tumors with immunomodulatory agents can promote local tumor inflammation leading to improved T cell responses in injected tumors. Addition of systemic ICB increases response frequency and immune-mediated clearance of injected and distal non-injected lesions, and this promising approach is being widely investigated clinically. In this work, we evaluate and characterize the local and systemic antitumor immunotherapeutic activity of VAX014, a novel non-viral targeted oncolytic agent based on recombinant bacterial minicells, following intratumoral administration and in combination with systemic ICB.

METHODS

The immunotherapeutic activity of VAX014 following weekly intratumoral administration was investigated in multiple preclinical tumor models with B16F10 murine melanoma serving as the primary model for evaluation of immune desert tumors. Mice bearing a single intradermal tumor were used to evaluate tumor response and overall survival (OS), assess changes in immune cell populations, and explore global changes to immunotranscriptomes of injected tumors. Mice bearing bilateral intradermal tumors were then used to evaluate non-injected tumors for changes in TIL populations and phenotypes, compare immunotranscriptomes across treatment groups, and assess distal non-injected tumor response in the context of monotherapy or in combination with ICB.

RESULTS

VAX014 demonstrated strong immune-mediated tumor clearance of injected tumors coinciding with significantly elevated CD8+ TILs and upregulation of multiple immune pathways essential for antitumor immune responses. Modest activity against distal non-injected immune desert tumors was observed despite elevated levels of systemic antitumor lymphocytes. Combination with systemic CTLA-4 blockade improved survival and elevated TILs but did not improve clearance rates of non-injected tumors. Immunotranscriptomes of non-injected tumors from this treatment combination group exhibited upregulation of multiple immune pathways but also identified upregulation of PD-1. Further addition of systemic PD-1 blockade led to rapid clearance of non-injected tumors, enhanced OS, and provided durable protective immunological memory.

CONCLUSIONS

Intratumoral administration of VAX014 stimulates local immune activation and robust systemic antitumor lymphocytic responses. Combination with systemic ICB deepens systemic antitumor responses to mediate clearance of injected and distal non-injected tumors.

Structural analysis of cancer-relevant TCR-CD3 and peptide-MHC complexes by cryoEM

The recognition of antigenic peptide-MHC (pMHC) molecules by T-cell receptors (TCR) initiates the T-cell mediated immune response. Structural characterization is key for understanding the specificity of TCR-pMHC interactions and informing the development of therapeutics. Despite the rapid rise of single particle cryoelectron microscopy (cryoEM), x-ray crystallography has remained the preferred method for structure determination of TCR-pMHC complexes. Here, we report cryoEM structures of two distinct full-length α/β TCR-CD3 complexes bound to their pMHC ligand, the cancer-testis antigen HLA-A2/MAGEA4 (230-239). We also determined cryoEM structures of pMHCs containing MAGEA4 (230-239) peptide and the closely related MAGEA8 (232-241) peptide in the absence of TCR, which provided a structural explanation for the MAGEA4 preference displayed by the TCRs. These findings provide insights into the TCR recognition of a clinically relevant cancer antigen and demonstrate the utility of cryoEM for high-resolution structural analysis of TCR-pMHC interactions.

Glucocorticoid activation by HSD11B1 limits T cell-driven interferon signaling and response to PD-1 blockade in melanoma

BACKGROUND

Immune responses against tumors are subject to negative feedback regulation. Immune checkpoint inhibitors (ICIs) blocking Programmed cell death protein 1 (PD-1), a receptor expressed on T cells, or its ligand PD-L1 have significantly improved the treatment of cancer, in particular malignant melanoma. Nevertheless, responses and durability are variables, suggesting that additional critical negative feedback mechanisms exist and need to be targeted to improve therapeutic efficacy.

METHODS

We used different syngeneic melanoma mouse models and performed PD-1 blockade to identify novel mechanisms of negative immune regulation. Genetic gain-of-function and loss-of-function approaches as well as small molecule inhibitor applications were used for target validation in our melanoma models. We analyzed mouse melanoma tissues from treated and untreated mice by RNA-seq, immunofluorescence and flow cytometry to detect changes in pathway activities and immune cell composition of the tumor microenvironment. We analyzed tissue sections of patients with melanoma by immunohistochemistry as well as publicly available single-cell RNA-seq data and correlated target expression with clinical responses to ICIs.

RESULTS

Here, we identified 11-beta-hydroxysteroid dehydrogenase-1 (HSD11B1), an enzyme that converts inert glucocorticoids into active forms in tissues, as negative feedback mechanism in response to T cell immunotherapies. Glucocorticoids are potent suppressors of immune responses. HSD11B1 was expressed in different cellular compartments of melanomas, most notably myeloid cells but also T cells and melanoma cells. Enforced expression of HSD11B1 in mouse melanomas limited the efficacy of PD-1 blockade, whereas small molecule HSD11B1 inhibitors improved responses in a CD8+ T cell-dependent manner. Mechanistically, HSD11B1 inhibition in combination with PD-1 blockade augmented the production of interferon-γ by T cells. Interferon pathway activation correlated with sensitivity to PD-1 blockade linked to anti-proliferative effects on melanoma cells. Furthermore, high levels of HSD11B1, predominantly expressed by tumor-associated macrophages, were associated with poor responses to ICI therapy in two independent cohorts of patients with advanced melanomas analyzed by different methods (scRNA-seq, immunohistochemistry).

CONCLUSION

As HSD11B1 inhibitors are in the focus of drug development for metabolic diseases, our data suggest a drug repurposing strategy combining HSD11B1 inhibitors with ICIs to improve melanoma immunotherapy. Furthermore, our work also delineated potential caveats emphasizing the need for careful patient stratification.

Tumor antigen-loaded AAV vaccine drives protective immunity in a melanoma animal model

We previously described therapeutic opportunities provided by capsid- and expression cassette-optimized adeno-associated virus serotype 6 (AAV6) vectors to suppress tumor growth in both solid and metastatic mouse models by using artificial ovalbumin (OVA) immunogen. In the current study, we further elucidated the mechanism of function of a novel AAV-based vaccine loaded with the melanoma tumor-associated antigens premelanosome protein gp100, tyrosinase (Tyr), tyrosinase-related protein 1 (TRP1), and dopachrome tautomerase (TRP2). We showed that the AAV6-based vaccine creates cellular and humoral antigen-specific responses, while antigen expression at the site of vaccine injection was temporal, and the clearance of antigen coincided with T cell infiltration. Our data revealed the superior protective immune response of optimized AAV6-TRP1 compared with other self-antigens in a disease-free mouse model. We further assessed the ability of AAV6-TRP1 to protect animals from metastatic spread in the lungs and to extend animal survival by inhibiting solid tumor growth. Flow cytometry-based analysis indicated significant infiltration of CD8⁺ T cells and natural killer (NK) cells in the tumor site, as well as changes in the polarization of intratumoral macrophages. Altogether, our data strongly support the use of optimized AAV vectors for cancer vaccine development.

Intracavitary adoptive transfer of IL-12 mRNA-engineered tumor-specific CD8+ T cells eradicates peritoneal metastases in mouse models

Previous studies have shown that local delivery of tumor antigen-specific CD8⁺ T lymphocytes engineered to transiently express single-chain IL-12 mRNA is highly efficacious. Peritoneal dissemination of cancer is a frequent and often fatal patient condition usually diagnosed when the tumor burden is too large and hence uncontrollable with current treatment options. In this study, we have modeled intracavitary adoptive T cell therapy with OVA-specific OT-I T cells electroporated with IL-12 mRNA to treat B16-OVA and PANC02-OVA tumor spread in the peritoneal cavity. Tumor localization in the omentum and the effects of local T-cell encounter with the tumor antigens were monitored, the gene expression profile evaluated, and the phenotypic reprogramming of several immune subsets was characterized. Intraperitoneal administration of T cells promoted homing to the omentum more effectively than intravenous administration. Transient IL-12 expression was responsible for a favorable reprogramming of the tumor immune microenvironment, longer persistence of transferred T lymphocytes in vivo, and the development of immunity to endogenous antigens following primary tumor eradication. The efficacy of the strategy was at least in part recapitulated with the adoptive transfer of lower affinity transgenic TCR-bearing PMEL-1 T lymphocytes to treat the aggressive intraperitoneally disseminated B16-F10 tumor. Locoregional adoptive transfer of transiently IL-12-armored T cells appears to offer promising therapeutic advantages in terms of anti-tumor efficacy to treat peritoneal carcinomatosis.

Inflammatory Cytokines That Enhance Antigen Responsiveness of Naïve CD8(+) T Lymphocytes Modulate Chromatin Accessibility of Genes Impacted by Antigen Stimulation

Naïve CD8⁺ T lymphocytes exposed to certain inflammatory cytokines undergo proliferation and display increased sensitivity to antigens. Such 'cytokine priming' can promote the activation of potentially autoreactive and antitumor CD8⁺ T cells by weak tissue antigens and tumor antigens. To elucidate the molecular mechanisms of cytokine priming, naïve PMEL-1 TCR transgenic CD8⁺ T lymphocytes were stimulated with IL-15 and IL-21, and chromatin accessibility was assessed using the assay for transposase-accessible chromatin (ATAC) sequencing. PMEL-1 cells stimulated by the cognate antigenic peptide mgp100_25-33 served as controls. Cytokine-primed cells showed a limited number of opening and closing chromatin accessibility peaks compared to antigen-stimulated cells. However, the ATACseq peaks in cytokine-primed cells substantially overlapped with those of antigen-stimulated cells and mapped to several genes implicated in T cell signaling, activation, effector differentiation, negative regulation and exhaustion. Nonetheless, the expression of most of these genes was remarkably different between cytokine-primed and antigen-stimulated cells. In addition, cytokine priming impacted the expression of several genes following antigen stimulation in a synergistic or antagonistic manner. Our findings indicate that chromatin accessibility changes in cytokine-primed naïve CD8⁺ T cells not only underlie their increased antigen responsiveness but may also enhance their functional fitness by reducing exhaustion without compromising regulatory controls.

Divergent outcomes of anti-PD-L1 treatment coupled with host-intrinsic differences in TCR repertoire and distinct T cell activation states in responding versus non-responding tumors

Differential responses to immune checkpoint inhibitors (ICI) may be attributed to tumor-intrinsic factors or environmental cues; however, these mechanisms cannot fully explain the variable ICI responses in different individuals. Here, we investigate the potential contribution of immunological heterogeneity with a focus on differences in T-cell receptor (TCR) repertoire to ICI responses, which has not been defined previously. To reveal additional factors underlying heterogeneous responses to ICI, we employed a squamous cell carcinoma (SCC) mouse model in which tumor-bearing recipients unambiguously diverged into responders (R) or non-responders (NR) upon anti-PD-L1 treatment. Treatment efficacy absolutely required CD8 T-cells and correlated positively with effector functions of CD8 tumor-infiltrating lymphocytes (TILs). We showed that TCR repertoires exhibited a similar magnitude of clonal expansion in R vs. NR CD8 TILs. However, the top expanded TCR clonotypes appeared to be mutually exclusive between R and NR CD8 TILs, which also occurred in a recipient-specific manner, demonstrating preferential expansion of distinct TCR clonotypes against the same SCC tumor. Unexpectedly, R vs. NR CD8 TILs reached all activation clusters and did not exhibit substantial global differences in transcriptomes. By linking single-cell transcriptomic data with unique TCR clonotypes, CD8 TILs harboring top TCR clonotypes were found to occupy distinct activation clusters and upregulate genes favoring anti-tumor immunity to different extents in R vs. NR. We conclude that stochastic differences in CD8 TIL TCR repertoire and distinct activation states of top TCR clonotypes may contribute to differential anti-PD-L1 responses. Our study suggests that host-intrinsic immunological heterogeneity may offer a new explanation for differential ICI responses in different individuals, which could impact on strategies for personalized cancer immunotherapy.

Transient cell-in-cell formation underlies tumor relapse and resistance to immunotherapy

Despite the remarkable successes of cancer immunotherapies, the majority of patients will experience only partial response followed by relapse of resistant tumors. While treatment resistance has frequently been attributed to clonal selection and immunoediting, comparisons of paired primary and relapsed tumors in melanoma and breast cancers indicate that they share the majority of clones. Here, we demonstrate in both mouse models and clinical human samples that tumor cells evade immunotherapy by generating unique transient cell-in-cell structures, which are resistant to killing by T cells and chemotherapies. While the outer cells in this cell-in-cell formation are often killed by reactive T cells, the inner cells remain intact and disseminate into single tumor cells once T cells are no longer present. This formation is mediated predominantly by IFNγ-activated T cells, which subsequently induce phosphorylation of the transcription factors signal transducer and activator of transcription 3 (STAT3) and early growth response-1 (EGR-1) in tumor cells. Indeed, inhibiting these factors prior to immunotherapy significantly improves its therapeutic efficacy. Overall, this work highlights a currently insurmountable limitation of immunotherapy and reveals a previously unknown resistance mechanism which enables tumor cells to survive immune-mediated killing without altering their immunogenicity.

Cancer immunotherapies use the body’s own immune system to fight off cancer. But, despite some remarkable success stories, many patients only see a temporary improvement before the immunotherapy stops being effective and the tumours regrow.

It is unclear why this occurs, but it may have to do with how the immune system attacks cancer cells. Immunotherapies aim to activate a special group of cells known as killer T-cells, which are responsible for the immune response to tumours. These cells can identify cancer cells and inject toxic granules through their membranes, killing them. However, killer T-cells are not always effective. This is because cancer cells are naturally good at avoiding detection, and during treatment, their genes can mutate, giving them new ways to evade the immune system. Interestingly, when scientists analysed the genes of tumour cells before and after immunotherapy, they found that many of the genes that code for proteins recognized by T-cells do not change significantly. This suggests that tumours’ resistance to immune attack may be physical, rather than genetic.

To investigate this hypothesis, Gutwillig et al. developed several mouse tumour models that stop responding to immunotherapy after initial treatment. Examining cells from these tumours revealed that when the immune system attacks, they reorganise by getting inside one another. This allows some cancer cells to hide under many layers of cell membrane. At this point killer T-cells can identify and inject the outer cell with toxic granules, but it cannot reach the cells inside.

This ability of cancer cells to hide within one another relies on them recognising when the immune system is attacking. This happens because the cancer cells can detect certain signals released by the killer T-cells, allowing them to hide. Gutwillig et al. identified some of these signals, and showed that blocking them stopped cancer cells from hiding inside each other, making immunotherapy more effective.

This new explanation for how cancer cells escape the immune system could guide future research and lead to new cancer treatments, or approaches to boost existing treatments. Understanding the process in more detail could allow scientists to prevent it from happening, by revealing which signals to block, and when, for best results.

Targeting Endothelial Connexin37 Reduces Angiogenesis and Decreases Tumor Growth

Connexin37 (Cx37) and Cx40 form intercellular channels between endothelial cells (EC), which contribute to the regulation of the functions of vessels. We previously documented the participation of both Cx in developmental angiogenesis and have further shown that loss of Cx40 decreases the growth of different tumors. Here, we report that loss of Cx37 reduces (1) the in vitro proliferation of primary human EC; (2) the vascularization of subcutaneously implanted matrigel plugs in Cx37−/− mice or in WT using matrigel plugs supplemented with a peptide targeting Cx37 channels; (3) tumor angiogenesis; and (4) the growth of TC-1 and B16 tumors, resulting in a longer mice survival. We further document that Cx37 and Cx40 function in a collaborative manner to promote tumor growth, inasmuch as the injection of a peptide targeting Cx40 into Cx37−/− mice decreased the growth of TC-1 tumors to a larger extent than after loss of Cx37. This loss did not alter vessel perfusion, mural cells coverage and tumor hypoxia compared to tumors grown in WT mice. The data show that Cx37 is relevant for the control of EC proliferation and growth in different tumor models, suggesting that it may be a target, alone or in combination with Cx40, in the development of anti-tumoral treatments.

Nanovesicle-Mediated Targeted Delivery of Immune Checkpoint Blockades to Potentiate Therapeutic Efficacy and Prevent Side Effects

Despite the clinically proven efficacies of immune checkpoint blockades, including anti-cytotoxic T lymphocyte-associated protein 4 antibody (αCTLA-4), the low response rate and immune-related adverse events (irAEs) in cancer patients represent major drawbacks of the therapy. These drawbacks of αCTLA-4 therapy are mainly due to the suboptimal activation of tumor-specific cytotoxic T lymphocytes (CTLs) and the systemic nonspecific activation of T cells. To overcome such drawbacks, αCTLA-4 is delivered by dendritic cell-derived nanovesicles presenting tumor antigens (DCNV-TAs) that exclusively interact with tumor-specific T cells, leading to selective activation of tumor-specific CTLs. Compared to conventional αCTLA-4 therapy, treatment with αCTLA-4-conjugated DCNV-TAs significantly inhibits tumor growth and reduces irAEs in syngeneic tumor-bearing mice. This study demonstrates that the spatiotemporal presentation of both αCTLA-4 and tumor antigens enables selective activation of tumor-specific T cells and potentiates the antitumor efficacy of αCTLA-4 without inducing systemic irAEs.

An In Vivo Screen to Identify Short Peptide Mimotopes with Enhanced Antitumor Immunogenicity

Tumor-associated self-antigens are potential cancer vaccine targets but suffer from limited immunogenicity. There are examples of mutated, short self-peptides inducing epitope-specific CD8⁺ T cells more efficiently than the wild-type epitope, but current approaches cannot yet reliably identify such epitopes, which are referred to as enhanced mimotopes ("e-mimotopes"). Here, we present a generalized strategy to develop e-mimotopes, using the tyrosinase-related protein 2 (Trp2) peptide Trp2180-188, which is a murine major histocompatibility complex class I (MHC-I) epitope, as a test case. Using a vaccine adjuvant that induces peptide particle formation and strong cellular responses with nanogram antigen doses, a two-step method systematically identified e-mimotope candidates with murine immunization. First, position-scanning peptide micro libraries were generated in which each position of the wild-type epitope sequence was randomized. Randomization of only one specific residue of the Trp2 epitope increased antitumor immunogenicity. Second, all 20 amino acids were individually substituted and tested at that position, enabling the identification of two e-mimotopes with single amino-acid mutations. Despite similar MHC-I affinity compared to the wild-type epitope, e-mimotope immunization elicited improved Trp2-specific cytotoxic T-cell phenotypes and improved T-cell receptor affinity for both the e-mimotopes and the native epitope, resulting in better outcomes in multiple prophylactic and therapeutic tumor models. The screening method was also applied to other targets with other murine MHC-I restriction elements, including epitopes within glycoprotein 70 and Wilms' Tumor Gene 1, to identify additional e-mimotopes with enhanced potency.

Antitumor Peptide-Based Vaccine in the Limelight

The success of the immune checkpoint blockade has provided a proof of concept that immune cells are capable of attacking tumors in the clinic. However, clinical benefit is only observed in less than 20% of the patients due to the non-specific activation of immune cells by the immune checkpoint blockade. Developing tumor-specific immune responses is a challenging task that can be achieved by targeting tumor antigens to generate tumor-specific T-cell responses. The recent advancements in peptide-based immunotherapy have encouraged clinicians and patients who are struggling with cancer that is otherwise non-treatable with current therapeutics. By selecting appropriate epitopes from tumor antigens with suitable adjuvants, peptides can elicit robust antitumor responses in both mice and humans. Although recent experimental data and clinical trials suggest the potency of tumor reduction by peptide-based vaccines, earlier clinical trials based on the inadequate hypothesis have misled that peptide vaccines are not efficient in eliminating tumor cells. In this review, we highlighted the recent evidence that supports the rationale of peptide-based antitumor vaccines. We also discussed the strategies to select the optimal epitope for vaccines and the mechanism of how adjuvants increase the efficacy of this promising approach to treat cancer.

In Vitro Cell Impedance Assay to Examine Antigen-Specific T-Cell-Mediated Melanoma Cell Killing to Support Cancer Immunotherapy Drug Discovery

The design of cancer immunotherapy drugs is essential for the continued investigation of novel drug regimens to improve responses and increase the survival of cancer patients. Methods to examine the interaction of effector immune cells with target cancer cells are limited by labor-intensive labeling that can be examined at specific time points. In this report, we examine an antigen-dependent model of effector cytotoxic (CD8+) T-cell-mediated cytotoxicity of target murine melanoma cells using a real-time cell impedance assay. The real-time monitoring allows measurement of viability and kinetics, allowing for a better understanding of effector/target cell interactions to support drug discovery.

RKIP Pleiotropic Activities in Cancer and Inflammatory Diseases: Role in Immunity

Simple Summary

The human body consists of tissues and organs formed by cells. In each cell there is a switch that allows the cell to divide or not. In contrast, cancer cells have their switch on which allow them to divide and invade other sites leading to death. Over two decades ago, Doctor Kam Yeung, University of Toledo, Ohio, has identified a factor (RKIP) that is responsible for the on/off switch which functions normally in healthy tissues but is inactive or absent in cancers. Since this early discovery, many additional properties have been ascribed to RKIP including its role in inhibiting cancer metastasis and resistance to therapeutics and its role in modulating the normal immune response. This review describes all of the above functions of RKIP and suggesting therapeutics to induce RKIP in cancers to inhibit their growth and metastases as well as inhibit its activity to treat non-cancerous inflammatory diseases.

Abstract

Several gene products play pivotal roles in the induction of inflammation and the progression of cancer. The Raf kinase inhibitory protein (RKIP) is a cytosolic protein that exerts pleiotropic activities in such conditions, and thus regulates oncogenesis and immune-mediated diseases through its deregulation. Herein, we review the general properties of RKIP, including its: (i) molecular structure; (ii) involvement in various cell signaling pathways (i.e., inhibition of the Raf/MEK/ERK pathway; the NF-kB pathway; GRK-2 or the STAT-3 pathway; as well as regulation of the GSK3Beta signaling; and the spindle checkpoints); (iii) regulation of RKIP expression; (iv) expression’s effects on oncogenesis; (v) role in the regulation of the immune system to diseases (i.e., RKIP regulation of T cell functions; the secretion of cytokines and immune mediators, apoptosis, immune check point inhibitors and RKIP involvement in inflammatory diseases); and (vi) bioinformatic analysis between normal and malignant tissues, as well as across various immune-related cells. Overall, the regulation of RKIP in different cancers and inflammatory diseases suggest that it can be used as a potential therapeutic target in the treatment of these diseases.

A therapeutic cancer vaccine delivers antigens and adjuvants to lymphoid tissues using genetically modified T cells

Therapeutic vaccines that augment T cell responses to tumor antigens have been limited by poor potency in clinical trials. In contrast, the transfer of T cells modified with foreign transgenes frequently induces potent endogenous T cell responses to epitopes in the transgene product, and these responses are undesirable, because they lead to rejection of the transferred T cells. We sought to harness gene-modified T cells as a vaccine platform and developed cancer vaccines composed of autologous T cells modified with tumor antigens and additional adjuvant signals (Tvax). T cells expressing model antigens and a broad range of tumor neoantigens induced robust and durable T cell responses through cross-presentation of antigens by host DCs. Providing Tvax with signals such as CD80, CD137L, IFN-β, IL-12, GM-CSF, and FLT3L enhanced T cell priming. Coexpression of IL-12 and GM-CSF induced the strongest CD4⁺ and CD8⁺ T cell responses through complimentary effects on the recruitment and activation of DCs, mediated by autocrine IL-12 receptor signaling in the Tvax. Therapeutic vaccination with Tvax and adjuvants showed antitumor activity in subcutaneous and metastatic preclinical mouse models. Human T cells modified with neoantigens readily activated specific T cells derived from patients, providing a path for clinical translation of this therapeutic platform in cancer.

Novel EGFRvIII-CAR transgenic mice for rigorous preclinical studies in syngeneic mice

BACKGROUND

Rigorous preclinical studies of chimeric antigen receptor (CAR) immunotherapy will require large quantities of consistent and high-quality CAR-transduced T (CART)-cells that can be used in syngeneic mouse glioblastoma (GBM) models. To this end, we developed a novel transgenic (Tg) mouse strain with a fully murinized CAR targeting epidermal growth factor receptor variant III (EGFRvIII).

METHODS

We first established the murinized version of EGFRvIII-CAR and validated its function using a retroviral vector (RV) in C57BL/6J mice bearing syngeneic SB28 GBM expressing EGFRvIII. Next, we created C57BL/6J-background Tg mice carrying the anti-EGFRvIII-CAR downstream of a Lox-Stop-Lox cassette in the Rosa26 locus. We bred these mice with CD4-Cre Tg mice to allow CAR expression on T-cells and evaluated the function of the CART-cells both in vitro and in vivo. To inhibit immunosuppressive myeloid cells within SB28 GBM, we also evaluated a combination approach of CART and an anti-EP4 compound (ONO-AE3-208).

RESULTS

Both RV- and Tg-CART-cells demonstrated specific cytotoxic activities against SB28-EGFRvIII cells. A single intravenous infusion of EGFRvIII-CART-cells prolonged the survival of glioma-bearing mice when preceded by a lymphodepletion regimen with recurrent tumors displaying profound EGFRvIII loss. The addition of ONO-AE3-208 resulted in long-term survival in a fraction of CART-treated mice and those survivors demonstrated delayed growth of subcutaneously re-challenged both EGFRvIII + and parental EGFRvIII - SB28.

CONCLUSION

Our new syngeneic CAR Tg mouse model can serve as a useful tool to address clinically relevant questions and develop future immunotherapeutic strategies.

PMEL as a Prognostic Biomarker and Negatively Associated With Immune Infiltration in Skin Cutaneous Melanoma (SKCM)

Premelanosome protein (PMEL) is crucial for the formation of melanosomal fibrils through the transition from stage I to stage II melanosomes. It was used as a target antigen in some adoptive T-cell therapy of melanoma. The correlation of PMEL to prognosis and immune cell infiltration level are unknown in melanoma. The PMEL expression was evaluated via Tumor Immune Estimation Resource, Oncomine and Gene Expression Profiling Interactive Analysis (GEPIA). We also evaluate the influence of PMEL on overall survival via GEPIA, PrognoScan, and immunohistochemistry in human tissue microarray. The correlation between PMEL expression level and immune cell or gene markers of immune infiltration level was explored on Tumor Immune Estimation Resource and GEPIA. PMEL expression was significantly higher in skin cutaneous melanoma (SKCM) and SKCM-metastasis in comparison with the other cancers. In SKCM, PMEL expression in high levels was associated with poor overall survival. In both SKCM and SKCM-metastasis patients, PMEL expression is negatively correlated with the infiltration cells of CD8+ T cells, macrophages, and neutrophils. Programmed cell-death protein 1 just showed response rates ranging from 20% to 40% in patients with melanoma, so it is critical to discover a new therapeutic target. PMEL is negatively associated with immune cell infiltration and can be as a negative prognosis marker or new immunotherapy target in SKCM and SKCM-metastasis.

Combination of metabolic intervention and T cell therapy enhances solid tumor immunotherapy

Treatment of solid tumors with T cell therapy has yielded limited therapeutic benefits to date. Although T cell therapy in combination with proinflammatory cytokines or immune checkpoints inhibitors has demonstrated preclinical and clinical successes in a subset of solid tumors, unsatisfactory results and severe toxicities necessitate the development of effective and safe combinatorial strategies. Here, the liposomal avasimibe (a metabolism-modulating drug) was clicked onto the T cell surface by lipid insertion without disturbing the physiological functions of the T cell. Avasimibe could be restrained on the T cell surface during circulation and extravasation and locally released to increase the concentration of cholesterol in the T cell membrane, which induced rapid T cell receptor clustering and sustained T cell activation. Treatment with surface anchor-engineered T cells, including mouse T cell receptor transgenic CD8⁺ T cells or human chimeric antigen receptor T cells, resulted in superior antitumor efficacy in mouse models of melanoma and glioblastoma. Glioblastoma was completely eradicated in three of the five mice receiving surface anchor-engineered chimeric antigen receptor T cells, whereas mice in other treatment groups survived no more than 64 days. Moreover, the administration of engineered T cells showed no obvious systemic side effects. These cell-surface anchor-engineered T cells hold translational potential because of their simple generation and their safety profile.

APOBEC and Cancer Viroimmunotherapy: Thinking the Unthinkable

The apolipoprotein B mRNA editing enzyme catalytic polypeptide (APOBEC) family protects against infection by degrading incoming viral genomes through cytosine deamination. Here, we review how the potential to unleash these potent DNA mutagens comes at a price as APOBEC DNA mutagenesis can contribute to development of multiple types of cancer. In addition, because viral infection induces its expression, APOBEC is seen as the enemy of oncolytic virotherapy through mutation of the viral genome and by generating virotherapy-resistant tumors. Therefore, overall APOBEC in cancer has received very poor press. However, we also speculate how there may be silver linings to the storm clouds (kataegis) associated with APOBEC activity. Thus, although mutagenic genomic chaos promotes emergence of ever more aggressive subclones, it also provides significant opportunity for cytotoxic and immune therapies. In particular, the superpower of cancer immunotherapy derives in part from mutation, wherein generation of tumor neoantigens-neoantigenesis-exposes tumor cells to functional T-cell repertoires, and susceptibility to immune checkpoint blockade. Moreover, APOBECs may be able to induce suprathreshold levels of cellular mutation leading to mitotic catastrophe and direct tumor cell killing. Finally, we discuss the possibility that linking predictable APOBEC-induced mutation with escape from specific frontline therapies could identify mutated molecules/pathways that can be targeted with small molecules and/or immunotherapies in a Trap and Ambush strategy. Together, these considerations lead to the counterintuitive hypothesis that, instead of attempting to expunge and excoriate APOBEC activity in cancer therapy, it might be exploited-and even, counterintuitively, encouraged.

Adjuvant-free peptide vaccine targeting Clec9a on dendritic cells can induce robust antitumor immune response through Syk/IL-21 axis

Dendritic cells (DCs) can process the antigens of cancer vaccine and thus stimulate the CD8+ T cells to recognize and kill the tumor cells that express these antigens. However, lack of promising carriers for presenting the antigens to DCs is one of the main barriers to the development of clinically effective cancer vaccines. Another limitation is the risk of inflammatory side effects induced by the adjuvants. It is still unclear how we can develop ideal adjuvant-free DC vaccine carriers without adjuvants. Methods: A 12-mer peptide carrier (CBP-12) with high affinity for Clec9a expressed on DCs was developed using an in silico rational optimization method. The therapeutic effects of the adjuvant-free vaccine comprising CBP-12 and exogenous or endogenous antigenic peptides were investigated in terms of antigen cross-presentation efficacy, specific cytotoxic T lymphocyte response, and antitumor activity. We also explored the mechanism involved in the antitumor effects of the adjuvant-free CBP-12 vaccine. Finally, we assessed the effects of the CBP-12 conjugated peptide vaccine combined with radiotherapy. Results: Here, we developed CBP-12 as a vaccine carrier that enhanced the uptake and cross-presentation of the antigens, thus inducing strong CD8+ T cell responses and antitumor effects in both anti-PD-1-responsive (B16-OVA) and -resistant (B16) models, even in adjuvant-free conditions. CBP-12 bound to and activated Clec9a, thereby stimulating Clec9a+ DC to product IL-21, but not IL-12 by activating of Syk. The antitumor effects of the CBP-12 conjugated peptide vaccines could be blocked by an IL-21 neutralizing antibody. We also observed the synergistic antitumor effects of the CBP-12 conjugated peptide vaccine combined with radiotherapy. Conclusions: CBP-12 could serve as an adjuvant-free peptide vaccine carrier for cancer immunotherapy.

Oncolytic virotherapy induced CSDE1 neo-antigenesis restricts VSV replication but can be targeted by immunotherapy

In our clinical trials of oncolytic vesicular stomatitis virus expressing interferon beta (VSV-IFNβ), several patients achieved initial responses followed by aggressive relapse. We show here that VSV-IFNβ-escape tumors predictably express a point-mutated CSDE1P5S form of the RNA-binding Cold Shock Domain-containing E1 protein, which promotes escape as an inhibitor of VSV replication by disrupting viral transcription. Given time, VSV-IFNβ evolves a compensatory mutation in the P/M Inter-Genic Region which rescues replication in CSDE1P5S cells. These data show that CSDE1 is a major cellular co-factor for VSV replication. However, CSDE1P5S also generates a neo-epitope recognized by non-tolerized T cells. We exploit this predictable neo-antigenesis to drive, and trap, tumors into an escape phenotype, which can be ambushed by vaccination against CSDE1P5S, preventing tumor escape. Combining frontline therapy with escape-targeting immunotherapy will be applicable across multiple therapies which drive tumor mutation/evolution and simultaneously generate novel, targetable immunopeptidomes associated with acquired treatment resistance.

The intrinsic immunogenic properties of cancer cell lines, immunogenic cell death, and how these influence host antitumor immune responses

Modern cancer therapies often involve the combination of tumor-directed cytotoxic strategies and generation of a host antitumor immune response. The latter is unleashed by immunotherapies that activate the immune system generating a more immunostimulatory tumor microenvironment and a stronger tumor antigen-specific immune response. Studying the interaction between antitumor cytotoxic therapies, dying cancer cells, and the innate and adaptive immune system requires appropriate experimental tumor models in mice. In this review, we discuss the immunostimulatory and immunosuppressive properties of cancer cell lines commonly used in immunogenic cell death (ICD) studies being apoptosis or necroptosis. We will especially focus on the antigenic component of immunogenicity. While in several cancer cell lines the epitopes of endogenously expressed tumor antigens are known, these intrinsic epitopes are rarely determined in experimental apoptotic or necroptotic ICD settings. Instead by far the most ICD research studies investigate the antigenic response against exogenously expressed model antigens such as ovalbumin or retroviral epitopes (e.g., AH1). In this review, we will argue that the immune response against endogenous tumor antigens and the immunopeptidome profile of cancer cell lines affect the eventual biological readouts in the typical prophylactic tumor vaccination type of experiments used in ICD research, and we will propose additional methods involving immunopeptidome profiling, major histocompatibility complex molecule expression, and identification of tumor-infiltrating immune cells to document intrinsic immunogenicity following different cell death modalities.

Differences in TCR repertoire and T cell activation underlie the divergent outcomes of antitumor immune responses in tumor-eradicating versus tumor-progressing hosts

Background

Antitumor immunity is highly heterogeneous between individuals; however, underlying mechanisms remain elusive, despite their potential to improve personalized cancer immunotherapy. Head and neck squamous cell carcinomas (HNSCCs) vary significantly in immune infiltration and therapeutic responses between patients, demanding a mouse model with appropriate heterogeneity to investigate mechanistic differences.

Methods

We developed a unique HNSCC mouse model to investigate underlying mechanisms of heterogeneous antitumor immunity. This model system may provide a better control for tumor-intrinsic and host-genetic variables, thereby uncovering the contribution of the adaptive immunity to tumor eradication. We employed single-cell T-cell receptor (TCR) sequencing coupled with single-cell RNA sequencing to identify the difference in TCR repertoire of CD8 tumor-infiltrating lymphocytes (TILs) and the unique activation states linked with different TCR clonotypes.

Results

We discovered that genetically identical wild-type recipient mice responded heterogeneously to the same squamous cell carcinoma tumors orthotopically transplanted into the buccal mucosa. While tumors initially grew in 100% of recipients and most developed aggressive tumors, ~25% of recipients reproducibly eradicated tumors without intervention. Heterogeneous antitumor responses were dependent on CD8 T cells. Consistently, CD8 TILs in regressing tumors were significantly increased and more activated. Single-cell TCR-sequencing revealed that CD8 TILs from both growing and regressing tumors displayed evidence of clonal expansion compared with splenic controls. However, top TCR clonotypes and TCR specificity groups appear to be mutually exclusive between regressing and growing TILs. Furthermore, many TCRα/TCRβ sequences only occur in one recipient. By coupling single-cell transcriptomic analysis with unique TCR clonotypes, we found that top TCR clonotypes clustered in distinct activation states in regressing versus growing TILs. Intriguingly, the few TCR clonotypes shared between regressors and progressors differed greatly in their activation states, suggesting a more dominant influence from tumor microenvironment than TCR itself on T cell activation status.

Conclusions

We reveal that intrinsic differences in the TCR repertoire of TILs and their different transcriptional trajectories may underlie the heterogeneous antitumor immune responses in different hosts. We suggest that antitumor immune responses are highly individualized and different hosts employ different TCR specificities against the same tumors, which may have important implications for developing personalized cancer immunotherapy.

MEK inhibition reprograms CD8+ T lymphocytes into memory stem cells with potent antitumor effects

Regenerative stem cell-like memory (TSCM) CD8+ T cells persist longer and produce stronger effector functions. We found that MEK1/2 inhibition (MEKi) induces TSCM that have naive phenotype with self-renewability, enhanced multipotency and proliferative capacity. This is achieved by delaying cell division and enhancing mitochondrial biogenesis and fatty acid oxidation, without affecting T cell receptor-mediated activation. DNA methylation profiling revealed that MEKi-induced TSCM cells exhibited plasticity and loci-specific profiles similar to bona fide TSCM isolated from healthy donors, with intermediate characteristics compared to naive and central memory T cells. Ex vivo, antigenic rechallenge of MEKi-treated CD8+ T cells showed stronger recall responses. This strategy generated T cells with higher efficacy for adoptive cell therapy. Moreover, MEKi treatment of tumor-bearing mice also showed strong immune-mediated antitumor effects. In conclusion, we show that MEKi leads to CD8+ T cell reprogramming into TSCM that acts as a reservoir for effector T cells with potent therapeutic characteristics.

The Glutaminase Inhibitor CB-839 (Telaglenastat) Enhances the Antimelanoma Activity of T-Cell-Mediated Immunotherapies

Immune-checkpoint inhibitors and adoptive tumor-infiltrating lymphocyte (TIL) therapies have profoundly improved the survival of patients with melanoma. However, a majority of patients do not respond to these agents, and many responders experience disease relapse. Although numerous innovative treatments are being explored to offset the limitations of these agents, novel therapeutic combinations with immunotherapies have the potential to improve patient responses. In this study, we evaluated the antimelanoma activity of immunotherapy combinations with Telaglenastat (CB-839), a potent glutaminase inhibitor (GLSi) that has favorable systemic tolerance. In in vitro TIL:tumor coculture studies, CB-839 treatment improved the cytotoxic activity of autologous TILs on patient-derived melanoma cells. CB-839 treatment decreased the conversion of glutamine to alpha-ketoglutarate (αKGA) more potently in tumor cells versus TILs in these cocultures. These results suggest that CB-839 may improve immune function in a tumor microenvironment by differentially altering tumor and immune cell metabolism. In vivo CB-839 treatment activated melanoma antigen-specific T cells and improved their tumor killing activity in an immune-competent mouse model of adoptive T-cell therapy. Additionally, the combination of CB-839 with anti-PD1 or anti-CTLA4 antibodies increased tumor infiltration by effector T cells and improved the antitumor activity of these checkpoint inhibitors in a high mutation burden mouse melanoma model. Responsiveness to these treatments was also accompanied by an increase of interferon gamma (IFNγ)-associated gene expression in the tumors. Together, these results provide a strong rationale for combining CB-839 with immune therapies to improve efficacy of these treatments against melanoma.

Heterologous Prime Boost Vaccination Induces Protective Melanoma-Specific CD8+ T Cell Responses

Cancer vaccination aims at inducing an adaptive immune response against tumor-derived antigens. In this study, we utilize recombinant human adenovirus serotype 5 (rAd5) and recombinant lymphocytic choriomeningitis virus (rLCMV)-based vectors expressing the melanocyte differentiation antigen gp100. In contrast to single or homologous vaccination, a heterologous prime boost vaccination starting with a rAd5-gp100 prime immunization followed by a rLCMV-gp100 boost injection induces a high magnitude of polyfunctional gp100-specific CD8⁺ T cells. Our data indicate that an optimal T cell induction is dependent on the order and interval of the vaccinations. A prophylactic prime boost vaccination with rAd5- and rLCMV-gp100 protects mice from a B16.F10 melanoma challenge. In the therapeutic setting, combination of the vaccination with low-dose cyclophosphamide showed a synergistic effect and significantly delayed tumor growth. Our findings suggest that heterologous viral vector prime boost immunizations can mediate tumor control in a mouse melanoma model.

An Antigen-Delivery Protein Nanoparticle Combined with Anti-PD-1 Checkpoint Inhibitor Has Curative Efficacy in an Aggressive Melanoma Model

Immune checkpoint inhibition is a promising alternative treatment to standard chemotherapies; however, it fails to achieve long-term remission in a significant portion of patients. A previously developed protein nanoparticle-based platform (E2 nanoparticle) delivers cancer antigens to increase antigen-specific tumor responses. While prior work has focussed on prophylactic conditions, the objectives in this study are therapeutic. It is hypothesized that immune checkpoint inhibition, when augmented by antigen delivery using E2 nanoparticles containing CpG oligonucleotide 1826 (CpG) and a glycoprotein 100 (gp100) melanoma antigen epitope (CpG-gp-E2), would synergistically elicit antitumor responses. To identify a regimen primed for obtaining effective treatment results, immune benchmarks in the spleen and tumor are examined. Conditions that lead to significant immune activation, including increases in gp100-specific interferon gamma (IFN-𝜸), CD8 T cells in the spleen, tumor-infiltrating CD8 T cells, and survival time are identified. Based on the findings, the resulting combination of CpG-gp-E2 and anti-programmed cell death protein 1 (anti-PD-1) treatment in tumor-challenged mice yield significantly increased long-term survival; more than 50% of the mice treated with combination therapy were tumor-free, compared with 0% and ≈5% for CpG-gp-E2 and anti-PD-1 alone, respectively. Evidence of a durable antitumor response is also observed upon tumor rechallenge, pointing to long-lasting immune memory.

EZH2 as a Regulator of CD8+ T Cell Fate and Function

Enhancer of zeste 2 (EZH2) is the catalytic subunit of the Polycomb Repressive Complex 2 (PRC2) that mediates di- and trimethylation of histone 3 lysine 27 effectively precluding successful gene transcription at these loci. This class of epigenetic modifications facilitates the maintenance of tissue-specific cellular transcriptional programs as cells undergoing successive rounds of proliferation. CD8+ T cells are effective mediators of adaptive immunity and function to eliminate virus- and bacteria-infected cells as well as tumor cells. Upon recognition of cognate antigen, T cells undergo activation/proliferation to clear the target cells. The heterogeneous population of responding T cells formed during these proliferative events thus rely on epigenetic modifications to ensure identity and confer functional capabilities. In this review, we will focus on the role of the dynamic expression EZH2 in shaping the epigenetic landscape of CD8+ T cell fate and function, with a particular emphasis on infection and cancer. We also explore competing hypotheses pertaining to EZH2 function and the prospects of clinical EZH2 inhibitors in fine-tuning T cell responses.

Robust Iterative Stimulation with Self-Antigens Overcomes CD8+ T Cell Tolerance to Self- and Tumor Antigens

The immune system adapts to constitutive antigens to preserve self-tolerance, which is a major barrier for anti-tumor immunity. Antigen-specific reversal of tolerance constitutes a major goal to spur therapeutic applications. Here, we show that robust, iterative, systemic stimulation targeting tissue-specific antigens in the context of acute infections reverses established CD8⁺ T cell tolerance to self, including in T cells that survive negative selection. This strategy results in large numbers of circulating and resident memory self-specific CD8⁺ T cells that are widely distributed and can be co-opted to control established malignancies bearing self-antigen without concomitant autoimmunity. Targeted expansion of both self- and tumor neoantigen-specific T cells acts synergistically to boost anti-tumor immunity and elicits protection against aggressive melanoma. Our findings demonstrate that T cell tolerance can be re-adapted to responsiveness through robust antigenic exposure, generating self-specific CD8⁺ T cells that can be used for cancer treatment.

Adaptable antigen matrix platforms for peptide vaccination strategies and T cell-mediated anti-tumor immunity

Injection of antigenic peptides has been widely used as a vaccine strategy to boost T cell immunity. However, the poor immunogenicity of single peptides can potentially be strengthened through modification of the tertiary structure and the selection of the accompanying adjuvant. Here, we generated antigenic peptides into non-linear trimers by solid phase peptide synthesis, thereby enhancing antigen presentation by dendritic cells to CD8⁺ T cells in vitro and in vivo. CD8⁺ T cells from mice vaccinated with trimers showed an KLRG1⁺ effector phenotype and were able to recognize and kill antigen-expressing tumor cells ex vivo. Importantly, trimers outperformed synthetic long peptide in terms of T cell response even when equal number of epitopes were used for immunization. To improve the synthesis of trimers containing difficult peptide sequences, we developed a novel small molecule that functions as conjugation platform for synthetic long peptides. This platform , termed Antigen MAtriX (AMAX) improved yield, purity and solubility of trimers over conventional solid phase synthesis strategies. AMAX outperformed synthetic long peptides in terms of both CD8⁺ and CD4⁺ T cell responses and allowed functionalization with DC-SIGN-binding carbohydrates for in vivo dendritic cell targeting strategies, boosting T cell responses even further. Moreover, we show that agonistic CD40 antibody combined with MF59 (AddaVax) emulsion synergistically improves the antigen-specific T cell response of the AMAX in vivo. Also, tumor-associated antigens and neo-antigens could be incorporated in AMAX for tumor-specific CD8⁺ T cell responses. Importantly, immunization with a mix of neoantigen AMAX could reduce tumor growth in a pre-clinical syngeneic mouse model. Hence, we provide pre-clinical support for the induction of effector CD8⁺ T cells through the adaptable AMAX platform as easy implementable peptidic vaccination strategy against any antigen of choice, including neoantigens for anti-tumor immunity.

Inosine is an alternative carbon source for CD8+-T-cell function under glucose restriction

T cells undergo metabolic rewiring to meet their bioenergetic, biosynthetic and redox demands following antigen stimulation. To fulfil these needs, effector T cells must adapt to fluctuations in environmental nutrient levels at sites of infection and inflammation. Here, we show that effector T cells can utilize inosine, as an alternative substrate, to support cell growth and function in the absence of glucose in vitro. T cells metabolize inosine into hypoxanthine and phosphorylated ribose by purine nucleoside phosphorylase. We demonstrate that the ribose subunit of inosine can enter into central metabolic pathways to provide ATP and biosynthetic precursors, and that cancer cells display diverse capacities to utilize inosine as a carbon source. Moreover, the supplementation with inosine enhances the anti-tumour efficacy of immune checkpoint blockade and adoptive T-cell transfer in solid tumours that are defective in metabolizing inosine, reflecting the capability of inosine to relieve tumour-imposed metabolic restrictions on T cells.

Induced pluripotent stem cell-derived myeloid cells expressing OX40 ligand amplify antigen-specific T cells in advanced melanoma

Immune checkpoint inhibitors improved the survival rate of patients with unresectable melanoma. However, some patients do not respond, and variable immune-related adverse events have been reported. Therefore, more effective and antigen-specific immune therapies are urgently needed. We previously reported the efficacy of an immune cell therapy with immortalized myeloid cells derived from induced pluripotent stem cells (iPS-ML). In this study, we generated OX40L-overexpressing iPS-ML (iPS-ML-Zsgreen-OX40L) and investigated their characteristics and in vivo efficacy against mouse melanoma. We found that iPS-ML-Zsgreen-OX40L suppressed the progression of B16-BL6 melanoma, and prolonged survival of mice with ovalbumin (OVA)-expressing B16 melanoma (MO4). The number of antigen-specific CD8⁺ T cells was higher in spleen cells treated with OVA peptide-pulsed iPS-ML-Zsgreen-OX40L than in those without OX40L. The OVA peptide-pulsed iPS-ML-Zsgreen-OX40L significantly increased the number of tumor-infiltrating T lymphocytes (TILs) in MO4 tumor. Flow cytometry showed decreased regulatory T cells but increased effector and effector memory T cells among the TILs. Although we plan to use allogeneic iPS-ML in the clinical applications, iPS-ML showed the tumorgenicity in the syngeneic mice model. Incorporating the suicide gene is necessary to ensure the safety in the future study. Collectively, these results indicate that iPS-ML-Zsgreen-OX40L therapy might be a new method for antigen-specific cancer immunotherapy.

Personalized neoantigen vaccination with synthetic long peptides: recent advances and future perspectives

Therapeutic cancer vaccines are one of the most promising strategies of immunotherapy. Traditional vaccines consisting of tumor-associated antigens have met with limited success. Recently, neoantigens derived from nonsynonymous mutations in tumor cells have emerged as alternatives that can improve tumor-specificity and reduce on-target off-tumor toxicity. Synthetic peptides are a common platform for neoantigen vaccines. It has been suggested that extending short peptides into long peptides can overcome immune tolerance and induce both CD4⁺ and CD8⁺ T cell responses. This review will introduce the history of long peptide-based neoantigen vaccines, discuss their advantages, summarize current preclinical and clinical developments, and propose future perspectives.

T-cell receptor affinity in the age of cancer immunotherapy

The strength of the interaction between T-cell receptors (TCRs) and their ligands, peptide/major histocompatibility complex complexes (pMHCs), is one of the most frequently discussed and investigated features of T cells in immuno-oncology today. Although there are many molecules on the surface of T cells that interact with ligands on other cells, the TCR/pMHC is the only receptor-ligand pair that offers antigen specificity and dictates the functional response of the T cell. The strength of the TCR/pMHC interaction, along with the environment in which this interaction takes place, is key to how the T cell will respond. The TCR repertoire of T cells that interact with tumor-associated antigens is vast, although typically of low affinity. Here, we focus on the low-affinity interactions between TCRs from CD8+ T cells and different models used in immuno-oncology.

CX3CR1-CD8+ T cells are critical in antitumor efficacy but functionally suppressed in the tumor microenvironment

Although blockade of the programmed cell death 1/programmed cell death ligand 1 (PD-1/PD-L1) immune checkpoint has revolutionized cancer treatment, how it works on tumor-infiltrating CD8+ T cells recognizing the same antigen at various differentiation stages remains elusive. Here, we found that the chemokine receptor CX3CR1 identified 3 distinct differentiation states of intratumor CD8+ T cell subsets. Adoptively transferred antigen-specific CX3CR1-CD8+ T cells generated phenotypically and functionally distinct CX3CR1int and CX3CR1hi subsets in the periphery. Notably, expression of coinhibitory receptors and T cell factor 1 (Tcf1) inversely correlated with the degree of T cell differentiation defined by CX3CR1. Despite lower expression of coinhibitory receptors and potent cytolytic activity, in vivo depletion of the CX3CR1hi subset did not alter the antitumor efficacy of adoptively transferred CD8+ T cells. Furthermore, differentiated CX3CR1int and CX3CR1hi subsets were impaired in their ability to undergo proliferation upon restimulation and had no impact on established tumors upon second adoptive transfer compared with the CX3CR1- subset that remained effective. Accordingly, anti-PD-L1 therapy preferentially rescued proliferation and cytokine production of the CX3CR1- subset and enhanced antitumor efficacy of adoptively transferred CD8+ T cells. These findings provide a better understanding of the phenotypic and functional heterogeneity of tumor-infiltrating CD8+ T cells and can be exploited to develop more effective immunotherapy.

Melanoma-Secreted Lysosomes Trigger Monocyte-Derived Dendritic Cell Apoptosis and Limit Cancer Immunotherapy

The recent success of checkpoint blockade therapies has established immunotherapy as one of the most promising treatments for melanoma. Nonetheless, a complete curative response following immunotherapy is observed only in a fraction of patients. To identify what factors limit the efficacy of immunotherapies, we established mouse models that cease to respond to immunotherapies once their tumors exceed a certain stage. Analysis of the immune systems of the organisms revealed that the numbers of tumor-infiltrating dendritic cells (TIDC) drastically decreased with time. Further, in contrast to the current paradigm, once melanoma was established, TIDC did not migrate into sentinel lymph nodes. Instead, they underwent local cell death due to excessive phagocytosis of lysosomes. Importantly, TIDC were required to license the cytotoxic activity of tumor CD8⁺ T cells, and in their absence, T cells did not lyse melanoma cells. Our results offer a paradigm shift regarding the role of TIDC and a framework to increase the efficacy of immunotherapies. SIGNIFICANCE: This work redefines the role of monocyte-derived dendritic cells in melanoma and provides a novel strategy to increase the efficacy of T-cell-based immunotherapies in nonresponding individuals. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/10/1942/F1.large.jpg.

APOBEC3B-mediated corruption of the tumor cell immunopeptidome induces heteroclitic neoepitopes for cancer immunotherapy

APOBEC3B, an anti-viral cytidine deaminase which induces DNA mutations, has been implicated as a mediator of cancer evolution and therapeutic resistance. Mutational plasticity also drives generation of neoepitopes, which prime anti-tumor T cells. Here, we show that overexpression of APOBEC3B in tumors increases resistance to chemotherapy, but simultaneously heightens sensitivity to immune checkpoint blockade in a murine model of melanoma. However, in the vaccine setting, APOBEC3B-mediated mutations reproducibly generate heteroclitic neoepitopes in vaccine cells which activate de novo T cell responses. These cross react against parental, unmodified tumors and lead to a high rate of cures in both subcutaneous and intra-cranial tumor models. Heteroclitic Epitope Activated Therapy (HEAT) dispenses with the need to identify patient specific neoepitopes and tumor reactive T cells ex vivo. Thus, actively driving a high mutational load in tumor cell vaccines increases their immunogenicity to drive anti-tumor therapy in combination with immune checkpoint blockade.