Vikram R. Juneja

Melanoma Cell-Intrinsic PD-1 Receptor Functions Promote Tumor Growth

Therapeutic antibodies targeting programmed cell death 1 (PD-1) activate tumor-specific immunity and have shown remarkable efficacy in the treatment of melanoma. Yet, little is known about tumor cell-intrinsic PD-1 pathway effects. Here, we show that murine and human melanomas contain PD-1-expressing cancer subpopulations and demonstrate that melanoma cell-intrinsic PD-1 promotes tumorigenesis, even in mice lacking adaptive immunity. PD-1 inhibition on melanoma cells by RNAi, blocking antibodies, or mutagenesis of melanoma-PD-1 signaling motifs suppresses tumor growth in immunocompetent, immunocompromised, and PD-1-deficient tumor graft recipient mice. Conversely, melanoma-specific PD-1 overexpression enhances tumorigenicity, as does engagement of melanoma-PD-1 by its ligand, PD-L1, whereas melanoma-PD-L1 inhibition or knockout of host-PD-L1 attenuate growth of PD-1-positive melanomas. Mechanistically, the melanoma-PD-1 receptor modulates downstream effectors of mTOR signaling. Our results identify melanoma cell-intrinsic functions of the PD-1:PD-L1 axis in tumor growth and suggest that blocking melanoma-PD-1 might contribute to the striking clinical efficacy of anti-PD-1 therapy.

Response to BRAF Inhibition in Melanoma Is Enhanced When Combined with Immune Checkpoint Blockade

Cooper, Juneja, Sage, and colleagues show that combining BRAF and PD-1/PD-L1 blockade slowed tumor growth and prolonged survival in a melanoma mouse model, with increased number and activity of tumor-infiltrating lymphocytes similar to that in a human melanoma patient treated with this regimen. BRAF-targeted therapy results in objective responses in the majority of patients; however, the responses are short lived (∼6 months). In contrast, treatment with immune checkpoint inhibitors results in a lower response rate, but the responses tend to be more durable. BRAF inhibition results in a more favorable tumor microenvironment in patients, with an increase in CD8+ T-cell infiltrate and a decrease in immunosuppressive cytokines. There is also increased expression of the immunomodulatory molecule PDL1, which may contribute to the resistance. On the basis of these findings, we hypothesized that BRAF-targeted therapy may synergize with the PD1 pathway blockade to enhance antitumor immunity. To test this hypothesis, we developed a BRAF(V600E)/Pten−/− syngeneic tumor graft immunocompetent mouse model in which BRAF inhibition leads to a significant increase in the intratumoral CD8+ T-cell density and cytokine production, similar to the effects of BRAF inhibition in patients. In this model, CD8+ T cells were found to play a critical role in the therapeutic effect of BRAF inhibition. Administration of anti-PD1 or anti-PDL1 together with a BRAF inhibitor led to an enhanced response, significantly prolonging survival and slowing tumor growth, as well as significantly increasing the number and activity of tumor-infiltrating lymphocytes. These results demonstrate synergy between combined BRAF-targeted therapy and immune checkpoint blockade. Although clinical trials combining these two strategies are ongoing, important questions still remain unanswered. Further studies using this new melanoma mouse model may provide therapeutic insights, including optimal timing and sequence of therapy. Cancer Immunol Res; 2(7); 643–54. ©2014 AACR.

In vivo CRISPR screening identifies Ptpn2 as a cancer immunotherapy target

Immunotherapy with PD-1 checkpoint blockade is effective in only a minority of patients with cancer, suggesting that additional treatment strategies are needed. Here we use a pooled in vivo genetic screening approach using CRISPR–Cas9 genome editing in transplantable tumours in mice treated with immunotherapy to discover previously undescribed immunotherapy targets. We tested 2,368 genes expressed by melanoma cells to identify those that synergize with or cause resistance to checkpoint blockade. We recovered the known immune evasion molecules PD-L1 and CD47, and confirmed that defects in interferon-γ signalling caused resistance to immunotherapy. Tumours were sensitized to immunotherapy by deletion of genes involved in several diverse pathways, including NF-κB signalling, antigen presentation and the unfolded protein response. In addition, deletion of the protein tyrosine phosphatase PTPN2 in tumour cells increased the efficacy of immunotherapy by enhancing interferon-γ-mediated effects on antigen presentation and growth suppression. In vivo genetic screens in tumour models can identify new immunotherapy targets in unanticipated pathways.

BRAF inhibition is associated with increased clonality in tumor-infiltrating lymphocytes

There have been significant advances with regard to BRAF-targeted therapies against metastatic melanoma. However, the majority of patients receiving BRAF inhibitors (BRAFi) manifest disease progression within a year. We have recently shown that melanoma patients treated with BRAFi exhibit an increase in melanoma-associated antigens and in CD8+ tumor-infiltrating lymphocytes in response to therapy. To characterize such a T-cell infiltrate, we analyzed the complementarity-determining region 3 (CDR3) of rearranged T-cell receptor (TCR) β chain-coding genes in tumor biopsies obtained before the initiation of BRAFi and 10–14 d later. We observed an increase in the clonality of tumor-infiltrating lymphocytes in 7 of 8 patients receiving BRAFi, with a statistically significant 21% aggregate increase in clonality. Over 80% of individual T-cell clones detected after initiation of BRAFi treatment were new clones. Interestingly, the comparison of tumor infiltrates with clinical responses revealed that patients who had a high proportion of pre-existing dominant clones after the administration of BRAFi responded better to therapy than patients who had a low proportion of such pre-existing dominant clones following BRAFi. These data suggest that although the inhibition of BRAF in melanoma patients results in tumor infiltration by new lymphocytes, the response to treatment appears to be related to the presence of a pre-existing population of tumor-infiltrating T-cell clones.

PD-L1 on tumor cells is sufficient for immune evasion in immunogenic tumors and inhibits CD8 T cell cytotoxicity

It is unclear whether PD-L1 on tumor cells is sufficient for tumor immune evasion or simply correlates with an inflamed tumor microenvironment. We used three mouse tumor models sensitive to PD-1 blockade to evaluate the significance of PD-L1 on tumor versus nontumor cells. PD-L1 on nontumor cells is critical for inhibiting antitumor immunity in B16 melanoma and a genetically engineered melanoma. In contrast, PD-L1 on MC38 colorectal adenocarcinoma cells is sufficient to suppress antitumor immunity, as deletion of PD-L1 on highly immunogenic MC38 tumor cells allows effective antitumor immunity. MC38-derived PD-L1 potently inhibited CD8+ T cell cytotoxicity. Wild-type MC38 cells outcompeted PD-L1–deleted MC38 cells in vivo, demonstrating tumor PD-L1 confers a selective advantage. Thus, both tumor- and host-derived PD-L1 can play critical roles in immunosuppression. Differences in tumor immunogenicity appear to underlie their relative importance. Our findings establish reduced cytotoxicity as a key mechanism by which tumor PD-L1 suppresses antitumor immunity and demonstrate that tumor PD-L1 is not just a marker of suppressed antitumor immunity.

Deletion of CTLA-4 on regulatory T cells during adulthood leads to resistance to autoimmunity

Paterson et al. demonstrate that, in contrast to CTLA-4 germline knockout mice, conditional deletion on T reg cells during adulthood confers protection from EAE and does not increase resistance to tumors.

Suppression by TFR cells leads to durable and selective inhibition of B cell effector function

Follicular regulatory T cells (TFR cells) inhibit follicular helper T cell (TFH cell)–mediated antibody production. The mechanisms by which TFR cells exert their key immunoregulatory functions are largely unknown. Here we found that TFR cells induced a distinct suppressive state in TFH cells and B cells, in which effector transcriptional signatures were maintained but key effector molecules and metabolic pathways were suppressed. The suppression of B cell antibody production and metabolism by TFR cells was durable and persisted even in the absence of TFR cells. This durable suppression was due in part to epigenetic changes. The cytokine IL-21 was able to overcome TFR cell–mediated suppression and inhibited TFR cells and stimulated B cells. By determining mechanisms of TFR cell-mediated suppression, we have identified methods for modulating the function of TFR cells and antibody production.

A phase II study of combined therapy with a BRAF inhibitor (vemurafenib) and interleukin-2 (aldesleukin) in patients with metastatic melanoma

ABSTRACT Background: Approximately 50% of melanomas harbor BRAF mutations. Treatment with BRAF +/− MEK inhibition is associated with favorable changes in the tumor microenvironment thus providing the rationale for combining targeted agents with immunotherapy. Methods: Patients with unresectable Stage III or IV BRAFV600E mutant melanoma were enrolled in a single-center prospective study (n = 6). Patients were eligible to receive two courses of HD-IL-2 and vemurafenib twice daily. The primary endpoint was progression-free survival (PFS) with secondary objectives including overall survival (OS), response rates (RR), and safety of combination therapy as compared to historical controls. Immune profiling was performed in longitudinal tissue samples, when available. Results: Overall RR was 83.3% (95% CI: 36%–99%) and 66.6% at 12 weeks. All patients eventually progressed, with three progressing on treatment and three progressing after the vemurafenib continuation phase ended. Median PFS was 35.8 weeks (95% CI: 16–57 weeks). Median OS was not reached; however, the time at which 75% of patients were still alive was 104.4 weeks. Change in circulating BRAFV600E levels correlated with response. Though combination therapy was associated with enhanced CD8 T cell infiltrate, an increase in regulatory T cell frequency was seen with HD-IL-2 administration, suggesting a potential limitation in this strategy. Conclusion: Combination vemurafenib and HD-IL-2 is well tolerated and associated with treatment responses. However, the HD-IL-2 induced increase in Tregs may abrogate potential synergy. Given the efficacy of regimens targeting the PD-1 pathway, strategies combining these regimens with BRAF-targeted therapy are currently underway, and the role of combination vemurafenib and HD-IL-2 is uncertain. Trial Registration: Clinical trial information: NCT01754376; https://clinicaltrials.gov/show/NCT01754376

Abstract 1019: In vivo CRISPR screening identifies Ptpn2 as a target for cancer immunotherapy

Despite the dramatic clinical success of cancer immunotherapy with PD-1 checkpoint blockade, most patients don’t experience sustained clinical benefit, suggesting that additional therapeutic strategies are needed. Functional genomic screens in cancer cells to discover new therapeutic targets are usually carried out in vitro where interaction with the immune system is absent. Here we report a pooled, loss-of-function genetic screening approach using CRISPR/Cas9 genome editing that is conducted in vivo in mouse transplantable tumors treated with vaccination and PD-1 checkpoint blockade. We tested 2,400 genes expressed by melanoma cells for those that synergize with or cause resistance to checkpoint blockade, and recovered the known immune evasion molecules, PD-L1 and CD47. Loss of function of multiple genes required to sense interferon-y caused resistance to immunotherapy. Deletion of Ptpn2, a pleotropic protein tyrosine phosphatase improved response to immunotherapy. In vivo, Ptpn2 deficient tumors showed increased infiltration of activated CD8+T cells. In vitro, Ptpn2 loss by tumor cells increased antigen presentation to T cells. Biochemical, transcriptional and genetic epistasis experiments demonstrated that loss of function of Ptpn2 sensitizes tumors to immunotherapy by enhancing interferon-y-mediated effects on the tumor cell. Thus, augmenting interferon-y signaling in tumor cells could increase the efficacy of immunotherapy. More generally, in vivo genetic screens in tumor models can identify new immunotherapy targets and rationally prioritize combination therapies. Citation Format: Robert T. Manguso, Hans W. Pope, Margaret D. Zimmer, Flavian D. Brown, Kathleen B. Yates, Brian C. Miller, Natalie B. Collins, Kevin Bi, Martin W. Lafleur, Vikram R. Juneja, Sarah A. Weiss, David E. Fisher, David E. Root, Arlene H. Sharpe, John G. Doench, W Nicholas Haining. In vivo CRISPR screening identifies Ptpn2 as a target for cancer immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1019. doi:10.1158/1538-7445.AM2017-1019

Enhancing the Efficacy of Checkpoint Blockade Through Combination Therapies

Antibodies targeting coinhibitory receptors on T cells (“checkpoint blockade”) have emerged as some of the most promising therapies for a broad range of malignancies, including melanoma, non-small cell lung cancer, renal cell carcinoma, Hodgkin’s lymphoma, and bladder cancer. These coinhibitory molecules include CTLA-4, PD-1, LAG-3, TIM-3, and others. The anti-CTLA-4 antibody ipilimumab was approved in 2011 and the anti-PD-1 antibodies pembrolizumab and nivolumab were approved in 2014 for patients with advanced melanoma. Single agent checkpoint blockade is associated with 20–40 % objective response rates in advanced melanoma with improved overall survival. The combination of anti-CTLA-4 and anti-PD-1 antibodies leads to an increased durable response rate compared to either antibody alone, supporting the concept that combination therapy may result in increased clinical benefit. An important goal in the field is to combine checkpoint blockade with other immunotherapies and other types of therapy (e.g., radiation, targeted therapy, chemotherapy, surgery) to increase the fraction of patients that have objective and durable responses. Here, we discuss the current understanding of the mechanisms underlying checkpoint blockade and the rationale for combination therapy. We then discuss potential immunotherapeutic and non-immunotherapeutic combination therapies. Finally, we discuss critical issues that need to be addressed in order to develop combination strategies to induce long-term clinical responses in patients with cancer.