Tissue-resident memory T cells (TRM cells) have been widely characterized in infectious disease settings; however, their role in mediating immunity to cancer remains unknown. Here we report that skin-resident memory T cell responses to melanoma are generated naturally as a result of autoimmune vitiligo. Melanoma antigen-specific TRM cells resided predominantly in melanocyte-depleted hair follicles and were maintained without recirculation or replenishment from the lymphoid compartment. These cells expressed CD103, CD69, and CLA, but lacked PD-1 or LAG-3, and were capable of making IFN-γ. CD103 expression on CD8 T cells was required for establishment of TRM cells in skin, but was dispensable for vitiligo development. Importantly, CD103+ CD8 TRM cells were critical for protection against melanoma re-challenge. This work establishes that CD103-dependent TRM cells play a key role in perpetuating anti-tumor immunity.

Summary Proteasomes degrade the majority of proteins in mammalian cells, are involved in the regulation of multiple physiological functions, and are established targets of anti-cancer drugs. The proteasome has three types of active sites. Chymotrypsin-like sites are the most important for protein breakdown and have long been considered the only suitable targets for anti-neoplastic drugs; however, our recent work demonstrated that inhibitors of caspase-like sites sensitize malignant cells to inhibitors of the chymotrypsin-like sites. Here we describe the development of specific cell-permeable inhibitors and an activity-based probe of the trypsin-like sites. These compounds selectively sensitize multiple myeloma cells to inhibitors of the chymotrypsin-like sites, including anti-myeloma agents bortezomib and carfilzomib. Thus, trypsin-like sites are co-targets for anti-cancers drugs. Together with inhibitors of chymotrypsin- and caspase-like sites developed earlier we provide the scientific community with a complete set of tools to separately modulate proteasome active sites in living cells.

Regulatory T cells (Treg) are critical mediators of immunosuppression in established tumors, although little is known about their role in restraining immunosurveillance during tumorigenesis. Here, we employ an inducible autochthonous model of melanoma to investigate the earliest Treg and CD8 effector T-cell responses during oncogene-driven tumorigenesis. Induction of oncogenic BRAF and loss of Pten in melanocytes led to localized accumulation of FoxP3 Tregs, but not CD8 T cells, within 1 week of detectable increases in melanocyte differentiation antigen expression. Melanoma tumorigenesis elicited early expansion of shared tumor/self-antigen-specific, thymically derived Tregs in draining lymph nodes, and induced their subsequent recruitment to sites of tumorigenesis in the skin. Lymph node egress of tumor-activated Tregs was required for their C-C chemokine receptor 4 (Ccr4)-dependent homing to nascent tumor sites. Notably, BRAF signaling controlled expression of Ccr4-cognate chemokines and governed recruitment of Tregs to tumor-induced skin sites. BRAF expression alone in melanocytes resulted in nevus formation and associated Treg recruitment, indicating that BRAF signaling is sufficient to recruit Tregs. Treg depletion liberated immunosurveillance, evidenced by CD8 T-cell responses against the tumor/self-antigen gp100, which was concurrent with the formation of microscopic neoplasia. These studies establish a novel role for BRAF as a tumor cell-intrinsic mediator of immune evasion and underscore the critical early role of Treg-mediated suppression during autochthonous tumorigenesis. This work provides new insights into the mechanisms by which oncogenic pathways impact immune regulation in the nascent tumor microenvironment. .

Acquired resistance to BRAFV600E inhibitors (BRAFi) in melanoma remains a common clinical obstacle, as is the case for any targeted drug therapy that can be developed given the plastic nature of cancers. While there has been significant focus on the cancer cell-intrinsic properties of BRAFi resistance, the impact of BRAFi resistance on host immunity has not been explored. Here we provide preclinical evidence that resistance to BRAFi in an autochthonous mouse model of melanoma is associated with restoration of myeloid-derived suppressor cells (MDSC) in the tumor microenvironment initially reduced by BRAFi treatment. In contrast to restoration of MDSC, levels of T regulatory cells remained reduced in BRAFi-resistant tumors. Accordingly, tumor gene expression signatures specific for myeloid cell chemotaxis and homeostasis reappeared in BRAFi-resistant tumors. Notably, MDSC restoration relied upon MAPK pathway reactivation and downstream production of the myeloid attractant CCL2 in BRAFi-resistant melanoma cells. Strikingly, while combination checkpoint blockade (anti-CTLA-4 + anti-PD-1) was ineffective against BRAFi-resistant melanomas, the addition of MDSC depletion/blockade (anti-Gr-1 + CCR2 antagonist) prevented outgrowth of BRAFi-resistant tumors. Our results illustrate how extrinsic pathways of immunosuppression elaborated by melanoma cells dominate the tumor microenvironment and highlight the need to target extrinsic as well as intrinsic mechanisms of drug resistance.

Antibodies targeting CTLA-4 induce durable responses in some patients with melanoma and are being tested in a variety of human cancers. However, these therapies are ineffective for a majority of patients across tumor types. Further understanding the immune alterations induced by these therapies may enable the development of novel strategies to enhance tumor control and biomarkers to identify patients most likely to respond. In several murine models, including colon26, MC38, CT26, and B16 tumors cotreated with GVAX, anti-CTLA-4 efficacy depends on interactions between the Fc region of CTLA-4 antibodies and Fc receptors (FcR). Anti-CTLA-4 binding to FcRs has been linked to depletion of intratumoral T regulatory cells (Treg). In agreement with previous studies, we found that Tregs infiltrating CT26, B16-F1, and autochthonous Braf V600E Pten À/À melanoma tumors had higher expression of surface CTLA-4 (sCTLA-4) than other T-cell subsets, and anti-CTLA-4 treatment led to FcR-dependent depletion of Tregs infiltrating CT26 tumors. This Treg depletion coincided with activation and degranulation of intratumoral natural killer cells. Similarly, in non-small cell lung cancer (NSCLC) and melanoma patient-derived tumor tissue, Tregs had higher sCTLA-4 expression than other intratumoral T-cell subsets, and Tregs infiltrating NSCLC expressed more sCTLA-4 than circulating Tregs. Patients with cutaneous melanoma who benefited from ipilimumab, a mAb targeting CTLA-4, had higher intratumoral CD56 expression, compared with patients who received little to no benefit from this therapy. Furthermore, using the murine CT26 model we found that combination therapy with anti-CTLA-4 plus IL15/IL15Ra complexes enhanced tumor control compared with either monotherapy.