SLAMF6​ deficiency augments tumor killing and skews toward an effector phenotype revealing it as a novel T cell checkpoint

Abstract

SLAMF6 is a homotypic receptor of the Ig-superfamily whose exact role in immune modulation has remained elusive. Its constitutive expression on resting and activated T cells precludes it from being a bona fide exhaustion marker. By breeding Pmel-1 mice with SLAMF6 -/- mice, we generated donors for T cells lacking SLAMF6 and expressing a transgenic TCR for gp100-melanoma antigen. Activated Pmel-1xSLAMF6 -/- CD8+ T cells displayed improved polyfunctionality and strong tumor cytolysis. T-bet was the dominant transcription factor in Pmel-1 x SLAMF6 -/- cells, and upon activation, they acquired an effector-memory phenotype. Adoptive transfer of Pmel-1 x SLAMF6 -/- T cells to melanoma-bearing mice resulted in lasting tumor regression in contrast to temporary responses achieved with Pmel-1 T cells. LAG-3 expression was elevated in the SLAMF6 -/- cells, and the addition of the LAG-3-blocking antibody to the adoptive transfer protocol improved the SLAMF6 -/- T cells and expedited the antitumor response even further. The results from this study support the notion that SLAMF6 is an inhibitory immune receptor whose absence enables powerful CD8+ T cells to eradicate tumors.

SLAMF6 expression associates with survival in triple negative breast cancer.

“…It is not understood why women of African descent (black women) are diagnosed with triple negative breast cancer more frequently than white women, and why this disease is more aggressive in black women. SLAMF6 is expressed on T-cells and appears similar to markers of exhaustion though it is constitutively expressed (5); though its deficiency is reported to enhance tumor killing (5), we found that it is expressed at higher levels in the tumors of patients who had superior survival outcomes ("alive" patients). SLAMF6 may be of relevance as a biomarker in identifying patients who are more likely to survive, perhaps as part of a panel of genes used for prognostic stratification, and the molecule itself may be important to the biology of triple negative breast cancer etiology or more likely its progression.…”

Alternative splicing of SLAMF6 in human T cells creates a co-stimulatory isoform that counteracts the inhibitory effect of the full-length receptor

“…In contrast to the paradigm of a bidirectional modulatory role of SFRs, we recently reported that genetic deletion of SLAMF6 in melanoma-specific murine CD8 T cells significantly enhances their functional capacity 13 . We showed that the SLAMF6 KO-melanoma antigen-specific T cells demonstrated a global improvement of effector function, including cytokine release, cytotoxicity, and post-transfer persistence.…”

“…Terhorst and our group have previously shown that canonical SLAMF6 serves as an inducer of T cell exhaustion and acts as an inhibitory checkpoint for T cells 13,22,23 . The results presented here with the fulllength SLAMF6 support this role.…”

Engineering Solutions for Mitigation of Chimeric Antigen Receptor T-Cell Dysfunction

“…In the absence of AP1, or in overactivation of NFAT in a highly activated T-cell, NFAT directs a transcriptional signature of genes that induce exhaustion. Three patterns of NFAT occupancy of target promoters can be envisioned: (1) Classical NFAT/AP1 dimers drive transcription of effector genes, whilst (2) NFAT dimerised with alternate bZIP members (e.g., JUNB or IRF4) drive exhaustion genes, or (3) NFAT is "partner-less" at the promoter, due to its overactivation ( Figure 3) [41,42]. Intriguingly, it has recently been shown that forced expression of C-JUN can reverse the exhaustion phenotype in epigenetically exhausted/dysfunctional cells, highlighting the new therapeutic opportunities of manipulation of key transcriptional regulators [42].…”

“…Several lines of evidence point to TOX being activated following TCR engagement to NFAT mediated transcription ( Figure 3). Once expressed, TOX hardwires T-cells into the exhaustion phenotype through epigenetic modification (e.g., interaction with the H3 and H4 acetylation complex HBO1 [31]) and regulation of other proteins driving exhaustion, such as the transcription factor NR4A [39,40] and the type 1 transmembrane protein SLAMF6 [41]. Oblong boxes represent consensus binding sites in promoters.…”