Inhibitory receptors like PD-1 and CTLA-4 contribute to T cell dysfunction in cancer1–3. Monoclonal antibodies (mAbs) blocking the interactions in trans of these receptors with their ligands on cancer cells or in the tumour microenvironment lead to clinical responses in some but not all types of cancer. Signalling lymphocytic activation molecule 6 (SLAMF6, also known as Ly108) is a homotypic receptor preferentially expressed on progenitor or stem-like exhausted T (Tpex) cells, but not on terminally exhausted T (Tex) cells, as demonstrated in mouse models4–9. In contrast to Tex cells, Tpex cells retain the capacity for functional restoration after immune checkpoint blockade10–12. The role of SLAMF6 in T cells remains ambiguous, as it has both activating and inhibitory effects, complicating its evaluation as a therapeutic target. Here we find that SLAMF6 was triggered in cis by homotypic interactions at the T cell surface. These interactions elicited inhibitory effects that suppressed activation of T cells and limited anti-tumour immunity, independently of SLAMF6 expression on tumour cells. mAbs against human SLAMF6 with a robust ability to disrupt the cis interactions strongly augmented T cell activation, reduced the proportions of exhausted T cells and inhibited tumour growth in vivo. Collectively, these findings show that SLAMF6 functions exclusively as a T cell inhibitory receptor, which is triggered by cis homotypic interactions. They also position SLAMF6 as a promising target for therapies aimed at enhancing anti-tumour immunity, regardless of SLAMF6 expression on tumour cells.
