Alternative splicing of the inhibitory immune checkpoint receptor SLAMF6 generates a dominant positive form, boosting T-cell effector functions

SLAMF6 is a homotypic receptor of the Ig-superfamily associated with progenitor-exhausted T cells. Here we show that in humans, SLAMF6 has three splice isoforms involving its V-domain. Although the canonical receptor inhibited T-cell activation through SAP recruitment, the short isoform SLAMF6^D17–⁶⁵ had a strong agonistic effect. The costimulatory action depended on protein phosphatase SHP1 and led to a cytotoxic molecular profile mediated by the expression of TBX21 and RUNX3. Patients treated with immune checkpoint blockade showed a shift toward SLAMF6^D17–65 in peripheral blood T cells. We developed splice-switching antisense oligonucleotides (ASO) designed to target the relevant SLAMF6 splice junction. Our ASOs enhanced SLAMF6^D17–⁶⁵ expression in human tumor-infiltrating lymphocytes and improved their capacity to inhibit human melanoma in mice. The yin-yang relationship of SLAMF6 splice isoforms may represent a balancing mechanism that could be exploited to improve cancer immunotherapy.