FcγRIIB is a T cell checkpoint in antitumor immunity.

In the setting of cancer, T cells upregulate coinhibitory molecules that attenuate TCR signaling and lead to the loss of proliferative capacity and effector function. Checkpoint inhibitors currently in clinical use have dramatically improved mortality from melanoma yet are not effective in all patients, suggesting that additional pathways may contribute to suppression of tumor-specific CD8+ T cell responses in melanoma. Here, we show that FcγRIIB, an inhibitory Fc receptor previously thought to be exclusively expressed on B cells and innate immune cells, is upregulated on tumor-infiltrating effector CD8+ T cells in an experimental melanoma model and expressed on CD8+ T cells in patients with melanoma. Genetic deficiency of Fcgr2b resulted in enhanced tumor-infiltrating CD8+ T cell responses and significantly reduced tumor burden. Adoptive transfer experiments of Fcgr2b-/- tumor antigen-specific T cells into FcγRIIB-sufficient hosts resulted in an increased frequency of tumor-infiltrating CD8+ T cells with greater effector function. Finally, FcγRIIB was expressed on CD8+ memory T cells isolated from patients with melanoma. These data illuminate a cell-intrinsic role for the FcγRIIB checkpoint in suppressing tumor-infiltrating CD8+ T cells.