Efficacy of CTLA-4 checkpoint therapy is dependent on IL-21 signaling to mediate cytotoxic reprogramming of PD-1CD8 T cells.

The mechanisms underlying the efficacy of anti-programmed cell death protein 1 (PD-1) and anti-cytotoxic T lymphocyte-associated protein 4 (CTLA-4) therapy are incompletely understood. Here, by immune profiling responding PD-1CD8 T (T) cell populations from patients with advanced melanoma, we identified differential programming of T cells in response to combination therapy, from an exhausted toward a more cytotoxic effector program. This effect does not occur with anti-PD-1 monotherapy. Single-cell transcriptome and T cell receptor repertoire analysis was used to identify altered effector programming of expanding PD-1CD8 T cell clones with distinct regulon usage, STAT1 and STAT3 utilization and antitumor specificity connected to interleukin (IL)-21 signaling in combination and anti-CTLA-4 monotherapy. Therapeutic efficacy of CTLA-4 blockade was lost in B16F10 melanoma models with either Il21r deficiency or anti-IL-21 receptor blockade. Together, these results show how IL-21 signaling to T is critical for anti-CTLA-4-based checkpoint therapies and highlight major signaling differences to anti-PD-1 monotherapy.