Immune awakening revealed by peripheral T cell dynamics after one cycle of immunotherapy

Our understanding of how checkpoint inhibitors (CPIs) affect T cell evolution is incomplete, limiting our ability to achieve full clinical benefit from these drugs. Here, we analyzed peripheral T cell populations after one cycle of CPI treatment and identified a dynamic awakening of the immune system, as revealed by T cell evolution in response to treatment. We sequenced T cell receptors in plasma cell-free DNA and peripheral blood mononuclear cells and performed phenotypic analysis of peripheral T cell subsets from patients with metastatic melanoma treated with CPIs. We found that early peripheral T cell turnover and T cell receptor repertoire dynamics identified which patients would respond to treatment. Additionally, the expansion of a subset of immune effector peripheral T cells we call TIE cells correlated with response. These events are prognostic and occur within 3 weeks of starting immunotherapy, raising the potential for monitoring patients’ responses by using minimally invasive liquid biopsies. Marais and colleagues report that checkpoint inhibitor treatment of patients with melanoma leads to dynamic changes in peripheral T cells and expansion of immune effector cells. This awakening of the immune system occurs early after treatment and could be exploited in the clinic.