Depletion of effector regulatory T cells associates with major response to induction dual immune checkpoint blockade

Abstract In a phase 2 trial, local-regionally advanced HPV-positive oropharyngeal carcinoma patients (N = 35) received ipilimumab (anti-CTLA-4) and nivolumab (anti-PD-1) as induction immunotherapy and concurrently with radiotherapy (NCT03799445). Co-primary endpoints included 6-month complete metabolic response rate (94%) and 2-year progression-free survival (84%). Induction yielded a 46% major histopathologic response rate. Single-cell profiling revealed responders had higher baseline intratumoral CD8+ T cells with a tumor-reactive, tissue-resident memory (TRM) phenotype and a treatment-related decrease in effector regulatory T (eTreg) cells. The eTreg decrease correlated with CD8+ T-cell clonotype transitioning from TRM to effector memory and IFNG+ effector cells. In nonresponders, clonotypes transitioned to exhausted TRM and proliferating cells. Multivariable regression modeling determined the baseline feature most associated with reduction in tumor viability was the proportion of FCGR3A-expressing NK cells, which are capable of ipilimumab-dependent depletion of CTLA4high eTregs. eTreg depletion may be critical for major response to induction dual immune checkpoint blockade.