Original findings reported in this study identified a circulating CD8 T cell population delineated by the co-expression of TIGIT and PD-1 inhibitory receptors as an early immune marker of anti-PD-1 efficacy in three independent cohorts of cancer patients (two melanoma patient’s cohorts and one Merkel-cell carcinoma patient’s cohort). The frequency of this double positive (DPOS) population even appeared predictive of PD-1 inhibitor therapy efficacy at baseline in the MCC cohort...Our findings provide a compelling rationale to measure PD-1+TIGIT+ CD8 T-cell subset in the blood of cancer patients to monitor early anti-PD1 mediated clinical efficacy.