Response to anti-PD-1 + anti-LAG-3 checkpoint blockade is associated with T regulatory cell functional reprogramming and instability

We previously identified and validated a pre-treatment peripheral blood biomarker, characterized by a high frequency of LAG-3+ lymphocytes, that predicts resistance in patients receiving anti-PD-1 immune checkpoint blockade. To better understand the mechanism of aPD-1 resistance and select rational therapies to overcome resistance, we identified murine tumor models with a high LAG-3+ lymphocyte frequency (LAG-3hi), which were resistant to aPD-1 therapy, and LAG-3lo murine tumor models that were aPD-1 sensitive, recapitulating the predictive biomarker we previously described in patients. As we hypothesized, LAG-3hi tumor-bearing mice were sensitive to aPD-1+aLAG-3 therapy and the benefit of combination therapy was CD8+ T cell-dependent. Unexpectedly, the benefit of combination therapy was enhanced in LAG-3hi (but not LAG-3lo) mice with depletion of CD4+ T cells. Furthermore, responses to aPD-1+aLAG-3 significantly correlated with T regulatory cell (Treg) phenotypic plasticity in LAG-3hi mice, suggesting a specific role for Tregs in the mechanism of response to this combination. Using Treg fate tracking Foxp3GFP-Cre-ERT2 x ROSAYFP reporter mice we demonstrate that expanded populations of unstable Tregs correlate with response to combination therapy in LAG-3hi mice. Complementing this preclinical data, an increased proportion of unstable Tregs also significantly correlated with higher response rate and improved survival after aPD-1+aLAG-3 therapy in a cohort of patients with metastatic melanoma (n=117). These data indicate that Treg phenotypic plasticity is a key mechanism in aPD-1+aLAG-3 responsiveness, which may represent both a novel biomarker to aid patient selection and a rationale therapeutic target to expand benefit for a subset of PD-1 refractory patients.