Differential effects of PD-L1 versus PD-1 blockade on myeloid inflammation in human cancer

BACKGROUND. PD-1 and PD-L1 have been studied interchangeably in the clinic as checkpoints to reinvigorate T cells in diverse tumor types. Data for biologic effects of checkpoint blockade in human premalignancy are limited. METHODS. We analyzed the immunologic effects of PD-L1 blockade in a clinical trial of atezolizumab in patients with asymptomatic multiple myeloma (AMM), a precursor to clinical malignancy. Genomic signatures of PD-L1 blockade in purified monocytes and T cells in vivo were also compared with those following PD-1 blockade in lung cancer patients. Effects of PD-L1 blockade on monocyte-derived DCs were analyzed to better understand its effects on myeloid antigenpresenting cells. RESULTS. In contrast to anti-PD-1 therapy, anti-PD-L1 therapy led to a distinct inflammatory signature in CD14+ monocytes and increase in myeloid-derived cytokines (e.g., IL-18) in vivo. Treatment of AMM patients with atezolizumab led to rapid activation and expansion of circulating myeloid cells, which persisted in the BM. Blockade of PD-L1 on purified monocyte-derived DCs led to rapid inflammasome activation and synergized with CD40L-driven DC maturation, leading to greater antigen-specific T cell expansion. CONCLUSION. These data show that PD-L1 blockade leads to distinct systemic immunologic effects compared with PD-1 blockade in vivo in humans, particularly manifest as rapid myeloid activation. These findings also suggest an additional role for PD-L1 as a checkpoint for regulating inflammatory phenotype of myeloid cells and antigen presentation in DCs, which may be harnessed to improve PD-L1-based combination therapies.