PD-L1 deletion or blockade regulate macrophage antigen presentation and checkpoint molecule surface levels
Macrophages in the tumor microenvironment are known to upregulate PD-L1 expression, thereby suppressing T cells through PD-1 ligation. However, the manner in which PD-L1 expression intrinsically impacts macrophages and their immunomodulatory phenotype is less clear. Clarifying this knowledge gap would yield insight into the mechanisms of immunosuppression within the tumor microenvironment. To characterize the macrophage intrinsic role of PD-L1, we used complementary genetic and pharmacological approaches by analyzing primary murine bone marrow-derived macrophages (BMDMs) with complete genetic PD-L1 deletion and wildtype BMDMs treated with anti-PD-L1 blocking antibodies. Macrophages were evaluated across naive, pro-inflammatory (M1), and tumor conditioned (TCM) polarization states in vitro. Unlike prior reports, neither genetic deletion nor antibody blockade dramatically altered the expression of macrophage polarization markers or in vitro phagocytic capacity. Both conditions consistently reduced surface levels of the M1-associated costimulatory molecule CD80, prompting further analysis of T cell interacting and antigen presenting proteins, in which we revealed disparate effects of genetic deletion and antibody blockade on the surface levels of MHCI, MHCII, PD-1, and PD-L2. These findings suggest that PD-L1 deletion and antibody-mediated blockade contribute to macrophage immune regulatory profiles in distinct manners. This difference supports a model in which PD-L1 functions in macrophages beyond its canonical role as a ligand for PD-1, influencing antigen presentation and checkpoint molecule levels and playing a broader role in immune regulation in the tumor microenvironment.