Sugiura et al. found that PD-L1 and CD80 bind in cis on primary dendritic cells, limiting the capacity of PD-L1 to interact with PD-1 on T cells, but not limiting the capacity of CD80 to interact with CD28 or CTLA-4. DC expression of mutated PD-L1 or CD80 incapable of cis binding significantly suppressed T cell responses in both murine and human co-cultures. In knock-in mice in which cis binding does not occur, antigen responses were reduced, antitumor activity following vaccination was suppressed, and autoimmune responses were minimized. DCs from the knock-in mice were less effective as vaccines than DCs from wild-type mice.
Targeted blockade of PD-1 using checkpoint inhibitor immunotherapies can activate T cells to destroy tumors. PD-1 is believed to function mainly at the effector but not in the activation phase of T cell responses, yet how PD-1 function is restricted at the activation stage is currently unknown. Here we demonstrate that CD80 interacts with PD-L1 in cis on antigen presenting cells (APCs) to disrupt PD-L1/PD-1 binding. Subsequently, PD-L1 cannot engage PD-1 to inhibit T cell activation when APCs express substantial amounts of CD80. Using knock-in mice where cis-PD-L1/CD80 interactions do not occur, tumor immunity and autoimmune responses were greatly attenuated by PD-1. These findings indicate that CD80 on APCs limits the PD-1 co-inhibitory signal, while promoting CD28-mediated co-stimulation, and highlights critical components for induction of optimal immune responses.