In situ and in silico kinetic analyses of programmed cell death-1 (PD-1) receptor, programmed cell death ligands, and B7-1 protein interaction network

Abstract

Edited by Norma AllewellProgrammed cell death-1 (PD-1) is an inhibitory receptor with an essential role in maintaining peripheral tolerance and is among the most promising immunotherapeutic targets for treating cancer, autoimmunity, and infectious diseases. A complete understanding of the consequences of PD-1 engagement by its ligands, PD-L1 and PD-L2, and of PD-L1 binding to B7-1 requires quantitative analysis of their interactions at the cell surface. We present here the first complete in situ kinetic analysis of the PD-1/PD-ligands/B7-1 system. Consistent with previous solution measurements, we observed higher in situ affinities for human (h) than murine (m) PD-1 interactions, stronger binding of hPD-1 to hPD-L2 than hPD-L1, and comparable binding of mPD-1 to both ligands. However, in contrast to the relatively weak solution affinities, the in situ affinities of PD-1 are as high as those of the T cell receptor for agonist pMHC and of LFA-1 (lymphocyte function-associated antigen 1) for ICAM-1 (intercellular adhesion molecule 1) but significantly lower than that of the B7-1/CTLA-4 interaction, suggesting a distinct basis for PD-1-versus CTLA-4-mediated inhibition. Notably, the in situ interactions of PD-1 are much stronger than that of B7-1 with PD-L1. Overall, the in situ affinity ranking greatly depends on the on-rate instead of the off-rate. In silico simulations predict that PD-1/PD-L1 interactions dominate at interfaces between activated T cells and mature dendritic cells and that these interactions will be highly sensitive to the dynamics of PD-L1 and PD-L2 expression. Our results provide a kinetic framework for better understanding inhibitory PD-1 activity in health and disease.PD-1 2 (CD279) is an immune checkpoint receptor expressed mainly on activated T cells and B cells. Its primary function is to maintain peripheral tolerance within the adaptive immune system. PD-1 deficiency results in spontaneous development of autoimmunity in mouse models (1, 2), and polymorphism of its gene PDCD1 in humans is associated with various autoimmune diseases including systemic lupus erythematosus, rheumatoid arthritis, type I diabetes, etc., among different populations (3). High surface expression of PD-1 has also been found to be a hallmark of T cell exhaustion, where antigen-specific CD8 ϩ T cells lose their ability to combat tumor cells or virus-infected cells (4, 5). Antibody blockade of the PD-1 pathway is able to restore the effector functions of exhausted CD8 ϩ T cells for tumor or viral clearance, and this approach is emerging as a promising immunotherapeutic strategy to treat a wide range of cancer and infectious diseases, e.g. nivolumab and pembrolizumab among others in the pipeline (3, 6). In addition, the more complex functions of PD-1 are evidenced by its important role in the generation and activity of induced regulatory T cells (7-9), its high expression on follicular helper CD4 ϩ T cells (10), the improved cognitive performance in an Alzheimer's mouse model with PD-1 blockade (11), and its expression ...

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“…In contrast, the PD‐1:PD‐L1 binding was characterized as a two state reaction model in which a conformation change is required for efficient binding of PD‐L1 to its receptor . Recently, the first complete in situ kinetic analysis of the PD‐1/PD‐1 ligand/B7‐1 system was performed at the cell surface by Li et al The measured in situ parameters exhibited a similar trend as the solution measurements, with PD‐1:PD‐L2 being stronger than PD‐1:PD‐L1 and PD‐1:PD‐L1/PD‐L2 being stronger than B7‐1:PD‐L1. The authors also stated that solution‐based affinity measurements in the micromolar range were weaker than the in situ parameters measured in their study, suggesting that the in vivo interactions could be in the nanomolar to submicromolar range.…”