Shimizu et al. generated murine T lymphoma cells expressing TCR variants with CDR amino acid substitutions that alter affinity for a defined peptide–MHC complex (pMHC). Using PD-L1-blocking antibodies and altered peptide ligands with varying affinities for a single MHC I or II, PD-1 negative signaling in vitro was shown to have greater impact on pMHC-stimulated T cell gene expression patterns of T cells expressing lower affinity TCRs, and be unaffected by MHC expression levels or affinity of peptide for MHC. In vivo, more lower-affinity T cells were activated in PD-1-deficient than control mice inoculated s.c. with pMHC-expressing cells.
Contributed by Paula Hochman
ABSTRACT: Anti-PD-1 therapies can activate tumor-specific T cells to destroy tumors. However, whether and how T cells with different antigen specificity and affinity are differentially regulated by PD-1 remain vaguely understood. Upon antigen stimulation, a variety of genes is induced in T cells. Recently, we found that T cell receptor (TCR) signal strength required for the induction of genes varies across different genes and PD-1 preferentially inhibits the induction of genes that require stronger TCR signal. As each T cell has its own response characteristics, inducibility of genes likely differs across different T cells. Accordingly, the inhibitory effects of PD-1 are also expected to differ across different T cells. In the current study, we investigated whether and how factors that modulate T cell responsiveness to antigenic stimuli influence PD-1 function. By analyzing TCRs with different affinities to peptide-MHC complexes (pMHC) and pMHCs with different affinities to TCR, we demonstrated that PD-1 inhibits the expression of TCR-inducible genes efficiently when TCR:pMHC affinity is low. In contrast, affinities of peptides to MHC and MHC expression levels did not affect PD-1 sensitivity of TCR-inducible genes although they markedly altered the dose responsiveness of T cells by changing the efficiency of pMHC formation, suggesting that the strength of individual TCR signal is the key determinant of PD-1 sensitivity. Accordingly, we observed a preferential expansion of T cells with low-affinity to tumor-antigen in PD-1-deficient mice upon inoculation of tumor cells. These results demonstrate that PD-1 imposes qualitative control of T cell responses by preferentially suppressing low-affinity T cells.