Negative costimulation constrains T cell differentiation

Abstract

Abstract T cell costimulation is a principal mechanism by which T cell activation is regulated, but it remains unclear whether costimulatory pathways also control T cell differentiation. This unresolved question is germane to our understanding of how T cell function is controlled and has therapeutic implications in the context of cancer immunotherapy. Using a mass cytometry based systems approach and murine models we investigated whether negative costimulation regulates T cell differentiation as well as activation. Unsupervised population identification reveals 30 T cell subsets in Ctla-4−/− and littermate control mice including 6 CD8, 3 Treg, 14 CD4 effector T cell populations. Expectedly, loss of CTLA-4 led to dramatic changes in the relative frequency of CD8 and CD4 T cell subsets. Strikingly, however, loss of CTLA-4 led to the generation of multiple non-canonical T cell populations outside of the normal boundaries of T cell phenotypes, which interestingly, were all in the CD4 T compartment. To determine if the role of negative costimulation in differentiation may be a generalizable phenomenon, we performed similar analyses of Pdcd-1−/− mice. This profiling revealed that loss of PD-1 leads to a subtle expansion of CD8 T cell phenotypes in addition to changes in the relative frequencies of CD4 and CD8 T cell populations. These observations indicate that CTLA-4 restricts CD4 T cell phenotypes while PD-1 restricts CD8 T cell phenotypes. Consistent with this model, loss of CTLA-4 differentially affects the expression of individual markers in CD4 and CD8 T cells. Together these findings indicate that in addition to attenuating T cell activation, negative costimulation acts to constrain T cell differentiation.