Wei and Sharma et al. investigated the role of negative costimulation on T cell differentiation and found that, in addition to their effects on activation, during peripheral proliferation CTLA-4 significantly imposes phenotypic boundaries on CD4+ T cell phenotypes and PD-1 subtly imposes phenotypic boundaries on CD8+ T cells. T cells engineered to lack these molecules are able to differentiate further down active pathways, leading to the acquisition of phenotypes not observed under natural circumstances. Antibody blockade of these molecules may similarly reduce some of the limitations on T cell differentiation.
Co-stimulation regulates T cell activation, but it remains unclear whether co-stimulatory pathways also control T cell differentiation. We used mass cytometry to profile T cells generated in the genetic absence of the negative co-stimulatory molecules CTLA-4 and PD-1. Our data indicate that negative co-stimulation constrains the possible cell states that peripheral T cells can acquire. CTLA-4 imposes major boundaries on CD4(+) T cell phenotypes, whereas PD-1 subtly limits CD8(+) T cell phenotypes. By computationally reconstructing T cell differentiation paths, we identified protein expression changes that underlied the abnormal phenotypic expansion and pinpointed when lineage choice events occurred during differentiation. Similar alterations in T cell phenotypes were observed after anti-CTLA-4 and anti-PD-1 antibody blockade. These findings implicate negative co-stimulation as a key regulator and determinant of T cell differentiation and suggest that checkpoint blockade might work in part by altering the limits of T cell phenotypes.