Highly self-reactive naive CD4 T cells are prone to differentiate into regulatory T cells.

Upon activation, naive CD4 T cells differentiate into a variety of T-helper-cell subsets characterized by different cytokine production and functions. Currently, lineage commitment is considered to depend mostly on the environmental context to which naive CD4 T cells are exposed. Here we challenge this model based on the supposed homogeneity of the naive CD4 T-cell compartment. We show that peripheral naive CD4 T cells can be subdivided into two subsets according to Ly-6C expression. Furthermore, the two newly defined subsets (Ly-6C(-) and Ly-6C(+) naive CD4 T cells) are not equal in their intrinsic ability to commit into the induced regulatory T-cell lineage. Finally, phenotypic analysis, imaging and adoptive transfer experiments reveal that Ly-6C expression is modulated by self-recognition, allowing the dichotomization of the naive CD4 T-cell compartment into two cell subsets with distinct self-reactivity. Altogether, our results show that naive CD4 T cells with the highest avidity for self are prone to differentiate into regulatory T cells.