Developmentally distinct Th cells control plasma cell production in vivo.

Differential Ly6C expression identifies a major phenotypic division in CD44loCD62LhiCD4+ Th cells. Using two separate models of single subset adoptive transfer, we demonstrate the unique capacity of Ly6Chi Th cells to promote antigen-specific plasma cell production in vivo. In contrast, both compartments support germinal center formation and proliferate to equivalent levels upon TCR triggering in vivo and in vitro. Developmentally, CD4+CD8- thymocytes leave the thymus expressing low levels of Ly6C; 3 days later approximately 50% stably upregulate Ly6C without cell division or TCR engagement in the periphery. Interestingly, antigen-specific Th cell clonotypes unevenly assort into these peripheral compartments, creating separate TCR repertoires that underpin peripheral functional diversity. Taken together, these data reveal a developmentally distinct Ly6Chi naive Th cell compartment subspecialized to regulate plasma cell production in vivo.