Differential requirements of MHC and of DCs for endogenous proliferation of different T-cell subsets in vivo.

T cells transferred into severe lymphopenic hosts undergo rapid proliferation known as "endogenous proliferation" that are distinct from conventional homeostatic proliferation. Unlike homeostatic proliferation, cytokines, such as IL-7 are dispensable, yet TCR:MHC interaction is essential for this process to occur. However, cell types inducing the proliferation have not formally been addressed. In this study, we report that CD11c+ conventional DCs play irreplaceable roles in inducing endogenous proliferation of both naive and memory phenotype CD4 T cells via TCR-MHC II interaction. By contrast, CD8 T-cell endogenous proliferation was independent of MHC I or CD11c+ DCs. Interestingly, MHC II was necessary to support naive CD8 T-cell proliferation within MHC I-deficient hosts. Depletion of both B cells and DCs was sufficient to abrogate the proliferation of naive but not of memory CD8 T cells. These results suggest that depending on the T-cell lineages, as well as the differentiation status, different mechanisms control endogenous proliferation, revealing in vivo complexity of T-cell proliferation under lymphopenic conditions.