Objectives: We sought to determine the mechanisms how TGF-β signaling through Smad4 regulates the pathogenic effector T cell subsets in CHS.
Methods: We used Cd4Cre-loxp system to delete Smad4 in T cell-specific manner (Cd4Cre;Smad4fl/fl,+/+). Cd4Cre;Smad4fl/fl,+/+ mice were immunized and sensitized by 1-fluoro-2,4-dinitrobenzene (DNFB).
Results: We found that T cell-specific deletion of Smad4 exacerbated DNFB-induced CHS with significant expansion and infiltration of CD8+ T cells in the draining lymph nodes and the skin lesions. Smad4 deficient CD8+ T cells upregulated Th2 differentiation, regardless of Smad4 genotypes of CD4+ T cells. Smad4 in combination with Smad3 suppressed the expression of a T-box transcription factor, Eomesodermin (Eomes) in CD8+ T cells. Expression of Eomes and the cytotoxic molecules in CD8+ T cells at the early phase of sensitization was significantly upregulated in Cd4Cre;Smad4fl/fl mice compared with control Cd4Cre;Smad4+/+ mice. Cytolytic molecules in CD8+ T cells upregulated by Smad4 deletion induced Th1 cell apoptosis, which resulted in increased Th2.
Conclusions: These data highlight CD8+ T cell-intrinsic Smad4 as the crucial regulator of effector CD4+ T cell subsets in CHS.