Resistance to CD4+CD25+ regulatory T cells and TGF-β in Cbl-b-/- mice

Cbl-b(-/-) mice have signaling defects that result in CD28-independent T cell activation, increased IL-2 production, hyper-reactive T cells, and increased autoimmunity. Although the increased autoimmunity in these mice is believed to result from the hyper-reactive T cells, the mechanisms leading from T cell hyper-reactivity to autoimmunity remain unclear. Specifically, the function and interaction of CD4(+)CD25(+) regulatory T cells (T-reg) and CD4(+)CD25(-) effector T cells (T-eff) in Cbl-b(-/-) mice have not been examined. We now report that Cbl-b(-/-) CD4(+)CD25(+) T-reg exhibit normal regulatory function in vitro. In contrast, the in vitro response of Cbl-b(-/-) CD4(+)CD25(-) T-eff is abnormal, in that it is not inhibited by either Cbl-b(-/-) or wild-type T-reg. This resistance of Cbl-b(-/-) T-eff to in vitro regulation is seen at the levels of both DNA synthesis and cell division. In addition to this resistance to CD4(+)CD25(+) T,,g, Cbl-b(-/-) T-eff demonstrate in vitro resistance to inhibition by TGF-beta. This second form of resistance in Cbl-b(-/-) T-eff is seen despite the expression of normal levels of type II TGF-beta receptors and normal levels of phosphorylated Smad3 after TGF-beta stimulation. Coupled with,recent reports of resistance to T-reg in T-eff exposed to LPS-treated dendritic cells, our present findings suggest that resistance to regulation may be a relevant mechanism in both normal immune function and autoimmunity.