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 hyperreactive 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_reg, Cbl-b^-/- T _eff demonstrate in vitro resistance to inhibition by TGF-β. This second form of resistance in Cbl-b^-/- T_eff is seen despite the expression of normal levels of type II TGF-β receptors and normal levels of phosphorylated Smad3 after TGF-β stimulation. Coupled with recent reports of resistance to T_reg in T_eff exposed to LPS-treated dendrific cells, our present findings suggest that resistance to regulation may be a relevant mechanism in both normal immune function and autoimmunity.