Resistance to CD4+CD25+ Regulatory T Cells and TGF-β in Cbl-b−/− Mice

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Abstract
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 (Treg) and CD4+CD25− effector T cells (Teff) in Cbl-b−/− mice have not been examined. We now report that Cbl-b−/− CD4+CD25+ Treg exhibit normal regulatory function in vitro. In contrast, the in vitro response of Cbl-b−/− CD4+CD25− Teff is abnormal, in that it is not inhibited by either Cbl-b−/− or wild-type Treg. This resistance of Cbl-b−/− Teff to in vitro regulation is seen at the levels of both DNA synthesis and cell division. In addition to this resistance to CD4+CD25+ Treg, Cbl-b−/− Teff demonstrate in vitro resistance to inhibition by TGF-β. This second form of resistance in Cbl-b−/− Teff 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 Treg in Teff 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.