T Cell Abnormalities in Cbl-b Deficient Mice

Cbl-b is an E3 ubiquitin ligase that negatively regulates T cell activation. Cbl-b-deficient (Cbl-b-/-) mice develop spontaneous autoimmunity, and Cbl-b dysregulation has been described in both murine and human autoimmune diseases. While the mechanisms underlying the development of autoimmunity in Cbl-b-/- mice are not yet clear, we have reported that Cbl-b -/- CD4+CD25- T cells (Teff) are resistant to CD4+CD25+ regulatory T cell (Treg)-mediated suppression in vitro and have suggested that this may be an important mechanism in the development of autoimmunity. To confirm the relevance of this resistance to autoimmune disease, we now show that Cbl-b-/- Teff are resistant to suppression by Tregs in vivo. Additionally, we show that Cbl-b-/- Tregs are functional in vivo, further suggesting that the regulatory abnormalities in Cbl-b-/- mice are related to defects in Teff, not Treg, function. ^ To characterize the relevance of TGF-β sensitivity in Treg resistance, we examined in vivo Th17 generation and report that Cbl-b-/- mice are able to mount a normal Th17 response in vivo. As Cbl-b-/- Teff have been shown to be insensitive to the suppressive effects of TGF-β in other in vivo models, the present results suggest that Cbl-b-/- Teff demonstrate a context-dependent sensitivity to TGF-β in vivo. Since IL-2 was found to modulate the TGF-β sensitivity of Cbl-b-/- Teff, we examined a role for cytokines in Treg resistance and found that in vitro resistance of Cbl-b-/- Teff is partially due to increased production of IL-2 and IL-4. We also found that WT Teff, which are typically suppressed by Tregs, do not display Treg suppression of cytokine when cultured with Cbl-b-/- Teff in vitro. The ability of one subpopulation of Teff to induce Treg resistance in another population may have significant implications for mechanisms underlying autoimmune disease. Overall, our results suggest that resistance to Tregs may be a bona fide mechanism underlying autoimmunity and that Cbl-b-/- mice offer unique approaches for studying the interrelationships between Treg function, TGF-β-mediated responses, and the development of autoimmunity.