Regulation of CD4 T cell activation, proliferation and anergy induction by Cblb

Though the role of Cbl-b in CD4+ T cell activation, proliferation, and clonal anergy induction has previously been explored, the precise function of Cbl-b in CD4+ T cells is still ambiguous. We utilized Rag2-/- 5C.C7 TCR-transgenic Cbl-b-/- mice as a model to study in depth the role of Cbl-b in CD4+ T cells both in vitro and in vivo. During clonal anergy induction in vitro, we found that Cbl-b-/- T cells were not refractory to anergy induction although, Cbl-b protein was shown up-regulated in anergic T cells, and Cbl-b-/- T cells remained more responsive to antigen even after anergy induction. Therefore, Cbl-b can be one potent anergy factor, but its function can be compensated by other factors. Upon initial TCR activation in naive Cbl-b-/- CD4+ T cells, major TCR signaling pathways remained intact. Nevertheless, the Akt/Foxo pathway was enhanced in the absence of Cbl-b. Moreover, Cbl-b protein was induced during TCR stimulation and enhanced Akt signal was best observed during late TCR signaling of Cbl-b-/- T cells. Consistently, PI-3 kinase-dependent activation of NF-kB was enhanced in the absence of Cbl-b. Thus, Cbl-b counter-regulates both early and late TCR signal transduction, mainly through inhibition of a PI-3 kinase/Akt/NFkB pathway to IL-2 production. During in vivo antigen stimulation, no Cbl-b effects were observed in the first 24 hr of T cell activation. However, Cbl-b-/- T cells demonstrated the capacity to maintain a higher level of CD71 expression and prolonged cell cycle progression. This suggests a role of Cbl-b in turning off late T cell activation and proliferation in vivo. To further support this, in vitro pre-activated T cells maintained hyper-responsiveness to late antigen stimulation in vivo. Therefore, Cbl-b regulates late TCR-dependent T cell activation and cell cycle progression in vivo. Since CD71 is regulated by PI-3 kinase/mTOR activity, it is speculated that Cbl-b regulates late T cell antigen stimulation through a PI-3 kinase pathway in vivo. In summary, Cbl-b functions in turning off late TCR signaling, mainly through the PI-3 kinase/Akt pathway, to avoid over-activation of T cells during immune responses.