The glucocorticoid-induced TNFR (GITR) is expressed at high levels on resting CD4+CD25+ T regulatory (TR) cells and regulates their suppressive phenotype. Accordingly, we show that anti-GITR mAb treatment of SJL mice with proteolipid protein 139–151-induced experimental autoimmune encephalomyelitis significantly exacerbated clinical disease severity and CNS inflammation, and induced elevated levels of Ag-specific T cell proliferation and cytokine production. Interestingly, prior depletion of TR cells failed to result in exacerbated experimental autoimmune encephalomyelitis suggesting alternative targets for the anti-GITR mAb treatment. Importantly, naive CD4+CD25− T cells up-regulated GITR expression in an activation-dependent manner and anti-GITR mAb treatment enhanced the level of CD4+ T cell activation, proliferation, and cytokine production in the absence of TR cells both in vivo and in vitro. Taken together, these findings suggest a dual functional role for GITR as GITR cross-linking both inactivates TR cells and increases CD4+CD25− T cell effector function, thus enhancing T cell immunity.

In vivo administration of anti-CD3 Ab induces both immune tolerance and undesirable side-effects resulting from nonspecific proinflammatory cytokine production. In the current study, we investigated the therapeutic potential of two structurally altered forms of the anti-CD3 Ab in ameliorating established experimental autoimmune encephalomyelitis. Administration of either a chimeric (NM-IgG3) or digestion product (NM-F(ab′)2) form of the anti-CD3 Ab during established experimental autoimmune encephalomyelitis conferred significant protection from clinical disease progression and was associated with decreased Ag-specific T cell proliferation, cytokine production, and CNS inflammation. Interestingly, while this protection correlated with an increase in the frequency of CD4+CD25+ regulatory T cells, neither prior depletion of regulatory T cells nor anti-TGF-β treatment abrogated the treatment’s efficacy. Importantly, both treatments induced normal levels of intracellular Ca2+-flux, but significantly diminished levels of TCR signaling. Consequent to this decreased level of TCR-mediated signaling were alterations in the level of apoptosis and CD4+ T cell trafficking resulting in a profound lymphopenia. Collectively, these results indicate that nonmitogenic anti-CD3 directly induces a state of immune unresponsiveness in primed pathogenic autoreactive effector cells via mechanisms that may involve the induction of T cell tolerance, apoptosis, and/or alterations in cell trafficking.

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