Abstract FTY720 is a sphingosine-1-phosphate receptor 1 (S1P1) modulator used to treat Multiple Sclerosis (MS). While FTY720 is postulated to mediate its effect by enhancing T cell S1P1 internalization and degradation, the mechanisms underlying the therapeutic effect of FTY720 in MS are not fully understood. To further clarify the functional effects of FTY720 we utilized Cbl-b-deficient (Cbl-b-/-) mice. Cbl-b is an E3 ubiquitin ligase that regulates the PI3K-Akt pathway in T cells and its absence leads to resistance to regulatory T cells (Tregs) and multi-organ autoimmunity in mice. Given that MS has been associated with both SNPs in the CBLB gene and resistance to Tregs, Cbl-b-/- mice represent an important model for studying MS. We now report that Cbl-b-/- T cells demonstrate multiple functional abnormalities in S1P1. These include significantly less lymphopenia induced by the S1P lyase inhibitor, THI, (Cbl-b-/- blood CD4+ T cells decreased 19% vs 59% in wild type (WT) at 48 hrs), suggesting abnormal regulation of Cbl-b-/- T cell S1P1 in response to the endogenous ligand S1P. Paradoxically, Cbl-b-/- T cells demonstrate enhanced lymphopenia induced by the therapeutic agent FTY720 (Cbl-b-/- blood CD4+ T cells decreased 51% vs 26% in WT at 120 hrs). These results for the first time identify a complex role for Cbl-b in regulating expression and turnover of T cell S1P1 and suggest that SNPs in the CBLB gene may associate with a differential response to FTY720 in MS patients.