Invariant natural killer T (iNKT) cells are a unique lymphocyte subtype implicated in the regulation of autoimmunity and a good source of protective Th2 cytokines. Agonist α-galactosylceramide (α-GalCer) of iNKT cells exert a therapeutical effect in type 1 diabetes. We investigated whether iNKT activation with α-GalCer was protective in collagen-induced arthritis (CIA) in DBA/1 mice, a standard model of rheumatoid arthritis. Here, we have shown that in vivo iNKT cell function was altered in DBA/1 mice since stimulation with α-GalCer led to decreased IL-4 and IFN-γ levels in sera, as compared with C57BL/6 mice. α-GalCer induced a clear-cut diminution of clinical and histological arthritides. An anti-IL-10 receptor antibody abrogated the protective effect of α-GalCer, suggesting a key role for IL-10 in the protection against CIA by activated iNKT cells. Confirming these data, disease protection conferred by α-GalCer correlated with the ability of LN CD4+ cells to secrete larger amounts of IL-10. These findings suggest that in CIA susceptibility to autoimmunity is associated with dysfunctions of iNKT cells. Our demonstration that iNKT cell activation by α-GalCer remains efficient in CIA-prone DBA/1 mice to provide protective IL-10 suggests that this could be used therapeutically to treat autoimmune arthritis.