It is currently acknowledged that T _H17 cells are critically involved in the pathogenesis of autoimmune diseases such as multiple sclerosis (MS). In this article, we demonstrate that signals delivered by the coinhibitory molecule B7-homologue 1 (B7-H1) via a B7-homologue 1 mouse-IgG2aFc (B7-H1-Ig) fusion protein nearly abolish T _H17, but not T _H1 and T _H2, differentiation via direct interaction with the T cell. These effects were equally pronounced in the absence of programmed death-1 or B7.1 and B7.2 on the T cell side, thus providing clear evidence that B7-H1 modulates T cell differentiation via a novel receptor. Mechanistically, B7-H1 interfered with early TCR-mediated signaling and cytokine-mediated induction of the T _H17-determining transcription factors retinoic acid-related orphan receptor g t and IFN regulator factor-4 in a programmed death-1 and B7-independent fashion. In an animal model of MS, active myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis, B7-H1-Ig exhibited a significant and long-lasting effect on disease severity upon administration during the first 5 d of the priming phase, which was accompanied by reduced T _H17 responses in the periphery and within the CNS. Importantly, B7-H1-Ig was even capable of interfering with T cell encephalitogenicity when interaction with the T cells occurred after priming using an adoptive transfer experimental autoimmune encephalomyelitis model. In line with this, both naive human CD4 ⁺ T cells and differentiated T _H17 effector cells from MS patients were highly sensitive toward B7-H1-Ig-mediated T _H17 suppression. Together, we propose the existence of a novel B7-H1-mediated immune-regulatory pathway in T cells, which selectively limits murine and human T _H17 cell responses and might be therapeutically exploited to control T _H17-mediated autoimmunity.