The clinical use of intravenous immunoglobulin (IVIg) based on its immunomodulatory and anti-inflammatory potential remains an ongoing challenge. Fc␥ receptor-mediated effects of IVIg, although well elucidated in certain pathologies, cannot entirely account for its proven benefit in several autoimmune disorders mediated by autoreactive T cells. In this study, we show that prophylactic infusion of IVIg prevents the devel- IntroductionNatural CD4 ϩ CD25 ϩ regulatory T cells (nTreg) expressing the lineage marker Foxp3 are the key players in controlling immune responses and in maintenance of T-cell homeostasis. 1,2 Therapeutic induction of the Treg represents a novel approach in the treatment of autoimmune pathology. 3,4 CD4 ϩ CD25 ϩ Foxp3 ϩ nTreg develop in thymus, in contrast to "adaptive" or "induced" Treg that develop in peripheral lymphoid tissues from CD4 ϩ conventional T cells (Tconv) and are frequently Foxp3 Ϫ . Although studies have highlighted the role of cytokines interleukin-2 (IL-2), transforming growth factor-␤ (TGF-␤), and IL-10 in Treg development, other factors or mechanisms crucial to Treg homeostasis are not elucidated. 5 Intravenous immunoglobulin (IVIg) is an established therapy for several immune disorders. [6][7][8][9] Several mutually nonexclusive mechanisms have been proposed to explain the beneficial effect of IVIg 7,8 ; however, the issue remains debated and an ongoing challenge. For instance, the Fc␥R-mediated effects of IVIg 10-12 cannot entirely account for its proven benefit in several peripheral and central demyelinating diseases such as GuillainBarré syndrome, chronic inflammatory demyelinating polyneuropathy (CIDP), and relapsing-remitting multiple sclerosis (MS), which are primarily mediated by autoreactive T cells. 6,7,13,14 Because the T cells do not express Fc␥R, 15 the observed effects raise certain speculations, that is, if these effects could be attributed to a direct interaction of the variable region of the immunoglobulin (Ig) G molecules with the T cell or an indirect influence via other cell types such as dendritic cells (DC).During the induction phase of experimental autoimmune encephalomyelitis (EAE), myelin reactive proinflammatory CD4 ϩ T cells proliferate in the periphery, infiltrate the central nervous system (CNS) during the effector phase and, in concert with other inflammatory mediators, lead to demyelination characterized by a progressive paralysis. 16 Natural remission and recovery from relapse in EAE is associated with the recruitment or generation of Treg in the CNS. 17,18 We and others have shown that IVIg protects against EAE development only when administered prophylactically. 14,19 We reasoned that IVIg manifests its protective effect in EAE through an early modulation of autoreactive T cells, and therefore we investigated the regulatory mechanisms, particularly the effect of IVIg on regulatory T cells. Methods Animals, antigen, and tissue culture mediumWe purchased C57BL/6J mice (females, 6-8 weeks of age) from Charles River Laboratories (L'Arbresle, F...