The administration of Th2 cytokines or immune deviation to a Th2 phenotypic response has been shown to protect against the autoimmune pathology of experimental autoimmune encephalomyelitis (EAE). To better understand the function of Th2 cytokines in the induction stage of EAE in the absence of an overt Th2 response, we immunized IL-4 receptor alpha-deficient (IL-4Ra À/À ) mice, which are unable to respond to either IL-4 or IL-13. Contrary to expectations, mice lacking IL-4Ra had a lower incidence of EAE and a delayed onset compared to WT BALB/c mice; however, this delay did not correlate to an alteration in the Th1/Th17 cytokine balance. Instead, IL-4Ra-responsive macrophages were essential promoters of disease as macrophagespecific IL-4Ra-deficient (LysM cre IL-4Ra À/lox ) mice were protected from EAE. The protection afforded by IL-4Ra-deficiency was not due to IL-10-, IFN-c-, NO-or IDO-mediated suppression of T-cell responses but was dependent upon the presence of regulatory T cells (Tregs). This investigation highlights the importance of macrophages and Tregs in regulating central nervous system inflammation and demonstrates that macrophages activated in the absence of Th2 cytokines can promote disease suppression by Tregs. Keywords: EAE; macrophage; IL-4Ra; T regulatory cell; immune regulation Experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), models aspects of MS including autoimmune Th response development and central nervous system (CNS) inflammation. Mice immunized with myelin proteins or immunodominant peptides of myelin develop an ascending progressive paralysis mediated by CD4 T cells, 1 and migration of these myelin-specific CD4 cells to the CNS induces inflammation, demyelination and axonal/ neuronal damage. 2 Autoreactive CD4 T cells are essential in triggering EAE; however, macrophages are required to facilitate the invasion of the autoreactive T cells into the CNS parenchyma and are thus also essential in the development of autoimmune pathology. 3 Macrophage depletion or deactivation completely blocks the egress of T cells from the perivascular spaces into the parenchyma thus preventing the disease-causing demyelination and tissue damage. 3 Conversely, mice that exhibit greater numbers of activated macrophages have been shown to develop a more rapid onset of EAE. 4 Cytokines produced by Th2 cells (especially IL-4) have been associated with remission from EAE, 5 and the levels of the Th2-associated cytokines, IL-4 and IL-10, are increased within the CNS during recovery from EAE. 6 Moreover, administration of IL-4 or immune deviation to enhance Th2 cell development can reduce or inhibit EAE. 7-9 Using mice deficient in the STAT6 pathway, which controls the differentiation of cells into a Th2 phenotype, a more severe course of EAE develops establishing a regulatory function for Th2 cytokines in vivo during EAE. 10 However, despite the protective effect of Th2 cytokines in EAE and MS, a recent study has shown that IL-4 receptor alpha-deficient (IL-4Ra À/À ) m...