Multiple sclerosis (MS) is a demyelinating disease of the central nervous system (CNS) that is characterized by an autoimmune inflammatory process involving myelin antigens . Experimental autoimmune encephalomyelitis is currently the most widely accepted animal model to investigate this pathology. Epidemiological studies have identified several environmental risk factors that contribute to the development of this disease, such as viral infections, smoking and depleted vitamin D serum levels . Although SAgs have been implicated in the pathogenesis of different autoimmune diseases, such as type I diabetes, Kawasaki disease and MS , their effects have not been systematically tested in these pathologies. In the present investigation, we evaluated the effect of previous infections with S. aureus on EAE development. To achieve this, female C57BL/6 mice were infected with 2 distinct S. aureus strains; one strain was a producer of the TSST-1 toxin (ATCC 51650), and the other strain did not produce any SAgs (ATCC 43300). Three days after infection, the animals were subjected to EAE induction, and the disease evolution was compared with a control group that had not been infected before EAE induction. Previous inoculation with both strains clearly induced a protective effect that was characterized by the appearance of a much more benign disease. Previously infected animals did not lose weight during the acute phase and also presented a less severe sickness with lower clinical scores. In addition to these less serious clinical manifestations, there was also a lower disease incidence. A comparison of all of these parameters indicated that the TSST-1+ strain was more protective than the TOX- strain. In addition, the first signals of paralysis in the TSST-1+ infected group were delayed. In this group, these clinical symptoms appeared only on the 21st day after EAE induction, compared to the EAE control group, which exhibited symptoms by the 14th day. To examine the mechanisms by which these two S. aureus strains protected against EAE development, cytokine production by the spleen and CNS infiltrating cell cultures were concomitantly evaluated. The similar IFN-γ and IL-17 levels in the MOG-stimulated spleen cultures from the three experimental groups suggest that previous infection is not down-regulating the peripheral production of encephalytogenic cytokines. However, a significant down-regulation of IL-5 and IL-10 was observed in the TOX-/EAE and TSST-1+/EAE-infected animals, respectively. In this context, we hypothesized that this decreased production of anti-inflammatory cytokines was due to a migration of specific T cell clones from the periphery to the CNS. However, an evaluation of IL-5 and IL-10 production by cells infiltrating the CNS was not consistent with this interpretation. Interestingly, there was a lower production of IL-17 in the CNS of mice that were previously infected with S. aureus compared to the positive control animals, i.e., the EAE group. We believe that this is an important finding because IL-17 is described as a relevant mediator of EAE and MS immunopathogenesis [18, 19]. Because the peripheral production of IL-17 stimulated by MOG was similar between the three groups, this lower local level of IL-17 could be due to a decreased migration of Th17-specific clones to the CNS or to a local down-modulation of IL-17 production. To test the last possibility, we analyzed the cytokine production specifically induced by S. aureus (SAC) in the periphery (spleen) and in the CNS. Previous infection was associated with an accentuated immune response characterized by a significant production of IFN-γ, IL-5 and IL-10. SAC-specific clones capable of producing anti-inflammatory cytokines could migrate to the CNS and down-modulate IL-17 production. A theoretical basis for this possibility was found in the literature. Initially described as a product of Th2 cells, IL-10 is now recognized as being secreted by nearly every cell type of the immune system and is able to inhibit the inflammatory process . In the context of MS and EAE, IL-10 clearly elicits beneficial effects on these diseases. In MS patients, for example, IL-10 levels are increased in the serum during disease remission . In addition, the efficacy of IFN-β and glatiramer acetate, two widely employed MS treatments, is partially attributed to the induction of IL-10 [21–23]. Genetic studies using the EAE model also showed that IL-10 deletion enhanced EAE disease severity while over-expression of this cytokine protected mice . Moreover, treatment of EAE with a herpes simplex virus type 1 vector expressing IL-5 ameliorated EAE and decreased the number of infiltrating lymphocytes in the brain . Similarly, treatment of experimental autoimmune neuritis with recombinant IL-5 markedly reduced clinical paralysis, weight loss, demyelination as well as infiltration of Th1, Th17 and Tc cells and macrophages in nerves . Despite the significant peripheral production of IL-10 in mice infected with both strains and IL-5 in mice infected with the TOX- strain, the levels of these cytokines were statistically similar in the CNS of the three experimental groups. Taken together, the cytokine results suggest that down-modulation of IL-17 in the brain was not mediated by IL-10 or IL-5 production in the CNS. Considering that the balance between Th17 and regulatory T cells is critical in autoimmune diseases  and that a high frequency of regulatory T cells migrate to the CNS during EAE recovery [28, 29], we analyzed the frequency of Foxp3+ regulatory T cells in the spleen and at the site of inflammation. The proportion of CD4+ CD25+ Foxp3+ cells in the spleen and CNS was significantly elevated in mice with EAE, as shown in this study and previous studies [18, 28]. The supposition that the protection by previous S. aureus infection was mediated by an increased number of Foxp3+ Treg cells was not confirmed. Previous infections did not increase the number of Foxp3+ cells in either the spleen or the CNS. In contrast with our expectations, the number of CD4+ CD25+ Foxp3+ cells was significantly decreased in the TOX-/EAE group. This finding was consistent with the presence of a strong inflammatory process in the CNS of this group, although the animals were clearly protected from the disease. The histopathological analysis of the infected animals that were not subjected to EAE induction showed that infection with the TOX- strain, but not with the TSST-1strain, triggered a clear inflammatory process in the brain. Taken together, these results suggest that the TOX- strain or its secreted components can cross the blood-brain barrier and elicit a local accumulation of inflammatory cells. Penetration of S. aureus across the blood-brain barrier has recently been described  and provides support for this idea. In this scenario, the lower number of Foxp3+ cells in TOX-infected animals could be due to a “dilutive effect” caused by the infiltration of phagocytic cells that are trying to eliminate a local S. aureus infection. Our initial hypothesis that the TSST-1 strain could increase disease severity due to its superantigenicity was not confirmed by our findings. Considering the various parameters (lower clinical score, lower disease incidence and very discrete inflammation in the CNS), we believe that this strain was more protective than the TOX- strain. Thus, we hypothesize that these two S. aureus strains are protecting the animals from EAE development using different molecular mechanisms or more than one immunomodulatory via. In this scenario, we could imagine that the TSST-1 strain was more effective because it employed a stronger immunoregulatory mechanism or multiple mechanisms. In this sense, it has been demonstrated that the TSST-1 toxin was able to cause apoptosis of myelin T cell clones . However, we could not make a direct comparison of these results with the literature because there were no experimental approaches similar to the one employed by our study. Thus, this is the first direct demonstration that an S. aureus infection was able to decrease the severity of EAE. Further studies are necessary to highlight the protective mechanisms that are triggered by S. aureus infection to protect against EAE development. It will be relevant to establish if these two strains present differences related to dissemination in the CNS as well as in the secretion of extracellular adherence protein (Eap). This protein is endowed with the ability to prevent EAE development by inhibiting infiltration of inflammatory cells into the CNS .