Mucosal tolerance has been considered a potentially important pathway for the treatment of autoimmune disease, including human multiple sclerosis and experimental conditions such as experimental autoimmune encephalomyelitis (EAE). There is limited information on the capacity of commensal gut bacteria to induce and maintain peripheral immune tolerance. Inbred SJL and C57BL/6 mice were treated orally with a broad spectrum of antibiotics to reduce gut microflora. Reduction of gut commensal bacteria impaired the development of EAE. Intraperitoneal antibiotic-treated mice showed no significant decline in the gut microflora and developed EAE similar to untreated mice, suggesting that reduction in disease activity was related to alterations in the gut bacterial population. Protection was associated with a reduction of proinflammatory cytokines and increases in IL-10 and IL-13. Adoptive transfer of low numbers of IL-10-producing CD25+CD4+ T cells (>75% FoxP3+) purified from cervical lymph nodes of commensal bacteria reduced mice and in vivo neutralization of CD25+ cells suggested the role of regulatory T cells maintaining peripheral immune homeostasis. Our data demonstrate that antibiotic modification of gut commensal bacteria can modulate peripheral immune tolerance that can protect against EAE. This approach may offer a new therapeutic paradigm in the treatment of multiple sclerosis and perhaps other autoimmune conditions.

Bacterial outer membrane vesicle (OMV)-mediated delivery of proteins to host cells is an important mechanism of host-pathogen communication. Emerging evidence suggests that OMVs contain differentially packaged short RNAs (sRNAs) with the potential to target host mRNA function and/or stability. In this study, we used RNA-Seq to characterize differentially packaged sRNAs in Pseudomonas aeruginosa OMVs, and to show transfer of OMV sRNAs to human airway cells. We selected one sRNA for further study based on its stable secondary structure and predicted mRNA targets. Our candidate sRNA (sRNA52320), a fragment of a P. aeruginosa methionine tRNA, was abundant in OMVs and reduced LPS-induced as well as OMV-induced IL-8 secretion by cultured primary human airway epithelial cells. We also showed that sRNA52320 attenuated OMV-induced KC cytokine secretion and neutrophil infiltration in mouse lung. Collectively, these findings are consistent with the hypothesis that sRNA52320 in OMVs is a novel mechanism of host-pathogen interaction whereby P. aeruginosa reduces the host immune response.

The intestinal microbiome may have a critical roll in susceptibility or resistance to immune-mediated diseases. Alterations of the gut microflora after oral antibiotic treatment can regulate encephalomyelitis (EAE), an animal model for human multiple sclerosis (MS). We now show that a zwitterionic capsular polysaccharide A (PSA) of Bacteroides fragilis can protect against central nervous system demyelinating disease. Oral administration with purified PSA protected mice against EAE prophylactic and therapeutically. PSA treatment enhanced CD103 expressing dendritic cells (DCs) that accumulated in the cervical lymph nodes. Exposure of naïve DCs to PSA induced the conversion of naïve CD4(+) T cells into interleukin (IL)-10-producing FoxP3(+)Treg cells. Protection against EAE was completely abrogated in IL-10-deficient mice. Our results show an important role for a molecule from human commensal bacteria in protecting against EAE and suggest the possibility for protection in MS.

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