Abstract
Commensal microorganisms colonize the nasal mucosa without inducing inflammation. Pathogens perturbing the commensal flora often invade evading immune defenses. The different types of adaptive responses that drive the distinct behaviors of commensals and pathogens, allowing one to persist at mucosal surfaces and the other to survive within tissues, are not yet clear. In the present work we demonstrate that although crossing epithelial barriers, the commensal Lactobacillus murinus stimulates epitope-specific CD4(+) T cells in nasal-associated lymphoid tissue (NALT) less efficiently than the pathogen Streptococcus pyogenes. In NALT antigen-presenting cells other than CCR6(+) CD11c(+) dendritic cells process and present the microbial antigens. Effector/memory CD4(+) T cells generated after intranasal priming with L. murinus and S. pyogenes surprisingly express similar proinflammatory cytokines and are not CD25+/FoxP3+ T-regulatory cells when recirculating in the spleen. These findings suggest that when a commensal crosses the nasal epithelial barrier it induces a proinflammatory response similar to a pathogen but without causing disease.