Commensal Candida Albicans Positively Calibrate Systemic Th17 Immunological Responses

Colonization with pathobiont microbes is a classical risk factor for invasive infection. However, the relative paucity of systemic infection despite ubiquitous pathobiont commensalism suggests colonization may also elicit protective host immunity. We show Candida albicans intestinal colonization of mice and humans drives systemic expansion of fungal-specific Th17 CD4+ T cells, and IL-17 responsiveness by circulating neutrophils, which synergistically protect against C. albicans invasive infection. Persistent C. albicans colonization is required, since protection is overturned by eradicating fungal colonization. Commensal C. albicans conferred protection extends to invasive infection by the extracellular bacterial pathogen, Staphylococcus aureus. However, positively calibrating systemic Th17 responses is not uniformly beneficial, as commensal C. albicans does not protect against intracellular influenza A virus infection, and exacerbates allergic airway inflammation susceptibility. Thus, systemic Th17 inflammation driven by CD4+ T cells responsive to commensal C. albicans tonic stimulation improves host defense against extracellular pathogens, but with harmful immunological consequences.