Gut mycobiota alteration contributes to the pathogenesis of Pneumocystis pneumonia

Pneumocystis is an opportunistic fungal pathogen that causes life-threatening pneumonia in immunocompromised hosts, with increasing incidence in HIV-negative individuals. Although the gut mycobiota has emerged as a critical regulator of distal immunity, its role in HIV-negative Pneumocystis pneumonia (PCP) remains entirely unexplored.

We established a murine model of Pneumocystis murina infection and performed full-length internal transcribed spacer (ITS) sequencing to characterize longitudinal changes in gut fungal communities over five weeks. Untargeted metabolomics was conducted on plasma samples to identify systemic metabolic alterations. To investigate causality, gut fungal communities were depleted using fluconazole, and fecal microbiota transplantation (FMT) was performed in germ-free mice to assess the functional role of gut fungi in modulating pulmonary immune responses.

While α diversity of the gut mycobiota remained unchanged, β diversity analysis revealed significant structural alterations beginning week 3 (w3) post-infection, coinciding with peak pulmonary fungal burden. Linear discriminant analysis effect size identified Purpureocillium lilacinum and Talaromyces verruculosus as enriched opportunistic taxa. Untargeted metabolomics demonstrated marked metabolic reprogramming at w3, with significant perturbations in glycine, serine, and threonine metabolism, as well as the tricarboxylic acid cycle. Fluconazole-mediated depletion of gut fungi significantly increased pulmonary Pneumocystis burden and exacerbated lung inflammation, accompanied by reduced pulmonary Th1 cell responses. Critically, FMT from fluconazole-treated donors into germ-free mice recapitulated the exacerbated phenotype, confirming that gut fungal dysbiosis is sufficient to impair Th1-mediated antifungal immunity and worsen disease severity.

This study establishes, for the first time, that gut fungal dysbiosis actively contributes to the pathogenesis of HIV-negative PCP via the gut-lung axis. Our findings reveal that commensal gut fungi support pulmonary Th1 immune responses essential for controlling PCP, and their disruption exacerbates disease. These results provide new insights into the gut mycobiota as a potential therapeutic target in PCP and caution against indiscriminate antifungal use in susceptible populations.