Immune-Modulatory Effects of Micrococcus yunnanensis, a Dominant Indoor Air Bacterium in Seoul Metropolitan City

초록

Rationale Humans inhale approximately 20–50 m^3 of air daily, establishing direct link between air quality and respiratory health. Airborne microorganisms have increasingly drawn scientific attention due to their potential impact to modulate immune responses within the respiratory tract. We hypothesized that the composition and biological activity of indoor airborne microorganisms significantly influence human respiratory immunity, particularly through dominant microbial taxa. Methods Indoor air samples from 272 diverse sites across Seoul including public facilities, transportation hubs, and residential environments. Airborne bacteria were captured via direct impaction, cultured, and taxonomically identified by 16S rDNA sequencing. Remarkably, Micrococcus yunnanensis (M. unnanensis) was ubiquitously abundant across all sampled locations. BALB/c mice which were sensitized and challenged with ovalbumin received intranasal M. yunnanensis (10^5 CFU) for examining immunomodulatory effects. Airway hyperresponsiveness (AHR) was evaluated via methacholine challenge. Cytokines, including IL-2, IL-4, IL-12, MDC, and IFN-γ in BALF and serum total IgE levels were measured. Results Exposure to M. yunnanensis significantly attenuated AHR compared to the OVA- controls. BALF concentrations of Th2 associated cytokines such as IL-4 and IL-2 were reduced, whereas Th1 cytokines, IFN-γ and IL-12 were elevated. Notably, MDC expression increased despite reduced IL-4, suggesting IgE regulation mechanisms beyond classical Th2 pathways. Conclusions These findings demonstrate that M. yunnanensis, a prevalent urban indoor bacterium, modulates respiratory immune responses by promoting Th1-skewed immunity and suppressing Th2-driven inflammation. This study highlights the potential protective role of indoor commensal microbes in allergic airway diseases and underscore the necessity of reevaluating the immunological impact of indoor microbial exposure on respiratory health.