Transdermal entry of a non-pathogenic filamentous fungus

Introduction: The immune response to non-pathogenic fungi entering the skin remains largely unknown. In this study, we focused on the fermentative filamentous fungus Methods: Mice were subcutaneously inoculated with the Ao conidia and skin-draining lymph nodes were harvested over time for flow cytometry to evaluate various immune cell subsets. Quantitative reverse transcription-PCR was also conducted to assess the cytokine expression (IFN-γ, IL-4, IL-6, IL-10, IL-12, IL-17, TGF-β, TNF-α) at 4 or 7 days after Ao inoculation. To detect antigen-specific antibody production, mice were immunized with Ao and/or ovalbumin in aluminum hydroxide adjuvant, and sera were collected at day -1, 14, and 35 for ELISA. Fluorescence immunohistostaining was employed to visualize the structural reorganization of the lymph nodes. Active cutaneous anaphylaxis reaction was assessed for the impact of Ao inoculation on allergic response. Results: We observed a marked enlargement of skin-draining lymph nodes and increased immune cell numbers within days. Notably, this was characterized by a marked increase in the number of activated B cells and type 2 resident dendritic cells. These responses were partially reproduced by Ao cell wall components and purified β-glucans; the Ao-dependent downregulation of Dectin-1 expression on dendritic cells supports these findings. However, live Ao elicited the most pronounced lymph node response, as heat inactivation and the cell wall fraction clearly attenuated it, suggesting that other components are also necessary. Ao appears to induce a relatively mild response in the lymph nodes, characterized by a marked increase in IL-4 expression, whereas other cytokines were suppressed or unaltered. In the long term, Ao entry elicited immune memory with antibody production specific to the conidial proteins and β-glucan, although it did not enhance the antibody against another antigen/adjuvant. Furthermore, we found that pre-inoculation with Ao inhibited allergic responses. This is consistent with the novel effects of Ao inoculation, which reduces the CD301b Conclusion: Transdermal Ao entry clearly induces adaptive immunity in draining lymph nodes. However, the response is not necessarily intense and shows an immunomodulatory aspect with less harmful or even beneficial effects, potentially suppressing allergic diseases.