Methylene Blue-Photodynamic Therapy for Microsporum canis Infection: Investigating a Dual Mechanism of Fungicidal Action and Neutrophil Homeostasis Restoration

Gaoyuan Peng^*

• The First Affiliated Hospital of Guangxi Medical University, Nanning, China

Background: Microsporum canis is an increasingly common cause of tinea capitis. Conventional antifungal therapies are limited by toxicity and resistance, creating a need for novel treatments. Antimicrobial photodynamic therapy (aPDT) is a promising alternative. We investigated the efficacy and dual mechanism of methylene blue-photodynamic therapy (MB-PDT) against M. canis, focusing on its effects on the host innate immune response. Methods: The in vitro susceptibility of clinical M. canis isolates was determined by broth microdilution. Fungal ultrastructural changes were examined using transmission electron microscopy. In vivo efficacy was assessed in a murine dermatophytosis model and compared to topical terbinafine. Systemic immunomodulatory effects were evaluated by flow cytometric analysis of peripheral blood neutrophil phenotypes (Dectin-1, Dectin-2) and functional markers (MPO, NOX2). Results: In vitro, MB-PDT demonstrated potent fungicidal activity (Geometric Mean MIC at 80 J/cm²: 0.367 µg/mL; 95% CI: 0.295–0.439 µg/mL). It induced severe ultrastructural damage, including mitochondrial collapse and cell wall disruption. In the murine model, MB-PDT achieved an 80% mycological cure rate, significantly outperforming topical terbinafine (20% cure rate). Mechanistically, M. canis infection induced systemic neutrophil dysfunction, evidenced by a population shift and suppressed MPO and NOX2 expression. MB-PDT treatment reversed this immune dysfunction, restoring neutrophil homeostasis and the expression of key functional markers (MPO, NOX2). Conclusions: MB-PDT is a highly effective treatment for M. canis infection. Its efficacy is based on a dual mechanism: direct fungicidal action through oxidative damage and restoration of host neutrophil function.