Contribution to the study of the immune response against dermatophytes using a murine model of Trichophyton benhamiae infection.

Dermatophytoses are widespread superficial skin mycosis known to affect more than 20% of the human population, making them the most common fungal infections. Pathogenic dermatophytes are specialized filamentous fungi that exhibit the capacity to invade keratinized tissues and produce infections that are generally restricted to the stratum corneum, hair, and nails. Despite their superficial localization, these fungi induce an immune response that remains poorly documented as compared to that observed with other fungal infections. Although the ability of dermatophytes to induce innate response is documented, little is known about the adaptive immune responses against these fungi. Furthermore, the inoculation route in most in vivo models used for studying the immune response against dermatophytes fails to mimic the natural epicutaneous infection or even induce observable skin lesions. Some clinical case studies, however, have shown that resolution of dermatophytoses is associated with the development of cell-mediated immunity with T helper (Th) type 1 polarization. At the beginning of this work, despite the crucial role of Th17 response in many other fungal diseases, particularly at mucosal surfaces, no study had yet experimentally documented Th17 response in dermatophytoses. In this regard, a relevant mouse model of dermatophytosis had just been developed at the Laboratory. In the first part of this work, by comparing Trichophyton benhamiae acute superficial dermatophytosis in WT and Rag2−/− mice, we evidenced that TCR-mediated immunity is critical for the optimal control of acute dermatophytosis and that adaptive immunity is polarized to both Th1 and Th17 responses. Using IL-17A-deficient, IFN-γ-deficient and IL-17A and IFN-γ double-deficient mice we next evidenced that theses two pathways function in a complementary manner, with the Th17 antifungal response acting on dermatophyte clearance and the Th1 one being involved in both fungal clearance and Th17-inflammation down-modulation. In mammals, it is generally accepted that, besides an increased epidermal turnover, the two main effector and regulatory mechanisms in the fight against opportunistic fungal skin infections are the cytotoxic and phagocytic activity of polymorphonuclear neutrophil (PMN)s, as well as the production of antimicrobial peptides (AMP)s at the site of infection. PMNs and AMP have long been known as key elements of the cutaneous innate immune system, but interest in these has drastically increased with the discovery of the Th17 pathway, particularly in fungal infections. Indeed, in skin and mucosal defence against fungi, IL-17 and IL-22, the major’s cytokines of Th17 family, have essential role for promoting PMN mobilization and regulating AMP production at the site of infection. Both attraction of PMNs and production of AMPs occur upon infection with dermatophytes and have been associated with host protection in human and experimental infections but the implication of the different immune components in the triggering of these two defences mechanisms had so far never been studied in acute superficial dermatophytosis. In the second part of this work, we evidenced for the first time that while in situ PMN attraction and AMP production can occur without the intervention of TCR-mediated immunity, this last significantly contributes to optimally strengthen these two defense mechanisms in acute superficial dermatophytosis. We next demonstrated that innate and Th17-secreted IL-17A significantly contributed to increase the in situ PMN attraction and that the actions of PMNs significantly contribute to decrease the fungal loads. We also evidenced that, while single IL-17A and IFN-γ deficiencies had no impact on the AMP production in our model of superficial T. benhamiae infection, a double IFN-γ and IL-17A deficiency significantly decreased it. These two lasts observations suggest not only a preponderant role of a Th17-secreted non-IL-17 cytokine, probably IL-22, in the AMPs production but also possible synergistic actions of this unknow cytokine with IL-17A and also with IFN-γ on the AMPs production, particulary in the case of IFN-γ and IL-17A deficiencies respectively. In parallel with these experiments, we infected the various aforementioned strains of mice after having depleted them in PMN, and we evidenced that the action of the PMNs contributed significantly to reduce the fungal loads but that other mechanisms, such as the local AMP production, certainly helped to reduce the fungal skin load and prevent the dissemination of the dermatophyte beyond the skin, especially in the case of deficiencies affecting the cutaneous PMNs infiltration. Taken together these data suggest that, during acute superficial dermatophytosis, Th17 immune response contributes to the fungal clearance by strengthening the innate in situ PMN infiltration and AMP production, with preponderant roles of IL-17A on the PMN recruitement and of an other Th17-secreted cytokine, probably IL-22, on AMP production, alone or in synergy with IL-17A or IFN-γ