FUNGIPSOME

Context: Chronic immune-mediated inflammatory diseases can attack many organs such as the skin, the joints, the central nervous system. They are on the rise, affecting an estimated 5% of western population1,2, and represent a public health issue. Although treatments are improving, with the use of targeted antibodies against cytokines, so called “biologics”, for severe forms, these diseases tend to relapse upon treatment withdrawal and the fear for relapse prolongs the duration of the therapies. Both the probability of relapse and its igniting events remain unclear. To optimize patient exposure to these very costly medications, a better understanding of relapse mechanisms and ways to prevent it is warranted.

In dermatology, psoriasis is a good model of a relapsing inflammatory disease. The relapses are related to a disease memory, that is best characterized by memory T cells that remain in the epidermis even after successful treatment, and are capable to release the pathogenic IL-17 protein upon reactivation by unknown triggers. Depleting this memory could help improve patient outcomes, but what makes the memory cells stay in the skin for years and how to modulate their functions is not elucidated. Among potential modulating factors, the ones that are present on the skin surface are the best candidates to external, simple interventions that would not diffuse to the whole body but concentrate where the memory lies. Skin microbiota is correlated to the T cell function in the skin. Fungi are of particular interest as T cell modulators since the host’s antifungal defences involve the Th17 axis that happens to be the major pathway

involved in psoriasis pathophysiology. Thus, a skin area after local infection or colonization by fungi could already harvest T cells prone to either stimulation or inhibition of IL-17–producing T cells.

Aim: to identify strains of the human mycobiota that are most likely to favour the skin colonisation by anti-inflammatory T cells to limit disease relapses.

Methods: thirteen patients with psoriasis in remission were sampled (skin swabs) at time of treatment cessation and followed up until relapse, at least a year. Skin swabs were frozen at the initial time-point and will be used to culture fungi on specific culture media. The strains isolated from the skin of late-relapsers (>6 months) and absent in early-relapsers will be selected for further characterisation using co-cultures with skin cell suspension from healthy donors for FACS analysis, and co-cultures with keratinocytes for RNA analysis. Proteomic and transcriptomic signals related to IL-17 and integrin cell attachment will help to further select the most relevant fungal strains. Lastly, selected fungal strains (n=3) will be tested on human skin explants in which T cells have been partially depleted using fractionated LASER (Light Amplification by Stimulated Emission of Radiation) technology. This will simulate ex vivo a T cell turnover event, and help select the most relevant fungus. Microscopy will objectify the recolonization of the epidermis by T cells, using immunohistochemistry and spatial transcriptomics.

Relevance to HARMI: We propose that modulating the host-fungi interactions facilitates the control of the relapse in psoriasis, e cellular level. Thus, direct fungi colonization or the use of their lipidic by-products as add-on therapies are new strategies that could induce long-term remission. The project is relevant to HARMI WP 1, 2, and 4, as developed later in the text.