P125 Histoplasma capsulatum modulates the immune response exerted by mesenchymal stromal cells

POSTER SESSION 1, SEPTEMBER 21, 2022, 12:30 PM - 1:30 PM: BACKGROUND: Mesenchymal stromal cells (MSCs) have become a tool not only for tissue regeneration but also for the treatment of inflammatory diseases. Several studies have demonstrated the therapeutic potential of MSCs for the treatment of noninfectious inflammatory diseases; however, they appear to play a dual role in infectious diseases. Histoplasmosis is a systemic mycosis caused by Histoplasma spp., which occurs mainly in immunosuppressed individuals; this mycosis can present a severe clinical picture with dissemination to various organs and is associated with an exacerbated inflammatory response and with anemia and pancytopenia if bone marrow is affected. So far, the effect of a possible interaction of Histoplasma with stem cells present in the bone marrow is unknown. OBJECTIVES: To examine, in vitro, the immunomodulatory effects of MSCs in response to H. capsulatum infection. METHODS: MSCs were obtained from bone marrow of C57BL/6 male mice; after isolation and purification, they were induced to mesodermal lineages and characterized by flow cytometry. Later, the basal expression of toll-like receptor (TLR)-2, TLR4, and Dectin-1 was determined using flow cytometry. MSCs were infected with H. capsulatum yeasts (isolate CIB 1980) in a multiplicity of infection (MOI) of 5 and incubated for 24 h. In addition, some of the co-cultures were previously treated with specific blocking antibodies for TLR2 and TLR4 or with a blocking peptide specific for Dectin-1 (CLEC7A). Furthermore, phagocytosis, microbicidal, and cell proliferation assays were done, and the expression of the genes encoding the cytokines IL-1β, IL-6, IL-10, IL-17, TNF-α, and TGF-β as well as of those for arginase-1 and iNOS were assessed. RESULTS: We observed that H. capsulatum has the capability to adhere and internalize within these MSCs; nonetheless, this process did not affect the survival of the fungus. The interaction of H. capsulatum with MSCs induced a slight but significantly increased expression of TLR2 but not TLR4 nor Dectin-1. In addition, this fungal interaction significantly induced an augmented expression of IL-6 and a decrease in the expression of IL-1β, IL-17, TNF-α, TGF-β, as well as the immune mediators Arg-1 and iNOS. Interestingly, blockade of these receptors did not affect phagocytosis, but increased IL-1β, IL-17, and TNF-α expression and reduced the expression of IL-6. Noteworthy, H. capsulatum induced apoptosis and inhibited the proliferation of these stem cells; furthermore, this fungus significantly reduced the expression of genes related to adipogenic differentiation and increased the expression of genes related to the osteogenic differentiation process. CONCLUSIONS: The above results indicate that MSCs do not exert a notable antifungal effect against H. capsulatum; on the contrary, this fungal pathogen not only modulates the expression of inflammatory mediators in MSCs, by a mechanism dependent on TLR2, TLR4, and Dectin-1, but also affects their viability and their ability to differentiate into a different type of specialized cells. These events could, in principle, affect both hematopoiesis and the immune response in the infected host, and in addition, these stem cells may provide a niche for this fungus, allowing it to persist and evade host immunity.