Signaling through Syk or CARD9 Mediates Species-Specific Anti- Protection in Bone Marrow Chimeric Mice.

The spleen tyrosine kinase (Syk) and the downstream adaptor protein CARD9 are crucial signaling molecules in antimicrobial immunity. Candida parapsilosis is an emerging fungal pathogen with a high incidence in neonates, while Candida albicans is the most common agent of candidiasis. While signaling through Syk/CARD9 promotes protective host mechanisms in response to C. albicans, its function in immunity against C. parapsilosis remains unclear. Here, we generated Syk and CARD9 bone marrow chimeric mice to study the role of Syk/CARD9 signaling in immune responses to C. parapsilosis compared to C. albicans. We demonstrate various functions of this pathway (e.g., phagocytosis, phagosome acidification, and killing) in -challenged, bone marrow-derived macrophages with differential involvement of Syk and CARD9 along with species-specific differences in cytokine production. We report that Syk or CARD9 chimeras rapidly display high susceptibility to C. albicans, while C. parapsilosis infection exacerbates over a prolonged period in these animals. Thus, our results establish that Syk and CARD9 contribute to systemic resistance to C. parapsilosis and C. albicans differently. Additionally, we confirm prior studies but also detail new insights into the fundamental roles of both proteins in immunity against C. albicans. Our data further suggest that Syk has a more prominent influence on anti- immunity than CARD9. Therefore, this study reinforces the Syk/CARD9 pathway as a potential target for anti- immune therapy. While C. albicans remains the most clinically significant species, C. parapsilosis is an emerging pathogen with increased affinity to neonates. Syk/CARD9 signaling is crucial in immunity to C. albicans, but its role in responses to other pathogenic species is largely unexplored. We used mice with hematopoietic systems deficient in Syk or CARD9 to comparatively study the function of these proteins in anti- immunity. We demonstrate that Syk/CARD9 signaling has a protective role against C. parapsilosis differently than against C. albicans. Thus, this study is the first to reveal that Syk can exert immune responses during systemic infections species specifically. Additionally, Syk-dependent immunity to a nonalbicans species in an murine model has not been reported previously. We highlight that the contribution of Syk and CARD9 to fungal infections are not identical and underline this pathway as a promising immune-therapeutic target to fight infections.