Glucocorticoids suppress NF-κB-mediated neutrophil control of Aspergillus fumigatus hyphal growth

Glucocorticoids are a major class of therapeutic anti-inflammatory and immunosuppressive drugs prescribed to patients with inflammatory diseases, to avoid transplant rejection, and as part of cancer chemotherapy. However, exposure to these drugs increases the risk of opportunistic infections such as with the fungus Aspergillus fumigatus. Prolonged glucocorticoid therapy is one of the main risks for invasive aspergillosis, which causes mortality in >50% of infected patients. The mechanisms by which glucocorticoids increase susceptibility to A. fumigatus are poorly understood. Here, we used a zebrafish larva-Aspergillus infection model to identify innate immune mechanisms altered by glucocorticoid treatment. Infected larvae exposed to dexamethasone succumb to the infection at a significantly higher rate than control larvae. However, both macrophages and neutrophils are still recruited to the site of infection and dexamethasone treatment does not significantly affect fungal spore killing. Instead, the primary effect of dexamethasone manifests later in infection with treated larvae exhibiting increased invasive hyphal growth. In line with this, dexamethasone predominantly inhibits neutrophil function, rather than macrophage function. Dexamethasone-induced mortality also depends on the glucocorticoid receptor. One pathway that glucocorticoids can inhibit is NF-{kappa}B activation and we report that dexamethasone partially suppresses NF-{kappa}B activation at the infection site by inducing the transcription of I{kappa}B via the glucocorticoid receptor. Independent CRISPR/Cas9 targeting of IKK{gamma} to prevent NF-{kappa}B activation also increases invasive A. fumigatus growth and larval mortality. However, dexamethasone treatment of IKK{gamma} crispant larvae further increases invasive hyphal growth, suggesting that dexamethasone may suppress other pathways in addition to NF-{kappa}B to promote host susceptibility. Collectively, we find that dexamethasone acts through the glucocorticoid receptor to suppress NF-{kappa}B-mediated neutrophil control of A. fumigatus hyphae in zebrafish larvae. Author SummaryGlucocorticoids are drugs that stop inflammation and suppress the immune system. Glucocorticoids are effective in treating inflammatory diseases such as asthma and arthritis, preventing organ rejection after transplant surgery, and in ameliorating the side effects of cancer chemotherapy. However, as these drugs suppress the immune system, patients taking glucocorticoids are more prone to infections such as with the environmental fungus Aspergillus fumigatus. The specific mechanisms that glucocorticoids inhibit to increase susceptibility to infection are largely unknown. Here, we used a larval zebrafish model of A. fumigatus infection to determine that glucocorticoids mainly suppress the ability of neutrophils to control the fungal hyphal growth that causes tissue damage. Our study provides insight into future strategies to treat A. fumigatus infection in patients undergoing glucocorticoid therapy.