GM-CSF orchestrates monocyte and granulocyte responses to Cryptococcus gattii

Cryptococcus gattii is an emerging fungal pathogen that is acquired through the respiratory tract and causes invasive infections in both immunocompromised and otherwise healthy people. Many of these apparently immunocompetent patients are subsequently found to have autoantibodies against the pleiotropic cytokine GM-CSF. In this study, we investigated the potential role of GM-CSF (or CSF2) in the host response to C. gattii using a murine model of infection. Interestingly, infected Csf2-/- mice were found to have significantly improved survival and decreased lung fungal burden compared to wild type (WT) mice. We determined that during C. gattii infection, GM-CSF promotes the differentiation of monocytes into alveolar and interstitial macrophages. When these macrophages are ablated in CCR2-DTR+ mice, there is a corresponding improvement in survival with decreased lung fungal burden, thus phenocopying Csf2-/- mice. WT bone-marrow derived macrophages challenged with C. gattii and interstitial and alveolar macrophages from infected WT mice are unable to undergo M1 polarization, suggesting that monocyte-derived macrophages (moMacs) are rendered permissive for fungal proliferation. Therefore, GM-CSF and moMacs mediate immune responses that are harmful to the host. We further found that GM-CSF and moMacs preferentially promote the influx of eosinophils over neutrophils into the infected lung which is associated with substantial inflammatory lung pathology. Ablation of neutrophils using Mrp8cretg iDTR+ mice significantly increased C. gattii burden in the lungs, indicating that GM-CSF and moMacs block the entry of these beneficial, fungal-clearing granulocytes during infection. Altogether, our results show that GM-CSF plays a key role in impeding host anti-fungal responses to C. gattii by coordinating monocyte-derived macrophages and granulocyte activity and crosstalk.