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 have poor antifungal activity, 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. This eosinophilia 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. Author SummaryCryptococcus gattii is an environmental fungus that can cause severe lung and brain infections after inhalation through the respiratory tract. C. gattii causes disease in patients with known immune deficits but also in individuals that are apparently healthy. Studies on otherwise healthy people who become infected with C. gattii suggest that they may have a previously unrecognized problem involving granulocyte macrophage-colony stimulating factor (GM-CSF), a cytokine, or messenger protein, that is an important part of the immune system. Here, we investigate the role of GM-CSF in the immune response to C. gattii using a mouse model of infection. We find that C. gattii increases GM-CSF in the lungs, leading to the influx of immune cells, including monocyte-derived macrophages and eosinophils, while inhibiting the entry of neutrophils. The macrophages and eosinophils allow the fungus to proliferate and cause inflammatory damage to the lungs, which is ultimately fatal. The absence of neutrophils also contributes to fungal growth, as these immune cells would otherwise be able to help kill the fungus. Our study provides new insight into how GM-CSF regulates immunity to C. gattii and has important implications as to the mechanisms that govern susceptibility to this infection.