Type II cytokines impair host defense against an intracellular fungal pathogen by amplifying macrophage generation of IL-33.

Interleukin (IL)-4 subverts protective immunity to multiple intracellular pathogens, including the fungus Histoplasma capsulatum. Previously, we reported that H. capsulatum-challenged CCR2(-/-) mice manifest elevated pulmonary fungal burden owing to exaggerated IL-4. Paradoxical to our anticipation in IL-33 driving IL-4, we discovered that the latter prompted IL-33 in mutant mice. In infected CCR2(-/-) animals, amplified IL-33 succeeded the heightened IL-4 response and inhibition of IL-4 signaling decreased IL-33. Moreover, macrophages, but not epithelial cells or dendritic cells, from these mice expressed higher IL-33 in comparison with controls. Dissection of mechanisms that promulgated IL-33 revealed type-II cytokines and H. capsulatum synergistically elicited an IL-33 response in macrophages via signal transducer and activator of transcription factor 6/interferon-regulatory factor-4 and Dectin-1 pathways, respectively. Neutralizing IL-33 in CCR2(-/-) animals, but not controls, enhanced their resistance to histoplasmosis. Thus we describe a previously unrecognized role for IL-4 in instigating IL-33 in macrophages. Furthermore, in the presence of intracellular fungal pathogens, the type-II cytokine-driven IL-33 response impairs immunity.