Complementing macrophage functions
Alveolar macrophages play an important role in the defense against a wide range of respiratory pathogens. Xu-Vanpala et al. have characterized the roles of alveolar macrophages in response to pulmonary fungal infections in mice. By studying antifungal immune responses in Cxcl2-gfp reporter mice, the authors found CXCL2+ alveolar macrophages to be pro-inflammatory, whereas CXCL2-negative alveolar macrophages produced immunoregulatory molecules, including interleukin-10 and complement component 1q (C1q). Although these two populations had distinct metabolic, epigenetic, and transcriptional profiles, these cell populations were plastic. By using C1q-deficient mice, the authors report that C1q plays an important role in dampening antifungal immune responses in the lung. The studies highlight a previously unappreciated link between tissue macrophages and the complement system.
Alveolar macrophages (AMs) are the major lung-resident macrophages and have contradictory functions. AMs maintain tolerance and tissue homeostasis, but they also initiate strong inflammatory responses. However, such opposing roles within the AM population were not known to be simultaneously generated and coexist. Here, we uncovered heterogeneous AM subpopulations generated in response to two distinct pulmonary fungal infections, Cryptococcus neoformans and Aspergillus fumigatus. Some AMs are bona fide sentinel cells that produce chemoattractant CXCL2, which also serves as a marker for AM heterogeneity, in the context of pulmonary fungal infections. However, other AMs do not produce CXCL2 and other pro-inflammatory molecules. Instead, they highly produce anti-inflammatory molecules, including interleukin-10 (IL-10) and complement component 1q (C1q). These two AM subpopulations have distinct metabolic profiles and phagocytic capacities. We report that polarization of pro-inflammatory and anti-inflammatory AM subpopulations is regulated at both epigenetic and transcriptional levels and that these AM subpopulations are generally highly plastic. Our studies have uncovered the role of C1q expression in programming and sustaining anti-inflammatory AMs. Our finding of the AM heterogeneity upon fungal infections suggests a possible pharmacological intervention target to treat fungal infections by tipping the balance of AM subpopulations.
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