The Dual Benefit of Sulfasalazine on Pneumocystis Pneumonia-Related Immunopathogenesis and Antifungal Host Defense Does Not Require IL-4R alpha-Dependent Macrophage Polarization

Pneumocystis is a respiratory fungal pathogen that is among the most frequent causes of life-threatening pneumonia (PcP) in immunocompromised hosts. Alveolar macrophages play an important role in host defense against Pneumocystis, and several studies have suggested that M2 polarized macrophages have anti-Pneumocystis effector activity. Our prior work found that the immunomodulatory drug sulfasalazine (SSZ) provides a dual benefit during PcP-related immune reconstitution inflammatory syndrome (IRIS) by concurrently suppressing immunopathogenesis while also accelerating macrophage-mediated fungal clearance. The benefits of SSZ were associated with heightened Th2 cytokine production and M2 macrophage polarization. Therefore, to determine whether SSZ improves the outcome of PcP through a mechanism that requires Th2-dependent M2 polarization, RAG2(-/-) mice lacking interleukin 4 receptor alpha chain (IL-4R alpha) on macrophage lineage cells were generated. As expected, SSZ treatment dramatically reduced the severity of PcP-related immunopathogenesis and accelerated fungal clearance in immune-reconstituted RAG2(-/-) mice. Similarly, SSZ treatment was also highly effective in immune-reconstituted RAG2/IL-4R alpha(-/-) and RAG2/gamma interferon receptor (IFN-gamma R)(-/-) mice, demonstrating that neither IL-4R alpha-dependent M2 nor IFN-gamma R-dependent M1 macrophage polarization programs were required for the beneficial effects of SSZ. Despite the fact that macrophages from RAG2/IL-4R alpha(-/-) mice could not respond to the Th2 cytokines IL-4 and IL-13, M2-biased alveolar macrophages were identified in the lungs following SSZ treatment. These data demonstrate that not only does SSZ enhance phagocytosis and fungal clearance in the absence of macrophage IL-4R alpha signaling, but also that SSZ promotes M2 macrophage polarization in an IL-4R alpha-independent manner. These findings could have implications for the treatment of PcP and other diseases in which M2 polarization is beneficial. Pneumocystis is a respiratory fungal pathogen that is among the most frequent causes of life-threatening pneumonia (PcP) in immunocompromised hosts. Alveolar macrophages play an important role in host defense against Pneumocystis, and several studies have suggested that M2 polarized macrophages have anti-Pneumocystis effector activity.