IL-33 and M2a Alveolar Macrophages Promote Lung Defense against the Atypical Fungal Pathogen Pneumocystis murina

Abstract

We have recently reported that mice deficient in the myeloid Src-family tyrosine kinases Hck, Fgr, and Lyn (Src triple knockout [TKO]) had augmented innate lung clearance of Pneumocystis murina that correlated with a higher ability of alveolar macrophages (AMs) from these mice to kill P. murina. In this article, we show that despite possessing enhanced killing, AMs from naive Src TKO mice did not demonstrate enhanced inflammatory responses to P. murina. We subsequently discovered that both AMs and lungs from P. murina-infected Src TKO mice expressed significantly greater levels of the M2a markers RELM-α and Arg1, and the M2a-associated chemokines CCL17 and CCL22 than did wild-type mice. IL-4 and IL-13, the primary cytokines that promote M2a polarization, were not differentially produced in the lungs between wild-type and Src TKO mice. P. murina infection in Src TKO mice resulted in enhanced lung production of the novel IL-1 family cytokine IL-33. Immunohistochemical analysis of IL-33 in lung tissue revealed localization predominantly in the nucleus of alveolar epithelial cells. We further demonstrate that experimental polarization of naive AMs to M2a resulted in more efficient killing of P. murina compared with untreated AMs, which was further enhanced by the addition of IL-33. Administration of IL-33 to C57BL/6 mice increased lung RELM-α and CCL17 levels, and enhanced clearance of P. murina, despite having no effect on the cellular composition of the lungs. Collectively, these results indicate that M2a AMs are potent effector cells against P. murina. Furthermore, enhancing M2a polarization may be an adjunctive therapy for the treatment of Pneumocystis.

Neither Classical nor Alternative Macrophage Activation Is Required for Pneumocystis Clearance during Immune Reconstitution Inflammatory Syndrome

“…In summary, while prior work suggests that an enhancement of M2 polarization accelerates Pneumocystis clearance and promotes disease resolution (18,22), our present findings demonstrate that restricting the ability of macrophages to polarize to either an M1 or M2 phenotype does not affect the ability of mice to clear Pneumocystis infection. Instead, the loss of IFN-␥R signaling exacerbates Pneumocystis immunopathogenesis, which is likely related to the impaired recruitment of CD8 ϩ T cell suppressor cells to the lung.…”

“…Importantly, the beneficial effects of SSZ on inflammation and fungal clearance were associated with its ability to promote an alternatively activated alveolar macrophage phenotype (18). The protective role of Th2 responses and alternatively activated macrophages against Pneumocystis infection has also been demonstrated in previous studies (22,33). While these data suggest that alternatively activated M2a macrophages possess anti-Pneumocystis effector function, the requirement for either classical or alternatively activated macrophages for the clearance of Pneumocystis during IRIS has not been determined.…”

“…The beneficial effects of SSZ were associated with an increase in the abundance of macrophages with the M2 phenotype and enhanced AM phagocytosis of Pneumocystis (18). Other groups have also found that Th2 and M2 macrophages are effector cells that mediate immunity to Pneumocystis infection (22).…”

Differential Macrophage Polarization from Pneumocystis in Immunocompetent and Immunosuppressed Hosts: Potential Adjunctive Therapy during Pneumonia

“…An additional study had shown that alternatively activated M2 macrophages are potent effector cells against Pneumocystis murina and suggests that enhancing M2a polarization may be an adjunctive therapy for the treatment of Pneumocystis infections (33). Parallel to our observation, these investigators observed that alveolar macrophages from P. murinainfected Src TKO mice expressed significantly greater levels of the M2a markers and possessed enhanced organism clearance activity and yet did not demonstrate enhanced inflammatory responses to P. murina (33). In addition, we observed that the lung microenvironment in immunocompetent animals that rapidly clear P. carinii organisms favors M2 polarization with increasing amounts of M2-driving cytokines.…”

“…Macrophages offer a novel approach to cell therapy in the treatment of infection, especially in the immunosuppressed hosts, but require additional investigation (26)(27)(28)(29)(30)(31). There are relatively few reports concerning macrophage polarization and the resulting antifungal and anti-inflammatory activity as a potential treatment approach for fungal infections (12,32,33).…”