Functional phenotype and role of resident and recruited bone marrow derived exudate macrophages in influenza virus-induced lung injury and repair

Abstract

Background: Alveolar macrophages (AM) play an important role in host defense and tissue homeostasis. Even though macrophage polarization has been extensively studied in different disease models, the generation of macrophage phenotypes with specific functional profiles, their lineage relation and particular roles during pathogen-induced acute lung injury (ALI), resolution and tissue repair has not been convincingly elucidated. Methods & Materials: C57BL/6 mice were infected with influenza virus (IV) PR8 by intra-tracheal (i.t) application. BAL and lungs were harvested during the acute and resolution and repair phases of infection. The polarization profiles of different AM subsets were characterized by FACS during the course of infection. Bone marrow transplantation (BMT) experiments using CD45.2/1 mice were performed to demonstrate lineage-relation between recruited and resident macrophages. Adoptive transfer of BMM with diverse polarization patterns (isolated from IV-challenged wild type mice) into the lungs of IV-infected CCR2-/- mice (lacking BMM recruitment) was performed to address their functional phenotypes during injury, resolution and lung repair. Gene expression profiles in different BMM phenotypes were screened by genome wide transcriptome analysis. Results: Our flow cytometry analyses demonstrated that alveolar and interstitial BMM show an M1 phenotype in the acute phase and shift to an M2 phenotype in the late phase of infection. BMT experiments revealed that M2 BMM substantially contributed to replenishment of the depleted resident alveolar macrophage (rAM) pool, indicating a high functional plasticity of BMM recruited after infection. In addition, M1 and M2BMM adoptive transfer experiments showed that intra-tracheal transferred M1BMM increased alveolar barrier dysfunction whereas M2BMM preserved the rAM pool and induced alveolar epithelial cell proliferation and barrier repair. Further, our genome-wide transcriptome analysis on flow-sorted M1/M2BMM showed significant up-regulation of a distinct set of growth factors, repair mediators and pro-survival genes in M2 when compared with M1BMM, some of which were found to be central mediators of the beneficial functions of M2BMM in vivo. Conclusion: These data support that mediators produced by M2BMM contribute to replenishment and preservation of the rAM pool and improved lung barrier function. In summary, our data demonstrate high functional plasticity of BMM during IV pneumonia and highlight these cells as targets for therapeutic approaches.