Differential roles of resident lung macrophages during control of murine alveolar and airway Mycobacterium abscessus infection

Introduction Mycobacterium abscessus ( Mabsc ), a nontuberculous mycobacterium (NTM), is readily cleared from healthy lungs but can cause infections in immunocompromised individuals and individuals with chronic airways diseases that disrupt mucociliary clearance, such as cystic fibrosis and bronchiectasis. In bronchiectatic airways, Mabsc can persist despite robust immune cell recruitment, raising the possibility that, in addition to impaired mucociliary clearance, local pulmonary immune defects contribute to NTM susceptibility. Since chronic infections result from failed eradication of acute infection, we sought to determine whether immune cell responses critical for control of acute Mabsc infection in healthy lungs remain relevant when Mabsc infection occurs in obstructed airways, rather than in the alveoli, as occurs in bronchiectasis. Methods Using an agar bead model of Mabsc infection that prolongs murine small airway infection, replicates factors associated with bronchiectasis, and mirrors pathology of human Mabsc lung disease, we tested the hypothesis that Mabsc infection in obstructed airways elicits a qualitatively different immune response that alveolar Mabsc infection. We compared myeloid cell responses in in the two infection models using flow cytometric analyses of lung cells, immunofluorescent imaging, and histopathologic evaluation of lung sections. Results During the first 2 weeks of the bead model of Mabsc infection, absolute abundance of neutrophils in the lungs was significantly higher and relative abundance of recruited macrophages was lower than in the alveolar Mabsc infection model. Additionally, most resident alveolar macrophages (CD11c + RAMs) in the bead model upregulated CD11b, a marker of inflammation, by 1-week post-infection and maintained high levels of CD11b expression at 3 weeks post infection despite infection occurring in airways, not alveoli. To understand the role of macrophage subsets in controlling Mabsc infection, clodronate liposomes were administered oropharyngeally to deplete RAMs. RAMs were essential for early control of alveolar Mabsc infection, but depletion of RAMs had no effect on control of Mabsc burden during bead infection. Conclusions These studies demonstrate that murine Mabsc airway infection induced by obstructing small airways with Mabsc embedded in agar beads generates an immune milieu distinct from that induced by Mabsc infection in alveoli, altering the importance of different macrophage populations for control of infection.