William J. Janssen

Influenza infections are common and result in substantial mortality, with secondary bacterial pneumonias contributing to worse outcomes. To prevent mortality associated with bacterial co-infections, antibiotics are often prescribed in the absence of microbiological data. Currently, there are conflicting society recommendations on antibiotic utilization in viral respiratory tract infections as we lack prospective studies to guide treatment. We developed an animal model to study the effects of antibiotic exposure during influenza infection. Our results suggest that amoxicillin started during influenza A infection increases mortality from influenza A and increases severity of secondary bacterial pneumonia, and this is more pronounced in males.

We found that MyD88 expression is decreased, that IRAK1 expression is increased, and that IRAK1 splicing is shifted in a proinflammatory direction, in lung immune cells in patients with ARDS. We also find that MyD88 expression levels may correlate with survival in patients with ARDS. Thus, changes in expression and splicing of these two genes offer potential novel prognostic and therapeutic targets for ARDS.

T-helper (Th)-17 lymphocytes are central mediators of adaptive type 17 immunity. Decreased type-17 signaling increases severity of infections in humans and mice. However, detrimental effects of excessive type 17 responses in autoimmune and other inflammatory diseases highlight a need for type-17 immune calibration to support beneficial host defense requirements. Mechanisms of type 17 calibration are poorly understood. A gut-lung axis has been proposed to coordinate homeostatic protection and acute host defense. Factors that acutely alter the gut microbiome are heterogeneous and include acute intestinal infections, non-infectious colitis, and medical treatments such as antibiotics. How changes in the gut microbiome affect lung immune tone during homeostasis and acute pulmonary infections are also poorly understood. Prior studies have shown that antibiotics reduce expression of IL-17-mediated host defense in the gut. Since gut microbial homeostasis influences Th17 cell numbers in both the intestine and remote tissues, we postulated that antibiotic treatment would result in gut dysbiosis and weakened type-17 host defense in the lungs. We found that amoxicillin induces significant dysbiosis that is long-lasting and that there is a long-term decrease in type-17 tone in the lungs. We also found that in mice lacking the gut mucin, Muc2, Th17 cells increased in the lungs following inflammatory challenge. These findings suggest that antibiotic-induced dysbiosis can decrease lung immune defenses for long periods of time after cessation of antibiotic treatment.

Respiratory surfaces are exposed daily to billions of inhaled particles that could cause injury to alveolar tissues where gas exchange occurs. Accordingly, robust defense is essential but must involve minimal physiologic disruption. Airspace macrophages (AMs) protect lung surfaces while also maintaining tissue integrity through non-inflammatory homeostatic responses in health. However, in chronic obstructive pulmonary disease (COPD), AMs contribute to emphysematous alveolar destruction through mechanisms that are poorly understood. We hypothesized that damaging effects of AMs in emphysema result from loss of mechanisms that normally restrain homeostatic AMs. We found that Siglec-F, a common marker of AMs, exerts suppressive effects on tissue resident AMs (RAMs), thereby supporting alveolar integrity in mouse lungs during health and in a model of elastase-induced alveolar destruction. Siglec-F-deficient mice exhibited decreased alveolar numbers at baseline, and they had worsened alveolar damage after elastase challenge that was unexpectedly RAM-mediated. Transcriptomic profiling revealed dysregulation of key pathways involved in tissue remodeling and repair, including extracellular matrix degradation, TGF-β signaling, and phagocytosis. These findings uncover previously unrecognized roles for RAMs and Siglec-F in preserving alveolar integrity. Our findings implicate RAMs and Siglecs as potential therapeutic targets for preserving alveolar integrity in health and for limiting alveolar damage in COPD.

Rationale: Transcriptionally-defined populations of interstitial macrophages (IMs) and airspace macrophages (AMs) have recently been identified in the human lung. However, the anatomic locations occupied by these populations (i.e. alveoli, pleura, airways, or arteries) have not been fully defined. Objectives: To determine the distribution of transcriptionally-defined human macrophages in the major anatomical lung structures and to identify alterations in their distribution and programming induced by cigarette smoking. Methods: Single-cell RNA sequencing was performed on lung tissue from eight human donors without pulmonary disease (four smokers and four nonsmokers). Microdissection was used to isolate distinct pulmonary anatomical structures from each lung: alveoli, pleura, airways, and arteries. Transcriptional profiles of subpopulations of interstitial macrophages (IMs) and alveolar macrophages (AMs) were analyzed based on their anatomical structure of origin and smoking status. Measurements and Main Results: Five major IM and five AM subpopulations in human lungs are identified. We demonstrate significant differences in the accumulation patterns of each macrophage subset within anatomical structures, though each subset was detected in each. Immunofluorescent microscopy confirmed anatomical structure-specific accumulation patterns of IMs. Conclusions: In this study, we highlight key differences in the accumulation of lung macrophage subpopulations in anatomical structures but find programming within macrophage subpopulations is largely conserved, regardless of structure of origin or smoking status. We also detect populations of inflammatory AMs and IMs which accumulate within the airways, but not the alveolar parenchyma, of human cigarette smokers. We introduce a novel three-tiered hierarchy nomenclature to distinguish transcriptionally defined human lung IM subsets as 1°) Monocyte-like vs Antigen Presenting, 2°) Quiescent vs Inflammatory, and 3°) FOLR2 ^high vs FOLR2 ^low . This study is the first to report the fractional accumulation of human lung macrophage subsets by lung anatomical structure. Summary: Lung anatomical structure-specific single cell RNA sequencing is introduced to identify and determine the local composition of human lung leukocytes, including 5 populations of human interstitial macrophages.

Macrophages are required for healthy repair of the lungs following injury, but they are also implicated in driving dysregulated repair with fibrosis. How these 2 distinct outcomes of lung injury are mediated by different macrophage subsets is unknown. To assess this, single-cell RNA-Seq was performed on lung macrophages isolated from mice treated with LPS or bleomycin. Macrophages were categorized based on anatomic location (airspace versus interstitium), developmental origin (embryonic versus recruited monocyte derived), time after inflammatory challenge, and injury model. Analysis of the integrated dataset revealed that macrophage subset clustering was driven by macrophage origin and tissue compartment rather than injury model. Gpnmb -expressing recruited macrophages that were enriched for genes typically associated with fibrosis were present in both injury models. Analogous GPNMB -expressing macrophages were identified in datasets from both fibrotic and nonfibrotic lung disease in humans. We conclude that this subset represents a conserved response to tissue injury and is not sufficient to drive fibrosis. Beyond this conserved response, we identified that recruited macrophages failed to gain resident-like programming during fibrotic repair. Overall, fibrotic versus nonfibrotic tissue repair is dictated by dynamic shifts in macrophage subset programming and persistence of recruited macrophages.

Shareable abstract Slamf7 is expressed by monocyte-derived macrophages recruited to the lungs during injury. Whole-body and macrophage-specific knockouts of Slamf7 had no effect on the degree of inflammation in three mouse models of acute lung injury.https://bit.ly/3KgTJg1