Identification of a nerve-associated, lung-resident interstitial macrophage subset with distinct localization and immunoregulatory properties

Abstract

Tissue-resident macrophages are a diverse population of cells that perform specialized functions including sustaining tissue homeostasis and tissue surveillance. Here, we report an interstitial subset of CD169+ lung-resident macrophages that are transcriptionally and developmentally distinct from alveolar macrophages (AMs). They are primarily localized around the airways and are found in close proximity to the sympathetic nerves in the bronchovascular bundle. These nerve- and airway-associated macrophages (NAMs) are tissue resident, yolk sac derived, self-renewing, and do not require CCR2+ monocytes for development or maintenance. Unlike AMs, the development of NAMs requires CSF1 but not GM-CSF. Bulk population and single-cell transcriptome analysis indicated that NAMs are distinct from other lung-resident macrophage subsets and highly express immunoregulatory genes under steady-state and inflammatory conditions. NAMs proliferated robustly after influenza infection and activation with the TLR3 ligand poly(I:C), and in their absence, the inflammatory response was augmented, resulting in excessive production of inflammatory cytokines and innate immune cell infiltration. Overall, our study provides insights into a distinct subset of airway-associated pulmonary macrophages that function to maintain immune and tissue homeostasis.

Airway-associated macrophages in homeostasis and repair

“…By combining the Arginase reporter allele with a Ccr2-based lineage tracing strategy, we provide evidence that some of the M2-like macrophages come from a Ccr2-expressing precursor. Similar to previous reports, we also observed Cx3cr1-expressing monocytes in homeostatic tracheas and in response to injury (Joshi et al, 2020;Ural et al, 2020).…”

“…Clusters 0, 1 and 3 were from the tracheal mesenchyme and could be identified as classic macrophages (cluster 0), monocytes (cluster 1) and cells with an intermittent signature (cluster 3) ( Figure 3E-G). These three populations are similar to previously identified IMs from the lung, due to their expression of Mertk, Fcgr1 and Cd68, combined with their negativity for SiglecF and Marco and variable expression of H2-ab1, H2-aa and Retnla (Gibbings et al, 2017;Ural et al, 2020). A similar signature was also observed for Cluster 8 that originated from the lung, presumably representing the captured lung IMs.…”

“…Tracheal macrophage dynamics during injury and repair of the tracheal epithelium Following infection, IMs expand through local proliferation and modulate lung inflammation (Kawano et al, 2016;Ural et al, 2020). We used a model of polidocanolinduced injury to determine how the tracheal macrophage populations respond to noninfectious injury of the airway epithelium (Borthwick et al, 2001).…”

“…This result presumably reflects the local proliferation and/or recruitment of CCR2-negative leukocytes, including neutrophils, lymphocytes and other monocyte subpopulations ( Figure 1 and (Joshi et al, 2020;Ural et al, 2020)). Consistent with our transcription and flow cytometry data, both the total number and proportion of YARG + cells were increased 3dpP compared to controls.…”

“…Importantly, the transcription profile of these cells was distinct from those reported for IMs (Gibbings et al, 2017), and could not be binned into a classical category. IAMs were also negative for the marker Cd169 , recently associated with a subpopulation of macrophages localized in close proximity to airway neurons (Ural et al, 2020). Cluster 5 expressed genes associated with neutrophils, and cluster 9 expressed markers of lymphocytes.…”