Tissue macrophages have a split personality: Resident tissue macrophages (RTMs) reside in various tissue-specific niches during development. They evince microenvironment-directed phenotypes that support host defense and tissue homeostasis. Chakarov et al. used single-cell RNA sequencing and fate-mapping of murine lung RTMs to interrogate RTM-subset heterogeneity, interrelationships, and ontogeny (see the Perspective by Mildner and Yona). In addition to alveolar macrophages, they identified two different interstitial macrophage populations. One population mostly abutted nerve fibers; the other population preferentially localized near blood vessels and appeared to support vessel integrity and inhibit inflammatory cell infiltration into tissues. Science , this issue p. eaau0964 ; see also p. 1154

Increases in eosinophil numbers are associated with infection and allergic diseases, including asthma, but there is also evidence that eosinophils contribute to homeostatic immune processes. In mice, the normal lung contains resident eosinophils (rEos), but their function has not been characterized. Here, we have reported that steady-state pulmonary rEos are IL-5–independent parenchymal Siglec-F^intCD62L⁺CD101^lo cells with a ring-shaped nucleus. During house dust mite–induced airway allergy, rEos features remained unchanged, and rEos were accompanied by recruited inflammatory eosinophils (iEos), which were defined as IL-5–dependent peribronchial Siglec-F^hiCD62L^–CD101^hi cells with a segmented nucleus. Gene expression analyses revealed a more regulatory profile for rEos than for iEos, and correspondingly, mice lacking lung rEos showed an increase in Th2 cell responses to inhaled allergens. Such elevation of Th2 responses was linked to the ability of rEos, but not iEos, to inhibit the maturation, and therefore the pro-Th2 function, of allergen-loaded DCs. Finally, we determined that the parenchymal rEos found in nonasthmatic human lungs (Siglec-8⁺CD62L⁺IL-3R^lo cells) were phenotypically distinct from the iEos isolated from the sputa of eosinophilic asthmatic patients (Siglec-8⁺CD62L^loIL-3R^hi cells), suggesting that our findings in mice are relevant to humans. In conclusion, our data define lung rEos as a distinct eosinophil subset with key homeostatic functions.

Mast cells (MCs) influence intercellular communication during inflammation by secreting cytoplasmic granules that contain diverse mediators. Here, we have demonstrated that MCs decode different activation stimuli into spatially and temporally distinct patterns of granule secretion. Certain signals, including substance P, the complement anaphylatoxins C3a and C5a, and endothelin 1, induced human MCs rapidly to secrete small and relatively spherical granule structures, a pattern consistent with the secretion of individual granules. Conversely, activating MCs with anti-IgE increased the time partition between signaling and secretion, which was associated with a period of sustained elevation of intracellular calcium and formation of larger and more heterogeneously shaped granule structures that underwent prolonged exteriorization. Pharmacological inhibition of IKK-β during IgE-dependent stimulation strongly reduced the time partition between signaling and secretion, inhibited SNAP23/STX4 complex formation, and switched the degranulation pattern into one that resembled degranulation induced by substance P. IgE-dependent and substance P–dependent activation in vivo also induced different patterns of mouse MC degranulation that were associated with distinct local and systemic pathophysiological responses. These findings show that cytoplasmic granule secretion from MCs that occurs in response to different activating stimuli can exhibit distinct dynamics and features that are associated with distinct patterns of MC-dependent inflammation.

Resident tissue macrophages (RTM) can fulfill various tasks during development, homeostasis, inflammation and repair. In the lung, non-alveolar RTM, called interstitial macrophages (IM), importantly contribute to tissue homeostasis but remain little characterized. Here we show, using single-cell RNA-sequencing (scRNA-seq), two phenotypically distinct subpopulations of long-lived monocyte-derived IM, i.e. CD206⁺ and CD206⁻IM, as well as a discrete population of extravasating CD64⁺CD16.2⁺ monocytes. CD206⁺ IM are peribronchial self-maintaining RTM that constitutively produce high levels of chemokines and immunosuppressive cytokines. Conversely, CD206⁻IM preferentially populate the alveolar interstitium and exhibit features of antigen-presenting cells. In addition, our data support that CD64⁺CD16.2⁺ monocytes arise from intravascular Ly-6C^lo patrolling monocytes that enter the tissue at steady-state to become putative precursors of CD206⁻IM. This study expands our knowledge about the complexity of lung IM and reveals an ontogenic pathway for one IM subset, an important step for elaborating future macrophage-targeted therapies.

Infection with SARS-CoV-2 is causing a deadly and pandemic disease called coronavirus disease–19 (COVID-19). While SARS-CoV-2–triggered hyperinflammatory tissue-damaging and immunothrombotic responses are thought to be major causes of respiratory failure and death, how they relate to lung immunopathological changes remains unclear. Neutrophil extracellular traps (NETs) can contribute to inflammation-associated lung damage, thrombosis, and fibrosis. However, whether NETs infiltrate particular compartments in severe COVID-19 lungs remains to be clarified. Here we analyzed postmortem lung specimens from four patients who succumbed to COVID-19 and four patients who died from a COVID-19–unrelated cause. We report the presence of NETs in the lungs of each COVID-19 patient. NETs were found in the airway compartment and neutrophil-rich inflammatory areas of the interstitium, while NET-prone primed neutrophils were present in arteriolar microthrombi. Our results support the hypothesis that NETs may represent drivers of severe pulmonary complications of COVID-19 and suggest that NET-targeting approaches could be considered for the treatment of uncontrolled tissue-damaging and thrombotic responses in COVID-19.

Targeting NETosis alleviates type-2 immunopathology induced by rhinovirus infection in a mouse model of airway hypersensitivity, and correlative data suggest that a similar mechanism may operate in human rhinovirus-exacerbated asthma. Respiratory viral infections represent the most common cause of allergic asthma exacerbations. Amplification of the type-2 immune response is strongly implicated in asthma exacerbation, but how virus infection boosts type-2 responses is poorly understood. We report a significant correlation between the release of host double-stranded DNA (dsDNA) following rhinovirus infection and the exacerbation of type-2 allergic inflammation in humans. In a mouse model of allergic airway hypersensitivity, we show that rhinovirus infection triggers dsDNA release associated with the formation of neutrophil extracellular traps (NETs), known as NETosis. We further demonstrate that inhibiting NETosis by blocking neutrophil elastase or by degrading NETs with DNase protects mice from type-2 immunopathology. Furthermore, the injection of mouse genomic DNA alone is sufficient to recapitulate many features of rhinovirus-induced type-2 immune responses and asthma pathology. Thus, NETosis and its associated extracellular dsDNA contribute to the pathogenesis and may represent potential therapeutic targets of rhinovirus-induced asthma exacerbations.