Coordinated chemokine expression defines macrophage subsets across tissues.

Lung-resident macrophages, which include alveolar macrophages and interstitial macrophages (IMs), exhibit a high degree of diversity, generally attributed to different activation states, and often complicated by the influx of monocytes into the pool of tissue-resident macrophages. To gain a deeper insight into the functional diversity of IMs, here we perform comprehensive transcriptional profiling of resident IMs and reveal ten distinct chemokine-expressing IM subsets at steady state and during inflammation. Similar IM subsets that exhibited coordinated chemokine signatures and differentially expressed genes were observed across various tissues and species, indicating conserved specialized functional roles. Other macrophage types shared specific IM chemokine profiles, while also presenting their own unique chemokine signatures. Depletion of CD206hi IMs in Pf4creR26EYFP+DTR and Pf4creR26EYFPCx3cr1DTR mice led to diminished inflammatory cell recruitment, reduced tertiary lymphoid structure formation and fewer germinal center B cells in models of allergen- and infection-driven inflammation. These observations highlight the specialized roles of IMs, defined by their coordinated chemokine production, in regulating immune cell influx and organizing tertiary lymphoid tissue architecture.

Human serous cavity macrophages and dendritic cells possess counterparts in the mouse with a distinct distribution between species.

In mouse peritoneal and other serous cavities, the transcription factor GATA6 drives the identity of the major cavity resident population of macrophages, with a smaller subset of cavity-resident macrophages dependent on the transcription factor IRF4. Here we showed that GATA6+ macrophages in the human peritoneum were rare, regardless of age. Instead, more human peritoneal macrophages aligned with mouse CD206+ LYVE1+ cavity macrophages that represent a differentiation stage just preceding expression of GATA6. A low abundance of CD206+ macrophages was retained in C57BL/6J mice fed a high-fat diet and in wild-captured mice, suggesting that differences between serous cavity-resident macrophages in humans and mice were not environmental. IRF4-dependent mouse serous cavity macrophages aligned closely with human CD1c+CD14+CD64+ peritoneal cells, which, in turn, resembled human peritoneal CD1c+CD14-CD64- cDC2. Thus, major populations of serous cavity-resident mononuclear phagocytes in humans and mice shared common features, but the proportions of different macrophage differentiation stages greatly differ between the two species, and dendritic cell (DC2)-like cells were especially prominent in humans.

Macrophages show up in style when Th2 lymphocytes organize their homecoming.

Monocytes can differentiate into tissue-resident pleural macrophages, but the mechanisms underlying this process are not yet fully understood. In this issue of Immunity, Finlay et al.1 show that Th2 cytokines promote this differentiation in resistant mice infected with Litomosoides sigmodontis.

Biology of lung macrophages in health and disease.

Tissue-resident alveolar and interstitial macrophages and recruited macrophages are critical players in innate immunity and maintenance of lung homeostasis. Until recently, assessing the differential functional contributions of tissue-resident versus recruited macrophages has been challenging because they share overlapping cell surface markers, making it difficult to separate them using conventional methods. This review describes how scRNA-seq and spatial transcriptomics can separate these subpopulations and help unravel the complexity of macrophage biology in homeostasis and disease. First, we provide a guide to identifying and distinguishing lung macrophages from other mononuclear phagocytes in humans and mice. Second, we outline emerging concepts related to the development and function of the various lung macrophages in the alveolar, perivascular, and interstitial niches. Finally, we describe how different tissue states profoundly alter their functions, including acute and chronic lung disease, cancer, and aging.

Immunogenicity Threshold in Allogeneic Cells Impacts CTL Response to Nondominant Congenic Antigens.

Transplantation and cancer expose the immune system to neoantigens, including immunogenic (dominant and subdominant) and nonimmunogenic Ags with varying quantities and affinities of immunodominant peptides. Conceptually, immunity is believed to mainly target dominant Ags when subdominant or nondominant Ags are linked within the same cell due to T cell interference. This phenomenon is called immunodominance. However, our previous study in mice showed that linked nonimmunogenic Ags (OVA and GFP) containing immunodominant peptides mount immunity irrespective of the MHC-matched allogeneic cell's immunogenicity. Consequently, we further explored 1) under what circumstances does the congenic marker CD45.1 provoke immunity in CD45.2 mice, and 2) whether linking two dominant or subdominant Ags can instigate an immune response. Our observations showed that CD45.1 (or CD45.2), when connected to low-immunogenic cell types is presented as an immunogen, which contrasts with its outcome when linked to high-immunogenic cell types. Moreover, we found that both dominant and subdominant Ags are presented as immunogens when linked in environments with lower immunogenic thresholds. These findings challenge the existing perception that immunity is predominantly elicited against dominant Ags when linked to subdominant or nondominant Ags. This study takes a fundamental step toward understanding the nuanced relationship between immunogenic and nonimmunogenic Ags, potentially opening new avenues for comprehending cancer immunoediting and enhancing the conversion of cold tumors with low immunogenicity into responsive hot tumors.

Natural Antibodies Alert the Adaptive Immune System of the Presence of Transformed Cells in Early Tumorigenesis.

Recent studies have revealed a critical role for natural Abs (NAbs) in antitumor immune responses. However, the role of NAbs in cancer immunosurveillance remains unexplored, mainly because of the lack of in vivo models that mimic the early recognition and elimination of transforming cells. In this article, we propose a role for NAbs in alerting the immune system against precancerous neoantigen-expressing cells immediately after they escape intrinsic tumor suppression mechanisms. We identify four distinct reproducible, trackable, MHC-matched neoantigen-expressing cell models that do not form tumors as the end point. This amplified readout in the critical window prior to tumor formation allows investigation of new mediators of cancer immunosurveillance. We found that neoantigen-expressing cells adoptively transferred in NAb-deficient mice persisted, whereas they were eliminated in wild-type mice, indicating that the circulating NAb repertoire alerts the immune system to the presence of transformed cells. Moreover, immunity is mounted against immunogenic and nonimmunogenic neoantigens contained in the NAb-tagged cells, regardless of whether the NAb directly recognizes the neoantigens. Beyond these neoantigen-expressing model systems, we observed a significantly greater tumor burden in chemically and virally induced tumor models in NAb-deficient mice compared with wild-type mice. Restoration of the NAb repertoire in NAb-deficient mice elicited the recognition and elimination of neoantigen-expressing cells and cancer. These data show that NAbs are required and sufficient for elimination of transformed cells early in tumorigenesis. These models can now be used to investigate how NAbs stimulate immunity via recognition receptors to eliminate precancerous cells.

Advancing Lung Immunology Research: An Official American Thoracic Society Workshop Report.

The mammalian airways and lungs are exposed to a myriad of inhaled particulate matter, allergens, and pathogens. The immune system plays an essential role in protecting the host from respiratory pathogens, but a dysregulated immune response during respiratory infection can impair pathogen clearance and lead to immunopathology. Furthermore, inappropriate immunity to inhaled antigens can lead to pulmonary diseases. A complex network of epithelial, neural, stromal, and immune cells has evolved to sense and respond to inhaled antigens, including the decision to promote tolerance versus a rapid, robust, and targeted immune response. Although there has been great progress in understanding the mechanisms governing immunity to respiratory pathogens and aeroantigens, we are only beginning to develop an integrated understanding of the cellular networks governing tissue immunity within the lungs and how it changes after inflammation and over the human life course. An integrated model of airway and lung immunity will be necessary to improve mucosal vaccine design as well as prevent and treat acute and chronic inflammatory pulmonary diseases. Given the importance of immunology in pulmonary research, the American Thoracic Society convened a working group to highlight central areas of investigation to advance the science of lung immunology and improve human health.

Localization of Macrophages in the Human Lung via Design-based Stereology.

Rationale: Interstitial macrophages (IMs) and airspace macrophages (AMs) play critical roles in lung homeostasis and host defense, and are central to the pathogenesis of a number of lung diseases. However, the absolute numbers of macrophages and the precise anatomic locations they occupy in the healthy human lung have not been quantified.Objectives: To determine the precise number and anatomic location of human pulmonary macrophages in nondiseased lungs and to quantify how this is altered in chronic cigarette smokers.Methods: Whole right upper lobes from 12 human donors without pulmonary disease (6 smokers and 6 nonsmokers) were evaluated using design-based stereology. CD206 (cluster of differentiation 206)-positive/CD43+ AMs and CD206+/CD43- IMs were counted in five distinct anatomical locations using the optical disector probe.Measurements and Main Results: An average of 2.1 × 109 IMs and 1.4 × 109 AMs were estimated per right upper lobe. Of the AMs, 95% were contained in diffusing airspaces and 5% in airways. Of the IMs, 78% were located within the alveolar septa, 14% around small vessels, and 7% around the airways. The local density of IMs was greater in the alveolar septa than in the connective tissue surrounding the airways or vessels. The total number and density of IMs was 36% to 56% greater in the lungs of cigarette smokers versus nonsmokers.Conclusions: The precise locations occupied by pulmonary macrophages were defined in nondiseased human lungs from smokers and nonsmokers. IM density was greatest in the alveolar septa. Lungs from chronic smokers had increased IM numbers and overall density, supporting a role for IMs in smoking-related disease.