Induction of the nuclear receptor PPAR-γ by the cytokine GM-CSF is critical for the differentiation of fetal monocytes into alveolar macrophages

Lung tumor immunity: redirecting macrophages through infection-induced inflammation

Macrophages play a central role in maintaining tissue homeostasis and in surveillance against pathogens and disease. In the lung, they can adopt either proinflammatory or anti-inflammatory states depending on the nature of the stimulus. As the predominant immune cells in both the lung tumor microenvironment and in sites of lung infection, the functional plasticity of macrophages makes them key players in determining disease outcome. Accurately defining their inflammatory profiles offers an opportunity to reprogram infection-associated macrophages towards enhanced tumor-killing phenotypes. This review explores how acute inflammation can drive macrophage-mediated antitumor immunity and highlights key molecules and signaling pathways that may be leveraged to therapeutically modulate macrophage function.

Potentials of peroxisome proliferator-activated receptor (PPAR) α, β/δ, and γ: An in-depth and comprehensive review of their molecular mechanisms, cellular Signalling, immune responses and therapeutic implications in multiple diseases

Peroxisome proliferator-activated receptors (PPARs), ligand-activated transcription factors, have emerged as a key regulator of various biological processes, underscoring their relevance in the pathophysiology and treatment of numerous diseases. PPARs are primarily recognized for their critical role in lipid and glucose metabolism, which underpins their therapeutic applications in managing type 2 diabetes mellitus. Beyond metabolic disorders, they have gained attention for their involvement in immune modulation, making them potential targets for autoimmune-related inflammatory diseases. Furthermore, PPAR's ability to regulate proliferation, differentiation, and apoptosis has positioned them as promising candidates in oncology. Their anti-inflammatory and anti-fibrotic properties further highlight their potential in dermatological and cardiovascular conditions, where dysregulated inflammatory responses contribute to disease progression. Recent advancements have elucidated the molecular mechanisms of different PPAR isoforms, including their regulation of key signalling pathways such as NF-κB and MAPK, which are crucial in inflammation and cellular stress responses. Additionally, their interactions with co-factors and post-translational modifications further diversify their functional roles. The therapeutic potential of various PPAR agonists has been extensively explored, although challenges related to side effects and target specificity remain. This growing body of evidence underscores the significance of PPARs in understanding the molecular basis of diseases and advancing therapeutic interventions, paving way for targeted treatment approach across a wide spectrum of medical conditions. Here, we provide a comprehensive and detailed perspective of PPARs and their potential across different health conditions to advance our understanding, elucidate underlying mechanisms, and facilitate the development of potential treatment strategies.

IL-10 sensing by lung interstitial macrophages prevents bacterial dysbiosis-driven pulmonary inflammation and maintains immune homeostasis

Crosstalk between the immune system and the microbiome is critical for maintaining immune homeostasis. Here, we examined this communication and the impact of immune-suppressive IL-10 signaling on pulmonary homeostasis. We found that IL-10 sensing by interstitial macrophages (IMs) is required to prevent spontaneous lung inflammation. Loss of IL-10 signaling in IMs initiated an inflammatory cascade through the activation of classical monocytes and CD4+ T cell subsets, leading to chronic lung inflammation with age. Analyses of antibiotic-treated and germ-free mice established that lung inflammation in the animals lacking IL-10 signaling was triggered by commensal bacteria. 16S rRNA sequencing revealed Delftia acidovorans and Rhodococcus erythropolis as potential drivers of lung inflammation. Intranasal administration of these bacteria or transplantation of human fecal microbiota elicited lung inflammation in gnotobiotic Il10-deficient mice. These findings highlight that IL-10 sensing by IMs contributes to pulmonary homeostasis by preventing lung inflammation caused by commensal dysbiosis.

KLF family members control expression of genes required for tissue macrophage identities

Tissue-resident macrophages adopt distinct gene expression profiles and exhibit functional specialization based on their tissue of residence. Recent studies have begun to define the signals and transcription factors that induce these identities. Here we describe an unexpected and specific role for the broadly expressed transcription factor Krüppel-like factor 2 (KLF2) in the development of embryonically derived large cavity macrophages (LCMs) in the serous cavities. KLF2 not only directly regulates the transcription of genes previously shown to specify LCM identity, such as retinoic acid receptors and GATA6, but also is required for induction of many other transcripts that define the identity of these cells. Our results suggest that KLF4 may similarly regulate the identity of alveolar macrophages in the lung. These data demonstrate that broadly expressed transcription factors, such as group 2 KLFs, can play important roles in the specification of distinct identities of tissue-resident macrophages.

Cell type-specific efferocytosis determines functional plasticity of alveolar macrophages

Resolution of lung injuries is vital to maintain gas exchange, but there is an increased risk of secondary bacterial infections during this stage. Alveolar macrophages (AMs) are crucial to clear bacteria and control the resolution of inflammation, but environmental cues that switch functional phenotypes of AMs remain incompletely understood. Here, we found that AMs lack the capacity to mount an effective immune response against bacteria during resolution of inflammation. Neutrophil (PMN)-derived myeloperoxidase (MPO) fueled canonical glutaminolysis via the mitochondrial membrane transporter uncoupling protein-2 (UCP2), resulting in decreased mtROS-dependent killing of bacteria and secretion of pro-inflammatory cytokines. MPO-enhanced UCP2 expression inhibited mitochondrial hyperpolarization and boosted efferocytosis irrespective of the presence of bacterial pathogens. Conversely, efferocytosis of other cell types resulted in a distinct anti-inflammatory AM phenotype while maintaining antibacterial phenotypic plasticity. Overall, our findings indicate that the uptake of apoptotic PMNs or MPO switches AMs to prioritize resolution of inflammation over antibacterial responses, a feature that is conserved in murine extrapulmonary macrophages and human AMs.

Peroxisomes are critical for a unique metabolic demand and survival of alveolar macrophages

Tissue-resident macrophages (TRMs) populate throughout various tissues, and their homeostatic metabolism is heavily influenced by these microenvironments. Peroxisomes are organelles that contribute to lipid metabolism. However, the involvement of these organelles in the bioenergetics of TRMs remains undetermined. We conducted a developmental screen of TRMs using a conditional peroxisomal biogenesis factor 5 (Pex5) knockout mouse model that lacks functional peroxisomes in all immune cell subsets. Pulmonary alveolar macrophages (AMs) appeared as the only subset of TRMs that required functional peroxisomes for their development. Pex5 deficiency resulted in reduced AM survival due to increased sensitivity to lipotoxicity, in line with an excess accumulation of ceramides. The absence of peroxisomes had a significant effect on overall mitochondrial fitness and altered their metabolic program, allowing them to engage in glycolysis in addition to oxidative phosphorylation. Our results revealed that AMs have a unique metabolic regulation, where peroxisomes play a central role in their homeostatic development and maintenance.

Endothelial-driven TGFβ signaling supports lung interstitial macrophage development from monocytes

Lung interstitial macrophages (IMs) are monocyte-derived parenchymal macrophages whose tissue-supportive functions remain unclear. Despite progress in understanding lung IM diversity and transcriptional regulation, the signals driving their development from monocytes and their functional specification remain unknown. Here, we found that lung endothelial cell-derived Tgfβ1 triggered a core Tgfβ receptor-dependent IM signature in mouse bone marrow-derived monocytes. Myeloid-specific impairment of Tgfβ receptor signaling severely disrupted monocyte-to-IM development, leading to the accumulation of perivascular immature monocytes, reduced IM numbers, and a loss of IM-intrinsic identity, a phenomenon similarly observed in the absence of endothelial-specific Tgfβ1. Mice lacking the Tgfβ receptor in monocytes and IMs exhibited altered monocyte and IM niche occupancy and hallmarks of aging including impaired immunoregulation, hyperinflation, and fibrosis. Our work identifies a Tgfβ signaling-dependent endothelial-IM axis that shapes IM development and sustains lung integrity, providing foundations for IM-targeted interventions in aging and chronic inflammation.

The Interplay Between Innate Immunity and Nonimmune Cells in Lung Damage, Inflammation, and Repair

As the site of gas exchange, the lung is critical for organismal survival. It is also subject to continual environmental insults inflicted by pathogens, particles, and toxins. Sometimes, these insults result in structural damage and the initiation of an innate immune response. Operating in parallel, the immune response aims to eliminate the threat, while the repair process ensures continual physiological function of the lung. The inflammatory response and repair processes are thus inextricably linked in time and space but are often studied in isolation. Here, we review the interplay of innate immune cells and nonimmune cells during lung insult and repair. We highlight how cellular cross talk can fine-tune the circuitry of the immune response, how innate immune cells can facilitate or antagonize proper organ repair, and the prolonged changes to lung immunity and physiology that can result from acute immune responses and repair processes.

A novel ubiquitin-related genes-based signature demonstrated values in prognostic prediction, immune landscape sculpture and therapeutic options in laryngeal cancer

Background

Laryngeal cancer (LC) is characterized by high mortality and remains challenging in prognostic evaluation and treatment benefits. Ubiquitin-related genes (UbRGs) are widely involved in cancer initiation and progression, but their potential value in LC is unknown.

Methods

RNA-seq and clinical data of LC were obtained from TCGA and GEO. UbRGs that independently influenced the overall survival (OS) of LC patients were screened with differential expression, COX and LASSO regression analyses. A prognostic signature was then established and assessed for its predictive value, stability and applicability using Kaplan-Meier analysis and receiver operating characteristic curves. The nomogram was further generated in combination with the signature and clinical characteristics. Characterization of immune properties and prediction of drug sensitivity were investigated on the signature-based subgroups using a panel of in silico platforms. Verification of gene expression was conducted with Western blot, qRT-PCR and ELISA, ultimately.

Results

PPARG, LCK and LHX1 were identified and employed to construct the UbRGs-based prognostic signature, showing a strong ability to discriminate LC patients with distinct OS in TCGA-LC and GSE65858, and excellent applicability in most clinical conditions. The nomogram showed higher predictive value and net clinical benefit than traditional indicators. As evaluated, the low-risk group had a more activated immune function, higher infiltration of anti-cancer immune cells and stronger expression of immune-promoting cytokines than the high-risk group. Immune properties were also correlated with individual signature genes. PPARG and LHX1 were negatively correlated, whereas LCK positively correlated, with the immuno-promoting microenvironment. Additionally, chemotherapy would be more effective in high-risk patients, while immune checkpoint inhibitors would be more effective in low-risk patients. Finally, dysregulation of the signature genes was confirmed in LC cell lines by Western blot, and PPARG knockdown significantly reduced the expression of the immunosuppressive cytokines IL6, TGFB1, TGFB2 and VEGFC by qRT-PCR and ELISA.

Conclusion

We have developed a UbRGs-based signature for LC prognostic evaluation that is valuable in clinical application, indicative of the immune microenvironment and beneficial for individualized treatment guidance.

Tissue macrophages: origin, heterogenity, biological functions, diseases and therapeutic targets

Macrophages are immune cells belonging to the mononuclear phagocyte system. They play crucial roles in immune defense, surveillance, and homeostasis. This review systematically discusses the types of hematopoietic progenitors that give rise to macrophages, including primitive hematopoietic progenitors, erythro-myeloid progenitors, and hematopoietic stem cells. These progenitors have distinct genetic backgrounds and developmental processes. Accordingly, macrophages exhibit complex and diverse functions in the body, including phagocytosis and clearance of cellular debris, antigen presentation, and immune response, regulation of inflammation and cytokine production, tissue remodeling and repair, and multi-level regulatory signaling pathways/crosstalk involved in homeostasis and physiology. Besides, tumor-associated macrophages are a key component of the TME, exhibiting both anti-tumor and pro-tumor properties. Furthermore, the functional status of macrophages is closely linked to the development of various diseases, including cancer, autoimmune disorders, cardiovascular disease, neurodegenerative diseases, metabolic conditions, and trauma. Targeting macrophages has emerged as a promising therapeutic strategy in these contexts. Clinical trials of macrophage-based targeted drugs, macrophage-based immunotherapies, and nanoparticle-based therapy were comprehensively summarized. Potential challenges and future directions in targeting macrophages have also been discussed. Overall, our review highlights the significance of this versatile immune cell in human health and disease, which is expected to inform future research and clinical practice.

Differences in glycolytic metabolism between tissue-resident alveolar macrophages and recruited lung macrophages

Immunometabolism has emerged as a key area of focus in immunology and has the potential to lead to new treatments for immune-related diseases. It is well-established that glycolytic metabolism is essential for adaptation to hypoxia and for macrophage inflammatory function. Macrophages have been shown to upregulate their glycolytic metabolism in response to pathogens and pathogen-associated molecular patterns such as LPS. As a direct link to the external environment, the lungs' distinctive nutrient composition and multiple macrophage subtypes provide a unique opportunity to study macrophage metabolism. This review aims to highlight how the steady-state airway and severely inflamed airway offer divergent environments for macrophage glycolytic metabolism. We describe the differences in glycolytic metabolism between tissue-resident alveolar macrophages, and other lung macrophages at steady-state and during inflammation/injury. We also provide an overview of experimental guidelines on how to assess metabolism at the cellular level using Seahorse-based bioenergetic analysis including a review of pharmacologic agents used to inhibit or activate glycolysis.

Longitudinal single-cell profiles of lung regeneration after viral infection reveal persistent injury-associated cell states

Functional regeneration of the lung's gas exchange surface following injury requires the coordination of a complex series of cell behaviors within the alveolar niche. Using single-cell transcriptomics combined with lineage tracing of proliferating progenitors, we examined mouse lung regeneration after influenza injury, demonstrating an asynchronously phased response across different cellular compartments. This longitudinal atlas of injury responses has produced a catalog of transient and persistent transcriptional alterations in cells as they transit across axes of differentiation. These cell states include an injury-induced capillary endothelial cell (iCAP) that arises after injury, persists indefinitely, and shares hallmarks with developing lung endothelium and endothelial aberrations found in degenerative human lung diseases. This dataset provides a foundational resource to understand the complexity of cellular and molecular responses to injury and correlations to responses found in human development and disease.

Mitochondrial Regulator CRIF1 Plays a Critical Role in the Development and Homeostasis of Alveolar Macrophages via Maintaining Metabolic Fitness

The importance of mitochondrial function in macrophages is well established. Alveolar macrophages (AMs), the tissue-resident macrophages (TRMs) of the lung, are particularly dependent on mitochondria-driven oxidative phosphorylation (OXPHOS) to support their functions and maintain homeostasis. However, the specific genes and pathways that regulate OXPHOS in AMs remain unclear. In this study, we investigated the role of CR6-interacting factor 1 (CRIF1), a mitochondrial regulator, as a key factor that specifically modulates the metabolic fitness and maintenance of AMs. Using single-cell RNA sequencing and transcriptomic analyses, we found CRIF1 to be highly expressed in AMs compared to TRMs from other tissues, correlating with enhanced OXPHOS activity. Genetic ablation of Crif1 in macrophages resulted in a marked reduction in AM populations exclusively in the lung, while other TRM populations were unaffected. CRIF1-deficient AMs exhibited an altered metabolic profile, including impaired mitochondrial function, increased glycolysis, and aberrant lipid accumulation. These findings underscore the essential role of CRIF1 in regulating mitochondrial functions and metabolic fitness in AMs, distinguishing it from broader mitochondrial regulators like mitochondrial transcription factor A, which operates across multiple TRM populations. Our study provides critical insights into the tissue-specific regulation of macrophage metabolism and suggests potential therapeutic avenues for lung diseases associated with AM dysfunction.

Synergy between pluripotent stem cell-derived macrophages and self-renewing macrophages: Envisioning a promising avenue for the modelling and cell therapy of infectious diseases

As crucial phagocytes of the innate immune system, macrophages (Mϕs) protect mammalian hosts, maintain tissue homeostasis and influence disease pathogenesis. Nonetheless, Mϕs are susceptible to various pathogens, including bacteria, viruses and parasites, which cause various infectious diseases, necessitating a deeper understanding of pathogen-Mϕ interactions and therapeutic insights. Pluripotent stem cells (PSCs) have been efficiently differentiated into PSC-derived Mϕs (PSCdMϕs) resembling primary Mϕs, advancing the modelling and cell therapy of infectious diseases. However, the mass production of PSCdMϕs, which lack proliferative capacity, relies on large-scale expansions of PSCs, thereby increasing both costs and culture cycles. Notably, Mϕs deficient in the MafB/c-Maf genes have been reported to re-enter the cell cycle with the stimulation of specific growth factor cocktails, turning into self-renewing Mϕs (SRMϕs). This review summarizes the applications of PSCdMϕs in the modelling and cell therapy of infectious diseases and strategies for establishing SRMϕs. Most importantly, we innovatively propose that PSCs can serve as a gene editing platform to creating PSC-derived SRMϕs (termed PSRMϕs), addressing the resistance of Mϕs against genetic manipulation. We discuss the challenges and possible solutions in creating PSRMϕs. In conclusion, this review provides novel insights into the development of physiologically relevant and expandable Mϕ models, highlighting the enormous potential of PSRMϕs as a promising avenue for the modelling and cell therapy of infectious diseases.

Lymphatic-derived oxysterols promote anti-tumor immunity and response to immunotherapy in melanoma

In melanoma, lymphangiogenesis correlates with metastasis and poor prognosis and promotes immunosuppression. However, it also potentiates immunotherapy by supporting immune cell trafficking. We show in a lymphangiogenic murine melanoma that lymphatic endothelial cells (LECs) upregulate the enzyme Ch25h, which catalyzes the formation of 25-hydroxycholesterol (25-HC) from cholesterol and plays important roles in lipid metabolism, gene regulation, and immune activation. We identify a role for LECs as a source of extracellular 25-HC in tumors inhibiting PPAR-γ in intra-tumoral macrophages and monocytes, preventing their immunosuppressive function and instead promoting their conversion into proinflammatory myeloid cells that support effector T cell functions. In human melanoma, LECs also upregulate Ch25h, and its expression correlates with the lymphatic vessel signature, infiltration of pro-inflammatory macrophages, better patient survival, and better response to immunotherapy. We identify here in mechanistic detail an important LEC function that supports anti-tumor immunity, which can be therapeutically exploited in combination with immunotherapy.

Menin orchestrates macrophage reprogramming to maintain the pulmonary immune homeostasis

Menin is a scaffold protein encoded by the Men1 gene, and it interacts with a variety of chromatin regulators to activate or repress cellular processes. The potential importance of menin in immune regulation remains unclear. Here, we report that myeloid deletion of Men1 results in the development of spontaneous pulmonary alveolar proteinosis (PAP). This is strongly correlated with impaired development of alveolar macrophages (AM) through inactivation of the granulocyte-macrophage colony-stimulating factor (GM-CSF/CSF2) pathway caused by Men1 deficiency. Mechanistically, menin directly interacts with the SET domain containing 2 (SETD2) through the N-terminal domain (NTD) and Palm domains to maintain protein stability and chromatin recruitment. SETD2 and menin collectively maintain CSF2 expression through H3K36me3, which orchestrates AM reprogramming and pulmonary immune homeostasis. Targeting H3K36me3 remodeling mitigated the aberrant activation of macrophages caused by lipopolysaccharide (LPS). Our results point to a nonredundant role of menin in the control of macrophage lineage maintenance via reinforcement of the H3K36me3 transcriptional program.

Path of differentiation defines human macrophage identity

Macrophages play central roles in immunity, wound healing, and homeostasis - a functional diversity that is underpinned by varying developmental origins. The impact of ontogeny on properties of human macrophages is inadequately understood. We demonstrate that definitive human fetal liver (HFL) hematopoietic stem cells (HSCs) possess two divergent paths of macrophage specification that lead to distinct identities. The monocyte-dependent pathway exists in both prenatal and postnatal hematopoiesis and generates macrophages with adult-like responses properties. We now uncover a fetal-specific pathway of expedited differentiation that generates tissue resident-like macrophages (TRMs) that retain HSC-like self-renewal programs governed by the aryl hydrocarbon receptor (AHR). We show that AHR antagonism promotes TRM expansion and mitigates inflammation in models of atopic dermatitis (AD). Overall, we directly connect path of differentiation with functional properties of macrophages and identify an approach to promote selective expansion of TRMs with direct relevance to inflammation and diseases of macrophage dysfunction.

PPARG/SPP1/CD44 signaling pathway in alveolar macrophages: Mechanisms of lipid dysregulation and therapeutic targets in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic and progressive interstitial lung disease. It is characterized by inflammation and fibrosis in the lung parenchyma and interstitium. Given its poor prognosis and limited treatment options, understanding the underlying molecular mechanisms is crucial. Recent evidence suggests that lipid metabolism plays a pivotal role in IPF pathogenesis, however, the precise mechanisms remain poorly understood. To address this, we analyzed 12 bulk RNA-seq and 2 single-cell RNA-seq datasets from the GEO database using machine learning approaches. As a result, we identified four key lipid-related genes-PPARG, SPP1, CASP3, and PECAM1-that are expressed across various cell types. Specifically, in alveolar macrophages (AMs), we observed that PPARG was significantly downregulated, while SPP1 was highly expressed. Importantly, PPARG serves as a transcriptional regulator of SPP1, which in turn mediates intercellular signaling via CD44. Based on these findings, we propose a novel PPARG/SPP1/CD44 signaling pathway in AMs, which modulates lipid metabolism and likely contributes to the progression of fibrosis in IPF. Moreover, network pharmacology analysis identified several herbal compounds that target PPARG, offering potential therapeutic opportunities. In conclusion, these findings highlight the critical role of lipid metabolism in IPF and present novel molecular targets for the development of future therapeutic strategies.

Alveolar Macrophages in Viral Respiratory Infections: Sentinels and Saboteurs of Lung Defense

Respiratory viral infections continue to cause pandemic and epidemic outbreaks in humans and animals. Under steady-state conditions, alveolar macrophages (AlvMϕ) fulfill a multitude of tasks in order to maintain tissue homeostasis. Due to their anatomic localization within the deep lung, AlvMϕ are prone to detect and react to inhaled viruses and thus play a role in the early pathogenesis of several respiratory viral infections. Here, detection of viral pathogens causes diverse antiviral and proinflammatory reactions. This fact not only makes them promising research targets, but also suggests them as potential targets for therapeutic and prophylactic approaches. This review aims to give a comprehensive overview of the current knowledge about the role of AlvMϕ in respiratory viral infections of humans and animals.

High-dose intravenous BCG vaccination induces enhanced immune signaling in the airways

Intradermal Bacillus Calmette-Guérin (BCG) is the most widely administered vaccine, but it does not sufficiently protect adults against pulmonary tuberculosis. Recent studies in nonhuman primates show that intravenous BCG administration offers superior protection against Mycobacterium tuberculosis (Mtb). We used single-cell analysis of bronchoalveolar lavage cells from rhesus macaques vaccinated via different routes and doses of BCG to identify alterations in the immune ecosystem in the airway following vaccination. Our findings reveal that high-dose intravenous BCG induces an influx of polyfunctional T cells and macrophages in the airways, with alveolar macrophages from high-dose intravenous BCG displaying a basal activation state in the absence of purified protein derivative stimulation, defined in part by interferon signaling. Enhanced intercellular immune signaling and stronger T helper 1-T helper 17 transcriptional responses were observed following purified protein derivative stimulation. These results suggest that high-dose intravenous BCG vaccination creates a specialized immune environment that primes airway cells for effective Mtb clearance.

Alveolar epithelial cells shape lipopolysaccharide-induced inflammatory responses and reprogramming of alveolar macrophages

Alveolar macrophages (AMs) are sentinels in the airways, where they sense and respond to invading microbes and other stimuli. Unlike macrophages in other locations, AMs can remain responsive to Gram-negative lipopolysaccharides (LPS) after they have responded to LPS in vivo (they do not develop "endotoxin tolerance"), suggesting that the alveolar microenvironment may influence their responses. Although alveolar epithelial cells (AECs) normally limit AMs' innate responses, preventing inflammation induced by harmless antigens in the lung, how AECs influence the innate responses of AMs to infectious agents has been uncertain. Here we report that (1) after exposure to aspirated (intranasal instillation) LPS, AMs increase their responses to TLR agonists and elevate their phagocytic and bactericidal activities in mice; (2) Aspirated LPS pre-exposure increases host resistance to pulmonary infection caused by Gram-negative bacteria and the protection effect lasts for at least 35 days; (3) LPS stimulation of AECs both increases AMs' innate immune responses and prevents AMs from developing tolerance in vitro; (4) Upon LPS stimulation, AMs secreted TNF-α induces AECs to release GM-CSF, which potentiates AMs' response. These experiments have revealed a previously unappreciated role that AECs may play in boosting the innate responses of AMs and promoting resistance to pulmonary infections.

Advancements in research and clinical management of interstitial lung injury associated with ADC drugs administration in breast cancer

Antibody-drug conjugates (ADCs) represent a novel class of targeted anti-tumor medications that utilize the covalent linkage between monoclonal antibodies and cytotoxic agents. This unique mechanism combines the cytotoxic potency of drugs with the targeting specificity conferred by antigen recognition. However, it is essential to recognize that many ADCs still face challenges related to off-target toxicity akin to cytotoxic payloads, as well as targeted toxicity and other potential life-threatening adverse effects, such as treatment-induced interstitial lung injury. Currently, of the four approved ADC drugs for breast cancer, several reports have documented post-treatment lung injury-related fatalities. As a result, treatment-induced interstitial lung injury due to ADC drugs has become a clinical concern. In this review article, we delve into the factors associated with ADC-induced interstitial lung injury in patients with advanced-stage breast cancer and highlight strategies expected to decrease the incidence of ADC-related interstitial lung injury in the years ahead. These efforts are directed at enhancing treatment outcomes in both advanced and early-stage cancer patients while also providing insights into the development and innovation of ADC drugs and bolstering clinicians' understanding of the diagnosis and management of ADC-associated interstitial lung injury.

Metformin improves Mycobacterium avium infection by strengthening macrophage antimicrobial functions

Introduction

The incidence and prevalence of infections with non-tuberculous mycobacteria such as Mycobacterium avium (Mav) are increasing. Prolonged drug regimens, inherent antibiotic resistance, and low cure rates underscore the need for improved treatment, which may be achieved by combining standard chemotherapy with drugs targeting the host immune system. Here, we examined if the diabetes type 2 drug metformin could improve Mav-infection.

Methods

Metformin was administered to C57BL/6 mice infected intranasally with Mav and C57BL/6 mice were infected intranasally with Mav and treated with metformin over 3 weeks. Organ bacterial loads and lung pathology, inflammatory cytokines and immune cell profiles were assessed. For mechanistic insight, macrophages infected with Mav were treated with metformin alone or in combination with inhibitors for mitochondrial ROS or AMPK and assessed for bacterial burden and phagosome maturation.

Results and discussion

Three weeks of metformin treatment significantly reduced the lung mycobacterial burden in mice infected with Mav without major changes in the overall lung pathology or immune cell composition. Metformin treatment had no significant impact on tissue inflammation except for a tendency of increased lung IFNγ and infiltration of Mav-specific IFNγ-secreting T cells. Metformin did, however, boost the antimicrobial capacity of infected macrophages directly by modulating metabolism/activating AMPK, increasing mitochondrial ROS and phagosome maturation, and indirectly by bolstering type I immunity. Taken together, our data show that metformin improved the control of Mav-infection in mice, mainly by strengthening antimicrobial defenses in macrophages, and suggest that metformin has potential as an adjunct treatment of Mav infections.

Tax1bp1 enhances bacterial virulence and promotes inflammatory responses during Mycobacterium tuberculosis infection of alveolar macrophages

Crosstalk between autophagy, host cell death, and inflammatory host responses to bacterial pathogens enables effective innate immune responses that limit bacterial growth while minimizing coincidental host damage. Mycobacterium tuberculosis (Mtb) thwarts innate immune defense mechanisms in alveolar macrophages (AMs) during the initial stages of infection and in recruited bone marrow-derived cells during later stages of infection. However, how protective inflammatory responses are achieved during Mtb infection and the variation of the response in different macrophage subtypes remain obscure. Here, we show that the autophagy receptor Tax1bp1 plays a critical role in enhancing inflammatory cytokine production and increasing the susceptibility of mice to Mtb infection. Surprisingly, although Tax1bp1 restricts Mtb growth during infection of bone marrow-derived macrophages (BMDMs) (Budzik et al. 2020) and terminates cytokine production in response to cytokine stimulation or viral infection, Tax1bp1 instead promotes Mtb growth in AMs, neutrophils, and a subset of recruited monocyte-derived cells from the bone marrow. Tax1bp1 also leads to increases in bacterial growth and inflammatory responses during infection of mice with Listeria monocytogenes, an intracellular pathogen that is not effectively targeted to canonical autophagy. In Mtb-infected AMs but not BMDMs, Tax1bp1 enhances necrotic-like cell death early after infection, reprogramming the mode of host cell death to favor Mtb replication in AMs. Tax1bp1's impact on host cell death is a mechanism that explains Tax1bp1's cell type-specific role in the control of Mtb growth. Similar to Tax1bp1-deficiency in AMs, the expression of phosphosite-deficient Tax1bp1 restricts Mtb growth. Together, these results show that Tax1bp1 plays a crucial role in linking the regulation of autophagy, cell death, and pro-inflammatory host responses and enhancing susceptibility to bacterial infection.

A pan-family screen of nuclear receptors in immunocytes reveals ligand-dependent inflammasome control

Ligand-dependent transcription factors of the nuclear receptor (NR) family regulate diverse aspects of metazoan biology, enabling communications between distant organs via small lipophilic molecules. Here, we examined the impact of each of 35 NRs on differentiation and homeostatic maintenance of all major immunological cell types in vivo through a "Rainbow-CRISPR" screen. Receptors for retinoic acid exerted the most frequent cell-specific roles. NR requirements varied for resident macrophages of different tissues. Deletion of either Rxra or Rarg reduced frequencies of GATA6+ large peritoneal macrophages (LPMs). Retinoid X receptor alpha (RXRα) functioned conventionally by orchestrating LPM differentiation through chromatin and transcriptional regulation, whereas retinoic acid receptor gamma (RARγ) controlled LPM survival by regulating pyroptosis via association with the inflammasome adaptor ASC. RARγ antagonists activated caspases, and RARγ agonists inhibited cell death induced by several inflammasome activators. Our findings provide a broad view of NR function in the immune system and reveal a noncanonical role for a retinoid receptor in modulating inflammasome pathways.

Mouse and human macrophages and their roles in cardiovascular health and disease

The past 15 years have witnessed a leap in understanding the life cycle, gene expression profiles, origins and functions of mouse macrophages in many tissues, including macrophages of the artery wall and heart that have critical roles in cardiovascular health. Here, we review the phenotypical and functional diversity of macrophage populations in multiple organs and discuss the roles that proliferation, survival, and recruitment and replenishment from monocytes have in maintaining macrophages in homeostasis and inflammatory states such as atherosclerosis and myocardial infarction. We also introduce emerging data that better characterize the life cycle and phenotypic profiles of human macrophages. We discuss the similarities and differences between murine and human macrophages, raising the possibility that tissue-resident macrophages in humans may rely more on bone marrow-derived monocytes than in mouse.

The role of alveolar macrophages in viral respiratory infections and their therapeutic implications

Alveolar macrophages are pivotal components of the lung's innate immune defense against respiratory virus infections. Their multifaceted role spans from viral clearance to modulation of immune responses, making them essential players in shaping disease outcomes. In this comprehensive review collection, we look into the intricate interplay between Alveolar macrophages and various respiratory viruses, shedding light on their dynamic contributions to immune resilience. From influenza to respiratory syncytial virus, Alveolar macrophages emerge as sentinels of the airways, actively participating in viral detection and initiating rapid antiviral responses. Their ability to recognize viral pathogens triggers a cascade of events, including cytokine and chemokine production that guides the recruitment and activation of immune effectors. Furthermore, Alveolar macrophages impact the fate of adaptive immune responses by modulating the activation of T lymphocytes and the secretion of key cytokines. These reviews encompass a range of insights, including the regulation of inflammasome activation, the influence of Alveolar macrophages on cytokine dysregulation, and their role in preventing secondary bacterial pneumonia post-infection. Collectively, they highlight the significance of Alveolar macrophages in preserving pulmonary integrity and immune homeostasis during viral challenges.

Lung Immune Cell Niches and the Discovery of New Cell Subtypes

Immune cells in the lungs are important for maintaining lung function. The importance of immune cells in defending against lung diseases and infections is increasingly recognized. However, a primary knowledge gaps in current studies of lung immune cells is the understanding of their subtypes and functional heterogeneity. Increasing evidence supports the existence of novel immune cell subtypes that engage in the complex crosstalk between lung-resident immune cells, recruited immune cells, and epithelial cells. Therefore, further studies on how immune cells respond to perturbations in the pulmonary microenvironment are warranted. This review explores the processes behind the formation of the immune cell niche during lung development, and the characteristics and cell interaction modes of several major lung-resident immune cells. It indicates that distinct lung microenvironments or inflammatory niches can mediate the formation of different cell subtypes. These findings summarize and clarify paths to identify new cell subtypes that originate from resident progenitor cells and recruited peripheral cells, which are remodeled by the pulmonary microenvironment. The development of new techniques combining transcriptome analysis and location information is essential for identifying new immune cell subtypes and their relative immune niches, as well as for uncovering the molecular mechanisms of immune cell-mediated lung homeostasis.

Cholesterol sensing and metabolic adaptation in tissue immunity

Cholesterol metabolites, particularly oxidized forms known as oxysterols, play crucial roles in modulating immune and metabolic processes across various tissues. Concentrations of local cholesterol and its metabolites influence tissue-specific immune responses by shaping the metabolic and spatial organization of immune cells in barrier organs like the small intestine (SI) and lungs. We explore recent molecular and cellular evidence supporting the metabolic adaptation of innate and adaptive immune cells in the SI and lung, driven by cholesterol and cholesterol metabolites. Further research should unravel the detailed molecular mechanisms and spatiotemporal adaptations involving cholesterol metabolites in distinct mucosal tissues in homeostasis or infection. We posit that pharmacological interventions targeting the generation or sensing of cholesterol metabolites might be leveraged to enhance long-term immune protection in mucosal tissues or prevent autoinflammatory states.

[Advances on physiology and pathology of subpopulations of macrophages in the lung tissue]

Macrophages are vital in maintaining tissue homeostasis in the lungs by modulating and regulating immune responses. Based on different origins and anatomical locations, macrophages in the lungs are categorized into alveolar macrophages, interstitial macrophages, perivascular macrophages, and inflammatory macrophages. Alveolar macrophages are located in the alveolar spaces and are primarily responsible for maintaining alveolar surfactant homeostasis, defending against pathogens and regulating immune responses. Interstitial macrophages can maintain homeostasis, regulate immunity and anti-inflammation in the lung tissue. Perivascular macrophages play a crucial role in inhibiting lung inflammation, improving pulmonary fibrosis, and regulating lung tumor progression due to antigen-presenting and immunomodulatory effects. Inflammatory macrophages, which are differentiated from monocytes during inflammation, regulate the inflammatory process. This article reviews the origins of various subpopulations of macro-phages in the lung tissue and their physiological and pathological functions as well as discusses the underlying mechanisms and potential therapeutic targets.

FPR1 signaling aberrantly regulates S100A8/A9 production by CD14+FCN1hi macrophages and aggravates pulmonary pathology in severe COVID-19

Excessive alarmins S100A8/A9 escalate the inflammation and even exacerbate immune-driven thrombosis and multi-organ damage. However, the regulatory mechanisms of S100A8/A9 expression in infectious diseases remain unclear. In this study, high-dimensional transcriptomic data analyses revealed a high proportion of CD14+FCN1hi macrophages within the pulmonary niche post-severe SARS-CoV-2 infection. By constructing the S100-coexpression gene list and supervised module scoring, we found that CD14+FCN1hi macrophages presented the highest scores of alarmin S100, and possibly served as the trigger and amplifier of inflammation in severe COVID-19. These CD14+FCN1hi cells lacked the positive regulatory activity of transcription factor PPARγ, and lost their differentiation ability towards mature macrophages. Ex vivo experiments further validated that the epithelial cells with high ORF-3a expression promoted the expression and secretion of S100A8/A9 through ANXA1/SAA1-FPR1 signaling. S100A8/A9 heterodimers, as well as the co-localization of S100A8/A9 with microtubules, were both diminished by the FPR1 inhibitor. Phospho-kinase protein array indicated that STAT3 promoted transcription, and PLC-γ and ERK1/2 pathways were involved in the hetero-dimerization and unconventional secretion of S100A8/A9. Our study highlights the pivotal role of FPR1 signaling in the excessive production of S100A8/A9 and provides a promising target for the prevention and control of severe COVID-19 and post-acute COVID-19 sequelae.

The frequency of CD38+ alveolar macrophages correlates with early control of M. tuberculosis in the murine lung

Tuberculosis, caused by Mycobacterium tuberculosis, remains an enduring global health challenge due to the limited efficacy of existing treatments. Although much research has focused on immune failure, the role of host macrophage biology in controlling the disease remains underappreciated. Here we show, through multi-modal single-cell RNA sequencing in a murine model, that different alveolar macrophage subsets play distinct roles in either advancing or controlling the disease. Initially, alveolar macrophages that are negative for the CD38 marker are the main infected population. As the infection progresses, CD38+ monocyte-derived and tissue-resident alveolar macrophages emerge as significant controllers of bacterial growth. These macrophages display a unique chromatin organization pre-infection, indicative of epigenetic priming for pro-inflammatory responses. Moreover, intranasal BCG immunization increases the numbers of CD38+ macrophages, enhancing their capability to restrict Mycobacterium tuberculosis growth. Our findings highlight the dynamic roles of alveolar macrophages in tuberculosis and open pathways for improved vaccines and therapies.

Interplay between pulmonary epithelial stem cells and innate immune cells contribute to the repair and regeneration of ALI/ARDS

Mammalian lung is the important organ for ventilation and exchange of air and blood. Fresh air and venous blood are constantly delivered through the airway and vascular tree to the alveolus. Based on this, the airways and alveolis are persistently exposed to the external environment and are easily suffered from toxins, irritants and pathogens. For example, acute lung injury/acute respiratory distress syndrome (ALI/ARDS) is a common cause of respiratory failure in critical patients, whose typical pathological characters are diffuse epithelial and endothelial damage resulting in excessive accumulation of inflammatory fluid in the alveolar cavity. The supportive treatment is the main current treatment for ALI/ARDS with the lack of targeted effective treatment strategies. However, ALI/ARDS needs more targeted treatment measures. Therefore, it is extremely urgent to understand the cellular and molecular mechanisms that maintain alveolar epithelial barrier and airway integrity. Previous researches have shown that the lung epithelial cells with tissue stem cell function have the ability to repair and regenerate after injury. Also, it is able to regulate the phenotype and function of innate immune cells involving in regeneration of tissue repair. Meanwhile, we emphasize that interaction between the lung epithelial cells and innate immune cells is more supportive to repair and regenerate in the lung epithelium following acute lung injury. We reviewed the recent advances in injury and repair of lung epithelial stem cells and innate immune cells in ALI/ARDS, concentrating on alveolar type 2 cells and alveolar macrophages and their contribution to post-injury repair behavior of ALI/ARDS through the latest potential molecular communication mechanisms. This will help to develop new research strategies and therapeutic targets for ALI/ARDS.

Optimized method for higher yield of alveolar macrophage isolation for ex vivo studies

Alveolar macrophages (AMs) are a fully differentiated lung-resident immune cell population and are a critical component of lung immunity. AMs can be easily isolated from mice via bronchoalveolar lavage fluid (BALF) collection. The quality and quantity of AMs in BALF isolation are critical for generating reliable and high-quality data for ex vivo studies. Traditional techniques use ice-cold (4°C) buffer to collect AMs in BALF and result in low yield. Hence, a new method that consistently gives a higher yield of AMs is needed. We demonstrate here an optimized method that significantly increases the quantity of AM recovery in BALF (>2.8 times than the traditional method). Our method uses a warm-buffer (37°C) containing EDTA. We compared the viability and functional parameters (cytokine/chemokine expression, phagocytosis) of AMs isolated by our new and traditional methods. Our study revealed that AMs collected using our method have similar viability and functional characteristics to those collected using traditional method. Hence, our new method can be used for the collection of a higher number of AMs without altering their function. This protocol might also be useful for isolating tissue-resident immune cells from other anatomical sites for ex vivo and other downstream applications.

EGR2 is an epigenomic regulator of phagocytosis and antifungal immunity in alveolar macrophages

Alveolar macrophages (AMs) act as gatekeepers of the lung's immune responses, serving essential roles in recognizing and eliminating pathogens. The transcription factor (TF) early growth response 2 (EGR2) has been recently described as required for mature AMs in mice; however, its mechanisms of action have not been explored. Here, we identified EGR2 as an epigenomic regulator and likely direct proximal transcriptional activator in AMs using epigenomic approaches (RNA sequencing, ATAC sequencing, and CUT&RUN). The predicted direct proximal targets of EGR2 included a subset of AM identity genes and ones related to pathogen recognition, phagosome maturation, and adhesion, such as Clec7a, Atp6v0d2, Itgb2, Rhoc, and Tmsb10. We provided evidence that EGR2 deficiency led to impaired zymosan internalization and reduced the capacity to respond to Aspergillus fumigatus. Mechanistically, the lack of EGR2 altered the transcriptional response, secreted cytokines (i.e., CXCL11), and inflammation-resolving lipid mediators (i.e., RvE1) of AMs during in vivo zymosan-induced inflammation, which manifested in impaired resolution. Our findings demonstrated that EGR2 is a key proximal transcriptional activator and epigenomic bookmark in AMs responsible for select, distinct components of cell identity and a protective transcriptional and epigenomic program against fungi.

TGFβ primes alveolar-like macrophages to induce type I IFN following TLR2 activation

Alveolar macrophages (AMs) are key mediators of lung function and are potential targets for therapies during respiratory infections. TGFβ is an important regulator of AM differentiation and maintenance, but how TGFβ directly modulates the innate immune responses of AMs remains unclear. This shortcoming prevents effective targeting of AMs to improve lung function in health and disease. Here we leveraged an optimized ex vivo AM model system, fetal-liver derived alveolar-like macrophages (FLAMs), to dissect the role of TGFβ in AMs. Using transcriptional analysis, we first globally defined how TGFβ regulates gene expression of resting FLAMs. We found that TGFβ maintains the baseline metabolic state of AMs by driving lipid metabolism through oxidative phosphorylation and restricting inflammation. To better understand inflammatory regulation in FLAMs, we next directly tested how TGFβ alters the response to TLR2 agonists. While both TGFβ (+) and TGFβ (-) FLAMs robustly responded to TLR2 agonists, we found an unexpected activation of type I interferon (IFN) responses in FLAMs and primary AMs in a TGFβ-dependent manner. Surprisingly, mitochondrial antiviral signaling protein and the interferon regulator factors 3 and 7 were required for IFN production by TLR2 agonists. Together, these data suggest that TGFβ modulates AM metabolic networks and innate immune signaling cascades to control inflammatory pathways in AMs.

Pioglitazone alleviates lacrimal gland impairments induced by high-fat diet by suppressing M1 polarization

A high-fat diet (HFD) contributes to the pathogenesis of various inflammatory and metabolic diseases. Previous research confirms that under HFD conditions, the extraorbital lacrimal glands (ELGs) can be impaired, with significant infiltration of pro-inflammatory macrophages (Mps). However, the relationship between HFD and Mps polarization in the ELGs remains unexplored. We first identified and validated the differential expression of PPAR-γ in murine ELGs fed ND and HFD through RNA sequencing. Tear secretion was measured using the Schirmer test. Lipid droplet deposition within the ELGs was observed through Oil Red O staining and transmission electron microscopy. Mps phenotypes were determined through quantitative RT-PCR, immunofluorescence, and flow cytometric analysis. An in vitro high-fat culture system for Mps was established using palmitic acid (PA), with supernatants collected for co-culture with lacrimal gland acinar cells. Gene expression was determined through ELISA, immunofluorescence, immunohistochemistry, quantitative RT-PCR, and Western blot analysis. Pioglitazone reduced M1-predominant infiltration induced by HFD by increasing PPAR-γ levels in ELGs, thereby alleviating lipid deposition and enhancing tear secretion. In vitro tests indicated that PPAR-γ agonist shifted Mps from M1-predominant to M2-predominant phenotype in PA-induced Mps, reducing lipid synthesis in LGACs and promoting lipid catabolism, thus alleviating lipid metabolic disorders within ELGs. Conversely, the PPAR-γ antagonist induced opposite effects. In summary, the lacrimal gland is highly sensitive to high-fat and lipid metabolic disorders. Downregulation of PPAR-γ expression in ELGs induces Mps polarization toward predominantly M1 phenotype, leading to lipid metabolic disorder and inflammatory responses via the NF-κb/ERK/JNK/P38 pathway.

Metabolic homeostasis of tissue macrophages across the lifespan

Macrophages are present in almost all organs. Apart from being immune sentinels, tissue-resident macrophages (TRMs) have organ-specific functions that require a specialized cellular metabolism to maintain homeostasis. In addition, organ-dependent metabolic adaptations of TRMs appear to be fundamentally distinct in homeostasis and in response to a challenge, such as infection or injury. Moreover, TRM function becomes aberrant with advancing age, contributing to inflammaging and organ deterioration, and a metabolic imbalance may underlie TRM immunosenescence. Here, we outline current understanding of the particular metabolic states of TRMs across organs and the relevance for their function. Moreover, we discuss the concomitant aging-related decline in metabolic plasticity and functions of TRMs, highlighting potential novel therapeutic avenues to promote healthy aging.

BootCellNet, a resampling-based procedure, promotes unsupervised identification of cell populations via robust inference of gene regulatory networks

Recent advances in measurement technologies, particularly single-cell RNA sequencing (scRNA-seq), have revolutionized our ability to acquire large amounts of omics-level data on cellular states. As measurement techniques evolve, there has been an increasing need for data analysis methodologies, especially those focused on cell-type identification and inference of gene regulatory networks (GRNs). We have developed a new method named BootCellNet, which employs smoothing and resampling to infer GRNs. Using the inferred GRNs, BootCellNet further infers the minimum dominating set (MDS), a set of genes that determines the dynamics of the entire network. We have demonstrated that BootCellNet robustly infers GRNs and their MDSs from scRNA-seq data and facilitates unsupervised identification of cell clusters using scRNA-seq datasets of peripheral blood mononuclear cells and hematopoiesis. It has also identified COVID-19 patient-specific cells and their potential regulatory transcription factors. BootCellNet not only identifies cell types in an unsupervised and explainable way but also provides insights into the characteristics of identified cell types through the inference of GRNs and MDS.

A detailed insight into macrophages' role in shaping lung carcinogenesis

Despite significant advancements in cancer treatment in recent decades, the high mortality rate associated with lung cancer remains a significant concern. The development and proper execution of new targeted therapies needs more deep knowledge regarding the lung cancer associated tumour microenvironment. One of the key component of that tumour microenvironment is the lung resident macrophages. Although in normal physiological condition the lung resident macrophages are believed to maintain lung homeostasis, but they may also initiate a vicious inflammatory response in abnormal conditions which is linked to lung cancer development. Depending on the activation pathway, the lung resident macrophages are either of M1 or M2 sub-type. The M1 and M2 sub-types differ significantly in various prospectuses, from phenotypic markers to metabolic pathways. In addition to this generalized classification, the recent advancement of the multiomics technology is able to identify some other sub-types of lung resident macrophages. Researchers have also observed that these different sub-types can manipulate the pathogenesis of lung carcinogenesis in a context dependent manner and can either promote or inhibit the development of lung carcinogenesis upon receiving proper activation. As proper knowledge about the role played by the lung resident macrophages' in shaping the lung carcinogenesis is limited, so the main purpose of this review is to bring all the available information under the same roof. We also elaborated the different mechanisms involved in maintenance of the plasticity of M1/M2 sub-type, as this plasticity can be a good target for lung cancer treatment.

Characterization of IL-6R-expressing monocytes in the lung of patients with chronic obstructive pulmonary disease

BACKGROUND

Monocytes play a crucial role in innate immune responses for host defense, however, their involvement in chronic obstructive pulmonary disease (COPD) remains poorly understood. We previously identified a subset of monocytes in COPD lung tissues characterized by high interleukin-6 receptor (IL-6R) expression. This study aimed to characterize the phenotypes of IL-6Rhi monocytes in the lungs of COPD patients.

METHODS

Using flow cytometry, we assessed the abundance of pulmonary CD14+IL-6Rhi cells in never smokers (CNS), control ex-smokers (CES) and COPD patients. IL-6 expression in CD14+ monocytes isolated from the peripheral blood of patients with COPD was also examined. CD45+CD206-CD14+IL-6Rhi and CD45+CD206-CD14+IL-6R-/lo cells were isolated from COPD lung tissues for transcriptome analysis. A monocyte line THP1 cell with constitutive IL-6R expression was stimulated with recombinant IL-6, followed by RNA sequencing to evaluate the IL-6 responsiveness of IL-6R+ monocytes.

RESULTS

The number of pulmonary CD14+IL-6Rhi monocytes was elevated in COPD patients compared to CNS, whereas CD14+ monocytes in the peripheral blood of COPD patients did not express IL-6R. Upregulated mRNA expression in CD14+IL-6Rhi monocytes was associated with chemotaxis, monocyte differentiation, fatty acid metabolism and integrin-mediated signaling pathway. Stimulation of THP1 cells with recombinant IL-6 induced changes in the expression of genes linked to chemotaxis and organism development.

CONCLUSION

In patients with COPD, CD14+IL-6Rhi monocytes are increased in lung tissues compared to those in CNS. They exhibit a transcriptome profile different from that of CD14+IL-6R-/lo monocytes.

Unravelling monocyte functions: from the guardians of health to the regulators of disease

Monocytes are a key component of the innate immune system. They undergo intricate developmental processes within the bone marrow, leading to diverse monocyte subsets in the circulation. In a state of healthy homeostasis, monocytes are continuously released into the bloodstream, destined to repopulate specific tissue-resident macrophage pools where they fulfil tissue-specific functions. However, under pathological conditions monocytes adopt various phenotypes to resolve inflammation and return to a healthy physiological state. This review explores the nuanced developmental pathways and functional roles that monocytes perform, shedding light on their significance in both physiological and pathological contexts.

CD38+ Alveolar macrophages mediate early control of M. tuberculosis proliferation in the lung

Tuberculosis, caused by M.tuberculosis (Mtb), remains an enduring global health challenge, especially given the limited efficacy of current therapeutic interventions. Much of existing research has focused on immune failure as a driver of tuberculosis. However, the crucial role of host macrophage biology in controlling the disease remains underappreciated. While we have gained deeper insights into how alveolar macrophages (AMs) interact with Mtb, the precise AM subsets that mediate protection and potentially prevent tuberculosis progression have yet to be identified. In this study, we employed multi-modal scRNA-seq analyses to evaluate the functional roles of diverse macrophage subpopulations across different infection timepoints, allowing us to delineate the dynamic landscape of controller and permissive AM populations during the course of infection. Our analyses at specific time-intervals post-Mtb challenge revealed macrophage populations transitioning between distinct anti- and pro-inflammatory states. Notably, early in Mtb infection, CD38- AMs showed a muted response. As infection progressed, we observed a phenotypic shift in AMs, with CD38+ monocyte-derived AMs (moAMs) and a subset of tissue-resident AMs (TR-AMs) emerging as significant controllers of bacterial growth. Furthermore, scATAC-seq analysis of naïve lungs demonstrated that CD38+ TR-AMs possessed a distinct chromatin signature prior to infection, indicative of epigenetic priming and predisposition to a pro-inflammatory response. BCG intranasal immunization increased the numbers of CD38+ macrophages, substantially enhancing their capability to restrict Mtb growth. Collectively, our findings emphasize the pivotal, dynamic roles of different macrophage subsets in TB infection and reveal rational pathways for the development of improved vaccines and immunotherapeutic strategies.

Pathogenesis-driven treatment of primary pulmonary alveolar proteinosis

Pulmonary alveolar proteinosis (PAP) is a syndrome that results from the accumulation of lipoproteinaceous material in the alveolar space. According to the underlying pathogenetic mechanisms, three different forms have been identified, namely primary, secondary and congenital. Primary PAP is caused by disruption of granulocyte-macrophage colony-stimulating factor (GM-CSF) signalling due to the presence of neutralising autoantibodies (autoimmune PAP) or GM-CSF receptor genetic defects (hereditary PAP), which results in dysfunctional alveolar macrophages with reduced phagocytic clearance of particles, cholesterol and surfactant. The serum level of GM-CSF autoantibody is the only disease-specific biomarker of autoimmune PAP, although it does not correlate with disease severity. In PAP patients with normal serum GM-CSF autoantibody levels, elevated serum GM-CSF levels is highly suspicious for hereditary PAP. Several biomarkers have been correlated with disease severity, although they are not specific for PAP. These include lactate dehydrogenase, cytokeratin 19 fragment 21.1, carcinoembryonic antigen, neuron-specific enolase, surfactant proteins, Krebs von Lungen 6, chitinase-3-like protein 1 and monocyte chemotactic proteins. Finally, increased awareness of the disease mechanisms has led to the development of pathogenesis-based treatments, such as GM-CSF augmentation and cholesterol-targeting therapies.

Cell Cross-Talk in Alveolar Microenvironment: From Lung Injury to Fibrosis

Alveoli are complex microenvironments composed of various cell types, including epithelial, fibroblast, endothelial, and immune cells, which work together to maintain a delicate balance in the lung environment, ensuring proper growth, development, and an effective response to lung injuries. However, prolonged inflammation or aging can disrupt normal interactions among these cells, leading to impaired repair processes and a substantial decline in lung function. Therefore, it is essential to understand the key mechanisms underlying the interactions among the major cell types within the alveolar microenvironment. We explored the key mechanisms underlying the interactions among the major cell types within the alveolar microenvironment. These interactions occur through the secretion of signaling factors and play crucial roles in the response to injury, repair mechanisms, and the development of fibrosis in the lungs. Specifically, we focused on the regulation of alveolar type 2 cells by fibroblasts, endothelial cells, and macrophages. In addition, we explored the diverse phenotypes of fibroblasts at different stages of life and in response to lung injury, highlighting their impact on matrix production and immune functions. Furthermore, we summarize the various phenotypes of macrophages in lung injury and fibrosis as well as their intricate interplay with other cell types. This interplay can either contribute to the restoration of immune homeostasis in the alveoli or impede the repair process. Through a comprehensive exploration of these cell interactions, we aim to reveal new insights into the molecular mechanisms that drive lung injury toward fibrosis and identify potential targets for therapeutic intervention.

Deciphering metabolic crosstalk in context: lessons from inflammatory diseases

Metabolism plays a crucial role in regulating the function of immune cells in both health and disease, with altered metabolism contributing to the pathogenesis of cancer and many inflammatory diseases. The local microenvironment has a profound impact on the metabolism of immune cells. Therefore, immunological and metabolic heterogeneity as well as the spatial organization of cells in tissues should be taken into account when studying immunometabolism. Here, we highlight challenges of investigating metabolic communication. Additionally, we review the capabilities and limitations of current technologies for studying metabolism in inflamed microenvironments, including single-cell omics techniques, flow cytometry-based methods (Met-Flow, single-cell energetic metabolism by profiling translation inhibition (SCENITH)), cytometry by time of flight (CyTOF), cellular indexing of transcriptomes and epitopes by sequencing (CITE-Seq), and mass spectrometry imaging. Considering the importance of metabolism in regulating immune cells in diseased states, we also discuss the applications of metabolomics in clinical research, as well as some hurdles to overcome to implement these techniques in standard clinical practice. Finally, we provide a flowchart to assist scientists in designing effective strategies to unravel immunometabolism in disease-relevant contexts.

Myeloid progenitor dysregulation fuels immunosuppressive macrophages in tumors

Monocyte-derived macrophages (mo-macs) drive immunosuppression in the tumor microenvironment (TME) and tumor-enhanced myelopoiesis in the bone marrow (BM) fuels these populations. Here, we performed paired transcriptome and chromatin analysis over the continuum of BM myeloid progenitors, circulating monocytes, and tumor-infiltrating mo-macs in mice and in patients with lung cancer to identify myeloid progenitor programs that fuel pro-tumorigenic mo-macs. Analyzing chromatin accessibility and histone mark changes, we show that lung tumors prime accessibility for Nfe2l2 (NRF2) in BM myeloid progenitors as a cytoprotective response to oxidative stress. NRF2 activity is sustained and increased during monocyte differentiation into mo-macs in the lung TME to regulate oxidative stress, in turn promoting metabolic adaptation, resistance to cell death, and contributing to immunosuppressive phenotype. NRF2 genetic deletion and pharmacological inhibition significantly reduced mo-macs' survival and immunosuppression in the TME, enabling NK and T cell therapeutic antitumor immunity and synergizing with checkpoint blockade strategies. Altogether, our study identifies a targetable epigenetic node of myeloid progenitor dysregulation that sustains immunoregulatory mo-macs in the TME.

Multiomic analysis of monocyte-derived alveolar macrophages in idiopathic pulmonary fibrosis

BACKGROUND

Monocyte-derived alveolar macrophages (Mo_AMs) are increasingly recognised as potential pathogenic factors for idiopathic pulmonary fibrosis (IPF). While scRNAseq analysis has proven valuable in the transcriptome profiling of Mo_AMs, the integration analysis of multi-omics may provide additional dimensions of understanding of these cellular populations.

METHODS

We performed multi-omics analysis on 116 scRNAseq, 119 bulkseq and five scATACseq lung tissue samples from IPF. We built a large-scale IPF scRNAseq atlas and conducted the Monocle 2/3 as well as the Cellchat to explore the developmental path and intercellular communication on Mo_AMs. We also reported the difference in metabolisms, tissue repair and phagocytosis between Mo_AMs and tissue-resident alveolar macrophages (TRMs). To determine whether Mo_AMs affected pulmonary function, we projected clinical phenotypes (FVC%pred) from the bulkseq dataset onto the scRNAseq atlas. Finally, we used scATATCseq to uncover the upstream regulatory mechanisms and determine key drivers in Mo_AMs.

RESULTS

We identified three Mo_AMs clusters and the trajectory analysis further validated the origin of these clusters. Moreover, via the Cellchat analysis, the CXCL12/CXCR4 axis was found to be involved in the molecular basis of reciprocal interactions between Mo_AMs and fibroblasts through the activation of the ERK pathway in Mo_AMs. SPP1_RecMacs (RecMacs, recruited macrophages) were higher in the low-FVC group than in the high-FVC group. Specifically, compared with TRMs, the functions of lipid and energetic metabolism as well as tissue repair were higher in Mo_AMs than TRMs. But, TRMs may have higher level of phagocytosis than TRMs. SPIB (PU.1), JUNB, JUND, BACH2, FOSL2, and SMARCC1 showed stronger association with open chromatin of Mo_AMs than TRMs. Significant upregulated expression and deep chromatin accessibility of APOE were observed in both SPP1_RecMacs and TRMs.

CONCLUSION

Through trajectory analysis, it was confirmed that SPP1_RecMacs derived from Monocytes. Besides, Mo_AMs may influence FVC% pred and aggravate pulmonary fibrosis through the communication with fibroblasts. Furthermore, distinctive transcriptional regulators between Mo_AMs and TRMs implied that they may depend on different upstream regulatory mechanisms. Overall, this work provides a global overview of how Mo_AMs govern IPF and also helps determine better approaches and intervention therapies.

Lung Interstitial Macrophages Can Present Soluble Antigens and Induce Foxp3+ Regulatory T Cells

Lung macrophages constitute a sophisticated surveillance and defense system that contributes to tissue homeostasis and host defense and allows the host to cope with the myriad of insults and antigens to which the lung mucosa is exposed. As opposed to alveolar macrophages, lung interstitial macrophages (IMs) express high levels of Type 2 major histocompatibility complex (MHC-II), a hallmark of antigen-presenting cells. Here, we showed that lung IMs, like dendritic cells, possess the machinery to present soluble antigens in an MHC-II-restricted way. Using ex vivo ovalbumin (OVA)-specific T cell proliferation assays, we found that OVA-pulsed IMs could trigger OVA-specific CD4+ T cell proliferation and Foxp3 expression through MHC-II-, IL-10-, and transforming growth factor β-dependent mechanisms. Moreover, we showed that IMs efficiently captured locally instilled antigens in vivo, did not migrate to the draining lymph nodes, and enhanced local interactions with CD4+ T cells in a model of OVA-induced allergic asthma. These results support that IMs can present antigens to CD4+ T cells and trigger regulatory T cells, which might attenuate lung immune responses and have functional consequences for lung immunity and T cell-mediated disorders.

Journey of monocytes and macrophages upon influenza A virus infection

Influenza A virus (IAV) infections pose a global health challenge that necessitates a comprehensive understanding of the host immune response to devise effective therapeutic interventions. As monocytes and macrophages play crucial roles in host defence, inflammation, and repair, this review explores the intricate journey of these cells during and after IAV infection. First, we highlight the dynamics and functions of lung-resident macrophage populations post-IAV. Second, we review the current knowledge of recruited monocytes and monocyte-derived cells, emphasising their roles in viral clearance, inflammation, immunomodulation, and tissue repair. Third, we shed light on the consequences of IAV-induced macrophage alterations on long-term lung immunity. We conclude by underscoring current knowledge gaps and exciting prospects for future research in unravelling the complexities of macrophage responses to respiratory viral infections.