Surfactant proteins A and D suppress alveolar macrophage phagocytosis via interaction with SIRP alpha

Neuroimmune pathways involvement in neurodegeneration of R6/2 mouse model of Huntington's disease

Mechanisms of tissue damage in Huntington's disease (HD) involve excitotoxicity, mitochondrial damage, and neuroinflammation, including microglia activation. CD47 is a membrane protein that interacts with the inhibitory immunoreceptor SIRPα. Engagement of SIRPα by CD47 provides a downregulatory signal that inhibits host cell phagocytosis, promoting a "don't-eat-me" signal. These proteins are involved in the immune response and are downmodulated in inflammatory diseases. The involvement of inflammation and of the inflammasome in HD has already been described. In this study, we focused on other factors that can be involved in the unregulated inflammatory response that accelerates and exacerbate the neurodegenerative process in HD. Our results show that CD47 on striatal neurons decreased in HD mice, while it increased in wild type mice with age. SIRPα, on the other hand, was present in neurons in the wild type and increases in the R6/2 mice at all stages. Recruitment of SIRPα and binding to CD47 promotes the activation through phosphorylating events of non-receptor protein tyrosine phosphatase SHP-1 and SHP-2 in neurons and microglia. SHP phosphatases are able to curb the activity of NLRP3 inflammasome thereby reducing the detrimental effect of neuroinflammation. Such activity is mediated by the inhibition (dephosphorylation) of the proteins signal transducer and activator of transcription (STAT). We found that activated SHP-1 was present in microglia and neurons of WT mice at 5 and 13 weeks, increasing with time; while in R6/2 it was not localized in neurons but only in microglia, where it decreases with time. Consequently, STAT1 was overexpressed in neurons of R6/2 mice, as an effect of lack of modulation by SHP-1. Thus, our results shed light on the pathophysiology of neuronal damage, on one hand, paving the way toward a modulation of signal transducer proteins by specific inhibitors to achieve neuroprotection in HD, on the other.

Surfactant protein D prevents mucin overproduction in airway goblet cells via SIRPα

Mucin overproduction is a common feature of chronic airway diseases such as asthma and chronic obstructive pulmonary disease (COPD), and exacerbates their underlying respiratory condition. Surfactant protein D (SP-D) protects against airway diseases through modulation of immune reactions, but whether it also exerts direct effects on airway epithelial cells has remained unclear. Therefore, we sought to investigate the inhibitory role of SP-D on mucin production in airway epithelial cells. We prepared air-liquid interface (ALI) cultures of human primary bronchial epithelial cells (HBECs), which recapitulated a well-differentiated human airway epithelium. Benzo(a)pyrene (BaP), a key toxicant in cigarette smoke, induced mucin 5AC (MUC5AC) production in ALI-cultured HBECs, airway secretory cell lines, and airway epithelia of mice. Then, the protective effects of SP-D against the BaP-induced mucin overproduction were examined. BaP increased MUC5AC production in ALI cultures of HBECs, and this effect was attenuated by SP-D. SP-D also suppressed the BaP-induced phosphorylation of extracellular signal-regulated kinase (ERK) and MUC5AC expression in NCI-H292 goblet-like cells, but not in NCI-H441 club-like cells. Signal regulatory protein α (SIRPα) was found to be expressed in HBECs and NCI-H292 cells but absent in NCI-H441 cells. In NCI-H292 cells, SP-D activated SH2 domain-containing tyrosine phosphatase-1 (SHP-1), downstream of SIRPα, and knockdown of SIRPα abolished the suppressive effects of SP-D on BaP-induced ERK phosphorylation and MUC5AC production. Consistent with these in vitro findings, intratracheal instillation of SP-D prevented the BaP-induced phosphorylation of ERK and Muc5ac expression in airway epithelial cells in a mouse model. SP-D acts directly on airway epithelial cells to inhibit mucin secretion through ligation of SIRPα and SHP-1-mediated dephosphorylation of ERK. Targeting of SIRPα is therefore a potential new therapeutic approach to suppression of mucin hypersecretion in chronic airway diseases such as COPD and asthma.

SIRPα: A key player in innate immunity

Signal regulatory protein alpha (SIRPα) is a crucial inhibitory regulator expressed on the surface of myeloid cells, including macrophages, dendritic cells, monocytes, neutrophils, and microglia. SIRPα plays an indispensable role in innate immune and adoptive immune responses in cancer immunology, tissue homeostasis, and other physiological or phycological conditions. This review provides an overview of the research history, ligands, signal transduction pathways, and functional mechanisms associated with SIRPα. Additionally, we summarize the therapeutic implications of targeting SIRPα as a promising novel strategy in immuno-oncology.

Efferocytosis and Respiratory Disease

Cells are the smallest units that make up living organisms, which constantly undergo the processes of proliferation, differentiation, senescence and death. Dead cells need to be removed in time to maintain the homeostasis of the organism and keep it healthy. This process is called efferocytosis. If the process fails, this may cause different types of diseases. More and more evidence suggests that a faulty efferocytosis process is closely related to the pathological processes of respiratory diseases. In this review, we will first introduce the process and the related mechanisms of efferocytosis of the macrophage. Secondly, we will propose some methods that can regulate the function of efferocytosis at different stages of the process. Next, we will discuss the role of efferocytosis in different lung diseases and the related treatment approaches. Finally, we will summarize the drugs that have been applied in clinical practice that can act upon efferocytosis, in order to provide new ideas for the treatment of lung diseases.

Alveolar macrophages in tissue homeostasis, inflammation, and infection: evolving concepts of therapeutic targeting

Alveolar macrophages (AMs) are the sentinel cells of the alveolar space, maintaining homeostasis, fending off pathogens, and controlling lung inflammation. During acute lung injury, AMs orchestrate the initiation and resolution of inflammation in order to ultimately restore homeostasis. This central role in acute lung inflammation makes AMs attractive targets for therapeutic interventions. Single-cell RNA-Seq and spatial omics approaches, together with methodological advances such as the generation of human macrophages from pluripotent stem cells, have increased understanding of the ontogeny, function, and plasticity of AMs during infectious and sterile lung inflammation, which could move the field closer to clinical application. However, proresolution phenotypes might conflict with proinflammatory and antibacterial responses. Therefore, therapeutic targeting of AMs at vulnerable time points over the course of infectious lung injury might harbor the risk of serious side effects, such as loss of antibacterial host defense capacity. Thus, the identification of key signaling hubs that determine functional fate decisions in AMs is of the utmost importance to harness their therapeutic potential.

Hijacking homeostasis: Regulation of the tumor microenvironment by apoptosis

Cancers are genetically driven, rogue tissues which generate dysfunctional, obdurate organs by hijacking normal, homeostatic programs. Apoptosis is an evolutionarily conserved regulated cell death program and a profoundly important homeostatic mechanism that is common (alongside tumor cell proliferation) in actively growing cancers, as well as in tumors responding to cytotoxic anti-cancer therapies. Although well known for its cell-autonomous tumor-suppressive qualities, apoptosis harbors pro-oncogenic properties which are deployed through non-cell-autonomous mechanisms and which generally remain poorly defined. Here, the roles of apoptosis in tumor biology are reviewed, with particular focus on the secreted and fragmentation products of apoptotic tumor cells and their effects on tumor-associated macrophages, key supportive cells in the aberrant homeostasis of the tumor microenvironment. Historical aspects of cell loss in tumor growth kinetics are considered and the impact (and potential impact) on tumor growth of apoptotic-cell clearance (efferocytosis) as well as released soluble and extracellular vesicle-associated factors are discussed from the perspectives of inflammation, tissue repair, and regeneration programs. An "apoptosis-centric" view is proposed in which dying tumor cells provide an important platform for intricate intercellular communication networks in growing cancers. The perspective has implications for future research and for improving cancer diagnosis and therapy.

Porcine Macrophage Markers and Populations: An Update

Besides its importance as a livestock species, pig is increasingly being used as an animal model for biomedical research. Macrophages play critical roles in immunity to pathogens, tissue development, homeostasis and tissue repair. These cells are also primary targets for replication of viruses such as African swine fever virus, classical swine fever virus, and porcine respiratory and reproductive syndrome virus, which can cause huge economic losses to the pig industry. In this article, we review the current status of knowledge on porcine macrophages, starting by reviewing the markers available for their phenotypical characterization and following with the characteristics of the main macrophage populations described in different organs, as well as the effect of polarization conditions on their phenotype and function. We will also review available cell lines suitable for studies on the biology of porcine macrophages and their interaction with pathogens.

After cell death: the molecular machinery of efferocytosis

Cells constituting a multicellular organism die in a variety of ways throughout life, and most of them die via apoptosis under normal conditions. The occurrence of apoptosis is especially prevalent during development and in tissues with a high cellular turnover rate, such as the thymus and bone marrow. Interestingly, although the number of apoptotic cells produced daily is known to be innumerable in a healthy adult human body, apoptotic cells are rarely observed. This absence is due to the existence of a cellular process called efferocytosis that efficiently clears apoptotic cells. Studies over the past decades have focused on how phagocytes are able to remove apoptotic cells specifically, swiftly, and continuously, resulting in defined molecular and cellular events. In this review, we will discuss the current understanding of the clearance of apoptotic cells at the molecular level.

Mechanisms and Effects of Macrophage Polarization and Its Specifics in Pulmonary Environment

Macrophages are a specific group of cells found in all body tissues. They have specific characteristics in each of the tissues that correspond to the functional needs of the specific environment. These cells are involved in a wide range of processes, both pro-inflammatory and anti-inflammatory ("wound healing"). This is due to their specific capacity for so-called polarization, a phenotypic change that is, moreover, partially reversible compared to other differentiated cells of the human body. This promises a wide range of possibilities for its influence and thus therapeutic use. In this article, we therefore review the mechanisms that cause polarization, the basic classification of polarized macrophages, their characteristic markers and the effects that accompany these phenotypic changes. Since the study of pulmonary (and among them mainly alveolar) macrophages is currently the focus of scientific interest of many researchers and these macrophages are found in very specific environments, given mainly by the extremely high partial pressure of oxygen compared to other locations, which specifically affects their behavior, we will focus our review on this group.

Special Issue: "Parasitic Infection and Host Immunity": Editorial

Parasite-host interactions depend on a complex interplay between the metabolism of the parasite, their antigens, and the host immune response system [...].

Surfactant Protein A Can Affect Macrophage Phagocytosis: An Important Pathogenic Mechanism of Otitis Media with Effusion

Otitis media with effusion (OME), also known as secretory otitis media, is a common condition in otorhinolaryngology. The main manifestations include middle ear effusion and conductive hearing loss. Recently, increasing attention has been paid to the etiology of OME, wherein immune dysfunction is one important pathogenic mechanism. However, it is unknown whether changes in surfactant protein A (SPA) secretion affect the phagocytic activity of macrophages in the Eustachian tube, thereby altering pathogen clearance, during the pathogenesis of OME. In our study, an OME animal model was established and evaluated. Differences in SPA levels in Eustachian tube lavage fluid between the experimental and control groups were analyzed. Cell-based experiments revealed that SPA decreased the expression of CD64 and SYK and inhibited phagocytosis by RAW264.7 cells. By using flow cytometry and immunofluorescence, we confirmed that macrophage phagocytosis decreased with increasing SPA levels. Finally, we concluded that SPA affects macrophage function and plays a role in the occurrence and development of OME.

Immunometabolic Signature during Respiratory Viral Infection: A Potential Target for Host-Directed Therapies

RNA viruses are known to induce a wide variety of respiratory tract illnesses, from simple colds to the latest coronavirus pandemic, causing effects on public health and the economy worldwide. Influenza virus (IV), parainfluenza virus (PIV), metapneumovirus (MPV), respiratory syncytial virus (RSV), rhinovirus (RhV), and coronavirus (CoV) are some of the most notable RNA viruses. Despite efforts, due to the high mutation rate, there are still no effective and scalable treatments that accompany the rapid emergence of new diseases associated with respiratory RNA viruses. Host-directed therapies have been applied to combat RNA virus infections by interfering with host cell factors that enhance the ability of immune cells to respond against those pathogens. The reprogramming of immune cell metabolism has recently emerged as a central mechanism in orchestrated immunity against respiratory viruses. Therefore, understanding the metabolic signature of immune cells during virus infection may be a promising tool for developing host-directed therapies. In this review, we revisit recent findings on the immunometabolic modulation in response to infection and discuss how these metabolic pathways may be used as targets for new therapies to combat illnesses caused by respiratory RNA viruses.

Gut microbiota-derived succinate aggravates acute lung injury after intestinal ischaemia/reperfusion in mice

INTRODUCTION

Acute lung injury (ALI) is a major cause of morbidity and mortality after intestinal ischaemia/reperfusion (I/R). The gut microbiota and its metabolic byproducts act as important modulators of the gut-lung axis. This study aimed to define the role of succinate, a key microbiota metabolite, in intestinal I/R-induced ALI progression.

METHODS

Gut and lung microbiota of mice subjected to intestinal I/R were analysed using 16S rRNA gene sequencing. Succinate level alterations were measured in germ-free mice or conventional mice treated with antibiotics. Succinate-induced alveolar macrophage polarisation and its effects on alveolar epithelial apoptosis were evaluated in succinate receptor 1 (Sucnr1)-deficient mice and in murine alveolar macrophages transfected with Sucnr1-short interfering RNA. Succinate levels were measured in patients undergoing cardiopulmonary bypass, including intestinal I/R.

RESULTS

Succinate accumulated in lungs after intestinal I/R, and this was associated with an imbalance of succinate-producing and succinate-consuming bacteria in the gut, but not the lungs. Succinate accumulation was absent in germ-free mice and was reversed by gut microbiota depletion with antibiotics, indicating that the gut microbiota is a source of lung succinate. Moreover, succinate promoted alveolar macrophage polarisation, alveolar epithelial apoptosis and lung injury during intestinal I/R. Conversely, knockdown of Sucnr1 or blockage of SUCNR1 in vitro and in vivo reversed the effects of succinate by modulating the phosphoinositide 3-kinase-AKT/hypoxia-inducible factor-1α pathway. Plasma succinate levels significantly correlated with intestinal I/R-related lung injury after cardiopulmonary bypass.

CONCLUSION

Gut microbiota-derived succinate exacerbates intestinal I/R-induced ALI through SUCNR1-dependent alveolar macrophage polarisation, identifying succinate as a novel target for gut-derived ALI in critically ill patients.

Cellular mechanisms underlying the impairment of macrophage efferocytosis

The phagocytosis and clearance of dying cells by macrophages, a process termed efferocytosis, is essential for both maintaining homeostasis and promoting tissue repair after infection or sterile injury. If not removed in a timely manner, uncleared cells can undergo secondary necrosis, and necrotic cells lose membrane integrity, release toxic intracellular components, and potentially induce inflammation or autoimmune diseases. Efferocytosis also initiates the repair process by producing a wide range of pro-reparative factors. Accumulating evidence has revealed that macrophage efferocytosis defects are involved in the development and progression of a variety of inflammatory and autoimmune diseases. The underlying mechanisms of efferocytosis impairment are complex, disease-dependent, and incompletely understood. In this review, we will first summarize the current knowledge about the normal signaling and metabolic processes of macrophage efferocytosis and its importance in maintaining tissue homeostasis and repair. We then will focus on analyzing the molecular and cellular mechanisms underlying efferocytotic abnormality (impairment) in disease or injury conditions. Next, we will discuss the potential molecular targets for enhanced efferocytosis in animal models of disease. To provide a balanced view, we will also discuss some deleterious effects of efferocytosis.

After the Storm: Regeneration, Repair, and Reestablishment of Homeostasis Between the Alveolar Epithelium and Innate Immune System Following Viral Lung Injury

The mammalian lung has an enormous environmental-epithelial interface that is optimized to accomplish the principal function of the respiratory system, gas exchange. One consequence of evolving such a large surface area is that the lung epithelium is continuously exposed to toxins, irritants, and pathogens. Maintaining homeostasis in this environment requires a delicate balance of cellular signaling between the epithelium and innate immune system. Following injury, the epithelium can be either fully regenerated in form and function or repaired by forming dysplastic scar tissue. In this review, we describe the major mechanisms of damage, regeneration, and repair within the alveolar niche where gas exchange occurs. With a focus on viral infection, we summarize recent work that has established how epithelial proliferation is arrested during infection and how the innate immune system guides its reconstitution during recovery. The consequences of these processes going awry are also considered, with an emphasis on how this will impact postpandemic pulmonary biology and medicine.

AL008 Enhances Myeloid Antitumor Function by Inhibiting SIRPα Signaling and Activating Fc Receptors

Antagonizing the CD47-signal regulatory protein (SIRP)α pathway, a critical myeloid checkpoint, promotes antitumor immunity. In this study, we describe the development of AL008, a pan-allelic, SIRPα-specific Ab that triggers the degradation of SIRPα and, concurrently, stimulates FcγR activation of myeloid cells through an engineered Fc domain. AL008 showed superior enhancement of phagocytosis of tumor cells opsonized with antitumor Ag Abs compared with another SIRPα Ab tested. Unlike ligand-blocking SIRPα Abs, AL008 demonstrated single-agent activity by increasing tumor cell engulfment by human monocyte-derived macrophages even in the absence of opsonizing agents. This effect was due to enhanced Fc function, as blocking FcγR2A abrogated AL008-mediated phagocytic activity. AL008 also promoted human monocyte-derived dendritic cell-mediated T cell proliferation. In humanized mouse models, AL008 induced internalization of SIRPα and increased expression of CD86 and HLA-DR on human tumor-associated macrophages, confirming that the mechanism of action is retained in vivo. Monotherapy treatment with AL008 significantly reduced tumor growth in humanized mice implanted with human MDA-MB-231 tumor cells. AL008 also significantly potentiated the effects of T cell checkpoint blockade with anti-programmed death ligand-1 in syngeneic tumor models. This dual and specific mechanism of AL008, to our knowledge, provides a novel therapeutic strategy for targeting myeloid cells for immune activation.

The underappreciated role of resident epithelial cell populations in metastatic progression: contributions of the lung alveolar epithelium

Metastatic cancer is difficult to treat and is responsible for the majority of cancer-related deaths. After cancer cells initiate metastasis and successfully seed a distant site, resident cells in the tissue play a key role in determining how metastatic progression develops. The lung is the second most frequent site of metastatic spread, and the primary site of metastasis within the lung is alveoli. The most abundant cell type in the alveolar niche is the epithelium. This review will examine the potential contributions of the alveolar epithelium to metastatic progression. It will also provide insight into other ways in which alveolar epithelial cells, acting as immune sentinels within the lung, may influence metastatic progression through their various interactions with cells in the surrounding microenvironment.

Lack of SIRP-alpha reduces lung cancer growth in mice by promoting anti-tumour ability of macrophages and neutrophils

OBJECTIVES

Signal regulatory protein-alpha (SIRPα) is a transmembrane glycoprotein specifically expressed on myeloid cells. Blockade of SIRPα/CD47 interaction is effective in combinational therapy of some cancers. This study aimed to explore into the role and underlying molecular mechanisms of SIRPα in lung cancer growth.

MATERIALS AND METHODS

A mouse model with lung cancer in wild-type (WT) and SIRPα-knockout mouse (KO) mice was established by subcutaneous injection of Lewis murine lung cancer cells (LLC). Circulating monocytes and neutrophils were depleted in mice by intraperitoneal administration of clodronate liposomes and anti-Ly6G antibody, respectively. Phenotypes and phagocytosis of macrophages and neutrophils were analysed by flow cytometry. Transwell assay was used to analyse LLC cells migration and invasion.

RESULTS

Lack of SIRPα inhibited LLC cells growth in KO mice, associated with reduced infiltrating PD-1⁺ CD8⁺ T cells and production of IL-6 from infiltrating macrophages and neutrophils in tumour tissues. Depletion of circulating monocytes and neutrophils reduced LLC cells growth in WT mice, which was abolished in KO mice. Studies in vitro showed that lack of SIRPα increased M1/M2 ratio, and reduced LLC cell migration and invasion via attenuated IL-6 secretion. Lack of SIRPα expression in neutrophils effectively increased the cytotoxic activity to LLC cells in vitro.

CONCLUSIONS

Lack of SIRPα suppressed lung cancer cell growth in mice, dependent on circulating macrophages and neutrophils, in association with improved phagocytosis and reduced IL-6 expression.

An Agent-Based Model of Radiation-Induced Lung Fibrosis

Early- and late-phase radiation-induced lung injuries, namely pneumonitis and lung fibrosis (RILF), severely constrain the maximum dose and irradiated volume in thoracic radiotherapy. As the most radiosensitive targets, epithelial cells respond to radiation either by undergoing apoptosis or switching to a senescent phenotype that triggers the immune system and damages surrounding healthy cells. Unresolved inflammation stimulates mesenchymal cells' proliferation and extracellular matrix (ECM) secretion, which irreversibly stiffens the alveolar walls and leads to respiratory failure. Although a thorough understanding is lacking, RILF and idiopathic pulmonary fibrosis share multiple pathways and would mutually benefit from further insights into disease progression. Furthermore, current normal tissue complication probability (NTCP) models rely on clinical experience to set tolerance doses for organs at risk and leave aside mechanistic interpretations of the undergoing processes. To these aims, we implemented a 3D agent-based model (ABM) of an alveolar duct that simulates cell dynamics and substance diffusion following radiation injury. Emphasis was placed on cell repopulation, senescent clearance, and intra/inter-alveolar bystander senescence while tracking ECM deposition. Our ABM successfully replicates early and late fibrotic response patterns reported in the literature along with the ECM sigmoidal dose-response curve. Moreover, surrogate measures of RILF severity via a custom indicator show qualitative agreement with published fibrosis indices. Finally, our ABM provides a fully mechanistic alveolar survival curve highlighting the need to include bystander damage in lung NTCP models.

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.

Pulmonary surfactants and the respiratory-renal connection in steroid-sensitive nephrotic syndrome of childhood

Steroid-sensitive nephrotic syndrome (SSNS) in childhood is usually due to minimal change disease (MCD). Unlike many glomerular conditions, SSNS/MCD is commonly precipitated by respiratory infections. Of interest, pulmonary inflammation releases surfactants in circulation which are soluble agonists of SIRPα, a podocyte receptor that regulates integrin signaling. Here, we characterized this pulmonary-renal connection in MCD and performed studies to determine its importance. Children with SSNS/MCD in relapse but not remission had elevated plasma surfactants and urinary SIRPα. Sera from relapsing subjects triggered podocyte SIRPα signaling via tyrosine phosphatase SHP-2 and nephrin dephosphorylation, a marker of podocyte activation. Further, addition of surfactants to MCD sera from patients in remission replicated these findings. Similarly, nasal instillation of toll-like receptor 3 and 4 agonists in mice resulted in elevated serum surfactants and their binding to glomeruli triggering proteinuria. Together, our data document a critical pulmonary-podocyte signaling pathway involving surfactants and SIRPα signaling in SSNS/MCD.

Flow Cytometric Analysis of Leukocyte Populations in the Lung Tissue of Dromedary Camels

Respiratory tract infections are among the most common infections in dromedary camels, with a high impact on animal health, production, and welfare. Tissue-specific distribution of immune cells is one of the important factors that influence the nature and outcome of the immune response to pathogens. Several protocols have recently been described for the flow cytometric analysis of immune cells in the lung tissue of several species. However, no such protocol currently exists for dromedary camels. The aim of the present study was, therefore, to establish a flow cytometric protocol for the identification of immune cell populations in the camel lung tissue and the evaluation of some of their phenotypic and functional properties. Combined staining of camel lung leukocytes with monoclonal antibodies to the pan-leukocyte marker CD45 and the myeloid cell marker CD172a allowed the identification of myeloid cells (CD45⁺CD172a⁺) and lymphoid cells (CD45⁺CD172a⁻) in the lung of healthy camels. The cell adhesion molecules CD11a and CD18 were found in a higher abundance on myeloid cells compared to lymphoid cells. Based on their differential expression of the LPS receptor CD14, macrophages (CD172a⁺CD14^high cells) were identified as the most abundant immune cell population in the camel lung tissue. In contrast to their dominance in camel peripheral blood, granulocytes (CD172a⁺CD14^low) presented only a minor population in the lung tissue. The higher frequency of γδ T cells in the lung tissue than in peripheral blood suggests a role for these cells in the pulmonary immune system. Flow cytometric analysis of bacterial phagocytosis and ROS production upon bacterial stimulation revealed high antimicrobial activity of camel lung phagocytes, which was comparable with the antimicrobial activity of blood granulocytes. Comparative analysis of immune cell distribution between the cranial and caudal lobes of the camel lung revealed a higher frequency of granulocytes and a lower frequency of macrophages in the cranial compared to the caudal lung lobe. In addition, the higher frequency of cells expressing the M2 macrophage marker CD163 in the caudal lung tissue, with a slightly higher fraction of MHCII-positive cells (M1 phenotype) in the cranial lung tissue, may suggest the distribution of different macrophage subtypes in the different lobes of the camel lung. Such differences between lung lobes could influence the effectiveness of the immune response to infection or vaccination with respiratory pathogens. Collectively, the present study identified some similarities and differences between camels and other farm animals regarding the distribution of the main immune cell populations in their lungs. Further studies are required for comprehensive immunophenotyping of the cellular pulmonary immune system in camels.

Soluble SIRP-Alpha Promotes Murine Acute Lung Injury Through Suppressing Macrophage Phagocytosis

Soluble signal regulatory protein-alpha (SIRP-alpha) is elevated in bronchoalveolar lavage (BAL) of mice with lipopolysaccharides (LPS)-induced acute lung injury (ALI). To define the role of soluble SIRP-alpha in the pathogenesis of ALI, we established murine ALI in wild-type (WT) and SIRP-alpha knock-out (KO) mice by intratracheal administration of LPS. The results indicated that lack of SIRP-alpha significantly reduced the pathogenesis of ALI, in association with attenuated lung inflammation, infiltration of neutrophils and expression of pro-inflammatory cytokines in mice. In addition, lack of SIRP-alpha reduced the expression of pro-inflammatory cytokines in LPS-treated bone marrow-derived macrophages (BMDMs) from KO mice, accompanied with improved macrophage phagocytosis. Blockade of soluble SIRP-alpha activity in ALI BAL by anti-SIRP-alpha antibody (aSIRP) effectively reduced the expression of TNF-alpha and IL-6 mRNA transcripts and proteins, improved macrophage phagocytosis in vitro. In addition, lack of SIRP-alpha reduced activation of Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP-1) and improved activation of signal transducer and activator of transcription-3 (STAT3) and STAT6. Suppression of SHP-1 activity by tyrosine phosphatase inhibitor 1 (TPI-1) increased activation of STAT3 and STAT6, and improved macrophage phagocytosis, that was effectively reversed by STAT3 and STAT6 inhibitors. Thereby, SIRP-alpha suppressed macrophage phagocytosis through activation of SHP-1, subsequently inhibiting downstream STAT3 and STAT6 signaling. Lack of SIRP-alpha attenuated murine ALI possibly through increasing phagocytosis, and improving STAT3 and STAT6 signaling in macrophages. SIRP-alpha would be promising biomarker and molecular target in the treatment of murine ALI and patients with acute respiratory distress syndrome (ARDS).

Targeting the human β c receptor inhibits inflammatory myeloid cells and lung injury caused by acute cigarette smoke exposure

Background and objective

Chronic obstructive pulmonary disease (COPD) is a devastating disease commonly caused by cigarette smoke (CS) exposure that drives tissue injury by persistently recruiting myeloid cells into the lungs. A significant portion of COPD patients also present with overlapping asthma pathology including eosinophilic inflammation. The β_c cytokine family includes granulocyte monocyte‐colony‐stimulating factor, IL‐5 and IL‐3 that signal through their common receptor subunit β_c to promote the expansion and survival of multiple myeloid cells including monocytes/macrophages, neutrophils and eosinophils.

Methods

We have used our unique human β_c receptor transgenic (hβ_cTg) mouse strain that expresses human β_c instead of mouse β_c and β_IL3 in an acute CS exposure model. Lung tissue injury was assessed by histology and measurement of albumin and lactate dehydrogenase levels in the bronchoalveolar lavage (BAL) fluid. Transgenic mice were treated with an antibody (CSL311) that inhibits human β_c signalling.

Results

hβ_cTg mice responded to acute CS exposure by expanding blood myeloid cell numbers and recruiting monocyte‐derived macrophages (cluster of differentiation 11b⁺ [CD11b⁺] interstitial and exudative macrophages [IM and ExM]), neutrophils and eosinophils into the lungs. This inflammatory response was associated with lung tissue injury and oedema. Importantly, CSL311 treatment in CS‐exposed mice markedly reduced myeloid cell numbers in the blood and BAL compartment. Furthermore, CSL311 significantly reduced lung CD11b⁺ IM and ExM, neutrophils and eosinophils, and this decline was associated with a significant reduction in matrix metalloproteinase‐12 (MMP‐12) and IL‐17A expression, tissue injury and oedema.

Conclusion

This study identifies CSL311 as a therapeutic antibody that potently inhibits immunopathology and lung injury caused by acute CS exposure.

Myeloid cells, including macrophages, neutrophils and eosinophils, are important cellular drivers of inflammation and injury. In this study, we blocked granulocyte monocyte‐colony stimulating factor, IL‐5 and IL‐3 signalling with an anti‐β_c receptor antibody (CSL311), which greatly reduced lung inflammation and injury in a pre‐clinical model of acute cigarette smoke exposure.

Modulation of Signal Regulatory Protein α (SIRPα) by Plasmodium Antigenic Extract: A Preliminary In Vitro Study on Peripheral Blood Mononuclear Cells

Signal regulatory protein α (SIRPα) is an immunoreceptor expressed in myeloid innate immune cells that signals for inhibition of both phagocytosis and inflammatory response. Malaria parasites have evolutionarily selected multiple mechanisms that allow them to evade host immune defenses, including the modulation of cells belonging to innate immunity. Notwithstanding, little attention has been given to SIRPα in the context of immunosuppressive states induced by malaria. The present study attempted to investigate if malaria parasites are endowed with the capacity of modulating the expression of SIRPα on cells of innate immune system. Human peripheral blood mononuclear cells (PBMC) from healthy individuals were incubated in the presence of lipopolysaccharide (LPS) or crude extracts of P. falciparum or P. vivax and then, the expression of SIRPα was evaluated by flow cytometry. As expected, LPS showed an inhibitory effect on the expression of SIRPα in the population of monocytes, characterized by cell morphology in flow cytometry analysis, while Plasmodium extracts induced a significant positive modulation. Additional phenotyping of cells revealed that the modulatory potential of Plasmodium antigens on SIRPα expression was restricted to the population of monocytes (CD14⁺CD11c⁺), as no effect on myeloid dendritic cells (CD14⁻CD11c⁺) was observed. We hypothesize that malaria parasites explore inhibitory signaling of SIRPα to suppress antiparasitic immune responses contributing to the establishment of infection. Nevertheless, further studies are still required to better understand the role of SIRPα modulation in malaria immunity and pathogenesis.

The Innate Cellular Immune Response in Xenotransplantation

Xenotransplantation is very attractive strategy for addressing the shortage of donors. While hyper acute rejection (HAR) caused by natural antibodies and complement has been well defined, this is not the case for innate cellular xenogeneic rejection. An increasing body of evidence suggests that innate cellular immune responses contribute to xenogeneic rejection. Various molecular incompatibilities between receptors and their ligands across different species typically have an impact on graft outcome. NK cells are activated by direct interaction as well as by antigen dependent cellular cytotoxicity (ADCC) mechanisms. Macrophages are activated through various mechanisms in xenogeneic conditions. Macrophages recognize CD47 as a "marker of self" through binding to SIRPα. A number of studies have shown that incompatibility of porcine CD47 against human SIRPα contributes to the rejection of xenogeneic target cells by macrophages. Neutrophils are an early responder cell that infiltrates xenogeneic grafts. It has also been reported that neutrophil extracellular traps (NETs) activate macrophages as damage-associated pattern molecules (DAMPs). In this review, we summarize recent insights into innate cellular xenogeneic rejection.

Pulmonary macrophages and SARS-Cov2 infection

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has led to the largest global pandemic in living memory, with between 4.5 and 15M deaths globally from coronavirus disease 2019 (COVID-19). This has led to an unparalleled global, collaborative effort to understand the pathogenesis of this devastating disease using state-of-the-art technologies. A consistent feature of severe COVID-19 is dysregulation of pulmonary macrophages, cells that under normal physiological conditions play vital roles in maintaining lung homeostasis and immunity. In this article, we will discuss a selection of the pivotal findings examining the role of monocytes and macrophages in SARS-CoV-2 infection and place this in context of recent advances made in understanding the fundamental immunobiology of these cells to try to understand how key homeostatic cells come to be a central pathogenic component of severe COVID-19 and key cells to target for therapeutic gain.

MicroRNAs and Efferocytosis: Implications for Diagnosis and Therapy

About 10-100 billion cells are generated in the human body in a day, and accordingly, 10-100 billion cells predominantly die for maintaining homeostasis. Dead cells generated by apoptosis are also rapidly engulfed by macrophages (Mθs) to be degraded. In case of the inefficient engulfment of apoptotic cells (ACs) via Mθs, they experience secondary necrosis and thus release intracellular materials, which display damage-associated molecular patterns (DAMPs) and result in diseases. Over the last decades, researchers have also reflected on the significant contribution of microRNAs (miRNAs) to autoimmune diseases through the regulation of Mθs functions. Moreover, miRNAs have shown intricate involvement with completely adjusting basic Mθs functions, such as phagocytosis, inflammation, efferocytosis, tumor promotion, and tissue repair. In this review, the mechanism of efferocytosis containing "Find-Me", "Eat-Me", and "Digest-Me" signals is summarized and the biogenesis of miRNAs is briefly described. Finally, the role of miRNAs in efferocytosis is discussed. It is concluded that miRNAs represent promising treatments and diagnostic targets in impaired phagocytic clearance, which leads to different diseases.

Cadherin-11 Regulates Macrophage Development and Function

Cadherin-11 (CDH11) is a cell-cell adhesion protein that has previously been reported to play an important role in the pathogenesis of pulmonary fibrosis. It is expressed on macrophages in the fibrotic lung. However, the role of CDH11 on macrophage biology has not yet been studied. We show using immunophenotypic analyses that Cdh11^-/- mice have fewer recruited monocyte-derived macrophages and Ly6C^hi monocytes in the lungs compared to wild-type mice in the intraperitoneal bleomycin-induced pulmonary fibrosis model. Additionally, fewer Ly6C^hi monocytes were detected in the bone marrow and peripheral blood of naive Cdh11^-/- mice. Given that macrophages are derived from monocytes, we investigated the precursors of the monocyte/macrophage lineage in the bone marrow. We found increased numbers of CMPs and reduced numbers of GMPs and MPs/cMoPs in Cdh11^-/- mice compared to wild-type mice, suggesting decreased differentiation towards the myeloid lineage in Cdh11^-/- mice. Furthermore, we show using bone marrow cells that loss of CDH11 impaired monocyte to macrophage differentiation. We also demonstrate that CDH11 deficiency repressed the M2 program and impaired the phagocytic function of bone marrow-derived macrophages. Overall, our findings demonstrate a role for CDH11 in macrophage development, M2 polarization, and phagocytic function.

Spatiotemporal Adaptations of Macrophage and Dendritic Cell Development and Function

Macrophages and conventional dendritic cells (cDCs) are distributed throughout the body, maintaining tissue homeostasis and tolerance to self and orchestrating innate and adaptive immunity against infection and cancer. As they complement each other, it is important to understand how they cooperate and the mechanisms that integrate their functions. Both are exposed to commensal microbes, pathogens, and other environmental challenges that differ widely among anatomical locations and over time. To adjust to these varying conditions, macrophages and cDCs acquire spatiotemporal adaptations (STAs) at different stages of their life cycle that determine how they respond to infection. The STAs acquired in response to previous infections can result in increased responsiveness to infection, termed training, or in reduced responses, termed paralysis, which in extreme cases can cause immunosuppression. Understanding the developmental stage and location where macrophages and cDCs acquire their STAs, and the molecular and cellular players involved in their induction, may afford opportunities to harness their beneficial outcomes and avoid or reverse their deleterious effects. Here we review our current understanding of macrophage and cDC development, life cycle, function, and STA acquisition before, during, and after infection. We propose a unified framework to explain how these two cell types adjust their activities to changing conditions over space and time to coordinate their immunosurveillance functions. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Revisiting the role of pulmonary surfactant in chronic inflammatory lung diseases and environmental exposure

Pulmonary surfactant is a crucial and dynamic lung structure whose primary functions are to reduce alveolar surface tension and facilitate breathing. Though disruptions in surfactant homeostasis are typically thought of in the context of respiratory distress and premature infants, many lung diseases have been noted to have significant surfactant abnormalities. Nevertheless, preclinical and clinical studies of pulmonary disease too often overlook the potential contribution of surfactant alterations - whether in quantity, quality or composition - to disease pathogenesis and symptoms. In inflammatory lung diseases, whether these changes are cause or consequence remains a subject of debate. This review will outline 1) the importance of pulmonary surfactant in the maintenance of respiratory health, 2) the diseases associated with primary surfactant dysregulation, 3) the surfactant abnormalities observed in inflammatory pulmonary diseases and, finally, 4) the available research on the interplay between surfactant homeostasis and smoking-associated lung disease. From these published studies, we posit that changes in surfactant integrity and composition contribute more considerably to chronic inflammatory pulmonary diseases and that more work is required to determine the mechanisms underlying these alterations and their potential treatability.

The Effect of Cigarette Smoke Exposure on Efferocytosis in Chronic Obstructive Pulmonary Disease; Molecular Mechanisms and Treatment Opportunities

Cigarette smoking-related inflammation, cellular stresses, and tissue destruction play a key role in lung disease, such as chronic obstructive pulmonary disease (COPD). Notably, augmented apoptosis and impaired clearance of apoptotic cells, efferocytosis, contribute to the chronic inflammatory response and tissue destruction in patients with COPD. Of note, exposure to cigarette smoke can impair alveolar macrophages efferocytosis activity, which leads to secondary necrosis formation and tissue inflammation. A better understanding of the processes behind the effect of cigarette smoke on efferocytosis concerning lung disorders can help to design more efficient treatment approaches and also delay the development of lung disease, such as COPD. To this end, we aimed to seek mechanisms underlying the impairing effect of cigarette smoke on macrophages-mediated efferocytosis in COPD. Further, available therapeutic opportunities for restoring efferocytosis activity and ameliorating respiratory tract inflammation in smokers with COPD were also discussed.

The journey of SARS-CoV-2 in human hosts: a review of immune responses, immunosuppression, and their consequences

Coronavirus disease 2019 (COVID-19) is a highly infectious viral disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Laboratory findings from a significant number of patients with COVID-19 indicate the occurrence of leukocytopenia, specifically lymphocytopenia. Moreover, infected patients can experience contrasting outcomes depending on lymphocytopenia status. Patients with resolved lymphocytopenia are more likely to recover, whereas critically ill patients with signs of unresolved lymphocytopenia develop severe complications, sometimes culminating in death. Why immunodepression manifests in patients with COVID-19 remains unclear. Therefore, the evaluation of clinical symptoms and laboratory findings from infected patients is critical for understanding the disease course and its consequences. In this review, we take a logical approach to unravel the reasons for immunodepression in patients with COVID-19. Following the footprints of the virus within host tissues, from entry to exit, we extrapolate the mechanisms underlying the phenomenon of immunodepression.

Alveolar Macrophages: Adaptation to Their Anatomic Niche during and after Inflammation

At the early stages of life development, alveoli are colonized by embryonic macrophages, which become resident alveolar macrophages (ResAM) and self-sustain by local division. Genetic and epigenetic signatures and, to some extent, the functions of ResAM are dictated by the lung microenvironment, which uses cytokines, ligand-receptor interactions, and stroma cells to orchestrate lung homeostasis. In resting conditions, the lung microenvironment induces in ResAM a tolerogenic programming that prevents unnecessary and potentially harmful inflammation responses to the foreign bodies, which continuously challenge the airways. Throughout life, any episode of acute inflammation, pneumonia being likely the most frequent cause, depletes the pool of ResAM, leaving space for the recruitment of inflammatory monocytes that locally develop in monocyte-derived alveolar macrophages (InfAM). During lung infection, the local microenvironment induces a temporary inflammatory signature to the recruited InfAM to handle the tissue injury and eliminate the pathogens. After a few days, the recruited InfAM, which locally self-sustain and develop as new ResAM, gain profibrotic functions required for tissue healing. After the complete resolution of the infectious episode, the functional programming of both embryonic and monocyte-derived ResAM remains altered for months and possibly for the entire life. Adult lungs thus contain a wide diversity of ResAM since every infection brings new waves of InfAM which fill the room left open by the inflammatory process. The memory of these innate cells called trained immunity constitutes an immunologic scar left by inflammation, notably pneumonia. This memory of ResAM has advantages and drawbacks. In some cases, lung-trained immunity offers better defense capacities against autoimmune disorders and the long-term risk of infection. At the opposite, it can perpetuate a harmful process and lead to a pathological state, as is the case among critically ill patients who have immune paralysis and are highly susceptible to hospital-acquired pneumonia and acute respiratory distress syndrome. The progress in understanding the kinetics of response of alveolar macrophages (AM) to lung inflammation is paving the way to new treatments of pneumonia and lung inflammatory process.

Poor clinical outcomes and immunoevasive contexture in SIRPα+ tumor-associated macrophages enriched muscle-invasive bladder cancer patients

OBJECTIVES

In tumor immune microenvironment, the functions of tumor-associated macrophages (TAMs), including phagocytosis and immunomodulatory, have attracted increasing attention recently. With the discovery of CD47-signal regulatory protein-α (SIRPα) as "don't eat me" signaling pathway, the role of novel subpopulation of TAMs expressing SIRPα has not been fully elucidated in a wide spectrum of solid tumors including bladder cancer. In this study, we investigated the prognostic and predictive implication of SIRPα⁺ TAMs regarding clinical outcomes and adjuvant chemotherapeutic benefit in muscle-invasive bladder cancer (MIBC), and preliminarily characterized the phenotypic features of SIRPα⁺ TAMs and its relationship with immune contexture.

MATERIALS AND METHODS

A total of 141 histochemical MIBC samples from Zhongshan Hospital (ZS), 45 fresh tissue samples, and 391 MIBC patients from TCGA database were enrolled in this study. SIRPα⁺ TAMs was evaluated by immunohistochemical staining of CD68 and SIRPα, and flow cytometry fluorescence staining.

RESULTS

Our results illustrated that SIRPα⁺ TAMs were enriched in MIBC specimens. Patients with high SIRPα⁺ TAMs infiltration suffered significant poor overall survival and recurrence-free survival (P = 0.0030 and P = 0.0282). SIRPα⁺ TAMs infiltration was an independent prognosticator in multivariate Cox model. Moreover, adjuvant chemotherapy (ACT) application showed significantly survival benefit in patients with low SIRPα⁺ TAMs infiltration (P = 0.0135). SIRPα⁺ TAMs with suppressive phenotype exhibited a positive correlation with immune tolerance and dysfunctional CD8⁺ T cells in MIBC.

CONCLUSIONS

SIRPα⁺ TAMs infiltration indicated poor prognosis and ACT resistance in MIBC. Immunosuppressive SIRPα⁺ TAMs is closely related to immune evasion with exhausted T cells states, suggesting the prospect of SIRPα⁺ TAMs as a potential therapeutic target in MIBC.

Pro-Apoptotic and Immunotherapeutic Effects of Carbon Nanotubes Functionalized with Recombinant Human Surfactant Protein D on Leukemic Cells

Nanoparticles are efficient drug delivery vehicles for targeting specific organs as well as systemic therapy for a range of diseases, including cancer. However, their interaction with the immune system offers an intriguing challenge. Due to the unique physico-chemical properties, carbon nanotubes (CNTs) are considered as nanocarriers of considerable interest in cancer diagnosis and therapy. CNTs, as a promising nanomaterial, are capable of both detecting as well as delivering drugs or small therapeutic molecules to tumour cells. In this study, we coupled a recombinant fragment of human surfactant protein D (rfhSP-D) with carboxymethyl-cellulose (CMC) CNTs (CMC-CNT, 10-20 nm diameter) for augmenting their apoptotic and immunotherapeutic properties using two leukemic cell lines. The cell viability of AML14.3D10 or K562 cancer cell lines was reduced when cultured with CMC-mwCNT-coupled-rfhSP-D (CNT + rfhSP-D) at 24 h. Increased levels of caspase 3, 7 and cleaved caspase 9 in CNT + rfhSP-D treated AML14.3D10 and K562 cells suggested an involvement of an intrinsic pathway of apoptosis. CNT + rfhSP-D treated leukemic cells also showed higher mRNA expression of p53 and cell cycle inhibitors (p21 and p27). This suggested a likely reduction in cdc2-cyclin B1, causing G2/M cell cycle arrest and p53-dependent apoptosis in AML14.3D10 cells, while p53-independent mechanisms appeared to be in operation in K562 cells. We suggest that CNT + rfhSP-D has therapeutic potential in targeting leukemic cells, irrespective of their p53 status, and thus, it is worth setting up pre-clinical trials in animal models.

Pattern Recognition Proteins: First Line of Defense Against Coronaviruses

The rapid outbreak of COVID-19 caused by the novel coronavirus SARS-CoV-2 in Wuhan, China, has become a worldwide pandemic affecting almost 204 million people and causing more than 4.3 million deaths as of August 11 2021. This pandemic has placed a substantial burden on the global healthcare system and the global economy. Availability of novel prophylactic and therapeutic approaches are crucially needed to prevent development of severe disease leading to major complications both acutely and chronically. The success in fighting this virus results from three main achievements: (a) Direct killing of the SARS-CoV-2 virus; (b) Development of a specific vaccine, and (c) Enhancement of the host's immune system. A fundamental necessity to win the battle against the virus involves a better understanding of the host's innate and adaptive immune response to the virus. Although the role of the adaptive immune response is directly involved in the generation of a vaccine, the role of innate immunity on RNA viruses in general, and coronaviruses in particular, is mostly unknown. In this review, we will consider the structure of RNA viruses, mainly coronaviruses, and their capacity to affect the lungs and the cardiovascular system. We will also consider the effects of the pattern recognition protein (PRP) trident composed by (a) Surfactant proteins A and D, mannose-binding lectin (MBL) and complement component 1q (C1q), (b) C-reactive protein, and (c) Innate and adaptive IgM antibodies, upon clearance of viral particles and apoptotic cells in lungs and atherosclerotic lesions. We emphasize on the role of pattern recognition protein immune therapies as a combination treatment to prevent development of severe respiratory syndrome and to reduce pulmonary and cardiovascular complications in patients with SARS-CoV-2 and summarize the need of a combined therapeutic approach that takes into account all aspects of immunity against SARS-CoV-2 virus and COVID-19 disease to allow mankind to beat this pandemic killer.

Lower Oligomeric Form of Surfactant Protein D in Murine Acute Lung Injury Induces M1 Subtype Macrophages Through Calreticulin/p38 MAPK Signaling Pathway

Surfactant protein D (SP-D) plays an important role in innate and adaptive immune responses. In this study, we found that the expression of total and de-oligomerized SP-D was significantly elevated in mice with lipopolysaccharide (LPS)-induced acute lung injury (ALI). To investigate the role of the lower oligomeric form of SP-D in the pathogenesis of ALI, we treated bone marrow-derived macrophages (BMDMs) with ALI-derived bronchoalveolar lavage (BAL) and found that SP-D in ALI BAL predominantly bound to calreticulin (CALR) on macrophages, subsequently increasing the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and expression of interleukin (IL)-6, tumor necrosis factor (TNF)-alpha, IL-10, and CD80. However, anti-SP-D (aSP-D) and anti-calreticulin (aCALR) pretreatment reversed the SP-D binding and activation of macrophages induced by ALI BAL or de-oligomerized recombinant murine SP-D (rSP-D). Lack of signal transducer and activator of transcription (STAT)6 in STAT6-/- macrophages resulted in resistance to suppression by aCALR. Further studies in an ALI mouse model showed that blockade of pulmonary SP-D by intratracheal (i.t.), but not intraperitoneal (i.p.), administration of aSP-D attenuated the severity of ALI, accompanied by lower neutrophil infiltrates and expression of IL-1beta and IL-6. Furthermore, i.t. administration of de-oligomerized rSP-D exacerbated the severity of ALI in association with more pro-inflammatory CD45+Siglec-F(-) M1 subtype macrophages and production of IL-6, TNF-alpha, IL-1beta, and IL-18. The results indicated that SP-D in the lungs of murine ALI was de-oligomerized and participated in the pathogenesis of ALI by predominantly binding to CALR on macrophages and subsequently activating the pro-inflammatory downstream signaling pathway. Targeting de-oligomerized SP-D is a promising therapeutic strategy for the treatment of ALI and acute respiratory distress syndrome (ARDS).

Characterizing macrophage diversity in metastasis-bearing lungs reveals a lipid-associated macrophage subset

While macrophages are among the most abundant immune cell type found within primary and metastatic mammary tumors, how their complexity and heterogeneity change with metastatic progression remains unknown. Here, macrophages were isolated from the lungs of mice bearing orthotopic mammary tumors for single-cell RNA sequencing. Seven distinct macrophage clusters were identified, including populations exhibiting enhanced differential expression of genes related to antigen presentation (H2-Aa, Cd74), cell cycle (Stmn1, Cdk1), and interferon signaling (Isg15, Ifitm3). Interestingly, one cluster demonstrated a profile concordant with lipid-associated macrophages (Lgals3, Trem2). Compared to non-tumor-bearing controls, the number of these cells per gram of tissue was significantly increased in lungs from tumor-bearing mice, with the vast majority co-staining positively with the alveolar macrophage marker Siglec-F. Enrichment of genes implicated in pathways related to lipid metabolism as well extracellular matrix remodeling and immunosuppression was observed. Additionally, these cells displayed reduced capacity for phagocytosis. Collectively, these findings highlight the diversity of macrophages present within metastatic lesions and characterize a lipid-associated macrophage subset previously unidentified in lung metastases.

A model of the innate immune response to SARS-CoV-2 in the alveolar epithelium

We present a differential equation model of the innate immune response to SARS-CoV-2 within the alveolar epithelium. Critical determinants of the viral dynamics and host response, including type I and type II alveolar epithelial cells, interferons, chemokines, toxins and innate immune cells, are included. We estimate model parameters, compute the within-host basic reproductive number, and study the impacts of therapies, prophylactics, and host/pathogen variability on the course of the infection. Model simulations indicate that the innate immune response suppresses the infection and enables the alveolar epithelium to partially recover. While very robust antiviral therapy controls the infection and enables the epithelium to heal, moderate therapy is of limited benefit. Meanwhile interferon therapy is predicted to reduce viral load but exacerbate tissue damage. The deleterious effects of interferon therapy are especially apparent late in the infection. Individual variation in ACE2 expression, epithelial cell interferon production, and SARS-CoV-2 spike protein binding affinity are predicted to significantly impact prognosis.

Monocytes and Macrophages in COVID-19

COVID-19 is a contagious viral disease caused by SARS-CoV-2 that led to an ongoing pandemic with massive global health and socioeconomic consequences. The disease is characterized primarily, but not exclusively, by respiratory clinical manifestations ranging from mild common cold symptoms, including cough and fever, to severe respiratory distress and multi-organ failure. Macrophages, a heterogeneous group of yolk-sac derived, tissue-resident mononuclear phagocytes of complex ontogeny present in all mammalian organs, play critical roles in developmental, homeostatic and host defense processes with tissue-dependent plasticity. In case of infection, they are responsible for early pathogen recognition, initiation and resolution of inflammation, as well as repair of tissue damage. Monocytes, bone-marrow derived blood-resident phagocytes, are recruited under pathological conditions such as viral infections to the affected tissue to defend the organism against invading pathogens and to aid in efficient resolution of inflammation. Given their pivotal function in host defense and the potential danger posed by their dysregulated hyperinflammation, understanding monocyte and macrophage phenotypes in COVID-19 is key for tackling the disease's pathological mechanisms. Here, we outline current knowledge on monocytes and macrophages in homeostasis and viral infections and summarize concepts and key findings on their role in COVID-19. While monocytes in the blood of patients with moderate COVID-19 present with an inflammatory, interferon-stimulated gene (ISG)-driven phenotype, cellular dysfunction epitomized by loss of HLA-DR expression and induction of S100 alarmin expression is their dominant feature in severe disease. Pulmonary macrophages in COVID-19 derived from infiltrating inflammatory monocytes are in a hyperactivated state resulting in a detrimental loop of pro-inflammatory cytokine release and recruitment of cytotoxic effector cells thereby exacerbating tissue damage at the site of infection.

Detection of Immune Checkpoint Receptors - A Current Challenge in Clinical Flow Cytometry

Immunological therapy principles are increasingly determining modern medicine. They are used to treat diseases of the immune system, for tumors, but also for infections, neurological diseases, and many others. Most of these therapies base on antibodies, but small molecules, soluble receptors or cells and modified cells are also used. The development of immune checkpoint inhibitors is amazingly fast. T-cell directed antibody therapies against PD-1 or CTLA-4 are already firmly established in the clinic. Further targets are constantly being added and it is becoming increasingly clear that their expression is not only relevant on T cells. Furthermore, we do not yet have any experience with the long-term systemic effects of the treatment. Flow cytometry can be used for diagnosis, monitoring, and detection of side effects. In this review, we focus on checkpoint molecules as target molecules and functional markers of cells of the innate and acquired immune system. However, for most of the interesting and potentially relevant parameters, there are still no test kits suitable for routine use. Here we give an overview of the detection of checkpoint molecules on immune cells in the peripheral blood and show examples of a possible design of antibody panels.

The Phagocytic Code Regulating Phagocytosis of Mammalian Cells

Mammalian phagocytes can phagocytose (i.e. eat) other mammalian cells in the body if they display certain signals, and this phagocytosis plays fundamental roles in development, cell turnover, tissue homeostasis and disease prevention. To phagocytose the correct cells, phagocytes must discriminate which cells to eat using a 'phagocytic code' - a set of over 50 known phagocytic signals determining whether a cell is eaten or not - comprising find-me signals, eat-me signals, don't-eat-me signals and opsonins. Most opsonins require binding to eat-me signals - for example, the opsonins galectin-3, calreticulin and C1q bind asialoglycan eat-me signals on target cells - to induce phagocytosis. Some proteins act as 'self-opsonins', while others are 'negative opsonins' or 'phagocyte suppressants', inhibiting phagocytosis. We review known phagocytic signals here, both established and novel, and how they integrate to regulate phagocytosis of several mammalian targets - including excess cells in development, senescent and aged cells, infected cells, cancer cells, dead or dying cells, cell debris and neuronal synapses. Understanding the phagocytic code, and how it goes wrong, may enable novel therapies for multiple pathologies with too much or too little phagocytosis, such as: infectious disease, cancer, neurodegeneration, psychiatric disease, cardiovascular disease, ageing and auto-immune disease.

Putting the brakes on phagocytosis: "don't-eat-me" signaling in physiology and disease

Timely removal of dying or pathogenic cells by phagocytes is essential to maintaining host homeostasis. Phagocytes execute the clearance process with high fidelity while sparing healthy neighboring cells, and this process is at least partially regulated by the balance of "eat-me" and "don't-eat-me" signals expressed on the surface of host cells. Upon contact, eat-me signals activate "pro-phagocytic" receptors expressed on the phagocyte membrane and signal to promote phagocytosis. Conversely, don't-eat-me signals engage "anti-phagocytic" receptors to suppress phagocytosis. We review the current knowledge of don't-eat-me signaling in normal physiology and disease contexts where aberrant don't-eat-me signaling contributes to pathology.

Critical Role of Lkb1 in the Maintenance of Alveolar Macrophage Self-Renewal and Immune Homeostasis

Alveolar macrophages (AMs) are pivotal for maintaining lung immune homeostasis. We demonstrated that deletion of liver kinase b1 (Lkb1) in CD11c+ cells led to greatly reduced AM abundance in the lung due to the impaired self-renewal of AMs but not the impeded pre-AM differentiation. Mice with Lkb1-deficient AMs exhibited deteriorated diseases during airway Staphylococcus aureus (S. aureus) infection and allergic inflammation, with excessive accumulation of neutrophils and more severe lung pathology. Drug-mediated AM depletion experiments in wild type mice indicated a cause for AM reduction in aggravated diseases in Lkb1 conditional knockout mice. Transcriptomic sequencing also revealed that Lkb1 inhibited proinflammatory pathways, including IL-17 signaling and neutrophil migration, which might also contribute to the protective function of Lkb1 in AMs. We thus identified Lkb1 as a pivotal regulator that maintains the self-renewal and immune function of AMs.

Invasive fungal disease and the immunocompromised host including allogeneic hematopoietic cell transplant recipients: Improved understanding and new strategic approach with sargramostim

In hosts with damaged or impaired immune systems such as those undergoing hematopoietic cell transplant (HCT) or intensive chemotherapy, breakthrough fungal infections can be fatal. Risk factors for breakthrough infections include severe neutropenia, use of corticosteroids, extended use of broad-spectrum antibiotics, and intensive care unit admission. An individual's cumulative state of immunosuppression directly contributes to the likelihood of experiencing increased infection risk. Incidence of invasive fungal infection (IFI) after HCT may be up to 5-8%. Early intervention may improve IFI outcomes, although many infections are resistant to standard therapies (voriconazole, caspofungin, micafungin, amphotericin B, posaconazole or itraconazole, as single agents or in combination). We review herein several contributing factors that may contribute to the net state of immunosuppression in recipients of HCT. We also review a new approach for IFI utilizing adjunctive therapy with sargramostim, a yeast-derived recombinant human granulocyte-macrophage colony-stimulating factor (rhu GM-CSF).

Neutrophils and IL-1α Regulate Surfactant Homeostasis during Cigarette Smoking

Cigarette smoke exposure induces inflammation marked by rapid and sustained neutrophil infiltration, IL-1α, release and altered surfactant homeostasis. However, the extent to which neutrophils and IL-1α contribute to the maintenance of pulmonary surfactant homeostasis is not well understood. We sought to investigate whether neutrophils play a role in surfactant clearance as well as the effect of neutrophil depletion and IL-1α blockade on the response to cigarette smoke exposure. In vitro and in vivo administration of fluorescently labeled surfactant phosphatidylcholine was used to assess internalization of surfactant by lung neutrophils and macrophages during or following cigarette smoke exposure in mice. We also depleted neutrophils using anti-Ly-6G or anti-Gr-1 Abs, or we neutralized IL-1α using a blocking Ab to determine their respective roles in regulating surfactant homeostasis during cigarette smoke exposure. We observed that neutrophils actively internalize labeled surfactant both in vitro and in vivo and that IL-1α is required for smoke-induced elevation of surfactant protein (SP)-A and SP-D levels. Neutrophil depletion during cigarette smoke exposure led to a further increase in SP-A levels in the bronchoalveolar lavage and increased IL-1α, CCL2, GM-CSF, and G-CSF release. Finally, macrophage expression of Mmp12, a protease linked to emphysema, was increased in neutrophil-depleted groups and decreased following IL-1α blockade. Taken together, our results indicate that neutrophils and IL-1α signaling are actively involved in surfactant homeostasis and that the absence of neutrophils in the lungs during cigarette smoke exposure leads to an IL-1α-dependent exacerbation of the inflammatory response.

A Plausible Role for Collectins in Skin Immune Homeostasis

The skin is a complex organ that faces the external environment and participates in the innate immune system. Skin immune homeostasis is necessary to defend against external microorganisms and to recover from stress to the skin. This homeostasis depends on interactions among a variety of cells, cytokines, and the complement system. Collectins belong to the lectin pathway of the complement system, and have various roles in innate immune responses. Mannose-binding lectin (MBL), collectin kidney 1, and liver (CL-K1, CL-L1) activate the lectin pathway, while all have multiple functions, including recognition of pathogens, opsonization of phagocytosis, and modulation of cytokine-mediated inflammatory responses. Certain collectins are localized in the skin, and their expressions change during skin diseases. In this review, we summarize important advances in our understanding of how MBL, surfactant proteins A and D, CL-L1, and CL-K1 function in skin immune homeostasis. Based on the potential roles of collectins in skin diseases, we suggest therapeutic strategies for skin diseases through the targeting of collectins and relevant regulators.