Trained immunity of alveolar macrophages requires metabolic rewiring and type 1 interferon signaling

Francisella tularensis Live Vaccine Strain training of murine alveolar and bone marrow-derived macrophages

Traditionally, successful vaccines rely on specific adaptive immunity by activating lymphocytes with an attenuated pathogen, or pathogen subunit, to elicit heightened responses upon subsequent exposures. However, recent work with Mycobacterium tuberculosis and other pathogens has identified a role for "trained" monocytes in protection through memory-like but non-specific immunity. Here, we used an in vitro co-culture approach to study the potential role of trained macrophages, including lung alveolar macrophages, in immune responses to the Live Vaccine Strain (LVS) of Francisella tularensis. F. tularensis is an intracellular bacterium that replicates within mammalian macrophages and causes respiratory as well as systemic disease. We vaccinated mice with F. tularensis LVS and then obtained lung alveolar macrophages, or derived macrophages from bone marrow. LVS infected and replicated comparably in both types of macrophages, whether naïve or from LVS-vaccinated mice. LVS-infected macrophages were then co-cultured with either naïve splenocytes, splenocytes from mice vaccinated intradermally, or splenocytes from mice vaccinated intravenously. For the first time, we show that immune (but not naïve) splenocytes controlled bacterial replication within alveolar macrophages, similar to previous results using bone marrow-derived macrophage. However, no differences in control of intramacrophage bacterial replication were found between co-cultures with naïve macrophages or macrophages from LVS-vaccinated mice; furthermore, nitric oxide levels and interferon-gamma production in supernatants were largely comparable across all conditions. Thus, in the context of in vitro co-cultures, the data do not support development of trained macrophages in bone marrow or lungs of mice vaccinated with LVS intradermally or intravenously.

IMPORTANCE

The discovery of non-specific "trained immunity" in monocytes has generated substantial excitement. However, to date, training has been studied with relatively few microbes (e.g., Mycobacterium bovis Bacille Calmette-Guérin, a live attenuated intracellular bacterium used as a vaccine) and microbial substances (e.g., LPS), and it remains unclear whether training during infection is common. We previously demonstrated that vaccination of mice with Francisella tularensis Live Vaccine Strain (LVS), another live attenuated intracellular bacterium, protected against challenge with the unrelated bacterium Listeria monocytogenes. The present study therefore tested whether LVS vaccination engenders trained macrophages that contributed to this protection. To do so, we used a previous in vitro co-culture approach with murine bone marrow-derived macrophages to expand and study lung alveolar macrophages. We demonstrated that alveolar macrophages can be productively infected and employed to characterize interactions with LVS-immune lymphocytes. However, we find no evidence that either bone marrow-derived or alveolar macrophages are trained by LVS vaccination.

Metabolic Regulation in the Induction of Trained Immunity

The innate immune system exhibits features of memory, termed trained immunity, which promote faster and more robust responsiveness to heterologous challenges. Innate immune memory is sustained through epigenetic modifications, affecting gene accessibility, and promoting a tailored gene transcription for an enhanced immune response. Alterations in the epigenetic landscape are intertwined with metabolic rewiring. Here, we review the metabolic pathways that underscore the induction and maintenance of trained immunity, including glycolysis, oxidative phosphorylation, the tricarboxylic acid cycle, and amino acid and lipid metabolism. The intricate interplay of these pathways is pivotal for establishing innate immune memory in distinct cellular compartments. We explore in particular the case of resident lung alveolar macrophages. We propose that leveraging the memory of the innate immune system may present therapeutic potential. Specifically, targeting the metabolic programs of innate immune cells is an emerging strategy for clinical interventions, either to boost immune responses in immunosuppressed conditions or to mitigate maladaptive activation in hyperinflammatory diseases.

Trained immunity inducers in cancer immunotherapy

While most of the cancer immunotherapy strategies engage adaptive immunity, especially tumor-associated T cells, the small fraction of responding patients and types of cancers amenable, and the possibility of severe adverse effects limit its usage. More effective and general interventions are urgently needed. Recently, a de facto innate immune memory, termed 'trained immunity', has become a new research focal point, and promises to be a powerful tool for achieving long-term therapeutic benefits against cancers. Trained immunity-inducing agents such as BCG and fungal glucan have been shown to be able to avert the suppressive tumor microenvironment (TME), enhance T cell responses, and eventually lead to tumor regression. Here, we review the current understating of trained immunity induction and highlight the critical roles of emergency granulopoiesis, interferon γ and tissue-specific induction. Preclinical and clinical studies that have exploited trained immunity inducers for cancer immunotherapy are summarized, and repurposed trained immunity inducers from other fields are proposed. We also outline the challenges and opportunities for trained immunity in future cancer immunotherapies. We envisage that more effective cancer vaccines will combine the induction of trained immunity with T cell therapies.

Apolipoprotein E controls Dectin-1-dependent development of monocyte-derived alveolar macrophages upon pulmonary β-glucan-induced inflammatory adaptation

The lung is constantly exposed to the outside world and optimal adaptation of immune responses is crucial for efficient pathogen clearance. However, mechanisms that lead to lung-associated macrophages' functional and developmental adaptation remain elusive. To reveal such mechanisms, we developed a reductionist model of environmental intranasal β-glucan exposure, allowing for the detailed interrogation of molecular mechanisms of pulmonary macrophage adaptation. Employing single-cell transcriptomics, high-dimensional imaging and flow cytometric characterization paired with in vivo and ex vivo challenge models, we reveal that pulmonary low-grade inflammation results in the development of apolipoprotein E (ApoE)-dependent monocyte-derived alveolar macrophages (ApoE+CD11b+ AMs). ApoE+CD11b+ AMs expressed high levels of CD11b, ApoE, Gpnmb and Ccl6, were glycolytic, highly phagocytic and produced large amounts of interleukin-6 upon restimulation. Functional differences were cell intrinsic, and myeloid cell-specific ApoE ablation inhibited Ly6c+ monocyte to ApoE+CD11b+ AM differentiation dependent on macrophage colony-stimulating factor secretion, promoting ApoE+CD11b+ AM cell death and thus impeding ApoE+CD11b+ AM maintenance. In vivo, β-glucan-elicited ApoE+CD11b+ AMs limited the bacterial burden of Legionella pneumophilia after infection and improved the disease outcome in vivo and ex vivo in a murine lung fibrosis model. Collectively these data identify ApoE+CD11b+ AMs generated upon environmental cues, under the control of ApoE signaling, as an essential determinant for lung adaptation enhancing tissue resilience.

New insights into ILC2 memory

Group 2 Innate Lymphoid Cells (ILC2s) are innate lymphocytes involved in type 2 immunity. ILC2s are abundant at the barrier tissues and upon allergen exposure, respond to epithelial-derived alarmins by producing type 2 cytokines (e.g., IL-5 and IL-13). Upon activation, some of these activated ILC2s acquire immunological memory and can mount enhanced responses upon further allergen encounters. Here, we review recent findings of the cellular and molecular mechanisms underlying immune memory in ILC2s both in mice and humans and discuss the implications of memory ILC2s in the context of allergic diseases.

Endotoxin tolerance and trained immunity: breaking down immunological memory barriers

For decades, innate immune cells were considered unsophisticated first responders, lacking the adaptive memory of their T and B cell counterparts. However, mounting evidence demonstrates the surprising complexity of innate immunity. Beyond quickly deploying specialized cells and initiating inflammation, two fascinating phenomena - endotoxin tolerance (ET) and trained immunity (TI) - have emerged. ET, characterized by reduced inflammatory response upon repeated exposure, protects against excessive inflammation. Conversely, TI leads to an enhanced response after initial priming, allowing the innate system to mount stronger defences against subsequent challenges. Although seemingly distinct, these phenomena may share underlying mechanisms and functional implications, blurring the lines between them. This review will delve into ET and TI, dissecting their similarities, differences, and the remaining questions that warrant further investigation.

Intranasal delivery of a self-adjuvanted nanovaccine composed of the curli filaments and the highly conserved M2e epitope confers protection against influenza a virus in mice

Intranasal administration of vaccines is an attractive delivery route to fight viral respiratory infections. However, there are only a few intranasal vaccines used in human, emphasizing the critical need to identify novel safe mucosal adjuvants and antigen delivery systems to expand their usage. We recently revealed an immunostimulating nanoparticle based on a fragment (R4R5) of the Curli-specific gene A (CsgA) protein that confers protection against influenza A virus (IAV) when conjugated to three repeats of the highly conserved M2e epitope and administrated intramuscularly. Herein, the efficacy of this 3M2e-R4R5 nanovaccine was investigated upon administration by intranasal instillation. By triggering robust M2e-specific humoral and cellular responses, both systemic and locally in the respiratory tract, and by priming alveolar macrophages, the intranasal vaccine protected mice against a lethal IAV challenge without the use of additional adjuvant. Thus, CsgA-based nanostructures could serve as a safe and self-adjuvanted antigen delivery system for mucosal immunization.

β-glucans from Agaricus bisporus mushroom products drive Trained Immunity

Introduction

Macrofungi, such as edible mushrooms, have been used as a valuable medical resource for millennia as a result of their antibacterial and immuno-modulatory components. Mushrooms contain dietary fibers known as β-glucans, a class of polysaccharides previously linked to the induction of Trained Immunity. However, little is known about the ability of mushroom-derived β-glucans to induce Trained Immunity.

Methods & results

Using various powdered forms of the white button mushroom (Agaricus bisporus), we found that mouse macrophages pre-treated with whole mushroom powder (WMP) displayed enhanced responses to restimulation with TLR ligands, being particularly sensitive to Toll-like receptor (TLR)-2 stimulation using synthetic lipopeptides. This trained response was modest compared to training observed with yeast-derived β-glucans and correlated with the amount of available β-glucans in the WMP. Enriching for β-glucans content using either a simulated in-vitro digestion or chemical fractionation retained and boosted the trained response with WMP, respectively. Importantly, both WMP and digested-WMP preparations retained β-glucans as identified by nuclear magnetic resonance analysis and both displayed the capacity to train human monocytes and enhanced responses to restimulation. To determine if dietary incorporation of mushroom products can lead to Trained Immunity in myeloid cells in vivo, mice were given a regimen of WMP by oral gavage prior to sacrifice. Flow cytometric analysis of bone-marrow progenitors indicated alterations in hematopoietic stem and progenitor cells population dynamics, with shift toward myeloid-committed multi-potent progenitor cells. Mature bone marrow-derived macrophages derived from these mice displayed enhanced responses to restimulation, again particularly sensitive to TLR2.

Discussion

Taken together, these data demonstrate that β-glucans from common macrofungi can train innate immune cells and could point to novel ways of delivering bio-available β-glucans for education of the innate immune system.

Role of IFN-α in Rheumatoid Arthritis

PURPOSE OF REVIEW

Type 1 interferons (IFN-I) are of increasing interest across a wide range of autoimmune rheumatic diseases. Historically, research into their role in rheumatoid arthritis (RA) has been relatively neglected, but recent work continues to highlight a potential contribution to RA pathophysiology.

RECENT FINDINGS

We emphasise the importance of disease stage when examining IFN-I in RA and provide an overview on how IFN-I may have a direct role on a variety of relevant cellular functions. We explore how clinical trajectory may be influenced by increased IFN-I signalling, and also, the limitations of scores composed of interferon response genes. Relevant environmental triggers and inheritable RA genetic risk relating to IFN-I signalling are explored with emphasis on intriguing data potentially linking IFN-I exposure, epigenetic changes, and disease relevant processes. Whilst these data cumulatively illustrate a likely role for IFN-I in RA, they also highlight the knowledge gaps, particularly in populations at risk for RA, and suggest directions for future research to both better understand IFN-I biology and inform targeted therapeutic strategies.

Role of trained innate immunity against mucosal cancer

Mucosal tissues are frequent targets of both primary and metastatic cancers. This has highlighted the significance of both innate and adaptive anti-cancer immunity at mucosal sites. Trained innate immunity (TII) is an emerging concept defined as enhanced reactivity of innate leukocytes long after a previous stimulation that induces prolonged epigenetic, transcriptional, and metabolic changes. Trained innate leukocytes can respond to heterologous targets due to their lacking of antigen-specificity in most cases. Emerging experimental and clinical data suggest that certain microbes or their products induce TII in mucosal-associated innate leukocytes which endows heterologous anti-tumor innate immunity, in both prophylactic and therapeutic scenarios. In this mini-review, we summarize updated findings on the significance of TII in mucosal cancers. We also attempt to raise a few key questions critical to our further understanding on the roles of TII in mucosal cancers, and to the potential application of TII as anti-cancer strategy.

Antiviral innate immune memory in alveolar macrophages following SARS-CoV-2 infection

Pathogen encounter results in long-lasting epigenetic imprinting that shapes diseases caused by heterologous pathogens. The breadth of this innate immune memory is of particular interest in the context of respiratory pathogens with increased pandemic potential and wide-ranging impact on global health. Here, we investigated epigenetic imprinting across cell lineages in a disease relevant murine model of SARS-CoV-2 recovery. Past SARS-CoV-2 infection resulted in increased chromatin accessibility of type I interferon (IFN-I) related transcription factors in airway-resident macrophages. Mechanistically, establishment of this innate immune memory required viral pattern recognition and canonical IFN-I signaling and augmented secondary antiviral responses. Past SARS-CoV-2 infection ameliorated disease caused by the heterologous respiratory pathogen influenza A virus. Insights into innate immune memory and how it affects subsequent infections with heterologous pathogens to influence disease pathology could facilitate the development of broadly effective therapeutic strategies.

Helicobacter pylori induces a novel form of innate immune memory via accumulation of NF-кB proteins

Helicobacter pylori is a widespread Gram-negative pathogen involved in a variety of gastrointestinal diseases, including gastritis, ulceration, mucosa-associated lymphoid tissue (MALT) lymphoma and gastric cancer. Immune responses aimed at eradication of H. pylori often prove futile, and paradoxically play a crucial role in the degeneration of epithelial integrity and disease progression. We have previously shown that H. pylori infection of primary human monocytes increases their potential to respond to subsequent bacterial stimuli - a process that may be involved in the generation of exaggerated, yet ineffective immune responses directed against the pathogen. In this study, we show that H. pylori-induced monocyte priming is not a common feature of Gram-negative bacteria, as Acinetobacter lwoffii induces tolerance to subsequent Escherichia coli lipopolysaccharide (LPS) challenge. Although the increased reactivity of H. pylori-infected monocytes seems to be specific to H. pylori, it appears to be independent of its virulence factors Cag pathogenicity island (CagPAI), cytotoxin associated gene A (CagA), vacuolating toxin A (VacA) and γ-glutamyl transferase (γ-GT). Utilizing whole-cell proteomics complemented with biochemical signaling studies, we show that H. pylori infection of monocytes induces a unique proteomic signature compared to other pro-inflammatory priming stimuli, namely LPS and the pathobiont A. lwoffii. Contrary to these tolerance-inducing stimuli, H. pylori priming leads to accumulation of NF-кB proteins, including p65/RelA, and thus to the acquisition of a monocyte phenotype more responsive to subsequent LPS challenge. The plasticity of pro-inflammatory responses based on abundance and availability of intracellular signaling molecules may be a heretofore underappreciated form of regulating innate immune memory as well as a novel facet of the pathobiology induced by H. pylori.

Trained immunity: Target for prophylaxis and therapy

Trained immunity is a de facto memory for innate immune responses, leading to long-term functional reprogramming of innate immune cells. In physiological conditions, trained immunity leads to adaptive states that enhance resistance against pathogens and contributes to immunosurveillance. Dysregulated trained immunity can however lead either to defective innate immune responses in severe infections or cancer or to inflammatory and autoimmune diseases if trained immunity is inappropriately activated. Here, we review the immunological and molecular mechanisms that mediate trained immunity induction and propose that trained immunity represents an important target for prophylactic and therapeutic approaches in human diseases. On the one hand, we argue that novel approaches that induce trained immunity may enhance vaccine efficacy. On the other hand, induction of trained immunity in cancer, and inhibition of exaggerated induction of trained immunity in inflammatory disorders, are viable targets amenable for new therapeutic approaches.

Immunobiotic Ligilactobacillus salivarius FFIG58 Confers Long-Term Protection against Streptococcus pneumoniae

Previously, we isolated potentially probiotic Ligilactobacillus salivarius strains from the intestines of wakame-fed pigs. The strains were characterized based on their ability to modulate the innate immune responses triggered by the activation of Toll-like receptor (TLR)-3 or TLR4 signaling pathways in intestinal mucosa. In this work, we aimed to evaluate whether nasally administered L. salivarius strains are capable of modulating the innate immune response in the respiratory tract and conferring long-term protection against the respiratory pathogen Streptococcus pneumoniae. Infant mice (3-weeks-old) were nasally primed with L. salivarius strains and then stimulated with the TLR3 agonist poly(I:C). Five or thirty days after the last poly(I:C) administration mice were infected with pneumococci. Among the strains evaluated, L. salivarius FFIG58 had a remarkable ability to enhance the protection against the secondary pneumococcal infection by modulating the respiratory immune response. L. salivarius FFIG58 improved the ability of alveolar macrophages to produce interleukin (IL)-6, interferon (IFN)-γ, IFN-β, tumor necrosis factor (TNF)-α, IL-27, chemokine C-C motif ligand 2 (CCL2), chemokine C-X-C motif ligand 2 (CXCL2), and CXCL10 in response to pneumococcal challenge. Furthermore, results showed that the nasal priming of infant mice with the FFIG58 strain protected the animals against secondary infection until 30 days after stimulation with poly(I:C), raising the possibility of using nasally administered immunobiotics to stimulate trained immunity in the respiratory tract.

Obesity dysregulates the pulmonary antiviral immune response

Obesity is a well-recognized risk factor for severe influenza infections but the mechanisms underlying susceptibility are poorly understood. Here, we identify that obese individuals have deficient pulmonary antiviral immune responses in bronchoalveolar lavage cells but not in bronchial epithelial cells or peripheral blood dendritic cells. We show that the obese human airway metabolome is perturbed with associated increases in the airway concentrations of the adipokine leptin which correlated negatively with the magnitude of ex vivo antiviral responses. Exogenous pulmonary leptin administration in mice directly impaired antiviral type I interferon responses in vivo and ex vivo in cultured airway macrophages. Obese individuals hospitalised with influenza showed dysregulated upper airway immune responses. These studies provide insight into mechanisms driving propensity to severe influenza infections in obesity and raise the potential for development of leptin manipulation or interferon administration as novel strategies for conferring protection from severe infections in obese higher risk individuals.

Alveolar macrophages in lung cancer: opportunities challenges

Alveolar macrophages (AMs) are critical components of the innate defense mechanism in the lung. Nestled tightly within the alveoli, AMs, derived from the yolk-sac or bone marrow, can phagocytose foreign particles, defend the host against pathogens, recycle surfactant, and promptly respond to inhaled noxious stimuli. The behavior of AMs is tightly dependent on the environmental cues whereby infection, chronic inflammation, and associated metabolic changes can repolarize their effector functions in the lungs. Several factors within the tumor microenvironment can re-educate AMs, resulting in tumor growth, and reducing immune checkpoint inhibitors (ICIs) efficacy in patients treated for non-small cell lung cancer (NSCLC). The plasticity of AMs and their critical function in altering tumor responses to ICIs make them a desirable target in lung cancer treatment. New strategies have been developed to target AMs in solid tumors reprograming their suppressive function and boosting the efficacy of ICIs. Here, we review the phenotypic and functional changes in AMs in response to sterile inflammation and in NSCLC that could be critical in tumor growth and metastasis. Opportunities in altering AMs' function include harnessing their potential function in trained immunity, a concept borrowed from memory response to infections, which could be explored therapeutically in managing lung cancer treatment.

Innate (learned) memory

With the growing body of evidence, it is now clear that not only adaptive immune cells but also innate immune cells can mount a more rapid and potent nonspecific immune response to subsequent exposures. This process is known as trained immunity or innate (learned) immune memory. This review discusses the different immune and nonimmune cell types of the central and peripheral immune systems that can develop trained immunity. This review highlights the intracellular signaling and metabolic and epigenetic mechanisms underlying the formation of innate immune memory. Finally, this review explores the health implications together with the potential therapeutic interventions harnessing trained immunity.

Tissue-specific macrophages: how they develop and choreograph tissue biology

Macrophages are innate immune cells that form a 3D network in all our tissues, where they phagocytose dying cells and cell debris, immune complexes, bacteria and other waste products. Simultaneously, they produce growth factors and signalling molecules - such activities not only promote host protection in response to invading microorganisms but are also crucial for organ development and homeostasis. There is mounting evidence of macrophages orchestrating fundamental physiological processes, such as blood vessel formation, adipogenesis, metabolism and central and peripheral neuronal function. In parallel, novel methodologies have led to the characterization of tissue-specific macrophages, with distinct subpopulations of these cells showing different developmental trajectories, transcriptional programmes and life cycles. Here, we summarize our growing knowledge of macrophage diversity and how macrophage subsets orchestrate tissue development and function. We further interrelate macrophage ontogeny with their core functions across tissues, that is, the signalling events within the macrophage niche that may control organ functionality during development, homeostasis and ageing. Finally, we highlight the open questions that will need to be addressed by future studies to better understand the tissue-specific functions of distinct macrophage subsets.

Epigenetic memory of coronavirus infection in innate immune cells and their progenitors

Inflammation can trigger lasting phenotypes in immune and non-immune cells. Whether and how human infections and associated inflammation can form innate immune memory in hematopoietic stem and progenitor cells (HSPC) has remained unclear. We found that circulating HSPC, enriched from peripheral blood, captured the diversity of bone marrow HSPC, enabling investigation of their epigenomic reprogramming following coronavirus disease 2019 (COVID-19). Alterations in innate immune phenotypes and epigenetic programs of HSPC persisted for months to 1 year following severe COVID-19 and were associated with distinct transcription factor (TF) activities, altered regulation of inflammatory programs, and durable increases in myelopoiesis. HSPC epigenomic alterations were conveyed, through differentiation, to progeny innate immune cells. Early activity of IL-6 contributed to these persistent phenotypes in human COVID-19 and a mouse coronavirus infection model. Epigenetic reprogramming of HSPC may underlie altered immune function following infection and be broadly relevant, especially for millions of COVID-19 survivors.

Gene Immunotherapy of Colon Carcinoma with IL-2 and IL-12 Using Gene Electrotransfer

Gene immunotherapy has become an important approach in the treatment of cancer. One example is the introduction of genes encoding immunostimulatory cytokines, such as interleukin 2 and interleukin 12, which stimulate immune cells in tumours. The aim of our study was to determine the effects of gene electrotransfer of plasmids encoding interleukin 2 and interleukin 12 individually and in combination in the CT26 murine colon carcinoma cell line in mice. In the in vitro experiment, the pulse protocol that resulted in the highest expression of IL-2 and IL-12 mRNA and proteins was used for the in vivo part. In vivo, tumour growth delay and also complete response were observed in the group treated with the plasmid combination. Compared to the control group, the highest levels of various immunostimulatory cytokines and increased immune infiltration were observed in the combination group. Long-term anti-tumour immunity was observed in the combination group after tumour re-challenge. In conclusion, our combination therapy efficiently eradicated CT26 colon carcinoma in mice and also generated strong anti-tumour immune memory.

The role of monocyte-derived macrophages in the lung: it's all about context

Macrophages are present in every tissue of the body where they play crucial roles in maintaining tissue homeostasis and providing front line defence against pathogens. Arguably, this is most important at mucosal barrier tissues, such as the lung and gut, which are major ports of entry for pathogens. However, a common feature of inflammation, infection or injury is the loss of tissue resident macrophages and accumulation of monocytes from the circulation, which differentiate, to different extents, into macrophages. The exact fate and function of these elicited, monocyte-derived macrophages in infection, injury and inflammation remains contentious. While some studies have documented the indispensable nature of monocytes and their macrophage derivatives in combatting infection and restoration of lung homeostasis following insult, observations from clinical studies and preclinical models of lung infection/injury shows that monocytes and their progeny can become dysregulated in severe pathology, often perpetuating rather than resolving the insult. In this Mini Review, we aim to bring together these somewhat contradictory reports by discussing how the plasticity of monocytes allow them to assume distinct functions in different contexts in the lung, from health to infection, and effective tissue repair to fibrotic disease.

Shaping of the alveolar landscape by respiratory infections and long-term consequences for lung immunity

Respiratory infections and especially viral infections, along with other extrinsic environmental factors, have been shown to profoundly affect macrophage populations in the lung. In particular, alveolar macrophages (AMs) are important sentinels during respiratory infections and their disappearance opens a niche for recruited monocytes (MOs) to differentiate into resident macrophages. Although this topic is still the focus of intense debate, the phenotype and function of AMs that recolonize the niche after an inflammatory insult, such as an infection, appear to be dictated in part by their origin, but also by local and/or systemic changes that may be imprinted at the epigenetic level. Phenotypic alterations following respiratory infections have the potential to shape lung immunity for the long-term, leading to beneficial responses such as protection against allergic airway inflammation or against other infections, but also to detrimental responses when associated with the development of immunopathologies. This review reports the persistence of virus-induced functional alterations in lung macrophages, and discusses the importance of this imprinting in explaining inter-individual and lifetime immune variation.

Fine particulate matter (PM2.5) induces inhibitory memory alveolar macrophages through the AhR/IL-33 pathway

Fine particulate matter (PM2.5) concentrations have decreased in the past decade. The adverse effects of acute PM2.5 exposure on respiratory diseases have been well recognized. To explore the long-term effects of PM2.5 exposure on chronic obstructive pulmonary disease (COPD), mice were exposed to PM2.5 for 7 days and rest for 21 days, followed by challenges with lipopolysaccharide (LPS) and porcine pancreatic elastase (PPE). Unexpectedly, PM2.5 exposure and rest alleviated the disease severity and airway inflammatory responses in COPD-like mice. Although acute PM2.5 exposure increased airway inflammation, rest for 21 days reversed the airway inflammatory responses, which was associated with the induction of inhibitory memory alveolar macrophages (AMs). Similarly, polycyclic aromatic hydrocarbons (PAHs) in PM2.5 exposure and rest decreased pulmonary inflammation, accompanied by inhibitory memory AMs. Once AMs were depleted, pulmonary inflammation was aggravated. PAHs in PM2.5 promoted the secretion of IL-33 from airway epithelial cells via the aryl hydrocarbon receptor (AhR)/ARNT pathway. High-throughput mRNA sequencing revealed that PM2.5 exposure and rest drastically changed the mRNA profiles in AMs, which was largely rescued in IL-33^-/- mice. Collectively, our results indicate that PM2.5 may mitigate pulmonary inflammation, which is mediated by inhibitory trained AMs via IL-33 production from epithelial cells through the AhR/ARNT pathway. We provide the rationale that PM2.5 plays complicated roles in respiratory disease.

Low-Dose Lipopolysaccharide Protects from Lethal Paramyxovirus Infection in a Macrophage- and TLR4-Dependent Process

Respiratory diseases are a major public health burden and a leading cause of death and disability in the world. Understanding antiviral immune responses is crucial to alleviate morbidity and mortality associated with these respiratory viral infections. Previous data from human and animal studies suggested that pre-existing atopy may provide some protection against severe disease from a respiratory viral infection. However, the mechanism(s) of protection is not understood. Low-dose LPS has been shown to drive an atopic phenotype in mice. In addition, LPS has been shown in vitro to have an antiviral effect. We examined the effect of LPS treatment on mortality to the murine parainfluenza virus Sendai virus. Low-dose LPS treatment 24 h before inoculation with a normally lethal dose of Sendai virus greatly reduced death. This protection was associated with a reduced viral titer and reduced inflammatory cytokine production in the airways. The administration of LPS was associated with a marked increase in lung neutrophils and macrophages. Depletion of neutrophils failed to reverse the protective effect of LPS; however, depletion of macrophages reversed the protective effect of LPS. Further, we demonstrate that the protective effect of LPS depends on type I IFN and TLR4-MyD88 signaling. Together, these studies demonstrate pretreatment with low-dose LPS provides a survival advantage against a severe respiratory viral infection through a macrophage-, TLR4-, and MyD88-dependent pathway.

Memory Macrophages

Immunological memory is a crucial part of the immune defense that allows organisms to respond against previously encountered pathogens or other harmful factors. Immunological memory is based on the establishment of epigenetic modifications of the genome. The ability to memorize encounters with pathogens and other harmful factors and mount enhanced defense upon subsequent encounters is an evolutionarily ancient mechanism operating in all animals and plants. However, the term immunological memory is usually restricted to the organisms (invertebrates and vertebrates) possessing the immune system. The mammalian immune system, with innate and adaptive branches, is the most sophisticated among vertebrates. The concept of innate memory and memory macrophages is relatively new and thus understudied. We introduce the concept of immunological memory and describe types of memory in different species and their evolutionary status. We discuss why the traditional view of innate immune cells as the first-line defenders is too restrictive and how the innate immune cells can accumulate and retain immunologic memory. We describe how the initial priming leads to chromatin remodeling and epigenetic changes, which allow memory macrophage formation. We also summarize what is currently known about the mechanisms underlying development of memory macrophages; their molecular and metabolic signature and surface markers; and how they may contribute to immune defense, diseases, and organ transplantation.

Trained Immunity as a Prospective Tool against Emerging Respiratory Pathogens

Although parental vaccines offer long-term protection against homologous strains, they rely exclusively on adaptive immune memory to produce neutralizing antibodies that are ineffective against emerging viral variants. Growing evidence highlights the multifaceted functions of trained immunity to elicit a rapid and enhanced innate response against unrelated stimuli or pathogens to subsequent triggers. This review discusses the protective role of trained immunity against respiratory pathogens and the experimental models essential for evaluating novel inducers of trained immunity. The review further elaborates on the potential of trained immunity to leverage protection against pathogens via the molecular patterns of antigens by pathogen recognition receptors (PPRs) on innate immune cells. The review also focuses on integrating trained innate memory with adaptive memory to shape next-generation vaccines by coupling each one’s unique characteristics.

The Ability of Postimmunobiotics from L. rhamnosus CRL1505 to Protect against Respiratory Syncytial Virus and Pneumococcal Super-Infection Is a Strain-Dependent Characteristic

Previously, we demonstrated that the non-viable strain Lacticaseibacillus rhamnosus CRL1505 (NV1505) or its purified peptidoglycan (PG1505) differentially modulated the respiratory innate antiviral immune response triggered by Toll-like receptor (TLR)-3 activation in infant mice, improving the resistance to primary respiratory syncytial virus (RSV) infection and secondary pneumococcal pneumonia. In this work, we evaluated the effect of other non-viable L. rhamnosus strains and their peptidoglycans on the respiratory immune response and their impact on primary and secondary respiratory infections. In addition, the duration of the protective effect induced by NV1505 and PG1505 as well as their ability to protect against different Streptococcus pneumoniae serotypes were evaluated. Our results showed that among the five selected L. rhamnosus strains (CRL1505, CRL498, CRL576, UCO25A and IBL027), NV1505 and NVIBL027 improved the protection against viral and pneumococcal infections by modulating the respiratory immune response. Of note, only the PG1505 presented immunomodulatory activities when compared with the other purified peptidoglycans. Studies on alveolar macrophages showed that NV1505 and PG1505 differentially modulated the expression of IL-6, IFN-γ, IFN-β, TNF-α, OAS1, RNAseL and IL-27 genes in response to RSV infection, and IL-6, IFN-γ, IL-1β, TNF-α, CCL2, CXCL2, CXCL10 and IL-27 in response to pneumococcal challenge. Furthermore, we demonstrated that NV1505 and PG1505 treatments protected mice against secondary pneumococcal pneumonia produced by different serotypes of S. pneumoniae until 30 days after stimulation with poly(I:C). This work advances the characterization of the protective effect of NV1505 and PG1505 by demonstrating that they increase resistance against the pneumococcal serotypes 3, 6B, 14 and 19F, with an effect that lasts up to 30 days after the primary viral inflammation. The results also confirm that the immunomodulatory properties of NV1505 and PG1505 are unique and are not shared by other members of this species, and suggest the existence of a capacity to stimulate trained immunity in alveolar macrophages.