Alveolar Macrophage Phagocytosis and Bacteria Clearance in Mice

Current Challenges to Align Inflammatory Key Events in Animals and Lung Cell Models In Vitro

With numerous novel and innovative in vitro models emerging every year to reduce or replace animal testing, there is an urgent need to align the design, harmonization, and validation of such systems using in vitro-in vivo extrapolation (IVIVE) approaches. In particular, in inhalation toxicology, there is a lack of predictive and prevalidated in vitro lung models that can be considered a valid alternative for animal testing. The predictive power of such models can be enhanced by applying the Adverse Outcome Pathways (AOP) framework, which casually links key events (KE) relevant to IVIVE. However, one of the difficulties identified is that the endpoint analysis and readouts of specific assays in in vitro and animal models for specific toxicants are currently not harmonized, making the alignment challenging. We summarize the current state of the art in endpoint analysis in the two systems, focusing on inflammatory-induced effects and providing guidance for future research directions to improve the alignment.

Mechanistic model of radiotherapy-induced lung fibrosis using coupled 3D agent-based and Monte Carlo simulations

BACKGROUND

Mechanistic modelling of normal tissue toxicities is unfolding as an alternative to the phenomenological normal tissue complication probability models. The latter, currently used in the clinics, rely exclusively on limited patient data and neglect spatial dose distribution information. Among the various approaches, agent-based models are appealing as they provide the means to include patient-specific parameters and simulate long-term effects in complex systems. However, Monte Carlo tools remain the state-of-the-art for modelling radiation transport and provide measurements of the delivered dose with unmatched precision.

METHODS

In this work, we develop and characterize a coupled 3D agent-based - Monte Carlo model that mechanistically simulates the onset of the radiation-induced lung fibrosis in an alveolar segment. To the best of our knowledge, this is the first such model.

RESULTS

Our model replicates extracellular matrix patterns, radiation-induced lung fibrosis severity indexes and functional subunits survivals that show qualitative agreement with experimental studies and are consistent with our past results. Moreover, in accordance with experimental results, higher functional subunits survival and lower radiation-induced lung fibrosis severity indexes are achieved when a 5-fractions treatment is simulated. Finally, the model shows increased sensitivity to more uniform protons dose distributions with respect to more heterogeneous ones from photon irradiation.

CONCLUSIONS

This study lays thus the groundwork for further investigating the effects of different radiotherapeutic treatments on the onset of radiation-induced lung fibrosis via mechanistic modelling.

Comparison of PET tracing and biodistribution between 64Cu-labeled micro-and nano-polystyrene in a murine inhalation model

INTRODUCTION

Recent studies showed the presence of microplastic in human lungs. There remains an unmet need to identify the biodistribution of microplastic after inhalation. In this study, we traced the biodistribution of inhaled micro-sized polystyrene (mPS) and/or nano-sized PS (nPS) using 64Cu with PET in mice.

METHODS

We used 0.2-0.3-µm sized mPS and 20-nm sized nPS throughout. 64Cu-DOTA-mPS, 64Cu-DOTA-nPS and/or 64CuCl2 were used to trace the distribution in the murine inhalation model. PET images were acquired using an INVEON PET scanner at 1, 12, 24, 48, and 72 h after intratracheal instillation, and the SUVmax for interesting organs were determined, biodistribution was then determined in terms of percentage injected dose/gram of tissue (%ID/g). Ex vivo tissue-radio thin-layer chromatography (Ex vivo-radioTLC) was used to demonstrate the existence of 64Cu-DOTA-PS in tissue.

RESULTS

PET image demonstrated that the amount of 64Cu-DOTA-mPS retained within the lung was significantly higher than 64Cu-DOTA-nPS until 72 h; SUVmax values of 64Cu-DOTA-mPS in lungs was 11.7 ± 5.0, 48.3 ± 6.2, 65.5 ± 2.3, 42.2 ± 13.1, and 13.2 ± 2.3 at 1, 12, 24, 48, and 72 h respectively whereas it was 31.2 ± 3.1, 17.3 ± 5.9, 10.0 ± 3.4, 8.1 ± 2.4 and 8.9 ± 3.6 for 64Cu-DOTA-nPS at the corresponding timepoints. The biodistribution data supported the PET data with a similar pattern of clearance of the radioactivity from the lung. nPS cleared rapidly post instillation in comparison to mPS within the lungs. Higher accumulation of %ID/g for nPS (roughly 2 times) were observed compared to mPS in spleen, liver, intestine, thymus, kidney, brain, salivary gland, ovary, and urinary bladder. Ex vivo-radioTLC was used to demonstrate that the detected gamma rays originated from 64Cu-DOTA-mPS or nPS.

CONCLUSION

PET image demonstrated the differences in accumulations of mPS and/or nPS between lungs and other interesting organs. The information provided may be used as the basis for future studies on the toxicity of mPS and/or nPS.

Resident macrophages of the lung and liver: The guardians of our tissues

Resident macrophages play a unique role in the maintenance of tissue function. As phagocytes, they are an essential first line defenders against pathogens and much of the initial characterization of these cells was focused on their interaction with viral and bacterial pathogens. However, these cells are increasingly recognized as contributing to more than just host defense. Through cytokine production, receptor engagement and gap junction communication resident macrophages tune tissue inflammatory tone, influence adaptive immune cell phenotype and regulate tissue structure and function. This review highlights resident macrophages in the liver and lung as they hold unique roles in the maintenance of the interface between the circulatory system and the external environment. As such, we detail the developmental origin of these cells, their contribution to host defense and the array of tools these cells use to regulate tissue homeostasis.

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.

Alveolar macrophages and airway hyperresponsiveness associated with respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is a ubiquitous pathogen of viral bronchiolitis and pneumonia in children younger than 2 years of age, which is closely associated with recurrent wheezing and airway hyperresponsiveness (AHR). Alveolar macrophages (AMs) located on the surface of the alveoli cavity are the important innate immune barrier in the respiratory tract. AMs are recognized as recruited airspace macrophages (RecAMs) and resident airspace macrophages (RAMs) based on their origins and roaming traits. AMs are polarized in the case of RSV infection, forming two macrophage phenotypes termed as M1-like and M2-like macrophages. Both M1 macrophages and M2 macrophages are involved in the modulation of inflammatory responses, among which M1 macrophages are capable of pro-inflammatory responses and M2 macrophages are capable of anti-proinflammatory responses and repair damaged tissues in the acute and convalescent phases of RSV infection. Polarized AMs affect disease progression through the alteration of immune cell surface phenotypes as well as participate in the regulation of T lymphocyte differentiation and the type of inflammatory response, which are closely associated with long-term AHR. In recent years, some progress have been made in the regulatory mechanism of AM polarization caused by RSV infection, which participates in acute respiratory inflammatory response and mediating AHR in infants. Here we summarized the role of RSV-infection-mediated AM polarization associated with AHR in infants.

Protease-activated receptor 2 promotes clearance of Pseudomonas aeruginosa infection by inducing cAMP-Rac1 signaling in alveolar macrophages

Efficient phagocytosis of pathogens by the innate immune system during infectious injury is vital for restoring tissue integrity. Impaired phagocytosis, such as in the case of infection with Pseudomonas aeruginosa, a broad-spectrum antibiotic-resistant Gram-negative bacterium, can lead to a life threatening lung disorder, acute lung injury (ALI). Evidence indicates that loss of protease-activated receptor 2 (PAR2) impaired Pseudomonas aeruginosa clearance leading to non-resolvable ALI, but the mechanism remains unclear. Here, we focused on the alveolar macrophages (AMs), the predominant population of lung-resident macrophages involved in sensing bacteria, to understand their role in PAR2-mediated phagocytosis of Pseudomonas aeruginosa. We found that upon binding Pseudomonas aeruginosa, PAR2-expressing but not PAR2-null AMs had increased cAMP levels, which activated Rac1 through protein kinase A. Activated Rac1 increased actin-rich protrusions to augment the phagocytosis of Pseudomonas aeruginosa. Administration of liposomes containing constitutively active Rac1 into PAR2-null mice lungs rescued phagocytosis and enhanced the survival of PAR2-null mice from pneumonia. These studies showed that PAR2 drives the cAMP-Rac1 signaling cascade that activates Pseudomonas aeruginosa phagocytosis in AMs, thereby preventing death from bacterial pneumonia.

Recruitment and training of alveolar macrophages after pneumococcal pneumonia

Recovery from pneumococcal pneumonia remodels the pool of alveolar macrophages so that they exhibit new surface marker profiles, transcriptomes, metabolomes, and responses to infection. Mechanisms mediating alveolar macrophage phenotypes after pneumococcal pneumonia have not been delineated. IFN-γ and its receptor on alveolar macrophages were essential for certain, but not all, aspects of the remodeled alveolar macrophage phenotype. IFN-γ was produced by CD4⁺ T cells plus other cells, and CD4⁺ cell depletion did not prevent alveolar macrophage remodeling. In mice infected or recovering from pneumococcus, monocytes were recruited to the lungs, and the monocyte-derived macrophages developed characteristics of alveolar macrophages. CCR2 mediated the early monocyte recruitment but was not essential to the development of the remodeled alveolar macrophage phenotype. Lineage tracing demonstrated that recovery from pneumococcal pneumonias converted the pool of alveolar macrophages from being primarily of embryonic origin to being primarily of adult hematopoietic stem cell origin. Alveolar macrophages of either origin demonstrated similar remodeled phenotypes, suggesting that ontogeny did not dictate phenotype. Our data reveal that the remodeled alveolar macrophage phenotype in lungs recovered from pneumococcal pneumonia results from a combination of new recruitment plus training of both the original cells and the new recruits.

SARS-CoV-2 treatment effects induced by ACE2-expressing microparticles are explained by the oxidized cholesterol-increased endosomal pH of alveolar macrophages

Exploring the cross-talk between the immune system and advanced biomaterials to treat SARS-CoV-2 infection is a promising strategy. Here, we show that ACE2-overexpressing A549 cell-derived microparticles (AO-MPs) are a potential therapeutic agent against SARS-CoV-2 infection. Intranasally administered AO-MPs dexterously navigate the anatomical and biological features of the lungs to enter the alveoli and are taken up by alveolar macrophages (AMs). Then, AO-MPs increase the endosomal pH but decrease the lysosomal pH in AMs, thus escorting bound SARS-CoV-2 from phago-endosomes to lysosomes for degradation. This pH regulation is attributable to oxidized cholesterol, which is enriched in AO-MPs and translocated to endosomal membranes, thus interfering with proton pumps and impairing endosomal acidification. In addition to promoting viral degradation, AO-MPs also inhibit the proinflammatory phenotype of AMs, leading to increased treatment efficacy in a SARS-CoV-2-infected mouse model without side effects. These findings highlight the potential use of AO-MPs to treat SARS-CoV-2-infected patients and showcase the feasibility of MP therapies for combatting emerging respiratory viruses in the future.

A Children's Health Perspective on Nano- and Microplastics

BACKGROUND

Pregnancy, infancy, and childhood are sensitive windows for environmental exposures. Yet the health effects of exposure to nano- and microplastics (NMPs) remain largely uninvestigated or unknown. Although plastic chemicals are a well-established research topic, the impacts of plastic particles are unexplored, especially with regard to early life exposures.

OBJECTIVES

This commentary aims to summarize the knowns and unknowns around child- and pregnancy-relevant exposures to NMPs via inhalation, placental transfer, ingestion and breastmilk, and dermal absorption.

METHODS

A comprehensive literature search to map the state of the science on NMPs found 37 primary research articles on the health relevance of NMPs during early life and revealed major knowledge gaps in the field. We discuss opportunities and challenges for quantifying child-specific exposures (e.g., NMPs in breastmilk or infant formula) and health effects, in light of global inequalities in baby bottle use, consumption of packaged foods, air pollution, hazardous plastic disposal, and regulatory safeguards. We also summarize research needs for linking child health and NMP exposures and address the unknowns in the context of public health action.

DISCUSSION

Few studies have addressed child-specific sources of exposure, and exposure estimates currently rely on generic assumptions rather than empirical measurements. Furthermore, toxicological research on NMPs has not specifically focused on child health, yet children's immature defense mechanisms make them particularly vulnerable. Apart from few studies investigating the placental transfer of NMPs, the physicochemical properties (e.g., polymer, size, shape, charge) driving the absorption, biodistribution, and elimination in early life have yet to be benchmarked. Accordingly, the evidence base regarding the potential health impacts of NMPs in early life remains sparse. Based on the evidence to date, we provide recommendations to fill research gaps, stimulate policymakers and industry to address the safety of NMPs, and point to opportunities for families to reduce early life exposures to plastic. https://doi.org/10.1289/EHP9086.

Probiotics protect against RSV infection by modulating the microbiota-alveolar-macrophage axis

Respiratory syncytial virus (RSV) is leading cause of respiratory tract infections in early childhood. Gut microbiota is closely related with the pulmonary antiviral immunity. Recent evidence shows that gut dysbiosis is involved in the pathogenesis of RSV infection. Therefore; pharmacological and therapeutic strategies aiming to readjust the gut dysbiosis are increasingly important for the treatment of RSV infection. In this study, we evaluated the therapeutic effects of a probiotic mixture on RSV-infected mice. This probiotic mixture consisted of Lactobacillus rhamnosus GG, Escherichia coli Nissle 1917 and VSL#3 was orally administered to neonatal mice on a daily basis either for 1 week in advance or for 3 days starting from the day of RSV infection. We showed that administration of the probiotics protected against RSV-induced lung pathology by suppressing RSV infection and exerting an antiviral response via alveolar macrophage (AM)-derived IFN-β. Furthermore, administration of the probiotics reversed gut dysbiosis and significantly increased the abundance of short-chain fatty acid (SCFA)-producing bacteria in RSV-infected mice, which consequently led to elevated serum SCFA levels. Moreover, administration of the probiotics restored lung microbiota in RSV-infected mice. We demonstrated that the increased production of IFN-β in AMs was attributed to the increased acetate in circulation and the levels of Corynebacterium and Lactobacillus in lungs. In conclusion, we reveal that probiotics protect against RSV infection in neonatal mice through a microbiota-AM axis, suggesting that the probiotics may be a promising candidate to prevent and treat RSV infection, and deserve more research and development in future.

Development and Functions of Alveolar Macrophages

Macrophages residing in various tissue types are unique in terms of their anatomical locations, ontogenies, developmental pathways, gene expression patterns, and immunological functions. Alveolar macrophages (AMs) reside in the alveolar lumen of the lungs and serve as the first line of defense for the respiratory tract. The immunological functions of AMs are implicated in the pathogenesis of various pulmonary diseases such as allergic asthma, chronic obstructive pulmonary disorder (COPD), pulmonary alveolar proteinosis (PAP), viral infection, and bacterial infection. Thus, the molecular mechanisms driving the development and function of AMs have been extensively investigated. In this review article, we discuss the roles of granulocyte-macrophage colony-stimulating factor (GM-CSF) and transforming growth factor (TGF)-β in AM development, and provide an overview of the anti-inflammatory and proinflammatory functions of AMs in various contexts. Notably, we examine the relationships between the metabolic status of AMs and their development processes and functions. We hope that this review will provide new information and insight into AM development and function.

Panton-Valentine Leukocidin Induces Cytokine Release and Cytotoxicity Mediated by the C5a Receptor on Rabbit Alveolar Macrophages

Necrotizing pneumonia caused by Panton-Valentine leukocidin (PVL)-positive community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) has high mortality rates and is currently a serious clinical issue. PVL is a two-component toxin (LukS-PV and LukF-PV). It can cause necrosis in target cells by forming pores consisting of an octamer comprised of LukS-PV and LukF-PV. However, considering the specificity of PVL towards several target cells and species, the specific effect of PVL remains controversial. Therefore, we focused on necrotizing pneumonia caused by PVL-positive S. aureus and clarified the effect of PVL on alveolar macrophages, which play a central role in innate immunity in the alveolar space. We constructed recombinant PVL (rPVL) components and stimulated alveolar macrophages isolated from rabbits to evaluate cytotoxicity and pro-inflammatory cytokine release. Recombinant LukS-PV (rLukS-PV), but not recombinant LukF-PV (rLukF-PV), induced pro-inflammatory cytokine release. Specifically, tumor necrosis factor (TNF)-α release was mediated by the C5a receptor (C5aR) expressed on rabbit alveolar macrophages, and the toxicity of rPVL, consisting of rLukS-PV and rLukF-PV, towards rabbit alveolar macrophages was mediated by the same receptor. Overall, our findings shed light on the C5aR-mediated cytotoxic effect of PVL on alveolar macrophages, which may be useful for understanding the mechanism of necrotizing pneumonia caused by PVL.

[Phagocytosis of alveolar macrophages is suppressed in a mouse model of lipopolysaccharide-induced acute lung injury]

OBJECTIVE

To investigate the changes in phagocytic function of alveolar macrophages (AMs) in mice with lipopolysaccharide (LPS)-induced acute lung injury (ALI) and explore the possible mechanism.

METHODS

Kunming mice were randomly divided into normal control group and ALI (induced by LPS instillation in the airway) model group. AMs were obtained from bronchoalveolar lavage fluid in both groups, and phagocytosis of the AMs was observed using flow cytometry and fluorescence microscopy. Western blotting and ELISA were used to detect the expression and secretion of IL-33 in the lung tissue of the mice. We also detected the secretion of IL-33 by an alveolar epithelial cell line MLE-12 in response to stimulation with different concentrations of LPS. The AMs from the normal control mice were treated with different concentrations of LPS and IL-33, and the changes in the phagocytic activity of the cells were observed.

RESULTS

Compared with those in normal control group, the percentage of AMs phagocytosing fluorescent microspheres was significantly decreased, and the expression of IL-33 in lung tissue and IL-33 level in the bronchoalveolar lavage fluid were significantly increased in ALI mice (P < 0.01). LPS (100-1000 ng/mL) obviously promoted the secretion of IL-33 in cultured MLE-12 cells (P < 0.01). Both LPS (10-500 ng/mL) and IL-33 (100 ng/mL) significantly inhibited the phagocytic activity of the AMs from normal control mice (P < 0.01).

CONCLUSIONS

The phagocytic activity of AMs is weakened in ALI mice possibly due to direct LPS stimulation and the inhibitory effect of the alarmin IL-33 produced by LPS-stimulated alveolar epithelial cells.