Neutrophil-Dependent Immunity During Pulmonary Infections and Inflammations

Dynamic Immune Indicator Changes as Predictors of ARDS in ICU Patients with Sepsis: A Retrospective Study

Background

Understanding the dynamic changes in immune indicators during sepsis and their predictive value for Acute respiratory distress syndrome (ARDS) is crucial for improving patient outcomes.

Methods

This single-center, observational retrospective study was conducted at Lishui Central Hospital, Zhejiang Province. Patients diagnosed with Sepsis-3 were categorized into non-ARDS and ARDS groups based on ARDS development. Data collection included demographics, clinical data, and immune parameters. Immune parameters were collected on days 1, 3, and 7 post-admission. Multivariate logistic regression analysis identified independent risk factors for ARDS, and a nomogram model was constructed. The predictive ability of the model was evaluated using ROC curves.

Results

Multivariate analysis identified key factors for the nomogram, including CD4, CD8, Treg, lymphocyte, IgG, and IgA levels on Days 3 and 7. On Day 3, CD8 (P < 0.001), Tregs (P = 0.021), IgG (P < 0.001), and IgA (P < 0.001) showed significant negative correlations with ARDS development. On Day 7, CD4 (P < 0.001), CD8 (P < 0.001), lymphocyte count (P < 0.001), and IgA (P < 0.001) similarly demonstrated significant negative correlations with ARDS risk. The nomogram model had an AUC of 0.998 (95% CI: 0.997-0.999), indicating high predictive ability.

Conclusion

Early dynamic changes in immune indicators, including CD8, CD4, Treg, IgA, IgG, and Lymphocyte, predict ARDS development in ICU sepsis patients.

Impact of microbial diversity on inflammatory cytokines and respiratory pattern measured in whole-body plethysmography in guinea pig models

OBJECTIVE

This study investigates the breathing patterns and immune status of guinea pigs raised under specific pathogen-free (SPF) conditions compared to conventionally bred (CON).

METHODS

Breathing pattern parameters were assessed using whole-body plethysmography (WBP) during quiet breathing and saline nebulisation. Blood and bronchoalveolar lavage fluid (BALF) were analysed for white blood cell, neutrophil and eosinophil counts, and cytokine levels (TNF-α, IL-1β, IL-4).

RESULTS

SPF guinea pigs exhibited higher tidal volume, expired volume, minute volume, and airflow parameters than CON guinea pigs. The immune analysis revealed lower white blood cell counts and IL-4 levels in SPF guinea pigs. These findings indicate that SPF guinea pigs have different respiratory and immune responses than CON guinea pigs.

CONCLUSION

The study highlights that the maturation processes affecting breathing pattern parameters in SPF guinea pigs differ significantly from those in CON guinea pigs. This suggests potential limitations of SPF animals in respiratory physiology research due to their different immune and respiratory responses.

A pan-immune panorama of bacterial pneumonia revealed by a large-scale single-cell transcriptome atlas

Bacterial pneumonia is a significant public health burden, contributing to substantial morbidity, mortality, and healthcare costs. Current therapeutic strategies beyond antibiotics and adjuvant therapies are limited, highlighting the need for a deeper understanding of the disease pathogenesis. Here, we employed single-cell RNA sequencing of 444,146 bronchoalveolar lavage fluid cells (BALFs) from a large cohort of 74 individuals, including 58 patients with mild (n = 22) and severe (n = 36) diseases as well as 16 healthy donors. Enzyme-linked immunosorbent and histological assays were applied for validation within this cohort. The heterogeneity of immune responses in bacterial pneumonia was observed, with distinct immune cell profiles related to disease severity. Severe bacterial pneumonia was marked by an inflammatory cytokine storm resulting from systemic upregulation of S100A8/A9 and CXCL8, primarily due to specific macrophage and neutrophil subsets. In contrast, mild bacterial pneumonia exhibits an effective humoral immune response characterized by the expansion of T follicular helper and T helper 2 cells, facilitating B cell activation and antibody production. Although both disease groups display T cell exhaustion, mild cases maintained robust cytotoxic CD8+T cell function, potentially reflecting a compensatory mechanism. Dysregulated neutrophil and macrophage responses contributed significantly to the pathogenesis of severe disease. Immature neutrophils promote excessive inflammation and suppress T cell activation, while a specific macrophage subset (Macro_03_M1) displaying features akin to myeloid-derived suppressor cells (M-MDSCs) suppress T cells and promote inflammation. Together, these findings highlight potential therapeutic targets for modulating immune responses and improving clinical outcomes in bacterial pneumonia.

Fatty Acids Derived from Royal Jelly Exert Anti-Inflammatory and Antibacterial Activities in the Treatment of Pseudomonas aeruginosa-Induced Acute Pneumonia

Pseudomonas aeruginosa, an opportunistic pathogen, commonly causes hospital-acquired pneumonia. Royal jelly fatty acids (RJFAs), a mixture of various fatty acids extracted from royal jelly, exhibit antibacterial and anti-inflammatory properties in treating many infectious diseases. Nevertheless, the therapeutic mechanisms of RJFAs in treatment of acute P. aeruginosa pulmonary infection are still unclear. Herein, we initially extracted the fatty acids from royal jelly and characterized their chemical constituents using headspace gas chromatography-mass spectrometry. Furthermore, we examined the antibacterial effect of RJFAs in vitro and explored its therapeutic effect and molecular mechanisms in treating acute P. aeruginosa pulmonary infection in vivo. The in vitro antibacterial studies revealed that RJFAs significantly inhibited P. aeruginosa growth. Moreover, the in vivo studies showed that the RJFAs effectively mitigated the lung damage and inflammation induced by P. aeruginosa through impairing neutrophil infiltration, reducing the bacterial load in lung and diminishing the production of proinflammatory cytokines, including tumor necrosis factor (TNF-α), interleukin (IL-1β), IL-6, and macrophage inflammatory protein-2 (MIP-2). In addition, the mice treated with RJFAs exhibited reduced phosphorylation of extracellular signal-regulated kinase (ERK), p38, c-Jun N-terminal kinase (JNK), c-Jun, and nuclear factor-kappa B (NF-κB) p65 in the lung tissues in comparison with that of the mice without drug treatment. These findings demonstrated that RJFAs exhibited significant antibacterial and anti-inflammatory effects in treating the P. aeruginosa-induced acute pneumonia, and the anti-inflammatory effects were exerted through suppressing the mitogen-activated protein kinase/activator protein-1 (MAPK/AP-1) pathway and NF-κB activation, suggesting a promising therapeutic potential of RJFAs against acute bacterial pneumonia.

Single-cell transcriptome atlas revealed bronchoalveolar immune features related to disease severity in pediatric Mycoplasma pneumoniae pneumonia

The mechanisms underlying protective immunity in mild Mycoplasma pneumoniae pneumonia (MPP) and the pathogenesis of severe MPP, characterized by dysregulated immune responses, remain unclear. Here, we performed single-cell RNA sequencing (scRNA-seq) to profile bronchoalveolar lavage fluid (BALF) samples from 13 healthy donors and 24 hospitalized pediatric patients with MPP, covering both mild and severe cases. Severe MPP patients exhibited high levels of exhausted T cells and M1-like macrophages, with the exhaustion of T cells attributed to persistent type I interferon signaling and inadequate assistance from CD4+ T cells. Significant cell-cell interactions between exhausted T cells and programmed death-ligand 1+ (PD-L1+) macrophages were detected in severe patients, potentially mediated through inhibitor molecules (e.g., PD1) and their receptors (e.g., PD-L1), as well as human leukocyte antigen class I molecules and their receptors (e.g., KLRC1/D2), resulting in the dysfunction of anti-MP immune responses. Mild MPP patients were featured by an increased abundance of neutrophils, coupled with enhanced activation, contributing to protective immunity. Together, our study provides a detailed characterization of the BALF immune landscape in MPP patients, revealing distinct immune characteristics between mild and severe cases, which offers a valuable resource for understanding MPP immunopathogenesis and formulating effective therapeutic strategies.

Immunotherapy for Pulmonary Arterial Hypertension: From the Pathogenesis to Clinical Management

Pulmonary hypertension (PH) is a progressive cardiovascular disease, which may lead to severe cardiopulmonary dysfunction. As one of the main PH disease groups, pulmonary artery hypertension (PAH) is characterized by pulmonary vascular remodeling and right ventricular dysfunction. Increased pulmonary artery resistance consequently causes right heart failure, which is the major reason for morbidity and mortality in this disease. Although various treatment strategies have been available, the poor clinical prognosis of patients with PAH reminds us that further studies of the pathological mechanism of PAH are still needed. Inflammation has been elucidated as relevant to the initiation and progression of PAH, and plays a crucial and functional role in vascular remodeling. Many immune cells and cytokines have been demonstrated to be involved in the pulmonary vascular lesions in PAH patients, with the activation of downstream signaling pathways related to inflammation. Consistently, this influence has been found to correlate with the progression and clinical outcome of PAH, indicating that immunity and inflammation may have significant potential in PAH therapy. Therefore, we reviewed the pathogenesis of inflammation and immunity in PAH development, focusing on the potential targets and clinical application of anti-inflammatory and immunosuppressive therapy.

Building a human lung from pluripotent stem cells to model respiratory viral infections

To protect against the constant threat of inhaled pathogens, the lung is equipped with cellular defenders. In coordination with resident and recruited immune cells, this defence is initiated by the airway and alveolar epithelium following their infection with respiratory viruses. Further support for viral clearance and infection resolution is provided by adjacent endothelial and stromal cells. However, even with these defence mechanisms, respiratory viral infections are a significant global health concern, causing substantial morbidity, socioeconomic losses, and mortality, underlining the need to develop effective vaccines and antiviral medications. In turn, the identification of new treatment options for respiratory infections is critically dependent on the availability of tractable in vitro experimental models that faithfully recapitulate key aspects of lung physiology. For such models to be informative, it is important these models incorporate human-derived, physiologically relevant versions of all cell types that normally form part of the lungs anti-viral response. This review proposes a guideline using human induced pluripotent stem cells (iPSCs) to create all the disease-relevant cell types. iPSCs can be differentiated into lung epithelium, innate immune cells, endothelial cells, and fibroblasts at a large scale, recapitulating in vivo functions and providing genetic tractability. We advocate for building comprehensive iPSC-derived in vitro models of both proximal and distal lung regions to better understand and model respiratory infections, including interactions with chronic lung diseases.

Inhibiting SIRT-2 by AK-7 restrains airway inflammation and oxidative damage promoting lung resurgence through NF-kB and MAP kinase signaling pathway

Introduction

Chronic obstructive pulmonary disease (COPD) is a major global cause of mortality with limited effective treatments. Sirtuins (SIRT) are histone deacetylases that are involved in the regulation of redox and inflammatory homeostasis. Hence, the present study aims to investigate the role of SIRT-2 in modulating inflammation in a murine model of COPD.

Methods

COPD in mice was established by cigarette smoke (CS) exposure for 60 days, and AK-7 was used as the specific SIRT-2 inhibitor. AK-7 (100 µg/kg and 200 µg/kg body weight) was administered intranasally 1 h before CS exposure. Molecular docking was performed to analyze the binding affinity of different inflammatory proteins with AK-7.

Results

Immune cell analysis showed a significantly increased number of macrophages (F4/80), neutrophils (Gr-1), and lymphocytes (CD4+, CD8+, and CD19+) in the COPD, group and their population was declined by AK-7 administration. Total reactive oxygen species, total inducible nitric oxide synthase, inflammatory mediators such as neutrophil elastase, C-reactive protein, histamine, and cytokines as IL4, IL-6, IL-17, and TNF-α were elevated in COPD and declined in the AK-7 group. However, IL-10 showed reverse results representing anti-inflammatory potency. AK-7 administration by inhibiting SIRT-2 decreased the expression of p-NF-κB, p-P38, p-Erk, and p-JNK and increased the expression of Nrf-2. Furthermore, AK-7 also declined the lung injury by inhibiting inflammation, parenchymal destruction, emphysema, collagen, club cells, and Kohn pores. AK-7 also showed good binding affinity with inflammatory proteins.

Discussion

The current study reveals that SIRT-2 inhibition mitigates COPD severity and enhances pulmonary therapeutic interventions, suggesting AK-7 as a potential therapeutic molecule for COPD medication development.

Bacteria-derived extracellular vesicles: endogenous roles, therapeutic potentials and their biomimetics for the treatment and prevention of sepsis

Sepsis is one of the medical conditions with a high mortality rate and lacks specific treatment despite several years of extensive research. Bacterial extracellular vesicles (bEVs) are emerging as a focal target in the pathophysiology and treatment of sepsis. Extracellular vesicles (EVs) derived from pathogenic microorganisms carry pathogenic factors such as carbohydrates, proteins, lipids, nucleic acids, and virulence factors and are regarded as "long-range weapons" to trigger an inflammatory response. In particular, the small size of bEVs can cross the blood-brain and placental barriers that are difficult for pathogens to cross, deliver pathogenic agents to host cells, activate the host immune system, and possibly accelerate the bacterial infection process and subsequent sepsis. Over the years, research into host-derived EVs has increased, leading to breakthroughs in cancer and sepsis treatments. However, related approaches to the role and use of bacterial-derived EVs are still rare in the treatment of sepsis. Herein, this review looked at the dual nature of bEVs in sepsis by highlighting their inherent functions and emphasizing their therapeutic characteristics and potential. Various biomimetics of bEVs for the treatment and prevention of sepsis have also been reviewed. Finally, the latest progress and various obstacles in the clinical application of bEVs have been highlighted.

Single-Cell Profiling of Premature Neonate Airways Reveals a Continuum of Myeloid Differentiation

Single-cell genomic technologies hold great potential to advance our understanding of lung development and disease. A major limitation lies in accessing intact cells from primary lung tissues for profiling human airway health. Sampling methods such as endotracheal aspiration that are compatible with clinical interventions could enable longitudinal studies, the enrollment of large cohorts, and the development of novel diagnostics. To explore single-cell RNA sequencing profiling of the cell types present at birth in the airway lumen of extremely premature neonates (<28 wk gestation), we isolated cells from endotracheal aspirates collected from intubated neonates within the first hour after birth. We generated data on 10 subjects, providing a rich view of airway luminal biology at a critical developmental period. Our results show that cells present in the airways of premature neonates primarily represent a continuum of myeloid differentiation, including fetal monocytes (25% of total), intermediate myeloid populations (48%), and macrophages (2.6%). Applying trajectory analysis to the myeloid populations, we identified two trajectories consistent with the developmental stages of interstitial and alveolar macrophages, as well as a third trajectory presenting an alternative pathway bridging the distinct macrophage precursors. The three trajectories share many dynamic genes (N = 5,451), but also have distinct transcriptional changes (259 alveolar-specific, 666 interstitial-specific, and 285 bridging-specific). Overall, our results define cells isolated within the so-called "golden hour of birth" in extremely premature neonate airways, representing complex lung biology, and can be used in studies of human development and disease.

PD-L1-Expressing Extracellular Vesicles for the Treatment of Pneumonia

Acute respiratory distress syndrome (ARDS) is a severe lung condition with a high mortality rate and a lack of effective drug therapy. In this work, we developed mesenchymal stem cell (MSC)-derived extracellular vesicles with high PD-L1 expression (MSC-EVs-PD-L1) for treating lipopolysaccharide (LPS)-induced pneumonia by intratracheal administration. We found an upregulation of PD-1 expression in the inflammatory region of murine lungs; hence, MSC-EVs-PD-L1 exerted immunosuppressive effects via the PD-1/PD-L1 signaling pathway. Furthermore, we treated LPS-induced pneumonia mice by intratracheal administration, which enabled heavy drug accumulation in the lungs of mice and better therapeutic efficacy compared to systemic administration. Our results suggest that MSC-EVs-PD-L1 has the potential to provide a universal platform technology for the immunotherapy of pneumonia.

Occupational exposure to potentially toxic elements alters gene expression profiles in formal and informal Brazilian workers

Chemical elements, such as toxic metals, have previously demonstrated their ability to alter gene expression in humans and other species. In this study, microarray analysis was used to compare the gene expression profiles of different occupational exposure populations: a) informal workers who perform soldering of jewelry inside their houses (n = 22) in São Paulo (SP) State; and b) formal workers from a steel company (n = 10) in Rio de Janeiro (RJ) state, Brazil. Control participants were recruited from the same neighborhoods without occupational chemical exposure (n = 19 in SP and n = 8 in RJ). A total of 68 blood samples were collected and RNA was extracted and hybridized using an Agilent microarray platform. Data pre-processing, statistical and pathway analysis were performed using GeneSpring software. Different expression was detected by fold-change analysis resulting in 16 up- and 33 down-regulated genes in informal workers compared to the control group. Pathway analysis revealed genes enriched in MAPK, Toll-like receptor, and NF-kappa B signaling pathways, involved in inflammatory and immune responses. In formal workers, 20 up- and 50 down-regulated genes were found related to antimicrobial peptides, defensins, neutrophil degranulation, Fc-gamma receptor-dependent phagocytosis, and pathways associated with atherosclerosis development, which is one of the main factors involved in the progression of cardiovascular diseases. The gene IFI27 was the only one commonly differentially expressed between informal and formal workers and is known to be associated with various types of cancer. In conclusion, differences in gene expression related to occupational exposure are mainly associated with inflammation and immune response. Previous research has identified a link between inflammation and immune responses and the development of chronic diseases, suggesting that prolonged occupational exposures to potentially toxic elements in Brazilian metal workers could lead to negative health outcomes. Further analysis should be carried out to investigate its direct effects and to validate causal associations.

Neutrophil diversity and plasticity: Implications for organ transplantation

Neutrophils, as the first defenders against external microbes and stimuli, are highly active and finely regulated innate immune cells. Emerging evidence has challenged the conventional dogma that neutrophils are a homogeneous population with a short lifespan that promotes tissue damage. Recent findings on neutrophil diversity and plasticity in homeostatic and disease states have centered on neutrophils in the circulation. In contrast, a comprehensive understanding of tissue-specialized neutrophils in health and disease is still lacking. This article will first discuss how multiomics advances have contributed to our understanding of neutrophil heterogeneity and diversification in resting and pathological settings. This discussion will be followed by a focus on the heterogeneity and role of neutrophils in solid organ transplantation and how neutrophils may contribute to transplant-related complications. The goal of this article is to provide an overview of the research on the involvement of neutrophils in transplantation, with the aim that this may draw attention to an underappreciated area of neutrophil research.

Xuanfei Baidu Decoction regulates NETs formation via CXCL2/CXCR2 signaling pathway that is involved in acute lung injury

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are life-threatening symptoms in Coronavirus Disease 2019 (COVID-19) patients. Xuanfei Baidu Decoction (XFBD) is a recommend first-line traditional Chinese medicine (TCM) formula therapeutic strategy for COVID-19 patients. Prior studies demonstrated the pharmacological roles and mechanisms of XFBD and its derived effective components against inflammation and infections through multiple model systems, which provided the biological explanations for its clinical use. Our previous work revealed that XFBD inhibited macrophages and neutrophils infiltration via PD-1/IL17A signaling pathway. However, the subsequent biological processes are not well elucidated. Here, we proposed a hypothesis that XFBD can regulate the neutrophils-mediated immune responses, including neutrophil extracellular traps (NETs) formation and the generation of platelet-neutrophil aggregates (PNAs) after XFBD administration in lipopolysaccharide (LPS)-induced ALI mice. The mechanism behind it was also firstly explained, that is XFBD regulated NETs formation via CXCL2/CXCR2 axis. Altogether, our findings demonstrated the sequential immune responses of XFBD after inhibiting neutrophils infiltration, as well as shedding light on exploiting the therapy of XFBD targeting neutrophils to ameliorate ALI during the clinical course.

Unsymmetrical Phosphodiesters as GPR84 Antagonists with High Blood Exposure for the Treatment of Lung Inflammation

GPR84 is a proinflammatory G protein-coupled receptor that mediates myeloid immune cell functions. Blocking GPR84 with antagonists is a promising approach for treating inflammatory and fibrotic diseases. Previously, a GPR84 antagonist 604c, with a symmetrical phosphodiester structure, has displayed promising efficacy in a mouse model of ulcerative colitis. However, the low blood exposure resulting from physicochemical properties prevented its uses in other inflammatory diseases. In this study, a series of unsymmetrical phosphodiesters with lower lipophilicity were designed and tested. The representative compound 37 exhibited a 100-fold increase in mouse blood exposure compared to 604c while maintaining in vitro activity. In a mouse model of acute lung injury, 37 (30 mg/kg, po) significantly reduced the infiltration of proinflammatory cells and the release of inflammatory cytokines and ameliorated pathological changes equally or more effectively than N-acetylcysteine (100 mg/kg, po). These findings suggest that 37 is a promising candidate for treating lung inflammation.

CD14 signaling mediates lung immunopathology and mice mortality induced by Achromobacter xylosoxidans

Objective and design

Our research aimed to investigate the role of CD14 in pulmonary infection by Achromobacter xylosoxidans in an experimental murine model.

Methods

C57Bl/6 or CD14-deficient mice were infected intratracheally with non-lethal inoculum of A. xylosoxidans. At times 1, 3 and 7 days after infection, lungs, bronchoalveolar lavage and blood were collected. CD14 gene expression was determined by RT-PCR. The bacterial load in the lungs was assessed by counting colony forming units (CFU). Cytokines, chemokines, lipocalin-2 and sCD14 were quantified by the ELISA method. Inflammatory infiltrate was observed on histological sections stained with HE, and leukocyte subtypes were assessed by flow cytometry. In another set of experiments, C57Bl/6 or CD14-deficient mice were inoculated with lethal inoculum and the survival rate determined.

Results

CD14-deficient mice are protected from A. xylosoxidans-induced death, which is unrelated to bacterial load. The lungs of CD14-deficient mice presented a smaller area of tissue damage, less neutrophil and macrophage infiltration, less pulmonary edema, and a lower concentration of IL-6, TNF-α, CXCL1, CCL2 and CCL3 when compared with lungs of C57Bl/6 mice. We also observed that A. xylosoxidans infection increases the number of leukocytes expressing mCD14 and the levels of sCD14 in BALF and serum of C57Bl/6-infected mice.

Conclusions

In summary, our data show that in A. xylosoxidans infection, the activation of CD14 induces intense pulmonary inflammatory response resulting in mice death.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00011-022-01641-8.

Effect of multiplicity of infection on the evasion of neutrophil killing by Streptococcus agalactiae isolated from clinical mastitis bovine

Streptococcus agalactiae (S. agalactiae) causes intramammary infection in dairy cows. Increased neutrophils and a high bacterial load are important characteristics of bovine bacterial mastitis. We hypothesized that the multiplicity of infection (MOI) of S. agalactiae in bovine mastitis plays an important role in bacterial pathogenicity by modulating the neutrophil response to promote bacterial survival. Neutrophils from BALB/c mice were infected with the bovine mastitis isolate of S. agalactiae SAG-FX17 at various MOIs, and neutrophil responses were investigated. Infecting neutrophils with SAG-FX17 at an MOI of 1 induced reactive oxygen species (ROS) and neutrophil extracellular traps (NETs) formation. Bacteria at an MOI of 10 suppressed neutrophil responses, including ROS bursts, NET formation, and cell necrosis, which are conducive to bacterial multiplication within 30 min postinfection. In addition, neutrophils are destroyed by SAG-FX17 at an MOI of 100 or greater. This study identified the MOIs related to the ROS and NET suppression caused by SAG-FX17, and the findings suggested that interventions to decrease bacterial loads before the MOI of 10 could be necessary and effective to harness the power of innate immune response to eliminate pathogens.

Neutrophil Recruitment in Pneumococcal Pneumonia

Streptococcus pneumoniae (Spn) is the primary agent of community-acquired pneumonia. Neutrophils are innate immune cells that are essential for bacterial clearance during pneumococcal pneumonia but can also do harm to host tissue. Neutrophil migration in pneumococcal pneumonia is therefore a major determinant of host disease outcomes. During Spn infection, detection of the bacterium leads to an increase in proinflammatory signals and subsequent expression of integrins and ligands on both the neutrophil as well as endothelial and epithelial cells. These integrins and ligands mediate the tethering and migration of the neutrophil from the bloodstream to the site of infection. A gradient of host-derived and bacterial-derived chemoattractants contribute to targeted movement of neutrophils. During pneumococcal pneumonia, neutrophils are rapidly recruited to the pulmonary space, but studies show that some of the canonical neutrophil migratory machinery is dispensable. Investigation of neutrophil migration is necessary for us to understand the dynamics of pneumococcal infection. Here, we summarize what is known about the pathways that lead to migration of the neutrophil from the capillaries to the lung during pneumococcal infection.

Neutrophil Functional Heterogeneity and Implications for Viral Infections and Treatments

Evidence suggests that neutrophils exert specialized effector functions during infection and inflammation, and that these cells can affect the duration, severity, and outcome of the infection. These functions are related to variations in phenotypes that have implications in immunoregulation during viral infections. Although the complexity of the heterogeneity of neutrophils is still in the process of being uncovered, evidence indicates that they display phenotypes and functions that can assist in viral clearance or augment and amplify the immunopathology of viruses. Therefore, deciphering and understanding neutrophil subsets and their polarization in viral infections is of importance. In this review, the different phenotypes of neutrophils and the roles they play in viral infections are discussed. We also examine the possible ways to target neutrophil subsets during viral infections as potential anti-viral treatments.

The "Elastic Perspective" of SARS-CoV-2 Infection and the Role of Intrinsic and Extrinsic Factors

Elastin represents the structural component of the extracellular matrix providing elastic recoil to tissues such as skin, blood vessels and lungs. Elastogenic cells secrete soluble tropoelastin monomers into the extracellular space where these monomers associate with other matrix proteins (e.g., microfibrils and glycoproteins) and are crosslinked by lysyl oxidase to form insoluble fibres. Once elastic fibres are formed, they are very stable, highly resistant to degradation and have an almost negligible turnover. However, there are circumstances, mainly related to inflammatory conditions, where increased proteolytic degradation of elastic fibres may lead to consequences of major clinical relevance. In severely affected COVID-19 patients, for instance, the massive recruitment and activation of neutrophils is responsible for the profuse release of elastases and other proteolytic enzymes which cause the irreversible degradation of elastic fibres. Within the lungs, destruction of the elastic network may lead to the permanent impairment of pulmonary function, thus suggesting that elastases can be a promising target to preserve the elastic component in COVID-19 patients. Moreover, intrinsic and extrinsic factors additionally contributing to damaging the elastic component and to increasing the spread and severity of SARS-CoV-2 infection are reviewed.