Parallel activities and interactions between antimicrobial peptides and complement in host defense at the airway epithelial surface

Role of Complement Components in Asthma: A Systematic Review

Background: Asthma is a chronic inflammatory airway disease characterized by recurrent symptoms in response to a wide range of external stimuli, including allergens, viral infections, and air pollution together with internal host-derived danger signals. The disease is traditionally associated with adaptive immune responses; recent research emphasizes the critical role of innate immunity in its pathogenesis. The complement system, activated as part of the defense mechanisms, plays a crucial role in bridging innate to adaptive immunity. While experimental models demonstrate complement cascade activation in asthma, human studies remain limited. Methods: This systematic review summarizes existing literature on the complement system in asthma patients, gathering data from PubMed, Web of Science, Scopus, and Google Scholar. The protocol was registered in the OSF. Results: Out of 482 initially identified articles, only 24 met the eligibility criteria, revealing disparities in sample origin, methodologies, and populations. Despite observed heterogeneity, a consistent result was found in the elevation of complement regulatory proteins, such as complement Factor H, in samples from patients with asthma compared to those from healthy subjects. Conclusions: The increased level of regulatory proteins, such as Factor H and I highlight that these may influence asthma pathophysiology. The role of complement factors as potential biomarkers of asthma activity and severity needs further evaluation.

Specific Inhibition of Orai1-mediated Calcium Signalling Resolves Inflammation and Clears Bacteria in an Acute Respiratory Distress Syndrome Model

Rationale: Acute respiratory distress syndrome (ARDS) has an unacceptably high mortality rate (35%) and is without effective therapy. Orai1 is a Ca2+ channel involved in store-operated Ca2+ entry (SOCE), a process that exquisitely regulates inflammation. Orai1 is considered a druggable target, but no Orai1-specific inhibitors exist to date. Objectives: To evaluate whether ELD607, a first-in-class Orai1 antagonist, can treat ARDS caused by bacterial pneumonia in preclinical models. Methods: ELD607 pharmacology was evaluated in HEK293T cells and freshly isolated immune cells from patients with ARDS. A murine acute lung injury model caused by bacterial pneumonia was then used: mice were infected with Pseudomonas aeruginosa, Staphylococcus aureus, methicillin-resistant S. aureus, or multidrug-resistant P. aeruginosa and then treated with ELD607 intranasally. Measurements and Main Results: ELD607 specifically inhibited SOCE in HEK293T cells with a half-maximal inhibitory concentration of 9 nM. ELD607 was stable in ARDS airway secretions and inhibited SOCE in ARDS immune cells. In vivo, inhaled ELD607 significantly reduced neutrophilia and improved survival. Surprisingly, Orai1 inhibition by ELD607 caused a significant reduction in lung bacteria, including methicillin-resistant S. aureus. ELD607 worked as an immunomodulator that reduced cytokine levels, reduced neutrophilia, and promoted macrophage-mediated resolution of inflammation and clearance of bacteria. Indeed, when alveolar macrophages were depleted with inhaled clodronate, ELD607 was no longer able to resolve inflammation or clear bacteria. Conclusions: These data indicate that specific Orai1 inhibition by ELD607 may be a novel approach to reduce multiorgan inflammation and treat antibiotic-resistant bacteria.

Activity of airway antimicrobial peptides against cystic fibrosis pathogens

Antimicrobial peptides are important players of the innate host defence against invading microorganisms. The aim of this study was to evaluate the activity of airway antimicrobial peptides against the common cystic fibrosis (CF) pathogen Pseudomonas aeruginosa, and to compare it to the emerging multi-drug resistant CF pathogens Achromobacter xylosoxidans and Stenotrophomonas maltophilia. Clinical bacterial isolates from CF patients were used, and the antimicrobial activity of human beta-defensin 2 and 3, LL37 and lysozyme was evaluated using radial diffusion assay and viable counts. The cell surface zeta potential was analysed to estimate the net charge at the bacterial surface. Of the bacterial species included in the study, A. xylosoxidans was the most resistant to antimicrobial peptides, whereas P. aeruginosa was the most susceptible. The net charge of the bacterial surface was significantly more negative for P. aeruginosa compared to A. xylosoxidans, which may in part explain the differences in susceptibility.

C5aR inhibition of nonimmune cells suppresses inflammation and maintains epithelial integrity in SARS-CoV-2-infected primary human airway epithelia

Background

Excessive inflammation triggered by a hitherto undescribed mechanism is a hallmark of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections and is associated with enhanced pathogenicity and mortality.

Objective

Complement hyperactivation promotes lung injury and was observed in patients suffering from Middle East respiratory syndrome-related coronavirus, SARS-CoV-1, and SARS-CoV-2 infections. Therefore, we investigated the very first interactions of primary human airway epithelial cells on exposure to SARS-CoV-2 in terms of complement component 3 (C3)-mediated effects.

Methods

For this, we used highly differentiated primary human 3-dimensional tissue models infected with SARS-CoV-2 patient isolates. On infection, viral load, viral infectivity, intracellular complement activation, inflammatory mechanisms, and tissue destruction were analyzed by real-time RT-PCR, high content screening, plaque assays, luminex analyses, and transepithelial electrical resistance measurements.

Results

Here, we show that primary normal human bronchial and small airway epithelial cells respond to SARS-CoV-2 infection by an inflated local C3 mobilization. SARS-CoV-2 infection resulted in exaggerated intracellular complement activation and destruction of the epithelial integrity in monolayer cultures of primary human airway cells and highly differentiated, pseudostratified, mucus-producing, ciliated respiratory tissue models. SARS-CoV-2–infected 3-dimensional cultures secreted significantly higher levels of C3a and the proinflammatory cytokines IL-6, monocyte chemoattractant protein 1, IL-1α, and RANTES.

Conclusions

Crucially, we illustrate here for the first time that targeting the anaphylotoxin receptors C3a receptor and C5a receptor in nonimmune respiratory cells can prevent intrinsic lung inflammation and tissue damage. This opens up the exciting possibility in the treatment of COVID-19.

Cross-reactive antibodies against dust mite-derived enolase induce neutrophilic airway inflammation

BACKGROUND AND AIMS

Neutrophilic inflammation is a hallmark of some specific asthma phenotypes; its aetiology is not yet fully understood. House dust mite (HDM) is the most common factor in the pathogenesis of airway inflammation. This study aims to elucidate the role of cross-antibodies against HDM-derived factors in the development of neutrophilic inflammation in the airway.

METHODS

Blood samples were collected from asthma patients with chronic neutrophilic asthma for analysis of HDM-specific cross-reactive antibodies. The role of an antibody against HDM-derived enolase (EnoAb) in the impairment of airway epithelial barrier function and induction of airway inflammation was assessed in a cell culture model and an animal model.

RESULTS

High similarity (72%) of the enolase gene sequences was identified between HDM and human. Serum EnoAb was detected in patients with chronic neutrophilic asthma. The EnoAb bound to airway epithelial cells to form complexes with enolase, which activated complement, impaired airway epithelial barrier functions and induced neutrophilic inflammation in the airway tissues.

CONCLUSIONS

HDM-derived enolase can induce specific cross-antibodies in humans, which induce neutrophilic inflammation in the airway.

E-cigarettes, nicotine, the lung and the brain: multi-level cascading pathophysiology

Tobacco smoking is highly addictive and causes respiratory disease, cardiovascular disease and multiple types of cancer. Electronic-cigarettes (e-cigarettes) are non-combustible tobacco alternatives that aerosolize nicotine and flavouring agents in a propylene glycol-vegetable glycerine vehicle. They were originally envisaged as a tobacco cessation aid, but whether or not they help people to quit tobacco use is controversial. In this review, we have compared and contrasted what is known regarding the effects of nicotine on the lungs vs. the effects of nicotine in the brain in the context of addiction. Critically, both combustible tobacco products and e-cigarettes contain nicotine, a highly addictive, plant-derived alkaloid that binds to nicotinic acetylcholine receptors (nAChRs). Nicotine's reinforcing properties are primarily mediated by activation of the brain's mesolimbic reward circuitry and release of the neurotransmitter dopamine that contribute to the development of addiction. Moreover, nicotine addiction drives repeated intake that results in chronic pulmonary exposure to either tobacco smoke or e-cigarettes despite negative respiratory symptoms. Beyond the brain, nAChRs are also highly expressed in peripheral neurons, epithelia and immune cells, where their activation may cause harmful effects. Thus, nicotine, a key ingredient of both conventional and electronic cigarettes, produces neurological effects that drive addiction and may damage the lungs in the process, producing a complex, multilevel pathological state. We conclude that vaping needs to be studied by multi-disciplinary teams that include pulmonary and neurophysiologists as well as behaviourists and addiction specialists to fully understand their impact on human physiology.

Host-Pathogen Responses to Pandemic Influenza H1N1pdm09 in a Human Respiratory Airway Model

The respiratory Influenza A Viruses (IAVs) and emerging zoonotic viruses such as Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2) pose a significant threat to human health. To accelerate our understanding of the host–pathogen response to respiratory viruses, the use of more complex in vitro systems such as normal human bronchial epithelial (NHBE) cell culture models has gained prominence as an alternative to animal models. NHBE cells were differentiated under air-liquid interface (ALI) conditions to form an in vitro pseudostratified epithelium. The responses of well-differentiated (wd) NHBE cells were examined following infection with the 2009 pandemic Influenza A/H1N1pdm09 strain or following challenge with the dsRNA mimic, poly(I:C). At 30 h postinfection with H1N1pdm09, the integrity of the airway epithelium was severely impaired and apical junction complex damage was exhibited by the disassembly of zona occludens-1 (ZO-1) from the cell cytoskeleton. wdNHBE cells produced an innate immune response to IAV-infection with increased transcription of pro- and anti-inflammatory cytokines and chemokines and the antiviral viperin but reduced expression of the mucin-encoding MUC5B, which may impair mucociliary clearance. Poly(I:C) produced similar responses to IAV, with the exception of MUC5B expression which was more than 3-fold higher than for control cells. This study demonstrates that wdNHBE cells are an appropriate ex-vivo model system to investigate the pathogenesis of respiratory viruses.

An update on immunologic mechanisms in the respiratory mucosa in response to air pollutants

Every day, we breathe in more than 10,000 L of air that contains a variety of air pollutants that can pose negative consequences to lung health. The respiratory mucosa formed by the airway epithelium is the first point of contact for air pollution in the lung, functioning as a mechanical and immunologic barrier. Under normal circumstances, airway epithelial cells connected by tight junctions secrete mucus, airway surface lining fluid, host defense peptides, and antioxidants and express innate immune pattern recognition receptors to respond to inhaled foreign substances and pathogens. Under conditions of air pollution exposure, the defenses of the airway epithelium are compromised by reductions in barrier function, impaired host defense to pathogens, and exaggerated inflammatory responses. Central to the mechanical and immunologic changes induced by air pollution are activation of redox-sensitive pathways and a role for antioxidants in normalizing these negative effects. Genetic variants in genes important in epithelial cell function and phenotype contribute to a diversity of responses to air pollution in the population at the individual and group levels and suggest a need for personalized approaches to attenuate the respiratory mucosal immune responses to air pollution.

Intracellular C3 Protects Human Airway Epithelial Cells from Stress-associated Cell Death

The complement system provides host defense against pathogens and environmental stress. C3, the central component of complement, is present in the blood and increases in BAL fluid after injury. We recently discovered that C3 is taken up by certain cell types and cleaved intracellularly to C3a and C3b. C3a is required for CD4⁺ T-cell survival. These observations made us question whether complement operates at environmental interfaces, particularly in the respiratory tract. We found that airway epithelial cells (AECs, represented by both primary human tracheobronchial cells and BEAS-2B [cell line]) cultured in C3-free media were unique from other cell types in that they contained large intracellular stores of de novo synthesized C3. A fraction of this protein reduced ("storage form") but the remainder did not, consistent with it being pro-C3 ("precursor form"). These two forms of intracellular C3 were absent in CRISPR knockout-induced C3-deficient AECs and decreased with the use of C3 siRNA, indicating endogenous generation. Proinflammatory cytokine exposure increased both stored and secreted forms of C3. Furthermore, AECs took up C3 from exogenous sources, which mitigated stress-associated cell death (e.g., from oxidative stress or starvation). C3 stores were notably increased within AECs in lung tissues from individuals with different end-stage lung diseases. Thus, at-risk cells furnish C3 through biosynthesis and/or uptake to increase locally available C3 during inflammation, while intracellularly, these stores protect against certain inducers of cell death. These results establish the relevance of intracellular C3 to airway epithelial biology and suggest novel pathways for complement-mediated host protection in the airway.

The role of the innate immune system on pulmonary infections

Inhalation is required for respiration and life in all vertebrates. This process is not without risk, as it potentially exposes the host to environmental pathogens with every breath. This makes the upper respiratory tract one of the most common routes of infection and one of the leading causes of morbidity and mortality in the world. To combat this, the lung relies on the innate immune defenses. In contrast to the adaptive immune system, the innate immune system does not require sensitization, previous exposure or priming to attack foreign particles. In the lung, the innate immune response starts with the epithelial barrier and mucus production and is reinforced by phagocytic cells and T cells. These cells are vital for the production of cytokines, chemokines and anti-microbial peptides that are critical for clearance of infectious agents. In this review, we discuss all aspects of the innate immune response, with a special emphasis on ways to target aspects of the immune response to combat antibiotic resistant bacteria.

Lung allograft donors with excessive alcohol use have increased levels of human antimicrobial peptide LL-37

The relatively low long-term survival rate of lung transplant recipients as compared to other organ recipients serves as an impetus to identify potential lung dysfunction as early as possible. There is an association between donor heavy alcohol use and acute lung injury in the lung allograft after transplant, known as primary graft dysfunction. Excessive alcohol use (EAU) can induce pulmonary immune dysregulation in response to an infection. Antimicrobial peptides (AMPs) are an important component of the innate immune response to pulmonary infections, but the impact of EAU on AMPs in the allograft lung has not been evaluated. Our hypothesis is that specific lung AMPs, LL-37, α-defensin-1,2,3, and β-defensin-2, are dysregulated in the lungs from organ donors who had EAU. In this prospective observational investigation, we measured AMPs via ELISA and inflammatory cytokines via multiplex bead array, in bronchoalveolar lavage (BAL) fluid of lung allograft donors, comparing results based on their alcohol consumption. LL-37 levels in lung donors with EAU were found to be increased compared to nondrinker (ND) donors [median 7.7 ng/mL (IQR 4.1-37.0) vs. 2.3 ng/mL (IQR 1.1-7.9), p = 0.004], whereas α-defensins-1,2,3 were decreased only in the presence of an infection in donors with EAU compared to ND donors [median 2.2 ng/mL (IQR 1.6-2.4) vs. 3.2 ng/mL (IQR 2.3-3.8), p = 0.049]. There was no difference in β-defensin-2 levels. Gene expression levels of these AMPs were not different. Elevated levels of CXCL8 were noted in bronchial washings of donors with EAU compared to ND donors, [median 4372 pg/mL (IQR 3352-13180) vs. 867.3 pg/mL (IQR 163.6-3675), p = 0.04], suggesting a potentially heightened inflammatory response. At 1 month post-transplant, LL-37 and CXCL8 levels are decreased compared to levels at time of transplant. In lung donors with EAU, LL-37 and α-defensins-1,2,3 dysregulated levels in the presence of an infection may be a harbinger of dysfunction of the lungs through the transplant process.