Complement activation occurs through both classical and alternative pathways prior to onset and resolution of adult respiratory distress syndrome

Complement and COVID-19: Three years on, what we know, what we don't know, and what we ought to know

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus was identified in China in 2019 as the causative agent of COVID-19, and quickly spread throughout the world, causing over 7 million deaths, of which 2 million occurred prior to the introduction of the first vaccine. In the following discussion, while recognising that complement is just one of many players in COVID-19, we focus on the relationship between complement and COVID-19 disease, with limited digression into directly-related areas such as the relationship between complement, kinin release, and coagulation. Prior to the 2019 COVID-19 outbreak, an important role for complement in coronavirus diseases had been established. Subsequently, multiple investigations of patients with COVID-19 confirmed that complement dysregulation is likely to be a major driver of disease pathology, in some, if not all, patients. These data fuelled evaluation of many complement-directed therapeutic agents in small patient cohorts, with claims of significant beneficial effect. As yet, these early results have not been reflected in larger clinical trials, posing questions such as who to treat, appropriate time to treat, duration of treatment, and optimal target for treatment. While significant control of the pandemic has been achieved through a global scientific and medical effort to comprehend the etiology of the disease, through extensive SARS-CoV-2 testing and quarantine measures, through vaccine development, and through improved therapy, possibly aided by attenuation of the dominant strains, it is not yet over. In this review, we summarise complement-relevant literature, emphasise its main conclusions, and formulate a hypothesis for complement involvement in COVID-19. Based on this we make suggestions as to how any future outbreak might be better managed in order to minimise impact on patients.

Complement as a vital nexus of the pathobiological connectome for acute respiratory distress syndrome: An emerging therapeutic target

The hallmark of acute respiratory distress syndrome (ARDS) pathobiology is unchecked inflammation-driven diffuse alveolar damage and alveolar-capillary barrier dysfunction. Currently, therapeutic interventions for ARDS remain largely limited to pulmonary-supportive strategies, and there is an unmet demand for pharmacologic therapies targeting the underlying pathology of ARDS in patients suffering from the illness. The complement cascade (ComC) plays an integral role in the regulation of both innate and adaptive immune responses. ComC activation can prime an overzealous cytokine storm and tissue/organ damage. The ARDS and acute lung injury (ALI) have an established relationship with early maladaptive ComC activation. In this review, we have collected evidence from the current studies linking ALI/ARDS with ComC dysregulation, focusing on elucidating the new emerging roles of the extracellular (canonical) and intracellular (non-canonical or complosome), ComC (complementome) in ALI/ARDS pathobiology, and highlighting complementome as a vital nexus of the pathobiological connectome for ALI/ARDS via its crosstalking with other systems of the immunome, DAMPome, PAMPome, coagulome, metabolome, and microbiome. We have also discussed the diagnostic/therapeutic potential and future direction of ALI/ARDS care with the ultimate goal of better defining mechanistic subtypes (endotypes and theratypes) through new methodologies in order to facilitate a more precise and effective complement-targeted therapy for treating these comorbidities. This information leads to support for a therapeutic anti-inflammatory strategy by targeting the ComC, where the arsenal of clinical-stage complement-specific drugs is available, especially for patients with ALI/ARDS due to COVID-19.

Plant polysaccharides with anti-lung injury effects as a potential therapeutic strategy for COVID-19

When coronavirus disease 2019 (COVID-19) develops into the severe phase, lung injury, acute respiratory distress syndrome, and/or respiratory failure could develop within a few days. As a result of pulmonary tissue injury, pathomorphological changes usually present endothelial dysfunction, inflammatory cell infiltration of the lung interstitium, defective gas exchange, and wall leakage. Consequently, COVID-19 may progress to tremendous lung injury, ongoing lung failure, and death. Exploring the treatment drugs has important implications. Recently, the application of traditional Chinese medicine had better performance in reducing fatalities, relieving symptoms, and curtailing hospitalization. Through constant research and study, plant polysaccharides may emerge as a crucial resource against lung injury with high potency and low side effects. However, the absence of a comprehensive understanding of lung-protective mechanisms impedes further investigation of polysaccharides. In the present article, a comprehensive review of research into plant polysaccharides in the past 5 years was performed. In total, 30 types of polysaccharides from 19 kinds of plants have shown lung-protective effects through the pathological processes of inflammation, oxidative stress, apoptosis, autophagy, epithelial–mesenchymal transition, and immunomodulation by mediating mucin and aquaporins, macrophage, endoplasmic reticulum stress, neutrophil, TGF-β1 pathways, Nrf2 pathway, and other mechanisms. Moreover, the deficiencies of the current studies and the future research direction are also tentatively discussed. This research provides a comprehensive perspective for better understanding the mechanism and development of polysaccharides against lung injury for the treatment of COVID-19.

Beneficial effects of Houttuynia cordata polysaccharides on "two-hit" acute lung injury and endotoxic fever in rats associated with anti-complementary activities

Houttuynia cordata Thunb. is a traditional herb used for clearing heat and eliminating toxins, and has also been used for the treatment of severe acute respiratory syndrome (SARS). In vitro, the crude H. cordata polysaccharides (CHCP) exhibited potent anti-complementary activity through both the classical and alternative pathways by acting on components C3 and C4 of the complement system without interfering with the coagulation system. This study was to investigate the preventive effects of CHCP on acute lung injury (ALI) induced by hemorrhagic shock plus lipopolysaccharide (LPS) instillation (two-hit) and LPS-induced fever in rats. CHCP significantly attenuated pulmonary injury in the “two-hit” ALI model by reducing pulmonary edema and protein exudation in bronchoalveolar lavage fluid (BALF). In addition, it reduced the deposit of complement activation products in the lung and improved oxidant-antioxidant imbalance. Moreover, CHCP administration inhibited fever in rats, reduced the number of leukocytes and restored serum complement levels. The inhibition on the inappropriate activation of complement system by CHCP may play an important role in its beneficial effects on inflammatory diseases. The anti-complementary polysaccharides are likely to be among the key substances for the heat-clearing function of H. cordata.

C5L2, the Second C5a Anaphylatoxin Receptor, Suppresses LPS-Induced Acute Lung Injury

LPS-induced lung injury in the mouse is one of the most robust experimental models used for studies of acute lung injury (ALI) and acute respiratory distress syndrome in humans. Prior clinical and experimental studies support an important role for complement activation, particularly production of C5a, in the pathophysiology of human ALI/acute respiratory distress syndrome. In the mouse model, however, the precise role of C5a and its receptors is unclear. C5L2, an enigmatic second receptor for C5a, has been characterized, and results have generated substantial debate regarding its in vivo function. Our previous work with human neutrophils revealed a unique role for C5L2 in negatively modulating C5a-C5a receptor (C5aR)-mediated cellular activation, in which antibody-mediated blockade of C5L2 resulted in augmented C5a-C5aR responses. Here, we demonstrate that C5L2^-/- mice (BALB/c background) administered intranasal LPS exhibit significantly more airway edema and hemorrhage than do wild-type animals. Bronchoalveolar lavage fluid and lung homogenates have significantly more neutrophils and myeloperoxidase activity, as well as proinflammatory cytokines and chemokines. When a blocking antibody against the C5aR was administered before LPS administration, the increased neutrophilic infiltration and cytokine levels were reversed. Thus, our data show not only that C5a contributes significantly to LPS-induced ALI in the mouse, but also that C5L2 plays an important antiinflammatory role in this model through actions resulting at least in part from negative modulation of C5a receptor activation.

Modulators of complement activation: a patent review (2008 - 2013)

INTRODUCTION

The architecture of the complement system has evolved during the last 600 - 700 million years to become an amazingly efficient and highly versatile alerting and cell killing device. Under physiological conditions, this system acts as a well-regulated cascade, protecting the organism against pathogens and participating during the initial defensive steps of humoral and cellular response. The unregulated activation of this system may cause or even aggravate diseases; therefore, its modulation is currently considered of high importance.

AREAS COVERED

This review is a critical examination on patent literature published between 2008 and 2013. An insight is provided about the discovery and development of novel therapeutic agents. These include macromolecules, polysaccharides and proteins, specific antibodies, and hybrid or chimeric products. Peptides and low molecular weight organic compounds (natural products, their derivatives and fully synthetic molecules) are covered as well.

EXPERT OPINION

The search of specific inhibitors of the complement cascade has become one of the Holy Grails of Medicinal Chemistry for the last 30 - 40 years, with very few cases of success. Some highly specific macromolecules are currently available as modulators of the complement. However, there is still a marked need to find new, more specific, efficient and convenient alternatives, especially suited for chronic administration, including novel inexpensive small molecule inhibitors. Analogously, despite the initial success with specific monoclonal antibodies, a vast territory is awaiting to be explored and conquered, regarding the regulation of complement activation by antibody-mediated blockage of specific polypeptides or receptor sites.

Design and synthesis of polyethylene glycol-modified biphenylsulfonyl-thiophene-carboxamidine inhibitors of the complement component C1s

Complement C1s protease inhibitors have potential utility in the treatment of diseases associated with activation of the classical complement pathway such as humorally mediated graft rejection, ischemia-reperfusion injury (IRI), vascular leak syndrome, and acute respiratory distress syndrome (ARDS). The utility of biphenylsulfonyl-thiophene-carboxamidine small-molecule C1s inhibitors are limited by their poor in vivo pharmacokinetic properties. Pegylation of a potent analog has provided compounds with good potency and good in vivo pharmacokinetic properties.

Biphenylsulfonyl-thiophene-carboxamidine inhibitors of the complement component C1s

Complement activation has been implicated in disease states such as hereditary angioedema, ischemia-reperfusion injury, acute respiratory distress syndrome, and acute transplant rejection. Even though the complement cascade provides several protein targets for potential therapeutic intervention only two complement inhibitors have been approved so far for clinical use including anti-C5 antibodies for the treatment of paroxysmal nocturnal hemoglobinuria and purified C1-esterase inhibitor replacement therapy for the control of hereditary angioedema flares. In the present study, optimization of potency and physicochemical properties of a series of thiophene amidine-based C1s inhibitors with potential utility as intravenous agents for the inhibition of the classical pathway of complement is described.

Complement levels and activity in the normal and LPS-injured lung

Complement, a complex protein system, plays an essential role in host defense through bacterial lysis, stimulation of phagocytosis, recruitment of immune cells to infected tissue, and promotion of the inflammatory response. Although complement is most well-characterized in serum, complement activity is also present in the lung. Here we further characterize the complement system in the normal and inflamed lung. By Western blot, C5, C6, and factor I were detected in bronchoalveolar lavage (BAL) at lower levels than in serum, whereas C2 was detected at similar levels in BAL and serum. C4 binding protein (C4BP) was not detectable in BAL. Exposure to lipopolysaccharide (LPS) elevated levels of C1q, factor B, C2, C4, C5, C6, and C3 in human BAL and C3, C5, and factor B in mouse and rat BAL. Message for C1q-B, C1r, C1s, C2, C4, C3, C5, C6, factor B, and factor H, but not C9 or C4BP, was readily detectable by RT-PCR in normal mouse lung. Exposure to LPS enhanced factor B expression, decreased C5 expression, and did not affect C1q-B expression in mouse and rat lung. BAL from rats exposed to LPS had a greater ability to deposit C3b onto bacteria through complement activation than did BAL from control rats. In summary, these data demonstrate that complement levels, expression, and function are altered in acute lung injury and suggest that complement within the lung is regulated to promote opsonization of pathogens and limit potentially harmful inflammation.

A novel series of arylsulfonylthiophene-2-carboxamidine inhibitors of the complement component C1s

Inhibiting the classical pathway of complement activation by attenuating the proteolytic activity of the serine protease C1s is a potential strategy for the therapeutic intervention in disease states such as hereditary angioedema, ischemia-reperfusion injury, and acute transplant rejection. A series of arylsulfonylthiophene-2-carboxamidine inhibitors of C1s were synthesized and evaluated for C1s inhibitory activity. The most potent compound had a Ki of 10nM and >1000-fold selectivity over uPA, tPA, FX(a), thrombin, and plasmin.

A novel series of potent and selective small molecule inhibitors of the complement component C1s

Activation of the classical pathway of complement has been implicated in disease states such as hereditary angioedema, ischemia-reperfusion injury and acute transplant rejection. The trypsin-like serine protease C1s represents a pivotal upstream point of control in the classical pathway of complement activation and is therefore likely to be a useful target in the therapeutic intervention of these disease states. A series of thiopheneamidine-based inhibitors of C1s has been optimized to give a 70 nM inhibitor that inhibits the classical pathway of complement activation in vitro.

White blood cell counts and plasma C3a have synergistic predictive value in patients at risk for acute respiratory distress syndrome

OBJECTIVE

To investigate and select nonassociated variables with predictive value for acute respiratory distress syndrome (ARDS) in patients at risk.

DESIGN

Prospective, observational study.

SETTING

A university hospital intensive care unit.

PATIENTS

Twenty-four critically ill patients with different risk factors for ARDS.

INTERVENTIONS

Arterial and mixed venous blood, as well as urine samples, were collected. Invasive hemodynamic measurements were performed.

MEASUREMENTS AND MAIN RESULTS

Fifty-nine variables pertaining to the cardiorespiratory, hepatic, immunologic, and renal systems and including plasma complement activation products C3a and SC5b-9 and polymorphonuclear elastase, were determined every 6 hrs for 3 days in patients at risk for ARDS. Associations among variables were investigated and the predictive value of nonassociated variables for ARDS was determined. Patients who developed ARDS (n=8) had lower white blood cell counts at the time they entered the study (p=.006) and during the first 24 hrs thereafter (p=.032). Also, plasma C3a concentrations were markedly higher during the first 24 hrs in patients who developed ARDS (p=.006). Plasma C3a had better predictive value than did white blood cell counts for cutoff points set by discriminant analysis at 1075 ng/mL (1.075 x 10(-3) g/L) and 5700 cells/mL, respectively. The combination of both variables in a discriminant function improved the predictive value for ARDS.

CONCLUSIONS

The most notable and nonassociated alterations observed in patients who developed ARDS were lower white blood cell counts and higher plasma C3a concentrations compared with counts and concentrations in patients who did not develop ARDS. Plasma C3a concentrations showed better predictive value than white blood cell counts. The combination of white blood cell counts with plasma C3a concentrations synergistically improved the predictive value for ARDS. This combination may prove useful for identifying subpopulations at highest risk for ARDS and may contribute to make treatment at an early stage of the syndrome possible.

A solid-phase antibody capture assay for the measurement of C1-inhibitor consumption in vivo

C1-inhibitor (C1-Inh) is a serine esterase proteinase inhibitor (serpin) which plays an important role in regulating serine proteinases of the early inflammatory response. In this study, we describe a novel and versatile polyclonal antibody capture assay to examine C1-Inh consumption in vivo. This assay has advantages over previously described methods of measuring C1-Inh consumption as it allows the assessment of the relative amounts of native, complexed and cleaved inhibitor circulating in plasma. By using polyclonal antibodies specific for other complement proteins, the C1-Inh capture assay was adapted to measure in vivo activation of C3, C4 and factor B. C1-Inh consumption and complement activation were examined in the plasma of 21 normal individuals, 24 individuals with systemic lupus erythematosus (SLE), nine individuals with adult respiratory distress syndrome (ARDS) and in the paired plasma and synovial fluid from 18 patients with rheumatoid arthritis (RA). The C1-Inh capture assay revealed native, cleaved and complexed C1-Inh migrating at 115 kDa, 96 kDa and 209-225 kDa respectively, in normal plasma. C1-Inh consumption was increased in the plasma of all the inflammatory disorders examined, in comparison to normal plasma. It is proposed that this serpin capture assay could be adapted to the study of serpin involvement in a wide variety of inflammatory disorders.

Adult respiratory distress syndrome in children

In spite of modern technological advances, ARDS continues to be an important cause of morbidity and mortality from a diverse group of disorders such as sepsis, trauma, and aspiration. ARDS represents a target organ injury resulting from activation of the host's inflammatory cells and uncontrolled liberation of inflammatory mediators. In most instances, therefore, ARDS is a localized manifestation of a widespread onslaught characteristic of SIRS. At this time, there are no proven interventions to prevent ARDS, and the management is mainly supportive. Modulation of the host's inflammatory response seems to hold the most promise for prevention and treatment of ARDS. Such strategies need to be explored with well-controlled clinical trials.

Evaluation of complement activation in premature newborn infants with hyaline membrane disease

Fifteen premature newborns with hyaline membrane disease causing acute respiratory distress were evaluated for complement activation. A high intrapulmonary right-to-left shunt and marked arterial-alveolar oxygen difference indicated the severity of the respiratory failure. Twenty preterm healthy infants served as controls. Total haemolytic activity, plasma concentrations of complement components and regulatory proteins (C3, C4, C1-inhibitor, factors H and I) as well as activation products (C3a, C3dg, C1rsC1-inhibitor, C3b(Bb)P) gave no evidence of significant complement activation. Functional activity of the ubiquitous regulatory protein C1-inhibitor was significantly reduced without impact on classical pathway activation. These data suggest that, in contrast to the adult form of respiratory distress syndrome, the low-pressure pulmonary oedema characterising hyaline membrane disease is not mediated by activation of the complement system.

Complement activation and the prognostic value of C3a in patients at risk of adult respiratory distress syndrome

In vivo and in vitro studies have shown that complement activation plays an important role in the pathogenesis of the adult respiratory distress syndrome (ARDS). In a prospective study of polytrauma patients at risk of ARDS (n = 38) complement parameters were determined over a period of 14 days in serial plasma samples (obtained every 6 h during the first 48 h). Polytrauma induced a rapid and remarkable complement activation. Low levels of the complement proteins C3, C4, C1 inhibitor (C1 INH) factor I and factor H during the first 48 h indicated complement consumption in all patients. Elevated C3a levels in the first few hours after injury were associated with the later development of ARDS. A more sensitive indicator than C3a alone was the calculated C3a:C3 ratio discriminating ARDS and non-ARDS patients. A second rise of C3a levels and C3a:C3 ratio from day 4 on paralleled the course of extravascular lung water. To assess the mode of complement activation, the activation-specific protein complexes C1rC1s-C1 INH and C3b(Bb)P were measured in some of the patients. We demonstrate that in the first 48 h complement activation occurred via the alternative pathway only and was later followed by an additional activation via the classical pathway. Our observations suggest that monitoring of C3a and C3 in plasma can identify polytrauma patients at high risk for ARDS at an early stage of the disease.

Phosphatidylinositol-specific phospholipase C, a possible virulence factor of Staphylococcus aureus

Phosphatidylinositol-specific phospholipase C (PIPLC), an enzyme that can specifically release phosphatidylinositol-linked proteins from host cells, is one of the extracellular enzymes produced by Staphylococcus aureus. To investigate whether PIPLC might be a virulence factor, we assessed PIPLC production by S. aureus strains that had been isolated from healthy carriers and from infected patients with or without toxic shock syndrome. Although none of five vaginal isolates from healthy women was a PIPLC producer, only 10 of 32 selected pathogenic strains that caused significant infections or toxic shock syndrome elaborated PIPLC enzyme activity. Seven of 24 toxic-shock-associated strains, compared with 3 of 8 non-toxic-shock-associated strains, were positive for PIPLC. The majority of strains that produced PIPLC were negative for toxic shock syndrome toxin 1 (P less than 0.05); this association between PIPLC production and strains negative for toxic shock syndrome toxin 1 was even stronger among strains isolated only from patients with toxic shock syndrome (P less than 0.01). Among all 32 pathogenic isolates, PIPLC-producing S. aureus strains were isolated from four of four patients developing adult respiratory distress syndrome and four of five patients with disseminated intravascular coagulation, suggesting a significant association between PIPLC production and adult respiratory distress syndrome and/or disseminated intravascular coagulation (P less than 0.002). On the basis of these results, we propose that PIPLC is a virulence factor of S. aureus and is implicated in the development of adult respiratory distress syndrome and disseminated intravascular coagulation.