Complement-Related Proteins and Their Measurements: The Current Status of Clinical Investigation

The role of gamma globulin, complement component 1q, factor B, properdin, body temperature, C-reactive protein and serum amyloid alpha to the activity and the function of the human complement system and its pathways

The complement system plays a crucial role in orchestrating the activation and regulation of inflammation within the human immune system. Three distinct activation pathways-classical, lectin, and alternative-converge to form the common lytic pathway, culminating in the formation of the membrane-attacking complex that disrupts the structure of pathogens. Dysregulated complement system activity can lead to tissue damage, autoimmune diseases, or immune deficiencies. In this study, the antimicrobial activity of human serum was investigated by using a bioluminescent microbe probe, Escherichia coli (pEGFPluxABCDEamp). This probe has previously been used to determine the antimicrobial activity of complement system and the polymorphonuclear neutrophils. In this study, blocking antibodies against key serum activators and components, including IgG, complement component 1q, factor B, and properdin, were utilized. The influence of body temperature and acute phase proteins, such as C reactive protein (CRP) and serum amyloid alpha (SAA), on the complement system was also examined. The study reveals the critical factors influencing complement system activity and pathway function. Alongside crucial factors like C1q and IgG, alternative pathway components factor B and properdin played pivotal roles. Results indicated that the alternative pathway accounted for approximately one third of the overall serum antimicrobial activity, and blocking this pathway disrupted the entire complement system. Contrary to expectations, elevated body temperature during inflammation did not enhance the antimicrobial activity of human serum. CRP demonstrated complement activation properties, but at higher physiological concentrations, it exhibited antagonistic tendencies, dampening the response. On the other hand, SAA enhanced the serum's activity. Overall, this study sheds a light on the critical factors affecting both complement system activity and pathway functionality, emphasizing the importance of a balanced immune response.

Humoral immunosuppression of exposure to polycyclic aromatic hydrocarbons and the roles of oxidative stress and inflammation

Previous studies have indicated adverse health effects of exposure to polycyclic aromatic hydrocarbons (PAHs), but evidence on the association between PAH exposure and immunity is scarce and its underlying mechanism is largely unknown. This study assessed human exposure to PAHs by determining the concentrations of PAHs in serum and their metabolites in paired urine. The oxidative stress and inflammation levels were evaluated by urinary DNA damage biomarker 8-hydroxydeoxyguanosine, white blood cell counts and C-reaction protein. We investigated the relationship between PAH exposure and seven immunological components, and explored the indirect roles of oxidative stress and inflammation by mediation and moderation analysis. Multivariate regression analysis revealed that 1-hydroxynaphthalene and 2-hydroxyfluorene were negatively associated with immunoglobulin A, and 3-hydroxyphenanthrene was negatively correlated with complement component 3. Restricted cubic spline analysis demonstrated nonlinear relationships between some individual PAHs or their metabolites with immunological components. Bayesian kernel machine regression and quantile g-computation revealed significant associations of higher PAH exposure with decreased immunoglobulin G and kappa light chain levels. Phenanthrene was the compound that contributed the most to reduced immunoglobulin G. Mediation analysis demonstrated significant indirect effects of 8-hydroxydeoxyguanosine and white blood cell counts on the association between higher PAH exposure and decreased immunological components. Moderation analysis revealed that PAH exposure and decreased immunological components are significantly associated with higher levels of C-reaction protein and white blood cell counts. The results demonstrated significant immunosuppression of PAH exposure and highlighted the indirect roles of oxidative stress and inflammation. Interventions to reduce systemic inflammation may mitigate the adverse immune effects of PAH exposure.

Recent Advances of Proteomics in Management of Acute Kidney Injury

Acute Kidney Injury (AKI) is currently recognized as a life-threatening disease, leading to an exponential increase in morbidity and mortality worldwide. At present, AKI is characterized by a significant increase in serum creatinine (SCr) levels, typically followed by a sudden drop in glomerulus filtration rate (GFR). Changes in urine output are usually associated with the renal inability to excrete urea and other nitrogenous waste products, causing extracellular volume and electrolyte imbalances. Several molecular mechanisms were proposed to be affiliated with AKI development and progression, ultimately involving renal epithelium tubular cell-cycle arrest, inflammation, mitochondrial dysfunction, the inability to recover and regenerate proximal tubules, and impaired endothelial function. Diagnosis and prognosis using state-of-the-art clinical markers are often late and provide poor outcomes at disease onset. Inappropriate clinical assessment is a strong disease contributor, actively driving progression towards end stage renal disease (ESRD). Proteins, as the main functional and structural unit of the cell, provide the opportunity to monitor the disease on a molecular level. Changes in the proteomic profiles are pivotal for the expression of molecular pathways and disease pathogenesis. Introduction of highly-sensitive and innovative technology enabled the discovery of novel biomarkers for improved risk stratification, better and more cost-effective medical care for the ill patients and advanced personalized medicine. In line with those strategies, this review provides and discusses the latest findings of proteomic-based biomarkers and their prospective clinical application for AKI management.

Complement therapeutics are coming of age in rheumatology

The complement system was described over 100 years ago, and it is well established that activation of this pathway accompanies the great majority of autoimmune and inflammatory diseases. In addition, over three decades of work in murine models of human disease have nearly universally demonstrated that complement activation is upstream of tissue injury and the engagement of pro-inflammatory mechanisms such as the elaboration of cytokines and chemokines, as well as myeloid cell recruitment and activation. With that background, and taking advantage of advances in the development of biologic and small-molecule therapeutics, the creation and clinical evaluation of complement therapeutics is now rapidly expanding. This article reviews the current state of the complement therapeutics field, with a focus on their use in diseases cared for or consulted upon by rheumatologists. Included is an overview of the activation mechanisms and components of the system, in addition to the mechanisms by which the complement system interacts with other immune system constituents. The various therapeutic approaches to modulating the system in rheumatic and autoimmune diseases are reviewed. To understand how best to clinically assess the complement system, methods of its evaluation are described. Finally, next-generation therapeutic and diagnostic advances that can be envisioned for the future are discussed.

The complement system testing in clinical laboratory

With the advancement in research in the field of the complement system, a more comprehensive understanding developed about the complement system's role in the life process of an organism. It is a system of innate immune surveillance. This system plays a pivotal role in host defense against pathogens, inflammation, B and T cell homeostasis. Complement system analysis has a significant advantage in the assessment of the immune system status, diagnosis and prognosis of diseases, and medication guidelines. Currently, complement system testing is neither yet widely used across all clinical laboratoriesnor are the testing protocols yet systematic. Based on the current research, it is suggested that the analysis of complement activator-activated complement activity and total complement activity would be comprehensively assessed to evaluate the complement system's immunological function, and combine of the detection of its components to establish a systematic protocol for the complement system testing in the clinical laboratory. This article reviews the complement system's physiological role, disease relevance and the current testing status in clinical laboratories. Further more, some suggestions have also been provided for the preparation of complement standards i.e., the standardized preparation process for complement standards seems to be a feasible option given the easy inactivation of complement.

Biomarkers and laboratory assessments for monitoring the treatment of patients with paroxysmal nocturnal hemoglobinuria: Differences between terminal and proximal complement inhibition

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, life-threatening, acquired disease in which blood cells lacking complement regulatory proteins are destroyed because of uncontrolled complement activity. Since 2007, terminal complement inhibitors have revolutionized the treatment of this disease. However, patients treated with these inhibitors can still experience anemia because of C3-mediated extravascular hemolysis and clinically relevant levels of breakthrough or residual intravascular hemolysis. Proximal complement inhibitors, which are only just beginning to emerge, have the potential to address this problem by targeting components of the pathway upstream of C5, thereby protecting patients against both intra- and extravascular hemolysis. In this review, we describe different biomarkers that can be used to monitor complement pathway blockade and discuss key laboratory assessments for evaluating treatment efficacy. We also consider how these assessments are affected by each class of inhibitor and highlight how evolving treatment goals may influence the relative importance of these assessments.

Association of complement components with the risk and severity of NAFLD: A systematic review and meta-analysis

Background

It is generally believed that complement system is strongly associated with the risk of nonalcoholic fatty liver disease (NAFLD). However, complement system contains a variety of complement components, and the relationship between complement components and the risk and severity of NAFLD is inconsistent. The aim of this meta-analysis was to evaluate the association of complement components with the risk and severity of NAFLD.

Methods

We searched PubMed, Embase, Cochrane Library, Google Scholar, Scopus, and ZhiWang Chinese databases from inception to May 2022 for observational studies reporting the risk of NAFLD with complement components. Random-effects meta-analysis was used to obtain pooled estimates of the effect due to heterogeneity.

Results

We identified 18 studies with a total of 18560 included subjects. According to recent studies, levels of complement component 3 (C3) (mean difference (MD): 0.43, 95% confidence interval (CI) 0.26-0.60), complement component 4 (C4) (MD: 0.04, 95% CI 0.02-0.07), complement component 5(C5) (MD: 34.03, 95% CI 30.80-37.27), complement factor B (CFB) (MD: 0.22, 95% CI 0.13-0.31) and acylation stimulating protein (ASP) (standard mean difference (SMD): 5.17, 95% CI 2.57-7.77) in patients with NAFLD were significantly higher than those in the control group. However, no statistical significance was obtained in complement factor D (CFD) levels between NAFLD and non-NAFLD (MD=156.51, 95% CI -59.38-372.40). Moreover, the levels of C3, C5, CFB, and ASP in patients with moderate and severe NAFLD were significantly higher than those in patients with mild NAFLD. Except for C4 and CFD, the included studies did not explore the changes in the severity of NAFLD according to the concentration of C4 and CFD.

Conclusions

This meta-analysis demonstrates that an increase in complement components including C3, C5, CFB, and ASP is associated with an increased risk and severity of NAFLD, indicating that they may be good biomarkers and targets for the diagnosis and treatment of NAFLD.

Systematic review registration

PROSPERO [https://www.crd.york.ac.uk/PROSPERO/], identifier CRD42022348650.

Complement Terminal Pathway Activation is Associated with Organ Failure in Sepsis Patients

Background

Complement plays a pivotal role in the immune response to infection. Several studies demonstrated complement activation in sepsis, yet little is known of the relationship of complement terminal pathway activation and the clinical characteristics of sepsis patients. Therefore, we investigated serum C5, soluble C5b-9 (sC5b-9), and soluble CD59 (sCD59) and their relation to organ failure in sepsis patients in the intensive care unit (ICU).

Methods

In this prospective cohort study, all available patients admitted to the adult ICUs between June 2020 and January 2021 were included. Patients were divided into sepsis and non-sepsis groups according to the Sepsis-3 criteria, serum samples from both groups were investigated for the levels of C5, sC5b-9, and sCD59 using commercial sandwich ELISA kits.

Results

We analyzed 79 serum samples, 36 were from sepsis patients. We found that sepsis patients had significantly lower C5 (83.6± 28.4 vs 104.4± 32.0 µg/mL, p = 0.004) and higher sCD59 (380.7± 170.5 vs 288.9± 92.5 ng/mL, p = 0.016). sC5b-9, although higher in sepsis patients, did not reach statistical significance (1.5± 0.8 µg/mL vs 1.3± 0.7 µg/mL, p = 0.293). Sepsis patients who died during their ICU stay had significantly higher sCD59 compared to those who survived (437.0 ± 176.7 vs 267.8 ± 79.7 ng/mL, p = 0.003, respectively). Additionally, C5 and sCD59 both correlated to SOFA score in the sepsis group (r_s = −0.44, P = 0.007 and = 0.43, P = 0.009, respectively), and a similar correlation was not found in the non-sepsis group.

Discussion

In sepsis patients, levels of C5 and sCD59, but not sC5b-9, correlated to the severity of organ damage measured by SOFA. A similar correlation was not found in non-sepsis patients. This indicated that organ damage associated with sepsis led to a more pronounced terminal pathway activation than in non-sepsis patients, it also indicated the potential of using C5 and sCD59 to reflect sepsis severity.

Complement multiplex testing: Concept, promises and pitfalls

Complement is a complex system. This complexity becomes more obvious when looking at complement analysis in health and disease, where one presentation can require a number of measurements to understand the full role of this cascade in the disease. The current state of clinical testing requires multiple tests to cover the whole of the complement cascade. There is a clear potential for multiplex testing to help address this need for comprehensive analysis of the state of complement deficiency, activation or inhibition. Fortunately, there are a number of potential methods for multiplex analysis, each with advantages and disadvantages that need to be considered in light of the intricacy of the complement cascade and its interconnection to other systems. Despite the complexities of such methods several groups have started utilizing multiplex analysis for research and even for diagnostic testing. The potential methods, current successes, and the type of testing that needs to be streamlined are reviewed in this text.

Expanding Horizons in Complement Analysis and Quality Control

Complement not only plays a key role in host microbial defense but also modulates the adaptive immune response through modification of T- and B-cell reactivity. Moreover, a normally functioning complement system participates in hematopoiesis, reproduction, lipid metabolism, and tissue regeneration. Because of its powerful inflammatory potential, multiple regulatory proteins are needed to prevent potential tissue damage. In clinical practice, dysregulation and overactivation of the complement system are major causes of a variety of inflammatory and autoimmune diseases ranging from nephropathies, age-related macular degeneration (AMD), and systemic lupus erythematosus (SLE) to graft rejection, sepsis, and multi-organ failure. The clinical importance is reflected by the recent development of multiple drugs targeting complement with a broad spectrum of indications. The recognition of the role of complement in diverse diseases and the advent of complement therapeutics has increased the number of laboratories and suppliers entering the field. This has highlighted the need for reliable complement testing. The relatively rapid expansion in complement testing has presented challenges for a previously niche field. This is exemplified by the issue of cross-reactivity of complement-directed antibodies and by the challenges of the poor stability of many of the complement analytes. The complex nature of complement testing and increasing clinical demand has been met in the last decade by efforts to improve the standardization among laboratories. Initiated by the IUIS/ICS Committee for the Standardization and Quality Assessment in Complement Measurements 14 rounds of external quality assessment since 2010 resulted in improvements in the consistency of testing across participating institutions, while extending the global reach of the efforts to more than 200 laboratories in 30 countries. Worldwide trends of assay availability, usage, and analytical performance are summarized based on the past years’ experiences. Progress in complement analysis has been facilitated by the quality assessment and standardization efforts that now allow complement testing to provide a comprehensive insight into deficiencies and the activation state of the system. This in turn enables clinicians to better define disease severity, evolution, and response to therapy.