Renal diseases and the role of complement: Linking complement to immune effector pathways and therapeutics

CR1 variants contribute to FSGS susceptibility across multiple populations

Focal segmental glomerulosclerosis (FSGS) is a common cause of nephrotic syndrome with an annual incidence in the United States in African-Americans compared to European-Americans of 24 cases and 5 cases per million, respectively. Among glomerular diseases in Europe and Latin-America, FSGS was the second most frequent diagnosis, and in Asia the fifth. We expand previous efforts in understanding genetics of FSGS by performing a case-control study involving ethnically-diverse groups FSGS cases (726) and a pool of controls (13,994), using panel sequencing of approximately 2,500 podocyte-expressed genes. Through rare variant association tests, we replicated known risk genes - KANK1, COL4A4, and APOL1. A novel significant association was observed for the gene encoding complement receptor 1 (CR1). High-risk rare variants in CR1 in the European-American cohort were commonly observed in Latin- and African-Americans. Therefore, a combined rare and common variant analysis was used to replicate the CR1 association in non-European populations. The CR1 risk variant, rs17047661, gives rise to the Sl1/Sl2 (R1601G) allele that was previously associated with protection against cerebral malaria. Pleiotropic effects of rs17047661 may explain the difference in allele frequencies across continental ancestries and suggest a possible role for genetically-driven alterations of adaptive immunity in the pathogenesis of FSGS.

Complement System and the Kidney: Its Role in Renal Diseases, Kidney Transplantation and Renal Cell Carcinoma

The crosstalk among the complement system, immune cells, and mediators of inflammation provides an efficient mechanism to protect the organism against infections and support the repair of damaged tissues. Alterations in this complex machinery play a role in the pathogenesis of different diseases. Core complement proteins C3 and C5, their activation fragments, their receptors, and their regulators have been shown to be active intracellularly as the complosome. The kidney is particularly vulnerable to complement-induced damage, and emerging findings have revealed the role of complement system dysregulation in a wide range of kidney disorders, including glomerulopathies and ischemia-reperfusion injury during kidney transplantation. Different studies have shown that activation of the complement system is an important component of tumorigenesis and its elements have been proved to be present in the TME of various human malignancies. The role of the complement system in renal cell carcinoma (RCC) has been recently explored. Clear cell and papillary RCC upregulate most of the complement genes relative to normal kidney tissue. The aim of this narrative review is to provide novel insights into the role of complement in kidney disorders.

Development and characterization of narsoplimab, a selective MASP-2 inhibitor, for the treatment of lectin-pathway-mediated disorders

Introduction

Overactivation of the lectin pathway of complement plays a pathogenic role in a broad range of immune-mediated and inflammatory disorders; mannan-binding lectin-associated serine protease-2 (MASP-2) is the key effector enzyme of the lectin pathway. We developed a fully human monoclonal antibody, narsoplimab, to bind to MASP-2 and specifically inhibit lectin pathway activation. Herein, we describe the preclinical characterization of narsoplimab that supports its evaluation in clinical trials.

Methods and results

ELISA binding studies demonstrated that narsoplimab interacted with both zymogen and enzymatically active forms of human MASP-2 with high affinity (KD 0.062 and 0.089 nM, respectively) and a selectivity ratio of >5,000-fold relative to closely related serine proteases C1r, C1s, MASP-1, and MASP-3. Interaction studies using surface plasmon resonance and ELISA demonstrated approximately 100-fold greater binding affinity for intact narsoplimab compared to a monovalent antigen binding fragment, suggesting an important contribution of functional bivalency to high-affinity binding. In functional assays conducted in dilute serum under pathway-specific assay conditions, narsoplimab selectively inhibited lectin pathway-dependent activation of C5b-9 with high potency (IC50 ~ 1 nM) but had no observable effect on classical pathway or alternative pathway activity at concentrations up to 500 nM. In functional assays conducted in 90% serum, narsoplimab inhibited lectin pathway activation in human serum with high potency (IC50 ~ 3.4 nM) whereas its potency in cynomolgus monkey serum was approximately 10-fold lower (IC50 ~ 33 nM). Following single dose intravenous administration to cynomolgus monkeys, narsoplimab exposure increased in an approximately dose-proportional manner. Clear dose-dependent pharmacodynamic responses were observed at doses >1.5 mg/kg, as evidenced by a reduction in lectin pathway activity assessed ex vivo that increased in magnitude and duration with increasing dose. Analysis of pharmacokinetic and pharmacodynamic data revealed a well-defined concentration-effect relationship with an ex vivo EC50 value of approximately 6.1 μg/mL, which was comparable to the in vitro functional potency (IC50 33 nM; ~ 5 μg/mL).

Discussion

Based on these results, narsoplimab has been evaluated in clinical trials for the treatment of conditions associated with inappropriate lectin pathway activation, such as hematopoietic stem cell transplantation-associated thrombotic microangiopathy.

Complosome - the intracellular complement system

The complement system is a recognized pillar of host defence against infection and noxious self-derived antigens. Complement is traditionally known as a serum-effective system, whereby the liver expresses and secretes most complement components, which participate in the detection of bloodborne pathogens and drive an inflammatory reaction to safely remove the microbial or antigenic threat. However, perturbations in normal complement function can cause severe disease and, for reasons that are currently not fully understood, the kidney is particularly vulnerable to dysregulated complement activity. Novel insights into complement biology have identified cell-autonomous and intracellularly active complement - the complosome - as an unexpected central orchestrator of normal cell physiology. For example, the complosome controls mitochondrial activity, glycolysis, oxidative phosphorylation, cell survival and gene regulation in innate and adaptive immune cells, and in non-immune cells, such as fibroblasts and endothelial and epithelial cells. These unanticipated complosome contributions to basic cell physiological pathways make it a novel and central player in the control of cell homeostasis and effector responses. This discovery, together with the realization that an increasing number of human diseases involve complement perturbations, has renewed interest in the complement system and its therapeutic targeting. Here, we summarize the current knowledge about the complosome across healthy cells and tissues, highlight contributions from dysregulated complosome activities to human disease and discuss potential therapeutic implications.

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.

With complements: C3 inhibition in the clinic

C3 is a key complement protein, located at the nexus of all complement activation pathways. Extracellular, tissue, cell-derived, and intracellular C3 plays critical roles in the immune response that is dysregulated in many diseases, making it an attractive therapeutic target. However, challenges such as very high concentration in blood, increased acute expression, and the elevated risk of infections have historically posed significant challenges in the development of C3-targeted therapeutics. This is further complicated because C3 activation fragments and their receptors trigger a complex network of downstream effects; therefore, a clear understanding of these is needed to provide context for a better understanding of the mechanism of action (MoA) of C3 inhibitors, such as pegcetacoplan. Because of C3's differential upstream position to C5 in the complement cascade, there are mechanistic differences between pegcetacoplan and eculizumab that determine their efficacy in patients with paroxysmal nocturnal hemoglobinuria. In this review, we compare the MoA of pegcetacoplan and eculizumab in paroxysmal nocturnal hemoglobinuria and discuss the complement-mediated disease that might be amenable to C3 inhibition. We further discuss the current state and outlook for C3-targeted therapeutics and provide our perspective on which diseases might be the next success stories in the C3 therapeutics journey.

Locally generated C3 regulates the clearance of Toxoplasma gondii by IFN-γ-primed macrophage through regulation of xenophagy

Exogenous pathogen infection can induce autophagy in cells. Autophagy is essential for cell survival, development, and homeostasis. It not only regulates cell defense and stress, but also has a close relationship with innate and adaptive immunity. Complement is an important part of innate immunity, which could be activated by three approaches, including classic, alternative, and lectin pathways. All the three pathways result in the activation of C3, and generate anaphylatoxin fragments C3a and C5a, and formation of the membrane attack complex. Either C3a or C5a induces the inflammatory cytokines through binding to C3aR or C5aR, respectively. However, it is still unknown whether the complement could regulate the autophagy of intracellular microorganisms or not. In this study, we constructed a Toxoplasma gondii (T. gondii) and macrophages co-culture experimental model using T. gondii expressing enhanced green fluorescence protein (EGFP) fluorescence and C3^−/-C57BL/6 J mice for that T. gondii invaded peritoneal macrophages in mice. Western blot, laser confocal microscopy (LCM), and transmission electron microscopy (TEM) were used to observe the changes of autophagy between the macrophages from wild-type (WT) and C3^−/− mice. Flow cytometry and LCM were used to investigate the effect of autophagy on the killing ability of macrophages against T. gondii. Here, we found that local C3 could suppress not only the canonical autophagy of macrophage, but also the xenophagy to T. gondii. Interestingly, the inhibition of C3 on host cell autophagy could significantly suppress the clearance of T. gondii by the IFN-γ-primed macrophage. Finally, we investigated the mechanism of the autophagy regulation of C3 that the effect of C3 on the macrophage-specific autophagy against T. gondii depends on mTOR. And, there is C3a but not C5a/C5aR involved in regulating macrophage xenophagy against T. gondii. Collectively, our findings suggest locally generated C3 regulates the clearance of T. gondii by Macrophage through the regulation of the non-canonical IFN-γ-dependent autophagy pathway, and paint a clearer picture in the regulation of autophagy by innate immune components.