In Silico Designed Gain-of-Function Variants of Complement C2 Support Cytocidal Activity of Anticancer Monoclonal Antibodies

Enhancing complement activation by therapeutic anti-tumor antibodies: Mechanisms, strategies, and engineering approaches

The complement system plays an integral role in both innate and adaptive immune responses. Beyond its protective function against infections, complement is also known to influence tumor immunity, where its activation can either promote tumor progression or mediate tumor cell destruction, depending on the context. One such context can be provided by antibodies, with their inherent capacity to activate the classical complement pathway. In recent years, our understanding of the mechanisms governing complement activation by IgG and IgM antibodies has expanded significantly. At the same time, preclinical and clinical studies on antibodies such as rituximab, ofatumumab, and daratumumab have provided evidence for the role of complement in therapeutic success, encouraging strategies to further enhance its activity. In this review we examine the main determinants of antibody-mediated complement activation, highlighting the importance of antibody subclass, affinity, valency, and geometry of antigen engagement. We summarize the evidence for complement involvement in anti-tumor activity and challenges of accurately estimating the extent of its contribution to therapeutic efficacy. Furthermore, we explore several engineering approaches designed to enhance complement activation, including increased Fc oligomerization and C1q affinity, bispecific C1q-recruiting antibodies, IgG subclass chimeras, as well as antibody and paratope combinations. Strategies targeting membrane-bound complement regulatory proteins to overcome tumor-associated complement inhibition are also discussed as a method to boost therapeutic efficacy. Finally, we highlight the potential of complement-dependent cellular cytotoxicity (CDCC) and complement-dependent cellular phagocytosis (CDCP) as effector mechanisms that warrant deeper investigation. By integrating advances in antibody and complement biology with insights from efforts to enhance complement activation in therapeutic antibodies, this review aims to provide a comprehensive framework of antibody design and engineering strategies that optimize complement activity for improved anti-tumor efficacy.

The complement system in clinical oncology: Applications, limitations and challenges

The complement system, a key component of innate immunity, is involved in seemingly contradictory aspects of tumor progression and cancer therapy. It can act as an immune effector against cancer and modulate the antitumor activity of certain therapeutic antibodies, but it can also contribute to a tumor-promoting microenvironment. Understanding this dual role should lead to the development of better therapeutic tools, strategies for cancer treatment and biomarkers for the clinical management of cancer patients. Here, we review recent advances in the understanding of the role of complement in cancer, focusing on how these findings are being translated into the clinic. We highlight the activity of therapeutic agents that modulate the complement system, as well as combination therapies that integrate complement modulation with existing therapies. We conclude that the role of complement activation in cancer is a rapidly evolving field with the potential to translate findings into new therapeutic strategies and clinically useful biomarkers.

A Cell-Based Assay to Measure the Activity of the Complement Convertases

Introduction

The complement system serves as a crucial defense mechanism against invading pathogens; however, dysregulation of this system can result in harmful consequences. Central to the complement cascade are the classical pathway (CP) or lectin pathway (LP) and the alternative pathway (AP) convertases. Aberrant regulation of the convertases is often implicated in the development of rare complement-related diseases. However, analyzing convertase activity poses a significant challenge due to their labile nature and intricate interactions with serum proteins.

Methods

In this study, we propose a novel assay for the functional evaluation of these complexes. Our approach leverages a widely available human lymphoma cell line, which when sensitized with antibodies, triggers activation of the CP with a substantial amplification by the AP. The combined action of 2, C5 blockers eculizumab and crovalimab let the cascade proceed up to the level of convertases but not further. In the next step, C5 inhibitors were washed away and guinea pig serum in ethylenediamine tetraacetic acid (EDTA) buffer supported the development of lytic sites on the platform of preexisting convertases.

Results

The assay detects recombinant gain-of-function (GoF) components of both convertase types within human serum or plasma. Furthermore, we demonstrate the assay's practical utility in analyzing nephrological patients harboring C3 genetic variants and illustrate its capacity to distinguish between patients and asymptomatic relatives carrying the same pathogenic C3 variant.

Conclusion

We provided a proof-of-concept of a new assay that detects convertase overactivity in individuals carrying variants of both pathogenic character or those of unknown significance in ubiquitous complement proteins such as C3.

The Acquisition of Complement-Dependent Cytotoxicity by the Type II Anti-CD20 Therapeutic Antibody Obinutuzumab

Rituximab, a prototypic anti-CD20 mAb, and the third-generation anti-CD20 mAb obinutuzumab differ in their ability to activate the complement system. According to recent studies, this contrast stems from the architecture of the antigen-antibody complex formed by these two mAbs that facilitates (rituximab) or disables (obinutuzumab) further oligomerization, leading to engagement of the initial classical complement pathway component C1q. We examined whether a gain-of-function C2 variant that acts downstream of C1q and enforces the formation of complement convertase resistant to physiological decay can impact complement activation by obinutuzumab. Co-application of the C2 variant with obinutuzumab and human serum resulted in complement-dependent cytotoxicity equal to or higher than attainable for rituximab. This effect was observed either in serum or hirudin-anticoagulated whole blood. Long-term (24 h) overall cytotoxicity of obinutuzumab was improved in target cells of moderate sensitivity to complement but diminished in cells of low sensitivity. Our results demonstrate that the ability of complement activation of a given antibody is not ultimately determined at the stage of initial interactions with its target antigen but is modulable at later stages of the cascade and that the benefit of the acquisition of this new effector mechanism by obinutuzumab depends on the target cell characteristics.

Identification of the pyroptosis-related gene signature and risk score model for esophageal squamous cell carcinoma

Advanced esophageal squamous cell carcinoma (ESCC) still has a dismal prognostic outcome. However, the current approaches are unable to evaluate patient survival. Pyroptosis represents a novel programmed cell death type which widely investigated in various disorders and can influence tumor growth, migration, and invasion. Furthermore, few existing studies have used pyroptosis-related genes (PRGs) to construct a model for predicting ESCC survival. Therefore, the present study utilized bioinformatics approaches for analyzing ESCC patient data obtained from the TCGA database to construct the prognostic risk model and applied it to the GSE53625 dataset for validation. There were 12 differentially expressed PRGs in healthy and ESCC tissue samples, among which eight were selected through univariate and LASSO cox regression for constructing the prognostic risk model. According to K-M and ROC curve analyses, our eight-gene model might be useful in predicting ESCC prognostic outcomes. Based on the cell validation analysis, C2, CD14, RTP4, FCER3A, and SLC7A7 were expressed higher in KYSE410 and KYSE510 than in normal cells (HET-1A). Hence, ESCC patient prognostic outcomes can be assessed by our PRGs-based risk model. Further, these PRGs may also serve as therapeutic targets.

Substitutions at position 263 within the von Willebrand factor type A domain determine the functionality of complement C2 protein

The complement system is one of the first defense lines protecting from invading pathogens. However, it may turn offensive to the body's own cells and tissues when deregulated by the presence of rare genetic variants that impair physiological regulation and/or provoke abnormal activity of key enzymatic components. Factor B and complement C2 are examples of paralogs engaged in the alternative and classical/lectin complement pathway, respectively. Pathogenic mutations in the von Willebrand factor A domain (vWA) of FB have been known for years. Despite substantial homology between two proteins and the demonstration that certain substitutions in FB translated to C2 result in analogous phenotype, there was a limited number of reports on pathogenic C2 variants in patients. Recently, we studied a cohort of patients suffering from rare kidney diseases and confirmed the existence of two gain-of-function and three loss-of-function mutations within the C2 gene sequences coding for the vWA domain (amino acids 254-452) or nearly located unstructured region (243-253) of C2 protein. Herein, we report the functional consequences of amino acid substitution of glutamine at position 263. The p.Q263G variant resulted in the gain-of-function phenotype, similarly to a homologous mutation p.D279G in FB. Conversely, the p.Q263P variant found in a patient with C3 glomerulopathy resulted in the loss of C2 function. Our results confirm that the N-terminal part of the vWA domain is a hot spot crucial for the complement C2 function.

The yin and the yang of early classical pathway complement disorders

The classical pathway of the complement cascade has been recognized as a key activation arm, partnering with the lectin activation arm and the alternative pathway to cleave C3 and initiate the assembly of the terminal components. While deficiencies of classical pathway components have been recognized since 1966, only recently have gain-of-function variants been described for some of these proteins. Loss-of-function variants in C1, C4, and C2 are most often associated with lupus and systemic infections with encapsulated bacteria. C3 deficiency varies slightly from this phenotypic class with membranoproliferative glomerulonephritis and infection as the dominant phenotypes. The gain-of-function variants recently described for C1r and C1s lead to periodontal Ehlers Danlos syndrome, a surprisingly structural phenotype. Gain-of-function in C3 and C2 are associated with endothelial manifestations including hemolytic uremic syndrome and vasculitis with C2 gain-of-function variants thus far having been reported in patients with a C3 glomerulopathy. This review will discuss the loss-of-function and gain-of-function phenotypes and place them within the larger context of complement deficiencies.