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.

Elevated expression of complement factor I in lung cancer cells associates with shorter survival-Potentially via non-canonical mechanism.

While numerous membrane-bound complement inhibitors protect the body's cells from innate immunity's autoaggression, soluble inhibitors like complement factor I (FI) are rarely produced outside the liver. Previously, we reported the expression of FI in non-small cell lung cancer (NSCLC) cell lines. Now, we assessed the content of FI in cancer biopsies from lung cancer patients and associated the results with clinicopathological characteristics and clinical outcomes. Immunohistochemical staining intensity did not correlate with age, smoking status, tumor size, stage, differentiation grade, and T cell infiltrates, but was associated with progression-free survival (PFS), overall survival (OS) and disease-specific survival (DSS). Multivariate Cox analysis of low vs. high FI content revealed HR 0.55, 95 % CI 0.32-0.95, p=0.031 for PFS, HR 0.51, 95 % CI 0.25-1.02, p=0.055 for OS, and HR 0.32, 95 % CI 0.12-0.84, p=0.021 for DSS. Unfavorable prognosis might stem from the non-canonical role of FI, as the staining pattern did not correlate with C4d - the product of FI-supported degradation of active complement component C4b. To elucidate that, we engineered three human NSCLC cell lines naturally expressing FI with CRISPR/Cas9 technology, and compared the transcriptome of FI-deficient and FI-sufficient clones in each cell line. RNA sequencing revealed differentially expressed genes engaged in intracellular signaling pathways controlling proliferation, apoptosis, and responsiveness to growth factors. Moreover, in vitro colony-formation assays showed that FI-deficient cells formed smaller foci than FI-sufficient NSCLC cells, but their size increased when purified FI protein was added to the medium. We postulate that a non-canonical activity of FI influences cellular physiology and contributes to the poor prognosis of lung cancer patients.

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.

Off-label use of eculizumab for neurological symptoms and progressive vision loss.

We describe the results of eculizumab treatment of a patient with pachymeningitis, inflammatory infiltration of the left frontal lobe, and cerebral hematoma, who presented with progressive vision loss, epileptic seizures, and abnormal pattern of the complement system parameters. A 30-year-old female patient, initially diagnosed with hypereosinophilia and a tumour of the left orbit, developed a significant visual impairment in the left eye, progressive vision loss in the right eye, and neurological symptoms in the form of epileptic seizures and behavioural changes. Magnetic resonance imaging (MRI) revealed thickening of the dura mater in the left frontal area, slight oedema of the cortex, and subcortical white matter. Orbit biopsy showed non-specific inflammatory infiltrates. Despite the initial good response, symptoms progressed during treatment with glucocorticoids and immunosuppressants. Increased activity of the alternative complement pathway accompanied by a low level of its main inhibitor, factor H (FH), and the presence of anti-FH autoantibodies, was found. Genetic analysis revealed several missense variants of complement proteins, including two disease-linked mutations in FH (p.H402Y) and FI (T300A). An attempt to apply a complement C5 blocker, eculizumab, has been made. Neurological symptoms subsided, vision loss was inhibited, laboratory parameters improved, and discontinuation of steroid therapy was possible. The case underlines the role of complement system dysregulation in neurological distress.