Structural basis for surface activation of the classical complement cascade by the short pentraxin C-reactive protein.

Human C-reactive protein (CRP) is a pentameric complex involved in immune defense and regulation of autoimmunity. CRP is also a therapeutic target, with both administration and depletion of serum CRP being pursued as a possible treatment for autoimmune and cardiovascular diseases, among others. CRP binds to phosphocholine (PC) moieties on membranes to activate the complement system via the C1 complex, but it is unknown how CRP, or any pentraxin, binds to C1. Here, we present a cryoelectron tomography (cryoET)-derived structure of CRP bound to PC ligands and the C1 complex. To gain control of CRP binding, a synthetic mimotope of PC was synthesized and used to decorate cell-mimetic liposome surfaces. Structure-guided mutagenesis of CRP yielded a fully active complex able to bind PC-coated liposomes that was ideal for cryoET and subtomogram averaging. In contrast to antibodies, which form Fc-mediated hexameric platforms to bind and activate the C1 complex, CRP formed rectangular platforms assembled from four laterally associated CRP pentamers that bind only four of the six available globular C1 head groups. Potential residues mediating lateral association of CRP were identified from interactions between unit cells in existing crystal structures, which rationalized previously unexplained mutagenesis data regarding CRP-mediated complement activation. The structure also enabled interpretation of existing biochemical data regarding interactions mediating C1 binding and identified additional residues for further mutagenesis studies. These structural data therefore provide a possible mechanism for regulation of complement by CRP, which limits complement progression and has consequences for how the innate immune system influences autoimmunity.

C1q/MASP Complexes-Hybrid Complexes of Classical and Lectin Pathway Proteins Are Found in the Circulation.

Complement pathways, traditionally regarded as separate entities in vitro, are increasingly noted for cross-communication and bypass mechanisms. Among these, the MBL/ficolin/CL-associated serine protease (MASP)-3, a component of lectin pathway pattern recognition molecules, has shown the ability to process critical substrates such as pro-factor D and insulin growth factor binding protein-5. Given shared features between lectin pathway pattern recognition molecules and C1q from the classical pathway, we hypothesized that C1q might be a viable in vivo binding partner for the MASPs. We used microscale thermophoresis, ELISA, and immunoprecipitation assays to detect C1q/MASP complexes and functionally assessed the complexes through enzymatic cleavage assays. C1q/MASP-3 complexes were detected in human serum and correlated well with MASP-3 serum levels in healthy individuals. The binding affinity between MASP-3 and C1q in vitro was in the nanomolar range, and the interaction was calcium-dependent, as demonstrated by their dissociation in the presence of EDTA. Furthermore, most of the circulating C1q-bound MASP-3 was activated. Based on immunoprecipitation, also C1q/MASP-2 complexes appeared to be present in serum. Finally, C1q/MASP-2 and C1q/MASP-3 in vitro complexes were able to cleave C4 and pro-factor D, respectively. Our study reveals the existence of C1q/MASP complexes in the circulation of healthy individuals, and both C1q/MASP-2 and C1q/MASP-3 complexes display proteolytic activity. Hence, this study uncovers a crosstalk route between complement pathways not previously described.

Complete Description of the Three Pathways of the Complement System in a Series of 430 Patients with Rheumatoid Arthritis.

The complement (C) system is implicated in the etiopathogenesis of rheumatoid arthritis (RA). However, there is a lack of studies characterizing all three C pathways in RA patients. This study aimed to evaluate the association between an in-depth examination of the C system and RA patient characteristics, focusing on disease activity and the presence of rheumatoid factor and anti-citrullinated protein autoantibodies (ACPA). In a cohort of 430 RA patients, functional assays of the three C pathways (classical, alternative, and lectin) and serum levels of their components were assessed. Components included C1q (classical); factor D and properdin (alternative); lectin (lectin); C1-inhibitor; C2, C4, and C4b (classical and lectin); C3, C3a, and C4b (common); and C5, C5a, and C9 (terminal). A multivariable linear regression analysis showed significant positive correlations between C-reactive protein and C system proteins and functional assays, especially in the terminal and common pathways. Disease activity, measured by scores with or without acute phase reactants, positively correlated with the classical pathway functional test and terminal pathway products. Conversely, rheumatoid factor or ACPA presence was associated with lower classical pathway values and decreased C3a and C4b levels, suggesting complement depletion. In conclusion, RA disease activity increases C molecules and functional complement assays, while rheumatoid factor or ACPA positivity is linked to C consumption. Our study offers a detailed analysis of the complement system's role in RA, potentially guiding the development of more targeted and effective treatment strategies.

Complement-Coagulation Cross-talk: Factor H-mediated regulation of the Complement Classical Pathway activation by fibrin clots.

The classical pathway of the complement system is activated by the binding of C1q in the C1 complex to the target activator, including immune complexes. Factor H is regarded as the key downregulatory protein of the complement alternative pathway. However, both C1q and factor H bind to target surfaces via charge distribution patterns. For a few targets, C1q and factor H compete for binding to common or overlapping sites. Factor H, therefore, can effectively regulate the classical pathway activation through such targets, in addition to its previously characterized role in the alternative pathway. Both C1q and factor H are known to recognize foreign or altered-self materials, e.g., bacteria, viruses, and apoptotic/necrotic cells. Clots, formed by the coagulation system, are an example of altered self. Factor H is present abundantly in platelets and is a well-known substrate for FXIIIa. Here, we investigated whether clots activate the complement classical pathway and whether this is regulated by factor H. We show here that both C1q and factor H bind to the fibrin formed in microtiter plates and the fibrin clots formed under in vitro physiological conditions. Both C1q and factor H become covalently bound to fibrin clots, and this is mediated via FXIIIa. We also show that fibrin clots activate the classical pathway of complement, as demonstrated by C4 consumption and membrane attack complex detection assays. Thus, factor H downregulates the activation of the classical pathway induced by fibrin clots. These results elucidate the intricate molecular mechanisms through which the complement and coagulation pathways intersect and have regulatory consequences.

Crystal structure of Trichinella spiralis calreticulin and the structural basis of its complement evasion mechanism involving C1q.

Helminths produce calreticulin (CRT) to immunomodulate the host immune system as a survival strategy. However, the structure of helminth-derived CRT and the structural basis of the immune evasion process remains unclarified. Previous study found that the tissue-dwelling helminth Trichinella spiralis produces calreticulin (TsCRT), which binds C1q to inhibit activation of the complement classical pathway. Here, we used x-ray crystallography to resolve the structure of truncated TsCRT (TsCRTΔ), the first structure of helminth-derived CRT. TsCRTΔ was observed to share the same binding region on C1q with IgG based on the structure and molecular docking, which explains the inhibitory effect of TsCRT on C1q-IgG-initiated classical complement activation. Based on the key residues in TsCRTΔ involved in the binding activity to C1q, a 24 amino acid peptide called PTsCRT was constructed that displayed strong C1q-binding activity and inhibited C1q-IgG-initiated classical complement activation. This study is the first to elucidate the structural basis of the role of TsCRT in immune evasion, providing an approach to develop helminth-derived bifunctional peptides as vaccine target to prevent parasite infections or as a therapeutic agent to treat complement-related autoimmune diseases.

Canonical and noncanonical functions of complement in systemic lupus erythematosus.

For many years complement activation in systemic lupus erythematosus (SLE) was viewed as a major cause of tissue injury. However, human and murine studies showed that complement plays a protective as well as a proinflammatory role in tissue damage. A hierarchy is apparent with early classical pathway components, particularly C1q, exerting the greatest influence. Understanding the mechanisms underlying the protective function(s) of complement remains an important challenge for the future and has implications for the use of complement therapy in SLE. We review recent advances in the field and give a new perspective on the complement conundrum in SLE.

A Significant Contribution of the Classical Pathway of Complement in SARS-CoV-2 Neutralization of Convalescent and Vaccinee Sera.

Although high titers of neutralizing Abs in human serum are associated with protection from reinfection by SARS-CoV-2, there is considerable heterogeneity in human serum-neutralizing Abs against SARS-CoV-2 during convalescence between individuals. Standard human serum live virus neutralization assays require inactivation of serum/plasma prior to testing. In this study, we report that the SARS-CoV-2 neutralization titers of human convalescent sera were relatively consistent across all disease states except for severe COVID-19, which yielded significantly higher neutralization titers. Furthermore, we show that heat inactivation of human serum significantly lowered neutralization activity in a live virus SARS-CoV-2 neutralization assay. Heat inactivation of human convalescent serum was shown to inactivate complement proteins, and the contribution of complement in SARS-CoV-2 neutralization was often >50% of the neutralizing activity of human sera without heat inactivation and could account for neutralizing activity when standard titers were zero after heat inactivation. This effect was also observed in COVID-19 vaccinees and could be abolished in individuals who were undergoing treatment with therapeutic anti-complement Abs. Complement activity was mainly dependent on the classical pathway with little contributions from mannose-binding lectin and alternative pathways. Our study demonstrates the importance of the complement pathway in significantly increasing viral neutralization activity against SARS-CoV-2 in spike seropositive individuals.

The molecular determinants of classical pathway complement inhibition by OspEF-related proteins of Borrelia burgdorferi.

The complement system serves as the first line of defense against invading pathogens by promoting opsonophagocytosis and bacteriolysis. Antibody-dependent activation of complement occurs through the classical pathway and relies on the activity of initiating complement proteases of the C1 complex, C1r and C1s. The causative agent of Lyme disease, Borrelia burgdorferi, expresses two paralogous outer surface lipoproteins of the OspEF-related protein family, ElpB and ElpQ, that act as specific inhibitors of classical pathway activation. We have previously shown that ElpB and ElpQ bind directly to C1r and C1s with high affinity and specifically inhibit C2 and C4 cleavage by C1s. To further understand how these novel protease inhibitors function, we carried out a series of hydrogen-deuterium exchange mass spectrometry (HDX-MS) experiments using ElpQ and full-length activated C1s as a model of Elp-protease interaction. Comparison of HDX-MS profiles between unbound ElpQ and the ElpQ/C1s complex revealed a putative C1s-binding site on ElpQ. HDX-MS-guided, site-directed ElpQ mutants were generated and tested for direct binding to C1r and C1s using surface plasmon resonance. Several residues within the C-terminal region of ElpQ were identified as important for protease binding, including a single conserved tyrosine residue that was required for ElpQ- and ElpB-mediated complement inhibition. Collectively, our study identifies key molecular determinants for classical pathway protease recognition by Elp proteins. This investigation improves our understanding of the unique complement inhibitory mechanism employed by Elp proteins which serve as part of a sophisticated complement evasion system present in Lyme disease spirochetes.

Inhibition of the C1s Protease and the Classical Complement Pathway by 6-(4-Phenylpiperazin-1-yl)Pyridine-3-Carboximidamide and Chemical Analogs.

The classical pathway (CP) is a potent mechanism for initiating complement activity and is a driver of pathology in many complement-mediated diseases. The CP is initiated via activation of complement component C1, which consists of the pattern recognition molecule C1q bound to a tetrameric assembly of proteases C1r and C1s. Enzymatically active C1s provides the catalytic basis for cleavage of the downstream CP components, C4 and C2, and is therefore an attractive target for therapeutic intervention in CP-driven diseases. Although an anti-C1s mAb has been Food and Drug Administration approved, identifying small-molecule C1s inhibitors remains a priority. In this study, we describe 6-(4-phenylpiperazin-1-yl)pyridine-3-carboximidamide (A1) as a selective, competitive inhibitor of C1s. A1 was identified through a virtual screen for small molecules that interact with the C1s substrate recognition site. Subsequent functional studies revealed that A1 dose-dependently inhibits CP activation by heparin-induced immune complexes, CP-driven lysis of Ab-sensitized sheep erythrocytes, CP activation in a pathway-specific ELISA, and cleavage of C2 by C1s. Biochemical experiments demonstrated that A1 binds directly to C1s with a Kd of ∼9.8 µM and competitively inhibits its activity with an inhibition constant (Ki) of ∼5.8 µM. A 1.8-Å-resolution crystal structure revealed the physical basis for C1s inhibition by A1 and provided information on the structure-activity relationship of the A1 scaffold, which was supported by evaluating a panel of A1 analogs. Taken together, our work identifies A1 as a new class of small-molecule C1s inhibitor and lays the foundation for development of increasingly potent and selective A1 analogs for both research and therapeutic purposes.

Activation of the classical complement pathway in myasthenia gravis with acetylcholine receptor antibodies.

INTRODUCTION/AIMS: Myasthenia gravis (MG) is an autoimmune disease affecting the neuromuscular junction (NMJ) of skeletal muscle. Complement activation is one of the mechanisms by which anti-acetylcholine receptor (anti-AChR) autoantibodies reduce synaptic transmission at the NMJ. In this study, we aimed to examine the activation of the complement pathways, including the classical pathway, as potential contributors to the pathogenesis of MG with anti-AChR antibodies. METHODS: In this single-center, observational study of 45 patients with anti-AChR-antibody-positive generalized MG, serum concentrations of major components of the complement pathways, including C1q, C5, C5a, soluble C5b-9 (sC5b-9), Ba, and complement factor H, were measured using an enzyme-linked immunosorbent assay. A total of 25 patients with a non-inflammatory neurological disorder served as controls. In addition, the relationships of complement activation with clinical characteristics were examined. RESULTS: The patients with MG exhibited lower serum levels of C5 (p = .0001) and higher serum levels of sC5b-9 (p = .004) compared with the control group. At about 6 months (range, 172-209 days) after the start of immunotherapy, serum levels of Ba were significantly higher than baseline levels (p = .002) and were associated with improvement in MG clinical scores. DISCUSSION: Herein, we provide evidence for the activation of the classical complement pathway and its association with disease activity in anti-AChR-antibody-positive generalized MG.

A distinct variant of the SzM protein of Streptococcus equi subsp. zooepidemicus recruits C1q independent of IgG binding and inhibits activation of the classical complement pathway.

Streptococcus equi subsp. zooepidemicus (SEZ) is a major equine pathogen that causes pneumonia, abortion, and polyarthritis. It can also cause invasive infections in humans. SEZ expresses the M-like protein SzM, which recruits host proteins such as fibrinogen to the bacterial surface. Equine SEZ strain C2, which binds only comparably low amounts of human fibrinogen in comparison to human SEZ strain C33, was previously shown to proliferate in equine and human blood. As the expression of SzM_C2 was necessary for survival in blood, this study investigated the working hypothesis that SzM_C2 inhibits complement activation through a mechanism other than fibrinogen and non-immune immunoglobulin binding. Loss-of-function experiments showed that SEZ C2, but not C33, binds C1q via SzM in IgG-free human plasma. Furthermore, SzM C2 expression is necessary for recruiting purified human or equine C1q to the bacterial surface. Flow cytometry analysis demonstrated that SzM expression in SEZ C2 is crucial for the significant reduction of C3b labelling in human plasma. Addition of human plasma to immobilized rSzM_C2 and immobilized aggregated IgG led to binding of C1q, but only the latter activated the complement system, as shown by the detection of C4 deposition. Complement activation induced by aggregated IgG was significantly reduced if human plasma was pre-incubated with rSzM_C2. Furthermore, rSzM_C2, but not rSzM_C33, inhibited the activation of the classical complement pathway in human plasma, as determined in an erythrocyte lysis experiment. In conclusion, the immunoglobulin-independent binding of C1q to SzM_C2 is associated with complement inhibition.

Differentiating between activation via the lectin or the classical complement pathway in patients with systemic lupus erythematosus.

Complement activation is a hallmark of systemic lupus erythematosus (SLE) and can proceed through the classical (CP), lectin (LP), or alternative pathway (AP). When managing SLE patients, pathway-specific complement activation is rarely monitored as clinical assays are unavailable. In this study, we aim to differentiate between CP- or LP-mediated complement activation in SLE patients by quantifying pathway-specific protein complexes, namely C1s/C1-inhibitor (C1-INH) (CP-specific activation) and MASP-1/C1-INH (LP-specific activation). Levels for both complexes were assessed in 156 SLE patients and 50 controls using two newly developed ELISAs. We investigated whether pathway-specific complement activation was associated with disease activity and lupus nephritis (LN). Disease activity stratification was performed using SLEDAI scores assessed at inclusion. C1s/C1-INH concentrations were significantly increased in active SLE patients (SLEDAI ≥6) when compared with SLE patients with low disease activity (SLEDAI <6, P < 0.01) and correlated with SLEDAI score (r = .29, P < 0.01). In active LN, MASP-1/C1-INH plasma concentrations were significantly increased compared with nonactive LN (P = 0.02). No differences in MASP-1/C1-INH plasma concentrations were observed between active SLE patients and patients with low disease activity (P = 0.11) nor did we observe a significant correlation with disease activity (r = 0.12, P = 0.15). Our data suggest that the CP and the LP are activated in SLE. The CP is activated in active SLE disease, whereas activation of the LP might be more specific to disease manifestations like LN. Our results warrant further research into specific complement pathway activation in SLE patients to potentially improve specific-targeted and tailored-treatment approaches.

Anti-C1s humanized monoclonal antibody SAR445088: A classical pathway complement inhibitor specific for the active form of C1s.

The objective of this study was to characterize the complement-inhibiting activity of SAR445088, a novel monoclonal antibody specific for the active form of C1s. Wieslab® and hemolytic assays were used to demonstrate that SAR445088 is a potent, selective inhibitor of the classical pathway of complement. Specificity for the active form of C1s was confirmed in a ligand binding assay. Finally, TNT010 (a precursor to SAR445088) was assessed in vitro for its ability to inhibit complement activation associated with cold agglutinin disease (CAD). TNT010 inhibited C3b/iC3b deposition on human red blood cells incubated with CAD patient serum and decreased their subsequent phagocytosis by THP-1 cells. In summary, this study identifies SAR445088 as a potential therapeutic for the treatment of classical pathway-driven diseases and supports its continued assessment in clinical trials.

Complement activation profiles in anti-acetylcholine receptor positive myasthenia gravis.

BACKGROUND AND PURPOSE: Complement component 5 (C5) targeting therapies are clinically beneficial in patients with acetylcholine receptor antibody+ (AChR-Ab+ ) generalized myasthenia gravis (MG). That clearly implicates antibody-mediated complement activation in MG pathogenesis. Here, classical and alternative complement pathways were profiled in patients from different MG subgroups. METHODS: In a case-control study, concentrations of C3a, C5a and sC5b9 were simultaneously quantified, indicating general activation of the complement system, whether via the classical and lectin pathways (C4a) or the alternative pathway (factors Ba and Bb) in MG patients with AChR or muscle-specific kinase antibodies (MuSK-Abs) or seronegative MG compared to healthy donors. RESULTS: Treatment-naïve patients with AChR-Ab+ MG showed substantially increased plasma levels of cleaved complement components, indicating activation of the classical and alternative as well as the terminal complement pathways. These increases were still present in a validation cohort of AChR-Ab+ patients under standard immunosuppressive therapies; notably, they were not evident in patients with MuSK-Abs or seronegative MG. Neither clinical severity parameters (at the time of sampling or 1 year later) nor anti-AChR titres correlated significantly with activated complement levels. CONCLUSIONS: Markers indicative of complement activation are prominently increased in patients with AChR-Ab MG despite standard immunosuppressive therapies. Complement inhibition proximal to C5 cleavage should be explored for its potential therapeutic benefits in AChR-Ab+ MG.

First-in-human study with SAR445088: A novel selective classical complement pathway inhibitor.

SAR445088 is an anti-C1s humanized monoclonal antibody that inhibits activated C1s in the proximal portion of the classical complement system and has the potential to provide clinical benefit in the treatment of complement-mediated diseases. A phase I, first-in-human, double-blind, randomized, placebo-controlled, dose-escalation trial of single and multiple doses of SAR445088 was conducted in 93 healthy participants to evaluate the safety, tolerability, and pharmacokinetic (PK) and pharmacodynamic (PD) profiles. Single (intravenous [i.v.] and subcutaneous [s.c.]) ascending doses (SAD) and multiple (s.c.) ascending doses (MAD) of SAR445088 were well-tolerated. The PK of SAR445088 was characterized by slow absorption after the s.c. dose and a long half-life (mean terminal half-life [t1/2 ] 8-15 weeks). Two PD assays were used to measure inhibition of the classical complement pathway (CP): Wieslab CP and complement mediated hemolytic capacity (CH50). The estimated half-maximal inhibitory concentration (IC50 ) and 90% inhibitory concentration (IC90 ) for the Wieslab CP assay were 96.4 and 458 µg/ml, respectively, and 16.6 and 57.0 µg/ml, respectively, for the CH50 assay. In summary, SAR445088 was well-tolerated and had favorable PK and PD profiles after SAD (i.v. or s.c.) and MAD (s.c.) in humans. These findings warrant further clinical investigations in patients with classical complement-mediated disorders.

A possible role of classical complement pathway activation in the pathogenesis of immunoglobulin G nephropathy: a case report.

Immunoglobulin G (IgG) nephropathy refers to a rare group of diseases characterized by deposits of IgG in the mesangial region. However, IgG nephropathy is controversial as a single disease entity, and its pathogenesis remains to be elucidated. In the present report, we discuss a case of IgG nephropathy in which we observed activation of the classical complement pathway.A 47-year-old woman was admitted to our hospital with nephrotic syndrome. Light-microscopic examination revealed neither proliferative nor sclerotic lesions in the glomeruli. However, unusual and large deposits were observed in the paramesangial area. An immunofluorescence study revealed predominant IgG and C1q and slight C3 deposits in the paramesangial area, suggesting immune-complex-type glomerular disease. An electron microscopic study also revealed different sizes of non-organized electron-dense deposits with a similar pattern of distribution, which were accompanied by foot process effacement. Clinically, there was no evidence of systemic diseases, such as infectious or autoimmune diseases (including systemic lupus erythematosus). Based on these findings, she was diagnosed with IgG nephropathy and treated with prednisolone. Steroid therapy was effective, and complete remission was maintained.Additional immunological examination revealed that IgG deposits were polyclonal and consisted mainly of the IgG1 and IgG3 subclasses. Furthermore, staining was positive for C4d and C5b-9. The present findings indicate that the pathogenesis of IgG nephropathy in our patient may have involved activation of the classical complement pathway.

Dendritic spine loss in epileptogenic Type II focal cortical dysplasia: Role of enhanced classical complement pathway activation.

Dendritic spines are the postsynaptic sites for most excitatory glutamatergic synapses. We previously demonstrated a severe spine loss and synaptic reorganization in human neocortices presenting Type II focal cortical dysplasia (FCD), a developmental malformation and frequent cause of drug-resistant focal epilepsy. We extend the findings, investigating the potential role of complement components C1q and C3 in synaptic pruning imbalance. Data from Type II FCD were compared with those obtained in focal epilepsies with different etiologies. Neocortical tissues were collected from 20 subjects, mainly adults with a mean age at surgery of 31 years, admitted to epilepsy surgery with a neuropathological diagnosis of: cryptogenic, temporal lobe epilepsy with hippocampal sclerosis, and Type IIa/b FCD. Dendritic spine density quantitation, evaluated in a previous paper using Golgi impregnation, was available in a subgroup. Immunohistochemistry, in situ hybridization, electron microscopy, and organotypic cultures were utilized to study complement/microglial activation patterns. FCD Type II samples presenting dendritic spine loss were characterized by an activation of the classical complement pathway and microglial reactivity. In the same samples, a close relationship between microglial cells and dendritic segments/synapses was found. These features were consistently observed in Type IIb FCD and in 1 of 3 Type IIa cases. In other patient groups and in perilesional areas outside the dysplasia, not presenting spine loss, these features were not observed. In vitro treatment with complement proteins of organotypic slices of cortical tissue with no sign of FCD induced a reduction in dendritic spine density. These data suggest that dysregulation of the complement system plays a role in microglia-mediated spine loss. This mechanism, known to be involved in the removal of redundant synapses during development, is likely reactivated in Type II FCD, particularly in Type IIb; local treatment with anticomplement drugs could in principle modify the course of disease in these patients.

Safety and efficacy of classical complement pathway inhibition with sutimlimab in chronic immune thrombocytopenia.

Chronic/refractory immune thrombocytopenia (ITP) is a rare and pathophysiologically heterogeneous disorder with variable responsiveness to available treatments. Sutimlimab, a first-in-class humanized monoclonal anti-C1s IgG4 antibody, selectively inhibits the classical pathway. This phase 1 study (NCT03275454) assessed the safety, efficacy, pharmacokinetics, and pharmacodynamics of biweekly sutimlimab in patients with chronic/refractory ITP with an inadequate response to ≥2 therapies (platelet count ≤ 30 × 109/L). Twelve patients (median age 42 years) received sutimlimab for a median of 20.5 weeks followed by a median 2-week washout period (part A). In part B, 7 of the 12 eligible patients received sutimlimab retreatment for a median of 113 weeks. In part A, the mean (standard deviation) platelet count increased from 25 × 109/L (17) to 54 × 109/L (60) 24 hours after starting sutimlimab, maintaining ≥50 × 109/L throughout part A. Five patients (42%) achieved durable platelet count responses (≥50 × 109/L in ≥50% of follow-up visits) and 4 achieved complete response (platelet count ≥100 × 109/L). The mean platelet count returned to baseline during washout and increased upon retreatment in part B. The mean platelet count improvements accompanied the rapid inhibition of the classical pathway. There were 74 treatment-emergent adverse events in part A (n = 10) and 70 in part B (n = 6). Five serious adverse events were observed; 1 event (migraine) was assessed by the investigator as related to sutimlimab. These results demonstrated that in some patients with ITP, autoantibodies activate the classical complement pathway, accelerating platelet destruction or impairing platelet production and contributing to treatment failure. Thus, C1s inhibition may be a safe and beneficial therapeutic approach for patients with chronic/refractory ITP.

C1q as a target molecule to treat human disease: What do mouse studies teach us?

The complement system is a field of growing interest for pharmacological intervention. Complement protein C1q, the pattern recognition molecule at the start of the classical pathway of the complement cascade, is a versatile molecule with additional non-canonical actions affecting numerous cellular processes. Based on observations made in patients with hereditary C1q deficiency, C1q is protective against systemic autoimmunity and bacterial infections. Accordingly, C1q deficient mice reproduce this phenotype with susceptibility to autoimmunity and infections. At the same time, beneficial effects of C1q deficiency on disease entities such as neurodegenerative diseases have also been described in murine disease models. This systematic review provides an overview of all currently available literature on the C1q knockout mouse in disease models to identify potential target diseases for treatment strategies focusing on C1q, and discusses potential side-effects when depleting and/or inhibiting C1q.