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Background and Objectives: IgA nephropathy represents the most prevalent form of primary glomerulonephritis around the world, with significant heterogeneity in management strategies and outcomes. We conducted a systematic review and meta-analysis to evaluate the efficacy and safety of pharmacological interventions for IgA nephropathy. Materials and Methods: We searched multiple databases through June 2025, identifying randomized controlled trials and observational studies evaluating pharmacological treatments in biopsy-proven IgA nephropathy. Primary outcomes included proteinuria reduction and estimated glomerular filtration ration (eGFR) preservation. Secondary outcomes included hard kidney endpoints and safety parameters. Random-effects meta-analyses were performed with comprehensive risk–benefit assessments. Results: Twenty-five studies were included. B-cell/plasma-cell-targeted therapies showed significant proteinuria reduction (−34.0% [95% CI: −45.7, −22.3%]), complement pathway inhibitors demonstrated superior eGFR preservation (+5.8 mL/min/1.73 m2/year [95% CI: 2.4, 9.2]). Systemic corticosteroids showed observed hard outcome benefits (HR 0.37 [95% CI: 0.26, 0.52]) but highest adverse event risk (RR 3.28 [95% CI: 2.11, 5.09]). Novel agents showed projected favorable effects (B-cell: HR 0.38; complement: HR 0.42) pending validation. Conclusions: Novel targeted therapies, especially B-cell/plasma-cell-targeted agents and complement pathway inhibitors, show promising risk–benefit profiles. However, longer-term data and standardized eGFR slope reporting are needed to confirm these findings compared to other immunosuppressive agents.

Dysregulation of the alternative complement pathway (AP) predisposes individuals to a number of diseases including paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, and C3 glomerulopathy. Moreover, glomerular Ig deposits can lead to complement-driven nephropathies. Here we describe the discovery of a highly potent, reversible, and selective small-molecule inhibitor of factor B, a serine protease that drives the central amplification loop of the AP. Oral administration of the inhibitor prevents KRN-induced arthritis in mice and is effective upon prophylactic and therapeutic dosing in an experimental model of membranous nephropathy in rats. In addition, inhibition of factor B prevents complement activation in sera from C3 glomerulopathy patients and the hemolysis of human PNH erythrocytes. These data demonstrate the potential therapeutic value of using a factor B inhibitor for systemic treatment of complement-mediated diseases and provide a basis for its clinical development.

Ecotin is a serine protease inhibitor produced by hundreds of microbial species, including pathogens. Here we show, that ecotin orthologs from Escherichia coli, Yersinia pestis, Pseudomonas aeruginosa and Leishmania major are potent inhibitors of MASP-1 and MASP-2, the two key activator proteases of the complement lectin pathway. Factor D is the key activator protease of another complement activation route, the alternative pathway. We show that ecotin inhibits MASP-3, which is the sole factor D activator in resting human blood. In pathway-specific ELISA tests, we found that all ecotin orthologs are potent lectin pathway inhibitors, and at high concentration, they block the alternative pathway as well. In flow cytometry experiments, we compared the extent of complement-mediated opsonization and lysis of wild-type and ecotin-knockout variants of two E. coli strains carrying different surface lipopolysaccharides. We show, that endogenous ecotin provides significant protections against these microbicidal activities for both bacteria. By using pathway specific complement inhibitors, we detected classical-, lectin- and alternative pathway-driven complement attack from normal serum, with the relative contributions of the activation routes depending on the lipopolysaccharide type. Moreover, in cell proliferation experiments we observed an additional, complement-unrelated antimicrobial activity exerted by heat-inactivated serum. While ecotin-knockout cells are highly vulnerable to these activities, endogenous ecotin of wild-type bacteria provides complete protection against the lectin pathway-related and the complement-unrelated attack, and partial protection against the alternative pathway-related damage. In all, ecotin emerges as a potent, versatile self-defense tool that blocks multiple antimicrobial activities of the serum. These findings suggest that ecotin might be a relevant antimicrobial drug target.

The complement system is central to first-line defense against invading pathogens. However, excessive complement activation and/or the loss of complement regulation contributes to the development of autoimmune diseases, systemic inflammation, and thrombosis. One of the three pathways of the complement system, the alternative complement pathway, plays a vital role in amplifying complement activation and pathway signaling. Complement factor D, a serine protease of this pathway that is required for the formation of C3 convertase, is the rate-limiting enzyme. In this review, we discuss the function of factor D within the alternative pathway and its implication in both healthy physiology and disease. Because the alternative pathway has a role in many diseases that are characterized by excessive or poorly mediated complement activation, this pathway is an enticing target for effective therapeutic intervention. Nonetheless, although the underlying disease mechanisms of many of these complement-driven diseases are quite well understood, some of the diseases have limited treatment options or no approved treatments at all. Therefore, in this review we explore factor D as a strategic target for advancing therapeutic control of pathological complement activation.

Complement is a complex innate immune surveillance system, playing a key role in defense against pathogens and in host homeostasis. The complement system is initiated by conformational changes in recognition molecular complexes upon sensing danger signals. The subsequent cascade of enzymatic reactions is tightly regulated to assure that complement is activated only at specific locations requiring defense against pathogens, thus avoiding host tissue damage. Here, we discuss the recent advances describing the molecular and structural basis of activation and regulation of the complement pathways and their implication on physiology and pathology. This article will review the mechanisms of activation of alternative, classical, and lectin pathways, the formation of C3 and C5 convertases, the action of anaphylatoxins, and the membrane-attack-complex. We will also discuss the importance of structure-function relationships using the example of atypical hemolytic uremic syndrome. Lastly, we will discuss the development and benefits of therapies using complement inhibitors.

The complement system, consisting of three initiating pathways—classical, lectin and alternative, is an important part of innate immunity. Dysregulation of the complement system is implicated in the pathogenesis of several autoimmune and inflammatory diseases. Therapeutic inhibition of the complement system has been recognized as a viable approach to drug development and has been successful with the approval of a small number of complement inhibitors for diseases such as paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, neuromyelitis optica, myasthenia gravis and geographic atrophy. More recently, therapies selectively targeting the alternative pathway (AP), which drives the amplification of the complement responses, are being evaluated for these complement-mediated diseases. Complement Factor B, a serine protease, is a unique component of the AP that is essential for the catalytic activity of AP C3 convertase and AP C5 convertase. Inhibition of Factor B blocks the activity of the alternative pathway and the amplification loop, and subsequent generation of the membrane attack complex downstream; however, it has no effect on the initial activation mediated by the classical and lectin complement pathways. Therefore, Factor B is an attractive target for diseases in which the AP is overactivated. In this review, we provide an overview of Factor B and its critical role in the AP, discuss the benefit-risk of Factor B inhibition as a targeted therapeutic strategy, and describe the various Factor B inhibitors that are approved and/or in clinical development.

Membranoproliferative glomerulonephritis (MPGN) is a rare chronic kidney disease associated with complement activation. Recent immunofluorescence-based classification distinguishes between immune complex (IC)-mediated MPGN, with glomerular IgG and C3 deposits, and C3 glomerulopathies (C3G), with predominant C3 deposits. Genetic and autoimmune abnormalities causing hyperactivation of the complement alternative pathway have been found as frequently in patients with immune complex-associated MPGN (IC-MPGN) as in those with C3G. In the last decade, there have been great advances in research into the autoimmune causes of IC-MPGN and C3G. The complement-activating autoantibodies called C3-nephritic factors (C3NeFs), which are present in 40–80% of patients, form a heterogeneous group of autoantibodies that stabilise the C3 convertase or the C5 convertase of the alternative pathway or both. A few patients, mainly with IC-MPGN, carry autoantibodies directed against the two components of the alternative pathway C3 convertase, factors B and C3b. Finally, autoantibodies against factor H, the main regulator of the alternative pathway, have been reported in a small proportion of patients with IC-MPGN or C3G. The identification of distinct pathogenetic patterns leading to kidney injury and of targets in the complement cascade may pave the way for tailored therapies for IC-MPGN and C3G, with specific complement inhibitors in the development pipeline.

C3 glomerulopathy is an ultra-rare, severe form of glomerulonephritis caused by overactivation of the alternative complement pathway. We aimed to assess efficacy and safety of iptacopan (LNP023), an oral, proximal complement inhibitor that targets factor B to selectively inhibit the alternative pathway of the complement cascade. APPEAR-C3G was a multicentre, randomised, double-blind, placebo-controlled, phase 3 study of iptacopan versus placebo (both in addition to supportive care [renin–angiotensin–aldosterone system (RAAS) inhibitors] and immunosuppression). Adult participants (aged 18–60 years) with biopsy-confirmed C3 glomerulopathy were enrolled from 35 hospitals or medical centres in 18 countries. Inclusion criteria included reduced serum C3 concentration (ie, <77 mg/dL [defined as <0·85 × lower limit of the central laboratory normal range]) at screening, urine protein–creatinine ratio (UPCR) of 1·0 g/g or higher at day –75 and day –15 before randomisation, estimated glomerular filtration rate (eGFR) of 30 mL/min per 1·73 m2 or higher at screening and day –15, and vaccination against Neisseria meningitidis and Streptococcus pneumoniae. All eligible participants were randomised 1:1 via interactive response technology to either the iptacopan or the placebo group, stratified by treatment with corticosteroids, mycophenolic acid, or both (yes or no). During the 6-month double-blind period, participants orally received either iptacopan 200 mg twice daily or placebo; this was followed by a 6-month open-label period in which all participants received iptacopan 200 mg twice daily. The primary endpoint was relative reduction in proteinuria (measured by log-transformed ratio to baseline in UPCR sampled from a 24-h urine collection) at 6 months. The primary analyses were done in the full analysis set (ie, all participants to whom study treatment was assigned by randomisation); all participants who received at least one dose of study treatment were included in the safety analysis. This trial was registered with ClinicalTrials.gov (NCT04817618) and the adult cohort has been completed. Between July 28, 2021, and Feb 15, 2023, 132 participants were screened, of whom 58 did not complete the screening period and 74 (64% male; 69% White) were randomised 1:1 to receive either iptacopan (n=38) or placebo (n=36). One participant in the placebo group discontinued treatment during the open-label period. The 24-h UPCR percentage change relative to baseline at 6 months was –30·2% (95% CI –42·8 to –14·8) in the iptacopan group and 7·6% (–11·9 to 31·3) in the placebo group. In the iptacopan group, the geometric mean of 24-h UPCR was 3·33 g/g (95% CI 2·79 to 3·97) at baseline and 2·17 g/g (1·62 to 2·91) at 6 months; in the placebo group, this was 2·58 g/g (2·18 to 3·05) at baseline and 2·80 g/g (2·37 to 3·30) at 6 months. The primary endpoint was met with a relative reduction in 24-h UPCR at 6 months for iptacopan versus placebo of 35·1% (13·8 to 51·1; p=0·0014). 30 (79%) of 38 participants in the iptacopan group had treatment-emergent adverse events, compared with 24 (67%) of 36 participants in the placebo group; most of these were of mild or moderate severity. There were no deaths, no treatment discontinuations due to treatment-emergent adverse events, and no meningococcal infections. Serious adverse events were reported in three (8%) participants in the iptacopan group and one (3%) participant in the placebo group. Iptacopan showed a statistically significant, clinically meaningful proteinuria reduction in addition to RAAS inhibitors and immunosuppression at 6 months. Iptacopan was well tolerated with an acceptable safety profile in patients with C3 glomerulopathy. Novartis Pharma.

The complement system is an intricate defense network that rapidly removes invading pathogens. Although many complement regulators are present to protect host cells under homeostasis, the impairment of Factor H (FH) regulatory mechanism has been associated with several autoimmune and inflammatory diseases. To understand the dynamics involved in the pivotal balance between activation and regulation, we have developed a comprehensive computational model of the alternative and classical pathways of the complement system. The model is composed of 290 ordinary differential equations with 142 kinetic parameters that describe the state of complement system under homeostasis and disorder through FH impairment. We have evaluated the state of the system by generating concentration-time profiles for the biomarkers C3, C3a-desArg, C5, C5a-desArg, Factor B (FB), Ba, Bb, and fC5b-9 that are influenced by complement dysregulation. We show that FH-mediated disorder induces substantial levels of complement activation compared to homeostasis, by generating reduced levels of C3 and FB, and to a lesser extent C5, and elevated levels of C3a-desArg, Ba, Bb, C5a-desArg, and fC5b-9. These trends are consistent with clinically observed biomarkers associated with complement-mediated diseases. Furthermore, we introduced therapy states by modeling known inhibitors of the complement system, a compstatin variant (C3 inhibitor) and eculizumab (C5 inhibitor). Compstatin demonstrates strong restorative effects for early-stage biomarkers, such as C3a-desArg, FB, Ba, and Bb, and milder restorative effects for late-stage biomarkers, such as C5a-desArg and fC5b-9, whereas eculizumab has strong restorative effects on late-stage biomarkers, and negligible effects on early-stage biomarkers. These results highlight the need for patient-tailored therapies that target early complement activation at the C3 level, or late-stage propagation of the terminal cascade at the C5 level, depending on the specific FH-mediated disease and the manifestations of a patient's genetic profile in complement regulatory function.

Atypical hemolytic uremic syndrome (aHUS) is a rare and severe thrombotic microangiopathy caused by genetic or acquired abnormalities leading to activation of the complement alternative pathway on cell surfaces. This process leads to endothelial dysfunction and microvascular thrombosis. The introduction of anti-C5 antibodies has dramatically improved aHUS prognosis; however, these treatments require regular intravenous infusions and block systemic complement activity, exposing the patient to risk of infections. Recently complement inhibitors have been developed to selectively bind injury-associated target molecules, thereby concentrating the drug at specific cellular or tissue sites while preserving systemic complement function. This study evaluated the local complement inhibitory activity of new molecules that exploit the natural localization of C3d at complement activation sites on cells: namely the anti-C3d monoclonal antibody 3d8b conjugated with the first 10 or 17 short consensus repeats (SCRs) of complement receptor 1 (CR1 1–10 and CR1 1-17 , respectively) or the first 5 SCRs of complement factor H (FH 1-5 ). To this purpose we tested their capability to block C3 deposition and C5b-9 formation on microvascular endothelial cells (HMEC-1) exposed to serum from patients with aHUS. We also assessed their ability to prevent loss of anti-thrombogenic properties in HMEC-1 pre-exposed to aHUS serum and then perfused with control blood. We demonstrate that anti-C3d-antibody conjugated with CR1 1-10 , or CR1 1-17 , or FH 1–5 effectively prevented aHUS serum-induced complement activation on HMEC-1, outperforming their non-targeted soluble counterparts. The efficacy of C3 convertase inhibition varied depending on the complement inhibitory component (CR1 1-17 > CR1 1-10 > FH 1-5 ). However, all the inhibitors successfully blocked C5 convertase activity and eliminated the pro-thrombogenic effects of aHUS patients’ serum. These findings support the potential of tissue-targeted complement inhibition as a novel, non-systemic therapeutic strategy for aHUS and other diseases characterized by localized complement dysregulation.