Complement Component C3 Research Articles

A Single-Domain Antibody Targeting Complement Component C5 Acts as a Selective Inhibitor of the Terminal Pathway of the Complement System and Thus Functionally Mimicks the C-Terminal Domain of the Staphylococcus aureus SSL7 Protein.

The complement system is an efficient anti-microbial effector mechanism. On the other hand abnormal complement activation is involved in the pathogenesis of multiple inflammatory and hemolytic diseases. As general inhibition of the complement system may jeopardize patient health due to increased susceptibility to infections, the development of pathway-specific complement therapeutics has been a long-lasting goal over the last decades. In particular, pathogen mimicry has been considered as a promising approach for the design of selective anti-complement drugs. The C-terminal domain of staphylococcal superantigen-like protein 7 (SSL7), a protein secreted by Staphylococcus aureus, was recently found to be a specific inhibitor of the terminal pathway of the complement system, providing selective inhibition of cell lysis mediated by the membrane attack complex (MAC). We describe here the selection by phage display of a humanized single-domain antibody (sdAb) mimicking the C-terminal domain of SSL7. The antibody, called sdAb_E4, binds complement C5 with an affinity in the low micromolar range. Furthermore, sdAb_E4 induces selective inhibition of MAC-mediated lysis, allowing inhibition of red blood cell hemolysis and inhibition of complement deposition on apopto-necrotic cells, while maintaining efficient bactericidal activity of the complement terminal pathway. Finally, we present preliminary results indicating that sdAb_E4 may also be efficient in inhibiting hemolysis of erythrocytes from patients with paroxysmal nocturnal hemoglobinuria. Our data provide a proof of concept for the design of a selective MAC inhibitor capable of retaining complement bacteriolytic activity and this study opens up promising perspectives for the development of an sdAb_E4-derived therapeutics with application in the treatment of complement-mediated hemolytic disorders.

An iTRAQ-Based Quantitative Proteomic Analysis of Plasma Proteins in Preterm Newborns With Retinopathy of Prematurity.

Retinopathy of prematurity (ROP) is a vision-threatening complication of a premature birth, in which the etiology still remains unclear. Importantly, the molecular processes that govern these effects can be investigated in a perturbed plasma proteome composition. Thus, plasma proteomics may add new insights into a better understanding of the pathogenesis of this disease. The cord and peripheral blood of neonates (≤30 weeks gestational age) was drawn at birth and at the 36th postmenstrual week (PMA), respectively. Blood samples were retrospectively subdivided into ROP(+) and ROP(-) groups, according to the development of ROP. The quantitative analysis of plasma proteome at both time points revealed 30 protein abundance changes between ROP(+) and ROP(-) groups. After standardization to gestational age, children who developed ROP were characterized by an increased C3 complement component and fibrinogen level at both analyzed time points. Higher levels of the complement C3 component and fibrinogen, present in the cord blood and persistent to 36 PMA, may indicate a chronic low-grade systemic inflammation and hypercoagulable state that may play a role in the development of ROP.

Peptidylarginine deiminase and deiminated proteins are detected throughout early halibut ontogeny - Complement components C3 and C4 are post-translationally deiminated in halibut (Hippoglossus hippoglossus L.)

Post-translational protein deimination is mediated by peptidylarginine deiminases (PADs), which are calcium dependent enzymes conserved throughout phylogeny with physiological and pathophysiological roles. Protein deimination occurs via the conversion of protein arginine into citrulline, leading to structural and functional changes in target proteins. In a continuous series of early halibut development from 37 to 1050° d, PAD, total deiminated proteins and deiminated histone H3 showed variation in temporal and spatial detection in various organs including yolksac, muscle, skin, liver, brain, eye, spinal cord, chondrocytes, heart, intestines, kidney and pancreas throughout early ontogeny. For the first time in any species, deimination of complement components C3 and C4 is shown in halibut serum, indicating a novel mechanism of complement regulation in immune responses and homeostasis. Proteomic analysis of deiminated target proteins in halibut serum further identified complement components C5, C7, C8 C9 and C1 inhibitor, as well as various other immunogenic, metabolic, cytoskeletal and nuclear proteins. Post-translational deimination may facilitate protein moonlighting, an evolutionary conserved phenomenon, allowing one polypeptide chain to carry out various functions to meet functional requirements for diverse roles in immune defences and tissue remodelling.

Effects of feeding time on complement component C7 expression in Pelteobagrus vachellii subject to bacterial challenge

Shifting the feeding time for fish from daytime to nighttime could alter their digestive behavior, disturb their metabolism, and may affect immune-related genes. This study aimed to clone complement component C7 and analyze the different expression of C7 mRNA in fish fed during either the day or at night and then challenged with Aeromonas hydrophila infection. The Pv-C7 cDNA of Pelteobagrus vachellii contained 2 647 bp with an open reading frame encoding a protein of 818 amino acids. Multiple sequences analysis indicated that Pv-C7 included eight domains, which was similar to results for other species. Quantitative PCR analysis showed that Pv-C7 was mainly expressed in the liver, spleen, intestine and head kidney tissues of healthy P. vachellii. Quantitative PCR analysis showed that C7 mRNA transcript in the liver, spleen and head kidney also increased significantly when the fish were fed at nighttime (20:00). In addition, the expression of Pv-C7 mRNA significantly increased with A. hydrophila challenge in the liver (48–96 h), spleen and head kidney (12–96 h) tissues of P. vachellii. Pv-C7 mRNA expression of the fish fed at nighttime showed significant higher than that in the fish fed at day time at 12–48 h in head kidney and 12–24 h in spleen. This study indicates that altering the feeding time from daytime to nighttime could increase Pv-C7 mRNA expression, and feeding time may affect the immune response involving C7.

Molecular characterization and expression analysis of complement component C3 in southern catfish (Silurus meridionalis) and a whole mount in situ hybridization study on its ontogeny

The complement system plays an important role in protecting fish against attack by pathogens early in life. Complement component C3 is a central component in the complement system. The present work aimed to clone the full length C3 cDNA sequence of southern catfish (Silurus meridionalis), detect the tissue expression patterns of C3, investigate the ontogeny of C3 in embryo and larva, and assess the expression of C3 in response to pathogen infection. The full length C3 cDNA sequence of 5157 bp with an open reading frame (ORF) of 4938 bp was cloned from southern catfish. The deduced amino acid sequence showed similarity with other teleost fish. The mRNA expression of C3 was detected in liver, spleen, stomach, intestine, and head kidney with RT-PCR and in situ hybridization. Whole mount in situ hybridization results revealed that C3 was first expressed in the yolk syncytial layer at 34 h post fertilization (hpf), followed by the liver at 36 h post hatching (hph). When challenged with Aeromonas hydrophila, the transcripts of C3 showed a significant up-regulation in liver and spleen at 24 h. The results suggested that complement C3 played a key role in defense against invading pathogens in the early development stages of southern catfish. Therefore, these results provide important information to understand the functions of C3 during fish early development in Siluriformes.

The essential role of complement in antibody-mediated resistance to Salmonella.

SummaryVaccines can serve as essential tools to prevent bacterial diseases via the induction of long‐lasting IgG responses. The efficacy of such vaccines depends on the effector mechanisms triggered by IgG. The complement system and Fc‐gamma receptors (FcγRs) can potentially play a crucial role in IgG‐mediated immunity against bacterial diseases. However, their relative importance in vivo is unclear, and has been the object of controversy and debate. In this brief study, we have used gene‐targeted mice lacking either Fcγ RI, II, II and IV or the C3 complement component as well as a novel mouse strain lacking both C3 and FcγRs to conclusively show the essential role of complement in antibody‐mediated host resistance to Salmonella enterica systemic infection. By comparing the effect of IgG2a antibodies against Salmonella O‐antigen in gene‐targeted mice, we demonstrate that the complement system is essential for the IgG‐mediated reduction of bacterial numbers in the tissues.

Complement Component C3 Is Highly Expressed in Human Pancreatic Islets and Prevents β Cell Death via ATG16L1 Interaction and Autophagy Regulation

We show here that human pancreatic islets highly express C3, which is both secreted and present in the cytosol. Within isolated human islets, C3 expression correlates with type 2 diabetes (T2D) donor status, HbA1c, and inflammation. Islet C3 expression is also upregulated in several rodent diabetes models. C3 interacts with ATG16L1, which is essential for autophagy. Autophagy relieves cellular stresses faced by β cells during T2D and maintains cellular homeostasis. C3 knockout in clonal β cells impaired autophagy and led to increased apoptosis after exposure of cells to palmitic acid and IAPP. In the absence of C3, autophagosomes do not undergo fusion with lysosomes. Thus, C3 may be upregulated in islets during T2D as a cytoprotective factor against β cell dysfunction caused by impaired autophagy. Therefore, we revealed a previously undescribed intracellular function for C3, connecting the complement system directly to autophagy, with a broadpotential importance in other diseases and cell types.

Young and Old Animals Use Different Strategies for Forming an Immune Response to Infectious Agents (Pseudomonas aeruginosa and Escherichia coli)

This paper presents the results of a study of the features of the immune response to the injection of bacterial suspension (Pseudomonas aeruginosa and Escherichia coli) in young (3 months) and old (20 months) Wistar rats at 3, 5, and 7 days after infection. We assessed the contents of circulating immune complexes (CICs) and complement components C3 in dynamics and the activity of oxygen-dependent and oxygen-independent phagocytosis. Bacterial infection was accompanied by a multiple increase in the content of circulating immune complexes in young and, especially, in old animals. It remained at a high level from the third to the seventh day of the pathology development in young animals, while their content decreased to the seventh day in old animals. In the presence of infectious process, the content of complement components C3 decreased in young animals, and it increased in old animals, in the case of infection with P. aeruginosa and E. coli. The activity of oxygen-dependent and oxygen-independent phagocytosis decreased in different ways in young and old infected animals, i.e., the ratio of the forms of phagocytosis in old and young animals was different. Old animals were not inferior to the young ones in terms of the ability of the immune system to respond to the presence of an infectious agent. They used a different strategy for eliciting the immune response.

Aspergillus fumigatus conidial metalloprotease Mep1p cleaves host complement proteins

Innate immunity in animals including humans encompasses the complement system, which is considered an important host defense mechanism against Aspergillus fumigatus, one of the most ubiquitous opportunistic human fungal pathogens. Previously, it has been shown that the alkaline protease Alp1p secreted from A. fumigatus mycelia degrades the complement components C3, C4, and C5. However, it remains unclear how the fungal spores (i.e. conidia) defend themselves against the activities of the complement system immediately after inhalation into the lung. Here, we show that A. fumigatus conidia contain a metalloprotease Mep1p, which is released upon conidial contact with collagen and inactivates all three complement pathways. In particular, Mep1p efficiently inactivated the major complement components C3, C4, and C5 and their activation products (C3a, C4a, and C5a) as well as the pattern-recognition molecules MBL and ficolin-1, either by directly cleaving them or by cleaving them to a form that is further broken down by other proteases of the complement system. Moreover, incubation of Mep1p with human serum significantly inhibited the complement hemolytic activity and conidial opsonization by C3b and their subsequent phagocytosis by macrophages. Together, these results indicate that Mep1p associated with and released from A. fumigatus conidia likely facilitates early immune evasion by disarming the complement defense in the human host.

Complement component C3 regulates cytoprotective autophagy in pancreatic islets via ATG16L1 interaction

Complement component C3 regulates cytoprotective autophagy in pancreatic islets via ATG16L1 interaction

Ravulizumab (ALXN1210) in patients with paroxysmal nocturnal hemoglobinuria: results of 2 phase 1b/2 studies

AbstractRavulizumab (ALXN1210), a humanized monoclonal antibody to complement component C5, was engineered from eculizumab to have a substantially longer terminal half-life, permitting longer dosing intervals for paroxysmal nocturnal hemoglobinuria (PNH) treatment. Two phase 1b/2 multicenter open-label studies evaluated efficacy and safety of multiple doses and regimens of ravulizumab in PNH patients naive to complement-inhibitor treatment. Patients in study 103 (n = 13) received ravulizumab 900 mg (lower trough exposure) or 1800 mg every 4 weeks (higher trough exposure); those in study 201 (n = 26) received 1000 mg every 4, 1600 mg every 6, 2400 mg every 8, or 5400 mg every 12 weeks. Trough exposure levels with study 201 dosing regimens were similar to the study 103 900-mg every-4-weeks regimen. Rapid sustained reduction of plasma lactate dehydrogenase (LDH) occurred across all cohorts (73%-90% at end point vs baseline). A greater proportion of patients had normalized LDH (<234 U/L) at least once from days 29 to 253 in the higher- (85.7%) vs lower-trough-exposure (50.0%-83.3%) cohorts; the weighted average of the proportion of instances of LDH normalization from days 29 to 253 was highest in higher- vs lower-trough-exposure cohorts (62.3% vs 31.4%-54.5%). No patients in the higher-trough-exposure cohort, but 1 to 2 patients in all lower-trough-exposure cohorts, experienced breakthrough hemolysis. Ravulizumab improved quality of life (QoL) measures in all cohorts. Two patients experienced meningococcal infections; both recovered and continued in the study. In summary, ravulizumab provided rapid and sustained reduction in complement-mediated hemolysis and improved QoL at dosing intervals up to 12 weeks. This trial was registered at www.clinicaltrials.gov as #NCT02598583 (study 103) and NCT02605993 (study 201).

Cutting Edge: Deletion of Ezrin in B Cells of Lyn-Deficient Mice Downregulates Lupus Pathology.

Genetic deletion of the Src family tyrosine kinase Lyn in mice recapitulates human systemic lupus erythematosus, characterized by hyperactive BCR signaling, splenomegaly, autoantibody generation, and glomerulonephritis. However, the molecular regulators of autoimmunity in Lyn-deficient mice and in human lupus remain poorly characterized. In this study, we report that conditional deletion of the membrane-cytoskeleton linker protein ezrin in B cells of Lyn-deficient mice (double knockout [DKO] mice) ameliorates B cell activation and lupus pathogenesis. B cells from DKO mice respond poorly to BCR stimulation, with severe downregulation of major signaling pathways. DKO mice exhibit reduced splenomegaly as well as significantly lower levels of autoantibodies against a variety of autoantigens, including dsDNA, histone, and chromatin. Leukocyte infiltration and deposition of IgG and complement component C3 in the kidney glomeruli of DKO mice are markedly reduced. Our data demonstrate that ezrin is a novel molecular regulator of B cell-associated lupus pathology.

The rational design of affinity-attenuated OmCI for the purification of complement C5

Complement component C5 is the target of the mAb eculizumab and is the focus of a sustained drug discovery effort to prevent complement-induced inflammation in a range of autoimmune diseases. The immune evasion protein OmCI binds to and potently inactivates C5; this tight-binding interaction can be exploited to affinity-purify C5 protein from serum, offering a vastly simplified protocol compared with existing methods. However, breaking the high-affinity interaction requires conditions that risk denaturing or activating C5. We performed structure-guided in silico mutagenesis to identify prospective OmCI residues that contribute significantly to the binding affinity. We tested our predictions in vitro, using site-directed mutagenesis, and characterized mutants using a range of biophysical techniques, as well as functional assays. Our biophysical analyses suggest that the C5–OmCI interaction is complex with potential for multiple binding modes. We present single mutations that lower the affinity of OmCI for C5 and combinations of mutations that significantly decrease or entirely abrogate formation of the complex. The affinity-attenuated forms of OmCI are suitable for affinity purification and allow elution under mild conditions that are nondenaturing or activating to C5. We present the rational design, biophysical characterization, and experimental validation of affinity-reduced forms of OmCI as tool reagents to enable the affinity purification of C5.

Assessment of Mean Platelet Volume in Patients with Systemic Lupus Erythematosus.

Introduction:The Mean Platelet Volume (MPV) is a platelet activation biomarker that has been recently correlated with disease activity in SLE. We aimed to evaluate the MPV in patients with SLE comparing it with healthy individuals, to study the correlation between MPV and SLE Disease Activity Index (SLEDAI) in SLE patients and to analyze possible correlation between MPV and Erythrocyte Sedimentation Rate (ESR), C-Reactive Protein (CRP), and complement components C3 and C4.Methods:This is a cross-sectional study in which 81 patients with SLE according to the American College of Rheumatology (ACR) diagnostic classification criteria and 58 healthy controls were included. Active disease was defined as SLEDAI>0.Results:Patients with active SLE had decreased MPV when compared to inactive disease group (10.0±0.7fL vs. 10.7±1.0fL, p=0.005, respectively) and when compared to control group (10.9±1.0fL, p<0.001). Our study found a weak negative correlation between the SLEDAI and the MPV (r=-0.29, p=0.009). There was no correlation between MPV and CRP, ESR, C3 and C4. Also, no correlation between SLEDAI and CRP, ESR, C3 and C4 was found.Conclusion: MPV decreases in patients with active SLE and is inversely correlated with SLEDAI.

Complement component consumption in sepsis correlates better with hemostatic system parameters than with inflammatory biomarkers

IntroductionThe aim of this study was to investigate the role of C3 and C4 complement components in prediction of sepsis outcome. The secondary aim was to determine relationship between complement components and other inflammatory parameters, and parameters of hemostasis. MethodsOne-hundred-thirty-seven patients with sepsis (Sepsis-3 criteria) were included in the study. Routine laboratory markers, predictive APACHEII and SOFA scores, concentrations of C3 and C4, activated partial thromboplastin time (aPTT), prothrombin time (PT), thrombin time (TT), fibrinogen, antithrombin (AT), protein C (PC), protein S (PS), endogenous thrombin potential (ETP), thrombomodulin, and D-dimer were available. Concentrations of C3 and C4 were correlated with the disease outcome, predictive scores, inflammatory markers and parameters of hemostasis. Statistical analysis was performed using the non-parametric approach and significance was set at p < 0.05. ResultsA significant depletion of the complement was observed in non-survivors (AUCROCC3 = 0.692, pC3 < 0.001,AUCROCC4 = 0.672, pC4 = 0.001). There was a significant negative correlation of C3and C4with APACHEII and SOFA (C3-APACHEII ρ = −0.364, p = 0.011, C3-SOFA ρ = −0.460, p < 0.001), aPTT (ρ = −0.407, p < 0.001), PT (ρ = −0.408, p < 0.001), and D-dimer (ρ = −0.274, p = 0.001). A significant positive correlation was observed with natural anticoagulants (C3-AT ρ = 0.493, p < 0.001; C3-PC ρ = 0.450, p < 0.001; C3-PS ρ = 0.345, p < 0.001), fibrinogen (ρ = 0.481, p < 0.001),and ETP (ρ = 0.384, p < 0.001). C3 and C4 correlated significantly only with CRP (ρ = 0.207, p = 0.015), while no significant correlations with procalcitonin and WBC were detected. Results were similar for C4 and C3, although C3 presented higher correlation coefficients. ConclusionIn septic patients with poorer outcome, a significant depletion of the complement system was observed. Concentrations of complement components demonstrated stronger correlations with coagulation parameters than with inflammatory biomarkers.

Dietary Inflammatory Index and Biomarkers of Lipoprotein Metabolism, Inflammation and Glucose Homeostasis in Adults.

Accumulating evidence identifies diet and inflammation as potential mechanisms contributing to cardiometabolic risk. However, inconsistent reports regarding dietary inflammatory potential, biomarkers of cardiometabolic health and metabolic syndrome (MetS) risk exist. Our objective was to examine the relationships between a food frequency questionnaire (FFQ)-derived dietary inflammatory index (DII®), biomarkers of lipoprotein metabolism, inflammation and glucose homeostasis and MetS risk in a cross-sectional sample of 1992 adults. Energy-adjusted DII (E-DII) scores derived from an FFQ were calculated. Lipoprotein particle size and subclass concentrations were measured using nuclear magnetic resonance (NMR) spectroscopy. Serum acute-phase reactants, adipocytokines, pro-inflammatory cytokines and white blood cell (WBC) counts were determined. Insulin resistance was calculated by homeostasis model assessment (HOMA-IR). Our data indicate that a more pro-inflammatory diet, reflected by higher E-DII scores, was associated with potentially pro-atherogenic lipoprotein profiles characterised by increased numbers of large very low density lipoprotein (VLDL), small dense low density lipoprotein (LDL) and high density lipoprotein (HDL) particles and less large LDL and HDL particles (all p < 0.001). Inflammatory profiling identified a range of adverse phenotypes among those with higher E-DII scores, including higher complement component C3 (C3), C-reactive protein (CRP), (both p < 0.05), interleukin 6 (IL-6) and tumour necrosis factor (TNF)-α concentrations, higher WBC counts and neutrophil to lymphocyte ratio (NLR) and lower adiponectin levels (all p < 0.001). MetS risk was increased among those with higher E-DII scores (OR 1.37, 95% CI (1.01, 1.88), p < 0.05), after adjusting for potential confounders. In conclusion, habitual intake of a more pro-inflammatory diet is associated with unfavourable lipoprotein and inflammatory profiles and increased MetS risk.

The complement component C3 is expressed by the endometrial ectopic tissue and is involved in the endometriotic lesion formation

The complement component C3 is expressed by the endometrial ectopic tissue and is involved in the endometriotic lesion formation

Crystal structure of the membrane attack complex assembly inhibitor BGA71 from the Lyme disease agent Borrelia bavariensis

Borrelia (B.) bavariensis, B. burgdorferi, B. afzelii, B. garinii, B. spielmanii, and B. mayonii are the causative agents in Lyme disease. Lyme disease spirochetes reside in infected Ixodes ticks and are transferred to mammalian hosts during tick feeding. Once transmitted, spirochetes must overcome the first line of defense of the innate immune system either by binding complement regulators or by terminating the formation of the membrane attack complex (MAC). In B. bavariensis, the proteins BGA66 and BGA71 inhibit complement activation by interacting with the late complement components C7, C8, and C9, as well as with the formed MAC. In this study, we have determined the crystal structure of the potent MAC inhibitor BGA71 at 2.9 Ǻ resolution. The structure revealed a cysteine cross-linked homodimer. Based on the crystal structure of BGA71 and the structure-based sequence alignment with CspA from B. burgdorferi, we have proposed a potential binding site for C7 and C9, both of which are constituents of the formed MAC. Our results shed light on the molecular mechanism of immune evasion developed by the human pathogenic Borrelia species to overcome innate immunity. These results will aid in the understanding of Lyme disease pathogenesis and pave the way for the development of new strategies to prevent Lyme disease.

Insights on the relationship between complement component C4 serum concentrations and C4 gene copy numbers in a Western Australian systemic lupus erythematosus cohort

The relationship between serum concentration of complement C4 ([C4]) and C4 gene copy number (GCN) was investigated in 56 systemic lupus erythematosus (SLE) patients and 33 age and sex-matched controls in a Western Australian population. C4A and C4B gene copy numbers (C4A & B GCN) together with the presence or absence of the ≈6.4-kb human endogenous retroviral element type K (hereafter HERV-K) in intron 9 were estimated by two TaqMan™ real-time PCR (RT-PCR) assays that measured total C4 and HERV-K GCNs, respectively. There was good correlation between the two methods; however, the HERV-K GCN method showed a positive bias (≈6%) relative to the C4A & B total GCN. Despite individual variation, excellent correlation between total C4 GCN and mean [C4] per GCN was observed for both the SLE and control cohorts ( R2 = 88% and R2 = 99%, respectively). It was noted that serum [C4] was significantly lower in the SLE patients than the controls ( p = 0.006) despite there being no difference between C4A and C4B GCN in both cohorts. The data therefore confirm previous reports that the C4A genes are preferentially associated with the presence of the HERV-K insertion relative to C4B genes and does not support the hypothesis that low [C4] in SLE is explained by low C4A GCNs. There was no evidence also that the presence of the HERV-K insertion in C4 genes influenced [C4]. This study supports the view that low [C4] in SLE patients is due to consumption rather than deficient synthesis related to lower C4A & B GCN.

ETIOLOGY AND CLINICO-MORPHOLOGICAL PRESENTATION OF MEMBRANOPROLIFERATIVE GLOMERULONEPHRITIS IN RUSSIAN POPULATION

THE AIM. Analysis of etiology, clinical and morphological manifestations, of membranoproliferative glomerulonephritis (MPGN).PATIENTS AND METHODS. Retrospective analysis focused on etiology, clinical presentation, light and electron microscopy, immunoglobulins (Ig) and C3 complement component (C3) deposits was done in the cohort of MPGN cases identified in the period 2000-2017. RESULTS. Two hundred and fourteen cases of MPGN were included in the study (mean age of 44 ± 16 years). Most patients had nephrotic syndrome and significant hematuria. In 58.4% of cases, eGFR was &lt;60 mL/min/1.73 m2, and every fifth patient had CKD stages 4 or 5. The prevalence of MPGN among all biopsy-confirmed glomerulopathies was 9.3%. Idiopathic MPGN (iMPGN) was diagnosed in 30.4% of cases, while the proportion of iMPGN cases significantly reduced along the study period. Secondary MPGN (sMPGN) was identified in 69.6% of cases (autoimmune diseases – 34.1%, infectious diseases – 16.4%, monoclonal gammopathies – 9.3%, complement-mediated damage – 9.8%). Ig+C3+MPGN was mainly associated with autoimmune diseases and infections, however 26,6% of such cases remained “idiopathic”. C3-glomerulopathy or thrombotic microangiopathy were most often causes of Ig-C3+MPGN. Ig-C3-/Ig+C3-MPGN had heterogeneous etiology. CONCLUSION. MPGN is a severe variant of glomerular damage with a heterogeneous etiological structure. Targeted routine clinical and morphological diagnostics of MPGN allows identifying the cause of the disease in most cases. This approach is reliable for the adequate treatment choice and improvement of outcomes in sMPGN. Further improvement in diagnostic and classification approaches in idiopathic MPGN relies on progress in understanding of molecular etiopathogenesis of the disease.