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C3b

C3b is a key proteolytic fragment generated from the cleavage of complement component C3, the most abundant protein in the human complement system, and serves as a central hub for innate immune defense by covalently tagging pathogens and host debris for clearance while amplifying the complement cascade.^[1] With a plasma concentration of approximately 1.2 mg/mL, C3b exhibits remarkable versatility, often likened to the "Swiss army knife" of immunity due to its multifaceted roles in opsonization, inflammation, and bridging innate and adaptive responses.^[1] Its activation exposes a reactive thioester bond that enables rapid, site-specific attachment to nearby surfaces, ensuring targeted immune action within a narrow radius of about 60 nm.^[2] C3b is produced through the activation of all three complement pathways—classical, lectin, and alternative—each converging on C3 convertases that cleave C3 into the anaphylatoxin C3a and the opsonic C3b fragment.^[3] In the classical and lectin pathways, the C4b2a complex acts as the C3 convertase, while the alternative pathway relies on spontaneous C3 hydrolysis to initiate C3(H₂O)Bb formation, followed by amplification via C3bBb.^[2] This tick-over mechanism maintains low-level C3b generation constitutively, poised for rapid escalation upon pathogen detection, with the alternative pathway contributing over 80% of terminal complement activity during infections.^[2] Evolutionarily conserved for over 500 million years, C3b's structure undergoes conformational changes upon activation, exposing binding sites that facilitate its integration into higher-order complexes like C5 convertases (e.g., C3bBbC3b).^[1] The primary functions of C3b revolve around enhancing pathogen elimination and immune coordination: it acts as an opsonin by binding microbes or apoptotic cells, promoting their phagocytosis through receptors such as CR1 on macrophages and CR3 on neutrophils.^[3] Additionally, C3b drives complement amplification by recruiting factor B to form the C3 convertase C3bBb, which is stabilized up to 10-fold by properdin, leading to exponential C3b deposition and subsequent C5 cleavage for membrane attack complex (MAC) assembly and C5a-mediated inflammation.^[2] Beyond direct antimicrobial effects, C3b modulates adaptive immunity by binding CR2 on B cells, lowering the threshold for antibody production and enhancing humoral responses.^[2] It also contributes to homeostasis, including tissue regeneration and synaptic pruning in the brain.^[1] To prevent host tissue damage, C3b activity is tightly regulated by soluble and membrane-bound inhibitors; factor I cleaves C3b into inactive iC3b with cofactors like factor H, which discriminates self from non-self surfaces by binding glycosaminoglycans on host cells and accelerating convertase decay.^[2] Other regulators, including membrane cofactor protein (MCP/CD46), complement receptor 1 (CR1), and decay-accelerating factor (DAF), further limit amplification, with the C3bBb complex having a short half-life of about 90 seconds.^[2] Dysregulation of C3b, as seen in deficiencies or autoantibodies, is linked to recurrent infections, autoimmune diseases like systemic lupus erythematosus, and atypical hemolytic uremic syndrome, underscoring its critical balance in immune surveillance.^[4]

Structure and Activation

Molecular Composition

C3b is the major activation product of complement component C3, formed by limited proteolysis that cleaves the native 185 kDa glycoprotein into the larger C3b fragment (approximately 176 kDa) and the smaller C3a anaphylatoxin (9 kDa). The C3b fragment retains the intact β-chain (75 kDa) and the modified α'-chain (101 kDa), connected by disulfide bonds, enabling its role in downstream complement amplification.^[5]^[6]^[1] Structurally, C3b comprises 13 domains characteristic of the α2-macroglobulin superfamily, including eight macroglobulin domains (MG1–MG8) that form a central ring-like core, a thioester-containing domain (TED), a CUB domain, a C345C domain, and multiple linker regions that facilitate interdomain flexibility. The MG domains, primarily β-sheet rich, provide structural rigidity, while the TED harbors the reactive thioester bond essential for covalent target attachment. Linker regions, such as LNK1–LNK5, connect these elements and allow conformational rearrangements during activation.^[7]^[8]^[9] Insights into C3b's architecture come from X-ray crystallography, with the structure resolved at 4 Å resolution by Janssen et al. in 2006, revealing a compact conformation where the TED docks against the MG1 domain, partially shielding the thioester; upon activation from native C3, a major rearrangement exposes this bond for nucleophilic attack by nearby surfaces. This open form contrasts with the metastable, thioester-protected state in native C3, highlighting the dynamic transition that activates C3b's reactivity. Subsequent studies have resolved C3b at higher resolutions, such as 2.8 Å (PDB: 5FO7, 2016) and 2.0 Å in complexes (e.g., PDB: 7BAG, 2022), refining the conformational details.^[10]^[11]^[12] C3b inherits post-translational modifications from native C3, notably N-linked glycosylation at two primary sites—Asn63 in the β-chain and Asn917 in the α-chain—which attach complex oligosaccharides that enhance protein stability by preventing aggregation, promoting proper folding, and protecting against proteolysis. These glycans, comprising up to 2–3% of C3's mass, modulate solubility and conformational integrity without directly influencing the thioester reactivity.^[1]^[13]^[14]

Cleavage and Activation Process

The activation of complement component C3 to C3b is initiated by proteolytic cleavage at the Arg77-Ser78 bond within the alpha chain, mediated by C3 convertases, which releases the anaphylatoxin fragment C3a and generates the larger C3b fragment.^[15] This precise scission exposes a buried reactive thioester bond located in the thioester-containing domain (TED) of C3b, enabling its subsequent covalent attachment to target surfaces.^[1] Cleavage induces a major conformational transition in C3, shifting from a compact, closed structure to an open, extended configuration in C3b, characterized by the translocation of the TED by approximately 100 Å away from the macroglobulin domains and significant rearrangement of the MG7 and MG8 domains.^[16] These structural dynamics reposition the TED to facilitate nucleophilic attack by surface groups, while the MG7-MG8 movements stabilize the extended form and expose additional binding interfaces essential for C3b functionality.^[17] The nascent thioester in C3b exhibits extreme lability, with a half-life of approximately 60 microseconds under physiological conditions, during which it preferentially undergoes transacylation with nearby hydroxyl or amino groups on pathogen surfaces to form ester or amide bonds, respectively.^[18] If no suitable nucleophile is present within this brief window, the thioester reacts with water via non-productive hydrolysis, yielding hydrolyzed C3b that cannot covalently bind targets but can support fluid-phase complement initiation.^[1] The balance between transacylation and hydrolysis is governed by local nucleophile density and steric factors, optimizing surface-directed activation while minimizing wasteful decay.^[18]

Production in Complement Pathways

Classical and Lectin Pathways

The classical pathway of complement activation is initiated when antigen-antibody complexes, typically involving IgM or IgG, bind to the recognition molecule C1q on the surface of pathogens or immune complexes. This binding activates the C1r and C1s serine proteases within the C1 complex (C1qrs), which then cleave C4 and C2. The resulting C4b fragment covalently attaches to nearby surfaces via thioester bond formation, while C2 is cleaved into C2a and C2b; C2b associates with surface-bound C4b to form the C3 convertase C4b2b. This enzyme cleaves C3 into C3a, an anaphylatoxin, and C3b, which deposits on the target surface and facilitates further amplification. Additionally, C3b binding to C4b2b generates the C5 convertase C4b2b3b, which cleaves C5 to initiate the terminal complement pathway.^[3]^[19]^[20] The lectin pathway shares downstream components with the classical pathway but is triggered independently by pattern recognition molecules such as mannose-binding lectin (MBL) or ficolins, which bind to specific carbohydrate structures on microbial surfaces. Upon binding, these lectins associate with MBL-associated serine proteases (MASPs), particularly MASP-1 and MASP-2; MASP-2 activates to cleave C4 and C2, leading to the formation of the identical C3 convertase C4b2b that generates C3b. This process enables targeted deposition of C3b without requiring antibodies, providing an antibody-independent arm of innate immunity.^[21]^[22]^[23] Both pathways converge at C3b generation, creating a shared amplification loop where surface-bound C3b recruits factor B from plasma. Factor B binds to C3b and is cleaved by factor D to form the alternative pathway C3 convertase C3bBb, which further cleaves C3 and amplifies C3b deposition exponentially on the target. This amplification enhances the efficiency of complement activation initiated by either pathway.^[19]^[20] The classical pathway was first identified in the early 20th century through hemolytic assays that demonstrated antibody-dependent complement-mediated lysis of red blood cells, with detailed component elucidation occurring in the 1930s. The lectin pathway was discovered later, around 1990, through studies by Turner and colleagues linking MBL to complement activation.^[24]^[25]

Alternative Pathway

The alternative pathway of the complement system provides an antibody-independent mechanism for initiating and amplifying C3b deposition on target surfaces, distinguishing it from the classical and lectin pathways that rely on specific pattern recognition. This pathway begins with the spontaneous, low-level hydrolysis of native C3 in plasma, a process known as the tickover mechanism, where a thioester bond in the C3 α-chain reacts with water to form C3(H₂O), a conformationally altered form that exposes binding sites for factor B.^[26] Once bound to factor B, this complex is cleaved by factor D, generating a fluid-phase C3 convertase, C3(H₂O)Bb, which cleaves additional C3 molecules into C3a and C3b, thereby producing the initial seed of surface-bound C3b.^[27] Surface-attached C3b initiates the amplification loop by recruiting factor B to form the alternative pathway C3 convertase, C3bB, which factor D then cleaves into C3bBb; this enzyme efficiently cleaves more C3, leading to exponential deposition of additional C3b on nearby surfaces and creating a positive feedback cycle that rapidly coats pathogens.^[26] The convertase C3bBb has a short half-life in solution but is markedly stabilized on surfaces by properdin, a plasma protein that binds directly to C3bBb and extends its functional lifetime by 5- to 10-fold, thereby enhancing the efficiency of the amplification loop.^[28] Properdin preferentially stabilizes convertases on pathogen surfaces, such as those containing sulfated glycosaminoglycans or bacterial polysaccharides, while showing reduced binding to host cell surfaces protected by sialic acids and regulatory proteins, thus directing C3b amplification toward foreign targets.^[29] The alternative pathway's spontaneous initiation and self-amplifying nature represent an ancient defense strategy, predominant in lower vertebrates like fish and amphibians where it serves primarily for opsonization without reliance on adaptive immunity components.^[30] Its discovery traces back to the 1950s identification of the properdin system by Louis Pillemer and colleagues, who described antibody-independent complement activation, with further elucidation in the 1960s through studies using cobra venom factor—a C3 homolog that forms stable convertases and helped dissect the pathway's components.^[31]

Biological Functions

Opsonization

C3b plays a central role in opsonization by covalently attaching to the surfaces of pathogens, apoptotic cells, immune complexes, and host debris, marking them for recognition and uptake by phagocytic cells. Upon activation, the thioester bond within C3b is exposed, enabling rapid and stable covalent linkage primarily to hydroxyl or amino groups on microbial surfaces such as carbohydrates or proteins.^[32] This attachment creates a focal point for immune recognition, preventing pathogen evasion and facilitating targeted clearance without relying solely on non-covalent interactions.^[33] The deposited C3b interacts with specific complement receptors on phagocytes, including CR1 (CD35), CR3 (CD11b/CD18), and CR4 (CD11c/CD18), which are expressed on macrophages, neutrophils, and dendritic cells. CR1 primarily binds C3b directly, promoting the binding and engulfment of opsonized particles by bridging them to the phagocyte surface.^[34] In contrast, CR3 and CR4 exhibit preferential binding to the iC3b derivative of C3b, enhancing phagocytic efficiency through integrin-mediated adhesion and internalization.^[35] These receptor interactions trigger cytoskeletal rearrangements in phagocytes, driving the formation of pseudopods around the opsonized target for engulfment.^[36] Further processing of surface-bound C3b by factor I, in the presence of cofactors like factor H or CR1, cleaves it into iC3b, a secondary opsonin that retains the covalent attachment site but exposes additional binding motifs. iC3b demonstrates higher affinity for CR3 than intact C3b, thereby amplifying phagocytic uptake.^[37] This stepwise degradation ensures prolonged opsonization, as iC3b sustains recognition even as C3b levels diminish.^[38] Deposition of C3b and its derivatives significantly boosts phagocytosis efficiency, with in vitro studies demonstrating increases ranging from 10- to 100-fold depending on the pathogen and phagocyte type. For instance, serum-mediated C3b opsonization of bacteria has been shown to enhance uptake by up to 40-fold in macrophages.^[39] This quantitative enhancement underscores C3b's critical role in bridging innate immune recognition to effective pathogen elimination.^[40]

Effector Mechanisms for Pathogen Clearance

C3b contributes to pathogen clearance by facilitating the formation of C5 convertases in both the classical and alternative complement pathways, which initiate the assembly of the membrane attack complex (MAC) and subsequent osmotic lysis of target cells. In the classical pathway, C3b covalently binds to the C3 convertase C4b2a, forming the trimolecular C5 convertase C4b2a3b, which enhances C5 cleavage efficiency by approximately 1000-fold to produce C5a and C5b.^[2] Similarly, in the alternative pathway, C3b binds to the C3 convertase C3bBb to generate C3bBb3b, stabilized by properdin, which cleaves C5 at a lower rate but sustains activity through an extended half-life of 3–30 minutes.^[2] The resulting C5b then recruits C6, C7, C8, and multiple C9 molecules to form the pore-forming MAC (C5b-9), disrupting the pathogen's membrane integrity and causing lysis.^[2] Upstream cleavages involving C3b also generate anaphylatoxins C3a and C5a, which amplify inflammatory responses and promote immune cell recruitment to infection sites. C3a, produced during C3 cleavage by C3 convertases, induces vasodilation, increases vascular permeability, and acts as a chemoattractant for mast cells, eosinophils, and hematopoietic stem cells, thereby initiating local inflammation.^[41] C5a, liberated by C5 convertases that incorporate C3b, serves as a more potent anaphylatoxin, recruiting neutrophils, macrophages, basophils, and T/B cells while stimulating cytokine release (e.g., IL-6, TNF-α) and oxidative bursts in granulocytes to enhance antimicrobial activity.^[41] Beyond bacterial lysis, C3b coats viral particles to neutralize infectivity by preventing host cell entry, as demonstrated in studies on HIV and influenza. For HIV-1, antibody-triggered complement activation deposits C3b on virion surfaces, enhancing complement-dependent virolysis and reducing infectivity in vitro and in vivo.^[42] In influenza, C3b opsonization via the alternative and lectin pathways tags virions for immune clearance, inhibiting replication by blocking attachment and entry mechanisms.^[43] Experimental models of C3 deficiency underscore C3b's essential role in bacterial killing during sepsis. In cecal ligation and puncture-induced sepsis, C3-deficient mice exhibit a fourfold increase in blood-borne bacteria at 24 hours compared to wild-type controls, attributed to impaired opsonization and phagocytic clearance, leading to reduced survival (14% vs. 46% at day 9).^[44] Similarly, in group B streptococcal infection models, C3-deficient mice show a 50-fold lower lethal dose and defective in vitro bacterial killing by leukocytes, confirming C3b's necessity for effective innate defense.^[45]

Regulation and Homeostasis

Inhibitory Proteins

The complement system employs several inhibitory proteins to tightly regulate C3b activity, preventing unintended damage to host tissues while allowing targeted responses against pathogens. These regulators include both soluble plasma proteins and membrane-bound molecules expressed on host cells, which collectively limit C3b deposition, convertase formation, and downstream amplification on self-surfaces.^[20] Factor H (FH) is the primary soluble regulator of the alternative complement pathway, binding directly to C3b with a dissociation constant (Kd) of approximately 1-2 μM via its C-terminal complement control protein (CCP) domains 19-20. This interaction serves dual functions: FH accelerates the decay of C3b-containing convertases and acts as a cofactor for factor I-mediated cleavage of C3b into inactive iC3b. FH enhances its affinity for C3b on host cells by recognizing sialylated glycans and other polyanionic structures, thereby discriminating self from non-self surfaces and preferentially inhibiting complement activation on healthy tissues.^[46]^[47]^[48] Factor I (FI), a soluble serine protease, inactivates C3b by proteolytic cleavage but requires cofactors for activity. FI cleaves C3b into iC3b in the presence of FH in the fluid phase or membrane-bound cofactors such as membrane cofactor protein (MCP, CD46) and complement receptor 1 (CR1, CD35) on host cell surfaces. MCP and CR1 bind C3b directly, positioning it for FI-mediated degradation and thereby protecting bystander cells from opsonization and lysis.^[27]^[49] Additional membrane-bound regulators fine-tune C3b-related processes. Decay-accelerating factor (DAF, CD55) binds to C3bBb convertases with lower affinity than FH or CR1 (Kd approximately 10-fold higher than CR1), promoting their rapid dissociation to limit further C3b generation on host membranes. Protectin (CD59) does not interact directly with C3b but inhibits the terminal complement complex (MAC) assembly by blocking C9 polymerization, thereby mitigating the lytic consequences of sustained C3b-initiated activation.^[50]^[51]

Inactivation Mechanisms

The inactivation of C3b is essential for preventing excessive complement activation and maintaining immune homeostasis, primarily through proteolytic cleavage mediated by factor I in conjunction with cofactors. Factor I sequentially cleaves C3b into inactive fragments: first to iC3b by removing the C3f peptide, which eliminates the thioester domain and convertase activity of C3b; further degradation of iC3b yields C3c and C3dg, both of which retain reduced opsonic potential but lack the ability to form convertases.^[16]^[52] The C3bBb convertase in the alternative pathway exhibits intrinsic instability, with a half-life of approximately 90 seconds due to spontaneous dissociation of Bb from C3b, limiting uncontrolled amplification. This decay is accelerated by regulatory factors such as factor H, which promotes dissociation, and factor I, which facilitates subsequent cleavage of the exposed C3b.^[53]^[54]^[55] Inactivation processes differ between soluble and surface-bound C3b, with host cells protected through enhanced recruitment of factor H via sialic acid residues on their surfaces, increasing factor H's affinity for C3b by about 10-fold and thereby promoting rapid cofactor activity for factor I-mediated cleavage. This surface-specific regulation discriminates host tissues from pathogens lacking such markers, significantly shielding self-cells from complement-mediated damage.^[56]^[57] Detection of C3b activation and inactivation relies on assays targeting neoepitopes exposed during cleavage, such as enzyme-linked immunosorbent assays (ELISA) using monoclonal antibodies specific to the C3b/iC3b/C3c neoantigen, which quantify activation levels in serum or on surfaces without cross-reactivity to native C3.^[58]^[59]

Clinical and Research Implications

Associated Disorders

Dysregulation of C3b, a central component of the complement system, is implicated in various disorders arising from genetic or acquired defects that impair its production, activation, or regulation. These conditions often manifest as increased susceptibility to infections, thrombotic microangiopathies, or chronic inflammatory diseases due to uncontrolled complement amplification or deficiency in opsonization and clearance mechanisms.^[60] C3 deficiency is a rare autosomal recessive disorder caused by mutations in the C3 gene, leading to absent or reduced levels of C3 and its cleavage product C3b, which severely compromises opsonization and immune complex clearance. Affected individuals experience recurrent pyogenic infections, particularly with encapsulated bacteria such as Streptococcus pneumoniae and Haemophilus influenzae, as well as increased vulnerability to meningococcal infections by Neisseria meningitidis. This condition was first described in the 1970s, with fewer than 50 cases reported worldwide, highlighting its rarity and the critical role of C3b in host defense.^[61]^[62]^[60] C3 glomerulopathy (C3G) is a rare renal disorder (incidence ~1-2 per million) driven by alternative complement pathway dysregulation, leading to excessive C3b deposition in the glomeruli and subsequent inflammation. It includes dense deposit disease and C3 glomerulonephritis, often caused by genetic mutations in complement regulators (e.g., CFH, CFI, C3) or autoantibodies like C3 nephritic factors that stabilize C3 convertases. Patients typically present with proteinuria, hematuria, and progressive kidney dysfunction, potentially leading to end-stage renal disease; uncontrolled C3b amplification promotes glomerular damage and fibrosis.^[63] Atypical hemolytic uremic syndrome (aHUS) is frequently associated with mutations in complement factor H (CFH), a key regulator that normally inhibits C3b convertase activity in the alternative pathway, resulting in uncontrolled C3b amplification and excessive complement activation on endothelial cells. These genetic defects, present in over 50% of aHUS cases, lead to thrombotic microangiopathy predominantly affecting the kidneys, causing microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. The dysregulation promotes C3b deposition on glomerular endothelium, driving inflammation and vascular damage that can progress to end-stage renal disease if untreated.^[64]^[65]^[66] The Y402H polymorphism in the CFH gene significantly increases the risk of age-related macular degeneration (AMD), the leading cause of vision loss in the elderly, by reducing CFH's ability to bind and regulate C3b on retinal surfaces. This common variant, present in up to 50% of AMD cases in certain populations, leads to heightened C3b deposition in the retina and choroid, promoting chronic inflammation, drusen formation, and degeneration of the retinal pigment epithelium. Studies of retinal tissue from AMD patients with the Y402H polymorphism show elevated complement activation products, underscoring the role of impaired C3b control in disease pathogenesis.^[67]^[68]^[69] Paroxysmal nocturnal hemoglobinuria (PNH) arises from somatic mutations in the PIGA gene, causing a deficiency in glycosylphosphatidylinositol (GPI)-anchored proteins, including decay-accelerating factor (DAF/CD55) and membrane inhibitor of reactive lysis (CD59), which normally restrict C3b convertase formation and membrane attack complex assembly on blood cells. The absence of these regulators results in unchecked C3b opsonization and subsequent complement-mediated intravascular hemolysis of erythrocytes, leading to chronic anemia, hemoglobinuria, and thrombosis. C3b fragments are prominently deposited on PNH red blood cells, exacerbating lysis during complement activation triggered by infections or other stressors.^[70]^[71]^[72]

Therapeutic Targeting

Therapeutic strategies targeting C3b primarily focus on modulating complement activation upstream or directly at C3 to prevent excessive C3b deposition and downstream effects like opsonization and membrane attack complex (MAC) formation in complement-mediated diseases. Monoclonal antibodies represent a cornerstone of these approaches, with eculizumab, a humanized anti-C5 monoclonal antibody, indirectly limiting C3b-driven MAC assembly by blocking C5 cleavage within the C5 convertase (C3bBbC3b).^[73] Approved for paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS), eculizumab reduces intravascular hemolysis and thrombosis risk in these conditions by inhibiting terminal complement pathway progression.^[74] In contrast, pegcetacoplan, a pegylated cyclic peptide that binds C3 and its activation fragment C3b to inhibit C3 convertase activity and block C3b generation, thereby addressing both intravascular and extravascular hemolysis in PNH.^[75] The U.S. Food and Drug Administration (FDA) approved pegcetacoplan in May 2021 for adult PNH patients, including those switching from C5 inhibitors or treatment-naive, and expanded approval in July 2025 for complement 3 glomerulopathy (C3G) and immune-complex membranoproliferative glomerulonephritis (IC-MPGN) in patients aged 12 years and older, demonstrating reductions in proteinuria.^[76]^[77] Small molecule inhibitors, such as analogs of compstatin—a cyclic peptide that binds C3 and C3b to sterically hinder substrate access to C3 convertases—offer an alternative for targeted C3b modulation, particularly in ocular diseases like age-related macular degeneration (AMD).^[78] These analogs prevent C3 cleavage and reduce C3b production by interfering with convertase formation in the alternative and classical pathways, showing promise in preclinical models of AMD where intravitreal administration diminished complement-mediated retinal damage without systemic effects.^[79] Phase I clinical trials of compstatin analogs, such as POT-4 (AL-6221), have demonstrated safety and preliminary efficacy in patients with wet AMD, with reduced drusen volume and stabilized visual acuity observed after intravitreal injection.^[80] Another small molecule, iptacopan (Fabhalta), is an oral inhibitor of factor B that prevents formation of the C3bBb convertase in the alternative pathway, thereby limiting C3b generation. The FDA approved iptacopan in March 2025 for adults with C3G to reduce proteinuria, based on phase III data showing slowed kidney function decline.^[81] Gene therapy approaches are emerging for conditions involving C3b dysregulation due to genetic defects, such as factor H mutations in aHUS, with CRISPR-Cas9-based editing in preclinical stages as of 2024.^[82] These strategies aim to correct complement factor H (CFH) mutations that impair C3b regulation, using adeno-associated virus (AAV) vectors or CRISPR to restore functional CFH expression in hepatocytes, thereby enhancing C3b inactivation and preventing renal endothelial damage in preclinical models.^[83] For instance, AAV-mediated CFH augmentation has shown reversal of complement deposition in factor H-deficient mice, supporting translation to human aHUS therapies.^[82] Clinical trials of C3 inhibitors have substantiated their efficacy in reducing hemolysis in PNH, with phase III data from the PEGASUS trial demonstrating pegcetacoplan's superiority over eculizumab, achieving hemoglobin stabilization in 85% of patients and normalizing lactate dehydrogenase (LDH) levels—a marker of hemolysis—in 71% versus 15% with eculizumab.^[84] In the PRINCE trial, pegcetacoplan reduced LDH by approximately 79% from baseline in treatment-naive PNH patients compared to a 17% increase with placebo, alongside 62.8% achieving hemoglobin levels ≥12 g/dL without transfusions.^[85] These results highlight C3b-targeted therapies' ability to provide broader hemolysis control than distal inhibitors, with sustained benefits observed up to three years in follow-up studies.^[86]

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Opsonization with C3b or C4b - Reactome Pathway Database
Opsonization by C3b or C4b leads to engagement of complement receptors (CR1, CR3 or Cr4) on acceptor phagocytic cells, thus targeting foreign particles for ...<|separator|>
[34]
Complement Receptor 3 Forms a Compact High-Affinity Complex ...
Jun 15, 2021 · The data suggest that the iC3b-CR3 interaction is of high affinity and relies on minor contacts formed between CR3 and regions outside the iC3b thioester ...
[35]
Complement receptor 3 forms a compact high affinity complex with ...
Dec 14, 2020 · We now show that the CR3-iC3b interaction is characterized by a 17-fold higher affinity than the minimal complex between the iC3b TE domain and ...
[36]
Macrophage Complement Receptor 3-mediates Phagocytosis and ...
In contrast, in the presence of serum, the uptake was significantly increased ∼40 fold confirming that serum opsonization is critical for efficient phagocytosis ...
[37]
Cleavage of Complement C3b to iC3b on the Surface of ... - PMC - NIH
We have also shown that complement-mediated opsonization greatly increases the phagocytosis efficiency of a capsule polysaccharide type 5 S. aureus isolate ...
[38]
The Role of the Anaphylatoxins in Health and Disease - PMC
The anaphylatoxins (AT) C3a, C5a and C5a-desArg are generally considered pro-inflammatory polypeptides generated after proteolytic cleavage of C3 and C5 in ...
[39]
The good and evil of complement activation in HIV-1 infection - PMC
The complement activation is further evidenced by the finding that C3 accumulates on the surface of HIV-1 virions from infected individuals. Also, HIV-1- ...
[40]
Recent Advances in Influenza, HIV and SARS-CoV-2 Infection ...
Activation of the complement system can result in the lysis of pathogens or the covering (opsonization) of viruses with complement fragments (mainly C3b and C4b) ...
[41]
Functions of the complement components C3 and C5 during sepsis
C3-deficient mice seem to struggle with a lethal combination of impaired bacterial clearance because of reduced phagocytotic activity, impaired oxidative ...Missing: killing | Show results with:killing
[42]
https://pmc.ncbi.nlm.nih.gov/articles/PMC4002684/
[43]
Characterization of Binding Properties of Individual Functional Sites ...
Measurement of the Kd of C3b for Different Factor H Sites. Ten different recombinant Factor H protein fragments were analyzed by SPR for their affinity for C3b.
[44]
Characterization of Binding Properties of Individual Functional Sites ...
Factor H regulates complement activation through its interactions with C3b and polyanions. Three binding sites for C3b and multiple polyanion binding sites ...<|control11|><|separator|>
[45]
Self‐nonself discrimination by the complement system - Meri - 2016
Jul 9, 2016 · Factor H recognized sialic acids and other polyanions, like heparan sulfate on surfaces, where prior deposition of C3b had occurred. Furthermore ...
[46]
Analysis of Binding Sites on Complement Factor I That Are Required ...
The central complement inhibitor factor I (FI) degrades activated complement factors C4b and C3b in the presence of cofactors such as C4b-binding protein, ...
[47]
Differences between the binding sites of the complement ... - PubMed
Thus, C3b binds to DAF with a 10-fold lower affinity than to CR1 and a 100-fold lower affinity than to Factor H. The appKa of C3b,Bb for the three proteins were ...
[48]
Deficiency of decay-accelerating factor and complement receptor 1 ...
Aug 15, 2008 · MCP acts as a cofactor for factor I–mediated degradation of surface-bound C3b and C4b,4 and CD59 works by inhibiting the assembly of the ...
[49]
Regulator-dependent mechanisms of C3b processing by factor ... - NIH
Jul 3, 2017 · We present a crystal structure of C3b in complex with FI and regulator factor H (FH; domains1–4 with 19–20). FI binds C3b–FH between FH domains 2 and 3.
[50]
Factor B as a therapeutic target for the treatment of complement ...
Both the initial AP fluid-phase [C3(H2O)Bb] and the C3bBb C3 convertases alone have very short half-lives (~90 seconds for C3bBb) and rapidly dissociate (3).Missing: instability | Show results with:instability
[51]
Low‐molecular weight inhibitors of the alternative complement ...
Oct 11, 2022 · C3bBb also has a short half-life of 90 seconds, and binding to properdin stabilizes the complex by a factor of ten. C3b formation can either ...Missing: instability | Show results with:instability
[52]
Insights into the Effects of Complement Factor H on the Assembly ...
Several studies have documented that Ni2+ cation prolongs the half-life of both C3bB and C3bBb (27, 32, 37), and for this reason Ni2+ has been widely used, ...
[53]
Hijacking Factor H for Complement Immune Evasion - PMC
Feb 25, 2021 · FH has a 10-fold increase in affinity towards C3b in the presence of host sialic acid (38, 39, 41). In addition to recognizing host cell ...
[54]
Complement control protein factor H: the good, the bad, and ... - PMC
The disease-protective complement factor H allotypic variant Ile62 shows increased binding affinity for C3b and enhanced cofactor activity. Hum. Mol. Genet ...
[55]
Quantification in enzyme-linked immunosorbent assay of a C3 ...
The monoclonal antibody MoAb bH6, which is specific for a C3 neoepitope expressed on C3b, iC3b, and C3c, was used as capture antibody. Detection antibody was a ...
[56]
Quantitation of components of the alternative pathway of ...
Sensitive enzyme-linked immunosorbent assays (ELISA) using monoclonal antibodies have been developed to specifically detect components of the alternative ...
[57]
Complement Deficiency - StatPearls - NCBI Bookshelf
[7] A deficiency of C3 is associated with severe recurrent pyogenic infections early in life. ... Recurrent Neisseria infection indicates possible late complement ...
[58]
Case Report: C3 deficiency in two siblings - PMC - NIH
Jul 24, 2024 · Primary total C3 deficiency is an extremely rare autosomal-recessive disease, with fewer than 50 cases reported worldwide since the first C3- ...<|separator|>
[59]
Clinical characteristics and genotype-phenotype correlations in C3 ...
Oct 2, 2015 · C3 deficiency is a rare autosomal-recessive inherited disorder that is characterized by susceptibility to recurrent bacterial infections, ...
[60]
aHUS caused by complement dysregulation: new therapies on the ...
Atypical hemolytic uremic syndrome (aHUS) is a heterogeneous disease that is caused by defective complement regulation in over 50% of cases.
[61]
Atypical hemolytic uremic syndrome associated with a factor B ...
Several studies have clarified that aHUS is caused by dysregulation of the alternative pathway of complement, resulting in excessive deposition of C3b and of ...
[62]
Complement factor H and the hemolytic uremic syndrome - PMC - NIH
Mutations in FI, the protease required for C3b cofactor activity, also predispose to aHUS. Heterozygous changes in FI account for ∼5% of the known mutations in ...
[63]
Human complement factor H Y402H polymorphism causes an age ...
Feb 11, 2019 · The complement factor H (CFH) Y402H polymorphism (rs1061170) imparts the strongest risk for age-related macular degeneration (AMD), the leading ...
[64]
Complement factor H regulates retinal development and its absence ...
Jan 31, 2019 · FH regulates inflammation. Further, there is a change of FH function that leads to an increase in C3b retinal deposition. Hence, the retina, ...
[65]
Complement factor H and age-related macular degeneration
Importantly, a common polymorphism in CFH (Y402H) has been strongly associated with an increased risk of AMD. This polymorphism, which causes a tyrosine to ...
[66]
Paroxysmal Nocturnal Hemoglobinuria: A Complement ... - PMC - NIH
PNH erythrocytes are highly vulnerable to complement-mediated lysis owing to a reduction, or absence, of 2 important GPI-anchored complement regulatory membrane ...
[67]
Complement in paroxysmal nocturnal hemoglobinuria - NIH
(A) Complement activation is effectively contained on the surface of normal erythrocytes that express the GPI-linked regulators CD55 (DAF) and CD59. Opsonic C3b ...
[68]
The pathophysiology of paroxysmal nocturnal hemoglobinuria
The GPI-anchored complement regulatory protein CD55 restricts the formation and stability of both the C3 and the C5 amplification convertases, whereas CD59 ...
[69]
Complement therapy in atypical haemolytic uraemic syndrome (aHUS)
Eculizumab binds C5 and prevents its entry into the C5 convertase (C3bBbC3b), thus precluding cleavage into the effector molecules, C5a and C5b and ultimately ...
[70]
Advances and challenges in the management of complement ...
The three pathways of complement lead to C3 activation, followed by the formation of the anaphylatoxin C5a and the terminal membrane attack complex (MAC) in ...
[71]
[PDF] 215014Orig1s000 - accessdata.fda.gov
May 14, 2021 · Pegcetacoplan binds to complement protein C3 and its activation fragment C3b ... approved by the FDA for the treatment of patients with PNH.<|separator|>
[72]
Apellis Announces U.S. Food and Drug Administration (FDA ...
May 14, 2021 · EMPAVELI, the first targeted C3 therapy, is approved for use in adults with PNH who are: Treatment naïve; Switching from C5 inhibitor ...Missing: anti- | Show results with:anti-
[73]
Compstatin: a C3-targeted complement inhibitor reaching its prime ...
Compstatin was discovered as a cyclic peptide that inhibits complement activation by binding C3 and interfering with convertase formation and C3 cleavage.
[74]
New analogs of the clinical complement inhibitor compstatin with ...
Notably, the intravitreal use of compstatin analogs has shown promising results in the treatment of age-related macular degeneration (AMD), both in non-human ...
[75]
Compstatins: The dawn of clinical C3-targeted complement inhibition
This review delineates the evolution of the compstatin family from its academic origins to the clinic, and highlights current and potential future applications.
[76]
The future of complement therapeutics - Open Exploration Publishing
Currently, there are 13 active clinical studies with iptacopan on ClinicalTrials.gov in multiple complement mediated diseases such as PNH, C3G, aHUS, immune ...
[77]
https://investors.apellis.com/news-releases/news-release-details/fda-approves-apellis-empavelir-pegcetacoplan-first-c3g-and
[78]
Pegcetacoplan versus Eculizumab in Paroxysmal Nocturnal ...
Mar 17, 2021 · Pegcetacoplan was superior to eculizumab in improving hemoglobin and clinical and hematologic outcomes in patients with PNH by providing broad hemolysis ...
[79]
Apellis and Sobi Report Positive Top-line Results from the Phase 3 ...
May 25, 2021 · EMPAVELI demonstrated statistical superiority on the co-primary endpoints of hemoglobin stabilization (p<0.0001) and reduction in lactate ...Missing: III | Show results with:III
[80]
Efficacy and Safety Maintained up to 3 Years in Adults with ...
Jul 28, 2025 · Pegcetacoplan efficacy and safety was sustained for up to 3 years in patients with PNH, with or without prior C5 inhibitor treatment, verifying ...