Complement C3: The Most Important Protein in the Complement System 

• Gene Information: The human C3 gene is located on chromosome 19p13.3. It encodes complement component 3, the central effector protein of the complement cascade, belonging to the α2-macroglobulin superfamily of thioester-containing proteins. 
• Protein Expression: C3 is predominantly synthesized and secreted by hepatocytes in the liver into peripheral blood. Smaller amounts are locally produced by renal tubular epithelial cells, macrophages and endothelial cells. 
• Signaling Pathway: C3 serves as the universal convergence point for the classical, lectin and alternative complement pathways. All three cascades assemble C3 convertase to cleave intact C3 into two bioactive fragments: anaphylatoxin C3a and opsonin C3b. C3b further participates in C5 convertase formation to initiate terminal complement activation and MAC assembly, driving inflammatory response and tissue injury. 
• Therapeutic Inhibition: These therapeutic candidates represent diverse strategies: small peptide inhibitors block the enzymatic cleavage of C3; siRNA molecules silence C3 expression; and antibodies directly neutralize C3 function. Their development underscores a multifaceted approach to curb pathological complement activation.

• Gene Information: CFB gene encodes complement factor B (CFB), a 93 kDa serine protease zymogen belonging to the trypsin-like serine protease family. CFB is an essential component of the alternative complement pathway, where it participates in the formation and stabilization of the C3 convertase complex. 
• Protein Expression: CFB is primarily synthesized and secreted by hepatocytes in the liver and circulates in plasma as an inactive precursor. 
• Signaling Pathway: Upon activation of the alternative complement pathway, complement factor B binds to C3b and is subsequently cleaved by complement factor D into two fragments: the non-catalytic fragment Ba and the catalytic serine protease subunit Bb, forming the active alternative pathway C3 convertase (C3bBb). 
• Therapeutic Inhibition: CFB inhibition represents a promising therapeutic strategy for complement-mediated diseases by selectively suppressing alternative pathway activation. Potential applications include autoimmune diseases (lupus and rheumatoid arthritis), ophthalmic disorders such as age-related macular degeneration (AMD), and renal diseases including atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy (C3G), where uncontrolled complement activation drives inflammation, tissue injury, and organ dysfunction.

C3*R102G 

• The genome sequence of human C3 gene with R102G mutation encodes the whole molecule (ATG to STOP codon), including 5’UTR and 3’UTR, were inserted into the Hipp11 (H11) locus . 
• The human C3 expression is driven by the human C3 promoter. 
• R102G was an SNP named C3F. The C3F variant is associated with several diseases, including IgA nephropathy, C3G, and age-related macular degeneration.

 CFB 

• The exons 1~18 of the mouse Cfb gene that encoded the full-length protein were replaced by human CFB exons 1~18 in B-hCFB plus/hC3*R102G mice.
•  The promoter, 5’UTR and 3’UTR region of the mouse gene are replaced by human counterparts. 
• The human CFB expression is driven by human CFB promoter, while mouse Cfb gene transcription and translation will be disrupted.

Note: B-hCFB plus/hC3*R102G mice were obtained by mating B-hC3*R102G mice (113286) and B-hCFB mice plus (113153). Mouse C3 gene was not knockout.

Strain specific C3 expression analysis in wild-type C57BL/6JNifdc mice and homozygous humanized B-hCFB plus/hC3*R102G mice by ELISA. Serum was collected from wild-type C57BL/6JNifdc mice (+/+) (male, n=3, 9-week-old) and homozygous B-hCFB plus/hC3*R102G mice (H/H; H/H) (female and male, n=3, 9-week-old). Expression level of mouse C3 and human C3 were analyzed by ELISA (mouse C3 ELISA kit: Abcam, ab157711; human C3 ELISA kit: Abcam, ab108823). Mouse C3 was only detectable in wild-type C57BL/6JNifdc mice. Human C3 was exclusively detectable in homozygous B-hCFB plus/hC3*R102G mice. Values are expressed as mean ± SEM.

Strain specific CFB expression analysis in wild-type C57BL/6JNifdc mice and homozygous humanized B-hCFB plus/hC3*R102G mice by ELISA. Serum was collected from wild-type C57BL/6JNifdc mice (+/+) (male, n=3, 9-week-old) and homozygous B-hCFB plus/hC3*R102G mice (H/H; H/H) (female and male, n=3, 9-week-old). Expression level of human CFB were analyzed by ELISA (human CFB ELISA kit: Abcam, ab137973). Human CFB was exclusively detectable in homozygous B-hCFB plus/hC3*R102G mice. Values are expressed as mean ± SEM.