CFB

CFB rs1270942 — Alternative Complement Pathway and Lupus Risk

Complement Factor B (CFB) is the central amplification enzyme of the alternative complement pathway | One of three complement activation pathways; the alternative pathway provides continuous low-level surveillance and amplifies the classical and lectin pathways, the arm of the innate immune system responsible for continuous immune surveillance and the amplification of inflammation. The rs1270942 variant sits within an intron of the CFB gene in the major histocompatibility complex (MHC) class III region on chromosome 6 and was identified in the landmark 2008 SLEGEN genome-wide association study | Harley et al. Nature Genetics 2008 — 720 SLE cases, 2,337 controls, >317,000 SNPs as one of the strongest non-HLA signals for systemic lupus erythematosus (SLE), with an odds ratio of 2.35 and p=1.3×10⁻⁵¹. This association has since been replicated in larger multi-ancestry meta-analyses with p-values reaching 2×10⁻¹⁶⁵.

The Mechanism

CFB encodes the complement factor B protein, which binds to activated C3b on foreign or damaged surfaces to form the alternative pathway C3 convertase (C3bBb). This convertase amplifies complement deposition on target surfaces, tags them with opsonins, activates inflammatory signaling, and triggers the terminal membrane attack complex. In healthy individuals, the alternative pathway operates at low constitutive levels to clear cellular debris, apoptotic cells, and immune complexes — functions essential for preventing autoimmunity.

The rs1270942 G allele is in tight linkage disequilibrium | LD means nearby variants are inherited together; rs1270942 is a tag SNP for the broader CFB haplotype with functional variants across the CFB locus that alter complement activity. The MHC class III haplotype bearing the G allele is associated with dysregulated alternative pathway amplification | Impaired regulation allows C3b to accumulate on self-surfaces and within glomerular immune complexes, reducing the precision of immune complex clearance and promoting chronic inflammatory responses. When immune complexes containing nuclear antigens (anti-dsDNA, anti-Smith antibodies) accumulate in glomeruli, the alternative pathway amplification loop intensifies local complement deposition, leading to glomerular inflammation and lupus nephritis.

The Evidence

The association between rs1270942 and SLE is among the strongest in the MHC class III region outside of classical HLA alleles. The original SLEGEN GWAS | Harley JB et al. 2008 — this was the first large-scale GWAS specifically in women with SLE found per-allele OR=2.35 (p=1.3×10⁻⁵¹) for SLE risk in European ancestry women, with the G allele present at ~13% frequency in European controls rising to ~25% in SLE cases. Subsequent meta-analyses across multi-ancestry cohorts showed p-values approaching 2×10⁻¹⁶⁵, making this one of the most replicated non-HLA associations in lupus genetics.

Functional evidence for CFB's causal role in lupus nephritis is compelling. CFB-deficient MRL/lpr mice | Watanabe et al. J Immunol 2000 develop significantly less proteinuria, less glomerular IgG deposition, and lower renal damage scores compared to CFB-sufficient lupus mice, establishing that alternative pathway amplification through factor B drives kidney damage. Mechanistically, review of complement pathway involvement in LN | Satyam et al. Transl Res 2022 shows intra-renal upregulation of CFB and complement factor D during renal flares, and elevated alternative pathway biomarkers in renal tissue predict worse renal outcomes | Activation at biopsy associated with faster progression to end-stage renal disease independent of classical pathway markers. Furthermore, antisense oligonucleotides that suppress factor B production | Grossman et al. 2016 — significant reduction in CFB levels achieved in mice, with dose-dependent improvement in renal scores substantially improve renal pathology in lupus nephritis models, directly validating factor B as a therapeutic target and mechanistic driver.

The G allele shows marked ancestral frequency variation: ~13% in Europeans, dropping to ~5% in Africans and <0.1% in East Asians, mirroring the pattern of SLE susceptibility across these groups. In European women of reproductive age — the demographic at highest absolute SLE risk — the G allele translates into approximately 2-fold increased population risk for SLE and a particularly elevated risk for lupus nephritis when SLE develops.

Practical Implications

Carriers of the G allele — especially GG homozygotes — face substantially elevated SLE risk. SLE predominantly affects women aged 15–45 (female-to-male ratio ~9:1), and the genetics of the MHC class III region including CFB predominantly contribute to this susceptibility. If you carry this variant and have personal or family history of SLE, early rheumatologic evaluation and kidney function monitoring are warranted.

For those with established SLE, the rs1270942 G allele is a marker of alternative pathway-mediated disease — these patients are candidates for complement monitoring (serum C3, C4, alternative pathway activation products like Bb) and may benefit from emerging complement-targeted therapies. The alternative pathway's role in lupus nephritis means that standard immunosuppressants may incompletely suppress renal damage driven by this pathway; CFB inhibitors are under active clinical investigation.

Hydroxychloroquine, the backbone of SLE management, does not directly inhibit complement but reduces flare frequency and may limit the triggers that activate complement cascades. Patients with the G allele who develop lupus nephritis should discuss aggressive management with a nephrologist, including regular urinalysis, 24-hour urine protein quantification, and renal biopsy timing decisions.

Interactions

The rs1270942 G allele operates within the broader SLE genetic architecture centered on the MHC region, where multiple complement genes (C2, C4A, C4B, CFB) form an ancestral MHC haplotype that dramatically increases lupus susceptibility. The extended MHC class III haplotype harboring the CFB G allele is in partial LD with HLA-DR3/DQ2 alleles, making disentanglement of independent effects complex.

For interactions with rs7574865 (STAT4) and rs10516487 (BANK1) — genes in the JAK-STAT and B-cell signaling pathways — the biological pathways are distinct from complement but converge on SLE pathogenesis. Epidemiological data show that individuals carrying risk alleles at multiple SLE loci have additive or supra-additive risk, consistent with the polygenic architecture of SLE. The combination of complement pathway dysregulation (CFB) with T-cell signaling variants (STAT4) or B-cell activation variants (BANK1) may particularly predispose to severe nephritis.

The C3 variant rs2230199 (C3 R102G) is also in this complement pathway family, affecting the downstream amplification substrate. When both CFB and C3 carry risk alleles, the alternative pathway amplification loop may be further dysregulated, though direct interaction data in SLE are limited.