Disease-linked mutations in factor H reveal pivotal role of cofactor activity in self surface-selective regulation of complement activation

Spontaneous activation enables the complement system to respond very rapidly to diverse threats. Activation is efficiently suppressed by complement factor H (CFH) on self surfaces but not foreign surfaces. The surface selectivity of CFH, a soluble protein containing 20 CCP modules (CCPs 1-20), may be compromised by disease-linked mutations. Which of the several functions of CFH drives its self-surface selectivity remains unknown. We made human CFH mutants in Pichia pastoris. In benchmark studies of CCP-1 variants, recombinant I62-CFH (protective against age-related macular degeneration) and V62-CFH functioned equivalently, matching or outperforming plasma-derived CFH while R53H-CFH, linked to atypical haemolytic uraemic syndrome (aHUS), was defective in C3bBb decay-accelerating activity (DAA) and factor I-cofactor activity (CA). The aHUS-linked CCP-19 mutant D1119G-CFH had virtually no CA on (self-like) sheep erythrocytes (ES) but retained measurable DAA. The aHUS-linked CCP-20 mutant S1191L/V1197A-CFH (LA-CFH) had dramatically reduced CA on ES but was less compromised with respect to DAA. D1119G-CFH and LA-CFH performed poorly at preventing complement-mediated haemolysis of ES. PspCN, a CFH-binding Streptococcus pneumoniae protein domain, increases accessibility of CCPs 19-20. It did not improve the DAA of any CFH variant on ES although it enhanced DAA in surface plasmon-resonance studies. Conversely, PspCN boosted the CA on ES of I62-CFH, R53H-FH and LA-CFH. PspCN also enhanced haemolysis protection by I62-CFH and LA-CFH. We concluded that CCPs 19-20 are critical for efficient CA on self-surfaces but less important for DAA. By exposing CCPs 19-20 and enhancing CA on self-surfaces, PspCN might therapeutically reverse deficiencies in some CFH variants.