Abstract
Deficiency of both mannan-binding lectin (MBL) and complement components C4 and C2 has been associated with increased risk of systemic lupus erythematosus (SLE). MBL can activate the complement system either through C4 and C2 or directly through C3. Circulating immune complexes (CICs) are believed to play a pathogenic role in SLE and MBL has been shown to bind certain forms of immunoglobulins, including IgM, IgG and IgA. Thus, MBL might promote CIC clearance. In order to evaluate this, six individuals with non-functional classical pathway due to the rare homozygous C2 deficiency were chosen, as the classical pathway is known to have a fundamental role in CIC clearance. Four of the six C2-deficient individuals had SLE, two of whom also had MBL deficiency. MBL serum levels and genotypes were compared with the serum levels of CICs, as measured by their content of kappa, lambda, IgM, IgA, IgG and C3 opsonization. The C2-deficient individuals had higher serum levels of CICs than 16 healthy controls (P < 0.0001). Furthermore, an inverse association was observed between MBL and CIC levels in the C2-deficient individuals, which was strongest for IgM-CICs (r = - 0.84, P = 0.037). Moreover, C3 opsonization of the CICs correlated positively with MBL levels in the C2-deficient individuals (r = 0.89, P = 0.017). In conclusion, individuals with C2 deficiency have increased levels of CICs and MBL may facilitate their clearance. Defective CIC clearance might partly explain the increased risk of SLE associated with low MBL.