Calcium‐independent haemolysis via the lectin pathway of complement activation in the guinea‐pig and other species*

We previously reported that complement‐dependent haemolysis of sheep erythrocytes (E) coated with mannan (M) and sensitized with human mannan‐binding lectin (MBL) via the lectin pathway in man occurs in Mg‐EGTA and requires alternative pathway amplification. Calcium was required for MBL binding to E‐M, but once the E‐M‐MBL intermediate was formed, MBL was retained and haemolysis occurred in the absence of calcium. Comparable or greater lectin pathway haemolysis in the absence of calcium was observed upon incubation of E‐M‐MBL in guinea‐pig, rat, dog and pig sera, and was further investigated in the guinea‐pig, in which titres were much higher (~14‐fold) than in man, and in contrast to humans, greater than classical pathway haemolytic activity. As in human serum, no lysis was observed in C4‐ or C2‐deficient guinea‐pig serum until purified C4 or C2, respectively, were restored. However, lectin pathway haemolytic activity in the guinea‐pig did not require the alternative pathway. Removal (>98%) of factor D activity by three sequential passages through Sephadex G‐75, resulting in serum which retained a normal classical pathway but no alternative pathway haemolytic activity, did not reduce the ability of guinea‐pig serum to mediate haemolysis via the lectin pathway. Further, the C3‐convertase formed via the lectin pathway (E‐M‐MBL‐C4,2) lysed in C2‐deficient guinea‐pig but not human serum chelated with EDTA, a condition which precludes alternative pathway amplification. Thus, lectin pathway haemolysis occurs efficiently in guinea‐pig serum, in the absence of calcium and without requirement for alternative pathway amplification. The guinea‐pig provides a model for studying the assembly and haemolytic function of a lectin pathway which contrasts with the lectin pathway of man, and allows for comparisons that may help clarify the role of this pathway in complement biology.