The down-stream effects of mannan-induced lectin complement pathway activation depend quantitatively on alternative pathway amplification

Complement activation plays an important role in human pathophysiology. The effect of classical pathway activation is largely dependent on alternative pathway (AP) amplification, whereas the role of AP for the down-stream effect of mannan-induced lectin pathway (LP) activation is poorly understood. In normal human serum specific activation of LIP was obtained after exposure to a wide concentration range of mannan on the solid phase. Reaction mechanisms in this system were delineated in inhibition experiments with monoclonal antibodies. Direct mannose-binding lectin (MBL) independent activation of AP was not observed even at high mannan concentrations since addition of the inhibiting anti-MBL mAb 3178 completely abolished generation of the terminal C5b-9 complex (TCC). However, selective blockade of AP by anti-factor D inhibited more than 80% of TCC release into the fluid phase after LIP activation showing that AP amplification is quantitatively responsible for the final effect of initial specific LIP activation. TCC generation on the solid phase was distinctly but less inhibited by anti-fD. C2 bypass of the LP pathway could be demonstrated, and AP amplification was also essential during C2 bypass in LP as shown by complete inhibition of TCC generation in C2-deficient serum by anti-fD and anti-properdin antibodies. In conclusion, the down-stream effect of LIP activation depends strongly on AP amplification in normal human serum and in the C2 bypass pathway. (C) 2009 Elsevier Ltd. All rights reserved.