The mechanism of carbohydrate-mediated complement activation by the serum mannan-binding protein.

M Ohta,M Okada,I Yamashina,T Kawasaki

doi:10.1016/s0021-9258(19)39928-4

Abstract

Serum mannan-binding protein (S-MBP), a lectin specific for mannose and N-acetylglucosamine, was documented to activate complement through the classical pathway. In this study, we examined the mechanism that initiates this activation. By a passive hemolysis test using sheep erythrocytes coated with yeast mannan, the activation of complement by human S-MBP was shown to proceed in the absence of C1q. The following binding studies using 125I-labeled C1r2s2 and C1s indicated that the activated form of C1r2s2 bound to S-MBP located on the surface of the cells with high affinity. The binding of C1s to the cell-bound S-MBP require the presence of C1r, suggesting that C1r2s2 binds to S-MBP through C1r. The activation of C1s from a proenzyme to a protease was mediated by cell-bound S-MBP in the presence of C1r and the activated protease remained associated with the cells and was not released into the medium. The activation of complement with S-MBP was a solid phase event and did not proceed in a fluid phase. On the basis of these results, it was concluded that S-MBP is responsible for the initiation of carbohydrate-mediated complement activation as C1q does in immune complex-mediated complement activation.

Highlights

From The Department of Biological Chemistry, Faculty of Pharmaceutical $Ehime College of Health Science, Iyo-gun, Ehime 791-21, Japan
We report that Serum mannan-binding protein (S-MBP) triggers the carbohydrate-mediated complement activation through the activation of Cls located on the cell surface bound S-MBP molecules, i.e. the function of S-MBP is remarkably similar to that of Clq in the immune complex-mediated complement activation
We demonstrated that complement activation occurs in heterologous combinations of S-MBP and complement as described before [9] and in the autologous combination: human S-MBP and human complement

Summary

Introduction

Sensitized with or without MBP was first incubated with 1 pg of Clq at 0 “C for 20 min and the cells recovered by centrifugation were treated with activated ‘251-ClrZs2 as above. (1 x 10s cells) coated with 10 pg of mannan were sensitized with 5 (m), 50 (0), 100 (A), or 500 (0) ng of human S-MBP and lysed with various amounts of human complement in a total volume of 1.5 ml during a l-h incubation at 37 “C.

Results

Conclusion