Mannan-binding lectin and C1q bind to distinct structures and exert differential effects on macrophages

While the interaction of complement component C1q with cellular proteins is extensively studied, much less is known about the binding of the structura lly related molecule, mannan-binding lectin (MBL) to various cells. Here we show by cytofluorimetry that the interaction of MBL with immunocompetent c ells is much more restricted than that of C1q. It is shown that under condi tions of physiological ionic strength MBL binds to human monocyte-derived m acrophages (M Phi) and monocytoid cell lines, but not to T and B lymphocyte s, in contrast to C1q, which interacts with all these cells under the same conditions. As opposed to the binding of C1q, low ionic strength does not i mprove the interaction of MBL with M Phi. No competition for cellular bindi ng sites was found when MBL and C1q were added simultaneously to the cells. Studying the functional consequences of the interaction, we found that the release of TNF-alpha from M Phi is induced by C1q but not by MBL. Producti on of complement C3 by M Phi is stimulated by C1q strongly, while the effec t of MBL is much weaker. C3 produced upon C1q-mediated triggering is shown to opsonize RBC, resulting in enhanced phagocytosis. These results suggest that cell membrane molecules binding MBL and C1q are not identical; moreove r, biological functions exerted by these proteins are also markedly differe nt.