RECOGNITION OF THE MAJOR CELL-SURFACE GLYCOCONJUGATES OF LEISHMANIA PARASITES BY THE HUMAN SERUM MANNAN-BINDING PROTEIN

Activation of complement on the surface of parasitic protozoa of the g enus Leishmania appears to be important for parasite infectivity in th e mammalian host, as it allows these parasites to attach to an invade macrophages via their surface complement receptors. Serum mannan-bindi ng protein (MBP) is a known activator of complement. Therefore, in the present study, we have investigated whether serum MBP binds to live L eishmania parasites, and to mannose-containing saccharides derived fro m the parasite cell surface. We have observed by fluorescence microsco py that biotinylated MBP binds to the surface of L. major and L. Mexic ana promastigotes. At this developmental stage the parasites are coate d by a mannose-containing lipophosphoglycan (LPG). We have observed th at radioiodinated MBP binds in a mannose-inhibitable manner to purifie d LPG which has been immobilized in plastic microwells, as well as to purified mannose-terminating ditri- and tetrasaccharide fragments ('ca p' structures) which have been released by mild acid hydrolysis from t he outer chains of the LPG, converted into neoglycolipids and resolved by thin-layer chromatography. I-125-MBP also binds in the chromatogra m-binding assay to the mannose-containing glycoinositol-phospholipids that are expressed in high copy number on both the promastigote and th e intracellular amastigote stages of most Leishmania species. These da ta suggest that MBP has the potential to opsonize the major developmen tal stages of Leishmania parasites, and provide a possible mechanism f or the antibody-independent activation of complement on their surface.