The structure and function of the surface lipophosphoglycan on different developmental stages of Leishmania promastigotes.

The differentiation of Leishmania promastigotes from a noninfective procyclic stage to an infective metacyclic stage during growth within the midgut of their sand fly vectors or within axenic culture is accompanied by structural modifications of the surface lipophosphoglycan (LPG). The modifications are of two sorts: (a) a two- to three fold increase in size due to an increase in the number of phosphorylated saccharide units expressed and (b) a change in the composition of the terminal sugars of some of these units. The elongation of LPG on metacyclics promotes complement activation and C3 deposition in a nonlethal manner, thus opsonizing the promastigotes for attachment and uptake via macrophage receptors appropriate for subsequent intracellular survival. The down-regulation of terminally exposed galactose residues on metacyclic LPG appears to permit the selective release of infective-stage organisms from adhesion to midgut epithelial cells so as to make them available for subsequent transmission by bite.