During growth within the sandfly and within axenic, Leishmania promastigotes undergo differentiation from a dividing noninfective stage to a resting infective or metacyclic stage which is uniquely adapted for life in the vertebrate. This development is accompanied by a substantial modification of the surface lipophosphoglycan (LPG) which is the major surface glycoconjugate of these cells. In the case of L. major and L. donovani promastigotes derived from culture, the structural polymorphism is expressed as a loss of terminal galactose residues on the LPG. Using stage specific monoclonal antibodies against L. major LPG, identical structural changes in LPG have now been demonstrated for metacyclics found within the natural vector, P. papatasi, and in particular those metacyclics found to emerge from the proboscis during force feeding experiments. The substitution or capping of terminally exposed sugars with other sugars with other sugar residues was shown to be the molecular mechanism underlying the attachment and release of promastigotes from midgut epithelial cells during metacyclogenesis in the fly. The interspecific polymorphisms of these terminal sugars appears to influence the species specificity of vectorial competence observed in nature. Thus, the ability of procyclic promastigotes of the purified procyclic LPG of various Leishmania species to bind to midgut epithelial cells of various sandfly species forcibly predicts vectorial competence.
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