Stage-specific variations in lectin binding to Leishmania donovani

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Abstract
Visceral leishmaniasis is caused by the dimorphic protozoan Leishmania donovani, which exists as an aflagellar amastigote within mammalian mononuclear phagocytes and as a flagellated extracellular promastigote in its sandfly vector. We have identified four plant lectins that bind to the L. donovani surface, and through these we have documented stage-specific differences in exposed surface carbohydrates. Concanavalin A bound to both promastigotes and amastigotes; binding was inhibited by mannose or alpha-methyl-mannoside, implying a mannose-containing residue on the surface of both parasite stages. Ricinus communis agglutinin, which binds to galactose-containing residues, also bound to both stages and was inhibited by lactose, implying a galactose-containing glycoconjugate on the parasite surface. Two other lectins, wheat germ agglutinin (WGA) and peanut agglutinin (PNA), exhibited stage specificity in their binding characteristics. Amastigotes bound WGA but not PNA. During the process of conversion from the amastigote to the promastigote stage, the WGA-binding glycoconjugate was lost, and a PNA-binding residue was newly displayed. WGA binding was inhibited by N-acetyl-D-glucosamine and was not altered by neuraminidase treatment, suggesting the presence of an exposed N-acetyl-D-glucosamine moiety on the amastigote surface. The PNA binding site is known to accommodate the oligosaccharide beta-D-galactose-(1----3)-N-acetyl-D-galactosamine; in our system, PNA may have identified an internal rather than a terminal galactose on the promastigote surface. Localized binding of WGA and PNA to the surface of intermediate phases of the parasite suggested inhomogeneous and changing surface characteristics during conversion from amastigote to promastigote stages. This evolution of L. donovani surface glycoconjugates may be important in the adaptation of the organism to its divergent mammalian host and arthropod vector environments.