SOMATIC AND EXCRETORY POLYSACCHARIDE ANTIGENS OF LEISHMANIA DONOVANI PROMASTIGOTES

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Abstract
Leishmania donovani promastigotes were found to release excretory-secretory antigens (ESA) in the culture medium which reacted against rabbit antiserum to live promastigotes (RA-LP). The active material was purified and found to contain mainly polysaccharides with about 23% of protein. A polysaccharide-rich antigen (PRA) was also isolated from the promastigote debris by the phenol-water extraction procedure. When tested against RA-LP in agar gel diffusion, both the preparations were found to be antigenically identical to each other as well as to a preparation of crude soluble antigen (CSA). The determinants involved were shown to be polysaccharide in nature. Gel filtration of all the three preparations showed that the antigenic activity was distributed in various eluted fractions. Both CSA and PRA also exhibited considerable electrophoretic heterogeneity in immunoelectrophoresis, although ESA showed only two overlapping faster moving bands. It is concluded that ESA represented a mixture of highly negatively charged mucopolysaccharides of varying sizes. The minimum molecular weight of the basic structural unit was around 2 104 daltons. These materials were probably derived from antigenically similar complexes of larger molecular weights located on the outer surface of the parasite and were released in the culture medium during their multiplication. Information available so far from this study suggests that the polysaccharide determinants of ESA/PRA did not elicit any marked immune response in kala-azar patients.