Cryptic epitope explains the failure of a monoclonal antibody to bind to certain isolates of Trypanosoma cruzi.

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Abstract
A mouse monoclonal antibody, WIC 29.26 Ab, has previously been characterized as recognizing a carbohydrate epitope on a 72,000 m.w. glycoprotein (GP72) expressed on the surface of Trypanosoma cruzi epimastigotes and metacyclic trypomastigotes. This molecule has been implicated as a receptor in the control of parasite transformation, and when used as an immunogen in mice, partially protects against T. cruzi infection. In previous experiments in which a radioimmunoassay was used, WIC 29.26 Ab was found to react with approximately 50% of T. cruzi strains and clones derived from a variety of sources. In this study, we attempted to determine whether the WIC 29.26 Ab-nonreactive isolates lack the entire GP72 or merely lack the epitope recognized by this monoclonal antibody. WIC 226.4 Ab, a monoclonal antibody raised against periodate-treated GP72, reacted in an immunofluorescence assay with all strains and clones studied, including those which had not reacted with WIC 29.26 Ab. Likewise, two polyvalent rabbit sera, directed specifically against GP72, bound to all T. cruzi isolates tested. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of detergent lysates of surface-labeled epimastigotes immunoprecipitated with WIC 29.26 Ab showed that the epitope bound by this antibody was present in all but one of the parasites that were surface-nonreactive, as well as in all those that were surface-reactive. WIC 29.26 Ab precipitated a single 72K Mr band from most strains and clones, but in several cases 79K Mr and 66K Mr bands were seen. Isolates from both the surface-reactive and the surface-nonreactive groups showed the latter pattern. These results demonstrate that GP72, or similar electrophoretic variants--and with one exception, the carbohydrate epitope bound by WIC 29.26 Ab--are present in the surface membrane of all strains and clones tested. This observation suggests that in intact epimastigotes of the surface-nonreactive isolates, the epitope is not accessible because of structural changes in the molecule itself or because of differences in the membrane environment of GP72.