Date of Award

1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Abstract

Antigenic variation is manifested by many prominent infectious organisms in adaptive response to host immunity. However, polymorphism in antigenic structure is often limited by the necessity to maintain functionality of the antigen. It follows that it may be possible to design molecularly defined vaccines capable of accounting for variable antigenic phenotype. For this reason, the focus of the present thesis was to develop a model system using trypanosome variant surface glycoprotein (VSG) for examining the effects of amino acid sequence variation on the specificity of antigenic determinants.;In order to assess the suitability for a model system of four VSGs (isoVAT VSGs) known to share common surface-exposed epitopes, a variety of tests were utilized to elucidate structural and immunochemical properties. isoVAT VSGs were subjected to structural analysis using SDS PAGE, N-glycanase digestion, peptide mapping and amino acid analysis. Results disclosed structural similarities but also distinctions. Furthermore, immunochemical characteristics examined with polyclonal and monoclonal antibodies suggest significant antigenic homology but non-identity. Two novel strategies were developed to map antigenic determinants recognized by monoclonal antibodies. Relative distribution of antigenic sites was determined by probing VSG peptides resolved on western blots and precise locations (i.e. to as little as six residues) of epitopes were elucidated using deletion analysis of expressed recombinant VSG.;Comparison of the aligned isoVAT VSG sequences revealed the presence of amino acid substitutions occurring in antigenic regions. Among the most radical replacements having no effect on antigenic specificity were those involving proline and others capable of creating significant charge differences. The only site for which specificity was affected by replacements displayed significant charge effect differences at two substituted positions.;Antigenic sites revealed in this study were positioned on a model VSG structure. Results provide the first evidence for positions of regions not exposed on the surface of the living trypanosome. Furthermore, findings suggest structural differences may exist between isolated VSG and VSG organized in the trypanosome coat.;Results of this thesis contribute to our understanding of how antigenic polymorphism can obstruct immune recognition and how the African trypanosome survives in its mammalian host.

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