RECENT STUDIES TOWARDS A VACCINE FOR CHOLERA BASED ON A SYNTHETIC OLIGOSACCHARIDE
ANTIGEN
Pavol Kovac
National Institutes of Health, Bethesda, MD 20892-081 S, USA.
The intracatenary part of the O-PS of Vibrio cholerae O1, serotypes
Inaba and Ogawa, consists of (1->2)-linked moieties of 4-amino-4,6-dideoxy-alpha-D-mannopyranose
(D-perosamine), the amino groups of which are acylated with 3-deoxy-L-glycero-tetronic
acid. Only the Ogawa O-PS has the O-2 at its nonreducing D-perosaminyl
group methylated. Serologically, three antigen factors have been associated
with the O-antigenic structure of V. cholerae O1: Major group antigen factor
A, antigen factor B (Ogawa factor), and antigen factor C (Inaba factor).
The chemical nature of these factors has not been clarified. Fragments
of O-PS of V. cholerae O1, serotypes Inaba and Ogawa, mono- through
hexasaccharides, were prepared using various synthetic strategies for construction
of oligosaccharide chains composed of D-perosamine, and for introduction
of the 3-deoxy-L-glycero-teronamido side chain. Oligosaccharides
were
obtained in the form of methyl a-glycosides, and their interaction with
two monoclonal anti V. cholerae O1 IgGs directed against the LPS
of V. cholerae Ol,
serotype Ogawa, was studied. The antibodies exhibited similar affinities
for fragments of Ogawa O-PS, but did not react with the corresponding fragments
of Inaba OPS which do not carry the 2-OCH3 group. The methyl a-glycoside
of the residue that forms the non reducing terminal residue of the Ogawa
O-PS contributed ~90% of the maxima binding energy, suggesting that the
4,6-dideoxy-4-(3-deoxy-L-glycero-tetronamido)-2-O-methyl-alpha-D-mannopyranosyl
group in the Ogawa LPS is the immunodominant epitope. Binding studies indicate
that the latter structural entity constitutes the Ogawa-specific antigen
factor B. On the other hand, the lack of reaction of the anti-Ogawa antibodies
with oligosaccharide fragments of the Inaba O-PS indicates that the internal
part of the O-PS, common to both O-PSs, is probably not the group-specific
antigen factor A.
Hexa- and dodecasaccharide fragments of the O-PS of Vibrio cholerae
O1, serotype Ogawa were also prepared in the form of alpha-glycosides bearing
aglycons suitable for linking to proteins.