Characterization and protective activity of a monoclonal antibody against a capsular epitope shared by Streptococcus suis serotypes 1, 2 and 1/2

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Abstract
A monoclonal antibody (mAb Z3) was produced using BALB/c mice immunized with whole cells ofStreptococcus suisserotype 2 reference strain S735. Screening by dot-ELISA showed that mAb Z3, of isotype lgG2b, reacted only with reference strains and field isolates ofS. suisserotypes 1, 2 and 1/2. The recognized epitope was demonstrated to be polysaccharide in nature by periodate oxidation, and located in the capsule, since mAb Z3 reacted with purified capsular material by immunoblotting and was able to stabilize the capsule as shown by electron microscopy. Further characterization indicated that mAb Z3 may react specifically with the sialic acid moiety of the capsule, a common constituent of the polysaccharidic capsular material of the three capsular types, since sialidase-treated cells did not react with mAb Z3 in immunoblotting or indirect EILISA. Purified mAb Z3 was shown to significantly increase the rate of phagocytosis ofS. suiscells by porcine monocytes and to activate the clearance of bacteria from the circulation in experimentally infected mice. However, mAb Z3 only offered partial protection to mice challenged with a minimal lethal dose. Thus, even though the capsule ofS. suisseems to be an important virulence factor, the epitope recognized by mAb Z3 does not appear to be involved in complete protection against infection.