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

Abstract

A monoclonal antibody (mAb 23) was produced using BALBIc mice immunized with whole cells of Streptococcus suis serotype 2 reference strain S735. Screening by dot-ELISA showed that mAb 23, of isotype IgG2b, reacted only with reference strains and field isolates of 5. suis serotypes 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 23 reacted with purified capsular material by immunoblotting and was able to stabilize the capsule as shown by electron microscopy. Further characterization indicated that mAb 23 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 23 in immunoblotting or indirect ELISA. Purified mAb 23 was shown to significantly increase the rate of phagocytosis of 5. suis cells by porcine monocytes and to activate the clearance of bacteria from the circulation in experimentally infected mice. However, mAb 23 only offered partial protection to mice challenged with a minimal lethal dose. Thus, even though the capsule of 5. suis seems to be an important virulence factor, the epitope recognized by mAb 23 does not appear to be involved in complete protection against infection.

Streptococcus suis serotype 2 mutants deficient in capsular expression

“…To our knowledge, the capsule of S. suis serotype 2 is the first critical virulence factor described so far for this bacterium. (Charland et al, 1997). Other factors, such as extracellular or cellwall-associated proteins, are probably required for successful establishment of the bacteria in the host.…”

Protection against Streptococcus suis Serotype 2 Infection Using a Capsular Polysaccharide Glycoconjugate Vaccine

“…These vaccines were named "glycoconjugate vaccines." Since it has been reported that anti-CPS antibodies, if produced, have a high protective potential against infection caused by S. suis (22,23), an interesting strategy for the development of an S. suis type 2 vaccine would be the use of a glycoconjugate vaccine made of CPS coupled to an immunogenic carrier protein, such as tetanus toxoid (TT), diphtheria toxoid, cross-reactive material 197 (CRM 197 ), or many more (24). As a matter of fact, glycoconjugate vaccines are very successful in the fight against encapsulated human pathogens such as Haemophilus influenzae (Hiberix), Neisseria meningitidis (MenACWY), and Streptococcus pneumoniae (PCV13) (20).…”

Serological Characterization of Streptococcus iniae Strains Isolated from Cultured Fish in Japan

“…Antigen A is probably the group antigen of S. iniae, because it is acid-extractable and has carbohydrate nature (Pier et al, 1978;Bachrach et al, 2001). It is known that specific antibodies stabilize bacterial capsules (Bayer and Thurow, 1977;Charland et al, 1997;Yoshida et al, 1997;Vanrobaeys et al, 1999 (Fig. 2c).…”

“…Samples for transmission electron microscopy were prepared as described by Charland et al (1997) with some modifications. Briefly, one loopful of bacterial colonies grown on TH agar was suspended in 2 mL PBS, added with 40 µL of rabbit anti-NUF38 or anti-NUF631 serum and incubated for 3 h at 4°C.…”