Immunogenicity of various Vibrio ordalii lipopolysaccharide fractions in coho salmon Oncorhynchus kisutch

The immunogenicity of intraperitoneally injected native Vibrio ordalii (Vo) lipopolysaccharide (LPS) and of various Vo LPS fractions obtained by preparative sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) was examined in coho salmon Oncorhynchus kisutch. Immunogenicity was assessed using a challenge with live Vo cells, the challenge being sufficient to kill 85 to 87 % of the control fish not receiving any Vo preparations. Native Vo LPS afforded the strongest protection, the relative percentage survival (RPS) being 85.3 %. LPS fractions were found to contain 2 or more LPS components, and protective immunity associated with the fractions appeared to decline (from 51.4 to 5.9% RPS) as the molecular weight of the slowest migrating components in the fractions decreased. In fact, only those fractions containing LPS with molecular weights of 20 kD or above resulted in significant protection. We conclude that the higher molecular weight species of Vo LPS were the ones that were largely responsible for the anti-Vo immunity induced In coho by native LPS, and, based on the ladderlike pattern produced by the LPS on SDS-PAGE, we think it likely that the different LPS species represented molecules differing from one another largely in the number of O-polysaccharide repeat units present. Aggregation of the LPS molecules also occurred, and the possibility that aggregation may have contributed to the immunogenicity of the protective LPS fractions cannot be ruled out.