SEROTYPE CONVERSION OF A SHIGELLA-FLEXNERI CANDIDATE VACCINE STRAIN VIA A NOVEL SITE-SPECIFIC CHROMOSOME-INTEGRATION SYSTEM

Shigella flexneri SFL124 (serotype Y) is a promising live oral vaccine candidate, which has been shown to be safe and immunogenic in human v olunteers. To change the serotype of this vaccine strain, we inserted a serotype conversion gene cluster into the chromosome of SFL124 by us ing a bacteriophage-based site-specific integration system. By cloning an integrase gene (int), an attachment site (attP) and a glucosyl tra nsfer gene cluster from bacteriophage SfX into a suicide vector, and s ubsequently introducing this construct into S. flexneri SFL124, we obt ained a S. flexneri strain (designated SFL1213) expressing the serotyp e X somatic antigen specificity. The strain retained other characteris tics of the parent strain, such as colony shape, growth rate, and Cong o red binding property. Stability test showed that the serotype X O-an tigen specificity in SFL1213 was 100% stable after being cultured appr oximately 72 successive hours under non-selective condition. In a mous e pulmonary model, the recombinant strain elicited a significant level of humoral antibodies which recognized the lipopolysaccharide (LPS) o f a wild-type S. flexneri serotype X strain. The site-specific inserti on system will be useful when stable expression of a cloned single cop y gene is desired in the chromosome of S. flexneri vaccine candidate, SFL124. (C) 1998 Federation of European Microbiological Societies. Pub lished by Elsevier Science B.V. All rights reserved.