Bruce A.D. Stocker

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Bruce A.D. Stocker, Professor Emeritus Active, Microbiology and Immunology, Beckman Center, Stanford University School of Medicine, Stanford, CA 94305-5124 Tel: (650) 723-2006, Fax: (650) 725-6757
We construct nutritionally-exacting derivatives of pathogenic bacterial strains for use as live vaccines. Our method is to block a biosynthetic pathway, such that the bacteria cannot grow unless provided with a metabolite not available in mammalian tissues, specifically p-aminobenzoate (pAB). Blocking aromatic biosynthesis by mutation of gene aroA causes virtually complete loss of virulence, so that the injection of even live 106 aroA into the mouse causes no ill-effects whereas the LD50 of the virulent Salmonella typhimurium parent strain is less than 20 bacteria. Strains of S. typhimurium and S. dublin with deletions at aroA have proven to be safe and effective live vaccines in mice, calves, and sheep and strains of S. typhi with aro deletions tested in volunteers are promising candidate typhoid vaccines. A strain of Shigella flexneri (a common cause of dysentery) with a deletion in gene aroD fed, as live vaccine, to monkeys protected them against later challenge with a virulent strain and when fed to volunteers did not cause disease in any of them. We are also concerned with the genetic determination of two Salmonella surface components, the O antigen (LPS) and the flagella. Live-vaccine strains of Salmonella can "present" antigens of other pathogens, so that the vaccine induces a protective response to the passenger antigen. Mice fed an aroA strain of S. dublin carrying a cloned gene for part of the M .surface protein of Streptococcus pyogenes type 5 survived a challenge dose of streptococci which killed nonvaccinated mice. We have also inserted synthetic oligonucleotides specifying antigenic determinants of foreign proteins into a cloned Salmonella flagellin gene, so that the foreign epitope forms part of the surface of the flagella. Live-vaccine Salmonella with flagella composed of flagellin (flagellar protein) made to include a short amino acid sequence of subunit B of cholera toxin when given to mice caused production of antibody to cholera toxin. This method makes it possible to construct a vaccine to protect against infections (bacterial, viral, parasitic or other) for which no method of immunization is at present available.

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