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The development of vaccines for protection against enteric bacteria

Enteropathogenic bacteria, including Vibrio cholerae. Salmonella typhi. Shigella spp., Yersinia enterocolitica and Esherichia coli remain a considerable health problem. Thus, the successful development of a means of inducing protective immunity against such pathogens is an important goal and has raised much interest.

Evidence suggests that the systemic and mucosal immune systems within the body are largely compartmentalised. In order to achieve immunity against enteropathogens by vaccination, the gut mucosal immune system must be stimulated. Parenteni vaccines

Early in vivo work suggested that parenteral vaccination afforded only poor protection; they elicited a humoral response but there was no evidence of cell-mediated immunity. However, despite limited success in in vivo studies, acetone-inactivated S. typhi vaccine provided 79-93% protection against typhoid fever in humans. 25% of vaccines developed severe systemic and local reactions. Similar unexpected results were observed with parenteral inactivated cholera vaccines but inactivated Shigella vaccines offered no such protection. It was suggested that the parenteral vaccines afforded protective efficacy by stimulating both humoral and cellular immunity, although others suggested that the outcome of vaccination was dependent on previous exposure to the administered antigen. The latter suggestion was supported by in vivo and clinical observations.

Thus, the efficacy of parenteral vaccines may be improved by the use of adjuvants or by improving inactivation methods and so preserving protective antigens. However, until parenteral vaccines can reliably stimulate cell­mediated immunity and/or local secretory IgA responses, they will be of limited use. Oral vaccines

With the limited success of parenteral vaccines accompanied by the realisation that a local immune response was important for protection against enteropathogens, interest turned towards the development of oral vaccines.

Inactivated oral vaccines: these vaccines induce a relatively poor response. In I study, inactivated oral cholera vaccine elicited a level of immunity similar to that observed after V. cholerae infection. Further, parenteral vaccination with inactivated vibrios offered higher levels of protection than oral vaccination.

_ 21 M., 1990 INPHtfIlMA.

It is known that antibacterial and antitoxic immunities act synergistically. Thus, it was found that combination inactivated vaccines capable of stimulating both immunity SUbtypes achieved higher levels of protection than previously observed. In 1 study, such an alcohol-inactivated V cholera vaccine achieved 67% protection. A more extensively studied cholera vaccine, cholera toxin-B/whole-cell vaccine (B-WCV) is highly immunogenic and has been shown to offer protective rates of 64-85%. Further, it is known to elicit long term immunological memory. Inactivated oral S. typhi vaccines have shown little or no efficacy.

Live oral vaccines: limitations of immunisation with inactivated oral vaccines and preliminary observations using live oral vaccines suggest that they may be the most effective method of providing immunity against enteropathogens. However, to be fully safe and effective. live oral vaccines must fulfil the following criteria: sufficient attenuation must be achieved to avoid symptomatic disease; the degree of attenuation must be correct so that colonisation of the intestine and interaction with the gut-associated lymphoid tissue occurs and the full potential of the vaccine is realised; and attenuation must be sufficiently stable so as to avoid reversion to the wild type.

In the construction of potential live oral vaccines, a number of techniques, including selection of suitable spontaneous mutants of enteropathogens, induction of nonspecific mutagenesis and the construction of hybrid ...trains or strains with defined mutations, have been used. • Spontaneous mutants have been used in in

vim and clinical trials with limited success. Despite being isolated from a number of different sources. strains appear to either lack immunogenicity or revert to the wild type and manifest virulence as a result of genetic instability.

• Mutants derived from nonspecific mutagenesis. N-methyl-N'-nitro-N­nitrosoguanidine or UV irradiation have been used to construct avirulent mutants of V. cholerae and S. typhi, respectively. Strain Texas-Star-SR (TS-R), a strain of Texas-Star derived in turn from a strain of V. cholerae by nonspecific mutagenesis, showed protective efficacy in vivo. In humans, oral TS-R was shown to achieve an efficacy rate of 62%. TS­R further caused mild diarrhoea in 25% of recipients. S. tJ'phi Ty21 a, another mutant derived by nonspecific mutagenesis, showed a high level of protection (87%) in 155 volunteers. Further, no adverse reactions were observed and the strain was thought to be

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genetically stable. Two large scale trials conducted in populations in Egypt and Chile further documented the efficacy of the vaccine: efficacy rates in the 2 countries were 95 and 67%, respectively. However, strain Ty21a showed no protective efficacy against typhoid when administered to travellers visiting typhoid-endemic areas in I study. Ty21a reversion has also been reported, but to date revertants do not appear to be more virulent than Ty2la in humans .

• Construction of hybrid strains. Homology between E. coli and Shigella spp. has enabled construction of hybrid Shigella strains with E. coli chromosomal DNA. While such hybrids have shown some promise, limited understanding of the attenuating lesions has resulted in unpredictable genetic stability and in some cases, severe adverse effects .

. • Mutants derived by recombinant DNA techniques. Mutant strains of V. cholerae. S. typhimurium and Salmonella spp. obtained by this technique have been studied. While DNA techniques allow construction of strains with defined mutations which are genetically stable, obtaining strains with a correct balance of hypo-hyperattenuation appears to be difficult.

• Bivalent vaccines. It has been suggested that an attenuated strain carrying a gene encoding an antigen of another enteropathogen may induce immunity against both carrier and secondary pathogen. The introduced antigen must however, be stable. The carrier strain must also be suitably selected; to date, attenuated Salmonella strains have been !lrooosed as potential carriers although with the development ot attenuatru E. cd; ;:~~ I/ cholerae strains, alternatives may become available. 'A /lilly IIttellllllted yet immunogellic deri,llti,e of till

ellteroplltluJgell ill,lISi,e for hUmlltu (such lIS Shigella or Salmonella) would be ;1I,tdullble IIOt Ollly lIS II 'llCciu IIgllillSt diulue CIlWed by the pIlrellt bIlcterium but lIS II ctUTiu for clolled tIIItigellic determilltlllts from other enteroplltluJgetu. The ill,lISi,e qUlllitiu of Shigella or Salmonella, which would be mllilled to some ate"t i" tile JIIlCCiu $1m", should IlIlgar well fo, i""""rwgenicity of tile foreip IlIItigellS upt'U#ll by the 1Iy6rid.'

Hone D. Hackett J. Vaccination apinst enteric bacterial diseases. Reviews oflnfectious Diseases II: 853-877. Nov-Dec 1989 (315 references] "J'

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21 INPH.4RMA® 14 Mar 1990 ..