Characterisation and in vivo behaviour of aSalmonella typhimurium aroA strain expressingEscherichia coliK1 polysaccharide

Salmonella enterica serovar typhimurium trxA mutants are protective against virulent challenge and induce less inflammation than the live-attenuated vaccine strain SL3261

In Salmonella enterica serovar Typhimurium, trxA encodes thioredoxin 1, a small, soluble protein with disulfide reductase activity, which catalyzes thiol disulfide redox reactions in a variety of substrate proteins. Thioredoxins are involved as antioxidants in defense against oxidative stresses, such as exposure to hydrogen peroxide and hydroxyl radicals. We have made a defined, complete deletion of trxA in the mouse-virulent S. Typhimurium strain SL1344 (SL1344 trxA), replacing the gene with a kanamycin resistance gene cassette. SL1344 trxA was attenuated for virulence in BALB/c mice by the oral and intravenous routes and when used in immunization experiments provided protection against challenge with the virulent parent strain. SL1344 trxA induced less inflammation in murine spleens and livers than SL3261, the aroA mutant, live attenuated vaccine strain. The reduced splenomegaly observed following infection with SL1344 trxA was partially attributed to a reduction in the number of both CD4(+) and CD8(+) T cells and B lymphocytes in the spleen and reduced infiltration by CD11b(+) cells into the spleen compared with spleens from mice infected with SL3261. This less severe pathological response indicates that a trxA mutation might be used to reduce reactogenicity of live attenuated vaccine strains. We tested this by deleting trxA in SL3261. SL3261 trxA was also less inflammatory than SL3261 but was slightly less effective as a vaccine strain than either the SL3261 parent strain or SL1344 trxA.

Live attenuated Salmonella vaccines and their potential as oral combined vaccines carrying heterologous antigens

Live attenuated salmonellae are protective, and are candidate vaccines against invasive salmonella infections in man and animals. Different attenuating mutations have been described, and more than one can be incorporated in a vaccine for added safety. Combined salmonella vaccines express target carbohydrate and protein antigens or epitopes from viruses, bacteria and eukaryotic parasites, either within or on the surface of the cell, as capsules, fimbriae, or in the flagellin. Humoral, secretory and cellular responses to the recombinant antigens has been demonstrated. Experimental protection against diseases including streptococcal infection, tetanus, influenza and malaria has been obtained.

Immunogenicity of foreign peptide epitopes expressed in bacterial envelope proteins

We have used two bacterial proteins from Escherichia coli to express heterologous peptides. Both proteins are situated in the E. coli cell envelope but have different properties: LamB is an integral outer membrane protein, and MalE a soluble periplasmic protein. The peptides were expressed as genetic inserts within "permissive sites" of these recipient proteins, i.e. sites which allow the insertion of foreign peptides without affecting the biological properties of the host protein. In this paper, we summarize preliminary rules governing the immunogenicity of resulting LamB and MalE hybrid proteins when expressed in E. coli. We focus on two model epitopes: either peptide 132-145 from the preS(2) region of hepatitis B virus or peptide 93-103 from poliovirus VP1 capsid protein. We also present first results obtained when the same hybrid proteins were expressed in attenuated Salmonella typhimurium. Plasmids encoding the hybrid proteins were transferred to aroA S.typhimurium by electroporation. In vitro, the hybrid proteins could be expressed at high levels by S. typhimurium. Mice were immunized by parenteral and oral routes. The effect of the carrier protein and the level of its expression on the in vivo behaviour of the immunizing bacteria and on the immune response induced will be discussed.