Reactive oxygen species are the major antibacterials against Salmonella Typhimurium purine auxotrophs in the phagosome of RAW 264.7 cells.

Intramacrophage survival appears to be a pathogenic trait common to Salmonellae and definition of the metabolic requirements of Salmonella within macrophages might provide opportunities for novel therapeutic interventions. We show that loss of PurG function in Salmonella enterica serovar Typhimurium SL1344 leads to death of the bacterium in RAW264.7 cells, which was due to unavailability of purine nucleotides but not thiamine in the phagosome of RAW264.7 cells. Phagosomal escape of purG mutant restored growth, suggesting that the phagosomal environment, but not the cytosol, is toxic to Salmonella purine auxotrophs. NADPH oxidase inhibition restored the growth of purG mutant in RAW264.7 cells, implying that the Salmonella-containing vacuole acquires reactive oxygen species (ROS) that are lethal to purine auxotrophs. Under purine limiting conditions, purG mutant was unable to repair the damage caused by hydrogen peroxide or UV irradiation, suggesting that ROS-mediated DNA damage may have been responsible for the attenuated phenotype of purG mutant in RAW264.7 cells and in mice. These studies highlight the possibility of utilizing the Salmonella purine nucleotide biosynthetic pathway as a prospective therapeutic target and also underline the importance of metabolic pathways in assembling a comprehensive understanding of the host-pathogen interactions inside phagocytic cells.

Vector-primed mice display hypo-responsiveness to foreign antigen presented by recombinant Salmonella regardless of the route of delivery.

Our previous studies have shown that mice which have been orally primed with an attenuated Salmonella vector [S. enterica serovar Stanley] are hypo-responsive to foreign antigens later delivered orally by the same vector strain, responding with significantly impaired serum and intestinal antibody responses compared with those seen in unprimed controls. Initial vector priming of the gut-associated lymphoid tissue (GALT) is likely to result in impaired persistence of recombinant Salmonella later administered orally. Delivery of recombinant bacteria by the intra-peritoneal or intra-nasal route, to avoid exposure to a primed GALT, did not allow vector-primed recipients to mount normal antibody responses to the foreign pilus protein K88. The negative impact of vector priming could be largely overcome, however, if mice were exposed to the foreign protein just prior to priming with the vector strain. Using this strategy, vector-primed mice displayed normal gut IgA and intermediate serum IgG responses to K88 following oral administration of recombinant Salmonella. Our findings are compatible with the concept of epitopic suppression, in which failure to respond to the foreign vaccine antigen reflects the clonal dominance of B cells specific for epitopes associated with the vector strain.

Characterization of immune responses induced by intramuscular vaccination with DNA vaccines encoding measles virus hemagglutinin and/or fusion proteins.

Measles virus (MV) hemagglutinin (MV-H) and fusion (MV-F) proteins induce plaque reduction neutralizing (PRN) antibodies and cell-mediated immune responses that protect against clinical measles. DNA vaccines that encode MV-H and MV-F are being investigated as a new generation of measles vaccine to protect infants too young to receive currently licensed attenuated measles vaccines. However, it is unclear whether DNA vaccines encoding both MV-H and MV-F act synergistically to induce stronger immunity than immunization with plasmids encoding MV-H or MV-F alone. To address this question, we generated Sindbis virus-based pSINCP DNA vaccines that encode either MV-H or MV-F alone or bicistronic or fusion system vectors that encode both MV-H and MV-F (to mimic MV infection where both MV-H and MV-F proteins are expressed by the same mammalian cell). Mice immunized with DNA vaccine encoding MV-H alone developed significantly greater PRN titers than mice immunized with bicistronic constructs. Interestingly, the presence of MV-F in the bicistronic constructs stimulated serum MV-specific immunoglobulin G of reduced avidity. By contrast, mice immunized with bicistronic constructs induced equivalent or higher levels of MV-specific gamma interferon responses than mice immunized with DNA vaccine encoding MV-H alone. These data will help guide the design of DNA-based MV vaccines to be used early in life in a heterologous prime-boost strategy.

Recombinant Salmonella enterica serovar Typhi in a prime-boost strategy.

This study investigated the utility of attenuated Salmonella enterica serovar Typhi strain CVD 908-htrA (908 h) in a heterologous prime-boost strategy. Mice primed intranasally (i.n.) with 908 h expressing fragment C (Frag C) of tetanus toxin and boosted intramuscularly (i.m.) with tetanus toxoid (TT) mounted enhanced and accelerated serum IgG anti-Frag C responses in comparison to unprimed, vector-primed and homologously-primed and boosted mice. Serum antitoxin responses were also determined; mice that were vaccinated following a heterologous prime-boost regimen exhibited the highest levels of Frag C-specific toxin neutralizing antibodies 1 week after boosting. Mice primed and boosted i.m. with TT developed a significantly greater proportion of serum IgG1 antibodies and weaker IFN-gamma levels in contrast to those primed intranasally (i.n.) with rS. Typhi that were homologously or heterologously boosted. These encouraging pre-clinical data provide a rational basis for undertaking a pilot clinical trial to evaluate this strategy. An ability to stimulate enhanced, accelerated responses to parenteral vaccination following mucosal priming may be advantageous in the immunoprophylaxis of many infectious diseases, including those of biodefense importance.

Vector priming reduces the immunogenicity of Salmonella-based vaccines in Nramp1+/+ mice.

The present studies in Nramp1(-/-) BALB/c and Nramp1(+/+) CBA mice question the significance of this genotype as a determinant of the level of gut colonization following oral administration of naturally attenuated or highly virulent Salmonella strains. In line with previous results in BALB/c hosts, vector priming of CBA mice with Salmonella enterica serovar Stanley was found to significantly compromise the immunogenicity of a recombinant construct expressing a foreign pilus protein.

Impact of vector priming on the immunogenicity of recombinant Salmonella vaccines.

There are conflicting reports concerning the impact of prior vector priming on the immunogenicity of recombinant-Salmonella-based vaccines. A comparison of experimental protocols identified two variables which might account for this inconsistency: the potential of the vector strain to colonize the murine gut-associated lymphoid tissue (GALT) and the nature of the foreign antigen subsequently delivered by the recombinant Salmonella construct. The former was investigated by constructing an aroA mutant of the Salmonella enterica serovar Stanley vector previously used in our laboratory. Although the introduction of an aroA mutation had surprisingly little effect on GALT colonization, it did reduce the strength of antilipopolysaccharide (anti-LPS) antibody responses and the impact of vector priming. Studies were also performed to ascertain the extent to which any observed hyporesponsiveness consequent upon vector priming might be determined by the characteristics of the foreign antigen. S. enterica serovar Stanley was used to deliver either of two Escherichia coli antigens, K88 pilus protein or the LT-B toxin subunit, to vector-primed mice. Both serum immunoglobulin G (IgG) and intestinal IgA responses to K88 were completely abolished, and those to LT-B were significantly reduced, as a consequence of vector priming. When similar experiments were performed with an aroA S. enterica serovar Dublin vector, responses to K88 were significantly reduced but those to LT-B were unaffected by vector priming. Paradoxically, a priming infection with this vector induced stronger anti-LPS antibody responses but was less likely to elicit a state of hyporesponsiveness to subsequently presented foreign antigen. The impact of vector priming thus depends on both the Salmonella strain used and the nature of the foreign antigen, but our present data strengthen concerns that preexisting antivector immunity represents a serious threat to the Salmonella-based vaccine strategy.

Molecular and genetic characterization of the capsule biosynthesis locus of Streptococcus pneumoniae type 23F.

The authors have previously reported the nucleotide sequence of the 5' and 3' portions of the Streptococcus pneumoniae type 23F capsular polysaccharide biosynthesis locus (cps23f) (from dexB to cps23fB and from cps23fL to aliA). These regions of cps23f were very similar to the sequence reported for cps19f, the capsule locus of S. pneumoniae type 19F. However, Southern hybridization analysis indicated that no other genes closely related to cps19f are present in the cps23f locus. In this study long-range PCR was used to amplify and clone the section of the S. pneumoniae type 23F capsule locus between cps23fB and cps23fL. This region is 13 kb in size and contains 12 new ORFs, designated cps23fC-E, I, J, and T-Z. Functions are proposed for all of the protein products, including functional homologues of Cps19fC-E, Cps19fI and Cps19fJ. A biosynthetic pathway for type 23F capsular polysaccharide is also proposed.