Impact of Vector Priming on the Immunogenicity of RecombinantSalmonellaVaccines

Abstract

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.

Vector Priming Reduces the Immunogenicity ofSalmonella-Based Vaccines inNramp1+/+Mice

“…This observation extends our previous findings in BALB/c mice (1,18) to hosts that might better reflect the responses of human vaccine recipients (5). This study therefore supports other recent data (18) indicating that vector priming could prejudice the use of recombinant Salmonella vaccines in regions where these bacteria may be endemic. Further studies will be needed to elucidate the mechanism of hyporesponsiveness seen in vectorprimed hosts before compensating strategies can be designed and evaluated.…”

“…2A) and anti-LPS (Fig. 2B) responses comparable to those observed previously (1,18). Naïve animals displayed strong, consistent immunoglobulin G (IgG) responses to K88, with comparatively weak and delayed responses to vector LPS.…”

“…Serovar Stanley carries the same O-4,5,12 lipopolysaccharide (LPS) as serovar Typhimurium but is naturally attenuated for mice; after oral administration, it shows prolonged GALT colonization (1) but is unable to spread systemically (data not shown). Groups of BALB/c (Nramp1 Ϫ/Ϫ ) and CBA (Nramp1 ϩ/ϩ ) mice were orally dosed with 1.1 ϫ 10 9 CFU of serovar Stanley, and the levels of GALT colonization were compared by determining Peyer's patch burdens at various intervals, as done previously (18). Similar numbers of bacteria were recovered from the two strains at each of the first three time points, with mean burdens of ca.…”

“…Studies in three other laboratories employed aroA S. enterica serovar Typhimurium (11,14) or the naturally attenuated S. enterica serovar Stanley (1,18) as vectors and concluded that prior exposure to the vector significantly compromises both serum and intestinal antibody responses to a range of foreign antigens. The factors that determine the impact of vector priming remain unclear but include the nature of the foreign antigen and the Salmonella vector (18).…”

“…Ten weeks after primary infection, all animals were dosed with 1.3 ϫ 10 9 CFU of serovar Stanley-K88, a recombinant expressing the Escherichia coli pilus protein K88 (1). Serum and fecal pellet samples were collected at intervals for the determination of anti-K88 and anti-LPS antibody responses by enzyme-linked immunosorbent assay (ELISA), as detailed elsewhere (18).…”