The nature of the attenuation of Salmonella typhimurium strains expressing human papillomavirus type 16 virus-like particles determines the systemic and mucosal antibody responses in nasally immunized mice

We have recently shown by using a recombinant Salmonella typhimurium PhoP(c) strain in mice the feasibility of using a Salmonella-based vaccine to prevent infection by the genital human papillomavirus type 16 (HPV16). Here, we compare the HPV16-specific antibody responses elicited by nasal immunization with recombinant S. typhimurium strains harboring attenuations that, in contrast to PhoPc, are suitable for human use. For this purpose, chi 4989 (Delta cya Delta crp) and chi 4990 [Delta cya Delta(crp-cdt)] were constructed in the ATCC 14028 genetic background, and comparison was made with the isogenic PhoP(c) and PhoP(-) strains. Although the levels of expression of HPV16 virus-like particle (VLP) were similar in all strains, only PhoP(c) HPV16 induced sustained specific antibody responses after nasal immunization, while all strains induced high antibody responses with a single nasal immunization when an unrelated viral hepatitis B core antigen was expressed. The level of the specific antibody responses induced did not correlate with the number of recombinant bacteria surviving in various organs 2 weeks after immunization. Our data suggest that the immunogenicity of attenuated Salmonella vaccine strains does not correlate with either the number of persisting bacteria after immunization or the levels of in vitro expression of the antigen carried. Rather, the PhoP(c) phenotype appears to provide the unique ability in Salmonella to induce immune responses against HPV16 VLPs.