Construction and immunological evaluation of Salmonella vector vaccines that express HIV-1 envelope protein, gp120

Abstract

Since the human immunodeficiency virus (HIV-1) is transmitted either parenterally or sexually, both mucosal and systemic immune responses may be required to provide protective immunity. Attenuated Salmonella vectors expressing heterologous antigen can stimulate responses in both compartments. To evaluate the utility of Salmonella vectors as a HIV-1 vector vaccine, a gene expression cassette encoding recombinant HIV-1 gp120 (rgp120) was integrated into the aroC locus of an attenuated vaccine strain of S. typhi. Immunoblot analysis using anti-gp120 monoclonal antibodies (mAb's) shows that the S. typhi strains containing the integrated cassette express a protein that is the appropriate size for non-glycosylated full-length gp120 (52 kDa). Immunoblot analysis also demonstrated that the recombinant S. typhi strains express the rgp120 as insoluble monomers and multimers. Antigen-capture ELISA, using antibodies specific for continuous epitopes on gp120, revealed that the S. typhi-expressed rgp120 is folded differently from a rgp120 derived from baculovirus that binds CD4. Immunization of BALB/c mice with S. typhi::rgp120 did not stimulate env-specific cytotoxic T lymphocytes (CTLs) or a significant rise in anti-gp120 antibody titers as compared to controls. Since mice may be a poor animal model for evaluating S. typhi vector vaccines, S. typhimurium strains were constructed that expressed rgp120 from a chromosomally integrated cassette (SL3261::120). To test if increased antigen expression potentiates immunogenicity, strains were constructed that express rgp120 from a multicopy asd stabilized plasmid (SL7207 pYA:120). Immunoblot analysis demonstrated that SL7207 pYA:120 expressed approximately 50 times more rgp120 than SL3261::120. Oral immunization of BALB/c mice with these strains did not stimulate an env-specific CTLs or a significant rise in anti-gp120 antibody titer as compared to controls. However, splenic T cells from SL7207 pYA:120 immunized mice proliferated upon restimulation with gp120 in vitro while splenocytes from SL3261::120 immunized mice did not. gp120 restimulated splenic T cells from SL7207 pYA:120 immune mice also produced IFN-{dollar}\gamma{dollar} but no IL-5. Three conclusions can be drawn from these results. First, Salmonella expresses rgp120 as a misfolded insoluble protein. Second, high level expression of rgp120 in Salmonella vectors is necessary to stimulate a gp120-specific immune response in mice. Third, Salmonella::rgp120 stimulates a gp120-specific Th1 response in mice. This report is the first to describe the construction of a Salmonella::rgp120 vector vaccine that is immunogenic in mice.