Despite recent progress, there remain considerable obstacles to the development of a safe and effective HIV vaccine. It is currently believed that elicitation of a multi-faceted immune response will be required for full protection against both free and cell-associated virus. Salmonella has recently stimulated great interest as a live oral vaccine vector because of its ability to elicit potent humoral, cell- mediated, and mucosal immune responses to cloned foreign antigens. The long-term objective of this proposal is to evaluate the potential role of Salmonella as a vaccine vector for the delivery of human retroviral antigens, while simultaneously clarifying the variables relating to antigen expression which determine the various components of the host immune response to Salmonella. The specific aims of this study are to define systematically the importance of Salmonella strain and mode of antigen gene regulation in eliciting humoral, cell-mediated, and mucosal immune response to HIV-1 antigens expressed in Salmonella. The gene encoding the envelope protein gp120 of HIV-1 has been cloned, fused to the ribosomal binding site and proximal sequence of a Salmonella gene, and introduced into attenuated Salmonella vaccine strains. Plasmids have been constructed in which the cloned gene is either expressed constitutively, or under the control of Salmonella virulence gene regulatory systems. A specific mucosal IgA response has been successfully elicited using some of these constructs in a preliminary study. Different components of the host immune response will be measured in a murine model: a) Humoral- ELISA of serum b) Proliferative- elicitation of delayed hypersensitivity by intradermal antigen, measurement of lymphocyte proliferation in response to HIV-1 antigens c) Cytotoxic- assay of CTL activity d) Mucosal- ELISA of small intestinal secretions for detection of secretory IgA. In addition, the elicitation of neutralizing antibodies will be assessed in vitro. Various methods to achieve stable HIV-1 antigen expression from Salmonella in vivo will be evaluated. Candidate vaccine constructs which are identified as highly immunogenic will be suitable for testing in protection experiments. In summary, this study will examine systematically the determinants of the different types of immune response to Salmonella, while addressing the feasibility of utilizing a Salmonella vector as the basis for an AIDS vaccine.