Background: Vaccines against HIV have not been developed. We observed that HSV vectors induced durable host immunity in mice and macaques. In addition, some macaques vaccinated with HSV vectors expressing SIV proteins were protected against SIV challenge. These results indicated that HSV might be a good vector for generation of protective and durable immunity to HIV. Our current work aims to generate improved vaccine candidates through mutation of the HSV vector and enhanced expression of foreign antigens.

Methods: We constructed HSV vectors by homologous recombination into a virus encoding the green fluorescent protein (gfp) and screened for gfp-negative progeny. Protein expression was verified by Western blot and immunoprecipitation. Vectors were characterized for growth and cytotoxicity in tissue culture. Immunogenicity of the vector was evaluated in a mice by analyzing serum IgG ELISA titers.

Results: HSV vectors expressing E. coli ß-galactosidase, SIV gag, or SIV env proteins were generated in a multiple-deletion strain of HSV type 1 (d106) that lacks four viral proteins: ICP 4, ICP 22, ICP 27, and ICP 47. Because several of these genes have been implicated in host immune modulation following wt HSV infection, deletion of these proteins are expected to result in enhanced vector efficacy. The parental d106 virus was non-cytotoxic and was defective for growth in normal cells. Therefore, all vectors were passaged on complementing cells expressing ICP4 and ICP 27. Recombinant d106-ßgal, d106-SIV.gag, and d106-SIV.env were characterized by Western blot and immunoprecipitation and each showed high-level expression of antigen. Using a mouse model of infection, IgG antibody responses to the d106-ßgal vector indicated that this construct was immunogenic in vivo. Ongoing studies in mice will evaluate the immunogenicity of the d106-SIV.gag and d106-SIV.env constructs and determine the durability of host responses to these vectors. Studies in macaques are also being used to verify the utility of d106 as a vector system.

Conclusions: HSV vectors have shown promise for induction of durable host immune responses in animal models, but generation of more effective vaccines is essential. Using HSV-1 strain d106, we constructed and characterized novel SIV vectors, and we are currently testing their efficacy in vivo. The durability of responses generated by HSV vectors may make them a useful addition to prime-boost strategies currently being evaluated.