Although it is a major target of group-specific neutralizing antibodies, gp120-based vaccines have been weak immunogens. Most have been incapable of eliciting antibody titers even 1/50th as great as those induced by infection. One way to enhance the response to gp120 would be to incorporate it into an attenuated live viral vector. Prior infection with a mutant SIV can protect against a subsequent challenge with wild type SIV, suggesting that a truly attenuated vector expressing the same viral antigens could be a successful vaccine. However, the risk of incomplete attenuation of current vaccines has been amply demonstrated by lethal infections in newborn macaques. The key to a live attenuated vector would be to separate viral propagation and immunogenicity from pathogenicity. Based on this principle, we are working to develop a safe live viral vector against HIV. We are using another enveloped, positive strand RNA virus, rubella, as a carrier of HIV proteins. Rubella vaccine is highly effective against mucosal as well as systemic infection. Protection starts with the first dose and lasts a lifetime. By incorporating gp120 into an infectious cDNA clone of rubella virus, we hope to induce a comparable level and duration of immunity to HIV. Safety is the chief advantage of rubella, which has been given safely to millions of children around the world, including asymptomatic HIV+ children. Because the virus has only RNA intermediates and does not integrate into host DNA, it cannot persist indefinitely in the host. Rubella has not changed serotype in over 40 years, nor does the vaccine strain revert to wild type, so it is unlikely to lose the gp120 insert through mutation. It is minimally pathogenic for children or adults. Ideally, a rubella/HIV hybrid would combine the growth properties and safety profile of rubella vaccine with the antigenicity of HIV gp120, but without the pathogenicity of HIV. We are using a full length, infectious cDNA clone of rubella, provided by Dr. Teryl Frey. We have identified two sites for inserting HIV antigens without disrupting functional rubella genes. One of these insertional mutants is being tested for expression of green fluorescent protein. Viral infectivity and GFP expression will be tested in vitro. Ultimately, we will insert gp120 at this site to produce infectious rubella/HIV hybrid virus. The hybrid will be tested for growth and gp120 expression in vitro. It will then be tested for propagation in rhesus macaques and the ability to elicit anti-gp120 antibodies in vivo, as well as safety and viral persistence. Macaques are the ideal host for demonstrating protection, since they can be challenged with a SHIV challenge strain expressing envelope glycoproteins of the same HIV type or different from the vaccine strain.
