The correlates of immune protection against HIV infection and disease are still not well understood. However, many scientists believe that an efficacious vaccine against AIDS will need to induces neutralizing antibodies against the virus, and that to be protective, antibodies will need to neutralize the types of viruses believed to be representative of those encountered through natural exposure e.g. primary isolates rather than laboratory-adapted strains) as well as multiple HIV-1 strains. The experimental HIV vaccines tested to date seem to come up short on both accounts, leading to the pessimistic feeling that designing a neutralization-based vaccine that will be effective against the multitude of virus strains circulating in the world may be possible. But a study published recently by LaCasse et al 1 has created a stir in the AIDS vaccine community, breathing life back into the notion that creating a vaccine that will induce a potent, broadly reactive responses may be achievable.
Gp120 and gp120-CD4 complex vaccines
Most of the virus neutralizing activity found in sera from HIV-infected individuals is due to antibodies directed against the virus surface envelope protein, gp120. A number of candidate vaccines have been designed to present this target protein, either as purified recombinant protein, or through delivery of the envelope gene within recombinant vaccine vectors. Recombinant protein vaccines have been shown to be efficient at eliciting a neutralizing response, but the antibodies produced are poorly reactive against primary isolates and relatively vaccine. Since all HIV-1 isolates bind to CD4 and one of (a few) second receptors to initiate infection, there may be conserved epitopes in or around these binding sites that are targets for neutralizing antibodies. Binding of virus to the cell receptors results in conformational changes in the virus envelope proteins and then fusion of the viral and cell membranes.
Several laboratories have attempted to take advantage of this fact to design improved immunogens. Kang, et al 2 immunized mice with gp120/CD4 complexes and reported that some of the antibodies produced were targeted to newly formed epitopes that resulted from CD4 binding. DeVico, et al 3 immunized goats with covalently crosslinked omplexes of gp120 and CD4 and found, surprisingly, that the neutralizing antibodies produced were more reactive against the primary MN strain than the laboratory-adapted version of this strain. The antibodies produced also had some neutralizing capacity against more diverse primary isolates.
The recent identification of chemokine receptors that are involved in the HIV infection process has opened up additional avenues of investigation to potentially improve on these complex immunogens. An important proof-of-concept experiment demonstrating the potential of such an immunogen has been reported by LaCasse and colleagues in Dr. Jack Nunberg's laboratory at the University of Montana. LaCasse et al 1 created a complex immunogen by mixing simian fibroblasts (COS cells) engineered to express HIV envelope glycoprotein and human neuroblastoma (U87) cells expressing CD4 and the secondary receptor CCR5. After a short incubation period, these cells began to fuse with one another as the envelope proteins expressed by the COS-env cells bound to the receptors on the U87-CD4-CCR5 cells. These researchers hypothesized that they could freeze the envelope glycoprotein in the conformational state that occurs during receptor binding. They created what they termed a "fusion-competent vaccine" by fixing the fusing cells with formaldehyde. This vaccine was used to immunize mice transgenic for CD4 and CCR5 to eliminate immune responses to these host components. The resulting antisera neutralized not only the virus strain from which the envelope protein was derived, but also neutralized an impressive array of primary isolates representing multiple genetic subtypes. Not all virus strains were neutralized to equal degree. In fact one strain, the clade A isolate 92RW008, was not neutralized at all. LaCasse viewed this latter finding as a positive, since it suggested that the antibodies were not targeting cellular components that are packaged into virus particles, but virus-specific epitopes. Several controls were included in the study to further help rule out the possibility of anti-cell antibodies. Antisera pre-adsorbed with COS-env and U97-CD4-CCR5 cells retained neutralizing activity. Mice immunized with non-fused cells contained no HIV-neutralizing activity. Dr. Nunberg and colleagues have recently initiated imunizations of rhesus macaques. Analysis of the macaque sera will permit the identification of epitopes recognized by the neutralizing antibodies. Whether the antibody response elicited by this immunogen is protective against virus challenge can also be tested in this model system.
Isolates not neutralized by the mouse sera may represent a separate neutralization immunotype. The gene encoding the envelope glycoprotein derived from the totally neutralization resistant strain, 92RW008, is currently being cloned, and this gene will be used to create an additional COS-env cell line for production of another fusion-competent immunogen to determine if the resulting antisera have a different specificity than observed above.
Fusion competent vaccines
If results from these follow-up experiments are positive, the next challenge will be to exploit these findings to generate a practical vaccine against HIV-1. Dr. William Olson and colleagues at Progenics Pharmaceuticals have expressed and purified HIV envelope proteins and the cellular receptors. Their goal is to evaluate the immuogenicity of a purified CD4-gp120-CCR5/CXCR4 complex. http://www.niaid.nih.gov/daids/vaccine/meetings/envelope.htm#M
In addition, Dr. Meg Trahey in Nunberg's laboratory is attempting to purify HIV envelope protein-CD4-CCR5 complexes directly from fused and fixed cells to evaluate the immunogenicity of these complexes in mice. http://www.niaid.nih.gov/daids/vaccine/meetings/envelope.htm#E. Positive results from any of these or other related experiments in the coming months would justify the renewed optimism that induction of broadly neutralizing antisera is an attainable goal.
References LaCasse RA; Follis KE; Trahey M; Scarbouough JD; Littman DR; Nunberg JH. 1999. Fusion-competent vaccines: Broad neutralization of primary isolates of HIV. Science 283: 357-362.
Kang C-Y; Hariharan K; Nara PL; Sodroski J; Moore JP. 1994. Immunization with a soluble CD4-gp120 complex preferentially induces neutralizing anti-human immunodeficiency virus type 1 antibodies directed to conformation-dependent epitopes of gp120. J Virol 68 (9):5854-5862.
DeVico A; Silver A; Thornton AM; Sarngadharan MG; Pal R. 1996. Covalently crosslinked complexes of human immunodeficiency virus type 1 (HIV-1) gp120 and CD4 receptor elicit a neutralizing immune response that includes antibodies selective for primary virus isolates. Virology 218 (1): 258-263.