Annual and Interim Progress Report Summaries
Principal Investigator: Robert Gallo
Project: Sterilizing Protection Elicited by an HIV-1 Vaccine
Submitted April 15, 2010 (Interim Report)
Our program continues to the hypothesis that antibodies to epitopes associated with the co-receptor binding domain that is exposed on the HIV-1 envelope glycoprotein consequent to CD4 binding (CD4i epitopes) can mediate sterilizing protection against HIV-1. Two active immunization projects are underway to test this hypothesis.
In the first project, we are optimizing a parenteral DNA-prime, protein-boost protocol in rhesus macaques to elicit high titers of CD4i antibodies. We have primed rhesus macaques with DNA vaccines encoding single chain protein (rhFLSC) in which gp120 of the HIV-1Ba-L isolate is fused with the outer two domains of rhesus CD4 that selectively elicits antibody responses to CD4i epitopes. In two independent studies, rhFLSC elicits non-sterilizing protection against a rectal challenge with SHIV162P3. A key element of optimizing the DNA-prime, protein-boost strategy is the choice of adjuvant for the protein boost. We established a collaboration with investigators at the Infectious Disease Research Institute (IDRI) who provided an optimal adjuvant for our studies. We have obtained boosting with the adjuvant and developed new data that its use might supplant the need for DNA priming. If so, this would greatly simplify the immunization scheme. Studies are underway to determine the longevity of the antibody responses elicited by immunization with rhFLSC as a protein only vaccine using this adjuvant. These results will be compared with those obtained by a DNA-prime, rhFSLC protein-boost protocol to design a protection study to be carried out in the next years of our program.
In the second project, we are optimizing a mucosal immunization protocol using rhFLSC as the immunogen. Based on the unexpectedly poor mucosal immunogenicity of rhFSLC when delivered with cholera-toxin, we have altered our original approach in order to to more selectively target rhFLSC to mucosal surfaces by expressing it on the surface vesicular stomatitis virus (VSV), which is a potent mucosal vaccine vector. This alteration was approved after our annual review and the rhFSLC-VSV vaccine is under development. We anticipate having it in hand and the necessary small animal immunogenicity studies carried out as defined in our modified schedule to set the stage for a protection study to be carried out in the next years of our program.
In the third project, we are directly testing above hypothesis by passive immunization using both human and rhesus macaque monoclonal antibodies (mAbs) specific for CD4i epitopes. We have developed new methods to census memory B cells (BMem) for reactivity with specific Env epitopes and to capture the desired reactivities as mAbs. We are isolating a large panel of CD4i mAbs to obtain a detailed picture of the fine specificities that populate that anti-CD4i antibody response and, of equal importance, to determine if there is an association between specificity and effector function. We have a number of candidate mAbs in hand where there appears to be such a relationship. The goal is to develop a mAb panel for passive immunization against a mucosal challenge with SHIV162P3 in the last two years of our program.