Resistance of chimpanzees immunized with recombinant gp120_SF2 to challenge by HIV-1_SF2

Objective: To determine whether vaccination with recombinant HIV-1_SF2 gp120 in a novel oil-in-water adjuvant emulsion, MF59, protects chimpanzees against challenge with HIV-1_SF2, the homologous virus isolate. Methods: Two vaccinated chimpanzees and two control animals were challenged with 25-50 animal infectious doses of a stock of HIV-1_SF2 that had been grown in mitogen-activated human peripheral blood mononuclear cells (PBMC). The animals were monitored by a series of serologic [enzyme-linked immunosorbent assay (ELISA), Western blot, and neutralization assays] and virologic [virus culture, RNA and DNA polymerase chain reaction (PCR)] assays for infection. Results: Both control animals showed evidence of seroconversion in ELISA and Western blot assays. In addition, virus was detected in the early, acute phase of infection of both control animals by (1) plasma RNA PCR, (2) virus culture, and (3) PBMC DNA PCR assays. One vaccinated animal showed no serologic or virologic evidence of infection. The other vaccinated animal has not seroconverted, and there was no evidence of plasma viremia. However, virus was detected at early timepoints in this animal's PBMC, and transient lymphoproliferation to HIV-1 proteins not in the vaccine was observed. These observations suggest that the former animal was protected from challenge while the latter may have experienced a transient or curtailed infection. Conclusion: Two types of vaccine-induced protective immune responses were observed when chimpanzees immunized with rgp120_SF2 were challenged with the homologous virus isolate: a response consistent with the 'sterilizing immune response' documented in the chimpanzee model in previous studies, as well as one that did not completely protect from infection, showing curtailment of the acute phase and a failure of the animal to seroconvert.