Background:  Several lines of evidence indicate that CD4⁺ T cells contribute to an efficient immune control of HIV replication. However, most current candidate vaccines focus on the induction of CD8⁺ T cells, while inducing only modest CD4⁺ T cell responses. We present final results of the first prospective randomized observer-blind clinical trial of a recombinant purified fusion protein (F4) comprised of the 4 HIV antigens p17 and p24 Gag, reverse transcriptase, and Nef, derived from clade B strains and formulated with GSK Biologicals proprietary Adjuvant System AS01.

Methods:  We vaccinated 180 subjects with escalating doses of 10, 30, or 90 mg F4 adjuvanted or not with AS01. Local and systemic reactogenicity as well as serious adverse events were evaluated. Peripheral blood mononuclear cells were collected before and after immunization and analyzed by flow cytometry for expression of CD40L and production of interleukin-2 (IL-2), interferon-g (IFN-g), and tumor necrosis factor-a (TNF-a) using peptide pools from clade A, B, and C HIV strains.  Continuous measurements and percentages of responders were summarized per study group for each relevant time point. Analyses of variance were performed for each antigen to compare the 3 doses of F4 with or without adjuvant two weeks after the second vaccination.

Results:  There were no vaccine related serious adverse events. Reactogenicity was not antigen dose level related but was higher after the second than first dose of the F4/AS01 vaccine. Elevated frequencies of persistent CD4⁺ T cells were detected in all F4/AS01 vaccinated volunteers. The highest frequencies and responder rates (100% to 3 HIV antigens and 80% to all 4, indicating broad reactivity) were observed in the 10-mg F4/AS01 group. In this group, the geometric mean frequency of F4-specific CD4⁺ T cells reached 1.2 %. The majority of specific CD4⁺ T cells expressed CD40L and produced IL-2 alone or in combination with TNF-a or IFN-g, indicating the polyfunctional properties of these vaccine-induced specific CD4⁺ T cells. In addition, these responses persisted up to the end of the study (month 12).

Conclusions:  The reactogenicity profile of the vaccine was acceptable and there were no safety concerns. The vaccine elicited high levels of HIV-specific persistent polyfunctional CD4⁺ T cells. Given the importance of a strong CD4⁺-mediated immune response, the vaccine candidate will be further evaluated for safety, immunogenicity, and efficacy in HIV-infected subjects.