Background:  We have recently been developing replication-defective adenovirus type 5 (MRKAd5)-based vaccines that express reasonably conserved viral antigens (gag, pol, nef) of the B clade in several phase I studies. It is important to assess the potential of this vaccine to elicit potent cytotoxic activity as well as to provide broad coverage against viruses both intraclade and interclade.

Methods:  Peripheral mononuclear cells (PBMC) were obtained from subjects in 3 phase I studies, each evaluating a different MRKAd5-based vaccine. MRKAd5 HIV-1 gag and the trivalent MRKAd5 gag/pol/nef vaccines were independently tested in healthy adults at low risk of HIV infection (18 to 50 years of age). HIV-specific T-cell responses were evaluated by IFN-γ ELISpot and intracellular cytokine staining (ICS) against full antigen peptide pools. The responses were also dissected by testing PBMC against series of smaller pools of the gag, pol, or nef peptides. PBMC from responders were also analyzed for cross-reactivity against peptide pools representing near-consensus sequences of clade A and C antigens.

Results:  The MRKA5 vaccines elicited predominantly antigen-specific CD8⁺ T cells; a smaller fraction (20 to 30%) of patients with positive ICS responses contained detectable levels of virus-specific helper T cells. Cross-clade gag- and nef-specific responses were evaluated using the ELIspot method; cross-clade responses against pol were not evaluated because of its high protein sequence conservation (90% across clades vs 80% for gag and 70% for nef). About 66% of individuals (73 of 110 receiving either the MRKAd5 gag or MRKAd5 trivalent vaccine) with responses to the vaccine CAM1 HIV-1 gag (clade B) peptide pool cross-reacted with either clade A gag or clade C gag sequences. Of those receiving the MRKAd5 trivalent vaccine who had responses against JRFL HIV-1 nef (clade B), 38% (10 of 26) were able to respond to the clade A nef pool, while 19% (5 of 26) cross-reacted with clade C nef. PBMC were also assayed against a series of smaller pools (mini-pools) each consisting of 8 sequence-consecutive 9-aa peptides from gag, pol, and nef. The median numbers of positive mini-pools for clade B gag, pol, and nef were 2, 2, and 1, respectively

Conclusions:  The MRKAd5 vaccines are potent in eliciting CD8⁺ and to a lesser degree CD4⁺ T cells in human clinical trials. The vaccine antigens exhibited levels of immune coverage across multiple clades although at varying levels depending on the antigen.

Keywords: HIV ; Vaccine; Cellular Immunity