Protection from pathogenic SIV challenge using multigenic DNA vaccines

To assess DNA immunization as a strategy for protecting against HIV infecti on in humans, we utilized SIVmne infection of Macaca fascicularis as a vacc ine challenge model with moderate pathogenic potential. We compared the eff icacy of DNA immunization alone and in combination with subunit protein boo sts. All of the structural and regulatory genes of SIVmne clone 8 were clon ed into mammalian expression vectors under the control of the CMV IE-1 prom oter. Eight M. fascicularis were immunized twice with 3 mg of plasmid DNA d ivided between two sites; intramuscular and intradermal. Four primed macaqu es received a further two DNA immunizations at weeks 16-36, while the secon d group of four were boosted with 250 mu g recombinant gp160 plus 250 mu g recombinant Gag-Pol particles formulated in MF-59 adjuvant. Half of the con trols received four immunizations of vector DNA; half received two vector D NA and two adjuvant immunizations. As expected, humoral immune responses we re stronger in the macaques receiving subunit boosts, but responses were su stained in bath groups. Significant neutralizing antibody titers to SIVmne were detected in one of the subunit-boosted animals and in none of the DNA- only animals prior to challenge. T-cell proliferative responses to gp160 an d to Gag were detected in all immunized animals after three immunizations, and these responses increased after four immunizations. Cytokine profiles i n PHA-stimulated PBMC taken on the day of challenge showed trends toward Th 1 responses in 2/4 macaques in the DNA vaccinated group and in 1/4 of the D NA plus subunit vaccinated macaques; Th2 responses in 3/4 DNA plus subunit- immunized macaques; and Th0 responses in 4/4 controls. In bulk CTL culture, SIV specific lysis was low or undetectable, even after four immunizations. However, stable SIV Gag-Pol- and env-specific T-cell clones (CD3(+) CD8(+) ) were isolated after only two DNA immunizations, and Gag-Pol- and Nef-spec ific CTL lines were isolated on the day of challenge. All animals were chal lenged at week 38 with SIVmne uncloned stock by the intrarectal route. Base d on antibody anamnestic responses (western, ELISA, and neutralizing antibo dies) and virus detection methods (co-culture of PBMC and LNMC, nested set PCR- of DNA from PBMC and LNMC, and plasma QC-PCR), there were major differ ences between the groups in the challenge outcome. Surprisingly, sustained low virus loads were observed only in the DNA group, suggesting that four i mmunizations with DNA only elicited more effective immune responses than tw o DNA primes combined with two protein boosts. Multigenic DNA vaccines such as these, bearing all structural and regulatory genes, show significant pr omise and may be a safe alternative to live-attenuated vaccines. (C) 1999 P ublished by Elsevier Science B.V. All rights reserved.