Comparison of protection afforded by whole virus ISCOM versus MDP adjuvanted formalin-inactivated SIV vaccines from IV cell-free or cell-associated homologous challenge.

A SIV-ISCOM and a SIV-MDP adjuvanted vaccine were tested for their potential to induce protection from intravenous cell-free or cell-associated homologous SIV challenge in rhesus monkeys (Macaca mulatta). Seven monkeys vaccinated four times over a four-month period with either the SIV-ISCOM or the SIV-MDP vaccine were challenged intravenously with approximately 10 MID50 cell-free SIVmac251 (32H). They all were protected from developing viremia during a three-month observation period. Two other groups of four monkeys were vaccinated essentially in the same way with either of these vaccines. They were challenged intravenously with approximately 10 MID50 of infected PBMC of a rhesus monkey that had been infected with SIVmac251 (32H) 11 months earlier (stock prepared by J. Heeney). Two monkeys of each of these two groups proved to be protected from developing viremia during a two-month observation period. For both the cell-free and the cell-associated SIV challenge, monkeys vaccinated with measles virus ISCOMS or MDP adjuvanted measles virus antigen, served as controls. They all became viremic within two weeks after SIV challenge. This is the first demonstration that vaccinated previously unchallenged nonhuman primates can be protected from infection with lentivirus-infected PBMC from another animal. Serological analysis indicated that SIV-specific serum antibody titers were considerably higher in SIV-ISCOM vaccinated animals than in the SIV-MDP vaccinated animals. The serology also confirmed the protection data, by showing the absence of increase in SIV-specific serum antibodies in apparently protected animals after challenge.