Incomplete Protection, butSuppression ofVirusBurden, Elicited bySubunit Simian Immunodeficiency VirusVaccines

andanti-Env (gp120/gpl60) antibodies inallvaccinated macaques andanti-Gag (p27) antibodies ingroupsimmunizedwith WIV or Gag-Env. OnlyWIV-immunized macaques developed anticell (HuT78)antibodies. Afterhomologous low-dose intravenous viruschallenge, we usedfrequency ofvirusisolation, provirus burden, andchangein antibody titers todefine fourlevels ofresistance toSIVinfection as follows. (i)No infection ("sterilizing" immunity) was induced onlyinWIV-immunized animals. (ii) Abortive infection (strong immunity) was defined whenvirus or provirus were detected early inthepostchallenge period butnotthereafter and no evidence of virus orprovirus was detected interminal tissues. Thisresponsewas observed intwoanimals(oneW-Envand one Gag-Env). (iii) Suppression ofinfection (incomplete or partial immunity) described a gradient ofvirus suppression manifested bytermination ofviremia, declining postchallenge antibody titers, and lowlevels (composite mean = 9.1copies per106cells) ofprovirus detectable inperipheral bloodmononuclearcells or lymphoid tissues attermination (40weekspostchallenge). Thisresponseoccurred inthemajority (8of12)of subunit-vaccinated animals. (iv) Active infection (noimmunity) was characterized bypersistent virusisolation frombloodmononuclear cells, increasing viral antibody titers postchallenge, andhighlevels (composite mean = 198copies per106cells) ofprovirus interminal tissues andblood. Activeinfection developed inallcontrols andtwoofthreeW-Gag-Env-immunized animals. Theresults ofthisstudyrestate theprotective effect of inactivated wholevirusvaccines produced inheterologous cells butmore importantly demonstrate thata gradient ofsuppression ofchallenge virusgrowth, reflecting partial resistance toSIVinfection, isinducedby subunitvaccination. The latter finding may bepertinent tostudies withhuman immunodeficiency virus vaccines, inwhichitisplausible thatvaccination may elicit significant suppression ofvirusinfection and pathogenicity ratherthansterilizing immunity.