Vaccine protection from CD4 + T-cell loss caused by simian immunodeficiency virus (SIV) mac251 is afforded by sequential immunization with three unrelated vaccine vectors encoding multiple SIV antigens Gerrit Koopman, 1 Daniella Mortier, 1 Sam Hofman, 1 Henk Niphuis, 1 Zahra Fagrouch, 1 Steve Norley, 2 Gerd Sutter, 3 3 Peter Liljestro ¨m 4 and Jonathan L. Heeney 1 Correspondence Gerrit Koopman koopman@bprc.nl 1 Department of Virology, Biomedical Primate Research Centre, Lange Kleiweg 139, 2288 GJ Rijswijk, the Netherlands 2 Robert Koch Institut, Nordufer 20, D-13353 Berlin, Germany 3 GSF-Institut fu ¨ r Molekulare Virologie, Trogerstrasse 4b, 81675 Mu ¨ nchen, Germany 4 Karolinska Institute, PO Box 280, SE-171 77 Solna, Sweden Received 26 April 2004 Accepted 23 June 2004 Candidate human immunodeficiency virus (HIV) vaccine strategies that induce strong cellular immune responses protect rhesus macaques that are infected with recombinant simian/human immunodeficiency virus SHIV89.6p from acute CD4 + T-cell loss and delay progression to AIDS. However, similar strategies have not proven as efficacious in the simian immunodeficiency virus (SIV)mac model of AIDS, an infection that causes a slow, steady loss of CD4 + T-cell function and numbers in rhesus macaques similar to that caused by HIV-1, the principal cause of AIDS in humans. Efforts to increase vaccine efficacy by repeated boosting with the same vector are quickly limited by rising anti-vector immune responses. Here, the sequential use of three different vectors (DNA, Semliki Forest virus and modified vaccinia virus Ankara) encoding the same SIVmac structural and regulatory antigens was investigated and demonstrated to prevent or slow the loss of CD4 + T-cells after mucosal challenge with the highly pathogenic SIVmac251 strain. Of particular interest was an inverse association between the extent of T-helper 2 cytokine responses and steady-state virus load. Although limited in the number of animals, this study provides important proof of the efficacy of the triple-vector vaccine strategy against chronic, progressive CD4 + T-cell loss in the rigorous SIVmac/rhesus macaque model of AIDS. INTRODUCTION To combat the rapidly spreading AIDS epidemic, a safe and effective prophylactic human immunodeficiency virus (HIV) vaccine is urgently needed (UNAIDS, 2003). Despite many efforts, an ideal vaccine candidate has not yet emerged. Infection of rhesus macaques with simian immunodeficiency virus (SIV) or, more recently, recom- binant HIV/SIV (SHIV) strains has been used as a model for AIDS and has provided important information for vaccine development. A number of evolving HIV vaccine candidates are based on a two-vector prime/boost approach. Although unable to protect from infection, these have been demonstrated to protect from acute CD4 + T-cell loss in the SHIV89.6p model (Robinson et al., 1999; Amara et al., 2001; Barouch et al., 2001; Rose et al., 2001; Shiver et al., 2002). Prophylactic vaccine efficacy in the SHIV pre- clinical model has been correlated with either neutralizing antibodies or strong CD8 + T-cell responses and, more recently, results have suggested the growing importance of T-helper (Th) cell responses in protection in this model (Jin et al., 1999; Schmitz et al., 1999; Verschoor et al., 1999; Mascola et al., 2000; Nehete et al., 2001; Heeney, 2002). In rhesus macaques, SIVmac251 infection more closely mimics the chronic, progressive T-cell loss that is observed in HIV-1-infected humans, suggesting that other immune mechanisms may be necessary to protect from pathogenic pathways that lead to chronic CD4 + T-cell loss. When evaluated in this more rigorous SIVmac/ rhesus macaque model, the same or similar vaccine strategies have in general not been efficacious (Yasutomi et al., 1995; Nilsson et al., 2001; Allen et al., 2002; Horton 3Present address: Paul-Ehrlich-Institute, Langen, Germany. 0008-0226 G 2004 SGM Printed in Great Britain 2915 Journal of General Virology (2004), 85, 2915–2924 DOI 10.1099/vir.0.80226-0