title: Live attenuated HIV vaccines: pitfalls and prospects.
abstract: OBJECTIVE
  When simian immunodeficiency virus (SIV) deleted in the nef gene caused no disease in macaques and provided protection against wild-type SIV challenge, hopes were high that the removal of nef would convert a pathogenic immunodeficiency virus into a live attenuated vaccine. We seek to highlight recent studies focused on several major issues regarding live attenuated AIDS viruses as vaccine candidates: (1). safety, (2). efficacy, (3). the correlates of immune protection, and (4) the molecular determinants for lentiviral virulence or attenuation.
  
  
  RESULTS
  Nef-deletion mutants have retained virulence; compared with wild-type SIV, disease progression was slowed but not abrogated. After long-term observation, all adult macaques given SIVmac239delta3 exhibited immune dysfunction; over 50% had T-cell depletion, and 18% developed AIDS. Vaccine efficacy has been disappointing, with limited or no cross-protection and no protection against homologous virus challenge years after initial vaccination. To date, the correlates of protective immunity have defied precise definition; no dominant mechanism has yet emerged. Data from passive serum transfer and CD8+ T-cell depletion studies have raised the possibility that alternate mechanism of protection may be operative. Due to relentless viral replication and continuous selective pressure, initially benign viruses can generate virulent progeny with unpredictable genotypes.
  
  
  CONCLUSIONS
  Neither safety nor efficacy of the current live attenuated primate immunodeficiency virus vaccines has withstood the test of time. However, such viruses are invaluable tools to address two key questions: (1). what are the correlates of protection, and (2). what are the molecular determinants of viral immunopathogenesis?
authors.affiliation: Department of Cancer Immunology and AIDS, Dana-Farber Cancer Institute, Boston, Massachusetts 02115, USA.
authors.forename: James B
authors.initials: JB
authors.lastname: Whitney
authors.forename: Ruth M
authors.initials: RM
authors.lastname: Ruprecht
mesh_terms: D016915:AIDS Vaccines; D000818:Animals; D015754:Genes, nef; D015658:HIV Infections / Q000517:prevention & control*; D015497:HIV-1 / Q000235:genetics / Q000276:immunology*; D006801:Humans; D008251:Macaca; D016097:Simian Acquired Immunodeficiency Syndrome / Q000821:virology; D015302:Simian Immunodeficiency Virus / Q000235:genetics / Q000276:immunology; D014613:Vaccines, Attenuated
chemical_list: D016915:AIDS Vaccines; D014613:Vaccines, Attenuated