[1] Gladstone Inst Virol & Immunol, San Francisco, CA 94141 USA
[2] Rockefeller Univ, Aaron Diamond AIDS Res Ctr, New York, NY 10016 USA
[3] Inst Mol Med, MRC, Human Immunol Unit, Oxford OX3 9DS, England
[4] NIH, Vaccine Res Ctr, Bethesda, MD 20892 USA
[5] Univ Calif San Francisco, Dept Med, San Francisco, CA 94143 USA
[6] Univ Calif San Francisco, Dept Microbiol & Immunol, San Francisco, CA 94143 USA