1. Department of Molecular Microbiology and Immunology, Johns Hopkins University School of Hygiene and Public Health, Baltimore, Maryland 21205,1
2. Lady Davis Institute for Medical Research and McGill AIDS Centre, Jewish General Hospital,2 and
3. Basic Research Laboratory, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892,3 and
4. Departments of Medicine and Immunology and Microbiology, McGill University,4 Montreal, Quebec, Canada H3T 1E2