1. From the Department of Immunology, University of Toronto, Toronto, Ontario M5S 1A8, Canada; The Wellesley Hospital Research Institute, Toronto, Ontario M4Y 1J3, Canada; Department of Molecular Cell Biology, Research Institute for Microbial Diseases, Osaka University, Yamada-Oka 3-1, Suita, Osaka 565, Japan; and the Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, N