1. Department of Microbiology and Immunology, Department of Biochemistry and Biophysics, and Lineberger Comprehensive Cancer Center, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7290, and Institut Pasteur, Unite de Biologie Moleculaire du Gene, Departement d'Immunologie, 25 rue du Dr Roux, 75724, Paris Cedex 15, France