1. Department of Microbiology and Immunology, Stanford University School of Medicine; Stanford CA 94305 USA
2. Present address: Los Alamos National Laboratory, Health Research Laboratory (HRL 1), MS 888, LS-5; Los Alamos NM 87545 USA
3. Departments of Pathology and Microbiology-Immunology, Northwestern University; 303 E Chicago Avenue Chicago IL 60611 USA
4. Life Science Division, Lawrence Berkeley National Laboratory; 1 Cyclotron Road Berkeley CA 94720 USA