1. Howard Hughes Medical Institute and the Departments of Internal Medicine and Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Ml 48109, USA.
2. Department of Pathology, University of Michigan Medical Center, Ann Arbor, Ml 48109, USA.
3. Department of Internal Medicine, University of Michigan Medical Center, Ann Arbor, Ml 48109, USA.
4. Department of Microbiology and Immunology, University of Michigan Medical Center, Ann Arbor, Ml 48109, USA.