1. Department of Medicine, Washington University School of Medicine, St. Louis, MO 63110, USA.
2. Department of Anatomy & Neurobiology, Washington University School of Medicine, St. Louis, MO 63110, USA.
3. Department of Immunology, University of Washington School of Medicine, Seattle, WA 98195, USA.
4. ZymoGenetics, a Bristol-Myers Squibb Company, Seattle, WA 98102, USA.
5. Department of Pathology & Immunology, Washington University School of Medicine, St. Louis, MO 63110, USA.
6. Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, MO 63110, USA.