1. Howard Hughes Medical Institute at Denver, Department of Medicine, National Jewish Center for Immunology and Respiratory Medicine, Denver, CO 80206.
2. Departments of Microbiology and Immunology, of Medicine, and of Biochemistry, Biophysics and Genetics, University of Colorado Health Sciences Center, Denver, CO 80262.
3. Department of Microbiology and Immunology, University of Colorado Health Sciences Center, Denver, CO 80262.