1. Department of Immunology and Microbiology, University of Colorado School of Medicine, Aurora, CO, USA.
2. Division of Infectious Diseases, University of Colorado School of Medicine, Aurora, CO, USA.
3. Department of Microbiology and Immunology, Carver College of Medicine, The University of Iowa, Iowa City, IA, USA.
4. Veterans Administration Eastern Colorado Health Care System, Aurora, CO, USA.