1. Department of Microbiology, Medical School, University of Minnesota, Minneapolis, MN 55455, USA.
2. Department of Veterinary and Biomedical Sciences, University of Minnesota, St. Paul, MN55108, USA.
3. Emory Vaccine Center, Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, GA 30322, USA.
4. Immunobiology Section, National Institute of Allergy and Infectious Diseases, Bethesda, MD 20892, USA.
5. Division of Biostatistics, School of Public Health, University of Minnesota, Minneapolis, MN 55455, USA.