1. Immunology Laboratory, Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892
2. Center for Infectious Medicine, Department of Medicine, Karolinska Institutet, S-141 86 Stockholm, Sweden
3. BSL-3 Core Virology Laboratory, Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892