1. Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
2. Division of Infectious Diseases, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
3. Department of Epidemiology and Preventive Medicine, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
4. Research Service, Department of Veterans Affairs, Baltimore, MD 21201, USA.