1. M. Sarzotti, Research Service, Veterans Affairs Medical Center, 10 North Greene Street, Baltimore, MD 21201, USA, and Department of Microbiology and Immunology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
2. D. S. Robbins, Research Service, Veterans Affairs Medical Center, 10 North Greene Street, Baltimore, MD 21201, USA, and Department of Pathology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
3. P. M. Hoffman, Research Service, Veterans Affairs Medical Center, 10 North Greene Street, Baltimore, MD 21201, USA, and Department of Neurology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.