1. J. A. Baker Institute, Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853;
2. Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin 53706;
3. Institute of Medical Sciences, University of Tokyo, Tokyo 108-8639, Japan
4. CREST, Japan Science and Technology Corporation, Saitama, 332-0012, Japan