1. Laboratory of Host Defense & Germfree Life, Research Institute for Disease Mechanism and Control, Nagoya University School of Medicine, Nagoya 466, Japan
2. Laboratory of Transgenic Animals, Institute of Molecular Embryology and Genetics, Kumamoto University School of Medicine, Kumamoto 862, Japan
3. Ontario Cancer Institute, Departments of Medical Biophysics and Immunology, University of Toronto, Toronto, Ontario, Canada, M5G 2M9