1. Institute for Molecular and Cellular Biology, Osaka University, 1-3 Yamadaoka, Suita-shi, Osaka 565, Japan.
2. Department of Immunology, The Tokyo Metropolitan Institute of Medical Science, Bunkyo-ku, Tokyo 113, Japan.
3. Department of Biology, Kyushu University Faculty of Science, Higashiku, Fukuoka 812, Japan.