1. Schering-Plough Laboratory for Immunological Research, 69571 Dardilly, France
2. Department of Experimental Oncology, Istituto Nazionale per lo Studio e la Cura dei Tumori, 20133 Milano, Italy
3. Institut National de la Sante et de la Recherche Medicale U271, 69424 Lyon, France
4. DNAX Research Institute, Palo Alto, CA 94304