1. Department of Experimental Oncology, European Institute of Oncology, 20141 Milan, 1 and
2. Istituto di Medicina Interna e Scienze Oncologiche, Università degli Studi di Perugia, Policlinico Monteluce, 06100 Perugia, 2 Italy, and
3. Microbiology and Tumor Biology Center, Karolinska Institute, S-171 77 Stockholm, Sweden3