[1] CNR,IST MED SPERIMENTALE,VIA CARLO MARX 15,I-00137 ROME,ITALY
[2] UNIV ROME LA SAPIENZA,INST INFECT DIS,I-00185 ROME,ITALY
[3] UNIV ROME LA SAPIENZA,IST SUPER SANITA,I-00185 ROME,ITALY
[4] UNIV ANCONA,INST MICROBIOL,I-60100 ANCONA,ITALY
[5] INST PASTEUR,MICROBIAL IMMUNOL UNIT,F-75724 PARIS 15,FRANCE