1. Cellular Immunology Laboratory, National Institute for Infectious Diseases “Lazzaro Spallanzani” I.R.C.C.S., Rome, Italy
2. Becton Dickinson Italia, Buccinasco (MI), Italy
3. Epidemiology Laboratory, National Institute for Infectious Diseases “Lazzaro Spallanzani” I.R.C.C.S., Rome, Italy