1. Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, California 94305-5124
2. Department of Pathology, Stanford University School of Medicine, Stanford, California 94305-5324
3. UMR 5539 CNRS, Université de Montpellier 2, CP 107, Place Eugène Bataillon, 34090 Montpellier, France