1. The Immune Disease Institute and Program in Molecular and Cellular Medicine, Children's Hospital and Harvard Medical School, Boston, Massachusetts 02115;
2. Department of Medical Microbiology and Immunology, Aarhus University Graduate School of Health Sciences and the Danish Graduate School of Immunology, Aarhus University, DK-8000 Aarhus C, Denmark
3. Graduate Program in Immunology, Harvard University, Cambridge, Massachusetts 02138
4. Graduate Program of Life Sciences, Utrecht University, 3508 TC Utrecht, The Netherlands