1 Department of Immunology, Max Planck Institute for Infection Biology, Charitéplatz 1, 10117 Berlin, Germany. Electronic address: kaufmann@mpiib-berlin.mpg.de.
2 London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, England.
3 Department of Vaccinology and Applied Microbiology, Helmholtz-Zentrum für Infektionsforschung, Inhoffenstraße 7, 38124 Braunschweig, Germany.
4 Department of Infectious Diseases, Leiden University Medical Center, Leiden, The Netherlands.