1. Max Planck Institute for Infection Biology, Department of Immunology; Charitéplatz 1 10117 Berlin Germany
2. Department of Biology; Institute of Molecular Systems Biology, ETH Zurich; Wolfgang-Pauli Strasse 16 8093 Zurich Switzerland
3. Faculty of Science; University of Zurich; Winterthurerstrasse 190 8057 Zurich Switzerland