1. Institute of Molecular Biology I, University of Zürich, Hönggerberg, 8093 Zürich, Switzerland.
2. Institute of Experimental Immunology, University of Zürich, 8057 Zürich, Switzerland.
3. Department of Pharmaceutical Research New Technologies, F. Hoffmann-La Roche AG, 4002 Basel, Switzerland.
4. Department of Microbiology, New York University Medical Center, New York, NY 10016.