1. Institute for Medical Microbiology, Immunology, and Parasitology, University of Bonn, 53105 Bonn, Germany.
2. Department of Microbiology and Immunology, Indiana University School of Medicine, Indianapolis, Indiana 46202.
3. Laboratory of Haematology and Immunology, University Centre Hospital (CHU), Yaoundé, Cameroon.
4. Institute of Virology, University of Cologne, 50935 Köln, Germany.