1. Research group of Cellular and Molecular Immunology, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium, 2 Department of Structural Biology, VIB, B-1050 Brussels, Belgium and 3 Research group of Microbiology, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium