1. J. Alferink, A. Tafuri, G. J. Hämmerling, B. Arnold, Tumor Immunology Program, German Cancer Research Center, 69120 Heidelberg, Germany. D. Vestweber, Institute of Cell Biology, University of Münster, 48149 Münster, Germany. R. Hallmann, Institute for Experimental Medicine, University of Erlangen-Nürnberg, 91054 Erlangen, Germany.