This is publication no. 3266 from the Department of Immunology, Scripps Clinic and Research Foundation, La Jolla, California 92037. This work was supported by National Multiple Sclerosis Society Research grant nos. 1285, 1287 and 1307, United States Public Health Service grant nos. NS-12428 and NS-14069, National Institutes of Health grant no. RR00833 to the General Clinical Research Center at Scripps Clinic and Research Foundation, and grants from the Leiper Trust and the McDonald and Hearst Foundations. Robert L. Knobler was the Ralph I. Straus Fellow of the National Multiple Sclerosis Society during this study and currently is a recipient of Teacher Investigator Development Award no. NS-00803, from the National Institute of Neurological and Communicative Disorders and Stroke. George P. A. Rice is a recipient of the Centennial Fellowship from the Medical Research Council of Canada. Gordon S. Francis was the recipient of a Medical Research Council of Canada Fellowship.