1. Baylor Institute for Immunology Research, Dallas, Texas 75205
2. Department of Medical Microbiology, Dartmouth Hitchcock Medical Center, Lebanon, New Hampshire 03756
3. Montana State University Veterinary Molecular Biology, Bozeman, Montana 59717
4. Université Joseph Fourier, 38041 Grenoble cedex 9, France
5. and Tulane University Medical School, New Orleans, Louisiana 70112