1. Division of Immunology, Medical Biology Institute, La Jolla, CA 92037.
2. Veterans Administration Medical Center, La Jolla, CA 92161, and Department of Pathology, University of California at San Diego, La Jolla, CA 92093.
3. Department of Biology, University of California at San Diego, La Jolla, CA 92093.
4. Department of Pediatrics, University of California at San Diego, La Jolla, CA 92093.