1. Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, U.S.A.
2. Graduate Group in Immunology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, U.S.A.
3. Department of Pharmacology, University of North Carolina, Chapel Hill, NC 27599, U.S.A.
4. Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, NC 27599, U.S.A.