1. Department of Microbiology and Immunology, University of Texas Medical Branch at Galveston, Galveston, Texas 77555-0610
2. Present address: Department of Pharmacology, University of Texas Medical Branch at Galveston, Galveston, TX 77555-1059.
3. Present address: Department of Medicine, Center for Translational Immunology, and Lineberger Comprehensive Cancer Research Center, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7295.