The T Cell Receptor -Chain Second Complementarity Determining Region Loop (CDR2 ) Governs T Cell Activation and V Specificity by Bacterial Superantigens*

A. K. M. Nur-ur Rahman, Daniel A. Bonsor, Christine A. Herfst , Fraser Pollard, Michael Peirce, Aaron W. Wyatt, Katherine J. Kasper, Joaquín Madrenas**, Eric J. Sundberg, and John K. McCormick 2 From the Department of Microbiology and Immunology and the Centre for Human Immunology, The University of Western Ontario, London, Ontario N6A 5C1, Canada, the Boston Biomedical Research Institute, Watertown, Massachusetts 02472, the Lawson Health Research Institute, St. Joseph’s Health Care, London, Ontario N6A 4V2, Canada, and **Robarts Research Institute, The University of Western Ontario, London, Ontario N6A 5K8, Canada