1. Present address: Department of Microbiology and Immunology, Stanford University Medical School, Fairchild Building, D305, Stanford, CA 94305,USA
2. The Department of Medical Microbiology and Immunology, University of Wisconsin-Madison Medical School, Madison, WI 53706, USA
3. Present address: StemCo Biomedical Inc., 2810 Meridian Parkway, Suite 148,Durham, NC 27713, USA