1. Department of Pathology and Laboratory Medicine, University of Pennsylvania , 3450 Hamilton Walk Philadelphia, PA 19104 , USA
2. Department of Microbiology, University of Pennsylvania , 3450 Hamilton Walk Philadelphia, PA 19104 , USA
3. Department of Biochemistry and Biophysics, University of Pennsylvania , 3620 Hamilton Walk Philadelphia, PA 19104 , USA
4. Division of Viral Diseases, Centers for Disease Control and Prevention , 1600 Clifton Rd. Atlanta, GA 30329 , USA
5. Division of Allergy and Immunology, Children's Hospital of Philadelphia , 3615 Civic Center Blvd., Philadelphia, PA 19104 , USA