1. Sealy Center for Vaccine Development, University of Texas Medical Branch, Galveston TX 77555, USA
2. Department of Pathology, University of Texas Medical Branch, Galveston, TX 77555, USA
3. Present address: The Binding Site, Inc., San Diego, CA, USA.
4. Present address: Viral Diseases Branch, Walter Reed Army Institute of Research, Silver Spring, MD, USA.
5. Department of Microbiology, Immunology and Pathology, Colorado State University, Fort Collins, CO, 80523, USA