1. Duke Human Vaccine Institute, Durham, NC 27710, USA.
2. Department of Medicine, Duke University, Durham, NC 27710, USA.
3. Department of Surgery, Duke University, Durham, NC 27710, USA.
4. Department of Molecular Genetics and Microbiology, Duke University, Durham, NC 27710, USA.
5. Department of Immunology, Duke University, Durham, NC 27710, USA.
6. Theoretical Biology and Biophysics, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
7. Department of Biochemistry, Duke University, Durham, NC 27710, USA.