1. Center for Virology and Vaccine Research, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA.
2. Harvard Medical School, Boston, MA 02115, USA.
3. Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA 01536, USA.
4. Wisconsin National Primate Research Center and University of Wisconsin, Madison, WI 53711, USA.
5. BIOQUAL, Rockville, MD 20852, USA.
6. Ragon Institute of MGH, MIT, and Harvard, Cambridge, MA 02139, USA.