1. Department of Immunology, Duke University School of Medicine, Durham, NC 27710, USA.
2. Amgen Research, Amgen Inc., South San Francisco, CA 94080, USA.
3. Department of Pediatrics, Duke University Medical Center, Durham, NC 27710, USA.
4. Department of Pharmacology and Cancer Biology, Duke University School of Medicine, Durham, NC 27710, USA.
5. Department of Molecular Genetics and Microbiology, Duke University School of Medicine, Durham, NC 27710, USA.