1. Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
2. U.S. Military HIV Research Program, Walter Reed Army Institute of Research, Silver Spring, MD 20910, USA.
3. The Research Institute at Nationwide Children’s Hospital and Department of Pediatrics, The Ohio State University College of Medicine, Columbus, OH 43205, USA.
4. Department of Biochemistry, Geisel School of Medicine at Dartmouth, Hanover, NH 03755, USA.