Institute for Immunity, Transplantation and Infection, Stanford University School of Medicine, Stanford University, Stanford, CA, USA. bpulend@stanford.edu. Department of Pathology, Stanford University School of Medicine, Stanford University, Stanford, CA, USA. bpulend@stanford.edu. Department of Microbiology & Immunology, Stanford University School of Medicine, Stanford University, Stanford, CA, USA. bpulend@stanford.edu. Chemistry, Engineering & Medicine for Human Health, Stanford University School of Medicine, Stanford University, Stanford, CA, USA. bpulend@stanford.edu. Institute for Immunity, Transplantation and Infection, Stanford University School of Medicine, Stanford University, Stanford, CA, USA. GSK Vaccines, Rockville, MD, USA.