1. Department of Microbiology, Harvard Medical School, Boston, MA 02115, USA.
2. Department of Cancer Immunology and Virology, Dana-Farber Cancer Institute, Boston, MA 02115, USA.
3. Department of Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel.
4. Harvard Center for Cryo-Electron Microscopy, Harvard Medical School, Boston, MA 02115, USA.
5. Parker Institute for Cancer Immunotherapy, Dana-Farber Cancer Institute, Boston, MA 02115, USA.