[1] Univ London Imperial Coll Sci & Technol, Dept Biol Sci, London SW7 2AZ, England
[2] Univ Calif Los Angeles, Howard Hughes Med Inst, Dept Physiol, Los Angeles, CA USA
[3] Univ Calif Los Angeles, Howard Hughes Med Inst, Dept Microbiol, Los Angeles, CA USA
[4] Univ Calif Los Angeles, Howard Hughes Med Inst, Dept Immunol, Los Angeles, CA USA
[5] Univ Calif Los Angeles, Howard Hughes Med Inst, Dept Mol Genet, Los Angeles, CA USA
[6] Univ London Imperial Coll Sci & Technol, Div Biomed Sci, London SW7 2AZ, England