1. Microbiology and Tumor Biology Center, Nobels väg 16, Box 280, Karolinska Institutet, SE-171 77 Stockholm, Sweden. Department of Medical Biochemistry and Biophysics, Department of Cell and Molecular Biology, Department of Neuroscience, Karolinska Institutet, SE-171 77 Stockholm, Sweden. Swedish Institute for Infectious Disease Control, SE-171 82 Solna, Sweden. Molecular Medicine Program, Mayo Clinic, Guggenheim 1838, 200 First Street SW, Rochester, MN 55905, USA.