[1] KAROLINSKA INST, DEPT IMMUNOBIOL, WALLENBERG LAB, S-10405 STOCKHOLM, SWEDEN
[2] NATL BACTERIOL LAB, DEPT VIROL, S-10521 STOCKHOLM, SWEDEN
[3] CATHOLIC UNIV LEUVEN, REGA INST MED RES, DEPT HUMAN BIOL, DIV MICROBIOL, B-3000 LOUVAIN, BELGIUM
[4] MAX VON PETTENKOFER INST, MUNICH, FED REP GER