[1] Stanford Univ, Sch Med, Dept Microbiol & Immunol, Stanford, CA 94305 USA
[2] Scripps Res Inst, Dept Immunol, La Jolla, CA 92037 USA
[3] Univ Texas, Sect Mol Genet & Microbiol, Austin, TX 78712 USA
[4] Univ Texas, Inst Mol & Cellular Biol, Austin, TX 78712 USA
[5] Michigan State Univ, Dept Microbiol, E Lansing, MI 48824 USA