[1] STANFORD UNIV, SCH MED, DEPT MICROBIOL, STANFORD, CA 94305 USA
[2] STANFORD UNIV, SCH MED, DEPT IMMUNOL, STANFORD, CA 94305 USA
[3] STANFORD UNIV, SCH MED, DEPT PATHOL, STANFORD, CA 94305 USA
[4] CTR BIOL MOLEC, DEPT VET AFFAIRS, PALO ALTO, CA USA