[1] Georgetown Univ, Med Ctr, Dept Neurosurg, Washington, DC 20007 USA
[2] Georgetown Univ, Med Ctr, Dept Immunol & Microbiol, Washington, DC 20007 USA
[3] Georgetown Univ, Med Ctr, Dept Pathol, Washington, DC 20007 USA
[4] Georgetown Univ, Med Ctr, Div Comparat Med, Washington, DC 20007 USA