The number of reactivating splenocytes was determined as described previously [34] . Briefly, 21,000 NIH 3T3 cells were plated per well in a 24-well plate the day before infection. 2.5610 6 , 5610 5 , or 1610 5 splenocytes were overlayed onto the NIH 3T3 cells and incubated for 4 days. Cells were then fixed with 10% formaldehyde and stained with 0.04% methylene blue to visualize plaques.