Next, we assessed the relative functional avidity of CTL populations primed by native S598 and S598 Q600Y determinants, as a surrogate measure of the potency of the anti-virus CTL response in vivo. CNS-derived mononuclear cells were harvested from mice infected with rJ or rJ.S Q600Y and examined for IFN-c expression after stimulation ex vivo in the presence of 10-fold dilutions of the appropriate peptide. Cells primed to the native S598 epitope (cells harvested from rJ-infected mice) required approximately 100-fold more peptide than did S598 Q600Y -primed cells to elicit a half maximal response (100 nM vs. 1 nM, Figure 3A ,B).