Prf^−/− mice display heightened immune activation independently of virus control. (A) Serum IFN-γ levels were measured in WT and prf^−/− mice infected with the persistent strain of LCMV, clone 13 (LCMV-Cl13), at the indicated time points. Serum IFN-γ levels at days 6, 8, and 10 were significantly elevated (P < .01) in prf^−/− mice compared with WT. (B) Plasma viral levels were measured by plaque assay at the indicated time points in LCMV-Cl13–infected mice. Dashed line indicates limit of detection of the assay. *P < .01.