Quantification of viral titers and serum cytokine levels. Spleens were homogenized [14] , and viral titers were determined by plaque assay using mouse embryonic fibroblasts with centrifugal enhancement or NIH-3T3 cells [73] . Serum was collected at the indicated time points and cytokine levels were determined by ELISA for IFN-a (PBL Biomedical Laboratories, http://www.interferonsource.com/) and IL-12p70 (R&D Systems, http://www.rndsystems.com/) per the manufacturer's instructions.