RAW 264.7 cells were maintained as previously described [28] . Monoclonal antibodies (MAbs) specific to CD4 (YTS191.1 [61] ), CD8 (H35 [62] ) and SFR3-DR5 (ATCC HB-151 [63] ) were produced from hybridoma cell lines in INTEGRA Celline CL1000 flasks (Integra Biosciences, Ijamsville, MD) using CD Hybridoma media (Gibco, Carlsbad, CA) as previously described [64] . All mice (or cage sentinel mice for mice deficient in antibody production) were tested by ELISA for the presence of MNV antibody prior to experiments [27] . All mice used in these studies were seronegative at the initiation of experiments.