Role of TNFRs in viral clearance during a chronic viral infection. Groups of +/+, p55−/−, p75−/−, and DKO mice were infected with LCMV-clone 13, and the levels of infectious LCMV in the liver (A), lung (B), and serum (C) were determined by a plaque assay using Vero cell monolayers. Each data point represents the viral titer of an individual mouse. ND, not available.