Both CD8⁺ T cells and IFNγ are necessary for the development of an HLH-like disorder in LCMV-infected pfp^–/– mice. (A) Pfp^–/– mice were treated with control, CD4, CD8, or NK cell–depleting antibodies starting 6 days after LCMV infection and were monitored for survival. RAG/pfp^–/– mice were infected with LCMV and monitored for survival. (B) Pfp^–/– mice were infected with LCMV and given cytokine-neutralizing antibodies (against TNF-α, IFNγ, M-CSF, GM-CSF, IL-10, IL-12, and IL-18) starting on day 6 and were monitored for survival. (C) Serum IFNγ levels were assayed on day 12 from wild-type, control antibody–treated pfp^–/– mice, and pfp^–/– mice that had received anti-CD8 antibody. Data (± standard error) are representative of at least 2 experiments with at least 3 mice in each group.