As early as day four or five post infection, CD8 + T cells are present in the lung tissue and exhibit several effector functions, including the ability to secrete antiviral cytokines [interferon-γ (IFN-γ) and tumor necrosis factor α (TNFα)] and initiate cell killing of virusinfected epithelial cell through Fas, TRAIL or perforin-mediated lysis (14) .