Cytotoxic T lymphocytes (CTLs) play a prominent role in the resolution of viral infections through their capacity both to mediate contact-dependent lysis of infected cells and to release soluble proinflammatory cytokines and chemokines. The factors controlling these antiviral effector activities in vivo at infection sites are ill defined. Using a mouse model of influenza infection, we observed that the expression of CTL effector activity in the infected lungs is dictated by the target cell type encountered. CD45(+) lung infiltrating inflammatory mononuclear cells, particularly CD11c(hi) dendritic cells, trigger both CTL cytotoxicity and release of inflammatory mediators, whereas CD45(-) influenza-infected respiratory epithelial cells stimulate only CTL cytotoxicity. CTL proinflammatory mediator release is modulated by co-stimulatory ligands (CD80 and CD86) expressed by the CD45(+) inflammatory cells. These findings suggest novel mechanisms of control of CTL effector activity and have potentially important implications for the control of excess pulmonary inflammation and immunopathology while preserving optimal viral clearance during respiratory virus infections.