Dendritic cells and CD8+ T cells participate in the pathology of chronic obstructive pulmonary disease, including emphysema, but little is known of the involvement of the CD40/CD40L pathway. We investigated the role of the CD40/CD40L pathway in Tc1 cell differentiation induced by dendritic cells in a mouse model of emphysema, and in vitro. C57BL/6J wildtype and CD40-/- mice were exposed to cigarette smoke (CS) or not (control), for 24 weeks. In vitro experiments involved wildtype and CD40-/- dendritic cells treated with CS extract (CSE) or not. Compared with the control groups, the CS mice (both wildtype and CD40-/-) had a greater percentage of lung dendritic cells and higher levels of major histocompatability complex (MHC) class I molecules and costimulatory molecules CD40 and CD80. Relative to the CS CD40-/- mice, the CS wildtype showed greater signs of lung damage and Tc1 cell differentiation. In vitro, the CSE-treated wildtype cells evidenced more cytokine release (IL-12/p70) and Tc1 cell differentiation than did the CSE-treated CD40-/- cells. Exposure to cigarette smoke increases the percentage of lung dendritic cells and promotes Tc1 cell differentiation via the CD40/CD40L pathway. Blocking the CD40/CD40L pathway may suppress development of emphysema in mice exposed to cigarette smoke.