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Accessory cell function of airway epithelial cells. Am J Physiol Lung Cell Mol Physiol 287: L318-L331, 2004; 10.1152/ajplung.00174.2003.--We previously demonstrated that airway epithelial cells (AECs) have many features of accessory cells, including expression of class II molecules CD80 and CD86 and functional Fc[gamma] receptors. We have extended these studies to show that freshly isolated AECs have mRNA for cathepsins S, V, and H [proteases important in antigen (Ag) presentation], invariant chain, human leukocyte antigen (HLA)-DM-[alpha] and HLA-DM-[beta], and CLIP, an invariant chain breakdown product. A physiologically relevant Ag, ragweed, was colocalized with HLA-DR in AECs, and its uptake was increased by granulocyte-macrophage colony-stimulating factor and IFN-[gamma] treatments, which had no effect on CD80 and CD86 expression. We demonstrate the presence of other costimulatory molecules, including B7h and B7-H1, on AECs and the increased expression of B7-H1 on AECs after treatment with granulocyte-macrophage colony-stimulating factor and IFN-[gamma]. Finally, we compared T cell proliferation after allostimulation with AECs and dendritic cells (DCs). The precursor frequency of peripheral blood T cells responding to AECs was 0.264% compared with 0.55% for DCs. DCs stimulated CD45R[O.sup.+], CD45R[A.sup.+], CCR[7.sup.+] and CCR[7.sup.-]CD[4.sup.+], and CD[8.sup.+] T cells, whereas AECs stimulated only CD45R[O.sup.+], CD45R[A.sup.-], CCR[7.sup.-], CD[4.sup.+], and CD[8.sup.+] T cells. There was no difference in cytokine production, type of memory T cells stimulated (effector vs. long-term memory), or apoptosis by T cells cocultured with AECs and DCs. The localization of AECs exposed to the external environment may make them important in the regulation of local immune responses. dendritic cells; immune responses; ragweed; lymphocytes; bronchoalveolar lavage