HLA-DR (major histocompatibility complex [MHC] class II) is often expressed by epithelial cells in gingival tissues with periodontal disease but not by cells in healthy gingival tissues. Confocal microscopic analyses revealed that gingival epithelial cells (GEC) from tissue with periodontal disease express both HLA-DR and B7-1 (CD80) costimulatory molecules. Rat GEC lines were established to elucidate the possible role of MHC class II and B7-1 expression by GEC. Stimulation of a rat GEC line with gamma interferon (IFN-gamma) induced the expression of MHC class II, whereas the cell line constitutively expressed B7-1 costimulatory molecules as determined by reverse transcription-PCR and flow cytometry. Actinobacillus actinomycetemcomitans Omp29-specific CD4(+) Th1 clone cells proliferated in response to pretreatment of GEC with fixed A. actinomycetemcomitans and IFN-gamma. However, the Th1 cells did not respond to pretreatment of GEC with the bacteria alone or IFN-gamma alone. The activation of Th1 clone cells induced by the GEC was inhibited by antibody to MHC class II or by CTLA4 immunoglobulin (CTLA4-Ig). Lymph node T cells did not demonstrate superantigen activity to A. actinomycetemcomitans, although both lymph node T cells and Th1 clone cells were sensitive to superantigen activity of staphylococcal enterotoxin A as cultured in the presence of IFN-gamma-treated GEC. These results suggested that GEC can take up bacterial antigen and consequently process and present the bacterial antigen to CD4(+) T cells by MHC class II in conjunction with B7 costimulation. GEC appeared to play a role in the adaptive immune response by stimulating antigen-specific CD4(+) T cells.