Keratocytes express MHC class I molecules constitutively, and keratocytes stimulated with IFN-+ express MHC class II molecules. Unstimulated keratocytes constitutively express B7-1 and ICAM-1, as well as low levels of CD40 and 4-1BBL. These findings indicate that keratocytes may deliver both antigen-specific and costimulatory signals to CD4 + and CD8 + T cells. To demonstrate that keratocytes expressing B7-1 provide a costimulatory signal to T cells, CD4 + or CD8 + mouse T cells were incubated with anti-CD3 mAb and irradiated keratocytes. Enhanced proliferation of both CD4 + and CD8 + T cells occurred, and could be inhibited by anti-B7-1 mAb, indicating T cell costimulatory activity by B7-1 on the keratocytes. To demonstrate that keratocytes can deliver an antigen-specific signal, CD4 + and CD8 + T cells from herpes-infected mice were incubated with HSV-1-infected, irradiated keratocytes. The resulting T cell proliferation and production of Th1 cytokines (IL-2, IFN-+ ) indicated T cell activation by antigens presented by the infected keratocytes. These results show that keratocytes in the corneal stroma of the mouse can function as antigen-presenting cells and, thus, may play a role in immune-mediated stromal inflammation such as herpetic stromal keratitis.