Distribution of Ia antigen-expressing nonlymphoid cells in various stages of induced periapical lesions in rat molars.

Periapical lesions were experimentally produced in rat lower first molars by exposing the pulp to the oral environment for 1 to 56 days. Temporal changes in the number and distribution of la antigen-expressing nonlymphoid cells in the periapical tissue were examined immunohistochemically on decalcified cryostat sections using OX6, a monoclonal antibody against rat la antigen. Influx of la-positive exudative cells into the periapical tissue was observed from 1 day postoperatively. Between 14 and 28 days when expansion of the periapical lesion was most evident, numerous la-positive macrophage-like and dendritic cell-like cells of diverse morphologies were found in the periapical tissue. The number of these cells showed further increase at 56 days postoperatively, when the lesion expansion had ceased. These results suggest the involvement of la antigen-expressing nonlymphoid cells in the development and perpetuation of periapical pathosis. They may act primarily as antigen-presenting cells, which are essential for the initiation of antigen-specific immune defense.