Ultrastructural analysis of MHC class II molecule-expressing cells in experimentally induced periapical lesions in the rat

Periapical lesions were induced by making 28 days of unsealed pulp exposure s in the lower first molars of Wistar rats. Major histocompatibility comple x class II molecule-expressing cells were then demonstrated by means of imm unoperoxidase staining using a monoclonal antibody OX6, and the ultrastruct ure of these cells was analyzed under electron microscopy. OX6+ cells were classified into two major populations, (i.e. macrophages and dendritic cell (DC)-like cells. DC-like cells had elongated cytoplasmic processes, contai ned a few lysosomal structures, lacked distinct phagosomes, and were the mo st predominant cell type in the established lesion. Some of lymphocytes and plasma cells also showed a positive immunoreactivity. Both OX6+ macrophage s and DC-like cells often showed a cell-to-cell attachment with lymphocytes . These findings suggested that major histocompatibility complex class II m olecule-expressing macrophages and DC-like cells may play a crucial role in periapical lesion development by acting as antigen-presenting cells to mem ory T lymphocytes.