Objective: We have previously reported that dendritic cell (DC)-like cells are prevalent in the chronic, non-expanding phase of induced rat apical periodontitis. The aim of the present study was to characterize these cells by means of immunohistochemistry and electron microscopy using monoclonal antibodies that are reactive to DC-associated determinants. Methods: Apical periodontitis was induced in mandibular first molars by making unsealed pulp exposures. Specimens were dissected out at 28 days and subjected to immunoperoxidase staining and electron microscopy using OX62 (reactive to DCs and gd T cells), CD11c (reactive to DCs and macrophages), and OX6 (reactive to class II MHC molecules). The ratio of DCs in labeled cells and the density of DCs per unit area were determined under the electron microscope. Results: The three antibodies caused the immunoreaction along the plasma membrane and recognized DCs, which were mainly distributed in the fibroblasts rich area of the outer part of the lesion and distinguished from macrophages by their poorly-developed lysosomal structures and phagosomes. Quantitative analysis revealed that OX62 was most selectively reactive to DCs, although the density of OX62+ DCs was significantly lower than that of OX6+ and CD11c+ DCs. OX62+ DCs never contained phagosomes and had less pronounced cytoplasmic processed compared with OX6+ and CD11c+ DCs. Conclusion: These results suggest that DCs in rat apical periodontitis may be composed of heterogeneous populations that exhibit different phenotypes.

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