ELECTRON-MICROSCOPIC STUDIES OF THE EARLY CELLULAR EVENTS OCCURRING IN ORAL-MUCOSA OF RATS AFFECTED BY GRAFT-VERSUS-HOST DISEASE (GVHD)

Previous histologic studies of tongue lesions in graft-versus-host dis ease (GVHD) in non-irradiated (Brown NorwayxLewis) Fl rats given paren tal spleen cells have demonstrated an increase in the number of MHC cl ass II+ cells with dendritic shape in the lamina propria to be the ear liest event in the development of the lesion. We studied this histolog ic finding by electron microscopy to increase understanding of the ear ly cellular events occurring in the lesion. Electron microscopically, the most prominent cell type observed in the lamina propria was the ce ll with a dendritic shape. These dendritic cells possessed large nucle i that often showed irregular indentations and ample cytoplasm contain ing numerous filaments and mitochondria. A few lysosomal or phagocytic structures were also seen in the cytoplasm. No Birbeck granules were identified. These findings were very similar to those of indeterminate cells in the epidermis and dermis. Our data strongly support the hypo thesis that MHC class II+ cells with dendritic share present antigen d uring the induction of local immunological responses in the tongue of GVHD rats.