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 disease (GVHD) in non-irradiated (Brown Norway x Lewis) F1 rats given parental spleen cells have demonstrated an increase in the number of MHC class II+ cells with dendritic shape in the lamina propria to be the earliest event in the development of the lesion. We studied this histologic finding by electron microscopy to increase understanding of the early cellular events occurring in the lesion. Electron microscopically, the most prominent cell type observed in the lamina propria was the cell with a dendritic shape. These dendritic cells possessed large nuclei that often showed irregular indentations and ample cytoplasm containing 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 hypothesis that MHC class II+ cells with dendritic shape present antigen during the induction of local immunological responses in the tongue of GVHD rats.