Fig. 1

a, b, c Foci of inflammatory cell infiltration in three cases of PD-1 myopathy. In the inflammatory foci, lymphoid follicle-like structures with area like germinal center (arrow). Some blood vessels in the lymphoid follicle-like structures have endothelial cells with ample cytoplasm simulating high endothelial venules (arrowhead). d CD8 + cells were seen in the endomysium. Many of them were in close contact with the muscle fiber surface (case 2). e CD20+ B lymphocytes are seen in the foci of inflammation in all three cases (case 2). f MHC class I antigen is expressed on the surface of almost all muscle fibers (Case 1). g Electron micrograph of Case 2. In the foci of inflammation, two mononuclear cells (1,2) are located under the basal lamina of a muscle fiber and in contact with the plasma membrane (arrows) of a muscle fiber (3). h In the Case 2, the transverse view of a blood vessel located in a focus of inflammation shows a mononuclear cell (arrow) in direct contact with two endothelial cells protruding its tip into the vessel wall. The vessel has a basal lamina, which is irregular in thickness and rough in texture with many spaces (white arrowheads) and occasional fibrous structures (black arrowhead) in its matrix. i Higher magnification of the highlighted area in Fig. 1e. The endothelial cells have ample cytoplasm containing abundant Golgi complexes (G), mitochondria (M) and ribosomes. j Immuno-electron micrograph of Case 2. A cell underneath the cell surface of a muscle fiber has numerous electron dense particle of size consistent with diameters of gold particles labeled to the anti-CD8 antibody. k-m Immunofluorescence study of Case 2. Anti-laminin antibody labeled with FITC (green) shows the basal lamina of muscle fibers and blood vessels. The merged image shows CD8+ cells (orange) in and around muscle fibers, including some located inside the basal lamina of the muscle fibers (arrows)