PM and IBM staining patterns. A representative case of PM (a-d; subject #10) shows endomysial lymphocytic inflammation and muscle fiber invasion but no chronic myopathic features (a; H&E stain of the frozen material). There is no significant sarcoplasmic staining with LC3 (b), p62 (c), or TDP-43 (d), but TDP-43 stain highlights a subset of myofiber and inflammatory cell nuclei (internal positive control), while p62 faintly stains a subset of lymphocytes. A representative case of IBM (e-h, subject #22) shows endomysial inflammation accompanied by moderate to severe endomysial fibrosis, muscle fiber size variation, and RVs (white arrowhead) (e; H&E, frozen material). Staining for LC3 (f) and p62 (g) highlights RV rims; p62 also labels RV-associated protein aggregates (arrow) and scattered lymphocytes. TDP-43 immunostain (h) labels sarcoplasmic threads/skeins (black arrowheads), large protein aggregates (arrows), and coarse background puncta. Scale bars, 50 μM for a-c and e-g; 20 μM for d and h.