pIBM staining patterns. pIBM cases were histologically similar to classic IBM, with endomysial inflammation, fiber invasion, and moderate-severe chronic myopathic features, but without classic RVs (a and f; H&E, frozen material); “rimmed cracks” (white arrowhead in e) were present in a subset of specimens. The majority of pIBM samples (designated pIBM high) showed well-developed labeling for LC3 (b) and p62 (c); TDP-43 immunopositivity was less commonly observed (d; arrow marks a single TDP-43 positive fiber). In the example shown (subject #52), many fibers showed dense coarse puncta and rare RV-like structures (black arrowhead in b). In a smaller subset of samples (designated pIBM low), little or no labeling was seen with all three markers (f- h). a-d, subject #52, e-h, subject #51; scale bar, 50 μM.