Tissue-based detection of macrophage-associated YAP1-positive material in human acute liver injury: an exploratory immunopathological biopsy series

BACKGROUND: Yes-associated protein 1 (YAP1), a downstream effector of the Hippo pathway, is implicated in hepatocellular stress, ductular reaction, and tissue repair. Experimental studies suggest that injured YAP1-activated hepatocytes can be cleared by Kupffer-cell phagocytosis, but corresponding human biopsy evidence remains limited. METHODS: We retrospectively analyzed seven adults who underwent clinically indicated liver biopsy within 10 days of initial presentation or referral admission for acute liver injury. Liver biopsy sections were assessed by hematoxylin and eosin staining, periodic acid-Schiff-diastase staining, YAP1 immunohistochemistry, and double immunofluorescence for YAP1 and CD68. Macrophage-associated YAP1-positive structures were operationally defined as YAP1-positive signals located within or immediately adjacent to CD68-positive macrophages in portal or periportal areas. The definition was intended to capture spatial association and was not intended to prove intracellular phagocytosis, molecular co-expression, or endogenous YAP1 expression by macrophages. Associations with peak Model for End-Stage Liver Disease (MELD) score and days to alanine aminotransferase (ALT) normalization were evaluated as exploratory analyses. RESULTS: The median age was 52 years, and etiologies included autoimmune hepatitis, primary biliary cholangitis-autoimmune hepatitis overlap, drug-induced liver injury, acute hepatitis B, and idiopathic acute liver injury. Liver biopsy was performed at a median of 5 days (range, 2-10 days) after initial presentation or referral admission. A median of nine portal tracts and five macrophage-associated YAP1-positive structures were evaluated per biopsy, yielding a median YAP1/CD68-positive portal-tract index of 58.3%. ALT normalized after a median of 26 days. The index showed a nominal inverse association with days to ALT normalization (Spearman rho = -0.86, p = 0.014; Pearson r = -0.79, p = 0.034), whereas MELD score was not significantly associated with the index or recovery time. Leave-one-out analyses retained the inverse direction but showed unstable significance because of the very small sample size. CONCLUSIONS: In this small exploratory biopsy series, YAP1-positive material was observed in spatial association with CD68-positive macrophages in early human acute liver injury. The YAP1/CD68-positive portal-tract index showed a hypothesis-generating relationship with biochemical recovery but should not be interpreted as a validated prognostic marker or proof of macrophage-mediated phagocytosis. Larger, etiology-specific studies with standardized biopsy timing, lineage markers, multiplex immunofluorescence, high-resolution imaging, and digital pathology are warranted.