This guideline has been approved by the American Association for the Study of Liver Diseases (AASLD) and represents the position of the association.

The term veno-occlusive disease of the liver refers to a form of toxic liver injury characterized clinically by the development of hepatomegaly, ascites, and jaundice, and histologically by diffuse damage in the centrilobular zone of the liver. The cardinal histologic features of this injury are marked sinusoidal fibrosis, necrosis of pericentral hepatocytes, and narrowing and eventual fibrosis of central veins. Recent studies suggest that the primary site of the toxic injury is sinusoidal endothelial cells, followed by a series of biologic processes that lead to circulatory compromise of centrilobular hepatocytes, fibrosis, and obstruction of liver blood flow. Thus we propose a more appropriate name for this form of liver injury--sinusoidal obstruction syndrome. This review encompasses historical perspectives, clinical manifestations of sinusoidal obstruction syndrome in the setting of hematopoietic cell transplantation, histologic features of centrilobular injury, and a discussion of the pathophysiology of sinusoidal injury, based on both animal and clinical investigations.

Capillarization, lack of liver sinusoidal endothelial cell (LSEC) fenestration and formation of an organized basement membrane, not only precedes fibrosis, but is also permissive for hepatic stellate cell activation and fibrosis. Thus dysregulation of the LSEC phenotype is a critical step in the fibrotic process. Both a VEGF-stimulated, NO-independent pathway and a VEGF-stimulated NO-dependent pathway are necessary to maintain the differentiated LSEC phenotype. The NO-dependent pathway is impaired in capillarization and activation of this pathway downstream from NO restores LSEC differentiation in vivo. Restoration of LSEC differentiation in vivo promotes HSC quiescence, enhances regression of fibrosis, and prevents progression of cirrhosis.

BACKGROUND & AIMS Capillarization, characterized by loss of differentiation of liver sinusoidal endothelial cell (LSEC), precedes the onset of hepatic fibrosis. We investigated whether restoring differentiation to LSEC in liver affects their interactions with hepatic stellate cells (HSCs) and thereby promotes quiescence of HSCs and regression of fibrosis. METHODS Rat LSECs were cultured with inhibitors and/or agonists and examined by scanning electron microscopy for fenestrae in sieve plates. Cirrhosis was induced in rats using thioacetamide, followed by administration of BAY 60-2770, an activator of soluble guanylate cyclase (sGC). Fibrosis was assessed by Sirius red staining; expression of α-smooth muscle actin was measured by immunoblot analysis. RESULTS Maintenance of LSEC differentiation requires vascular endothelial growth factor-A stimulation of nitric oxide (NO)-dependent signaling (via sGC and cGMP) and NO-independent signaling. In rats with thioacetamide-induced cirrhosis, BAY 60-2770 accelerated the complete reversal of capillarization (restored differentiation of LSEC) without directly affecting activation of HSC or fibrosis. Restoration of differentiation to LSEC led to quiescence of HSC and regression of fibrosis, in the absence of further exposure to BAY 60-2770. Activation of sGC with BAY 60-2770, prevented progression of cirrhosis, despite continued administration of thioacetamide. CONCLUSIONS Differentiation of LSEC has an important role in activation of HSC and the fibrotic process in rats.

Capillarization precedes hepatic fibrosis. We hypothesize that capillarization of sinusoidal endothelial cells (SEC) is permissive for hepatic stellate cell (HSC) activation and therefore permissive for fibrosis. We examined whether freshly isolated SECs prevent activation of HSCs and promote reversion to quiescence, and whether this effect was lost in capillarization. HSCs were cultured alone or co-cultured with differentiated or capillarized SECs. Results: Co-culture with freshly isolated SECs markedly decreased HSC activation after 3 days in culture, but co-culture with capillarized SEC had no effect.