Scar-associated macrophages are a major source of hepatic matrix metalloproteinase-13 and facilitate the resolution of murine hepatic fibrosis.

Both the identity and source of the rodent collagenase(s) that mediates matrix remodeling in liver fibrosis remain elusive. We have recently demonstrated an unequivocal role for scar-associated macrophages (SAMs) in the spontaneous resolution of liver fibrosis and sought to determine whether SAMs are the source of matrix metalloproteinase (MMP) 13 (collagenase 3), considered to be the primary interstitial collagenase in rodents. In this study, we demonstrate an association between MMP13 expression and the presence of SAMs in the regression of experimental liver fibrosis. mmp13 gene expression was restricted to regions of fibrosis that were rich in SAMs. Both MMP13 mRNA and protein colocalized to large phagocytes within and directly apposed to hepatic scars. Using the CD11b-DTR-transgenic mouse to deplete SAMs in a model of chronic CCl(4) injury, we found that SAM depletion resulted in a 5-fold reduction in mmp13 message (p = 0.005). Furthermore, resolution of CCl(4)-induced fibrosis was retarded in MMP13-deficient mice. Thus, SAMs selectively, during resolution of fibrosis induce and use the major collagenase MMP13 to mediate the resorption of interstitial matrix and successfully remodel the fibrotic liver.

Selective depletion of macrophages reveals distinct, opposing roles during liver injury and repair.

Macrophages perform both injury-inducing and repair-promoting tasks in different models of inflammation, leading to a model of macrophage function in which distinct patterns of activation have been proposed. We investigated macrophage function mechanistically in a reversible model of liver injury in which the injury and recovery phases are distinct. Carbon tetrachloride---induced liver fibrosis revealed scar-associated macrophages that persisted throughout recovery. A transgenic mouse (CD11b-DTR) was generated in which macrophages could be selectively depleted. Macrophage depletion when liver fibrosis was advanced resulted in reduced scarring and fewer myofibroblasts. Macrophage depletion during recovery, by contrast, led to a failure of matrix degradation. These data provide the first clear evidence that functionally distinct subpopulations of macrophages exist in the same tissue and that these macrophages play critical roles in both the injury and recovery phases of inflammatory scarring.

Spontaneous recovery from micronodular cirrhosis: evidence for incomplete resolution associated with matrix cross-linking.

BACKGROUND & AIMS: Liver fibrosis and cirrhosis result from the excessive secretion of matrix proteins by hepatic stellate cells (HSCs). Previously considered irreversible, we have studied a model of cirrhosis to determine the mechanisms mediating and limiting spontaneous recovery. METHODS: A micronodular cirrhosis was induced in rats after 12 weeks of CCl(4) intoxication. Livers were analyzed for evidence of matrix degradation, matrix metalloproteinase (MMP) and tissue inhibitor of metalloproteinase (TIMP) expression, stellate cell apoptosis, tissue transglutaminase (tTg) expression, and matrix cross-linking during spontaneous recovery of up to 366 days. RESULTS: Over 366 days of recovery, micronodular cirrhosis underwent significant remodeling to a macronodular cirrhosis. Expression of collagen-1 and TIMP messenger RNA (mRNA) decreased significantly and active MMPs were shown in livers during remodeling of fibrosis. Resolution also was characterized by apoptosis of HSCs, predominantly at the margins of fibrotic septa. Residual septa, not remodeled at 366 days, were characterized by tTg-mediated cross-linking and relative hypocellularity. CONCLUSION: Recovery from comparatively advanced cirrhosis is possible and results in remodeling from a micronodular cirrhosis to a macronodular cirrhosis. We suggest resolution is limited by tTg-mediated matrix cross-linking and a failure of HSC apoptosis.

Hepatocytes express nerve growth factor during liver injury: evidence for paracrine regulation of hepatic stellate cell apoptosis.

A key feature of recovery from liver fibrosis is hepatic stellate cell (HSC) apoptosis, which serves the dual function of removing the major source of neomatrix and tissue inhibitors of metalloproteinases thereby facilitating matrix degradation. The mechanisms regulating HSC apoptosis remain undefined but may include the interaction of nerve growth factor (NGF) with its receptor, p75, on HSC. In this study, by TaqMan polymerase chain reaction in situ hybridization and immunohistochemistry, we demonstrate that NGF is expressed by hepatocytes during fibrotic injury. Peak hepatocyte expression of NGF (48 hours after CCl(4) injection) coincides with maximal rate of apoptosis of HSC by terminal dUTP nick-end labeling staining. Addition of recombinant NGF to HSC in tissue culture causes a dose-dependent increase in apoptosis. NGF regulates nuclear factor (NF)-kappaB activity, reducing p50/p65 binding detected by electromobility shift assay and reduced NF-kappaB CAT reporter activities from both basal unstimulated levels and after NF-kappaB induction by tumor necrosis factor. In each case, a relative reduction in NF-kappaB binding was associated with a significant increase in caspase 3 activity. These data provide evidence that NGF is expressed during fibrotic liver injury and may regulate number of activated HSCs via induction of apoptosis.