Emergence of Different Macrophage Populations in Hepatic Fibrosis Following Thioacetamide-induced Acute Hepatocyte Injury in Rats

Abstract

Macrophages may play a role in fibrogenesis. The kinetics and distribution of different macrophage populations were investigated immunohistochemically in hepatic lesions following acute hepatocyte injury induced in F344 rats by a single injection of thioacetamide (TAA) (300mg/kg body weight, intraperitoneally). Hepatocyte degeneration or necrosis induced by TAA occurred mainly in the perivenular areas of hepatic lobules as early as post-injection (PI) days 1 and 3; fibrotic lesion development began in the damaged areas on day 1, and peaked on day 5; thereafter (PI days 7 and 10), the fibrotic areas decreased and were replaced by regenerated hepatocytes on PI days 15 and 20, indicating a remodelling process. In this rat model, the number of macrophages reacting with ED1 antibody (specific for exudate macrophages), ED2 (recognizing cell membrane antigens of resident macrophages, including Kupffer cells) and OX6 (recognizing MHC class II antigens expressed in antigen-presenting macrophages and dendritic cells) began to increase on PI day 1, peaking on PI day 3. The numbers gradually decreased on PI days 5 and 7; however, the statistically significant increase was maintained in respect of ED1-positive cells up to PI day 20, whereas no significant increase in ED2- and OX6-positive cells remained from PI day 10 onwards. Interestingly, of the ED1-, ED2- and OX6-positive cells, the OX6-positive cells were the least numerous. ED1- and OX6-positive cells appeared exclusively in the injured perivenular areas, whereas ED2-positive cells were present mainly in the mid-zonal areas and in smaller numbers in the perivenular areas. These findings indicated differences in kinetics and distribution between macrophage populations appearing in hepatic fibrosis. In addition, RT-PCR revealed that mRNA expression of osteopontin, a factor for induction and maintenance of macrophages in inflammation, was markedly increased on PI days 5, 7 and 10, suggesting a role in the pathogenesis of hepatic fibrosis.