Hepatocytes modify macrophage polarization by reducing the availability of active TGFβ

The macrophage populations of the liver are heterogeneous, consisting of self-renewing tissue-resident macrophages and monocyte-derived macrophages (moMΦ) recruited from the circulation. After liver injury particularly the recruitment of moMΦ population is considered to be important for undisturbed regeneration. However, up to now it is unclear in how far an environment that is predominantly shaped by hepatocyte-derived factors influence macrophage polarization and function.

The main aim of the present study is to clarify whether a microenvironment determined by hepatocytes influences polarization of moMΦ and to identify components of the intercellular communication network that play a role in this context. Therefore, a co-culture model was established using primary murine hepatocytes, which are co-cultivated with bone-marrow derived macrophages (BMDM) separated by a membrane that allows mediator exchange but no direct cell-cell contact. Results of these studies were further verified by in vivo experiments in the regenerating liver.

Evidence is provided that hepatocytes induce the up-regulation of CD163, CD206 and MHC class II in BMDM. This is accompanied by substantial changes of the basal mRNA expression of Arg1, IL-10, Fizz, Stab1 and IFN-beta. In addition, the inflammatory response of BMDM towards LPS is substantially altered in the presence of hepatocytes resulting in a strong increase of the expression of IL-10 and IFN-beta. Analysis of transcriptome, proteome and secretome resulted in the identification of active TGF-beta as an important signaling intermediate of the intercellular communication network. Inhibition of TGF-beta signaling in BMDM either by antibodies, inhibitors or by deletion of the TGF-beta-receptor II resulted in an upregulation of the expression of CD163, CD206, IL-10, Stab1 and Arg1, indicating that reduced activation of TGF-beta signaling may indeed explain the observed effects of hepatocytes on BMDM polarization. Consistent with the in vitro data, myeloid cell-specific deletion of the TGF-beta receptor II results in increased expression of CD163 and CD206 in macrophages with high expression of CD14. In addition, the proliferative phase was prolonged during PHx liver regeneration in these animals.

The data provided indicate that hepatocytes have a strong impact on the polarization and function of BMDM and that in this context the reduction of the availability of active TGF-beta plays an important role in vitro and in vivo.