Inhibitory function of Tregs via soluble FGL2 in chronic hepatitis B

FGL1 and FGL2: emerging regulators of liver health and disease

Liver disease is a complex group of diseases with high morbidity and mortality rates, emerging as a major global health concern. Recent studies have highlighted the involvement of fibrinogen-like proteins, specifically fibrinogen-like protein 1 (FGL1) and fibrinogen-like protein 2 (FGL2), in the regulation of various liver diseases. FGL1 plays a crucial role in promoting hepatocyte growth, regulating lipid metabolism, and influencing the tumor microenvironment (TME), contributing significantly to liver repair, non-alcoholic fatty liver disease (NAFLD), and liver cancer. On the other hand, FGL2 is a multifunctional protein known for its role in modulating prothrombin activity and inducing immune tolerance, impacting viral hepatitis, liver fibrosis, hepatocellular carcinoma (HCC), and liver transplantation. Understanding the functions and mechanisms of fibrinogen-like proteins is essential for the development of effective therapeutic approaches for liver diseases. Additionally, FGL1 has demonstrated potential as a disease biomarker in radiation and drug-induced liver injury as well as HCC, while FGL2 shows promise as a biomarker in viral hepatitis and liver transplantation. The expression levels of these molecules offer exciting prospects for disease assessment. This review provides an overview of the structure and roles of FGL1 and FGL2 in different liver conditions, emphasizing the intricate molecular regulatory processes and advancements in targeted therapies. Furthermore, it explores the potential benefits and challenges of targeting FGL1 and FGL2 for liver disease treatment and the prospects of fibrinogen-like proteins as biomarkers for liver disease, offering insights for future research in this field.

Role of Regulatory T Cells (Treg) and the Treg Effector Molecule Fibrinogen-like Protein 2 in Alloimmunity and Autoimmunity

CD4(+)CD25(+)Foxp3(+) regulatory T cells (Treg) are critical to the maintenance of immune tolerance. Treg are known to utilize a number of molecular pathways to control immune responses and maintain immune homeostasis. Fibrinogen-like protein 2 (FGL2) has been identified by a number of investigators as an important immunosuppressive effector of Treg, which exerts its immunoregulatory activity by binding to inhibitory FcγRIIB receptors expressed on antigen-presenting cells including dendritic cells, endothelial cells, and B cells. More recently, it has been suggested that FGL2 accounts for the immunosuppressive activity of a highly suppressive subset of Treg that express T cell immunoreceptor with Ig and ITIM domains (TIGIT). Here we discuss the important role of Treg and FGL2 in preventing alloimmune and autoimmune disease. The FGL2-FcγRIIB pathway is also known to be utilized by viruses and tumor cells to evade immune surveillance. Moving forward, therapies based on modulation of the FGL2-FcγRIIB pathway hold promise for the treatment of a wide variety of conditions ranging from autoimmunity to cancer.

Soluble FGL2, a novel effector molecule of activated hepatic stellate cells, regulates T-cell function in cirrhotic patients with hepatocellular carcinoma

PURPOSE

To investigate the effects of soluble FGL2 (sFGL2) secreted by hepatic stellate cells (HSCs) on immune suppression in cirrhotic patients with hepatocellular carcinoma (HCC).

METHODS

Serum sFGL2 levels were examined by ELISA in 40 patients with HCC, liver cirrhosis (LC) or chronic HBV (CHB) infection. A double staining of the immunofluorescence analysis of α-SMA and FGL2 was performed in two cirrhotic liver specimens. The expression of FGL2 in the LX2 cell line was analyzed by immunofluorescence, Western blot and flow cytometry. T-cells purified from HCC patients using magnetic beads were cultured with LX2 cells at different ratios with anti-CD3-stimulating or FGL2-blocking antibodies. The proliferation index (PI) of CD8 + T cells was assessed by flow cytometry, and the secretion of IFN-γ was measured by ELISA.

RESULTS

sFGL2 levels are significantly higher in patients with HCC or LC compared with those with CHB (p = 0.0039/p = 0.0020). Among HCC patients, those with cirrhosis exhibited significantly higher levels of sFGL2 compared with non-cirrhotic individuals (p = 0.0108). The expressions of FGL2 and α-SMA overlapped in HSCs in liver specimens. FGL2 protein secreted by LX2 cells inhibited T-cell proliferation of HCC patients in a dose-dependent manner in vitro. The PI of CD8 + T cells was significantly enhanced following addition of FGL2 antibody to the culture system (LX2/T-cell ratio of 1:10, p = 0.002). The level of IFN-γ in mixed cultures was inversely correlated with the number of HSCs and was reversed by incubation with FGL2 blocking antibody.

CONCLUSION

sFGL2 protein is a novel effector molecule of activated HSCs, which suppresses CD8 + T cell proliferation and interferon-γ production, and it subsequently might contribute to immune suppression during fibrosis and tumorigenesis in the liver.