Liver cirrhosis is a condition characterized by diffuse fibrosis and hyperplasia of fibrous tissue1, which occurs after extensive hepatocyte necrosis and originates from various chronic liver injuries and inflammations, such as hepatitis B virus (HBV), hepatitis C virus (HCV), alcoholic liver disease (ALD), and non-alcoholic fatty liver disease (NAFLD)2. The primary symptoms of liver cirrhosis include fatigue, loss of appetite, weight loss, abdominal pain or distension, jaundice, palmar erythema, spider nevi, ascites, and gastrointestinal bleeding3. The primary burden that liver cirrhosis imposes on patients is due to the severe prognosis of complications during the decompensated phase, with a 5-year survival rate of only 30%4. Therefore, studies have suggested that the decompensated phase of liver cirrhosis is a crucial factor contributing to the mortality of cirrhosis patients globally. Furthermore, it is predicted that the number of deaths and cases attributed to decompensated liver cirrhosis will increase over the next decade, posing a significant challenge to public health5. In fact, given the pandemic of obesity, the global prevalence of NAFLD is 23.4%, and it is showing an annual upward trend6. Non-alcoholic steatohepatitis (NASH)-related cirrhosis has been demonstrated to be the predominant cause of liver cirrhosis, accounting for approximately 59.5% of cases7, highlighting the urgency of early diagnosis and treatment of liver cirrhosis and the necessity of developing effective lipid management strategies.
Choline is a water-soluble quaternary amine that is an essential nutrient for human health. It is now recognized that choline is associated with liver disease, atherosclerosis, and possibly neurological disorders. The primary fate of choline in eukaryotic cells is its conversion to PC, also known as lecithin. PC is the major storage form of choline, accounting for 40–50% of the total phospholipids in mammalian cell membranes and is thus abundant8,9. PC is primarily synthesized via the CDP-choline pathway10,11. Moreover, most choline must be supplied through the diet, absorbed through the intestinal tract, and then taken up by choline transporters12. Due to the remodeling of the acyl chain components of PC by phospholipases and lysophospholipid acyltransferases, the diversity of PC molecular species is crucial for membrane integrity and function. PC can provide a substantial supply of choline and maintain membrane integrity, playing a crucial role in the structure and function of cell membranes in various tissues, including as a key component in energy metabolism and lipid transport13,14. Studies have confirmed that a deficiency in PC can negatively impact liver function, leading to hepatic steatosis and fibrosis15–17. Mice placed on a choline-deficient diet die from fatty liver and liver failure12. Supplementation with PC can improve liver injury, fibrosis, and hepatocarcinogenesis caused by the absence of SREBP cleavage-activating protein (SCAP) and endoplasmic reticulum stress18. This emphasizes the importance of maintaining optimal PC levels for liver health. Additionally, both excessively high and low levels of PC are associated with the development of NAFLD in mice, suggesting that the optimal PC level is crucial for liver protection and the treatment of liver disease10,19.
Recent studies have indicated that in IL-10 transgenic mice, the relative increase of intrahepatic T lymphocytes during liver fibrosis is suppressed, suggesting that IL-10 has an anti-fibrotic effect20. Additionally, research has systematically described the different manifestations of innate immune cells during the progression of LC21. Myeloid-derived suppressor cells (MDSCs) are a heterogeneous immune cell population with immunosuppressive functions, divided into monocytic MDSCs (mMDSCs) and granulocytic MDSCs (gMDSCs)22. In humans, CD33 serves as a marker for subpopulations of MDSCs and is typically induced under various inflammatory conditions, including hepatitis, viral infections, and hepatocellular carcinoma (HCC)23–25. It can suppress T cell proliferation and cytokine production, potentially weakening T cell-mediated immune responses26.Some studies have found that the liver is an organ where MDSCs accumulate. MDSCs play a direct or indirect role in promoting immunosuppression in various pathological states. In mice with various types of cancer, the number of MDSCs in the liver increases, regardless of whether the liver shows signs of metastasis27. Beyond tumors, MDSCs have protective functions in both acute and chronic liver inflammation28, with the mMDSC subpopulation being more T cell-suppressive and limiting the liver damage mediated by concanavalin A (ConA)-induced hepatitis compared to gMDSCs29,30.MDSCs derived from bone marrow can promote tumor-promoting type 2 responses by interacting with macrophages to increase IL-10 and decrease IL-12 production, thereby inhibiting T cell activation31. Therefore, MDSCs play a key role in the prevention and limitation of immune-mediated hepatitis32. Given that the terminal stage of chronic liver injury and liver fibrosis is liver cirrhosis, there is currently a void in research on the role of immune cells in the development of liver cirrhosis. Moreover, current studies have not directly revealed whether changes in PC metabolism levels affect the onset of liver cirrhosis. Based on current research10,13,19,21,32,33, we hypothesize that immune cells may mediate the effects of PC on LC. Since the majority of research on the pathogenesis of PC, immune cells, and LC is observational and may involve confounding factors, it limits the ability to infer causal relationships. Furthermore, existing epidemiological data are biased when assessing the risk of liver cirrhosis in patients due to the recording of cases and the use of International Classification of Diseases codes5,34. Therefore, it is necessary to find more effective methods to explore the causal relationship among these three factors.
MR is a potential causal inference approach that adheres to Mendelian laws to eliminate confounding biases. It uses genetically determined variations associated with modifiable exposures as IVs to derive the impact of exposure factors on outcomes from observational data35,36. This method is not influenced by common confounding factors such as postnatal environmental and behavioral lifestyle, and it can maximize the reduction of reverse causal effects37,38. In recent years, with the development of GWAS and the flourishing of molecular mechanism identification, a solid foundation has been provided for the implementation of MR research. This means that MR offers a means to directly uncover the potential mechanisms by which PC, mediated by immune cells, influences LC. Therefore, we have decided to use the MR method to explore whether there is a causal relationship between PC and LC, and to what extent immune cells mediate the impact of PC on LC.