IM is an acute infectious disease caused by EBV, mainly affecting children and adolescents. It is clinically characterized by fever, angina, hepatosplenomegaly, and lymphadenopathy (1, 2). In Europe and the United States, IM mainly affects adolescents and adults aged 10–30 years (3). In China, IM tends to occur in children aged 4–6 (4, 5). The hepatic injury is one of the most common manifestations of IM. Studies have shown that up to 80%–90% of patients have mild or moderate hepatic injury, most of which is temporary, and a few may have severe hepatitis or liver failure (6–8). Bile acid is the main component of bile, the general name of a class of hydroxyl derivatives of 24-carbon cholestane acid. It belongs to endogenous organic anions. The enterohepatic circulation of bile acid belongs to “closed” circulation. Although the bile acid concentration in portal vein blood is very high, the content of bile acid entering the systemic circulation is usually shallow. When the liver is slightly damaged, the amount of bile acid entering the peripheral blood increases and the composition changes. The quantitative detection of the bile acid spectrum plays an important role in assessing liver disease (9, 10). There are few reports on the relationship between the bile acid spectrum and hepatic injury in children with infectious mononucleosis. In this study, total bile acid (TBA) and 15 bile acid subfractions in the serum of IM children were quantitatively detected by UPLC-MS/MS to explore their role in the early assessment of hepatic injury.

IM is an acute infectious diseases caused by EBV infection, which can invade all body systems, among which hepatic injury is the most common (14). Most children suffer from mild to moderate hepatic injury, and the liver function returns to normal within the first few months(6, 8, 15). In a small number of patients with chronic active EBV infection, hepatic injury can show an irreversible chronic development process, leading to cirrhosis and even liver failure (16–19). However, this is outcome is absolutely exceptional in the immunocompetent child. At present, the pathophysiological mechanism of how EBV infection leads to hepatic injury has not been fully clarified.

Some researchers believe that after EBV infects cells, it will lead to the free radicals generated by lipid peroxidation, which will produce toxic effects and lead to liver cell damage. At the same time, EBV itself has no direct cytotoxic effect on liver cells (20). It has also been reported that EBV does not infect liver cells, bile duct epithelial cells or vascular endothelial cells, but the infiltration of CD8+ T cells indirectly leads to hepatic injury (21). In the acute phase of IM, the number of CD4+ regulatory T cells decreased significantly, and the immunosuppressive function of the body decreased, which contributed to the activation and proliferation of effector T cells and CD8+ T cells and the release of inflammatory cytokines, and promoted the host immune system to clear EBV. In the recovery period of IM, CD4+ regulates the number of T cells to return to normal, and the immune suppression function of the body is enhanced, which can inhibit the excessive proliferation of effector T cells and the release of inflammatory cytokines and avoid the extreme immune damage of the body (22, 23). In this study, it was found that there were statistically significant differences in the percentage of NK cells, CD4+, CD8+ and CD4+/CD8+ values between IM hepatic injury group, without hepatic injury group and the healthy control group. ROC curve analysis showed that CD8+ had a high predictive value for liver injury in IM patients. The results of this study are similar to those of previous studies (21–23). However, how do these changes in lymphocyte subsets affect liver function? This study found that the percentage of NK cells was positively correlated with TCA and CA/CDCA, and negatively correlated with CDCA, DCA, LCA, GCDCA, GDCA, GLCA, TDCA, UDCA, GUDCA, TUDCA. The CD4+/CD8+ was positively correlated with CA, TCA, TCDCA and CA/CDCA; it was negatively correlated with CDCA, DCA, LCA, GCDCA, GDCA, GLCA, TDCA, UDCA, GUDCA, TUDCA. These results suggest that the changes in lymphocyte subsets in IM patients may lead to changes in the composition of bile acids in vivo. It has been found that bile acid not only plays an important role in the digestion and absorption of lipids and the steady metabolism of cholesterol, but also plays an important regulatory role in metabolism and liver regeneration as a signal molecule (24). The hepatic injury of IM patients is likely to be caused by the influence of immune cell changes on the secretion and metabolism of bile acid subfractions, thus leading to the damage of liver cells.

Research shows that the order of hydrophilicity of common free and conjugated bile acids in the human body is: UDCA > CA > CDCA > DCA > LCA, taurine conjugated bile acid > glycine conjugated bile acid > free bile acid (25). The contents of hydrophobic and hepatotoxic bile acids in the bile acid spectrum of patients with liver cirrhosis increased significantly, which may lead to changes in the bacterial flora in the bile, thus transforming primary bile acids into secondary bile acids, thus leading to a vicious cycle (26). In this study, ROC curve analysis shows that GDCA and GLCA have a high predictive value for hepatic injury in IM patients, which may be related to the low hydrophilicity and high hepatotoxicity of GDCA and GLCA, which may be a new clue for the development of new liver function evaluation indicators in the future. In addition, many foreign studies have shown that in the process of a pathological state, the content of total bile acid and some hydrophobic bile acids in the liver increased significantly. Through molecular biology experiments, it was found that the transcriptional level expression of bile acid synthase CYP7A1 and some transporter genes BSEP, ASBT, NTCP, FXR, FGF15 decreased significantly, further supporting the possible change rule of the bile acid spectrum (9, 27). These results indicate that the changes in bile acid spectrum are consistent with the changes in clinical indicators, suggesting that bile acid plays an important role in assessing liver disease. The results of this study showed that there were significant differences in CA, CDCA, DCA, LCA, GCDCA, GDCA, GLCA, TCA, TCDCA, TDCA, UDCA, GUDCA, TUDCA, CA/CDCA between IM hepatic injury, without hepatic injury group and the healthy control group. We found that the bile acid composition of IM patients with normal liver function changed significantly compared with the healthy control group, suggesting that bile acid spectrum detection is more sensitive than routine liver function examination to detect potential damage to liver function at an early stage, which is convenient for clinicians to timely assess the condition.

This study has several deficiencies, which need to be improved in the future. First, bile acid spectrum and lymphocyte subsets were detected only in the acute phase of IM, and the sample size of the study was too small. Secondly, in the future, we can further study the evaluation value of bile acid spectrum on liver function after other human herpes virus infections, and further explore the change characteristics of bile acid spectrum in different herpes virus infections.

UPLC-MS/MS method can sensitively detect the changes in serum bile acid spectrum before liver function damage in children with IM, which is helpful for early assessment of hepatic injury in children with IM. The changes of lymphocyte subsets in children with IM were correlated with some bile acid subfractions, which provided a new idea for studying the mechanism of hepatic injury in children with IM in the future.