In children and adolescents, the main clinical manifestations of EBV-IM include fever, pharyngitis, superficial lymphadenopathy, hepatosplenomegaly, and skin rash. EBV-IM is primarily transmitted through saliva contact, but it can also spread through blood or sexual transmission, usually accompanied by an increase in peripheral blood heterotypic lymphocytes. HLH is a kind of histiocytosis with reactive hyperplasia of the monocyte-macrophage system. This condition and the resulting clinical syndrome are due mainly to a dysfunction of cytotoxic killer (CTL) and NK cells in which they fail to remove antigen properly, and the resulting sustained antigen stimulation over-activates the mononuclear-macrophage system, producing a large number of inflammatory cytokines. Hemophagocytic syndrome can be divided into primary/familial HLH and secondary HLH, EBV-HLH is the most important type of secondary HLH. The majority of EBV-IM cases are self-limited and mild with a good prognosis. In contrast, most cases of EBV-HLH are severe, involve multiple organs, and are life-threatening[7]. Therefore, the early identification of EBV-HLH by a simple laboratory examination is of great significance.
After a host is infected with EBV, the virus invades the lymphocyte system, inducing the production of a large number of inflammatory factors that subsequently cause enlargement of the liver, spleen, and superficial lymph nodes. The inflammatory factor IL-1, IL-6, TNF-α, among others, acts on the thermoregulatory center, causing a high fever[8]. By inducing immune system factors, EBV infection can damage the functions of various organs, with liver damage being the most common. Rather than being caused by a direct invasion of the liver by the virus, the liver damage is often due to an infiltration of mononuclear cells in the hepatic lobule and portal area[9], free radical hyperactivity in intracellular lipid peroxidation caused by EBV infection, and hepatomegaly caused by B lymphocyte activation[10]. Here, it was found that liver, spleen, and lymph node enlargement all manifested in both EBV-IM and EBV-HLH cases, but the peak fever temperature and duration of fever differed, as did the incidence of liver, spleen and lymph node enlargement. Children with EBV-HLH had a longer duration of pre-hospital fever and overall duration of fever compared with those with EBV-IM; furthermore, elevated levels of alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, triglyceride, and ferritin were more obvious in the EBV-HLH cases. The possibility of EBV-HLH should be strongly considered in cases with persistent high fever, significant enlargement of the liver, spleen, and changes in the levels of alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, triglyceride, and ferritin.
If the disease progresses further after the initial EBV infection, the accompanying excessive cytokine secretion leads to cytokine storms, such that an immune disorder and uncontrolled inflammatory reactions coexist[11]. In these cases, IFN-γ and TNF-α inhibit the hematopoietic function of bone marrow, and the function of activated macrophages is out of control, together resulting in hemophagocytosis[12]. The inadequacy and deficiency of cellular immunity function after EBV infection are the main causes of severe diseases, such as malignant histiocytosis and lymphoma[13]. The results of this study show that the absolute values of CD3+, CD4+, CD8+, CD3-CD19+, and NK cell counts were significantly lower in the EBV-HLH group than in the EBV-IM group, indicating a serious immunodeficiency in the EBV-HLH cases. The CD4+/CD8+ lymphocyte ratio of the EBV-HLH group was higher than that of the EBV-IM group, indicating that the ability of CD8+ lymphocytes to activate and proliferate was lower in the EBV-HLH group than in the EBV-IM group, and, consequently, the virus clearance ability of the EBV-HLH group was weaker[11]. NK cells are important immune cells involved in immune surveillance and early anti-infection, and they can directly kill target cells that are infected by a virus. A decreased amount of NK cells in the peripheral blood and the corresponding decreased ability to clear viruses indicate that the disease may worsen[14], requiring active treatment.
EBV antigen can induce the body to produce specific antibodies, which combine with EBV antigen to form antigen–antibody immune complexes that cause an abnormal proliferation of humoral immunity[15]. Here, the EBV-HLH group had more serious immune defects compared with the EBV-IM group, and the levels of IgA and IgM in peripheral blood of the children in the EBV-HLH group were significantly lower than those in EBV-IM group. Compared with the EBV-IM group, the IgG trended lower in the EBV-HLH group children, but this difference was not statistically significant, indicating that the affected humoral immunity was mainly due to a change of immunoglobulin in the acute phase.
The EBV DNA loads in whole blood and plasma for the EBV-HLH group were significantly higher than those in the EBV-IM group. This finding indicates that EBV was better able to evade the host immune system in the EBV-HLH group, and the cellular and humoral immune functions of the patients in this group were significantly lower, thus allowing the proliferation of EBV throughout the body, further damaging organs and leading to the occurrence of EBV-HLH. The possibility of EBV-HLH should be strongly considered in children with a high EBV DNA load and fever, clearly abnormal liver function, and abnormal levels of lactate dehydrogenase, triglyceride, and ferritin.
In this study, the plasma EBV DNA load in children with EBV-IM was positively correlated with the level of D-Dimer, and the plasma EBV DNA load in children with EBV-HLH was positively correlated with the levels of PT and TT. During EBV infection, monocytes, macrophages, and endothelial cells not only produce inflammatory factors, but also express tissue factor and initiate exogenous coagulation reactions, thereby consuming a large amount of coagulation factors and leading to fibrinolytic hyperactivity and decreased coagulation factor synthesis. The levels of Scr, BUN, UA, TG, and EBV DNA load in children with EBV-HLH were positively correlated, indicating that inflammatory factors affected liver synthesis and secretion as well as kidney function and directly inhibited the level of lipoprotein lipase, causing an increase of triglyceride[16]. Inflammatory factors can invade all organs of the body. Here, the plasma EBV DNA load in children with EBV-HLH was positively correlated with the levels of α-HBDH, LDH, CK, and AST, suggesting that children with EBV-HLH had different degrees of organ damage. Additionally, the plasma EBV DNA load was positively correlated with the CRP level, suggesting that children with EBV-HLH suffer from multiple organ damage and severe immune dysfunction, as well as severe inflammatory reactions.
D-Dimer is a specific degradation product under the action of plasmin after cross-linking of fibrin, and its increased level indicates that the body's secondary fibrinolytic activity is enhanced, reflecting the body's hypercoagulable and fibrinolytic state, and can be used for diagnosis Thromboembolic disease. And previous studies have shown that the elevation of D-Dimer is an important independent and persistent risk factor for cardiovascular events and cancer events[17], as well as an early sign of impending MAS in febrile patients with active rheumatism and monitoring indicator for severe infections[18-19]. In this study, a ROC curve was used to analyze the diagnostic efficiency of various markers for EBV-HLH. It revealed that the diagnostic values of the D-Dimer, IgA, and IgM levels and the CD3+, CD4+, CD8+, CD3-CD19+, and NK cell counts were all better than the plasma EBV DNA load and whole blood EBV DNA load, which suggests that the immune function changes after EBV infection are a main factor for the occurrence of EBV-HLH. After EBV infection, perforin and granzyme are released from CD8+ and NK cells to kill all kinds of infected or tumor cells. However, the sensitivity and specificity of the IgA and IgM levels as diagnostic markers were low in the EBV-infected children, and the CD3+, CD4+, CD8+, CD3-CD19+, and NK cell counts could not be used to distinguish between EBV-IM and EBV-HLH because of their low sensitivity. D-Dimer level was the most valuable indicator for distinguishing between EBV-IM and EBV-HLH in this study. The sensitivity and specificity of the D-Dimer level (cut-off point of 1721.500 µg/L) for use in the diagnosis of EBV-HLH were 88.90% and 90.20%, respectively. However, this study is a small sample study, and the study is limited to 1-3 year-old children, which has certain limitations. Subsequent studies should be further in-depth and expand the sample size.