Liver fibrosis is the main protective pathological process of liver tissue in continuous stimulation E. granulosus infection. The early stage of liver fibrosis can be reversed, the damaged liver tissue can be repaired by collagen tissue and the growth of worm can be limited. As the lesion is continuously expand, the degree of fibrosis in the middle and late stages becomes more serious, even irreversible, which can further develop into cirrhosis and liver cancer.
Monocyte-macrophage system is a subgroup of leukocyte system, which originates from bone marrow hematopoietic cells and exists in blood circulation. After the pathogen infected the body, Kupffer Cells (KCs) in the liver firstly recognized it, then recruited many circulating monocyte-macrophages to play a role at the infection site [16]. In the early stage of parasite infection, the liver microenvironment was dominated by Th1 type immune response. With the deposition of parasite eggs, the immune response gradually changed to Th2 type in the liver [17]. Human and mouse studies indicated that KCs and MoMFs both had tissue-destructive and pro-restorative/tissue repair functions in APAP injury [18, 19].
The expression of CX3CR1 and CCR2 is closely related to the function of monocyte-macrophages. In our study, in the early stage of E. granulosus infection, the non-parenchymal cells in the liver expressed CX3CR1 and CCR2, which indicated that there were a large number of monocyte-macrophages infiltrating into the liver, and many CX3CR1 positive cells mainly located in the hepatic sinusoids; in the middle and late stages, the positive expression of CX3CR1 in the hepatic lobules gradually weakened, and the positive expression mainly existed around the lesions, which proved that after receiving the chemokine signal from KCs cells, the peripheral blood derived monocytes were collected from the blood to the liver, and then migrated to the lesions from the liver sinusoid and other hepatocyte spaces, indicating the infiltration of monocytes [20]. As for CXCR4, it is mainly expressed in monocytes and neutrophils, which is closely related to inflammatory response, liver fibrosis, tumor progression and metastasis [14, 15]. In this study, the expression of CXCR4 first showed a significant difference at the 30 days after infection, while pathology showed that at the 30 days after infection, lesions caused by E. granulosus infection gradually occurred in the liver. Whether there is a correlation between the two remains to be further studied.
α-SMA and Desmin, as indicators of HSCs activation [21], had a significant difference at the 180 days after infection, and a large number of HSCs were activated. Masson and Sirus red staining both showed the collagen fiber layer around the focus thickened and the collagen fibers penetrated deeply into the surrounding liver tissue with the aggravation of E. granulosus infection, however, the increased geen-expression only happened at the 8 days after infection, this might be due to the small number of infection models in each group, which led to statistical difference. IHC results showed that the expression level of HSCs in different stages of infection was higher than that in the control group, and mainly located around the lesion, it indicated that cytokines secreted by monocytes might activated HSCs with α-SMA expression was increased.
MMP-8 belongs to MMPS family, is a kind of protease that mainly degrades ECM components and mainly expressed on monocyte-macrophages [22]. In this study, we found that the expression of MMP8 in the model group was significantly higher than that in the control group at all stages, which indicated that the stimulation of E. granulosus promoted the release of chemokines, recruited a large number of monocytes to activate HSCs, and the fibrogenic microenvironment reversely promoted monocytes, it produced MMP-8 and other secreted proteins to degrade the ECM. In the middle and late stages, the expression of MMP-8 was mainly concentrated around the hepatic lesions, located in the wall layer of the fibrous sac, which proved that its main role was to degrade the matrix protein components of the fibrous sac wall, however the balance of ECM generation and degradation was destroyed, gradually leading to fibrosis around the lesions [23].
Phagosomal recruitment of the lysosomal protein LAMP-1 is a widely used marker of phagosome maturation [24]. In order that phagocytes can exert microbicidal activities, their phagosomes must undergo a process called phagosome maturation, if the proteins are lacking, the maturation process is impaired [24]. In this study, the results showed that the expression of LAMP-1 in the liver of mice infected with E. granulosus was significantly higher than that in the control group from 2 to 8 days, and it seemly inhibited with a low level after 30 days of infection, it seemed that phagocytic lysosomes were activated in the early stage of infection, and played a role of phagocytic clearance of pathogens, which was inhibited in the late stage of infection, it seemed to evade its microbicidal activity along with the aggravation of E. granulosus infection. This conclusion seems to be consistent with Leishmania promastigotes infection [25, 26].
Protein kinase C (PKC) is a serine/threonine kinase that regulates diverse cellular responses including immune responses [27]. PKC isoforms was shown to regulate immune cell signaling, and their inhibition was shown to alter immune cell effector functions in Leishmania infection [28]. And Mukherjee et al demonstrated that as an immune evasion strategy, Leishmania modulated those responses to ensure its survival [29]. After activation PKC-α translocated toward the plasma membrane and co-localized with surrounding collagen fiber within the contact areas of the cells and PKC-α directly bound and phosphorylated filamin, and was responsible for cell adhesion, cell migration and actin arrangement in the cells [30, 31]. In our study, we found that PKC-α expression was always higher than that in the control group at the 300 days infection. We speculated that, due to the influence of E. granulosus infection on liver microenvironment, the recruitment of monocyte-macrophages upregulated PKC-α expression after receiving external signals and PKC-α also regulated fibrotic markers to promote the process of fibrosis. The expression change of LAMP-1 and PKC-α in our study might indicate an immune evasion strategy of E. granulosus to ensure its survival in the liver.