The spleen is one of the major parenchymal organs affected by PRV. After PRV infection, the spleen mainly exhibits small white coagulative or lytic necrotic lesions7. In this work, In this experiment, we observed congestion and even inflammatory cell infiltration in the red pulp area of the spleen after PRV infection. Research has shown that PRV elevates the signaling pathways of p38-MAPK and JNK/SAPK, inducing tumor necrosis factor-α (TNF-α) mRNA levels and promoting of TNF-α secretion, which results in cell apoptosis8. An in vitro study showed that, a huge ROS was generated in the PK15 cells infected with PRV and this caused DNA damage and cell apoptosis9. The spleen is a substantial organ rich in mitochondria, an organelle that serves as the main source of ROS production. However, the mitochondria are also susceptible to ROS damage. ROS promotes the formation of lipid peroxidation, in which MDA is a key marker10. The oxidative stress induced by the out-of-balance between oxidants and antioxidants is an important feature of the occurrence of a series of viral infectious diseases11. It reported that HSV-1, a member of the PRV genus, causes GSH depletion and increases ROS and lipid peroxidation levels12. It has been reported in both cellular and animal models that HSV-1 causes host damage through oxidative stress13. In this experiment, after PRV was subcutaneously inoculated into mice, basing on the microscopic examination, we further observed that PRV promoted the production of ROS and MDA in the spleen of mice, reduce the production of non-enzymatic substances GSH, and at the same time down-regulated the expression of antioxidant enzymes i.e CAT, SOD, GSH-Px in the terms of gene levels, indicating that PRV can break the balance between splenocyte oxidation factors and antioxidant factors, hence induce oxidative stress in the spleen.
High levels of ROS are the main cause of cell apoptosis under physiological and pathological conditions. PRV induces the oxidative stress and damage of DNA, thus affecting the expression of pro-apoptotic signaling molecules and key regulatory molecules of cell cycle9. Apoptosis is a mechanism for maintaining balance in vivo, and has a pivotal role in viral infection. Several viruses directly induce host cell apoptosis at the stage of infection, which may be a pathological factor related to infection. It is known that PRV infection triggered suicide or apoptosis of infected cells14. Generally, apoptosis is activated mainly in the late stage of infection by PRV. Moreover, studies have shown that PRV triggers ROS production and causing of apoptosis in autophagic-damaged cells, which indicated that ROS may be participated in the linking of autophagy and apoptosis in cells infected by PRV15. Oxidative stress is associated with various cell death processes, including apoptosis, caused by viral infection. Hepatitis B virus (HBV) combines with the mitochondria through the C-terminal of X protein (HBX) to cause damage to mitochondrial DNA, and over-express the SIRT-1 gene for ROS production16. Interestingly, the Hepatitis C virus (HCV) causes damage to host cells in mitochondria by activating oxidative stress, and the damaged mitochondria are eliminated by way of autophagy17. In this work, our results indicated that the PRV-HLJ strain caused oxidative stress in the spleen of mice, thereby inducing splenocyte apoptosis. Hu Wei et.al18 reported that the PRV Bartha strain up-regulated the expression of LMP2, LMP7, LMP10, and other immune-related protease sub-units. After infecting mice with the PRV-GXLB-2013 strain, they observed an increase in ROS production in the brain, spleen, and lung.19. From these results, we speculated that there is a certain connection between the immune response caused by PRV and an organ being in a state of oxidative stress, however, the specific mechanism is not yet clear, and requires further research.
Virus infection activates Caspase-3, leading to cell apoptosis20. Caspases, a series of enzymes in the cysteine protease family, plays a pivotal function in the maintaining of homeostasis and regulating of programmed cell death in vivo. Caspase-3 and caspase-9 have been widely classified based on their known role in cell apoptosis21. Caspase-3 activation is essential for the promotion of cell apoptosis. Bax and Bcl-2 are two regulators in the process of releasing cytochrome C and mitochondrial integrity. Research has shown that excessive levels of ROS have a crucial role in activating the mitochondrial-dependent apoptosis signal pathway, which promoted the release of cytochrome C, reduces levels of mitochondrial membrane potential (MMP), damages cell respiratory function, disturbances the imbalance of energy metabolism, and finally apoptosis22. Apoptosis is inevitably related to the change of MMP. The decline of mitochondrial membrane potential is an important landmark event for apoptosis23.
The studies on the mechanism of PRV invasion are still unclear. The oxidative stress caused by the excessive generation of ROS in the spleen after being invaded by the virus is closely related to tissue damage. This was consistent with our experimental results which confirm that PRV infection caused spleen oxidative stress, thereby inducing splenocyte apoptosis. This study provides certain scientific data for further exploring the spleen damage caused by PRV and its pathogenic mechanism.