Preliminary Study Of Porcine Circovirus Type 2 And Streptococcus Suis Type 2 Co-infection In 3D4/21 Porcine Alveolar Macrophage

Porcine circovirus type 2(PCV2) is the primary causative agent of porcine circovirus associated diseases(PCVAD) in swine. Due to its effect on the immune system, it would cause the immnue suppression. PCV2 has also been shown to infect with other swine pathogens causing the severity of PCVAD. Streptococcus suis is a common respiratory bacteria, the secondary infection of which can be detected in the cases infected with PCV2. However, the pathogenic mechanisms of co-infection of PCV2 and SS is still unkown. Obviously, the immune host cells play a key role in the co-infection of virus and bacteria by regulating the host immune system. The reserach would be of great importance in illuminating the pathogenesis of the co-infection of virus and bacteria.We studied the epidemiology of co-infection of Porcine circovirus type 2 and Streptococcus suis in the swine using PCR. A total of 200 healthy samples from Jiangsu province and 300 samples of diseased pigs obtained from Jiangsu, Zhejiang, Anhui, Huibei and Jiangxi provinces during 2014-2015 were detected for PCV2 and SS, respectively. The results showed that the positive rates of PCV2 and SS in the healthy pigs were 34.50% and 31.50%, respectively. The detection rates of PCV2 and SS in the diseased pigs were 43.67% and 57.00%, respectively. In addition, the co-infection rate of PCV2 with SS in the healthy pigs and diseased pigs were 17.00% and 35%, repectively. Moreover, the co-infection samples in the PCV2 and SS accounted for 69.50% and 59.40%, individually. The results indicated that the co-infection of PCV2 and SS was widespread in the swine, while SS secondary infection is easier to occur to the PCV2 infected pigs.To evaluate the mechanism of the co-infection of PCV2 and SS, PCV2 b strain, SS 2 strain and 3D4/21 porcine alveolar macrophage were used to construst the co-infection model. We observed the bacterial adherence and invasion rate of co-infected group higher than that of the SS 2 group and the SS 2 virulent strain was higher than the attenuated strain. The virus was able to replicate in the 3D4/21 cells. Furthermore, pro-inflammatory(IL-1β,IL-8,IL-18,TNF-α) and immune cytokines(IFN-β,IFN-γ) were evaluated using quantitative Real-time PCR. We found that significantly increased(p<0.01) gene expression levels of IL-1β, IL-8, IL-18, TNF-α, IFN-β and IFN-γ were detected especially in cells dually infected with PCV2 b and SS2-1(the virulent strain) on 12 hpi. The results suggested that the viral replication and the severely increase mRNA expression of IFN-β and TNF-α mRNA might weaken the ability of 3D4/21 cells to phagocytose and eliminate pathogens after infection with PCV2 and SS 2. The capsular polysaccharide(CPS) plays a role in protecting SS 2 against phagocytosis and macrophages, which enhance the SS 2 adhesion and invasion of virus-pre-infected 3D4/21 cells. The secondary infection of the bacteria also privides an apprporiate environment for the replication of virus.Our studies revealed that PCV2 might promote a secondary infection of SS 2 and the concurrent infection might be an important factor in modulating the immune response in vitro with the construction of the co-infection model of virus and bacteria. The research on the interaction mechanisms between PCV2 and SS 2 in pigs will provide a theoretical foundation for the better understanding of the pathogenesis of PCV2, the development of vaccines and the prevention and control of PCVAD.