Porcine Circovirus Type 2 (PCV2): Pathogenesis and Interaction with the Immune System
Xiang-Jin Meng
Annual Review of Animal Biosciences, Vol. 1: 43 -64 (Volume publication date January 2013)
Abstarct
Porcine circovirus type 2 (PCV2) is the primary causative agent of porcine circovirus?associated disease (PCVAD). The virus preferentially targets the lymphoid tissues, which leads to lymphoid depletion and immunosuppression in pigs. The disease is exacerbated by immunostimulation or concurrent infections with other pathogens. PCV2 resides in certain immune cells, such as macrophage and dendritic cells, and modulates their functions. Upregulation of IL-10 and proinflammatory cytokines in infected pigs may contribute to pathogenesis. Pig genetics influence host susceptibility to PCV2, but the viral genetic determinants for virulence remain unknown. PCV2 DNA and proteins interact with various cellular genes that control immune responses to regulate virus replication and pathogenesis. Both neutralizing antibodies and cell-mediated immunity are important immunological correlates of protection. Despite the availability of effective vaccines, variant strains of PCV2 continue to emerge. Although tremendous progress has been made toward understanding PCV2 pathogenesis and immune interactions, many important questions remain.
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It appears that PCV2 is evolved more rapidly than we originally thought possible for a DNA virus, and the emergence of genetically variant strains of PCV2 and their impact on the current vaccine efficacy need investigations. Although the first clinical cases of PCVAD were recognized only in the late 1990s, the virus has been in circulation since the 1960s (134); how the virus suddenly caused diseases in the late 1990s remains a mystery. Under laboratory conditions, PCV1 has been shown to infect human liver cancer cells (135), and circovirus-like DNA sequences have also been detected in stool samples from humans (136), although their biological significance remains unclear. The discovery of PCV contaminations in live-attenuated human rotavirus vaccines raises concerns for vaccine safety, although definitive evidence of zoonotic human infections by PCV is still lacking. Clearly, future studies are warranted to identify the cellular and viral factors that determine species and tissue tropisms and to assess the zoonotic potential of PCV2.
