Interaction between PCV2 and immune system. What changed after the introduction of vaccination
Porcine circovirus diseases (PCVD) are important causes of economic losses in intensive pig farming. Porcine circovirus type 2 (PCV2) causes systemic (post-weaning multisystemic wasting syndrome - PMWS) and localized (respiratory, enteric, dermatitis-nephritis and reproductive) diseases and is also responsible for subclinical forms of infection causing reduction of average daily gain, growth retardation and increased susceptibility to other infections. For these reasons, PCV2 infection control is an international priority. In recent years, the availability of safe and effective vaccines has made it possible to reduce the incidence and severity of clinical forms, in particular PMWS, to increase the productivity of growing animals and to improve the reproduction parameters of sows. However, despite the successes of vaccination, most of the pig population is still PCV2 chronically infected, even in farms that use indirect prophylaxis plans. In addition, available vaccines are not able to prevent subclinical infections that are currently the main health problem. In subclinical infections, function loss of the natural interferon-producing cells (NIPC) and downregulation of TNF- and IFN- take place. These changes allow an altered balance of pro-inflammatory, pro-immune and regulatory cytokines that result in the upregulation of IL-10 and down-regulation IFN-. The effect is the inefficiency in triggering of an adequate innate pro-inflammatory response, impairment of the recognition of viral and bacterial associated signals, and consequently impairment of the T and B cell responses. Due to these interactions between PCV2 and the immune system, an impairment of both innate and adaptive responses allow the occurrence of subclinical infections that are responsible for severe economic consequences. This impairment of innate and adaptive defenses also explains the role played by PCV2 in coinfections. The anti-PCV2 vaccination can therefore, indirectly, improve the health status also against other pathogens reducing, for example, the incidence of the of porcine respiratory diseases complex (PRDC). Furthermore, in farms where PCV2 and PRRSV are circulating simultaneously, vaccination against PCV2 increases the effectiveness of the anti-PRRSV vaccine. The understanding of the interaction between PCV2 and the immune system and the mechanisms underlying the development of a protective immune response are the necessary prerequisites for the testing of new vaccine formulations. The goal for the near future will be to make available vaccines that can not only decrease the clinical expression of PCVD, but also guarantee a significant reduction in the prevalence of subclinical infections, thus paving the way for the concrete possibility of eradication of PCV2 from pig farms.