Perspectives
The circulation of PEDV has caused huge economic damages to the pig industry in the world, while the emergence of its variant strains aggravates the challenge. An increased prevalence of PEDV has been witnessed in those years. For example, an outbreak of PEDV caused PED in China in 2010 was characterized by high mortality in suckling pigs (RQ et al., 2012). Though progress in the understanding of PEDV including its genome, viral structure, has been made by the efforts of numerous investigations during the past years, it seems we still stand a little far away from completely revealing the pathogenic mechanisms of PEDV. In addition, the development of effective and preventive measures like medicines and vaccines remains on the way.
To establish a productive infection, invading viruses need to overcome their host’s first line of defense, the innate immune response. Although this reaction can largely protect the host from attacking by most microorganisms, it is still susceptible to antagonism and destruction of pathogenic viruses. As an important porcine pathogen emerging in recent years, PEDV has evolved strategies to overcome host innate immunity, including avoiding recognition by PRRs, inhibiting IFN production, and/or antagonizing IFN signal transduction and antiviral action mechanism of the host. Relative to extensive studies and in-depth information on other coronaviruses like SARS-CoV, the host-PEDV interactions just receive increased attention recently. One typical example is that PEDV targets intestinal epithelial cells (IECs) in the intestinal villi of pigs, while the most common in vitro cell culture system used to study PEDV does not derive from IECs of pigs. The development of new pig IEC derived cell lines will undoubtedly provide an alternative, more physiologically relevant model for future studies of the PEDV-host interactions.
After an outbreak of PED in the United States in 2014, Zoetis has developed PEDV inactivated whole virus vaccine, which can provide passive immunity to piglets by immunizing sows before delivery. Before the new PED outbreak, the attenuated virus vaccine has played an effective role in the prevention and control of PED (CH, BJ, JG, GO, & YB, 1999; DS et al., 2007). Besides this, PEDV genetic engineering vaccines based on the S protein which can induce the body to produce neutralizing antibodies, also display broad prospects. As the most ideal vaccine for oral immunization, transgenic plant vaccine similarly have great development space (TJ, SC, MS, & YS, 2006). The numerous properties of nanoparticles, including possessing immunoadjuvant activity, generating natural immune response of antigens, easily reaching antigen-presenting cells to regulate the immune response through a variety of ways, targeting to present antigen, and releasing slowly, allow it to be an ideal candidate for preparing new vaccines against the occurrence of PED (DM, JK, & JR, 2013; T et al., 2020). Notably, although vaccine immunization is an effective way to prevent and control PED, for piglets infected with PEDV, it often cannot solve all of the issues. Therefore, some therapeutic drugs are urgently needed as well. In this regard, the validly inhibitory effect of IFN-L on the proliferation of PEDV in pig intestinal epithelial cells together with better action than type I interferon, empowers it to receive more attention (L. L et al., 2017). Likewise, PEDV specific yolk antibody has also been reported to increase the survival rate of infected piglets (DH et al., 2015). Besides these, prokaryotic expression of single stranded variable region of PEDV monoclonal antibody or small peptides identified by phage screening, which is able to bind to PEDV receptor, can also prevent PEDV from invading host cells (M. F et al., 2014). Additionally, the key role of proteases in virus release from cell surface and enhancement of PEDV infection, render them to be important drug targets. Therefore, protease inhibitors may also be good candidates for developing anti-PEDV compounds to fight this infectious disease (S. K et al., 2011). Similarly, some Chinese herbal extracts, such as quercetin and ginkgo peel extract, can effectively suppress the infection process of PEDV in vitro (HJ et al., 2009; L. JH et al., 2015; S. JH, JK, & HJ, 2011; T et al., 2020). Although some drugs have been proved to have anti-PEDV activity, they have not been used in clinical treatment due to its cost and safety. Therefore, it is of far-reaching significance to deeply study the pathogenesis of PEDV, identify PEDV receptors and related host factors, thereby providing more druggable targets for the prevention and treatment of PEDV.