Introduction
African swine fever (ASF) is an acute hemorrhagic disease of domestic pigs. This disease often results in devastating economic losses to the pig industries of suffered countries because of the high rates of morbidity and mortality, and it is therefore classified as a notifiable disease by the World Organization for Animal Health (OIE). An effective vaccine is still not available so far, and control of the disease relies mainly on rapid diagnosis and culling of infected pigs and close contacted with them (Rowlands et al., 2008).
ASF was first recognized in early 1900’s in East Africa. Since then, it has spread to most sub-Saharan African countries(Boshoff et al., 2007). However, when the disease introduced in Georgia in the Caucasus in 2007 (Rowlands et al., 2008), it has spread subsequently toward the northern and eastern areas of Europe with many countries affected, such as the Russian Federation, Ukraine, and Poland (M. C. Gallardo et al., 2015; Goller et al., 2015; Smietanka et al., 2016). In 2018, the first ASF outbreak was reported in Liaoning Province in China (Ge et al., 2018; Zhou et al., 2018). Since then, the disease has occurred in almost every major pig production area of China (Tao et al., 2020). A recent surveillance showed that naturally attenuated ASFVs have been identified in several provinces of China, which makes the control situation of the disease even more complex in the future (Sun et al., 2021). The disease has also been reported in some other Asian countries recently (Mighell et al., 2021).
ASF virus (ASFV) is the only member of Asfarviridae family (Alonso et al., 2018). An ASFV virion has an overall icosahedral morphology with a diameter of 260-300 nm. It is composed of five layers including a viral core, a core shell, an inner lipid membrane, an icosahedral capsid, and an external envelope (Wang et al., 2019). The viral genome is a double-stranded DNA molecule of 170-190 kbp encoding more than 150 viral proteins. About 50 of the viral proteins are structural proteins that response for many important roles in the virus life cycle, such as viral particle assembly and the infection of a host cell; while the rest of them are non-structural proteins that are expressed during viral replication and function mainly as regulators of replication. Still, the functions of about 40 proteins of ASFV are unknown yet (Alejo et al., 2018).
The pB602L protein of ASFV is encoded by B602L gene, which contains a central variable region(CVR) and always severs as one of the targets for sub-genotype classification of ASFV isolates (Atuhaire et al., 2013; C. Gallardo et al., 2009; Mai et al., 2021; Owolodun et al., 2010). pB602L is one of the non-structural proteins and functions as a molecular chaperon the major structural protein p72, which formed aberrant ”zipper-like” structures instead of icosahedral virus particles in the absent of pB602L during the viral capsid assembly (Epifano et al., 2006; Liu et al., 2019). However, by which means that pB602L helps p72 to assemble correctly is still a mystery. Previous studies showed that pB602L has strong antigenicity and can be used to develop diagnostic tools for ASFV(Gutierrez-Castaneda et al., 2008). Given that live-attenuated ASFVs have been believed the most promising vaccination strategies against ASFV (Chen et al., 2020; Sang et al., 2020), pB602L is probability a suitable target for developing diagnostic tool for evaluating the humoral immune responses of these vaccines because of the antibodies against pB602Lproduced only after the expression of the protein in the host cells.
In this study, we expressed and purified a recombinant pB602L of ASFV strain HLJ/2018, which was then used as antigen to immunize mice for monoclonal antibody (mAb) development. A total of eight mAbs were obtained and they bound to three linear epitopes in pB602L. These results provide biological materials and molecular basis for the basic and applied researches on ASFV.