**1. Introduction**

Circovirus is one of the smallest non-enveloped viruses with a single-stranded circular genomic DNA and classified in the genus *Circovirus* of family *Circoviridae* [1]. It infects various species ranging from plants to animals with different pathogenicities [2]. Porcine circovirus type 2 (PCV2) is a prototype circovirus that causes significant morbidity and mortality in swine [3–5], and is associated with different syndromes, such as the post-weaning multisystemic wasting syndrome [6,7] and reproductive disorder [8]. The genome of PCV2 contains two major open reading frames (ORFs). *ORF1* encodes the replicase protein (Rep) involved in rolling-circle viral DNA replication, while *ORF2* encodes the unique viral structural protein capsid [9], which is involved in diverse and essential biological events during virus infection, such as virion attachment [10].

The primary attachment of the non-enveloped virus on the host cells is based on a low-affinity interaction between viral structural protein and cell surface glycosaminoglycans (GAGs)

receptors [11,12]. GAGs include heparan sulfate (HS), keratan sulfate (KS), and chondroitin sulfate (CS), which are characterized by disaccharide units forming blocks of polysaccharides [13]. Among these GAGs, HS is the most common polysaccharide molecule utilized by viruses for attachment to cell surfaces, such as the hepatitis C virus [14], herpes simplex virus 1 [15], human enterovirus 71 [16] and rabies virus [17]. PCV2 has also been verified to use cell surface HS as attachment receptor, and adopts a distinct non-symmetrical receptor distributed pattern on the virion with a multitude of weak HS binding sites [10,18]. Chemical molecules acting as receptor mimics that interfere with HS-virion interactions might exhibit antiviral activities [19]. However, there are very few reports of PCV2 infection inhibition available for pertaining to such natural small molecule compounds.

The green tea catechins (GTCs) are polyphenolic compounds extracted from the leaves of *Camellia sinensis* [20], and have shown numerous beneficial properties, such as anticancer activity [21], anti-inflammatory ability [22], antioxidative properties [23], antibacterial function [24] and antiviral effects [25]. GTCs are predominantly comprised of four constituents: epicatechin (EC), epigallocatechin, epicatechin gallate, and epigallocatechin gallate (EGCG) [23], among which the EGCG is the major component of GTCs accounting for approximately 59% of the total polyphenols, and it is also the most complicated and principal constituent, possessing the activity against infection of several viruses, such as human immunodeficiency virus (HIV) [26], influenza A virus [27], Zika virus [28], herpes simplex viruses 1 [29], and hepatitis C virus [30]. The mechanisms of EGCG antiviral activity are complicated and diverse, depending on the specific virus infection process or host cell response, one of which is that EGCG interacts with viral structural proteins, consequently blocking the latter from recognizing or binding with cellular receptors. In previous reports, EGCG has been shown to interact with the influenza A virus hemagglutinin, disrupting the binding of envelope glycoprotein to cell surface sialic acid receptor [25]. EGCG inhibited the interaction of HIV envelope glycoprotein gp120 with the cellular CD4 molecule [31]. Regarding the GAGs receptors, EGCG could bind to the herpes simplex virus-1 glycoprotein gB and gD, which should interact with HS and 3-O-sulfated HS of cellular glycans, respectively [29,32].

In previous studies, there have been no reports of catechins inhibiting circovirus invasion, we aimed to investigate the effect of EGCG on PCV2 infection.

#### **2. Materials and Methods**
