Trichinella spiralis is a tissue-dwelling parasitic nematode which infects over 150 kinds of mammals around the world (Pozio, 2005). Human Trichinella infection is principally caused by eating raw or semi-cooked animal meat containing infectious muscle larvae (ML). Pork and pork products of infected pigs are the dominant source of human trichinellosis in developing countries (Cui et al., 2013a; Jiang et al., 2016; Rostami et al., 2017). However, it is difficult to eliminate Trichinella infection in animals due to its broad distribution of animal hosts (Liu et al., 2015b). Trichinella infection is not only a major public health issue but also an important risk to meat food safety (Murrell and Pozio, 2011; Xu et al., 2021).

After being ingested, the encapsulated T. spiralis ML in skeletal muscles are released from collagen capsules in the stomach by means of digestion of gastric fluids, and the ML are activated into intestinal infectious larvae (IIL) following exposure to gut contents or bile (Liu et al., 2013; Ren et al., 2013). The IIL penetrate intestinal epithelium cells (IECs), molt four times and develop to adult worms (AWs) at 31 h post infection (hpi). Female AWs produce the newborn larvae (NBL) which migrate through the lymphatic and blood system to skeletal muscle tissues where they develop into the encapsulated larvae to complete the lifecycle (Despommier, 1998; Xu et al., 2018). Gut epithelium is the first natural defense barrier against Trichinella invasion (Hu et al., 2020b). However, the mechanism of gut epithelium invasion by Trichinella IIL is not yet completely elucidated (Lei et al., 2020). Characterization of invasion-related proteins will be valuable to understand Trichinella invasive mechanisms and further exploit new vaccines against larval invasion (Ren et al., 2021; Yue et al., 2020).

Previous studies showed that when the IIL were co-cultured with IECs monolayer, IIL invaded the monolayer and produced various types of proteases (Wang et al., 2012, 2013). Out of these proteases, a T. spiralis aminopeptidase (TsAP, GenBank: XP_003377703.1) has been identified and characterized. The results showed that TsAP participates in the invasion, development and fecundity of T. spiralis. Vaccination of mice with rTsAP only induced a partial protective immunity against a larval challenge, suggesting other aminopeptidases might also be involved in T. spiralis larval invasion, development and survival (Guo et al., 2020; Zhang et al., 2013). Therefore, it is needed to further characterize other novel aminopeptidases and investigate their biological functions in T. spiralis invasion and survival in host.

In this study, a novel T. spiralis aminopeptidase P (TsAPP, GenBank: EFV57850.1) was identified in the IIL co-cultivated with IECs (Wang et al., 2013). TsAPP belongs to peptidase M24 family. Bioinformatics analysis revealed that TsAPP has a functional domain of aminopeptidase P. However, the biological properties and functions of TsAPP in the T. spiralis lifecycle are not clear (Mitreva et al., 2011; Nagano et al., 2009). The aim of this study was to investigate the biological properties and roles of TsAPP in gut epithelium invasion of IECs.