Introduction
The 2019 novel coronavirus (COVID-19) has faced global pandemic. This is different from the past Severe Acute Respiratory Syndrome(SARS-CoV) and Middle East Respiratory Syndrome coronavirus (MERS-CoV), and may suggest that the COVID-19 situation may not be easily terminated through the second and third wave. Various strategies have been applied to control the rapidly expanding COVID-19 (Kannan, Shaik Syed Ali, Sheeza, & Hemalatha, 2020; Singhal, 2020). There is a need for the development of vaccines that allow the formation of antibodies capable of controlling COVID-19, which is why global pharmaceutical companies are in the process of clinical research on vaccines (L. Yang, Tian, & Liu, 2020). Vaccine development requires three technologies, an antigen, a vaccine adjuvant, and delivery technology. Antigen-related technologies relate to antigen design and production techniques that induce protective immune responses. Vaccine adjuvants are designed to maintain high protective immune responses to the antigen over long periods of time. Finally, the vaccine delivery technology determines the vaccination route{Ahmed, 2011 #1}. Presently, vaccine development has mainly focused on antigen development (Calzas & Chevalier, 2019; Martinon, Cisneros, & Villicana, 2019; Mohan, Zhu, Wang, & Wang, 2018). However, the importance of immunity-enhancing vaccine adjuvants has been emphasized to develop new preventive vaccines against infectious diseases or to improve the efficacy of present vaccines (Roy et al., 2014; Rydell & Sjoholm, 2004). The use of adjuvants may increase the long-term immunogenicity of the vaccine and thus, reduce or eliminate the need for additional inoculations (Ahmed, Plotkin, Black, & Coffman, 2011). Moreover, since immunity-enhancers tends to increase the cross-reactivity of the vaccine, it can also increase the protective effect against serotype strains not included in the vaccine. If a vaccine, such as an influenza vaccine, is temporarily needed due to a global pandemic, it is also possible to increase the vaccine supply by lowering the antigen dose(Ahmed et al., 2011; Cribbs et al., 2003).
Immune cells generate a specific immune response to each pathogenic bacterium or virus at the time of infection but they also have receptors called pattern recognition receptors (PRRs) that respond to commonly-encountered molecules, such as bacterial cell walls, LPS, proteins or viral RNA and DNA (Jahanban-Esfahlan, Seidi, & Majidinia, 2019; Joshi, Shaw, & Quagliarello, 2009). The Toll-like receptor (TLR) is a typical PRR and 11 species are known in humans. TLR agonists are highly active against immune cells and are being developed as vaccine adjuvants (Medvedev, 2013).
Polysaccharides derived from natural products have potential as immunomodulators because they are large molecules that affect the immune system and they have been widely used in clinical applications. Most polysaccharides present in natural products are non-toxic and do not cause the serious side effects associated with other bacterial polysaccharides or synthetic substances that are capable of immunomodulation. They also maintain homeostasis in the body (Harandi, 2018). Recent studies have shown that polysaccharides present in natural products exert immunosuppressive effects on antigens and vaccines in both cellular and humoral immunity, making them alternatives to conventional aluminum salts as vaccine adjuvants for vaccines (Liu et al., 2019; Tian & Liang, 2019). Specifically, plant polysaccharides can safely induce humoral immunity, as well as cellular immunity, as vaccine adjuvants. Their advantages are availability at relatively low cost and application to a wide range of vaccines against cancer and infectious diseases (Gu et al., 2019; Munoz, Porsch, & St Geme, 2019; Tian & Liang, 2019).
Morus alba L . is enriched in polysaccharides, anthocyanins, alkaloids, and flavonoids. Pharmacological functions attributed toM. alba L. include anti-oxidative, anti-inflammatory, anti-diabetic, as well as cardiovascular, hepatoprotective, and neuroprotective activities (Hwang & Kim, 2004; H. G. Kim et al., 2010). M. alba L. has been found to be immunostimulatory in vitroand in vivo . We also found that when M. alba L. was used in combination with anticancer drugs, such as fluorouracil (5FU), it decreased the side effects of the anticancer drug and increased immune function. We recently found that M. alba L. can activate Toll-like receptor 4 (TLR4) on antigen-presenting cells (APCs) (Chang, Kim, Lee, Park, & Kim, 2015; S. B. Kim et al., 2013; X. Y. Yang et al., 2009). However, the oral administration of M. alba L. to improve immunogenicity as a vaccine adjuvant has been rarely reported. If effective, the ingestion of the M. alba L. would be more a more readily accepted and convenient application than intramuscular or nasal administration.
In this study, we conducted studies in a mouse model to investigate the effects of orally administered M. alba L. on modulating the efficacy of ovalbumin and PR8 influenza vaccine immunizations.