3  Polysaccharides have unique antiviral mechanisms
The antiviral mechanism of polysaccharides is usually related to its specific structure and virus type. Coronaviruses are enveloped positive-stranded RNA viruses that replicate in the cytoplasm [81]. To deliver the nucleocapsid into host cell, the coronavirus-cell entry procedure involves the fusion of the envelope with the host cell membrane mediated by viral S proteins (Fig. 2) [7,81], which is the main determinant of virus entry.

3.1  Directly interacting with virus
Polysaccharides, especially sulfated polysaccharides, can interact with the surface of virus by negative charge, thereby inhibiting the infectious ability of the virus, or killing the virus directly. Pathogens use GAGs at almost every major entry portal to promote their attachment and invasion of host cells, to move from one cell to another, and to protect themselves from immune attack [22]. For example, fucosylated chondroitin sulfate was effective in blocking laboratory strain HIV-1IIIB entry and replication (4.26 μg/mL and 0.73 μg/mL, respectively), and inhibiting infection by clinic isolate HIV-1KM018 and HIV-1TC-2 (23.75 μg/mL and 31.86 μg/mL, respectively) as well as suppressing HIV-1 drug-resistant virus. Further studies indicated that fucosylated chondroitin sulfate can potently bind the recombinant HIV-1 gp120 protein to inhibit several strains of HIV-1 [82]. A cationically modified chitosan derivative, HTCC, has been shown to be an effective inhibitor of HCoV-NL63 replication. The analysis of the interaction between HTCC polymer and the recombinant ectodomain of the S protein from CoV showed binding, resulting in the formation of protein-polymer complexes. One may assume that such binding will result in the efficient inactivation of the virus [83]. Carrageenan acts primarily by preventing the binding or the entry of virions into cells [84,85]. Iota-carrageenan, a high molecular weight sulfated polysaccharide, is an approved antiviral drug that interacts with the viral surface [67]. The binding and inactivation of virus particles by iota-carrageenan are fast and highly effective. During the residence time of the iota-carrageenan containing lozenge in the mouth, the viral titer is reduced by 85% and 91% for IAV and HCoV-OC43, respectively [67]. Furthermore, animal experiments have shown that iota-carrageenan can reduce the spreading of influenza virus in surface epithelia of infected animals, and thereby provided sufficient benefits for animals to promote survival [86].

3.2  Inhibiting virus adsorption and invasion
The first step for virus to invade a cell is to bind to the cell surface by electrostatic interaction or a receptor, such as heparan sulfated proteoglycan on the cell surface. Polysaccharides, especially sulfated polysaccharides, have strong polyanionic properties, and can block the positive charge on the cell surface to prevent virus adsorption or invasion [12]. The invasion process of virus is often associated with the endocytosis of virus, the fusion of virus with cell membrane, and the translocation of virus [12]. Heparin or heparin-like materials with broad-spectrum antiviral properties [[87], [88], [89]] have been developed to mimic the cell surface carbohydrates responsible for initial viral attachment, such as HS and carrageenan [34,90]. The sulfated polysaccharide derived from marine microalga showed strong inhibition against IAV infection via the viral adsorption and internalization steps [91]. The antiviral effect of sulfated polysaccharides from seaweeds was mainly exerted during dengue virus (DENV)-2 adsorption and internalization [92]. Iota-carrageenan and its N-sulfonated derivatives of poly (allylamine) hydrochloride showed strong antiviral activities against human metapneumovirus (hMPV), a kind of respiratory infections RNA virus, by blocking virus release from the cellular membrane and inhibiting virus adsorption [93]. Iota-carrageenan also effectively prevents the replication of HRV in primary human nasal epithelial cells in culture. The data suggest that iota-carrageenan acts primarily by preventing the binding or the entry of virions into the cells [85]. Fucoidan can bind to the neuraminidase (NA) of IAV, and inhibit the activity of NA to block the release of IAV. Additionally, fucoidan can also interfere with the activation of EGFR, PKCα, NF-κB, and Akt, and inhibit both IAV endocytosis and EGFR internalization in IAV-infected cells [73]. The antiviral mechanism of the fucoidans may be through blocking herpes simplex virus (HSV)-2 virion adsorption to host cells [94]. Our team found that 3,6-O-sulfated chitosan (36S) possessed broad anti-HPV activities by directly targeting viral capsid protein and host PI3K/Akt/mTOR pathway to inhibit cell autophagy (Fig. 6 ) [95]. Interestingly, using HCoV-NL63 as a model system, it can be determined that HTCC polymer blocks the interaction between S protein and cell receptor, consequently blocking its entry into cells and preventing virus infection [68,96]. The nano/microspheres of N-(2-hydroxypropyl) -3-trimethyl chitosan (HTCC-NS/MS) were used for adsorption of the coronavirus HCoV-NL63 from aqueous virus suspensions. This nano/microspheres can be applied for the removal of coronaviruses and purification of water from pathogenic coronaviruses [97].
Fig. 6 The mechanism of 3,6-O-sulfated chitosan inhibiting HBV [95].

3.3  Inhibiting viral transcription and replication
Polysaccharides, especially sulfated polysaccharides, can directly interfere with viral replication related enzymes and relevant targets in host cells. Iota-carrageenan can effectively inhibit porcine reproductive and respiratory syndrome virus (PRRSV) replication at mRNA and protein levels in both Marc-145 cells and porcine alveolar macrophages [98]. Carrageenan oligosaccharide and its sulphated derivative have good inhibitory effects on IAV replication both in vitro and in vivo, while not seem to be dependent on the interferon system [99]. Sulfated polysaccharide from Gracilaria lemaneiformis shows anti-influenza virus activities in vitro by inhibiting viral adsorption and replication on host cells [100]. Polysaccharides isolated from Grifola frondosa showed resistance against enterovirus 71, a positive-stranded RNA virus, by blocking viral replication and inhibiting viral VP1 protein expression and genomic RNA synthesis [101]. The virus replication was inhibited by a sulfated polysaccharide from Angelica sinensis, which is a commonly used traditional Chinese herbal medicine, at the dose of 10 and 30 mg/kg (26% and 30% inhibition respectively) [102]. APS has a long-lasting inhibitory effect on HBV replication in vivo, which can be used as a supplementary modality to treat hepatitis B infection [103]. Furthermore, APS can inhibit the replication of avian IBV in vitro in a dose-dependent manner [78].

3.4  Activating host antiviral immunomodulatory system
After the virus invades the host, it will trigger the host's immune response, such as regulating the host NK and macrophages cells, inducing the production of immune cytokines, and indirectly exert antiviral effects by activating innate immunity. Chitosan can enhance antigen-specific immune responses by increasing the induction of regulatory T cells, lung resident T cells, and neutralizing antibodies while reversing Th2-skewed immune responses induced by inactivated respiratory syncytial virus (RSV) vaccine without affecting lung histopathology in mice [13]. The sulphated-carrageenan from red alga showed a strong effect on tobacco mosaic virus (TMV) infection by affecting virus accumulation/infectivity and enhancing locally plant immunity [104]. APS can significantly enhance the immunological function of chicken erythrocytes after infected with infectious bursa disease virus (IBDV) [105]. Additionally, APS can reduce the replication of H9N2 AIV and promote early humoral immune responses in young chickens [17]. LNT can significantly down-regulate the expression level of TNF-α, IL-2 and IL-11, and up-modulate the expression levels of IFN-1 and IFN-γ after challenging with infectious hematopoietic necrosis virus (IHNV), which is an RNA virus. The results indicate that the inhibitory effects of LNT on IHNV infection are possibly attributed to its regulation of the innate immune responses and specific immunity [51].
In addition, our team found that PGS, a sulfated derivative of alginate, can effectively inhibit the expression and secretion of HBsAg and HBeAg in HepG2.2.15cells. The anti-HBV mechanism of PGS may be associated with appropriate activation of NF-κB and Raf/MEK/ERK signaling pathways to enhance the interferon system, and interfere with HBV transcription (Fig. 7 ). This study suggested that PGS merits further investigation as a novel anti-HBV agent aimed at modulating the host innate immune system in the future [42]. These studies bring new ideas to the development of current anti-novel coronavirus drugs.
Fig. 7 The molecular mechanisms of PGS inhibits HBV replication. Cellular NF-κB and Raf/MEK/ERK signaling pathways are associated with the activation of innate immune system such as interferon system. PGS can bind and enter into HepG2.2.15 cells to activate the NF-κB and Raf/MEK/ERK pathways to enhance the interferon system, and indirectly suppress HBV transcription [42].