# Role of Protein Glycoslation in Host-Pathogen Interaction
## 7 Lectin, a Class of Protein Entangled with Glycoprotein, Affects Host-Pathogen Interactions
### Host Lectins  
Host lectins are normally beneficial in pathogen resistance through inhibiting pathogen growth and reproduction or inducing the host immune responses. The animal or plant lectins can inhibit the development of various pathogens including fungi, bacteria, and parasites. For example, a lectin with hemagglutinin activity extracted from _Phaseolus vulgaris_ can inhibit the growth of the fungi _Valsa mali_[218]; the lectin PSL isolated from _Pisum sativum_ seeds exhibits antifungal activity against _Aspergillus flavus_, _Trichoderma viride_ and _Fusarium oxysporum_[36]; a chitin-binding lectin isolated from _Solanum tuberosum_ shows antibacterial activity against _Listeria monocytogenes_, _E. coli_, _Salmonella enteritidis_[219]; the lectin BpLec from _Bothrops pauloensis_ snake venom decreases replication of the parasite _Toxoplasma gondii_[37]. Host lectins can bind peptidoglycans, polysaccharides, lipopolysaccharideacharides (LPSs) or teichuronic acids on the pathogen surface to disturb the cell wall function and thus restrain pathogens [220]. Membrane-bound lectins of hosts can recognize pathogens by the glycans on the pathogen surface, and transmit signals to trigger immune responses. One example is the micelle that can mediate immune responses to resist infection from various pathogens [221]. In mice, the mincle participates in the immune response against _Candida albicans_. The cell wall of _C. albicans_ contains \(\alpha\)-mannosyl or \(N\)-acetylglucosamine residues with specific geometry. It can be specifically bound by a common carbohydrate-binding domain consisting of Glu-Pro-Asn that presents in mice. These glycan residues activate mouse mincle and trigger the Syk/Card pathway to effectively activate Nuclear Factor kappa-B (NF-\(\kappa\)B) signaling. Such a process increases the cytokine expression levels, and consequently prevents the _C. albicans_ infection [38; 222]. In some cases, however, host lectins can also be deleterious when it is used by pathogens. Different from the aforementioned mouse mincle, the human mincle can act as a negative regulatory factor to inhibit immune responses. _Fonsecaa monophora_ is a causative agent of chronic fungal skin infection. The \(\beta\)-glucan on _F. monophora_ cell wall can trigger the host lectin Dectin-1 on the surface of dendritic cells (DCs). Then Dectin-1 activates the transcription factor interferon regulatory factor 1 (IRF1), which is vital for _IL-12A_ transcription, and consequently triggers T\({}_{\text{H}}\)1 and T\({}_{\text{H}}\)17 immune responses against the fungus [40,223,224]. However, in this process, the human mincle on the surface of DCs is simultaneously triggered by the pathogen via proper carbohydrate molecule(s). Activated mincle induces degradation of IRF1 in the nucleus through an E3 ubiquitin ligase Mdm2-dependent degradation pathway. This process is deleterious for antifungal defense, as it leads to the loss of nuclear IRF1 activity and blocks _IL-12A_ transcription [40]. Host lectins may act as spies to help pathogen transmission and dissemination. During the host antiviral response, the lectin DC-SIGN on the DCs surface binds the virus surface glycan, and subsequently \"eats\" and \"digests\" the virus by endocytosis and lysosomes [225]. Alternatively, after DCs catch the virus it migrates to lymphoid organs where DCs present the antigens of the virus to CD4\({}^{+}\) T cells, and then trigger the adaptive immune response to effectively prevent virus infection [226, 227, 228]. This mechanism can be also utilized by some viruses. For instance, the host expressing DC-SIGN can enhance the transmission and dissemination of human immunodeficiency virus type 1 (HIV-1). HIV-1 causes acquired immune deficiency syndrome in humans by targeting CD4\({}^{+}\) T cells [229,230]. In the early stages of infection, this targeting is less efficient, as the concentration for virus or CD4\({}^{+}\) T cells is often very low at the infection sites [231].