Defensins are one major class of antimicrobial cationic pept
Defensins are one major class of antimicrobial, cationic peptides that are released from luteolin and imbedded in the mucus layer on mucosal surfaces . They can be categorized into two major families; α- and β-defensins. In humans there are two α-defensin members, defensin 5 (HD5) and defensin 6 (HD6), that are produced by the Paneth cells in the small intestine [20,45]. HD5 is the most abundant defensin in the Paneth cell granules and it is known to be able to kill a wide range of microbes [46,47]. HD6 is produced to a lower level by intestinal Paneth cells (4 to 6–fold lower) and it produces nanonets that traps microbes and blocks access to the host-cell surface [46,47]. β-defensins, like human β-defensin-1 (HBD-1), are ubiquitously expressed throughout the epithelium in the human small intestine and colon . Based on the intestinal distribution of the different antimicrobial peptides, it is conceivable that Paneth cell α-defensins and the constitutively expressed β-defensins (e.g. HBD-1) could have important roles in the anti-Giardia host defense. In this study we showed that Giardia CPs indeed degrade human defensins (HD6 and HBD-1), showing that the secreted CPs can have a role in interference with defensing activity during Giardia infections. We could not see any degradation of the most highly expressed defensin in the small intestine, HD5, but the predicted site is very close to the C-terminal so it might be difficult to see in gel analyses. Cleavage in this site might affect dimerization of HD5 and in that way function. Further experiments in vitro and in vivo are needed to show if the defensins are important factors in the protection against Giardia infections. A few other interesting potential CP targets were also identified in the in silico screen using the CP cleavage consensus sequence (Table 1). Of these, several chemokines and receptors that play an important role in Giardia infection were identified. IL-6 is important for Giardia clearance in mice [49,50], as is IL-17 receptor A (IL-17RA) and IL-17 A [51,52]. One putative target is toll-like receptor 4 (TLR4), which has been suggested to be important for activation of dendritic cells during Giardia infections . Here, CPs may have a role in reducing immune signaling and the level of inflammation during Giardia infections. Another very interesting group of targets are the mucins, the main components of mucus . The mucus layer acts as the first defense against entero-pathogens throughout the gastrointestinal tract. Here, we identified the membrane-bound MUC4 and MUC16 and the secreted, gel-forming MUC2 as putative CP targets (Table 1). The major component of the small intestinal mucus layer is MUC2 [20,21]. A recent study in Giardia found that cysteine protease activities play a role in the disruption of the intestinal mucus layer by degrading MUC2 . Infection of MUC2 knock-out mice caused higher levels of trophozoites in the small intestine and impaired weight gain. The CP activities of Giardia also regulate mucin secretion and mucin gene expression , but no specific giardial CP causing this was identified. Our study suggest that the three major Giardia secreted CPs studied here might have a role in this process, an interesting question to be addressed experimentally in further experiments. Currently it is not possible to easily produce genetically modified parasites with different genes knocked-out but recent progress in this area  will hopefully make it possible to produce parasites lacking certain proteases and to study these in vivo. In summary, we have performed a detailed analysis of the cleavage specificity of a major Giardia secreted cysteine protease (CP16160) by both phage display analysis and a panel of recombinant protein substrates to give a quantitative estimate on the involvement of the amino acid at and around the cleavage sites. The cleavage specificity of two other major secreted cysteine proteases (CP14019 and CP16779) was also studied using the recombinant substrates. Our results showed that the major secreted G. intestinalis CPs have relatively broad substrate specificity and show both chymase and tryptase activity. Potential in vivo substrates were identified and we showed that all three enzymes efficiently cleave immunoglobulins, including IgA (both IgA1 and IgA2) and IgG, and human defensins. This suggests that the three major secreted Giardia CPs play a role in the interference with the mucosal immune responses during Giardia infections.