# Antimicrobial Activity of Human C-Type Lectin Domain Family 3 Member A (CLEC3A)
## 4 Discussion  
Facing the global problem of increasing antibiotic resistance, many researchers are focusing on finding alternatives to the conventional antibiotic treatment of bacterial infections. CTLs have various immunological functions such as pathogen recognition, opsonization, and direct bacterial killing [6]. Mannan-binding lectin (MBL), a human CTL, was applied to decrease the susceptibility to bacterial infections of immune and MBL-deficient patients. Patients who received MBL-infusions did not show allergic reactions and MBL-antibodies were observed [20]. This example of MBL-infusions displays the potential of CTLs to be used in clinical practice [20,21]. For the problem of combatting the two common nosocomial bacteria _S. aureus_ and _P. aeruginosa_, we investigated the antimicrobial activity of the CTL CLEC3A and its protein domains CLEC3A Ex23 and CLEC3A Ex3 against these bacteria. _S. aureus_ is known to induce soft tissue infections in particular [22] while _P. aeruginosa_ is frequently detected in infections of the respiratory tract [23]. In immune-deficient or critically ill patients, both _S. aureus_ and _P. aeruginosa_ can lead to lethal bloodstream infections [22,24]. Due to the clinical relevance of _MRSA_, we also tested CLEC3A and its protein domains against this bacterial strain. We could show that CLEC3A and its protein domains have an antimicrobial effect against _P. aeruginosa_. Notably, the antimicrobial activity of CLEC3A against _P. aeruginosa_ is significantly stronger than that of the CLEC3A-derived AMP Ex12 and even than that of the AMP LL-37. For _S. aureus_, we were able to demonstrate that CLEC3A Ex23 and CLEC3A Ex3 possess antimicrobial activity. Recently CLEC3A has been shown as being highly susceptible to proteases [25] and is a precursor of AMPs [15]. The double function of CLEC3A being a precursor of AMPs and functioning as an antimicrobial CTL is exceptional. RegIII\(\alpha\) is another human, antimicrobial CTL [7]. CLEC3A's and RegIII\(\alpha\)'s antibacterial activity differ for gram-positive and gram-negative bacteria. RegIII\(\alpha\) shows bactericidal activity against gram-positive but not against gram-negative bacteria [7] whereas CLEC3A does not kill gram-positive but does kill gram-negative bacteria. CTLs recognize and bind bacteria using different membrane surface molecules. These vary for gram-positive and gram-negative bacteria, with peptidoglycans [8,12,26] and lipoteichoic acid (LTA) [27] frequently used in gram-positive, and lipopolysaccharidesaccharides (LPS) in gram-negative bacteria [8,10,12,26]. RegIII\(\alpha\) binds to peptidoglycans, but the antibacterial, pore-forming activity was inhibited by LPS, which explains why RegIII\(\alpha\) could affect gram-positive bacteria only [7]. In contrast, the binding of CLEC3A to LPS has been shown [15], which could explain its strong effect against gram-negative _P. aeruginosa_. In addition, CLEC3A, in contrast to the CLEC3A-derived peptide Ex12, did not bind to LTA [15], which could explain its absent effect against _S. aureus_. CLEC3A and RegIII\(\alpha\) share comparable structural characteristics as they consist of a CRD linked to a charged N-terminal segment, which is cationic for CLEC3A and anionic for RegIII\(\alpha\)[14,28]. RegIII\(\alpha\) bactericidal activity is suppressed by the N-terminal prosegment. When the N-terminal prosegment is removed, RegIII\(\alpha\) partially changes the conformation of one of its loops and can then form a hexameric pore in the membrane, which induces bacterial lysis [7]. We could demonstrate that CLEC3A, encompassing the N-terminal segment, kills _P. aeruginosa_ and the charged N-terminal segment of CLEC3A does not have to be removed for its antimicrobial activity. In contrast, only the CLEC3A protein domains CLEC3A Ex23 and CLEC3A Ex3, which do not possess the charged N-terminal segment, exhibit an antibacterial effect against _S. aureus_.