Defensins represent an ancient, highly conserved part of the innate immune system. Most of these small endogenous peptides possess broad-spectrum antimicrobial activity and immunomodulatory functions. In humans, granulocytes and Paneth cells secrete different alpha-defensins, whereas beta-defensins are expressed by epithelial surfaces throughout the body. Beta-defensin 2 consists of 64 amino acids and has a molecular weight of 7 kDa.
Defensins exert antimicrobial activity against bacteria, fungi, and some enveloped viruses. Vertebrate defensins are classified as α- or β-defensins based on their pattern of disulfide bridges. Nine human defensins of epithelial origin have been found, three being beta-defensins (HBD-1, -2, and -3). The pro-inflammatory cytokines and microorganisms (e.g., E. coli, H. pylori, or P. aeruginosa) induce the expression of beta-defensins.
As shown in vitro, HBD2 has antimicrobial solid and immunomodulatory functions and is induced by inflammatory stimuli or exogenous microbial substances. HBD2 promotes intestinal wound healing and angiogenesis in vitro and can act as a chemoattractant for dendritic cells (DCs), monocytes, and T-cells through interaction with the chemokine receptor 2 (CCR2) and 6 (CCR6). Thus, in addition to a lack of mucosal antibacterial activity, low defensin expression may also translate into a repressed anti-inflammatory activity. Together, these data provide evidence for the essential role of defensins, including hBD2, in IBD disease pathogenesis and potential therapy.
For example, a beta-defensin 2 deficiency can be observed in the intestinal mucous of Crohn’s disease patients. Therefore, the mucous membrane's defense system is restricted, allowing an increased invasion of bacteria, which could lead to a typical infection in Crohn’s disease patients.
