ß-defensins

ß-defensins were discovered in the early 1990´s as antimicrobial peptides in epithelial cells of the airways in cattle (DIAMOND et al. 1991). During the last 25 years ß-defensins have also been detected in epithelial surfaces of the skin, respiratory tract, urogenital tract and gastrointestinal tract. For a long period of time only two human ß-defensins, ß-defensin-1 and -2 (hBD-1 and -2) were known (GANZ and LEHRER 1998), however, another two ß-defensins were discovered later on.

Human ß-defensin 3 (hBD-3) has been isolated from human keratinocytes, epithelial cells of the respiratory tract (HARDER et al. 2001; WEHKAMP et al. 2003) and recently from the placenta, endometrium, pharynx, and from intestinal epithelial cells (DHOPLE et al. 2006; SALZMAN et al. 2007; MUKHERJEE et al. 2008). In the intestinal epithelial cells, the Paneth cells are the major producers of defensins (DANN and ECKMANN 2007). Moreover, the peptide was identified in stratified squamous epithelia, such as the epidermis; it was also detected in the epithelial root sheath of hair follicles and their glands of wild mammals (MEYER et al. 2003;

MEYER and SEEGERS 2004).

In the year 2001, hBD-4 was discovered, and it is proven that it is expressed in several tissues (testis, gastric antrum, uterus, lung, kidney). hBD-4, -5 and -6 are the latest three defensins being isolated. hBD-4 and -5 are specifically expressed in human epididymidis, and hBD-6 was found in Escherichia coli (HUANG et al. 2008).

ß-defensins can either be constitutively expressed or are induced by several pathogens. SALZMANN et al. (2007) mentioned that in general ß-defensin expression is inducible at sites of inflammation or infection, whereas DALE and FREDERICKS (2005) distinguished more precisely between the different groups of ß-defensins: They pointed out that hBD-1 is constitutively expressed in epithelial cells in lots of tissues (integument, gut, urinary tract), whereas hBD-2 and -3 is up-regulated by bacteria and proinflammatory stimuli. A remarkable finding was that hBD-2 is also expressed in human uninflammed gingival tissue (DALE and FREDERICKS 2005). The authors suggested that the high level of hBD-2 expression is a result of the exposure of the tissue to commensal, nonpathogenic bacteria.

Regarding this fact, it can be assumed that this ß-defensin has a normal surveillance function (DALE and FREDERICKS 2005). The same findings were obtained by

(VORA et al. 2004) for intestinal epithelial cells, as they described hBD-2 production being induced by activation of TLR through commensal bacteria in the intestine.

Moreover hBD-2 is expressed in human uninflammed skin. It can be localised in the uppermost layers and the stratum corneum of the epidermis. It has been suggested that hBD-2 is synthesised and stored in the lamellar bodies of the keratinocytes of the stratum spinosum and granulosum. Differentiation of the keratinocytes and barrier disruption leads to release of hBD-2. As a result, hBD-2 can be found in the intercellular spaces of the stratum corneum (HUH et al. 2002; OREN et al. 2003).

JIA et al. (2000) and GARCIA et al. (2001) independently confirmed or discussed hBD-3 expression in keratinocytes, as well as in epithelia of the gastrointestinal and respiratory tract, including the human esophagus epithelium. Additionally it was found in tonsils, trachea, placenta, adult heart and skeletal muscle.

Due to economic interest, researchers mainly focus on production animals and studies conducted on companion animals are scarce. LINDE et al. (2008) summed up the occurrence of ß-defensins in different species.

In cattle, ß-defensin was found in the trachea, lung, tongue, mammary gland and intestine. ß-defensin has also been detected in the ovine and caprine respiratory and gastrointestinal tract.

Pigs evoked the interest of scientists due to the fact that the antibiotics, used as growth promoters in sub-therapeutic levels for many decades, were prohibited in the European Union in 2006, thus resulting in the necessity for alternative strategies.

VELTHUIZEN et al. (2008) isolated ß-defensin 2 from the porcine intestine and studied its effect and up-regulation upon infection with different pathogens. They found out that ß-defensin 2 is expressed in the intestine, and that ß-defensin 2 is upregulated upon infection with Salmonella typhimurium. Additionally, LINDE et al.

(2008) demonstrated that ß-defensin 2 is also expressed in liver and kidney, though the peptide is mostly present in the ileum.

Information on innate immune mechanisms in companion animals is rare. Up to now ß-defensins have only been found in the canine testis (SANG et al. 2005).

As mouse and rat are the most common species used as scientific animal models, the occurrence of defensins has been intensively studied in these species. Thus, defensins have been detected in almost every organ (BALS et al. 1999).

Interestingly, the mouse is the only species until now in which expression of ß-defensin has been reported in the esophagus epithelium (JIA et al. 2000).

ß-defensins interact with pathogens. The microbicidal properties of defensins can be explained by their ability to interact with a bacterial surface. Membrane disruption through pore-forming activities is the major mechanism. This mode of action is described in detail by the Shia-Matsuzaki-Huang model (Fig. 2.3). MATSUZAKI et al.

(1999), YANG et al. (2000) and ZASLOFF et al. (2000) agree on the fact that the microbicidal activity of defensins is due to their cationic and amphiphilic nature. The composition of the cell membrane of microorganisms is characterised by an abundance of negatively charged phospholipids and an absence of cholesterol. This fact allows defensins to insert themselves into the phospholipid membranes. The hydrophobic regions are within the lipid membrane and the cationic region is located on the outside. Thus, a carpet-like structure is formed by defensins on the microbial surface. Subsequently the contiguity of the microbial membrane is disrupted and pores are formed. This leads to the complete collapse of the bacterial membrane.

21 Fig 2.3: Shia-Matsuzaki-Huang model. The model displays the general consensus for HDPs’ antimicrobial mode of action. 1: Host is initially exposed to microorganisms. 2: The innate immune response involves recruitment of cationic HDPs, which are immediately attracted toward the anionic microbial membrane. 3: The HDPs form a carpet-like structure on the microbial membrane, instituting channel formations. 4: The channels lead to pore-formation membrane destabilization and microbial demise. HDP, host defence peptides. (modificated after LINDE et al. 2008)