**6. Mechanisms of action of β-defensin-2**

Several mechanisms of action have been proposed for defensins; however, the main mechanism of action of the HBD2 is to eliminate microorganisms

**57**

charges [37].

**Figure 3.**

cells [69, 70].

is given below.

*Multifunctional Activity of the β-Defensin-2 during Respiratory Infections*

directly through the interaction with the microorganism membrane. The first step is given by the electrostatic attraction between the cationic defensin with positive charge and the microorganism's membrane components with negative

*Mechanisms of action of HBD2. The defensin with its positive charge is attached by electrostatic attraction to* 

Components of the bacteria membrane have been identified as targets for the HBD2. Lipopolysaccharides are targets for Gram-negative bacteria, teichoic acid for Gram-positive bacteria and phospholipids for both bacteria. In Gram-negative bacteria, the peptides are inserted into the membrane by hydrophobic interactions. It is thought that it possibly involves a folding of the peptide within the structure of the membrane [38, 66, 67]. After the electrostatic interaction of the peptide with the membrane and the displacement of the lipids, the defensin is added to the surface of membrane. There are several modes of action that have been proposed to describe how defensins are oriented to form pores and how the structure of the membrane is altered, becomes permeable, such as cell lysis and finally results in the

The high content of negatively charged amino acids in the membranes of bacteria is the main factor that makes them more susceptible to being targeted by defensins. The membrane of the eukaryotic cells predominates lipids with neutral charge without net charge, they have a high level of cholesterol, and bacteria do not contain cholesterol in their membranes. Cholesterol causes the membrane to condense and prevents the peptide from penetrating; this also has an asymmetric distribution of phospholipids contributing to the resistance against defensing. These mechanisms explain why these peptides are not toxic in eukaryotic

HBD2 was isolated and characterized by its antimicrobial activity. Currently, several studies have described different biological activities of HBD2, and it is considered as a multifunctional protein. Some of the account on its biological activities

death of the microorganism (**Figure 3**) [3, 37, 68].

**7. Multifunctional activities of β-defensin-2**

*DOI: http://dx.doi.org/10.5772/intechopen.80611*

*the membrane of the pathogen forming pores [3].*

*Multifunctional Activity of the β-Defensin-2 during Respiratory Infections DOI: http://dx.doi.org/10.5772/intechopen.80611*

#### **Figure 3.**

*Immune Response Activation and Immunomodulation*

**5. Expression and regulation of β-defensin-2**

crobial activity is secreted on the surface of the epithelial cells [3, 52].

phosphatidylinositol-3-kinase (PI3K) and protein kinase C (PKC) [59].

HBD2 is expressed mainly in all the epithelia of human body (respiratory, digestive, urogenital, conjunctive epithelium), mucous, peripheral blood and skin. In the respiratory system, HBD2 is expressed in mucous from the mouth to the epithelium of the lungs and is induced by bacteria, fungi and virus infections and by proinflammatory stimuli such as interleukin 1α (IL-1α), interleukin 1β (IL-1β), tumor necrosis factor alpha (TNF-α), interferon gamma (IFN-γ) and interleukin IL-6 (IL-6) [29, 47–51]. Recently, a study showed that the cytokines IL-17 and IL-22 are produced by Th17 cells, which are reportedly known to regulate the expression of HBD2 on mucosal surfaces [32]. HBD2 is produced as a functionally inactive peptide (prepro-defensin), and to achieve its biological activity, it must go through a post-translational modification process and form a mature peptide. Prepro-defensin is composed of a highly hydrophobic pre-peptide signal, a propeptide and the mature peptide. The prepropeptide is cut by proteases in the Golgi apparatus, and once removed, the mature peptide with antimi-

The regulation of the expression of the HBD2 in the respiratory epithelium involves multiple signaling pathways; most of these have been studied with bacterial infections. The bacterial proteins induce the expression of HBD2 in the respiratory epithelium through transcription factors (NF-kB) [53], the myeloid ELF-1-like factor (MEF) [54], the nuclear factor interleukin 6 (NF-IL6) [55] or the activated protein 1–3 (AP1–3) [56]. Signaling pathways involve mitogen-activated protein kinase (MAPKs) [49, 57, 58],

Another form to induce HBD2 expression is through cellular receptors, the respiratory epithelium expresses various receptors on its surface. Toll-like receptors (TLRs) 1–6 are expressed on the epithelial surface, and in the intracellular vesicles, the TLR3 and TLR7–9 are expressed in the endosomes or endoplasmic reticulum. These receptors recognize pathogens and have a cytoplasmic domain that is homologous to the IL-1 receptor and is responsible for initiating intracellular signaling pathways. This signaling cascade includes the activation of NF-kB. This transcription factor promotes the gene expression that contributes to the cytokines, chemokines, adhesion molecules, co-stimulatory molecules release as well as the

In a study with lung epithelial cells infected with *L. pneumophila*, it was observed that the infection induces the release of HBD2, and its expression is mediated by the receptors TLR2 and TLR5 and activation of MAPKs (p38, JNK) and

Another way to regulate the expression of HBD2 is through the interaction with receptors; its chemotactic property was initially discovered in different types of cells such as monocytes, T cells and immature dendritic cells. The activity is carried

Other receptors involved in the regulation of the HBD2 expression are the recep-

Nowadays, HBD2 is considered a multifunctional molecule; the ability to bind to several ligands suggests that there may be more receptors and signaling pathways still to be discovered and that may have important biological activities in the

Several mechanisms of action have been proposed for defensins; however, the main mechanism of action of the HBD2 is to eliminate microorganisms

transcription factors NF-kB and AP-1 [50, 57, 58, 61].

tors for vitamin D and protease-coupled receptors (PARs) [62–65].

through the CCR6 receptor binding [32].

**6. Mechanisms of action of β-defensin-2**

**56**

immune system.

expression of HBD2 [60].

*Mechanisms of action of HBD2. The defensin with its positive charge is attached by electrostatic attraction to the membrane of the pathogen forming pores [3].*

directly through the interaction with the microorganism membrane. The first step is given by the electrostatic attraction between the cationic defensin with positive charge and the microorganism's membrane components with negative charges [37].

Components of the bacteria membrane have been identified as targets for the HBD2. Lipopolysaccharides are targets for Gram-negative bacteria, teichoic acid for Gram-positive bacteria and phospholipids for both bacteria. In Gram-negative bacteria, the peptides are inserted into the membrane by hydrophobic interactions. It is thought that it possibly involves a folding of the peptide within the structure of the membrane [38, 66, 67]. After the electrostatic interaction of the peptide with the membrane and the displacement of the lipids, the defensin is added to the surface of membrane. There are several modes of action that have been proposed to describe how defensins are oriented to form pores and how the structure of the membrane is altered, becomes permeable, such as cell lysis and finally results in the death of the microorganism (**Figure 3**) [3, 37, 68].

The high content of negatively charged amino acids in the membranes of bacteria is the main factor that makes them more susceptible to being targeted by defensins. The membrane of the eukaryotic cells predominates lipids with neutral charge without net charge, they have a high level of cholesterol, and bacteria do not contain cholesterol in their membranes. Cholesterol causes the membrane to condense and prevents the peptide from penetrating; this also has an asymmetric distribution of phospholipids contributing to the resistance against defensing. These mechanisms explain why these peptides are not toxic in eukaryotic cells [69, 70].
