The innate immune system provides the first line of defense against microorganisms pervasive in the environment. Unlike the adaptive immune system, innate immunity is non-specific, lacks memory, and is not influenced by previous exposure. Antimicrobial peptides (AMPs) are cationic endogenous antibiotic proteins expressed throughout the epithelium that are effectors of the innate immune system. AMPs exert antimicrobial activity in a concentration-dependent manner, making their expression a critical factor in host defense [
14]. The amphiphathic nature of AMPs contributes to their effectiveness at interacting with hydrophobic and anionic components of the bacterial membrane [
15]. Cathelicidins, α-defensins, β-defensins, and θ-defensins are among the major classes of human AMPs [
16].
Beta-defensins are at the interface between the adaptive and innate immune systems; beta-defensins exhibit chemotactic function towards immature dendritic cells, memory T cells expressing the chemokine receptor CCR6, neutrophils primed with tumor necrosis factor (TNF)-α, and mast cells [
17,
18]. Individual beta-defensins have specific antimicrobial activity. Among the various types of defensin AMPs, only the expression of human beta-defensin-2 (hBD-2) and human beta-defensin-3 (hBD-3) is increased following stimulation by pro-inflammatory cytokines; all other defensin AMPs are continuously expressed [
19]. However, although the expression of hBD-2 and hBD-3 can be stimulated by pro-inflammatory cytokines, e.g., TNF-α, interleukin (IL)-1β, IL-17, and IL-22, these antimicrobial peptides are still expressed in unstimulated cells in basal amounts [
20,
21]. An additional difference between these two AMPs that are induced by humoral stimulation is that hBD-2 primarily targets Gram-negative bacteria, such as
P. aeruginosa, while hBD-3 exerts broad bacteriostatic activity against both Gram-positive and Gram-negative bacteria [
22]. hBD-2, like all defensins, is found throughout the epithelium of mammals. However, hBD-2 is most concentrated in the epithelia of the lung, tonsils, and trachea, and therefore plays a critical role in the prevention of pulmonary infection [
23,
24]. The inducible properties of hBD-2 suggest it plays a significant role in innate immune defense.
Human beta-defensin-2 is a cationic, 41 amino acid, 4 kDa, AMP intricately involved in the innate immune response of vertebrates that works synergistically with other antimicrobial molecules, such as lactoferrin and lysozyme [
24,
25]. Like other beta-defensins, hBD-2 is a monomeric protein containing six conserved cysteine residues forming three core disulfide bonds [
26]. The initial contact between hBD-2 and invading microorganisms is an electrostatic amphipathic attraction between the cationic AMP and the negatively charged phospholipid groups of the bacterium’s phospholipid bilayer [
27,
28]. Following initial electrostatic attraction, hBD-2 exerts its antimicrobial effects through insertion within the phospholipid bilayer disrupting the membrane integrity of the invading bacteria resulting in the collapse of membrane potential and death of the invading pathogen [
29]. Nuclear magnetic resonance (NMR) analysis of the crystal structures of hBD-2 suggests that the formation of a hBD-2 octamer is a prerequisite to the binding of the bacteria cell surface and subsequent increases in membrane permeability [
30].