Defensin is a cysteine-rich, cationic peptide expressed by higher organisms and plays an important role in innate immunity. Mammalian defensin is classified into alpha, beta, and theta depending on the pattern of disulfide-bridge formation between six conserved cysteine residues. In 1985, Selsted et al. first isolated a mature form of α-defensins family from human blood denoting them as human neutrophil peptides (HNPs) in accordance to their source, property, and size. Shortly, three types of HNPs (HNP-1, HNP-2, and HNP-3) having nearly identical amino acid sequences were isolated. In 1989, a fourth type of HNPs, HNP-4, was reported after being purified to homogeneity by chromatographic methods. In 1989 and 1992, α-defensin genes were extracted from mouse and human paneth cells, respectively, and termed as HD-5 and HD-6, which are tissue-specific peptides and only restricted to human intestines. Members of human β-defensin family were soon discovered with human β defensin-1 (hBD-1) being the first to be extracted by Bensch et al. from human plasma in 1995. Subsequently, hBD-2 was identified from lesional psoriatic skin with two variants hBD-3 and hBD-4 documented in year 2001 (Wang 2014). Till now, 28 additional human and 43 mouse β-defensin genes have been identified with the help of bioinformatic studies. In the meantime, a different type of defensin called θ-defensin also was identified in neutrophils and monocytes of rhesus monkey and other nonhuman primates (Tang et al. 1999). Humans including chimpanzees and gorillas also expressed θ-defensin pseudogenes, which prevent the translation of θ-defensin precursor due to the presence of a premature stop codon (Garcia et al. 2008).
Gene Structure and Its Transcription
Human α-defensin peptides are encoded by five DEFA (Defensin-α) genes. HNP1, 2, and 3 share almost identical sequences, i.e., XC1YC2RIPAC3IAGERRYGTC4IYQGRLWAFC5C6 with HNP 1 and 3 differing only by a single N-terminal residue (“X” is alanine in HNP1 and aspartic acid in HNP3). The proteolytic removal of X residue from N-terminal results in HNP2 explaining the absence of its individual gene. DEFA1 and DEFA3 genes are located on chromosome 8p23.1 where they show extensive copy number of polymorphism. In contrast, genes for HNP4, HD5, and HD6 do not undergo duplication with only one copy of each existing per haploid genome (Lehrer and Lu 2012). Human enteric α-defensin genes consist of two exons, whereas human neutrophils α-defensin genes consist of three exons, two of which (second and third) are homologous to enteric defensins. β-defensin gene coding consists of two exons located on human chromosome 8p23–p22. The first exon includes 5′ untranslated region and encodes the leader domain of the preproprotein, whereas the second exon encodes mature peptide with the six-cysteine domain (Meade et al. 2014).
Structure and Distribution
Tissue-specific expression of important defensins in humans
Expression in tissues
Type of expression
Nature killer cells
Paneth cells, urogenital tracts
Constitutive and inducible expression
Keratinocytes (gingiva, buccal mucosa, tongue)
Salivary glands (ductal cells of parotid and submandibular gland)
Kidney (epithelial layers of the loops of Henle, distal tubules, and the collecting ducts)
Occular tissue (cornea, conjunctiva, lacrimal gland)
Female genital tract (epithelial layers of the vagina, ectocervix, endocervix, uterus, and fallopian tubes)
Constitutive and inducible expression (induced by IFN-γ,IL-1β, IL-10, TNF-α and inhibited by IL-13 & 14)
HBD-1 is unaffected by cytokines
HBD-2 induced only by IL-1β
Gastrointestinal tract, respiratory tract, skin, salivary glands, oral cavity (stratum spinosum and granulosum)
Gingiva, tongue, buccal mucosa, labial mucosa, floor of the mouth (stratum basale), dental follicle
Skin (malphighian layer of epidermis, stratum corneum), trachea, pharynx, kidney, thymus, colon, stomach
Placenta, uterus endometrium
Submandibular gland, minor salivary gland (labial gland)
Lungs, testis, stomach, neutrophils
Defensin in Physiology
Various researches have revealed extended cellular functions of defensin like chemoattraction and innate and adaptive immune-mediated response beyond its well-established antimicrobial role.
Role of Defensin in Immunity
Continuous expression of defensin ranging from lower to higher organisms indicates its development as an integral part of innate immunity. Moreover, its constitutive expression in protective barriers (skin and mucous membrane) makes defensin act within first line of defense in innate immunity. Newborns deficient in neutrophil defensin have shown to be at a larger risk of bacterial infections; activation of classical complement pathway while expression of beta defensin to inflammatory cytokines supports pivotal role of defensin in innate immunity. Defensin also contributes enormously in cross-linking of innate to adaptive immunity through its receptor, specific ligation ability to pathogen-associated molecular pattern (lipopolysaccharides, peptidoglycans, etc.), and activation of appropriate adaptive immune response against the pathogens. They contribute in a larger extent to adaptive immune response via various mechanisms like (i) facilitation of chemotaxis (chemotactic ability of α-defensin toward monocytes, dendritic cells, and T-lymphocytes; β-defensin for dendritic cells and memory T cells via chemokine cell surface receptors like CCR-6 also for myeloid progenitors via CCR2) and (ii) maturation of dendritic cells and monocytes (toll-like receptor-mediated activation of antigen-presenting cells and cytokine-mediated activation of monocytes) (Raj and Dentino 2002; Weinberg et al. 2012).
Antimicrobial and Antiviral Role
Both in vivo and in vitro studies support broad-spectrum antimicrobial activity of defensin. Some experimental observations showed increased defensin concentration in response to the presence of microorganisms/inflammation and synergistic interaction with other antimicrobial peptides like lactoferrin and cathelicidins, signifying its antimicrobial mechanisms. A selective detrimental effect on cell membrane is the principal mode of antibacterial action of defensin. Defensin being cationic peptide mainly interacts with negatively charged constituents of microbial cell membrane (lipopolysachharide in gram-negative bacteria and techoic acid in gram-positive bacteria). It mediates bactericidal activity through increased permeability of inner and outer cell membrane by formation of multimeric pores in gram-positive bacteria along with competitive displacement of divalent cations through its high affinity for LPS in gram-negative bacteria. Other possible antibacterial mechanisms are stimulation of autolytic enzymes and interference with bacterial DNA and/or protein synthesis. In case of fungus, a cascade of events are orchestrated by defensin as an antifungal agent like binding to the cell wall, increasing membrane permeability, triggering receptor-mediated internalization, and interacting with intracellular targets which causes the formation of reactive oxygen species and ultimately induces apoptosis (Silva et al. 2014). All six human alpha defensins and HBD 1 to 3 also play a major role in antiviral activity through mechanisms like direct neutralization or aggregations of virions.
Nonimmune Regulatory Role
Defensin is also involved in various other regulatory functions other than its much acclaimed immunological role. Functions like (i) modulation of tissue-type plasminogen activator and plasminogen binding to fibrin and endothelial cells thus inhibiting fibrinolysis, (ii) interaction with ACTH, thus preventing ACTH-induced steroidogenesis, (iii) induction of histamine release from mast cells, (iv) regulation of proteoglycan-dependent catabolism of low-density lipoprotein (LDL) by vascular cells, and (v) proliferation of epithelial cell during wound healing broaden the range of biological role of defensin and establish these peptides as multifunctional cell effectors in addition to their much acclaimed role in integration of innate and adaptive immunity.
Defensin in Pathology
As defensin is closely associated with immunity and inflammation, it has attracted many researchers to explore its role in inflammatory diseases of human. Etiopathogenesis of inflammatory diseases of human was found to be directly related to either its antimicrobial properties or immunomodulation ability. Beside its immune-related function, defensin has also extended its contribution in pathophysiology of various diseases and disorders.
In Gastrointestinal Disease
Because of its much explored antimicrobial role, defensin is thought to maintain a well-balanced microbiota, prevention of pathogenic bacterial growth and invasion, thereby ensuring normal health/function of intestinal tract. Altered levels of defensin were mainly observed in intestinal diseases like inflammatory bowel disease (IBD), Crohn’s disease (CD), and Ulcerative colitis (UC). In IBD, a drastic decrease in α-defensin (HD-5&6) was noticed, suggesting its direct association with its etiopathogenesis. In CD, reduction of both α and β defensin concentration in association with mutation of an intracellular receptor nucleotide oligomerization domain (NOD) 2 indicates probable association of genetic mutation in its pathogenesis. NOD2 is normally expressed by intestinal epithelial cells (paneth cells) and acts as a recipient of bacterial cell component like muramyl diapeptide (MDP). A recent research work suggested functional mutation of CARD15 gene encoding NOD2, thus preventing NOD2/NF-κB-mediated transcription and translation of defensin. In UC, consistent observations like increase in mRNA concentration of inducible β-defensins (HBD2 and 3) and decrease in HBD-1 (constitutional defensin) clearly indicates about the regulatory role of defensin in response to inflammation (Ramasundara et al. 2009).
In Cardiovascular Diseases
Defensin, mainly HNPs, pursue various physiological role in relation to heart such as maintaining vascular tone, mediating thrombolytic activity, and involvement in lipid metabolism. Hence, possible mechanisms for HNP-mediated cardiovascular diseases like atherosclerosis and coronary disease are: (i) promotion of accumulation of LDL by its binding to endothelial cell surface, (ii) inhibition of fibrinolytic activity, and (iii) formation of stable, multivalent complexes with LDL (Maneerat et al. 2016).
In Skin Diseases
Since its discovery, altered expression of defensin protein has been recognized in some skin diseases/lesions like psoriasis, atopic dermatitis, Kostman’s syndrome, and skin injury, thus substantiating its role in determining susceptibility of patients with skin disorders to the pathogens. Conflicting observation of defensin levels in psoriasis (high) and atopic dermatitis (low) was found to be inversely proportional to their susceptibility to superadded infections. In Kostman’s syndrome (severe congenital neutropenia), decreased defensin level (HNP1 to 3) suggests the importance of its proper expression for the antimicrobial function of neutrophils. In skin injury, besides protection from infections, defensin is involved in wound healing with β-defensins, i.e., hBD-2, -3, and -4, accelerating keratinocyte migration and proliferation through calcium influx and EGFR phosphorylation. Similarly in skin injury, HNP1 is said to promote extracellular matrix deposition and control its degradation by enhancing the expression of pro-α1 collagen and inhibiting the expression of matrix metalloproteinase-1 (Kenshi and Richard 2008).
Defensin is well established in its roles as antimicrobial peptide, important component of innate immunity, and as an effective regulator of adaptive immunity. However, these beneficial roles need to be explored more for defensin could emerge as a novel adjuvant to amend the immunomodulatory strategies so that host responses could be enforced against the microbial challenge. Though its potential as tumor suppressor or promoter needs to be clarified, mounting evidences support defensin as a potent biomarker at least in epithelial tumors, thus necessitating an exploration in possibility of it as a candidate in targeted individualized therapy in near future.
- Maneerat Y, Prasongsukarn K, Benjathummarak S, Dechkhajorn W, Chaisri U. Increased alpha-defensin expression is associated with risk of coronary heart disease: a feasible predictive inflammatory biomarker of coronary heart disease in hyperlipidemia patients. Lipids Health Dis. 2016;15:117. doi: 10.1186/s12944-016-0285-5.PubMedPubMedCentralCrossRefGoogle Scholar