Antimicrobial Peptides and Acne
Michael P. Philpott
Centre for Cutaneous Research, Institute of Cell and Molecular Science, Barts and the London, Queen Mary’s School of Medicine and Dentistry, University of London, London, U.K.
It is well known that the epidermis forms an effective structural barrier and is also highly resistant to pathological infection by the many different types of microorgan- ism that colonize its surface. If the epidermal barrier is breached by pathogens, the first line of defence is the hosts innate immune response followed by the adaptive immune response. The innate immune response consists of a number of pre-existing defence mechanisms including phagocytic and natural killer cells, mast cells as well as epithelial cells themselves (1). These cells respond to microbial pathogens in a number of ways including the release of antimicrobial peptides. More than 500 antimicrobial peptides have been described in plants, insects, amphibians, and mammals, with broad-spectrum activity against bacteria, fungi, and viruses, representing an integral part of innate immunity (2,3). Of these antimi- crobial peptides, the defensins, adrenomedullin (AM), and cathelicidins are perhaps the most widely studied in skin, although to date only defensins and AM appear to have been investigated in acne.
Mammalian defensins are a family of cationic antimicrobial peptides, 28 to 42 amino acids long, containing three disulfide bonds. They have been divided into two subtypes, the a-defensins and the b-defensins (4). The a-defensins are found in neutrophil granules [human neutrophil proteins (HNP)1 – 3] or in the paneth cells [human defensin (HD)5 – 6] of the small intestine (5). The four b-defensins so far identified, human beta-defensin 1 to 4 (hBD1 – 4) are produced in various epithelia including keratinocytes of the epidermis (6,7). In keratinocytes, hBD1 is constitutive, whereas hBD2 – 4 are inducible and are produced by keratinocytes in response to pro-inflammatory stimuli such as interleukin 1 (IL-1), tumor necrosis factor (TNF), and lipopolysaccharide (LPS).
The most widely studied defensins in skin are hBD1 and hBD2 (8,9). Strong constitutive expression of both hBD1 and hBD2 mRNA and protein can be detected in the distal outer root sheath (ORS) of the hair follicle, surrounding the hair canal and in the pilosebaceous duct (Fig. 1). It is of particular interest that hBD1 and hBD2 proteins appear to be strongly expressed in more suprabasal cells. These patterns of expression are consistent with the concept that these regions are highly exposed to microbial organisms, and it is most likely that hBD1 and hBD2 play a key role in protecting the pilosebaceous unit from microbial invasion. In contrast, hair follicle compartments that are rarely exposed to microbial invasion such as the proximal ORS and inner root sheath (IRS) as well as the hair follicle bulb, including the
FIGURE 1 (See color insert.) Human beta-defensin 1 (hBD1) and hBD2 immunoreactivity in human hair follicles. hBD1 (A) and hBD2 (B) immunoreactivity is found in the suprabasal layers of the epidermis and the distal outer root sheath (ORS) of the hair follicle. Strong basal expression is seen in the bulge area, which contains a population of epidermal stem cells. Strong b-defensin expression is also found in the sebaceous gland and duct. Weaker expression is present in the suprabasal layers of the central and proximal ORS and in the proximal inner root sheath. hBD1 and hBD2 IR are not detected in the hair matrix or the dermal papilla. Abbreviations: APM, arrector pili muscle; DP, dermal papilla; IRS, inner root sheath; ORS, outer root sheath. Source: From Ref. 9.
dermal papilla (DP), showed only very weak hBD1 and hBD2 expression. More- over, as we shall discuss below it would also appear that as well as defensin expression in compartments with high exposure to microorganisms, defensins are also expressed in potential hair follicle stem cell compartments.
Perhaps, one of the most striking observations with regard to defensin expression in normal skin is that in contrast to the hair canal, pilosebaceous duct and interfollicular epithelium (8,9), where defensin expression is restricted to the cells of the suprabasal layers. In the central ORS and the buzlge region of the hair follicle, strong defensin expression is found in basal keratinocytes. This may be very important as it is accepted the central ORS and bulge of the hair follicle contain a population of epidermal stem cells (10 – 12). It is therefore tempting to speculate that the role of b-defensins in this region of the ORS may be to protect stem cells from microbial invasion. A similar role for defensins has recently been proposed in the gut, where marked defensins expression is detected in the paneth cells of the small intestine (2). Since paneth cells are located in the intestinal crypts, it has been suggested that paneth cell secretions might protect stem cells from pathogenic microbes. Whether defensins play an important role in protecting stem cells in the hair follicle remains to be investigated. However, it is also of note that the distal hair follicle containing the DP and hair follicle matrix are very weak expressers of defensins. The lower hair follicle is transient and during the hair growth cycle undergoes apoptotic driven regression (11,13,14). Moreover, if surgically removed, the lower follicle including the DP and matrix are able to regenerate (15,16). Therefore, lack of or weak defensin expression in these regions may reflect the transient nature of this part of the hair follicle, and the fact that if lost through mechanical injury and presumably infection they can regenerate. Whereas, the proximal hair follicle including the stem cell compartment cannot regenerate and must therefore be protected.
AM is a 52-amino acid ringed structure peptide with C-terminal amidation that mediates vasodilatory and natriuretic properties through the second messenger cAMP, nitric oxide, and the renal prostaglandin system. In addition to these cardi- ovascular and renal effects, AM is involved in a remarkable range of other functions involving growth regulation, modulation of hormone secretion, neurotransmission, and antimicrobial defence (17). AM is synthesized as part of the larger precursor molecule preproAM, which contains an additional biologically active peptide termed preproAM N-terminal 20 peptide or PAMP (18). The effects of AM appear to be mediated by two different receptors: L1, a previously identified orphan receptor and calcitonin receptor like receptor (CRLR) which can bind either calcitonin gene related peptide or AM, depending on whether receptor activity modifying proteins are co-associated with CRLR (19,20).
In normal human skin, AM is expressed in suprabasal layers of the epidermis, in melanocytes, and in sweat and sebaceous glands (8,21). In the hair follicle, AM protein is expressed in the basal and suprabasal layers of the hair bulb and the proximal ORS. Whereas, in the distal ORS AM becomes increasingly supra- basal, especially in proximity to the bulge region. In contrast to defensins, AM immunoreactivity (IR) is absent from the basal cells of the bulge. The CRLR is expressed in a similar pattern to that of AM. In contrast, the L1 IR receptor is only expressed in suprabasal cells (21).
AM has antimicrobial effects against both Gram-positive and -negative bac- teria isolated from the skin and oral cavity. Moreover, antimicrobial activity is most marked for Propionibacterium acnes and Micrococcus luteus with minimal inhibi- tory concentrations close to AM concentrations measured in sweat (22). AM also acts as an autocrine/paracrine growth factor in different tumor cell lines (23), Swiss 3T3 fibroblasts (24), vascular smooth muscle cells (25), as well as keratino- cytes of skin (26), and oral mucosa (27). Based on these data, possible roles for AM in the innate unspecific immune system of the skin and a possible participation of AM in the regulation of skin proliferation, wound repair, hair growth, and tumor progression have been postulated.