© Springer International Publishing Switzerland 2017
Anthony A. Gaspari, Stephen K. Tyring and Daniel H. Kaplan (eds.)Clinical and Basic Immunodermatology10.1007/978-3-319-29785-9_2424. Immunology of Acne
(1)
Department of Dermatology, Pennsylvania State Hershey Medical Center, Hershey, PA, USA
(2)
Department of Dermatology, Pennsylvania State Hershey College of Medicine, 500 University Drive, C7801 BMR; MC HU14, Hershey, PA 17033, USA
Abstract
Activation of the immune system is a central event in the development of acne, and our understanding of this process is rapidly expanding. In this chapter, we describe how P. acnes, sebaceous glands, and keratinocytes contribute to inflammation in acne and how current acne treatments modulate this immune response. Specifically, we address the role of anti-microbial peptides, Toll-Like Receptors (TLRs), sebaceous lipids as well as cytokines and the role of the inflammasome as driving inflammation in acne. The anti-inflammatory activities of retinoids, tetracyclines and other therapies are discussed.
Keywords
AcneP. acnesInflammationInnate immunityImmunologyCytokinesInflammasomeSebaceous glandKeratinocytesAnti-microbial peptidesAcne
Clinical Presentation and Etiology
Acne is one of the most prevalent skin conditions encountered by dermatologists, affecting nearly 85 % of the people between the ages of 12 and 24 years including 40–50 million people in the United States each year [1, 2]. Acne has a significant psychosocial and economic impact with an estimated average cost per episode of $690 dollars and a total burden of $12 billion dollars per year in the United States [3–7].
Acne is a disease of the pilosebaceous unit (PSU) and begins with the formation of the microcomedone. The severity of acne is determined by the type, number and distribution of lesions and the presence of scarring. Acne is a multifactorial disease that results from the interaction of four main pathogenic factors: (1) the production of sebum by androgen-mediated stimulation of sebaceous glands, (2) abnormal hyperkeratinization of the follicles leading to comedone formation, (3) colonization of the PSU by Propionibacterium acnes (P. acnes), and (4) inflammation, which is trigged by the interaction of immune cells, keratinocytes, and sebocytes with P. acnes after follicle disruption.
P. acnes, a gram positive, anaerobic, pleomorphic diphtheroid is the predominant organism in the follicular flora, although aerobic Staphylococcus epidermidis may also be present [8, 9]. P. acnes relies on sebaceous lipids, specifically triglycerides, as a nutrient source and metabolizes these into free fatty acids [10].
Experimental Models for Acne Vulgaris
Acne research is limited by the lack of a model system that mimics all aspects of acne pathogenesis. Cell culture, whole organ culture and animal models (mouse, hamster and rat) are used to study the individual features of acne, such as sebum production and hyperkeratinization. However, animal models for inflammation in acne, in which P. acnes plays a major role, are not available because P. acnes does not colonize the PSU. One attempt to mimic the microenvironment of acne lesions was done by T. Nakatsuji and colleagues, in which a sebocyte-filled tissue chamber implanted into ICR mice was injected with P. acnes and the immune response was measured in the chamber fluids [11]. Thus, studies of acne inflammation rely on in vitro methods and acne patient populations.
Activation of the immune system is a central event in the development of acne, and our understanding of this process is rapidly expanding. In this chapter, we describe how P. acnes, sebaceous glands, and keratinocytes contribute to inflammation in acne and how current acne treatments modulate this immune response.
Innate and Acquired Immune Systems in Acne
Anti-microbial Peptides (AMPs): Good or Bad?
Increasing evidence indicates that AMPs, produced by neutrophils and epithelial cells, play a pivotal role in acne pathogenesis. The most important skin-derived AMPs are beta-defensins, cathelicidins and S100 proteins. For each of these, both pro-inflammatory and anti-inflammatory activities are reported. In general, the presence of P. acnes triggers the production of AMPs, which leads to the direct killing of the microbe and reduction in bacterial burden. However, AMPs also have immunomodulatory properties such as promoting chemotaxis and cytokine release, leading to the outstanding question of whether or not AMPs themselves substantially contribute to inflammation in acne.
Beta-Defensins (BD)
Both human BD-1 (hBD-1) and hBD-2 are expressed within the PSU and hBD-2 is significantly increased in acne vulgaris and hidradenitis suppurativa [12–14]. hBD-2 expression is increased in areas of high sebum secretion including the forehead, nose and chin when compared to the cheeks [15]. P. acnes triggers hBD-2 expression in keratinocytes and sebocytes [16, 17]. hBD-2 has antimicrobial activity against P. acnes and could limit the P. acnes-induced inflammatory response [18].
However, hBD-2 itself likely contributes to inflammation. It induces chemotaxis of immune cells and triggers the release of pro-inflammatory cytokines, histamine and prostaglandins [19]. In addition, hBD-2 enhances keratinocyte proliferation and migration, implying that hBD-2 contributes to possible comedone formation in acne [20].
Cathelicidin
Cathelicidin (LL-37) is increased in acne lesions as well as in hidradenitis suppurativa [21]. The functional LL-37 peptide is present within human sebaceous glands and P. acnes induces expression in sebocytes in vitro [22]. Similar to hBDs, cathelicidin peptides have direct antimicrobial activity against P. acnes, but also initiate cytokine production and inflammation in the host organism [22, 23]. In rosacea, elevated LL-37 expression causes increased inflammation characterized by increases in IL-8, neutrophilic infiltrate and erythema [24]. A similar process may occur in acne.
S100 Proteins
S100A7 (also known as psoriasin) is increased in the overlying epidermis and sebaceous duct of acne lesions and hidradenitis suppurativa [14, 21]. In vitro, S100A7 is bactericidal against P. acnes and is even more effective when combined with cathelicidin [22]. As with other AMPs, S100A7 may be partially responsible for inflammation in acne, as it is a powerful chemokine for neutrophils and lymphocytes.
Other AMPs
Lactoferrin, lysozyme, RNase 7, and both splice variants of koebnerisin, (S100A15L and S100A15S) are all elevated in inflammatory acne lesions compared to non-lesion control skin [21]. Granulysin, RANTES (CCL5), perforin, CXCL9, substance P, chromogranin B, and dermcidin are not elevated in acne lesions in this study [21], but are reported as elevated in others [13]. The presence of all these AMPs is likely protective and simultaneously pro-inflammatory.
Toll-Like Receptors (TLR)
TLR2 is expressed within sebaceous glands as well as basal and infundibular keratinocytes [25]. Studies have demonstrated increased TLR2 expression within acne lesions and hidradenitis suppuritiva compared to normal skin [26, 27]. Moreover, patients with acne have increased TLR2 expression on peripheral monocytes compared to those unaffected [28]. TLR4 expression is also increased in epidermal keratinocytes of acne skin compared to healthy skin [17].
TLR2 activation by P. acnes is an important step in the pathogenesis of acne [27–29]. Activation of TLR leads to activation of NFkB transcription factor and up-regulation of numerous cytokines and chemokines that trigger inflammation through recruitment of immune cells and AMP production. For example, through TLR2, P. acnes induces IL-12 and IL-8 cytokine production in monocytes, which likely triggers neutrophil chemotaxis [27].
Sebaceous Glands and Lipids
Sebocytes play a key role in the pathogenesis of acne and actively participate in regulation of the immune system [30]. They express TLRs and have been demonstrated to produce various inflammatory cytokines in response to P. acnes [27–29]. Additionally, the synthetic pathways for the production of prostaglandins and leukotrienes from arachidonic acid are intact within sebaceous glands and sebocytes in vitro. These lipid mediators affect lipid synthesis, neutrophil chemotaxis and cytokine production [31].
Increased sebum production, altered sebum and skin surface lipid composition also contribute to inflammatory acne. Triglycerides within sebum are hydrolyzed to free fatty acids (FFA) by P. acnes lipase enzymes. These FFA including oleic, palmitic and lauric acids, act as “damage associated molecular patterns (DAMPs)” and can activate TLR2 and TLR4 resulting in inflammatory cytokine and AMP production [32]. The levels of lineolic acid (C18:2) are lower in acne patients compared to controls which may cause changes in keratinization of the follicle and increased neutrophil activity [33–35].
Sphingolipids and ceramides may also play a role in acne. Acid Sphingomyelinase (ASMase) is one enzyme responsible for catalyzing the breakdown of sphingomyelin on cell membranes to ceramide; ceramide, in turn, can induce apoptosis, differentiation and proliferation. Nakatsuji and colleagues demonstrated that P. acnes and its release of CAMP factor in combination with host ASMase activity was cytotoxic to keratinocytes in vitro and increased inflammation in vivo [36]. Interestingly, retinoid treatment of keratinocytes suppressed the expression of ASMase and numerous genes involved in ceramide biosynthesis [37], perhaps explaining some of its potency in the treatment of acne.
Sebaceous lipids are not all bad, some have potent antimicrobial effects. Lauric acid (C12:0) has strong direct antimicrobial activities against P. acnes [18]. In addition, lauric acid, along with palmitic acid (C16:0) and oleic acid (C18:1, cis-9), enhanced the expression of hBD2 in human sebocytes, suggesting indirect killing of P. acnes through hBD-2 [18]. Both palmitoleate (C16:1) and oleic (C18:1), produced by stearoyl-CoA desaturase-1 (SCD1) in the sebaceous gland in a TLR2 dependent manner, are bactericidal against gram-positive, but not gram-negative, organisms [38], although their activity on P. acnes has not been exclusively demonstrated.
Cytokines and T-cells
Gene expression studies demonstrate inflammatory acne lesions have increased levels of TNFα, IL-1β, IL-10 and IL-8 as well as increased levels of neutrophils and lymphocytes compared to non-acne skin [13, 39]. Additional cytokines involved in the pathogenesis of acne include IL-6, IL-12, IL-17, IFNγ, TGFα and epidermal growth factor (EGF).
IL-1 Family of Cytokines
Release of IL-1β propagates and perpetuates the inflammatory signal [40–43]. IL-1β release is also mediated by TLR2 activation. Inhibiting monocyte TLR2 activity reduces the secretion of IL-1β by half after stimulation with P. acnes [40]. IL-1β has been shown to induce the production of IL-6 and IL-8, a powerful neutrophil attractant, in sebocytes [29, 40, 41, 44]. IL-1β and IL-6, together with Transforming Growth Factor β (TGFβ), induce naïve helper T cells to undergo differentiation into Th17 cells which can amplify inflammatory signals through the production of IL-17 (See below) [45].
IL-1α is constitutively produced by sebocytes and keratinocytes, but levels increase rapidly following P. acnes stimulation early in the formation of microcomedones. High levels of IL-1α results in hypercornification of the follicular infundibulum, mimicking that seen in microcomedones, and may promote keratinocyte terminal differentiation through up-regulation of small proline rich protein 1 [43, 46, 47]. IL-1α has been found in high concentration in open comedones, and the production of the cytokine is increased with FGFR2 mutations [30]. FGFR2 mutations underlie Apert Syndrome, a key feature of which is acne [48, 49]. In one study, an uncommon single nucleotide polymorphism (SNP) in the IL-1α gene, a substitution of serine for alanine called the T allele, may be associated with more severe acne [49].
The Interleukins
IL-6 is a marker of activated monocytes, and participates in directing Th17 differentiation [27, 45]. IL-8 acts to attract neutrophils, whose arrival and degranulation can promote rupture of the PSU [29, 43]. The release of both IL-6 and 8 is promoted by activation of the TLR2, by sebocytes following IL-1β stimulation, and may play a role in stress- induced acne [27, 30, 40]. Corticotropin-Releasing Hormone (CRH) levels rise during stress, and CRH receptor activation on sebocytes promotes IL-6 and 8 secretion [30, 43]. IL-12 is a powerful driver of differentiation toward Th1 axis responses, is a principal inflammatory mediator in the response to gram positive organisms, and P. acnes induces increased secretion of IL-12 via TLR2 binding [27, 43]. IL-17 is involved in the Th17 axis and will be discussed in depth separately, but promotes the release of TNFα, IL-6, and matrix metalloproteinases [45]. P. acnes derived proteases can also stimulate the secretion of IL-1α, IL-8 and TNFα by activation of the protease-activated receptor-2 (PAR2) on the cell membrane of keratinocytes [43].
TNFα, IFNγ, EGF, TGF
Among other roles, TNFα may increase keratinocyte expression of adhesion molecules in the follicular infundibulum and thus may contribute to the formation of the microcomedone through abnormal desquamation [46, 50]. Furthermore, one report suggests that a SNP in the TLR2 gene, Asp299Gly, may increase TNFα secretion following bacterial activation. SNPs in the TNFα promoter region have been studied to varying results, but some hold these polymorphisms may drive acne by increasing production of TNFα in response to NFkB activation [49]. IFNγ is produced by monocytes in response to P. acnes and drives differentiation of T cells toward the Th1 axis [45]. EGF and TGFα together appear to promote rupture of the pilosebaceous unit [46]. TGFβ induces the differentiation of Treg cells in the presence of IL-2 [51].
The P. acnes induced cytokine cascade may explain the early perivascular and perifollicular infiltration of lymphocytes seen at the initiation of the microcomedone, prior to clinical evidence of inflammation, and preceding the arrival of neutrophils [46].
Th1 and Th17 Cells
Acne is a Th1/Th17 associated disease. Th1 and Th17 cells are detected within acne lesions [45, 52] and the corresponding cytokine profiles are induced by P. acnes.
Th17 cells are powerful drivers of inflammation and are thought to play a critical role in the development of several immunologically mediated diseases such as multiple sclerosis, psoriasis, and rheumatoid arthritis [45, 51]. This subset of helper T-cells produces IL-6, IL-17, IL-21 and IL-22 which act on tissues to promote degradation of the extracellular matrix and maintain intense chronic inflammation. Th17 cells are identified by the production of IL-17 family cytokines (IL-17A and IL-17 F) and by the presence of cell markers RORα and RORc. P. acnes drives the production of IL-1β and IL-6 through mechanisms discussed above which appear to then promote the differentiation of naïve helper T-cells to Th17 cells. The bacterium promotes the production and secretion of IL-17, as well as RORα and RORc.
The Inflammasome
Recently elucidated patterns of disease, termed autoinflammatory conditions, manifest from dysfunctional up-regulation of innate immunity and several of these disorders share acne as a distinctive feature. Characterization of the inflammasome, an intracellular multi-protein complex capable of activating innate immunity, and its role in auto-inflammatory disease has informed several recent advancements in our understanding of acne [42, 48, 53].
Structure and Function of the Inflammasome
Inflammasome signaling begins with the activation of a Nucleotide-Binding Domain Leucine-Rich Repeat-Containing Protein, also referred to as a Nucleotide Oligomerization Domain-like Receptor Protein (both abbreviated NLRP) and culminates with the secretion of mature, active IL-1β, IL-6 and IL-18 which propagate inflammation [40–42, 53]. The NLRPs are cytosolic, and thought to be non-adaptable pattern recognition receptors, much like TLR’s [41, 42, 54]. However, one study has questioned whether NLRPs respond to pattern molecules directly, respond to indirect signals or perhaps both [42]. Four NLRPs are well described: NLRP1, NLRP3 (also termed cryopyrin or NALP3), NLRC4 (also termed IPAF) and AIM2 [41, 42, 54]. These proteins can respond to various threats such as bacterial Pathogen Associated Molecular Patterns (PAMPs), viral and fungal pathogens, and Reactive Oxygen Species (ROS) [40–42, 54]. NLRP activation results in the recruitment of pro-caspase 1 through a Caspase Recruitment Domain (CARD) [41, 42]. In the case of NLRP3, the protein lacks a CARD and instead relies on the Apoptosis-associated Speck-like protein containing a Caspase recruitment domain (ASC) to bind pro-caspase 1 [42, 54]. Binding of pro-caspase 1 results in proteolytic cleavage into active caspase 1 subunits which subsequently convert the precursor of IL-1β into its active form [40, 42, 54]. Caspase 1 has also been shown to increase secretion of IL-1α [41].
P. acnes Activates the NLRP3 Inflammasome
The role of the inflammasome in acne vulgaris is currently being elucidated. First, P. acnes has been shown to induce the production of IL-1β and caspase 1 in human monocytes, as well as increasing the production of NLRP1 and NLRP3 [40]. Interfering with the production of NLRP3, but not NLRP1, results in dramatic decreases in the production of IL-1β, as does inhibition of caspase 1 [40, 55]. The production of AIM2 and NLRC4 (IPAF) are not affected by exposure to P. acnes [40, 54]. This information suggests that P. acnes stimulates the production of IL-1β via activation of the NLRP3 inflammasome in human monocytes in vitro, and in vivo. Though P. acnes is an extracellular pathogen, phagocytosis transmits the bacterium or its products to the cytosol where NLRPs reside [40, 55]. This internalization of bacteria appears to be necessary for NLRP3 inflammasome-dependent IL-1β production [55]. Bacterial muramyl dipeptide has been implicated as an activator of NLRP3, and P. acnes produces this molecule [40]. Direct binding of muramyl dipeptide and activation have not been demonstrated in acne.
A recent study shows that sebocytes not only express the constituents of the NLRP3 pathway, but that this pathway is activated upon exposure to P. acnes. Exposure to P. acnes led to increased levels of active caspase 1 and IL-1β in an NLRP3 dependent manner. The stimulated IL-1β production was dramatically impaired by the presence of N-acetyl cysteine, which the authors suggest as evidence for ROS mediating the activation of NLRP3 in some manner [54]. Other authors have suggested that NLRP3 can be activated by thioredoxin-interacting protein, which is itself released in the presence of ROS.
Autoinflammatory Disorders Involving Acne
The role of the inflammasome in the development of acne is suggested by PAPA (Pyogenic Arthritis, Pyoderma gangrenosum and Acne) syndrome [48, 53]. PAPA syndrome is a monogenic autoinflammatory disorder that involves severe nodulocystic acne as a hallmark feature. The syndrome develops from a mutation in the Proline-Serine-Threonine-Phosphatase-Interactive-Protein 1 (PSTPIP1) [41, 48, 53]. In PAPA syndrome, the aberrant PSTPIP1 protein develops an increased binding affinity for the protein pyrin, which results in increasing rates of complexation of ASC with the inflammasome and thus recruitment of pro-caspase 1. It is unclear whether PSTPIP1 and pyrin binding activate pyrin to stimulate ASC complexation, or whether PSTPIP1 disinhibits inflammasome assembly by sequestering pyrin [41, 48, 53]. Regardless, as a result of PSTPIP1’s increased affinity for pyrin, increased levels of IL-1β and Tumor Necrosis Factor α (TNFα) have been observed in patients with PAPA syndrome, and treatments antagonizing IL-1 or TNFα have been successful in ameliorating disease symptoms [48, 53]. The increase in ASC recruitment to inflammasomes in PAPA syndrome may implicate NLRP3, which appears to play a role in the inflammatory response to P. acnes [40, 42, 54]. NLRP3 mutations also underlie the cryopyrin associated periodic fever syndromes such as Familial Cold Autoinflammatory Syndrome, Muckle-Wells Syndrome, NOMID and CINCA syndromes [41, 42].
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
