Skin immunity and its dysregulation in psoriasis

ABSTRACT

The skin is a peripheral lymphoid organ, being the first immunological defense against infections as the initial interface between the organism and the external background. The maintenance of the skin immune homeostasis depends on a finely equilibrium of well-regulated relations between different cells and exogenous pathogens. Inflammatory skin diseases are directly linked to the dysregulation of this equilibrium. The present review discusses the role of the immune system, of T cells, in the etiopathogenesis of psoriasis, illustrating a potential rationale for innovative therapeutic intervention.

Introduction

The skin is the first interface between the organism and the external background, either acting as a mechanical barrier, restricting water loss and limiting the entry of exogenous materials, and also as an active barrier providing the initial immunological protection against infections [1,2]. Mammalian skin consists of two stratified layers known as dermis and epidermis [3]. The dermis is composed of immunocompetent mast cells, macrophages and fibroblasts, which produce collagen fibers, elastin and structural proteoglycans. A basement membrane anchors the epidermis to the dermis. It includes the basal lamina containing laminin and integrins in direct contact with the epidermal cells, forming dermo-epidermal junctions. The epidermis is rich in stratified keratinocytes which are highly proliferating in the basal layer becoming than more and more differentiated through the upper spinous, granular and corneous layers, harboring also hair stem cells [4,5]. Proliferating basal keratinocytes are characterized by the expression of the structural proteins Keratin 5 (K5) and Keratin 14 (K14) forming cells cytoskeleton together with Tubulin and Actin [6–8]. Moreover, in the basal layer transglutaminases (TG) are expressed; these are Ca2+-dependent enzymes which catalyze the formation of Nϵ-(γ-glutamyl) lysine bonds between proteins [9,10]. The first step of skin cornification takes place in the spinous layer were transglutaminase type 1 (TG1) and TG5 crosslink envoplakin and periplakin under the cell membrane. In the granular layer K5 and K14 are substituted by Keratin 1 and Keratin 10, while loricrin and small proline-rich proteins (SPRs) are expressed and crosslinked by TG3 and TG1. Consequently, the cornified envelope is predominantly composed by loricrin, involucrin, filaggrin as structural proteins and by a complex lipid envelope, both conferring elasticity and impermeability to the skin [3]. Acting as a physical barrier, the skin is continuously exposed to exogenous danger, including pathogens, allergens and ultraviolet radiation (UVR). External stimuli exposition may cause an alteration of cellular physiology, leading to unregulated reactive oxygen species (ROS) production and activation of programmed cell death. While skin development and homeostasis are regulated by p63 [11–13], like in other organs, apoptosis [2,14] is mainly regulated by protective proteins such as p53 [15–18], and its family members [19,20], powerful transcription factors [21,22] which regulation [23–26] protects from the development of tumors [27–30]. So far, there is no evidence in the skin of the third member of the p53/p63/p73 family, p73 itself [31–38]. A second major regulator of cell death is in fact the pro-survival BCL-2 protein family (BCL-2, BCL-XL, MCL-1, BCL-W and A1/BFL1) [27,39–41], counterbalanced by the BH3-only proteins (BAX and BAK) [31,42] causing the depolarization of the mitochondrial outer membrane permeabilization (MOMP)[43] and caspase-dependent death [44–47].

The epidermis and the dermis function as a secondary lymphoid organ with a powerful immunological setting composed by numerous cells, such as antigen-presenting cells (APC or Langherans cells), macrophages, mast cells, resident T-lymphocytes, local endothelial cells, tissue granulocytes, as well as the classic fibroblasts and keratinocytes (Figure 1). The cross-talk among these cells are mediated by a complex interactions of several cytokines, produced on the basis of cell stimulation by pathogens, chemicals or other irritating compounds, and ultraviolet light[48]. The maintenance of the skin immune homeostasis depends on a finely equilibrium of well-regulated relations between different cells and exogenous pathogens. Different skin diseases show a predominant inflammatory phenotype, and in particular, psoriasis shows a directly link to the dysregulation of this delicate balance with loss of immune tolerance[49].

Figure 1.

Skin cells. Skin resident cells in the steady state can be divided in innate immune cells: Langerhans cells, dermal dendritic cells, macrophages, other innate cells (mast cell), and adaptative immune cells: memory T cells. Non-immune cells are keratinocytes, fibroblasts and endothelial cells which also participate in immune responses by sensing tissue damage and producing inflammatory cytokines. On activation of the skin-resident immune system, additional immune cells are recruited to help contain and fight infection and/or to remove cellular debris to aid in the healing process (recruited immune cells). These include additional innate cells like neutrophils and eosinophils, as well as adaptive populations like naive or central memory T cells and B cells. T cell activation and differentiation in psoriasis. The figure shows the differentiation of Th lymphocytes in Th 17 producing IL-17, Th22 producing IL-22, Th9 producing IL-9A. Other cytokines implicate in developing psoriasis, acting on skin keratinocytes are IL-23, IL-12, IL-6, TNFalfa and IFN-gamma.

Microorganisms colonizing the skin layers including bacteria, archaea, viruses and fungi collectively compose the human skin microbiota. The role of microorganisms on the skin surface has not been properly analyzed. It has been hypothesized that, as the numerous species of microbes habiting the gastro-intestinal tract, in a mutualistic relationship with their host, even the skin microbiota composition has a crucial role in preventing infections by microbial pathogens trough the immune system activation. Complex interactions between the skin microbiota and TDCs induce immunological tolerance toward commensal antigens, protecting against skin inflammation. Any change in the skin microbiota can cause dysbiosis. Staphylococcus epidermidis is the most common microbe on skin surface. Even if S. epidermidis is hardly responsible of the keratinocyte’s impairment, it generates peptides toxic to other bacteria, such as S. aureus and group A Streptococcus (GAS, S. pyogenes)[50]. Modifications in microbial composition have been founded in several skin inflammatory disorders – ie: S. Aureus, producing delta- toxin, stimulates the local allergic response, inducing both innate and Th2 immune response[51]. Hence, genetic factors, infections, some metabolic syndromes and constant inflammatory conditions can alter the normal microbiome and allow the increase of bacteria which can modify the local immunological settings. In these circumstances, abnormalities in immune reactions to skin microorganism could promote a vicious inflammatory circle inducing finally tissue homeostasis collapse and development of different dermatological diseases[52].

The aim of this review is to understand how the immune cells affects the development of psoriasis, analyzing how T cells and their cytokines can trigger the development of this disease.

The role of the immune system in psoriasis

Psoriasis is a chronic inflammatory disease, with irregular differentiation of basal keratinocytes, which contributes to their increased proliferation. The etiology of the disorder is uncertain, but it is theorized an ensemble of genetic and environmental aspects leading to an impaired immunological activation in psoriatic patients (Figure 1), Figures 2 and 3 show the structure of some the protein indicated in Figure 1[53]. Indeed, despite the important role of keratinocytes in mediating inflammation, the activation of these cells is secondary to the activation of immune cells. However, a recent study has supposed a proper keratinocytes immune relevance showing that keratinocyte genetic defects yield mice are more susceptible to specific IL-17-mediated psoriasis-like inflammation.

Figure 2.

Structure of the Tumor Necrosis Factor. Mouse Tumor Necrosis Factor in a trimer conformation. The original PDB file is coded as 2TNF. The Color Legend shows he hydrophilic (red)/hydrophobic (green) surface residues for the left panel; while the secondary structure is shown in the central and left panel.

Figure 3.

Structure of the major immunological proteins. (a) Interferons gamma as a single chain, bound to its receptor. The original PDB file is coded as 2TNF 1FYH. (b) Interleukin-23 in a soluble form unbound. The original PDB file is coded as 5MXA. (c) mouse IL-23. The original PDB file is coded as 2L3Y.

Particularly, among all the different immunological cellular elements, T cells are the main protagonists in the development of the disease. Nowadays, in fact, the IL-23/Th17/IL-17 axis seems to show a determinant effect on the initiation of the inflammation in psoriasis[45]. The predominance of Th1/IFN-γ and pro-inflammatory cytokines, instead, occurs in the chronic phase [46,47]. Indeed, the disease is characterized by the increase of Th1 pathway cytokines (IL-2, IFN-γ, IL-12 and TNF-α) [54,55]. Moreover, peripheral levels of IFN-γ, TNF-α, IL-18, and IL-12 are proportionally correlated with psoriasis severity[48]. IFN-γ, especially, inhibits the apoptosis of keratinocytes, thus determining their hyper-proliferation in psoriatic skin [56–58].

On the other side, TNF-α contributes to the development and activation of numerous cell sets, stimulating apoptosis and enhancing the synthesis of some cytokines, including as C-reactive protein and IL-6, and the synthesis of adhesion molecule-1[59]. TNF-α induces the expression of IL-8, which is responsible for the recruiting neutrophils[60]. Finally, both TNF-a and IFN-γ promote skin inflammation through the recruitment of T cells and monocytes.

Regarding the Th17 lineage, several studies found that the hyper-activation of Th17 alters the Th17/Treg balance in psoriasis[61]. Psoriatic patients present, indeed, abnormalities in T cell phenotypes with, as already mentioned, an increase in Th1/Th17 and a relative decrease in Th2/Treg compared to healthy ones. The Th1/Th17 percentage increases proportionally with the severity of PASI. Actually, IL-17 is considered the key cytokine in the beginning and continuation of inflammation, stimulating the expression of pro-inflammatory cytokines by endothelial cells and macrophages[62]. Therefore the majority of IL-17-producing T cells express also the γδ T cell receptor. Thanks to IL-23 or IL-1β stimulation these cells are able to produce IL-17 and IL-22. Moreover T cell receptor γδ-deficient mice have shown a reduction of psoriasis after injection with IL-23 or imiquimod. Indeed human psoriatic skin has a marked infiltration of these cells, which are unique due to the constitutively expression ofIL-23 receptor, CLA, skin homing chemokine receptors (i.e. CCR6), and the transcription factor RORγt [63,64].

Of note, IL-23, secreted by macrophages and dendritic cells (DCs), is a vital factor in up-regulating the IL-17 production by stimulating Th17 survival and proliferation[65]. It also appears to be essential in the participation of keratinocytes in inflammation, inducing acanthosis and dermal infiltration by mixed inflammatory cells[66]. Furthermore, IL-23 is also responsible of enhancing TNF-α production in macrophages. Thus, the activation of IL23/Th17 inflammatory axis is critical for the formation and development of psoriatic lesions. Moreover, IL-22, a cytokine of the IL-10 family, produced by both Th17 and Th22, through the interface between immunity cells and epithelial ones, induces epidermal hyperplasia and hypogranulosis[67]. Another histological marker of psoriatic skin is the mobilization and degranulation of neutrophils, due to the IL-8 secretion by keratinocytes and Th17; IL-8 also mobilizes and stimulates T cells, natural killer (NK) cells, and basophils, induces angiogenesis and may stimulate the proliferation of keratinocytes[68].

Natural killer (NK) cells and NKT cells (which share features from both T cells and NK cells) represent a various subset of immune cells that seem to be implicated in psoriasis pathogenesis. Indeed, these cells are significantly increased in psoriatic lesional skin and are able to produce pathogenic cytokines, such as IFN-γ, IL-17, TNF-α, and IL-22. Especially NKT cells, through the expression of some chemokine receptors, such as CXCR3, CCR5, and CCR6, are recruited in lesional skin[69]. Even though these cells obviously have a role in inflammation, as indicated by the development of psoriasis driven by activated NKT cells in mice models grafted with normal skin or non-lesional skin, their function and their pathogenic role are not fully understood yet[70].

Another cytokine, implicated in psoriasis, is IL-9, which seems to be linked with other Th pathways, such as Th1, Th17 and also Th2. Indeed IL-9 together with IL- 6 and TGF-b1 increased the production of IL-17A from cultured and activated CD4 + T cells. Thus, IL-9 is a cytokine with pleiotropic activities, including activity as a growth factor for mast cells and T cells (e. g. Th17 cells), which can secrete pro-angiogenic factors such as IL-8, IL-17, TNF, HGF, FGF-2, and VEGF. Indeed, as mast cells may play a pathogenic role in psoriasis by augmenting VEGF release, IL-9 producing cells (Th9) may be the upstream cells of this stimulation. The ability of TH9 cells to enhance proliferation and the production of inflammatory cytokines from other T cell subsets and their increased presence in psoriasis skin lesions suggest that TH9 cells may also participate in initiating and maintaining cutaneous inflammation [71,72].

Finally, the dysfunction of Tregs, mainly involved in maintaining tolerance and preventing autoimmune disorders, is an essential aspect in psoriasis pathogenesis[73]. During the psoriasis, these Tregs exhibited an impaired function in counterbalance of pathogenic T cells. Notably, these Foxp3⁺Tregs from psoriasis patients can be more easily to differentiate into pathogenic Th17 cells. Consistently, IL-17A⁺/Foxp3⁺/CD4⁺ triple-positive cells can be observed in the skin lesions and be used to indicate the severity of psoriasis. Therefore, understanding of Treg properties in psoriasis can help to develop immunotherapeutics to treat diseases.

Conclusion

T cells are clearly implicated in several autoimmune diseases. In psoriasis, the Th1/Th17 axis seems to be the most implicated in initiating and/or maintaining the inflammatory loop. This evidence has been the basis for the development of bio-technologic therapies including anti-TNFs and anti-ILs.

Much effort has recently been made in defining the aetiopathogenesis of the inflammatory/autoimmune diseases of the skin. Now it is widely accepted that these are strictly associated to an imbalance between regulatory (Treg) and effector T (Th1/Th2 and Th17) responses. IL-10 producing T regs play a crucial role in restoring and maintaining the physiologic and delicate Th1/Th2 and Th17 balance.

This evidence, herein highlighted in psoriasis, has been the basis for the development of bio-technologic therapies, some of which are indicated in Tables 1 and 2, aimed at restoring the correct cytokine profiles, in the treatment of the inflammatory/autoimmune skin disorders.

Table 1.

Current clinical trials on psoriasis, based on regulators of the molecules shown in Figures 2 and 3.

Title	ClinicalTrials.gov Identifier	Location	Drug structure and target protein
TNF-alfa Inhibitors and Antibody Production in Patients With Psoriasis	NCT01657513	University Hospital, Gentofte, Copenhagen	Monoclonal TNF-alfa antibody → TNF-alfa inhibition
Study of Tumor Necrosis Factor Receptor Fusion Protein Etanercept (Enbrel) in Psoriasis of the Hands and/or Feet	NCT00585650	University of California, Irvine	Soluble TNF receptor-Fc fusion protein (TNFR2/p75 – IgG1 portion Fc) → TNF-alfa inhibition
Safety and Effectiveness of Live Zoster Vaccine in Anti-Tumor Necrosis Factor (TNF) Users (VERVE Trial)	NCT02538341	Birmingham, Alabama, USA	Monoclonal TNF-alfa antibody → TNF-alfa inhibition
A Study of the Safety and Efficacy of Ustekinumab in Patients With Psoriatic Arthritis With and Without Prior Exposure to Anti-TNF Agents	NCT01077362	Birmingham, Alabama, USA. Janssen Research & Development, LLC	recombinant fully human IgG1 kappa monoclonal antibody binding IL-12/23p40 → inhibition of IL-12/23p40
ASIS for Enbrel in Plaque Psoriasis (ASISinPP)	NCT02112097	Westminster, California, USA. ASIS Corporation	Soluble TNF receptor-Fc fusion protein (TNFR2/p75 – IgG1 portion Fc) → TNF-alfa inhibition
Study of CRx-191 to Assess Activity in Plaque Psoriasis	NCT00557739	Berlin, Germany. Bioskin GmbH	Mometasone furoate → immunosuppression
Phase 3 Study of M923 and Humira® in Subjects With Chronic Plaque-type Psoriasis	NCT02581345	Beverly Hills, California, USA. Momenta Pharmaceuticals, Inc.	Fully human IgG1 anti-TNF monoclonal antibody → TNF-alfa inhibition.
Secukinumab in TNF-IR Psoriasis Patients	NCT01961609	Cork, Ireland. Novartis Pharmaceuticals	Recombinant fully human IgG1/kappa monoclonal antibody binding IL-17 A → IL-17 A inhibition
A Study to Assess if Mirikizumab is Effective and Safe Compared to Secukinumab and Placebo in Moderate to Severe Plaque Psoriasis (OASIS-2)	NCT03535194	Birmingham, Alabama, USA. Eli Lilly and Company	Mirikizumab: humanized IgG4κ monoclonal antibody binding p19- IL-23 → IL-23p19 inhibition Sekukinumab: recombinant fully human IgG1/kappa monoclonal antibody binding IL-17 A → IL-17 A inhibition
Safety, Tolerability, Pharmacokinetics, Pharmacodynamics, and Efficacy of Repeat Doses of GSK2982772 in Subjects With Psoriasis	NCT02776033	Montreal, Quebec, Canada. GlaxoSmithKline	receptor-interacting protein-1 (RIP1) kinase inhibitor → RIP1 inhibition

Table 2.

Current available psoriasis therapies, based on regulators of the molecules shown in Figures 2 and 3[74].

Drug Name	Drug Structure	Target Protein	Mechanism of action
Infliximab	Chimeric (human-mouse) IgG1	TNF-alfa	Soluble TNF-alfa inhibition Activation of complement- dependent cytotoxicity and induction of cellular apoptosis, by interacting with membrane-bound TNF-alfa
Adalimumab	Human recombinant IgG1	TNF-alfa	Soluble TNF-alfa inhibition Activation of complement- dependent cytotoxicity and induction of cellular apoptosis, by interacting with membrane-bound TNF-alfa
Golimumab	Human recombinant IgG1	TNF-alfa	Soluble TNF-alfa inhibition Activation of complement- dependent cytotoxicity and induction of cellular apoptosis, by interacting with membrane-bound TNF-alfa
Etanercept	Dimeric fusion protein composed of the extracellular portion of two p75 TNF receptor linked to the Fc portion of IgG1	TNF-alfa	Soluble TNF-alfa inhibition.
Certolizumab	Pegylated Fab fragment of a human monoclonal antibody	TNF-alfa	Soluble TNF-alfa inhibition.
Ustekinumab	Human IgG1 monoclonal antibody	p40 subunit of IL-12 and IL-23	Soluble IL-12 and IL-23 inhibition,
Guselkumab	Human IgG1 monoclonal antibody	p19 subunit of IL-23	Soluble IL-23 inhibition.
Risankizumab	Human IgG1 monoclonal antibody	p19 subunit of IL-23	Soluble IL-23 inhibition.
Tildrakizumab	Human IgG1 monoclonal antibody	p19 subunit of IL-23	Soluble IL-23 inhibition.
Ixekizumab	Humanized IgG4 monoclonal antibody	IL-17 A	Soluble heterodimers of IL-17 A/F inhibition.
Sekukinumab	Human IgG1 kappa monoclonal antibody	IL-17 A	Soluble IL-17 A inhibition.
Brodalumab	Human IgG2 kappa monoclonal antibody	IL-17 receptor A	Inhibition of the interaction between IL-17 receptor A and IL-17 A/F and IL-17 E

In conclusion, increased knowledge of the complex aetiopathogenesis and immune mechanisms of psoriasis is the best way to increase our therapeutic potential against this disease. Therefore, the study of other cell populations, such as Th22 and Th9, represents a future perspective in the development of new drugs.

Funding Statement

This work has been supported by the Medical Research Council (to GM), Associazione Italiana per la Ricerca contro il Cancro (AIRC) to GM [IG#20473 (2018-2022)], Fondazione Luigi Maria Monti IDI-IRCCS (R.C. to M.M., M.A.P, G.M.), Ministry of Health & MAECI Italy-China Science and Technology Cooperation [#PGR00961] cooperation grant. Work has been also supported by Regione Lazio through LazioInnova Progetto Gruppo di Ricerca n [85-2017-14986].

Disclosure statement

No potential conflict of interest was reported by the authors.