**2. Psoriasis pathogenesis**

Psoriasis is usually identified by erythematous, raised, scaly skin lesions. These clinical features are explained by impressive growth and dilation of superficial blood vessels (elongated/hyperplastic capillaries in the papillary dermal region) and equally impressive hyperplasia of the epidermis. Epidermal growth occurs in a pattern termed ''psoriasiform'' hyperplasia, which describes both elongated rete pegs, thickening (acanthosis), and differentiation changes (Krueger & Bowcock, 2005). In psoriatic epidermis, keratinocytes proliferate and mature rapidly so that terminal differentiation, normally occurring in granular keratinocytes and then squamous corneocytes, is incomplete. Hence, squamous keratinocytes aberrantly retain intact nuclei (parakeratosis) and release few extracellular lipids that normally cement adhesions of corneocytes. The failure of psoriatic corneocytes to stack normally and to secrete extracellular lipids cause scaling and a break in the protective barrier whereas marked dilation of blood vessels in the dermis causes the visible redness of psoriatic skin lesions. The extensive infiltration of mononuclear immune cells in the dermis and epidermis (T cells and dendritic cells in the dermis and polymorphonuclear leucocytes such as neutrophils within small foci in the stratum corneum) is another defining feature of psoriasis histopathology and a key point of its pathogenesis. The pathogenesis of psoriasis is considered to be an immunologically mediated process that takes place upon a favourable

Pathogenesis of Psoriasis: The Role of Pro-Inflammatory Cytokines Produced by Keratinocytes 11

cells (NKT), contribute to the disease process. Key processes during disease maintenance are the presentation of putative (auto)-antigens to T cells, the release of IL-23 by dermal dendritic cells, the production of pro-inflammatory mediators such as IL-17A, IL-17F, IL-22 by Th17 and Tc17 cells and IFN-y and TNF-α by Th1 and Tc1 cells. These mediators act on keratinocytes leading to the activation, proliferation and production of antimicrobial peptides (AMPs) (e.g., LL-37 cathelicidin and β-defensins) and chemokines (e.g., CXCL1, CXCL9 through CXCL11 and CCL20), and S100 proteins (e.g., S100A7-9) (Nestle et al., 2010). These soluble mediators feed back into the pro-inflammatory disease cycle and shape the inflammatory infiltrate (Fig.1). In fact keratinocyte products influence immune activation, and products of activated lymphocytes alter keratinocyte responses, including the induction of new adhesion molecules for T cells. However although intrinsic alterations in keratinocytes are crucial for the development of psoriatic lesions, a deregulated function of other resident skin cells, such as fibroblasts and endothelial cells, may also contribute to the pathogenesis of psoriasis. Epidermal-dermal cell interaction is a determinant for the maintenance of the psoriatic phenotype because it guarantees the local production of growth factors and cytokines stimulating keratinocyte proliferation. An important paracrine loop

Fig. 1. Main actors of psoriasis pathogenesis.

genetic background. According to this view, the presence of a yet unknown (auto)-antigen causes the generation of effector T-cells that infiltrate the skin and initiate the inflammatory process (Wolk et al., 2009a). Over its course, cutaneous infiltration of various immune cell populations and, subsequently, an activation of numerous immune and tissue cells in the skin take place. Secreted cytokines from activated cells then induce keratinocyte alterations such as excessive growth and aberrant differentiation forming the basis of the epidermal acanthosis, hyperkeratosis and parakeratosis which characterize psoriasis plaques. The trigger of keratinocyte response is thought to be the activation of the cellular immune system, with T cells, dendritic cells and various immune-related cytokines and chemokines implicated in pathogenesis. Rather than viewing psoriasis as a disease caused by a single cell type or a single inflammatory cytokine, it is probably best to conceptualize the disease pathogenesis as linked to many interactive responses among infiltrating leucocytes, resident skin cells, and an array of pro-inflammatory cytokines, chemokines, and chemical mediators produced in the skin (Lowes et al., 2007). Fundamentally two different cell types interact in the formation of a psoriatic lesion: keratinocytes and mononuclear leukocytes. Whereas keratinocytes might be viewed only as bystander cells in terms of immune activation, it is more likely that they are active participants in the recruitment and activation of leukocytes in psoriatic lesions. Thus, there are two sets of interactive cellular responses in the psoriatic lesion that potentially create a ying/yang relationship; the balance between the activation of innate and acquired immune cell types, and the factors produced by epidermal keratinocytes that directly affect T cells and dendritic cells, and vice versa. Psoriasis is considered a T helper 1 (Th1) condition, characterized by the production of interferongamma (IFN-γ) and tumor necrosis factor alpha (TNF-α) under the influence of interleukin-12 (IL-12). However, there is increasing evidence of the importance of a novel T cell population, Th17 cells, in this inflammatory disease. Th17 cells are stimulated by IL-23 (which shares the p40 subunit with IL-12) to produce IL-17 and also IL-22, which has recently been shown to be a major driver of acanthosis in psoriasis, and so is a novel target for treatment. Effector cells of innate immunity including neutrophils, plasmacytoid dendritic cells (plasmocytoid DCs) and CD11c+ dendritic cells (myeloid DCs) are involved and present in psoriatic lesions creating a very intricate and complex network of interactions which is the base of the pathogenetic process of psoriasis (Nograles et al., 2010). An interplay between environmental and genetic factors sets the scene for disease-initiating events. Initial triggers such as physical trauma or bacterial products start a cascade of events that include the formation of DNA-LL-37 complexes, activation of plasmocytoid dendritic cells and secretion of interferon-α (IFN-α). IFN-α secreted by plasmocytoid dendritic cells promotes the activation of myeloid dendritic cells (Nestle et al., 2005). Activated myeloid dendritic cells migrate into draining lymph nodes and induce the differentiation of naive T cells into effector cells such as Th17 or type 17 cytotoxic T cells (Tc17) and Th1 or type 1 cytotoxic T cells (Tc1) (Nestle et al., 2010). Effector cells recirculate and slow down in skin capillaries in the presence of selectin-guided and integrin-guided receptor-ligand interactions. Immune cells expressing the chemokine receptors CCR6, CCR4, and CXCR3 emigrate into skin tissue along chemokine gradients. Dendritic cells and T cells form perivascular clusters and lymphoid-like structures around blood vessels in the presence of chemokines such as CCL19 produced by macrophages. A key checkpoint is the migration of T cells from the dermis into the epidermis; this migration is controlled through the interaction of α1β1 integrin (very late antigen 1 [VLA-1]) on T cells and collagen IV at the basement membrane (Conrad et al., 2007). Unconventional T cells, including natural killer T

genetic background. According to this view, the presence of a yet unknown (auto)-antigen causes the generation of effector T-cells that infiltrate the skin and initiate the inflammatory process (Wolk et al., 2009a). Over its course, cutaneous infiltration of various immune cell populations and, subsequently, an activation of numerous immune and tissue cells in the skin take place. Secreted cytokines from activated cells then induce keratinocyte alterations such as excessive growth and aberrant differentiation forming the basis of the epidermal acanthosis, hyperkeratosis and parakeratosis which characterize psoriasis plaques. The trigger of keratinocyte response is thought to be the activation of the cellular immune system, with T cells, dendritic cells and various immune-related cytokines and chemokines implicated in pathogenesis. Rather than viewing psoriasis as a disease caused by a single cell type or a single inflammatory cytokine, it is probably best to conceptualize the disease pathogenesis as linked to many interactive responses among infiltrating leucocytes, resident skin cells, and an array of pro-inflammatory cytokines, chemokines, and chemical mediators produced in the skin (Lowes et al., 2007). Fundamentally two different cell types interact in the formation of a psoriatic lesion: keratinocytes and mononuclear leukocytes. Whereas keratinocytes might be viewed only as bystander cells in terms of immune activation, it is more likely that they are active participants in the recruitment and activation of leukocytes in psoriatic lesions. Thus, there are two sets of interactive cellular responses in the psoriatic lesion that potentially create a ying/yang relationship; the balance between the activation of innate and acquired immune cell types, and the factors produced by epidermal keratinocytes that directly affect T cells and dendritic cells, and vice versa. Psoriasis is considered a T helper 1 (Th1) condition, characterized by the production of interferongamma (IFN-γ) and tumor necrosis factor alpha (TNF-α) under the influence of interleukin-12 (IL-12). However, there is increasing evidence of the importance of a novel T cell population, Th17 cells, in this inflammatory disease. Th17 cells are stimulated by IL-23 (which shares the p40 subunit with IL-12) to produce IL-17 and also IL-22, which has recently been shown to be a major driver of acanthosis in psoriasis, and so is a novel target for treatment. Effector cells of innate immunity including neutrophils, plasmacytoid dendritic cells (plasmocytoid DCs) and CD11c+ dendritic cells (myeloid DCs) are involved and present in psoriatic lesions creating a very intricate and complex network of interactions which is the base of the pathogenetic process of psoriasis (Nograles et al., 2010). An interplay between environmental and genetic factors sets the scene for disease-initiating events. Initial triggers such as physical trauma or bacterial products start a cascade of events that include the formation of DNA-LL-37 complexes, activation of plasmocytoid dendritic cells and secretion of interferon-α (IFN-α). IFN-α secreted by plasmocytoid dendritic cells promotes the activation of myeloid dendritic cells (Nestle et al., 2005). Activated myeloid dendritic cells migrate into draining lymph nodes and induce the differentiation of naive T cells into effector cells such as Th17 or type 17 cytotoxic T cells (Tc17) and Th1 or type 1 cytotoxic T cells (Tc1) (Nestle et al., 2010). Effector cells recirculate and slow down in skin capillaries in the presence of selectin-guided and integrin-guided receptor-ligand interactions. Immune cells expressing the chemokine receptors CCR6, CCR4, and CXCR3 emigrate into skin tissue along chemokine gradients. Dendritic cells and T cells form perivascular clusters and lymphoid-like structures around blood vessels in the presence of chemokines such as CCL19 produced by macrophages. A key checkpoint is the migration of T cells from the dermis into the epidermis; this migration is controlled through the interaction of α1β1 integrin (very late antigen 1 [VLA-1]) on T cells and collagen IV at the basement membrane (Conrad et al., 2007). Unconventional T cells, including natural killer T cells (NKT), contribute to the disease process. Key processes during disease maintenance are the presentation of putative (auto)-antigens to T cells, the release of IL-23 by dermal dendritic cells, the production of pro-inflammatory mediators such as IL-17A, IL-17F, IL-22 by Th17 and Tc17 cells and IFN-y and TNF-α by Th1 and Tc1 cells. These mediators act on keratinocytes leading to the activation, proliferation and production of antimicrobial peptides (AMPs) (e.g., LL-37 cathelicidin and β-defensins) and chemokines (e.g., CXCL1, CXCL9 through CXCL11 and CCL20), and S100 proteins (e.g., S100A7-9) (Nestle et al., 2010). These soluble mediators feed back into the pro-inflammatory disease cycle and shape the inflammatory infiltrate (Fig.1). In fact keratinocyte products influence immune activation, and products of activated lymphocytes alter keratinocyte responses, including the induction of new adhesion molecules for T cells. However although intrinsic alterations in keratinocytes are crucial for the development of psoriatic lesions, a deregulated function of other resident skin cells, such as fibroblasts and endothelial cells, may also contribute to the pathogenesis of psoriasis. Epidermal-dermal cell interaction is a determinant for the maintenance of the psoriatic phenotype because it guarantees the local production of growth factors and cytokines stimulating keratinocyte proliferation. An important paracrine loop

Fig. 1. Main actors of psoriasis pathogenesis.

Pathogenesis of Psoriasis: The Role of Pro-Inflammatory Cytokines Produced by Keratinocytes 13

(Nograles et al., 2008). The expression of another antimicrobial peptide, LL-37 cathelicidin, can also be enhanced by IL-17 in the presence of vitamin D3 (Peric et al., 2008). These proteins may function as key inflammation inducers in psoriasis, and at the same time decrease skin infections under conditions of a dysfunctional epidermal barrier. Infections or injury to the skin can promote lesion formation in susceptible individuals and these triggers have been shown to stimulate keratinocyte production of the antimicrobial LL-37 cathelicidin that, when complexed with self-DNA, binds to TLR9 on plasmocytoid DCs. These cells produce massive amounts of IFN-α and are implicated in the initiation of psoriasis lesions (Lande et al., 2007). Accordingly, patients treated with a topical plasmocytoid DCs agonist, imiquimod, up-regulate IFN-α and experience exacerbations in psoriasis. In addition to stimulating plasmocytoid DCs, LL-37 has been shown to complex with self-RNA to trigger the activation of myeloid dendritic cells (myeloid DCs) through TLR8. This leads to production of TNF-α and IL-6, and promotes their differentiation into mature dendritic cells (Ganguly et al., 2009). Because myeloid DCs in psoriasis have been shown to produce IL-23 it is plausible that self-RNA complexes might potentially initiate the inflammatory cascade leading to

Recent investigations identified high levels of osteopontin (OPN) in psoriatic plaques (Buommino et al., 2009). Osteopontin is produced by both keratinocytes and activated T cells. It is a phosphorylated acidic glycoprotein of pleiotropic properties and has been recently recognized as a potential inflammatory cytokine. A model for the role of OPN in Th1 Th17 psoriatic disease was so suggested. After activation of myeloid DCs that express OPN, they migrate to skin draining lymph nodes and polarize naive T cells towards a Th1 and Th17 phenotype. In addition OPN secreted by keratinocytes attracts additional inflammatory cells. Moreover OPN inhibits keratinocyte apoptosis thereby supporting enhanced epidermal proliferation, and, through a pro-angiogenic effect on microvascular endothelial cells, OPN also promotes vessel formation subsequently supporting the influx of

Cutaneous and systemic over-expression of various pro-inflammatory cytokines has been demonstrated in psoriasis. Psoriatic keratinocytes are able to produce and release IL-1α, IL-1β, IL-6, IL-15, IL-18 and IL-20, all of them involved in the development of different alterations which compose the complex and intricate net of psoriasis pathogenesis (Tab. 1).The cellular composition of the inflammatory infiltrate within the psoriatic plaques as well as hyperproliferation of keratinocytes and so the whole pathogenetic process of

IL-1 is a pro-inflammatory cytokine stimulating, among others, IL-2 and IFN-γ production through activated T cells. IL-1 activates neutrophils, monocytes, eosinophils and basophils, triggers production of TNF-α, IL-6, IL-8 by macrophages, and in autocrine fashion, IL-1 synthesis. IL-1 promotes proliferation of bone marrow cells, B lymphocytes, neutrophils, macrophages and platelets (Dinariello, 2002). In psoriasis, keratinocytes are the main source of IL-1α and IL-1β in the skin stored in the form of precursor particles (Zepter et al., 1997). Monocytes/macrophages, activated endothelial cells, fibroblasts and

psoriasis appears to be mediated by these cytokines (Wojas-Pelc et al., 2006).

expansion and activation of Th17 cells (Nograles et al., 2010).

inflammatory cells (Buback et al., 2009).

**3.1.1 Keratinocytes and IL-1α & IL-1β**

**3.1 Keratinocytes and cytokines** 

operating between keratinocytes and fibroblasts that culminates with keratinocyte proliferation is triggered by IL-1: IL-1α and IL-1β neutralization and IL-1 receptor antagonist significantly reduced keratinocyte growth through the abrogation of keratinocyte growth factor (KGF) production by fibroblasts. However, IL-1 is unlikely to be the only regulator of KGF production by fibroblasts, and indeed other keratinocyte-derived factors, such as parathyroid hormone–related protein (PTHrP), induce KGF expression. In addition fibroblast growth factor (FGF) family members and granulocyte-macrophage colony stimulating factor (GM-CSF) also play a particularly important role in the fibroblast-driven regulation of keratinocyte proliferation (Albanesi et al., 2007). Moreover, the activated phenotype of lesional endothelial cells are believed to play a central role in the pathogenesis of psoriasis and are determined by the expression of a variety of membrane and soluble factors mainly responsible for T-cell recruitment in the skin like adhesion molecule 1 (ICAM-1). A key point is the endothelium expression of certain chemokines involved in the arrest of circulating T lymphocytes at inflammatory sites: upon exposure to inflammatory signals, mainly represented by TNF-α and IL-1, endothelial cells express a broad array of chemokines, including CCL20/MIP-3α, CXCL12/SDF-1, CCL21/SLC, CCL17/TARC, CCL2/MCP-1, CXCL10/IL-8, CCL5/RANTES, CXCL1/Gro-α, and CCL4/MIP-1β (Girolomoni et al., 2004).

In summary, feedback loops involving keratinocytes, fibroblast and endothelial cells contribute to tissue reorganization with endothelial cell activation and proliferation and deposition of extracellular matrix. The hypothesis of cytokine/chemokine network in psoriasis proposed a central role of pro-inflammatory cytokines, including TNF-α.
