DIAGRAMA ESQUEMÁTICO [23]

The development of psoriatic lesions is associated with the presence of an inflammatory infiltrate in both the dermis and epidermis. In particular, CD4 and CDS positive T-cells and neutrophils are found to be recruited. A role for B-cells and antibody mediated immune responses has not been reported. The majority of T-cells found within psoriatic lesions express the cutaneous lymphocyte-associated antigen (CLA) (Picker et al., 1994; Pitzalis et al., 1996), a ligand for E- and P-selectin which enhances their extravasation in skin (Picker et al., 1993). It has been demonstrated that CLA positive T-cells accumulate in evolving psoriatic lesions prior to evident hyperproliferation of the epiderm is, as determined by numbers of Ki67 positive kératinocytes (Davison et al., 2001). In addition, it has been shown that the defects in keratinocyte differentiation present in psoriatic plaques are observed at the edges of lesions before characteristic abnormalities in the vasculature develop (Parent et al., 1990). Given that T-cells cloned from psoriatic lesions are able to stimulate keratinocyte proliferation via the production of soluble factors (Bata- Csorgo et al., 1995a; Bata-Csorgo et al., 1995b; Strange et al., 1993) and activated

Chapter 1. Introduction

kératinocytes can modulate development o f the vasculature via production of VEGF (Detmar et al., 1994), it is possible that infiltration of activated lymphocytes into the epidermis triggers the hyperproliferation and abnormal differentiation of the kératinocytes which then leads to vascular changes and hence to a fully developed psoriatic lesion (Norris et al., 1997; Ortonne, 1999).

A critical role for the immune system in the pathogenesis of psoriasis has been suggested following the observation that patients treated with an activated T-cell specific toxin (a fusion between IL-2 and fragments of the diptheria toxin) show clinical improvements in their psoriasis (Gottlieb et al., 1995). Treatment with anti-CD4 or anti-CD3 antibodies can also lead to reductions in the severity of psoriasis (Bachelez et al., 1998; Morel et al., 1992; Rizova et al., 1994; Weinshenker et al., 1989), as can antibodies directed against other proteins involved in activation of T-cells such as CD80 (Gottlieb et al., 2002) and GDI la (Gottlieb et al., 2000). Furtherm ore, potent im m unosuppressive drugs and regimens such as cyclosporin A and PU VA are standard treatments for severe psoriasis (Fry, 1992; Horroccks et al., 1989; Vallat et al., 1994).

It is known that systemic superantigen-mediated activation of T-cells through pharyngeal streptoccal infection can trigger acute guttate psoriasis which can also subsequently develop into chronic plaque psoriasis (Leung et al., 1995b; Valdimarsson et al., 1995). The cutaneous localisation of lymphocytes in this disease could be generated through the induction of the skin homing receptor (CLA) in T-cells by bacterial superantigen (Leung et al., 1995a). Streptoccal infection is also known to exacerbate plaque psoriasis but analysis of T-cells present in lesions has provided evidence of a classical antigen stimulation of

these cells (Chang et al., 1994; Mens sen et al., 1995; Norris et al., 1997). It is currently widely thought that psoriasis may be a T-cell mediated autoimmune disease directed at epidermal self-antigens. It has been postulated that an initial streptoccal infection activates T cells non-specifically through production of exotoxins which can act as super antigens. Subsequently, T cells specific for a cell wall component of streptoccus, the M protein, are generated which when present in skin are able to cross-react with epidermal keratins which have a high degree of homology to this bacterial component thus generating a chronic inflam m atory skin response (McFadden et al., 1991; Valdimarsson et al., 1995). Circulating T cells from psoriatic patients have been shown to be responsive to streptoccal M proteins with homology to, for example, keratin 14 (Brown et al., 2000; McFadden et al., 1991; Sigmundsdottir et al., 1997). In addition, T cells from psoriatic patients react to peptides from keratin 17 which share sequences with M -protein (Gudmundsdottir et al.,

1999).

Evidence for a central role of the immune system in psoriasis is also obtained from mouse models of the disease. For example, the spontaneous flaky skin mutant (fsn) shares many of the features of psoriasis, generating hyperproliferative skin lesions with a mixed inflammatory infiltrate and neovascularisation within the dermis (Sundberg et al., 1997). The exact nature of this autosomal recessive mutation is not known although it maps to the distal end of the mouse chromosome 17 (Beamer et al., 1995; Pelsue et al., 1995). It has recently been shown that the peripheral lymphoid compartment of these mice is greatly expanded and contains large numbers of activated T cells (A bem ethy et al., 2000a; Abem ethy et al., 2000b). Furthermore, bone marrow grafts from fs n homozygote mice

Chapter 1. Introduction

lesions (Sundberg et al., 1993) em phasizing the role of the im m une system in the development of the phenotype. However, skin grafts from fsn /fsn mice to athymic nude mice (nu) which lack T cells maintain the psoriatic-like phenotype for at least 10 weeks post-graft (Sundberg et al., 1994). Furthermore, scid/scid,fsn/fsn double mutant mice also have the skin phenotype despite lacking T-cells, B-cells and Thy-l-i- dendritic cells (Sundberg et al., 1993). These reports suggest that even \ h o u ^ fsn /fsn mice have large numbers of activated T-cells, these are not required for maintenance of the skin phenotype. However, for example, fsn/fsn skin grafts on nu/nu mice have a substantial recruitment of host innate immune cells, both macrophages and neutrophils (Sundberg et al., 1994) and depletion or blocking of neutrophil recruitment by antibodies in fsn/fsn mice substantially reduces the skin lesions (Schon et al., 2000). Therefore, even though the exact role of T cells in the flaky mouse skin phenotype is unclear, other elements of the immune system are implicated. This is of interest given that neutrophil infiltration into the epidermis of psoriatic lesions may have a role in production o f the observed keratinocyte hyperproliferation (Beurskens et al., 1989) and contribute to activation of intraepidermal T cells (Terui et al., 2000).

O ther mouse models of psoriasis have clearly implicated T cells in generation of the hyperproliferative skin lesions. It has been reported that when uninvolved skin from psoriatic patients is grafted onto scid/scid mice and autologous immunocytes are injected into the underlying dermis, the skin converts into a lesion with many psoriatic features (Wrone-Smith and Nickoloff, 1996). This conversion effect required the activation of the immunocytes with IL-2 and bacterial superantigen prior to intraderm al injection in the majority of cases. As a control, psoriatic-like lesions were not produced when skin and

immunocytes derived from normal, healthy, donors were used. It was additionally noted that intradermal injection of activated autologous CD4+ T cells alone could trigger the conversion to a psoriatic lesion but injection of activated CD8+ T cells could not (Nickoloff and Wrone-Smith, 1999), emphasizing the importance of CD4-I- rather than CD8+ T cells in initiating psoriatic lesions. In a further murine system, psoriatic - like skin lesions could be induced in scid/scid mice when injected with minor histocompatability mismatched naïve CD4-I- T cells (Schon et al., 1997).

These mouse models clearly implicate CD4+ T cells in the pathogenesis of psoriasis and fit well with the observed clinical benefits of anti-CD4 antibodies in the treatment of the disease (see above). It is of interest that CD4+ T cells predominate in early-phase psoriatic lesions whereas CD8-I- cells are the more abundant lym phocyte in chronic plaques (Onuma, 1994). Indeed, CD 8+ T cells are regarded as more of an effector cell in the developm ent of psoriatic lesions (Chang et al., 1995). The m ajority of CD4+ T cells present in psoriatic lesions exhibit a T hl cytokine expression profile and as such produce large quantities of interferon-y (IFNy) (Austin et al., 1999; Schlaak et al., 1994). The production of IFNy by lesional T cells is thought to be im portant in generating the hyperproliferation and differentiation defects observed in psoriatic kératinocytes. Kératinocytes are known to express the receptor for IFNy (Scheynius et al., 1992) and conditioned medium of T cell clones from psoriatic lesions can stimulate keratinocyte growth in vitro in an IFNy dependent fashion (Prinz et al., 1994). Furthermore, injection of recombinant IFNy into human skin is found to increase keratinocyte proliferation (Barker et al., 1993) and overexpression of IFNy in the epidermis of transgenic mice from the involucrin promoter induces hyperproliferation of the kératinocytes and focal parakeratosis

Chapter 1. Introduction

in severely affected animals (Carroll et al., 1997; Seery et al., 1997) (see Table 1.2). It has also been shown that several markers o f psoriatic kératinocytes, including intercellular adhesion m olecule-1 (ICAM-1), HLA-DR and keratin 17 are induced in kératinocytes following IFNy stimulation (Barker et al., 1990a; Barker et al., 1990b; Dustin et al., 1988; Jiang et al., 1994). It could be shown in the mouse model of psoriasis of (Schon et al., 1997) that IFNy production by the injected T cells contributed to the severity of the lesions although not to the incidence, for which a role was attributed to IL-12 (Hong et al., 1999).