**4.1.2 Neutrophilic granulocytes**

234 Psoriasis

Th1 and Th17 pro-inflammatory cytokines mediate keratinocyte hyperproliferation and trigger a 'vicious cycle' of inflammation. IL-23 secreted principally by keratinocytes, dendritic cells and macrophages is critical for maintenance of Th17 function. Low levels of anti-inflammatory cytokines released by Th2 and Tregs potentially counteract but cannot

From the foregoing it is clear that the contribution of both innate and adaptive immune responses are important in mediating the inflammatory psoriasis cascade (Gaspari, 2006)

The present knowledge of the possible role of innate immunity cells (Kcs, Dcs, neutrophils,

Over the last 20 years it has been regularly discussed if psoriatic skin lesions arise from a primary alteration in epidermal keratinocytes or in T cells (**Figure 1**); the current view is that infiltrating T cells initiate and maintain psoriasis. In this vision, cytokines (e.g., IL-1, IL-6 and IFN-) secreted by T cells and other inflammatory cells (DCs, macrophages and neutrophils) would trigger KCs hyperproliferation, inducing epidermal hyperplasia (J. G. Krueger, 2002). In particular, IL-23, whose expression is increased in psoriatic lesion (Piskin et al., 2006), has been implied in the development of epidermal acanthosis, most likely through the induction of IL-22 (Zheng et al., 2007); injection of IL-23 into mouse dermis induces as dermal inflammation as epidermal hyperplasia similarly to features seen in

Previous studies (J. G. Krueger, 2002) suggested an additional explanation for the chronic epidermal hyperplasia in psoriatic lesions: the migration of T cells in the epidermis would firstly break the basement membrane, which has been shown to have large areas with reduced staining intensity for collagen IV and laminins (Fleischmajer et al., 2000), and secondly disrupt desmosome connection between KCs. These two events could be interpreted by KCs as an injury and therefore induce a wound repair response. As a consequence many mitogenic cytokines would be released by KCs triggering a regenerative epidermal growth. Hence, T cells in psoriatic epidermis would be responsible for the chronic hyperplasia as by releasing pro-inflammatory cytokines as by disrupting epidermal

Another evidence against the fundamental role of T cells in psoriasis arise from the observation that HIV+ patients (which usually have reduced number of CD4+ T cells) develop psoriasis with similar frequency as the rest of the population (Namazi, 2004).

Finally, in 2005 Zenz et al. reported an interesting animal model with psoriasis-like features (Zenz et al., 2005); They knocked out JunB and c-Jun, two components of the AP-1 transcription factor, which control cell proliferation and differentiation , cytokine production, and stress responses in the skin. The importance of this animal model lies in the fact that JunB is located in the PSORS6 locus, which has been shown to be a psoriasis susceptibility region (Hensen et al., 2003). Interestingly, these mice developed not only

**4.1 Aberrant activation of the innate fraction of the skin immune system** 

macrophages, NK and NKT cells) in psoriasis will be discussed.

balance the effects of Th1/Th17 cytokines.

**4.1.1 Keratinocytes in psoriasis**

psoriasis (J. R. Chan et al., 2006).

(**Figure 1**).

integrity.

Early studies in psoriasis (Jablonska, 1986), in which the initial (pre-pinpoint) lesions were studied histologically,indicated that the primary abnormality in a developing lesion of psoriasis is the perivascular accumulation of neutrophils as well as their epidermis invasion (Kogoj phenomenon). Next, these neutrophils accumulations lead to microscopically detectable microabscesses (Munro abscesses). In many patients with psoriasis, these micropustules may enlarge and become clinically visible as sterile 2–3-mm pustules. In some patients, pustules are the primary visible abnormality, often on an erythematous base.

The presence of neutrophils must be important for the formation of psoriatic skin lesions, in fact agranulocytosis has been reported to result in the remission of psoriasis (Toichi et al., 2000). Next, neutrophils can contribute to the hyperproliferation of keratinocytes by the effects of human leucocyte-derived elastase (Rogalski et al., 2002).

#### **4.1.3 Natural killer T cells**

The potential role of cells expressing NK receptors in psoriatic skin was firstly proposed by Nickoloff (Nickoloff et al., 1999b); in further experiments, he showed that a CD94+/CD161+ NKT cell line (isolated from a psoriatic patient) injected in prepsoriatic skin of a SCID

The Role of Immune Response and the Impact of Biological Drugs in Psoriasis Patients 237

but their percentage remained significantly lower in comparison with healthy control subjects. In contrast, Langewouters group showed that the number of circulating CD94+ an CD161+ cells was significantly higher in patients with moderate-to-severe as compared to patients with mild psoriasis (Langewouters et al., 2008). It has been also reported that circulating CD161+ NKT cells in people with severe psoriasis belong to Th1 cells (W. L. Chan et al., 2003) and that treatment with alefacept or efalizumab causes significant reduction in their number (Larsen et al., 2007; van Lingen et al., 2008). These observations imply that distinct subsets of NKT cells differentially regulate immune responses and that their relative imbalance might be of importance in the psoriasis pathogenesis, but the precise impact of

DCs are involved in the development of tolerance (Steinman et al., 2003) and are the unique professional antigen-presenting able to take up antigen in the tissue they reside in and to migrate to the draining lymph nodes; here activate naive T cells, generating specific T-cell

In the peripheral tissues DCs receive all kinds of microenvironment signals, that influence the their maturation process, determining the phenotype and function of mature DCs and their type 1 type 2 polarizing potential. In case of risk (e.g. infection, cancer), DCs will transform into strong stimulatory antigen presenting cells, whereas under non pathological steady-state conditions DCs do not reach full maturation and they will present self-peptides MHC complexes in the presence of insufficient costimulatory molecules, inducing T-cell anergy or expansion of regulatory T cells (peripheral tolerance). If this delicate balance of immune reactivity vs. tolerance is broken, chronic inflammatory diseases, like psoriasis,

In the absence of pathogenic substances, stress signals from neighbouring cells (e.g. necrotic cells) can activate DCs enabling them to stimulate naive T cells (Gallucci et al., 1999). In relation to this, Krueger has expressed an remarkable view concerning traumatic injuries (breaches in the basement membrane and disruption of desmosome connections between adjacent keratinocytes) caused by migrating T cells and DCs in the epidermis, that are possibly involved in the development of psoriasis lesions (J. G. Krueger, 2002). Keratinocytes will respond to these defects by overproduction of cytokines, among others

Another exciting point to mention is the observation that DC development is also affected by mutual interaction with NKT cells (Taniguchi et al., 2003): weak responses by NKT cells to glycolipid CD1d complexes can be enhanced by DC-derived IL-12, resulting in up-

DCs are numerous in the dermal part of psoriasis skin and many of them exhibit an activated phenotype (CD80+, CD83+, CD86+ and DC-LAMP+) (Abrams et al., 2000; Teunissen, 2005) and may be an important source of TNF-a (Nickoloff et al., 1991; Zhou et

Recent evidences from different genetic mouse models show that also the macrophages can contribute to T-cell-mediated and epidermis-mediated psoriasis-form skin inflammation

expression of IFN-c by NK T cells. This interaction may be relevant in psoriasis.

these findings remains unclear.

responses (Teunissen, 2005).

may develop.

al., 2003).

**4.1.4 Accessory cells (Dendritic cells and macrophages)** 

TNF-a, a key cytokine to induce DC migration/maturation.

mouse, gave rise to creation of psoriatic plaque characterized by diffuse keratinocyte expression of CD1d, CD161+ T cells infiltration and marked presence of mRNA for IFN-c and IL-15 (Nickoloff et al., 2000).

In line with these data, it seems that constitutive CD1d expression on prepsoriatic skin keratinocytes represents a primary prerequisite that ensures their contact with the CD161 molecule on NKT cells. This interaction might also represent one of the critical events in the triggering of psoriasis as CD1d-bearing keratinocytes can present endogenous (self) or exogenous (bacterial, viral) glycolipids to NKT-cells. Consequently to the recognition of glycolipid antigens, NKT cells produce large amounts of IFN-c which induces stronger keratinocyte expression of CD1d; in this way the pathogenic mechanism of psoriatic plaques is not only initiated but also maintained.

Numerous studies have addressed the issue of NKT cells in the lesional skin of psoriasis patients and a constant result evidenced by independent groups of investigators is that CD161+ T cells appear in greater numbers in lesional skin of patients with psoriasis than in normal healthy skin and/or prepsoriatic skin (Bonish et al., 2000; Cameron et al., 2002; Curry et al., 2003; Vissers et al., 2004a).

However, Curry group shown a greater frequency of CD161+ T cells yet in the prepsoriatic skin in comparison with the normal skin, suggesting that certain immune response dysregulations exists in the uninvolved skin and creates a milieu, promoting the psoriatic lesions onset (Curry et al., 2003).

A flow cytometric analysis of psoriatic tissue-infiltrating T cell demonstrated the presence of CD3+ cells expressing also CD16, CD56, CD158b, CD94 or NKG2A and CD4–CD8–, the majority of these is a subset of NKT cells (Liao et al., 2006). In addiction a recent study (Zhao et al., 2008) confirmed that NKT cells as well as CD1d molecules were increased within psoriatic skin.

The supposed pathogenic role of NKT cells in psoriasis is also supported by the effects of different pharmacological treatments, that decreases NKT cell numbers in psoriatic plaques, in particular the clinical efficacy of betamethasone dipropionate (Bovenschen et al., 2007; Vissers et al., 2004b, 2008;) and similarly of alefacept that induce an improvement in plaque severity accompanied by significant reductions of dermal CD94+ and CD161+ (Bovenschen et al., 2007).

In general, these data strongly indicate that lesional CD94+ and CD161+ NKT cells actively participate in the development and/or maintenance of psoriatic lesions, but the relative relevance of each of these subsets remains elusive. However, it might be hypothesized that epidermal NKT cells play a major pathogenic role due to their direct interaction with CD1d+ keratinocytes and the resulting hyperproliferation while the dermal component of NKT cells participates in the immune response through the interaction with CD1d-bearing dermal dendritic cells and monocytes.

Likewise other Th1-mediated autoimmune diseases, psoriasis is associated with decreased numbers of circulating NKT cells (Cameron et al., 2003; Koreck et al., 2002; van der Vliet et al., 2001;); Koreck reported that decreased percentage of NKT cells population in blood of psoriasis patients tended to be even lower in those with frequently relapsing (Koreck et al., 2002). Moreover, different systemic therapy regimens resulted in the recovery of these cells, but their percentage remained significantly lower in comparison with healthy control subjects. In contrast, Langewouters group showed that the number of circulating CD94+ an CD161+ cells was significantly higher in patients with moderate-to-severe as compared to patients with mild psoriasis (Langewouters et al., 2008). It has been also reported that circulating CD161+ NKT cells in people with severe psoriasis belong to Th1 cells (W. L. Chan et al., 2003) and that treatment with alefacept or efalizumab causes significant reduction in their number (Larsen et al., 2007; van Lingen et al., 2008). These observations imply that distinct subsets of NKT cells differentially regulate immune responses and that their relative imbalance might be of importance in the psoriasis pathogenesis, but the precise impact of these findings remains unclear.
