| Summary: | Chemokines are central to the migration of leukocytes around the body, during both inflammatory and homeostatic conditions. Chemokines mediate their effects by binding to chemokine receptors found on the migrating cell’s surface. Chemokine binding to the chemokine receptor results in signaling, which allows the cell to migrate towards the epicenter of chemokine production. In addition to ‘classical’ chemokine receptors which are involved in leukocyte migration, a discrete family of chemokine receptors exist which are considered to be ‘atypical’, as binding to their cognate ligands does not result in classical signaling as detected by calcium flux assays. One of these atypical chemokine receptors is the chemokinescavenging receptor D6, which can bind to and internalize at least 14 inflammatory CC chemokines in vitro. In addition, an analysis of D6 function in vivo has shown that D6 is important for the resolution of the inflammatory response. D6 KO mice treated with phorbol ester to the shaved dorsal skin developed an inflammatory skin pathology that resembled the human condition psoriasis in many respects. In contrast, WT mice treated with phorbol ester developed a very mild inflammatory response, which quickly resolved. These data suggested that a loss of D6 expression ‘primed’ the mouse to develop a psoriasisform pathology, requiring only minor irritation/trauma to develop the pathology. Similarly, histologically normal (uninvolved) skin from a psoriatic patient has a propensity to developing inflammatory lesions upon minor trauma, and could also be suggested to be ‘primed’ for lesion development. Collectively, these data led us to the following hypothesis, ‘A loss of D6 expression in uninvolved psoriatic skin is associated with the development of psoriatic lesions’. To test this hypothesis, D6 expression in clinical samples from psoriasis patients was analysed. Full thickness biopsies from psoriasis patients were taken from a histologically normal site (uninvolved psoriatic skin), in addition to an elliptical biopsy covering the skin directly adjacent to the psoriatic lesion (peri-lesional psoriatic skin), in addition to the lesion itself (lesional psoriatic skin). D6 expression was analysed in these biopsies by QPCR and immuno-staining. It was observed that D6 expression was significantly elevated in psoriatic skin compared to healthy control skin. In particular, in uninvolved psoriatic skin D6 was significantly increased compared to healthy control skin, or peri-lesional 3 psoriatic skin or lesional psoriatic skin. The increase in D6 expression in uninvolved psoriatic skin localised to the epidermis and the LVs. A significant increase in PBMC-D6 expression was also noted in psoriatic patients compared to healthy control PBMCs. These data suggest that at sites not directly involved in the pathology, D6 is elevated in an attempt to limit inflammation-induced damage. Further immuno-staining showed the inflammatory CC chemokines CCL2 and CCL5 (both high affinity D6-binding ligands) were detected in uninvolved psoriatic epidermis, but were apparently unable to mediate their function due to the lack of significant leukocyte infiltration into the tissue. These data gave rise to the idea that D6 in uninvolved psoriatic skin was significantly elevated in an attempt to block the release of inflammatory CC chemokines into the dermis, and subsequent migration of inflammatory leukocytes into the tissue, and the onset of lesion formation. Interestingly, D6 expression on the epidermis was strongest towards the lower layers of the epidermis, which suggested a role for epidermal- D6 in ‘barrier function’, preventing the uncontrolled release of inflammatory CC chemokines into the dermis. In addition to inflammatory CC chemokines, a variety of inflammatory cytokines have been previously detected in uninvolved psoriatic skin. Several of these cytokines were shown to increase D6 expression in vitro in this study. Therefore, it is possible the significant increase in D6 expression in uninvolved psoriatic skin is partly mediated by cytokine stimulation. A loss of D6 expression was observed when comparing uninvolved psoriatic skin and perilesional psoriatic skin. These data suggested that a loss of D6 expression occurs directly before the onset of lesion formation. It was also shown in this study that a loss of D6 expression could occur after micro-trauma to uninvolved psoriatic skin, which suggests a possible mechanism of how D6 expression is lost in peri-lesional psoriatic skin. To analyze whether the increase in D6 expression in psoriatic skin was disease specific, or a generic response to cutaneous inflammation, D6 expression in eczema skin was studied. It was found that D6 expression in eczema skin is elevated compared to healthy control skin, but less so compared to psoriatic skin. Collectively these data suggest that increased D6 expression may be a feature of inflammatory skin diseases.
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