Assessment of the therapeutic potential of the atypical chemokine receptor, D6

Infiltration of inflammatory cells into the tissue during the inflammatory response is beneficial to the host. Chemokines and their receptors are instrumental in this process by influencing the migration and behaviour of leukocytes in the tissue. However, prolonged inflammation is associated with many diseases. In recent years, a family of atypical receptors have emerged which do not seem to signal. One of these receptors, D6, is able to internalise and degrade 12 pro-inflammatory CC chemokines and has a role in the resolution of the inflammatory response. Here, using a murine transgenic approach, the potential therapeutic role of D6 in suppressing cutaneous inflammation in vivo has been investigated, using a well-characterised model of skin inflammation. In addition, expression of D6 in a range of inflammatory disorders has also been characterised. Transgenic mice were generated (K14D6), using an epidermis-specific transgene, in which expression of the D6 transgene was driven by the human keratin 14 promoter in epidermal keratinocytes. K14D6 mice were validated and we have shown that D6 is expressed in K14D6 but not in wild-type epidermal keratinocytes. The K14D6 transgene was shown to be functional as only K14D6 keratinocytes were able to bind CCL2 and progressively deplete extracellular CCL3. K14D6 mice can dampen down cutaneous inflammation in response to a topical application of TPA. In addition, K14D6 mice displayed reduced infiltration of epidermal T cells and mast cells compared to wild-type mice. Using a microarray approach, we examined the transcriptional consequences of non-ligated D6 and after ligand binding in primary murine keratinocytes from K14D6 and wild-type mice. Although limited conclusions could be made from the microarray data, our results suggest the possibility that non-ligated D6 in murine keratinocytes may have a negative impact on the transcription of some genes, such as chemokines. In a previous study, D6 null mice displayed a human psoriasis-like pathology after chemical induced skin inflammation, suggesting a possible involvement of D6-dysfunction as a contributing factor in the pathogenesis of psoriasis. We have investigated the possible correlation between D6 expression levels and cutaneous disease development. Analysis of skin biopsies revealed that D6 mRNA levels were 8-fold higher in uninvolved psoriatic skin compared to matching psoriatic lesional skin, atopic dermatitis and control skin. In PBMCs, there was no significant difference in D6 mRNA expression in psoriasis patients compared to control. A preliminary study examining surface D6 expression on leukocytes from control and rheumatoid arthritis patients revealed enhanced D6 expression on B cells and myeloid DCs. In this study, we have shown for the first time that increased expression of D6 in vivo can limit cutaneous inflammation, therefore providing a rationale for exploring the therapeutic potential of D6 in human inflammatory diseases. In addition, we provide evidence that D6 expression is dysregulated in inflammatory disorders further suggesting an involvement of this receptor in the pathogenesis of these diseases.