Background: The genetic and immunological basis for psoriasis is unknown. Through the use of a severe combined immunodeficient mouse-human skin model, T cells have been shown to induce psoriasis, which points to a pathological role for such immunocompetent cells. During ongoing studies using this model, a previously overlooked subset of immunocytes expressing receptors typically present on natural killer (NK) cells was discovered, which may shed new light on the genetic susceptibility for psoriasis. Observations: Immunocytes from a psoriatic patient were injected into engrafted allogeneic normal human skin and produced a psoriatic plaque. Moreover, the disturbed epidermal environment spread to induce a greater than 20-fold increase in thickness of adjacent mouse epidermis with prominent elongation of rete pegs. Thus, rather than observing the predicted graft-vs- host reaction in the allogeneic human or xenogeneic mouse skin, injection of psoriatic immunocytes triggered psoriasis. To explore a potential mechanism to explain the lack of cytopathic effect by psoriatic T cells, immunostaining to detect inhibitory receptors normally present on NK cells was performed. These receptors include surface molecules that can inhibit NK cell proliferation, cytokine release, and/or cytotoxicity (ie, killer cell inhibitory receptors [KIRs]), as well as those that may activate NK cell cytotoxicity (ie. killer cell activating receptors [KARs]). The blood- derived psoriatic immunocytes in the skin graft expressed CD94, CD158a, CD158b, NKB1, and CD161. Furthermore, both hyperplastic human and murine keratinocytes express the major histocompatibility complex (MHC) class 1- like CD1d protein, which has been shown to be a specific ligand of T cells expressing CD161 and other NK cell-associated receptors. Conclusions: Since several KIRs and KARs are known to recognize various class I MHC alleles, and because psoriasis inheritance and susceptibility has been linked to various class I MHC molecules, we propose a novel hypothesis in which the pathogenic T cells are postulated to express an assortment of KIRs and KARs. These interactions may produce direct activation without any exogenous antigen, and at the same time block the cytotoxic effector function of these activated immunocytes in this allogeneic and xenogeneic experimental setting. In addition, human T cells expressing CD161 may be capable of interacting with human and murine CD1d expressed by the epidermal keratinocytes. These unexpected findings demonstrate that psoriasis is an immunological disease in which pathogenic T cells rather than epidermal keratinocytes are of primary importance. Functional studies will determine if targeting this previously overlooked population of immunocytes with blocking reagents will generate a novel immunotherapeutic strategic pathway for psoriasis, and whether disease susceptibility and/or incidence patterns can be explained by genetic abnormalities involving these ligand-receptor interactions.
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