Reduction of CD18 Promotes Expansion of Inflammatory γδ T Cells Collaborating with CD4+ T Cells in Chronic Murine Psoriasiform Dermatitis

IL-17 is a critical factor in the pathogenesis of psoriasis and other inflammatory diseases. The impact of γδ T cells, accounting for an important source of IL-17 in acute murine IL-23– and imiquimod-induced skin inflammation, in human psoriasis is still unclear. Using the polygenic CD18hypo PL/J psoriasis mouse model spontaneously developing chronic psoriasiform dermatitis due to reduced CD18/β2 integrin expression to 2–16% of wild-type levels, we investigated in this study the influence of adhesion molecule expression on generation of inflammatory γδ T cells and analyzed the occurrence of IL-17–producing γδ and CD4+ T cells at different disease stages. Severity of CD18hypo PL/J psoriasiform dermatitis correlated with a loss of skin-resident Vγ5+ T cells and concurrent skin infiltration with IL-17+, IL-22+, and TNF-α+ γδTCRlow cells preceded by increases in Vγ4+ T cells in local lymph nodes. In vitro, reduced CD18 levels promoted expansion of inflammatory memory-type γδ T cells in response to IL-7. Similar to IL-17 or IL-23/p19 depletion, injection of diseased CD18hypo PL/J mice with anti-γδTCR Abs significantly reduced skin inflammation and largely eliminated pathological γδ and CD4+ T cells. Moreover, CD18hypo γδ T cells induced allogeneic CD4+ T cell responses more potently than CD18wt counterparts and, upon adoptive transfer, triggered psoriasiform dermatitis in susceptible hosts. These results demonstrate a novel function of reduced CD18 levels in generation of pathological γδ T cells that was confirmed by detection of increases in CD18low γδ T cells in psoriasis patients and may also have implications for other inflammatory diseases.