Epidermal Î±6Î²4 integrin stimulates the influx of immunosuppressive cells during skin tumor promotion
BACKGROUND:Induction of Î±6Î²4 integrin in the differentiated epidermal cell layers in skin is a hallmark of human cutaneous squamous cell carcinoma (SCC) pathogenesis and stimulates chemically induced SCC formation in InvÎ±6Î²4 transgenic mice, which exhibit persistent expression of Î±6Î²4 in the suprabasal epidermal layers. However, the molecular basis for the support of SCC development by suprabasal Î±6Î²4 is not fully understood. OBJECTIVE:We examined the relevance for suprabasal Î±6Î²4 expression in the epidermis for the recruitment of immunosuppressive leukocytes during the early stages of tumor promotion. METHODS:In this study, we made use of the InvÎ±6Î²4 transgenic mouse model, which exhibits expression of Î±6Î²4 integrin in the suprabasal layers of the epidermis driven by the involucrin promoter. First, we examined protein lysates from InvÎ±6Î²4 transgenic skin using a pro-inflammatory cytokine array panel. Next, we immunofluorescence labeling of murine skin sections was employed to immunophenotype tumor promoter-treated InvÎ±6Î²4 transgenic skin. Finally, a macrophage colony stimulating factor (M-CSF) neutralizing antibody strategy was administered to resolve InvÎ±6Î²4 transgenic skin inflammation. RESULTS:Employing the InvÎ±6Î²4 transgenic mouse model, we show that suprabasal Î±6Î²4 integrin expression selectively alters the profile of secreted pro-inflammatory molecules by epidermal cells, in particular CXCL5 and M-CSF, in response to acute tumor promoter treatment. The induction of CXCL5 and M-CSF in InvÎ±6Î²4 transgenic epidermis was shortly followed by an exacerbated influx of CD200R(+) myeloid-derived suppressor cells (MDSCs), which co-expressed the M-CSF receptor, and FoxP3(+) Treg cells compared to wild-type mice. As a result, the levels of activated CD4(+) T lymphocytes were dramatically diminished in InvÎ±6Î²4 transgenic compared to wild-type skin, whereas similar levels of lymphocyte activation were observed in the peripheral blood. Finally, 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced CD200R(+) infiltrative cells and epidermal proliferation were suppressed in InvÎ±6Î²4 mice treated with M-CSF neutralizing antibodies. CONCLUSIONS:We conclude that aberrant expression of Î±6Î²4 integrin in post-mitotic epidermal keratinocytes stimulates a pro-tumorigenic skin microenvironment by augmenting the influx of immunosuppressive granular cells during tumor promotion.