Dysregulated expression of CD69 and IL-2 receptor α and β chains on CD8+ T lymphocytes in flaky skin mice

T-cell activation is considered to be an important element in the pathogenesis of psoriasis, a human skin disease characterized by keratinocyte hyperproliferation, altered keratinocyte differentiation and inflammation of the dermis and epidermis. Mice homozygous for the flaky skin (fsn) mutation develop a skin disorder that has histopathological and biochemical features resembling some forms of psoriasis. It has been reported recently that peripheral lymph nodes (PLN) in fsn/fsn mice exhibit various abnormalities in T-cell development suggestive of dysregulated T- and B-cell activation. In the present study, the expression of the inducible T-cell activation antigens CD69 and IL-2 receptor alpha chain (CD25) on PLN cells from fsn/fsn mice and their phenotypically normal littermates is examined. Expression of CD69 was significantly increased on PLN cells in fsn/fsn mice (mean +/- SD, 49.9 +/- 14.7% of cells) compared with control mice (14.6 +/- 4.2%). Analysis of CD4(+) and CD8(+) T cell subsets revealed that expression of CD69 in fsn/fsn PLN was significantly biased toward CD8(+) cells. Although expression of CD25 was preferentially associated with CD4(+) rather than CD8(+) cells in both fsn/fsn and control PLN, with most CD4(+)CD25(+) cells being CD25(hi), the proportion of CD4(+) cells expressing CD25 was higher in fsn/fsn than control PLN. In contrast, CD25 was expressed by 2-3 % of CD8(+) PLN cells in both fsn/fsn and control mice and CD25(hi) cells accounted for < 1 % of CD8(+) cells in fsn/fsn PLN. The paucity of CD25 on CD8(+) cells in fsn/fsn PLN did not appear to be due to a defect in the ability of these cells to upregulate CD25, because T cell receptor stimulation in vitro induced high expression of CD25 on both CD4(+) and CD8(+) cells. A striking and consistent finding was that most CD8(+) cells in fsn/fsn PLN expressed high levels of IL-2R beta chain (CD122). In contrast, CD122 was expressed at low levels on CD8(+) cells in control mice. Analysis of PLN cells from newborn fsn/fsn mice revealed that the high expression of CD122 on CD8(+) cells was established by 2 weeks of age, prior to the appearance of clinical skin disease. These data indicate that large numbers of T cells in fsn/fsn mice are activated and reinforce the view that fsn is an important regulator of lymphocyte development and function. The relationship between T-cell activation and flaky skin disease in these mice remains to be established.