Keratinocyte-Derived IL-7 Serves as a Growth Factor for Dendritic Epidermal T Cells in Mice

Dendritic epidermal T cells (DETC) are CD3⁺, CD45⁺, CD4^-, CD8^-, TCR-Vγ3/Vδ1⁺ T lymphocytes that reside in symbiosis with keratinocytes in mouse epidermis. To address mechanisms by which these cells survive and proliferate within the epidermal environment, we have tested the hypothesis that cytokines secreted by neighboring keratinocytes play relevant roles. The present study was conducted to determine whether keratinocytes produce biologically relevant amounts of IL-7, and, if so, to study its effects on DETC. The long term cultured DETC line, 7-17, and freshly isolated DETC exhibited dose- and time-dependent proliferative responses to rIL-7. These responses were blocked completely by anti-IL-7 antibodies, whereas anti-IL-2 had no effect, indicating that DETC respond to IL-7 by an IL-2-independent mechanism. Proliferative responses depended on the state of cell activation; DETC stimulated 2 to 5 days earlier with Con A proliferated optimally to added IL-7, whereas cells stimulated 10 days earlier did not proliferate. DETC that failed to proliferate responded to IL-7 by showing prolonged survival; cells maintained for 7 days with IL-7 alone retained their capacity to proliferate in response to Con A. Mouse epidermal cells and Pam 212 keratinocyte line both expressed IL-7 mRNA constitutively, as demonstrated by reverse transcription-polymerase chain reaction analyses. The production of IL-7 by mouse keratinocytes was also confirmed; Pam 212 culture supernatants supported DETC proliferation, and this activity was diminished by 50% with added anti-IL-7 antibodies. These results indicate that keratinocytes produce IL-7 in biologically relevant amounts, which, in turn, serve to promote the survival and growth of DETC. IL-7-mediated communication between epithelial cells and γδ T cells may represent one mechanism to sustain the indefinite residence of γδ T cells in epithelial tissues of mice.