Skin-derived interleukin-7 contributes to the proliferation of lymphocytes in cutaneous T-cell lymphoma

Abstract

Cutaneous T-cell lymphomas (CTCLs) are malignancies of T cells that have a special affinity for the skin. We have previously reported that much of the T-cell receptor repertoire is altered in CTCL, and both malignant and nonmalignant clones are numerically expanded, presumably in response to T-cell trophic cytokines. We therefore examined levels of the T-cell trophic cytokines IL-2, IL-4, IL-7, IL-12, IL-13, and IL-15 in plasma in 93 CTCL patients and healthy controls. Only IL-7 levels were elevated in CTCL. We next looked at lesional skin from patients with CTCL and found elevated levels of IL-7 mRNA. Explant cultures of normal and lesional CTCL skin biopsies revealed significantly more IL-7 protein production in CTCL skin. Additionally, cultures of CTCL skin released greater numbers of T cells than normal skin; this was blocked by the addition of an IL-7 neutralizing antibody. Finally, these cultures induced proliferation of normal peripheral skin-homing T cells that were added to the cultures. These observations led us to postulate that IL-7 produced by skin cells contributes to the survival and proliferation of T cells within skin lesions and is likely the source of elevated circulating IL-7 in CTCL.

Figure 1.

Levels of IL-7 in plasma from patients with CTCL. Plasma samples were collected from 93 CTCL patients and 20 healthy controls. IL-7 levels were measured by ELISA. IL-7 levels were significantly higher in plasma samples from patients with CTCL. ^*P < .001; ^**P < .01.

Figure 2.

Expression of IL-7 mRNA. Expression in normal and CTCL skin (A) and in normal keratinocytes and fibroblasts (B) was analyzed by quantitative PCR. The data are shown as a relative quantification of IL-7 mRNA expression levels divided by levels of β-actin mRNA. Expression of IL-7 mRNA was significantly higher in CTCL skin lesions than in samples of normal skin (A). ^*P < .001.

Figure 3.

IL-7 concentration in supernatants from skin explant cultures. Normal and CTCL skin explants were cultured under various conditions. The concentration of IL-7 in the supernatant was analyzed by ELISA. IL-7 levels were significantly higher in CTCL samples than in healthy controls. Treatment with the Th2 cytokines IL-4 and/or IL-13 did not significantly increase IL-7 production.

Figure 4.

Production of T cells by skin matrix explant cultures. CTCL and normal skin explants were cultured under various conditions, and T-cell production was assayed. The number of T cells produced from CTCL skin explants was significantly higher than from normal skin explants. Normal skin explants treated with Th2 cytokines IL-4 and/or IL-13 did not produce more T cells. Normal skin explants treated with recombinant human IL-7 produced more T cells than untreated cultures. The addition of anti-human IL-7 neutralizing antibody to CTCL skin explants decreased the numbers of T cells produced.

Figure 5.

Proliferation of normal blood CLA-positive T cells incubated in matrices colonized with skin cells from CTCL lesions or normal skin. CFSE-labeled CLA-positive T cells were isolated from peripheral blood and incubated for 1 week in matrices colonized with skin cells from either CTCL or normal skin. T cells were analyzed by flow cytometry. CD3⁺ CLA⁺ CFSE⁺ lymphocytes divided up to 3 times in CTCL skin matrices (open histograms, bold line); this cellular division of T cells could be blocked significantly by addition of antibodies to IL-7 (open histograms, broken line). Normal skin matrices did not support cell proliferation (filled histograms).