APRIL Promotes Survival and Proliferation of T Cells: Implications for T-Cell Lymphoma.

Background: Cutaneous T-cell lymphoma (CTCL) is an incurable lymphoproliferative disorder characterized by the clonal expansion of malignant CD4⁺ T cells. Currently there is a poor understanding of the underlying mechanisms in the development and progression of this disease. Members of the TNF superfamily have been shown to be important mediators in the formation and regulation of normal T cell responses. A proliferation-inducing ligand (APRIL), a newly identified member of the TNF superfamily, has been shown to stimulate tumor cell growth as well as proliferation of primary B and T lymphocytes. APRIL transcript levels are reported to be low in normal tissues with the highest levels found in peripheral blood leucocytes. In contrast, higher mRNA levels have been detected in tumor cell lines and primary tumor tissues and various tumor cell lines have been shown to respond to APRIL with increased tumor cell proliferation and decreased apoptosis. The striking effects of APRIL on tumor cell growth and survival raise the possibility that the APRIL and its receptors-TACI and BCMA may be involved in the pathogenesis and maintenance of T cell malignancies.

Goal: To explore the involvement of APRIL on normal and malignant T cell growth and survival.

Results: Under resting conditions, there was minimal APRIL binding to normal T-cells and no TACI or BCMA expression. However, stimulation of resting T cells with OKT3 (anti-CD3) or a combination of PMA and ionomycin resulted in an increase in APRIL binding and TACI expression on a subset of activated T-cells. When CD3⁺ resting human T cells were cultured in the presence of APRIL, IL-2, or OKT3 alone or a combination, normal T cells had greater viability compared to the nil control and APRIL mediated-T cell survival was similar level to that seen with IL-2 or OKT3 alone. CSFE labeled T cells were activated with suboptimal dose of OKT3 and incubated in the presence of APRIL and /or IL-2. Both APRIL and IL-2 alone induced cell proliferation, while treatment with the combination of APRIL and IL-2 had a synergistic effect on T cell proliferation. Malignant T cells isolated from the blood of patients with Sezary Syndrome exhibited a similar pattern of APRIL binding and APRIL also enhanced their survival. Gene array experiments were performed to identify genes involved in APRIL-mediated survival. In normal and primary malignant T cells the anti-apoptotic genes BCL2A1, BIRC6, CHK1, MCL-1, and MDM2 were upregulated and the pro-apoptotic genes CASP5, DR5, and TNFR2 were downregulated in response to APRIL.

Conclusion: APRIL promotes the survival of normal and malignant T cells and co-stimulates CD3-mediated T cell proliferation. The gene array experiments suggest that APRIL regulates the expression of anti-apoptotic and pro-apoptotic genes in both normal and malignant T cells. Taken together, these data indicate an important role for APRIL in the biology of normal T cells and in the pathophysiology of T cell lymphoma.