Involvement of interleukin-21 in the pathophysiology of aplastic anemia

Recently enhanced T-helper type 17 (Th17) immune responses and deficient CD4(+) CD25(hi) FoxP3(+) regulatory T cells (Tregs) have been reported in acquired aplastic anemia (AA). Interleukin-21 (IL-21), a CD4(+) T-cell-derived proinflammatory cytokine, modulates the balance between Th17 cells and Tregs. However, its role in AA remains unclear.

IL-21 gene expression was examined by quantitative real-time PCR. Cytokines in plasma and cell culture supernatants were detected by ELISA. Cytokines-producing T cells and Tregs were evaluated by flow cytometry.

IL-21 mRNA levels in circulating CD4(+) T cells and IL-21 levels in blood plasma were markedly increased in patients with newly diagnosed AA. Moreover, elevated IL-21-producing CD4(+) T cells were accompanied by Th17 cells accumulation and Tregs decrease, and correlated with AA activity. In vitro, IL-21 not only inhibited the expression of FoxP3, but also induced the expression of IL-17 in CD4(+) T cells of AA patients. More importantly, we found that T cells within the bone marrow (BM) of AA patients were in a heightened activation state, which may be related to IL-21.

Our data suggested a critical role of IL-21 in breaking immune homeostasis in AA by promoting Th17 cells, activating BM T cells and suppressing Tregs.