T regulatory cells control T-cell proliferation partly by the release of soluble CD25 in patients with B-cell malignancies

Interleukin‐2 (IL‐2) is one of the most studied cytokines driving T‐cell proliferation, activation and survival. It binds to the IL‐2 receptor consisting of three chains, the α (CD25), β and common γ (γc). The binding of the CD25 chain to IL‐2 is necessary to expose high‐affinity binding sites for the β and γc chains, which, in turn, are responsible for downstream signalling. A high level of soluble CD25 (sCD25) has been associated with a poor prognosis in patients with non‐Hodgkin’s lymphoma. The function and source of origin of this soluble receptor is not well investigated. In the present study we hypothesized that T regulatory (Treg) cells may release CD25 to act as a decoy receptor for IL‐2, thereby depriving T‐effector cells of IL‐2. Peripheral blood from patients with B‐cell malignancies (n = 26) and healthy controls (n = 27) was investigated for the presence and function of FoxP3⁺ Treg cells and sCD25 by multi‐colour flow cytometry and enzyme‐linked immunosorbent assay. Further, the proliferative capacity of T cells was evaluated with or without the presence of recombinant sCD25. The results demonstrate that Treg cells from patients had lower CD25 expression intensity and that they released CD25 in vitro. Further, high levels of Treg cells correlated with sCD25 plasma concentration. Recombinant sCD25 could suppress T‐cell proliferation in vitro. In conclusion, the release of sCD25 by Treg cells may be a mechanism to deprive IL‐2 and thereby inhibit anti‐tumour T‐cell responses.