Induction of cell surface interleukin 2 receptor alpha chain expression on non-T lymphoid leukemia cells

Non-IL-2-blocking anti-CD25 antibody inhibits tumor growth by depleting Tregs and has synergistic effects with anti-CTLA-4 therapy

CD25, also known as the interleukin-2 receptor α chain (IL-2Rα), is highly expressed on regulatory T cells (Tregs), but relatively lower on effector T cells (Teffs). This makes it a potential target for Treg depletion, which can be used in tumor immunotherapy. However, marketed anti-CD25 antibodies (Basiliximab and Daclizumab) were originally developed as immunosuppressive drugs to prevent graft rejection, because these antibodies can block IL-2 binding to CD25 on Teffs, which in turn destroys the function of Teffs. Recent studies have shown that non-IL-2-blocking anti-CD25 antibodies have displayed exciting antitumor effects. Here, we screened out a non-IL-2-blocking anti-CD25 monoclonal antibody (mAb) 7B7 by hybridoma technology, and confirmed its antitumor activity via depleting Tregs in a CD25 humanized mouse model. Subsequently, we verified that the humanized 7B7, named as h7B7-15S, has comparable activities to 7B7, and that its Treg depletion is further increased when combined with anti-CTLA-4, leading to enhanced remodeling of the tumor immune microenvironment. Moreover, our findings reveal that the Fab form of h7B7-15S has the ability to deplete Tregs, independent of the Fc region. Taken together, our studies expand the application of anti-CD25 in tumor immunotherapy and provide insight into the underlying mechanism.

CD25: A potential tumor therapeutic target

CD25 is the alpha-chain of the heterotrimer IL-2 receptor. CD25 is expressed on the surface of both immune and non-immune cells with different frequencies. For cancers, CD25 is expressed at high levels in many types of hematological malignancies, but at low levels in most solid tumors. CD25 is also highly expressed in activated circulating immune cells and regulatory T cells (Tregs). Infiltration of Tregs in the tumor microenvironment can lead to an imbalanced ratio of effector T cells (Teffs) and Tregs, which is associated with the progression of cancers. A rescued Teff/Treg cell ratio indicates an efficient anti-tumor response to immunotherapy. CD25 as a potential target for the depletion of Tregs is critical in developing new immunotherapeutic strategies. Few articles have summarized the relationships between CD25 and tumors, or the recent progress of drugs targeting CD25. In this paper, we will discuss the structures of IL-2 and IL-2R, the biological function of CD25 and its important role in tumor therapy. In addition, the latest research on drugs targeting CD25 has been summarized, providing guidance for future drug development.

The emerging role of anti-CD25 directed therapies as both immune modulators and targeted agents in cancer

CD25 (also termed IL2RA) forms one component of the high-affinity heterotrimeric interleukin 2 (IL2) receptor on activated T cells. Its affinity for IL2 and cellular function are tightly regulated and vary in different cell types. The high frequency of CD25 on the surface of many different haematological tumour cells is now well established and, apart from its prognostic significance, CD25 may be present on leukaemic stem cells and enable oncogenic signalling pathways in leukaemic cells. Additionally, high CD25 expression in activated circulating immune cells and Tregs is a factor that has already been exploited by IL2 immunotherapies for treatment of tumours and autoimmune disease. The relative clinical safety and efficacy of administering anti-CD25 radioimmunoconjugates and immunotoxins in various haematological tumour indications has been established and clinical trials of a novel CD25-directed antibody drug conjugate are underway.

Evaluation of CD25-positive cells in relation to the subtypes and prognoses in various lymphoid tumours in dogs

Interleukin-2 receptor alpha chain (CD25) expression has been reported in human lymphoid tumours and suggested to correlate with the prognosis. In this study, we detected CD25-positive cells in various types of lymphoid tumours in dogs. Immunohistochemical analyses of the tissues from diffuse large B-cell lymphoma (DLBCL) (n = 6), T-zone lymphoma (TZL) (n = 5), and follicular lymphoma (FL) (n = 2) revealed that cells strongly positive for CD25 were observed generally in accordance with lymphoma cell localization. CD25-positive cells were consistently detected in TZL and FL cases; however, the number of CD25-positive cells was variable among DLBCL cases. Furthermore, we evaluated the rate of CD25-positive cells by flow cytometric analysis in 29 dogs with lymphoid malignancies, including high-grade B-cell lymphoma (n = 17), TZL (n = 5), FL (n = 2), cutaneous lymphoma (n=2), and acute lymphoblastic leukaemia (ALL) (n = 3). CD25-positivity in the lymph node cells was significantly higher in dogs with high-grade B-cell lymphoma (mean ± SD, 49.6 ± 31.3%) or TZL (mean ± SD, 80.2 ± 10.0%) than that in healthy dogs (mean ± SD, 9.8 ± 2.8%). In prognostic analysis of 15 cases with high-grade B-cell lymphoma, the progression-free survival was significantly shorter in CD25-high group than that in CD25-low group. The results obtained in this study are useful for subtype differentiation and prognostic analysis of canine lymphomas and future development of molecular-targeted therapy directed at CD25.

Serum soluble interleukin 2 receptor alpha in human cancer of adults and children: a review

Cancer growth and development is associated with the stimulation of the innate immune system, including enhanced interleukin 2 receptor (IL-2R) expression in immune cells and its shedding into the circulation in a soluble form of sIL-2Ralpha. In most haematological malignancies, including different types of leukaemias and lymphomas, sIL-2Ralpha has been found to be released directly from the surface of neoplastic cells thus reflecting the tumour bulk, turnover and activity. Several studies have proved that not only lymphoid cancer cells, but also some non-lymphoid cancer cells, express IL-2R on their surface. They include malignant melanoma and carcinomas of the kidney, head and neck, oesophagus and lung. It is suggested that in most malignant solid tumours, elevated levels of sIL-2Ralpha are likely to be the product of normal peripheral mononuclear cells activated in response to the neoplasm's growth or that they are released from activated lymphoid cells infiltrating neoplastic tissues. This latter hypothesis has been proved by discovering the high expression of CD25 on the cell surface of most of these cells. Although the precise source and biological role of sIL-2Ralpha has not been clarified definitively, pretreatment serum levels of sIL-2Ralpha have been shown to reflect the activity, advancement and biological aggressiveness of many types of cancer in adults and children as well as to correlate with prognosis and overall survival. The possibility of enriching the diagnostic tools of oncologists with a new biochemical marker of activity of neoplasms resulted in numerous studies and reports concerning the clinical usefulness of sIL-2Ralpha measurements in adult and, less frequently, in paediatric malignancies. This article presents the actual knowledge concerning the structure, source and biological function of sIL-2Ralpha in patients with haematological and non-haematological malignancies. The authors review the published data on clinical applicability of soluble IL-2Ralpha determination in terms of diagnostics, prognosis and treatment monitoring of particular types of malignant disorders both in adults and in children. They also provide an insight into the clinical usefulness of sLL-2Ralpha-blocking antibodies in patients with cancer, and in those who reject organ transplants, develop graft-versus-host disease after allogeneic bone marrow transplantation and are affected with autoimmune disorders.

Elevated levels of soluble interleukin-2 receptor in serum of patients with hematological or non-hematological malignancies

We measured the levels of soluble interleukin-2 receptor (sIL-2R) in the serum of 53 patients with hematological malignancies (14 with acute leukemia, 10 myeloproliferative disorders, 21 non-Hodgkin's lymphoma and 8 multiple myeloma), 40 with non-hematological solid malignancies (24 with lung cancer, 11 digestive system cancer and 5 other cancers) and 95 healthy subjects as a control using an enzyme-linked immunosorbent assay. Both hematological and non-hematological neoplasms showed significantly higher sIL-2R levels than normal subjects (p<0.01). However, compared to solid malignancies, hematological neoplasms displayed a wide range of sIL-2R levels and extremely elevated values of sIL-2R were seen in certain cases. These results seem to suggest that sIL-2R levels may serve as one of non-invasive markers of differential diagnosis for patients with bulky mass lesions between hematological and solid malignancy.

DAB389IL2 (ONTAK) fusion protein therapy of chronic lymphocytic leukaemia

Despite multiple new therapeutic options for patients with chronic lymphocytic leukaemia (CLL), the prognosis of patients with relapsed disease is poor. After first-line therapy with fludarabine, alkylating agents, rituximab or combinations of these agents, most patients relapse within a few years. While second-line therapy with alemtuzumab, or other combinations of the above agents, have produced remissions, most of these are partial responses lasting months rather than years. Patients commonly die from progressive disease or infections related to the underlying disease or treatment. The authors sought to develop a novel therapeutic with the capacity to kill chemotherapy-resistant CLL cells and with reduced toxicities to normal tissues. Based on the presence of high affinity interleukin-2 receptor (IL-2R) on CLL cells, therapy of relapsed CLL patients with a diphtheria fusion protein targeting IL-2R - DAB(389)IL2 (ONTAK), Seragen, Inc., Hopkinton, MA, USA) - was tested in a pilot Phase II study. Biological activity and a partial remission were observed with modest drug-related side effects. Based on these encouraging findings, alternative schedules and combinations with agents that may enhance CLL cell expression of IL-2R are being tested. Hopefully, the use of these targeted therapeutic approaches will provide additional therapeutic options with fewer side effects for this common and incurable condition.