Effects of Daily Low Dose IL-2 Therapy on Homeostatic Regulation of CD4+Foxp3+ Regulatory T Cells In Patients with Chronic Graft-Versus-Host Disease

Abstract 895

CD4+FoxP3+ regulatory T cells (Treg) play a central role in the maintenance of tolerance after allogeneic hematopoietic stem cell transplantation (HSCT) and recent studies have demonstrated that Treg deficiency leads to the development of chronic GHVD (cGVHD). Interleukin-2 (IL-2) is known to promote thymic generation and maintenance of peripheral Treg and IL-2 deficiency results in a profound deficiency of Treg in vivo. Based on these findings we initiated a clinical trial to evaluate the safety, clinical efficacy and immunologic effects of low dose recombinant IL-2 in patients with steroid-refractory cGVHD. We recently reported the clinical outcomes of this trial demonstrating that IL-2 administration preferentially increased Treg in patients with active cGVHD and resulted in clinical improvement with only minor toxicities (Koreth et al, ASBMT 2010). However, the mechanisms responsible for Treg expansion in patients during IL-2 administration have not been characterized. To elucidate these mechanisms, we examined phenotypic and functional characteristics of Treg in 14 patients who received daily subcutaneous IL-2 (3×10⁵-3×10⁶IU/m²/day) for 8 weeks. Peripheral blood samples were obtained before and at 1, 2, 4, 6, 8, 10 and 12 weeks after starting IL-2. Treg were compared to conventional CD4+FoxP3- T cells (Tcon) within individual patient samples and examined for expression of Ki-67, PD-1 and BCL-2. In some experiments, Treg and Tcon were further divided into subpopulations by the expression of CD45RA and CD31. Absolute numbers of functionally suppressive Treg increased 5-fold in the first 4 weeks of therapy. Treg numbers then slowly decreased despite continued IL-2 therapy, but remained 2-fold higher than baseline at 8 weeks. Absolute numbers of Tcon increased 2-fold in the first 4 weeks and then returned to baseline levels at 8 weeks. This resulted in a sustained increase of Treg/Tcon ratio for the entire duration of therapy, which persisted for at least 4 weeks after treatment was completed. Plasma IL-2 levels peaked at 1 week and gradually declined despite continued treatment at the same dose. Nevertheless, IL-2 levels remained significantly higher than baseline throughout treatment (median 1.4pg/ml at baseline and 18.1pg/ml at 8 weeks, p<0.05). The proliferative response to IL-2 was examined by measuring expression of Ki-67 in each subset. Initially, Ki-67 expression in Treg rapidly increased in an IL-2 dose-dependent manner. Ki-67 also increased in Tcon but at a significantly lower level (median 20.0% Treg vs 6.7% Tcon, p=0.0001). Increased Ki-67 was seen in both CD45RA+CD31+ recent thymic emigrant Treg (RTE-Treg) and CD45RA- activated/memory Treg (MEM-Treg) at similar levels (median 20.1% and 18.5%, respectively, p=0.54). Treg proliferation peaked in the first week of IL-2, and rapidly returned to baseline levels in weeks 2–3. Despite changes in proliferation, the absolute number of RTE-Treg remained significantly elevated (median 1.05/ul at baseline, 9.43/ul at 4 weeks, p=0.001). In contrast, the absolute number of RTE-Tcon did not change. Phenotypic analysis of Treg showed that expression of both PD-1 and BCL-2 increased during IL-2 therapy (%PD-1+ Treg; median 15.7% at baseline and 38.3% at 8 weeks, p=0.02: relative BCL-2 MFI; median 1.00 at baseline and 1.59 at 8 weeks, p=0.04). To determine the functional effects of these changes on susceptibility to apoptosis, Treg and Tcon were purified and cultured with or without agonistic FAS antibody, and apoptosis was measured using Annexin-V staining. Remarkably, Treg obtained during IL-2 therapy were relatively resistant to apoptosis compared to baseline. In summary, these results indicate that the selective expansion of Treg during prolonged IL-2 administration is characterized by a series of homeostatic changes. Initial high levels of IL-2 lead to selective and rapid Treg proliferation. Treg proliferation is not maintained as numbers of Treg increase and IL-2 levels decrease. Subsequent maintenance of increased Treg appears to be mediated primarily by increased resistance to apoptosis and prolonged survival. Increased thymic output of Treg also appears to support this peripheral homeostatic process. These findings demonstrate the complex effects of IL-2 on Treg homeostasis and provide important information for developing strategies to promote immune tolerance.