Low-Dose Administration of Recombinant IL-2 Following T Cell Depleted Bone Marrow Transplantaion Induces Expansion of CD4+CD25+ FOXP3+ Regulatory T Cells in Vivo.

Regulatory T cells (Treg) play a key role in controlling immune responses following allogeneic hematopoietic stem cell transplantation (HSCT). In murine models, infusion of purified CD4+CD25+ Treg at the time of transplant is sufficient to prevent acute GVHD. In humans, development of acute as well as chronic GVHD has been associated with reduced numbers of Treg following allogeneic HSCT, suggesting that defective reconstitution of this functional cell type can contribute to exacerbation of alloimmune responses. Based on these results, adoptive cellular therapy using purified and in vitro expanded populations of Treg has been proposed as a therapeutic strategy to correct chronic GVHD. Treg are mainly characterized by the constitutive expression of the IL-2 receptor ? chain, CD25 and proliferate in response to IL-2 in vitro. In vivo, the effects of IL-2 on Treg populations are unknown. To examine this question we quantified changes in Treg in 9 patients with CML who previously received low dose IL-2 following allogeneic HSCT. Patients enrolled in this protocol received a daily intravenous infusion of 2 X 105 U IL-2/m2 for 3 months, starting 3 months after CD6 depleted allogeneic bone marrow transplantation (BMT). No patient developed GVHD following IL-2 administration and overall toxicity was minimal. The predominant immunologic effect of IL-2 reported in the initial study was a marked increase in NK cell populations characterized as CD3-CD16+CD56+ as well as total CD56+ cells. In this retrospective analysis we investigated populations of CD4+CD25+ T cells before and 1 to 2 months after the beginning IL-2 treatment. Using RNA extracted from patient PBMC we also assessed the level of expression of the specific transcription factor FOXP3 by quantitative PCR as an alternate marker of Treg in vivo. As initially reported, all 9 patients showed a marked increase in CD3-CD56+ cells 1 to 2 months post IL-2 administration. In contrast, the percent of CD3+ T cells were either unchanged or slightly decreased. The percent of CD4+CD25+ cells within the CD3+ T cell population increased during IL-2 treatment (median: 5.8 pre IL-2 vs 7.6 post IL-2, p-value=0.02). Likewise, FOXP3 expression in the CD3+ population showed 5 to 19 fold increase in 8 of 9 patients during this period (median: 3817 AU pre IL-2 vs. 18924 AU post IL-2, p-value=0.055). These results indicate that administration of low dose IL-2 can augment Treg cells in vivo as reflected by increased ratio of CD4+CD25+/CD3+ T cells as well as higher levels of FOXP3 expression. These studies suggest that prolonged treatment with low dose IL-2 can effectively expand CD4+CD25+ Treg in vivo. This represents a novel strategy for expanding regulatory T cells in vivo and may be useful alone or in conjunction with adoptive cellular therapy with Treg.