CD4+CD25+FOXP3+ regulatory T cells (Tregs) have an essential role in immune regulation and constitute a key target for cellular immunotherapies in transplantation. IL-2 is critical for Tregs function, and low-dose IL-2 therapy can exert beneficial effects in autoimmunity and GVHD. Calcineurin inhibitors (CNIs), which constitute the mainstay immunosuppression in liver transplantation, prevent rejection by inhibiting effector T cells, but also suppress Treg activity. CNIs also reduce the overall availability of IL-2. Our aims are to investigate the effects of CNIs on the Treg compartment by studying the homeostasis of the Treg subsets in human liver transplant recipients, and to elucidate if administration of low-dose IL-2 promotes the expansion and functional restoration of Tregs in the presence of CNIs both in humans and animal models.

We immunophenotyped PBMCs samples from 40 stable liver transplant recipients on CNI therapy >6 months post-transplant and 30 age-matched healthy controls. We assessed the effects of tacrolimus on Treg survival in vitro in a human whole blood culture system, and in vivo in a skin transplant mouse model (Bm12 into B6) in which allograft survival depends on tacrolimus treatment (5mg/kg/day).

Liver transplant recipients on CNIs displayed an increased cell turnover in the suppressive Treg subsets, with higher proliferation and apoptotic death than healthy controls. This was particularly striking in the activated Treg subset (CD4+CD45RA-Foxp3hi). Treg apoptosis correlated with soluble CD25 in serum. Tregs also exhibited reduced CD25 expression and lower IL-2 sensitivity. IL-2 therapy increased the number and function of Tregs in the presence of tacrolimus by upregulating the anti-apoptotic gene Bcl-2. Administration of tacrolimus to mice replicated the same phenotypic changes in Tregs. To confirm the beneficial effects of IL-2 in vivo we employed a CNI-dependent skin transplant model. Immunosuppression withdrawal was completed 4 weeks after transplantation. The allograft survival was significantly increased when IL-2 therapy (1ug rmIL-2+9ug mAbIL-2) was administered in combination with tacrolimus during the last week of treatment.

Our study provides a rationale for administering low-dose IL-2 to liver transplant recipients under CNI treatment to expand the pool of endogenous Tregs as a means to improve immunoregulation and potentially promote tolerance.

CITATION INFORMATION: Whitehouse G, Gray E, Mastoridis S, Kodela E, Sanchez-Fueyo A, Martinez-Llordella M. IL-2 Therapy Restores the Impaired Function of Regulatory T Cells Induced by Calcineurin Inhibitors in Liver Transplantation. Am J Transplant. 2016;16 (suppl 3).

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