Everolimus-based immunosuppression induces alloreactive regulatory T cell expansion combined with loss of alloreactive effector T cells

Background – Mammalian target of rapamycin inhibitors (mTORi) combined with tacrolimus and prednisolone enable tacrolimus dose reduction. This study evaluated everolimus–low-dose tacrolimus–prednisolone (EVR-IS) versus standard myco-phenolate mofetil (MMF)–tacrolimus–prednisolone (MMF-IS) on effector (Teff) and regulatory T cells (Tregs) in elderly kidney transplant recipients. Methods – Blood samples were obtained before, and 12 and 24 months after transplantation in the OPTIMIZE study (EudraCT 2018-003194-10). Recipients ≥ 65 years were randomized to EVR-IS or MMF-IS. Phenotype of circulating T cells (EVR-IS N = 86; MMF-IS N = 83) was assessed by multiparameter flow cytometry including transcription factors. Exploratory immune profiling in a limited cohort of kidney transplant recipients, involved detailed characterization of function and phenotype of donor antigen-specific alloreactive and virus-specific T cells, identified by activation-induced CD137 expression using multiparameter flow cytometry. Treg function was analyzed using a suppression assay after isolation and expansion of Tregs. Results – Effector memory T cells remained stable under EVR-IS but showed a small decline for MMF-IS. Treg frequencies increased in EVR-IS recipients (2.9% to 4.1%, P<0.01). Under EVR-IS, donor antigen-specific alloreactive CD4+ T cells remained stable whereas they declined under MMF-IS. Furthermore, donor antigen-specific alloreactive CD8+ T cells declined more rapidly under EVR-IS than for MMF-IS. Only under EVR-IS donor antigen-specific alloreactive Tregs (3.9% to 5.8%, P = 0.03) increased, which strongly suppressed donor antigen-specific alloreactive effector T-cell proliferation. This led to significantly higher Treg/CD4+Teff (3-fold, P<0.01) and Treg/CD8+Teff (10-fold, P = 0.01) ratios for EVR-IS at M24, respectively. Both regimens reduced polyfunctional donor antigen-specific alloreactive CD4+ T cells during the first year, while proportions of virus-specific T-cells remained unchanged. Conclusion – This study demonstrated that everolimus-combined with low dose tacrolimus and mycophenolate mofetil combined with standard dose of tacrolimus employ different mechanisms to effectively control the alloimmune response facilitating patient-tailored immunosuppression in elderly kidney transplant recipients.