BACKGROUND & AIMS: CD4+CD25+Foxp3+ regulatory T cells (Tregs) are essential to maintain immunological tolerance and have been shown to promote liver allograft tolerance both in rodents and in humans. Low dose interleukin-2 (LDIL-2) can expand human endogenous circulating Tregs in vivo, but its role in suppressing antigen-specific responses and promoting Treg trafficking to the sites of inflammation is unknown. Likewise, whether LDIL-2 facilitates the induction of allograft tolerance has not been investigated in humans. METHODS: We conducted a clinical trial in stable liver recipients 2-6 years post-transplant to determine the capacity of LDIL-2 to suppress allospecific immune responses and allow for the complete discontinuation of maintenance immunosuppression (clinicaltrials.gov NCT02949492). One month after initiating LDIL-2, patients exhibiting at least a 2-fold increase in circulating Tregs gradually discontinued immunosuppression over a 4-month period while continuing LDIL-2 for a total treatment duration of 6 months. RESULTS: All patients achieved a marked and sustained increase in circulating Tregs. However, this was not associated with the preferential expansion of donor-reactive Tregs and did not promote the accumulation of intra-hepatic Tregs. Furthermore, LDIL-2 induced a marked interferon-gamma orchestrated transcriptional response in the liver even before immunosuppression weaning was initiated. The trial was terminated after the first six participants failed to reach the primary endpoint due to rejection requiring immunosuppression reinstitution. CONCLUSIONS: The expansion of circulating Tregs in response to LDIL-2 is not sufficient to control alloimmunity and to promote liver allograft tolerance, due, at least in part, to off-target effects that increase the liver immunogenicity. Our trial provides unique insight into the mechanisms of action of immunomodulatory therapies such as LDIL-2 and their limitations in promoting alloantigen-specific effects and immunological tolerance. LAY SUMMARY: The administration of low-dose interleukin-2 to liver transplant patients increased the number of circulating lymphocytes with immunosuppressive properties (regulatory T cells), but did not promote their trafficking into the transplanted liver and failed to induce transplantation tolerance. CLINICAL TRIAL REGISTRATION: Clinicaltrials.gov NCT02949492.
Liver transplantation, regulatory T cells, rejection, transplant immunology