Low-dose IL-2 fails to expand Tregs after alemtuzumab: insight into impaired immune tolerance in multiple sclerosis.

Regulatory T cells (Tregs) are essential for preventing autoimmunity. They depend upon interleukin-2 (IL-2) for optimal function and due to high expression of the CD25 subunit of the IL-2 receptor, are 10-fold more sensitive to IL-2 than effector T cells (Teffs). Consequently low-dose IL-2 can be used to preferentially expand Tregs, a therapeutic strategy which has shown promise in a number of autoimmune and inflammatory conditions - including graft-versus-host disease and lupus, where IL-2 driven expansion correlates with improvements in some clinical markers of disease activity. Autoimmunity is a frequent delayed complication of treatment with the lymphocyte-depleting drug alemtuzumab in relapsing-remitting multiple sclerosis (RRMS). Here, using in vitro assays, a pre-clinical mouse model, and an experimental medicine study, we investigated whether low-dose IL-2 could selectively expand Tregs in alemtuzumab-treated RRMS patients. Six months after alemtuzumab treatment, the frequency of patient-derived naïve CD4+ Teffs expressing high-affinity IL-2 receptors increased from 30.11+/-5.09% to 72.88 +/-5.57%, and the density of receptors per cell increased, rendering them six times more sensitive to IL-2 in vitro, at concentrations that typically favour Tregs. Using a human CD52-expressing transgenic mouse model of alemtuzumab treatment, we found that IL-2 was still able to preferentially expand Tregs, but only when administered at a later time point, corresponding to more than 6 months post-treatment in patients. Guided by these findings, we evaluated low-dose IL-2 for Treg expansion in a prospective open-label mechanistic study of RRMS patients who had received alemtuzumab more than 6 months previously. IL-2, at a dose and frequency similar to that previously shown to expand Tregs in autoimmune diabetes (0.3 x 106 IU/m2 twice a week), was well tolerated and safe, but unexpectedly failed to expand Tregs. We discuss the potential reasons underlying this lack of response, ruling out sIL2RA-related neutralisation, and instead considering ceiling effects on Treg proliferation, Treg exhaustion and intrinsic Treg dysfunction in MS as possible contributors. Together, our findings demonstrate that low-dose IL-2 alone is not an effective strategy for promoting Treg expansion post-alemtuzumab, and is therefore not a viable approach by itself for preventing post-treatment autoimmune complications.