Background
Interleukin-2 (IL-2) has an essential role in the expansion and function of CD4+ regulatory T
cells (Tregs). Tregs reduce tissue damage by limiting the immune response following infection
and regulate autoreactive CD4+ effector T cells (Teffs) to prevent autoimmune diseases,
such as type 1 diabetes (T1D). Genetic susceptibility to T1D causes alterations in
the IL-2 pathway, a finding that supports Tregs as a cellular therapeutic target. Aldesleukin
(Proleukin; recombinant human IL-2), which is administered at high doses to activate the immune system in cancer immunotherapy, is now being repositioned to treat inflammatory
and autoimmune disorders at lower doses by targeting Tregs.
Methods and Findings
To define the aldesleukin dose response for Tregs and to find doses that increase Tregs
physiologically for treatment of T1D, a statistical and systematic approach was taken by
analysing the pharmacokinetics and pharmacodynamics of single doses of subcutaneous
aldesleukin in the Adaptive Study of IL-2 Dose on Regulatory T Cells in Type 1 Diabetes
(DILT1D), a single centre, non-randomised, open label, adaptive dose-finding trial with 40
adult participants with recently diagnosed T1D. The primary endpoint was the maximum
percentage increase in Tregs (defined as CD3+
CD4+
CD25highCD127low) from the baseline
frequency in each participant measured over the 7 d following treatment. There was an initial
learning phase with five pairs of participants, each pair receiving one of five preassigned
single doses from 0.04 × 106 to 1.5 × 106 IU/m2
, in order to model the doseresponse
curve. Results from each participant were then incorporated into interim statistical
modelling to target the two doses most likely to induce 10% and 20% increases in Treg frequencies.
Primary analysis of the evaluable population (n = 39) found that the optimal
doses of aldesleukin to induce 10% and 20% increases in Tregs were 0.101 × 106 IU/m2
(standard error [SE] = 0.078, 95% CI = −0.052, 0.254) and 0.497 × 106 IU/m2 (SE = 0.092,
95% CI = 0.316, 0.678), respectively. On analysis of secondary outcomes, using a highly
sensitive IL-2 assay, the observed plasma concentrations of the drug at 90 min exceeded
the hypothetical Treg-specific therapeutic window determined in vitro (0.015–0.24 IU/ml),
even at the lowest doses (0.040 × 106 and 0.045 × 106 IU/m2
) administered. A rapid
decrease in Treg frequency in the circulation was observed at 90 min and at day 1, which
was dose dependent (mean decrease 11.6%, SE = 2.3%, range 10.0%–48.2%, n = 37),
rebounding at day 2 and increasing to frequencies above baseline over 7 d. Teffs, natural
killer cells, and eosinophils also responded, with their frequencies rapidly and dose-dependently
decreased in the blood, then returning to, or exceeding, pretreatment levels. Furthermore,
there was a dose-dependent down modulation of one of the two signalling subunits
of the IL-2 receptor, the β chain (CD122) (mean decrease = 58.0%, SE = 2.8%, range
9.8%–85.5%, n = 33), on Tregs and a reduction in their sensitivity to aldesleukin at 90 min
and day 1 and 2 post-treatment. Due to blood volume requirements as well as ethical and
practical considerations, the study was limited to adults and to analysis of peripheral blood
only.
Conclusions
The DILT1D trial results, most notably the early altered trafficking and desensitisation of
Tregs induced by a single ultra-low dose of aldesleukin that resolves within 2–3 d, inform
the design of the next trial to determine a repeat dosing regimen aimed at establishing a
steady-state Treg frequency increase of 20%–50%, with the eventual goal of preventing
T1D.