Low-dose IL-2 expanded regulatory T cells but did not protect beta cells

Type 1 diabetes begins when the immune system destroys the insulin-producing beta cells of the pancreas, a process driven partly by a shortage of regulatory T cells (Tregs), the cells that normally keep immune attacks in check. Low-dose interleukin-2 (IL-2) is attractive precisely because it selectively wakes up Tregs. A phase 2b trial called DIABIL-2 tested whether using it at diagnosis could slow the destruction — and found that although the drug did exactly what it was designed to do to the immune system, it did not save the beta cells [s1].

The logic being tested

Researchers had previously shown that low-dose IL-2 expands Tregs in people with type 1 diabetes. The hope was that boosting those regulatory cells early, before most beta cells are gone, might preserve the body's own insulin production — a goal measured by how much C-peptide, a byproduct of natural insulin, a patient can still make [s1]. Preserving even modest residual insulin output matters, because it is linked to steadier blood sugar and fewer complications.

What the trial did

DIABIL-2 was a double-blind, multicentre, randomised, placebo-controlled trial run at 19 academic hospital centres across Belgium, France, Germany, Switzerland and the Netherlands, registered as NCT02411253 [s1][s3]. Eligible participants were aged 6-35 years, had been diagnosed with type 1 diabetes, had been on insulin for less than 3 months, and still had a peak C-peptide concentration of at least 0.2 pmol/mL [s1]. Participants were randomly assigned to one of two IL-2 dosing schedules or placebo; each schedule began with daily injections for 5 days, then switched to maintenance injections every 2 weeks or every week through to day 351 [s1].

Between June 18, 2015, and Nov 19, 2020, 141 people were enrolled — 48 to one regimen, 46 to the other and 47 to placebo [s1]. The primary endpoint was the change from baseline to month 12 in C-peptide output during a mixed meal tolerance test, analysed by intention to treat [s1]. Of the 141 participants, 125 (89%) completed the study [s1].

What it found

The drug engaged its target convincingly. A marked expansion of Tregs appeared soon after treatment began, peaking at day 5, with adjusted means of 1.31 in the IL-2 group versus 1.04 with placebo [s1]. But that biological success did not translate into a clinical one. For C-peptide output at month 12, there was no statistically significant difference between the IL-2 and placebo groups: the geometric mean ratio was 1.06 (95% CI 0.56 to 2.01; p=0.85), with similar null results when each regimen was examined separately and by pubertal stage [s1].

In plain terms, the treatment did everything to the immune system it was supposed to, and the beta cells declined anyway. The authors' own interpretation is blunt: despite robust Treg expansion confirming target engagement, low-dose IL-2 did not preserve beta-cell function in people with newly diagnosed type 1 diabetes [s1]. They suggest the inflammatory environment at diagnosis may simply be too intense for this drug used on its own [s1].

Safety

Low-dose IL-2 was well tolerated across all age groups and both schedules [s1]. Serious adverse events were reported in 9 (20%) of 46 participants in the placebo group and 9 (10%) of 94 in the IL-2 groups; one event in each group was judged probably related to treatment [s1]. That the drug was safe and clearly active, but ineffective on the endpoint that counts, is the crux: this is a well-run trial that returns a clean negative on efficacy rather than a signal to chase.

Reading it honestly

DIABIL-2 does not show that regulatory T cells are the wrong target in type 1 diabetes; it shows that expanding them with IL-2 monotherapy, started at diagnosis, is not enough on its own to protect beta cells over a year [s1]. An accompanying commentary places the result within a broader shift toward combination immunoregulatory strategies rather than single agents [s2]. The trial was funded by Assistance Publique-Hopitaux de Paris, the Investissements d'Avenir programme and the drug's developer, ILTOO Pharma — an industry tie worth noting, though the result published is a negative one [s1].

What to watch

The open question is whether low-dose IL-2 could still earn a role as part of a combination — paired with a drug that dampens the initial inflammatory assault, or given earlier, before symptoms appear, in people at high genetic risk [s1][s2]. For now, the practical message is cautionary: a therapy can hit its immune target precisely and still fail to change the disease, and that distinction is why C-peptide, not Treg counts, was the endpoint that mattered.

Sources

• [s1] Efficacy and safety of low-dose IL-2 in people with newly diagnosed type 1 diabetes (DIABIL-2). The Lancet Diabetes & Endocrinology, 1 October 2026. https://doi.org/10.1016/S2213-8587(26)00191-9
• [s2] IL-2 and immunoregulatory therapies in newly diagnosed type 1 diabetes. The Lancet Diabetes & Endocrinology, 1 October 2026. https://doi.org/10.1016/S2213-8587(26)00236-6
• [s3] Low-dose rhIL-2 in Patients With Recently-diagnosed Type 1 Diabetes. ClinicalTrials.gov, NCT02411253. https://clinicaltrials.gov/study/NCT02411253