Design: Phase 2b, randomised, double-blind, placebo-controlled (DIABIL-2)
Population: 141 people aged 6–35 with type 1 diabetes, under 3 months on insulin
Primary outcome: Change in mixed-meal C-peptide AUC at 12 months
Effect: Geometric mean ratio 1.06 (95% CI 0.56 to 2.01) vs placebo; p=0.85

DIABIL-2 was a double-blind, placebo-controlled phase 2b trial in 19 European centres. It randomised 141 people aged 6–35, within three months of starting insulin and with residual C-peptide, to two low-dose interleukin-2 regimens or placebo for about a year.

The drug did what it was designed to do biologically: regulatory T cells expanded sharply in the first days. But at 12 months C-peptide area under the curve did not differ from placebo (geometric mean ratio 1.06, 95% CI 0.56 to 2.01), whichever regimen and at any pubertal stage. Serious adverse events were 10% on drug and 20% on placebo.

Target engagement without clinical benefit is a familiar story in type 1 immunotherapy. The authors suggest the inflammatory environment at diagnosis may be too strong for this single agent and point to earlier-stage disease or combinations. For clinic, nothing changes.

Low-dose interleukin-2 is not a treatment for new type 1 diabetes; it failed its primary endpoint.
Families asking about immune therapy at diagnosis can be told this approach has not preserved insulin production.
Treg expansion on blood tests did not translate into C-peptide preservation.
Interleukin-2 was well tolerated, so trials in earlier, pre-symptomatic stages remain possible.

Why it matters

It shows that hitting the immune target is not enough once type 1 diabetes has declared itself.

The statistics, in plain English

A geometric mean ratio of 1.06 means C-peptide was essentially the same as placebo. The confidence interval, 0.56 to 2.01, is wide and crosses 1, so the trial cannot exclude a modest effect either way — but it shows no signal of benefit.