Low-dose interleukin-2 (IL-2)

Screening: finding T1D before it strikes

Low-dose IL-2 is being explored as a way to protect insulin-producing cells, which only matters if the right people are found in time — and that depends on screening. Type 1 diabetes develops along a defined three-stage path. Stage 1 is two or more islet autoantibodies (against insulin, GAD, IA-2, or ZnT8) with normal blood glucose and no symptoms; stage 2 adds abnormal glucose (dysglycemia) but still no symptoms; stage 3 is clinical disease, when insulin is needed.¹ The autoantibodies are powerful predictors: once a child has two or more, the lifetime risk of clinical diabetes approaches certainty, which is what makes early, pre-symptomatic intervention conceivable.¹

Screening can be offered to relatives, but roughly 90% of new cases have no family history, so finding most at-risk people requires general-population autoantibody (and genetic-risk) testing.¹ Early detection is valuable even before any therapy: people found through screening are monitored, so when they reach stage 3 they are far less likely to arrive in diabetic ketoacidosis (DKA), a dangerous and sometimes fatal emergency. Modern capillary and home-collected sampling is lowering the cost and burden that long blocked wide screening.¹

Therapy: tipping the immune balance back toward tolerance

T1D is driven by an imbalance between destructive effector T cells (Teffs) and protective regulatory T cells (Tregs). Tregs depend on interleukin-2 to survive and function but cannot make enough of their own, and T1D is genetically linked to a faulty IL-2 pathway.² Tregs carry the high-affinity IL-2 receptor, so very low doses of IL-2 — far below the toxic doses used in cancer — preferentially wake up Tregs without arming Teffs. The drug used is aldesleukin (recombinant human IL-2, Proleukin), an approved cancer medicine being repurposed at ~1/100th the oncology dose.³

The clinical record is consistent on mechanism and safety but modest on metabolic benefit. The first trial in 24 adults (NCT01353833) tested 0.33, 1, and 3 million-unit (MIU) daily doses: IL-2 was well tolerated with no serious adverse events and produced a clear dose-dependent Treg rise (4.8% vs 0.5% on placebo at the top dose), with only injection-site reactions and mild flu-like symptoms, and no worsening of glucose.³ Two Cambridge dose-finding studies refined this: DILT1D found single ultra-low doses of ~0.1 and ~0.5 MIU/m² raised Tregs by 10% and 20%,⁴ and DILfrequency identified a repeat regimen (~0.26 MIU/m² every 3 days) that holds a steady ~30% Treg increase without expanding Teffs.⁵

The most encouraging efficacy signal came from DF-IL2-Child (NCT01862120): in 24 children with recently diagnosed T1D, IL-2 drove a dose-dependent Treg increase (up to 77% at the highest dose), and the seven strongest "Treg high responders" showed better-preserved insulin (C-peptide) production at one year than low responders — a hint of beta-cell protection, though the trial was not powered to prove it.⁶ The phase-2 ITAD trial (NCT03782636) in 45 children then tested twice-weekly aldesleukin for 6 months with C-peptide as its primary endpoint.⁷ A key caveat surfaced in combination work: when IL-2 was paired with infused Tregs, it expanded Tregs but also expanded cytotoxic NK and CD8 T cells, a reminder that the safe therapeutic window is narrow.⁸ Durability is the central weakness — the Treg boost fades within days of stopping, so continuous dosing is needed and no lasting tolerance reset has been shown.⁵

Access and stages: aldesleukin exists and is affordable as a molecule, but it is not approved for T1D anywhere and is available only inside trials, given by repeated subcutaneous injection.³ Studied populations span new-onset adults and children (roughly age 6 and up), with prevention in stage 1-2 a logical but still-unproven extension.²

What's coming

The field is shifting from "does it move Tregs?" (answered: yes) to "does it protect beta cells?" Strategies now in trials combine ld-IL-2 with other agents to deepen and prolong the effect — for example ciclosporin followed by low-dose IL-2 in newly diagnosed patients (NCT05153070, with ILTOO Pharma), which aims to first quiet the autoimmune attack and then lock in tolerance with sustained Treg support.⁹

The bigger shift is from plain IL-2 to engineered IL-2 molecules ("muteins") built to hit Tregs more selectively and last longer in the body. This is no longer a general aspiration: in May 2026, TrialNet (NIDDK) began recruiting a phase-2 trial of rezpegaldesleukin (NKTR-358) — a PEGylated, Treg-selective IL-2 — in people with new-onset T1D (NCT07142252).¹⁰ The design tells you what the field wants from a mutein: 66 participants aged 8-45, randomised 2:1 against placebo, dosed by subcutaneous injection just once every 14 days for 26 weeks, versus the near-daily or every-3-day schedules plain low-dose IL-2 needs. The primary endpoint is stimulated C-peptide (a direct measure of surviving insulin production) on a mixed-meal test at 12 months, with primary completion expected in May 2028.¹⁰

Two cautions are worth stating plainly. First, enrolment is deliberately staged for safety: only adults aged 18 and over may be among the first 18 participants; ages 12-17 open only after a data safety monitoring board reviews those adults; and enrolment down to age 8 opens only after a further 17 participants aged 12-45 reach their 6-month visit and pass a second safety review. Second, this is a treatment trial in people who already have clinical (stage 3) diabetes — not a prevention trial. Whether IL-2 in any form can delay or prevent stage-3 onset in people at stage 1 or 2 remains untested, which is why this entry still scores low on delay and durability despite a decade of reliable immune-system results.

References

1. Sims EK, et al. Screening for Type 1 Diabetes in the General Population: A Status Report and Perspective. Diabetes (2022). https://doi.org/10.2337/dbi20-0054 ↩ ↩² ↩³ ↩⁴

2. Rosenzwajg M, et al. Interleukin 2 in the pathogenesis and therapy of type 1 diabetes. Curr Diab Rep (2014). https://doi.org/10.1007/s11892-014-0553-6 ↩ ↩²

3. Hartemann A, et al. Low-dose interleukin 2 in patients with type 1 diabetes: a phase 1/2 randomised, double-blind, placebo-controlled trial. Lancet Diabetes Endocrinol (2013). https://doi.org/10.1016/S2213-8587%2813%2970113-X ↩ ↩² ↩³

4. Todd JA, et al. Regulatory T Cell Responses in Participants with Type 1 Diabetes after a Single Dose of Interleukin-2 (DILT1D). PLoS Med (2016). https://doi.org/10.1371/journal.pmed.1002139 ↩

5. Seelig E, et al. The DILfrequency study is an adaptive trial to identify optimal IL-2 dosing in patients with type 1 diabetes. JCI Insight (2018). https://doi.org/10.1172/jci.insight.99306 ↩ ↩²

6. Rosenzwajg M, et al. Low-dose IL-2 in children with recently diagnosed type 1 diabetes: a Phase I/II randomised, double-blind, placebo-controlled, dose-finding study (DF-IL2-Child). Diabetologia (2020). https://doi.org/10.1007/s00125-020-05200-w ↩

7. Marcovecchio ML, et al. Interleukin-2 Therapy of Autoimmunity in Diabetes (ITAD): a phase 2, multicentre, double-blind, randomized, placebo-controlled trial. Wellcome Open Res (2020). https://doi.org/10.12688/wellcomeopenres.15697.1 ↩

8. Dong S, et al. The effect of low-dose IL-2 and Treg adoptive cell therapy in patients with type 1 diabetes. JCI Insight (2021). https://doi.org/10.1172/jci.insight.147474 ↩

9. U.S. National Library of Medicine. Ciclosporin Followed by Low-dose IL-2 in Patients With Recently Diagnosed Type 1 Diabetes (NCT05153070). ClinicalTrials.gov (2022). https://clinicaltrials.gov/study/NCT05153070 ↩

10. Type 1 Diabetes TrialNet / NIDDK. Rezpegaldesleukin (NKTR-358) in New Onset Type 1 Diabetes Mellitus (NCT07142252). ClinicalTrials.gov — recruiting, phase 2; start date 14 May 2026. https://clinicaltrials.gov/study/NCT07142252 ↩ ↩²