Background: Regulatory T cells (Tregs) are essential for immune homeostasis, suppressing excessive immune responses. Defects in Tregs, such as reduced numbers or impaired suppressive function, are linked to autoimmune diseases. Low-dose interleukin-2 (IL-2) therapy has emerged as a strategy to preferentially expand Tregs and enhance their suppressive capacity via the high-affinity IL-2 receptor (IL-2Rαβγ: CD25/CD122/CD132), while minimizing activation of effector T cells and natural killer (NK) cells that express the lower-affinity dimeric IL-2Rβγ (CD122/CD132).

Objectives: To enhance Treg selectivity, an IL-2 mutein with reduced affinity for IL-2Rβγ was developed, creating a CD25-dependent signaling profile. EGL-003 was created by linking two such muteins to an inert IgG1-derived Fc domain containing LALA mutations to minimize Fcγ receptor binding and extend its half-life. The preclinical journey preparing the clinical assessment of EGL-003 is reported here.

Methods: EGL-003’s selectivity and activity were assessed through in vitro studies using human peripheral blood mononuclear cells (PBMCs) and expanded Tregs (eTregs) and in vivo in humanized IL-2 receptor knock-in (TKI) mice. Immunosuppressive potential was evaluated in a lupus model and in a DSS-induced colitis model. Preclinical safety, pharmacokinetics, and pharmacodynamics were tested in non-human primates (NHPs). Ethical guidelines were followed in all animal studies.

Results: In vitro, EGL-003 demonstrated selective binding to IL-2Rαβγ, as confirmed by flow cytometry and bio-layer interferometry. Its dissociation rate (Koff) from IL-2Rβγ was 200-fold higher than wild-type IL-2 (IL2WT), maintaining affinity for IL-2Rα. Compared to IL2WT, EGL-003 induced p-STAT5 at significantly lower concentrations in Tregs than in Tconv, CD8+ T cells, and NK cells and selectively expanded Ki67+ Tregs without increasing proliferation of Tconv or CD8+ T cells. In NZB/NZW F1 mice, a surrogate of EGL-003 delayed appearance of severe proteinuria, decreased kidney histological score and improved survival. In TKI mice, EGL-003 dose-dependently increased Treg frequency in blood, lymphoid organs, and colon, with minimal effects on Tconv and CD8+ T cells. NK cell numbers increased modestly at the highest dose. In a dextran sodium sulfate colitis model, EGL-003 elevated Tregs in the colon, representing 10% of the infiltrating CD45+ cells, most of them expressing the tissue-residency marker ST2, and also expressing Helios and GATA3. EGL-003 treatment significantly improved clinical outcomes, including body weight, colon shrinkage, stool and colon histological scores. In NHPs, the no-observed-adverse-effect level (NOAEL) was set at 30 µg/kg (s.c., weekly for 12 weeks) coupled with up to a 7-fold increase in circulating Tregs, dominated by Ki67+ effector and memory effector phenotypes. At higher doses (120-200 µg/kg), only adverse effects related to an exaggerated IL-2 agonist effect have been observed. Based on pharmacokinetic data, the half-life in humans is estimated at 20-30 hours with a substantial distribution in lymphatic/tissues prior to its release into the blood.

Conclusion: EGL-003 is a novel IL-2 mutein Fc-fusion protein with potent, selective Treg-expanding activity and demonstrated safety and activity in preclinical models. These findings support its clinical evaluation for autoimmune diseases.

REFERENCES: NIL.

Acknowledgements: NIL.

Disclosure of Interests: Cindy Orvain Egle Therapeutics, Marine Dubois Egle Therapeutics, Caroline Janot-Sardet Egle Therapeutics, Erica Russo Egle Therapeutics, Elsa Lheriteau Egle Therapeutics, Karine Bailly Egle Therapeutics, Pauline Soulard Egle Therapeutics, Charlotte Muller Egle Therapeutics, Pamela Caudana Egle Therapeutics, Mercedes Tkach Egle Therapeutics, Ilgar Mamedov Egle Therapeutics, Yann Courbebaisse Egle Therapeutics, Jordan Denizeau: None declared, Annalisa Meola: None declared, Félix Rey: None declared, Pablo Guardado-Calvo: None declared, Alvaro Lladser Egle Therapeutics, Christine Sedlik: None declared, Reno Winter Egle Therapeutics, Eliane Piaggio Egle Therapeutics, Jonathan Bach Egle Therapeutics, Bernard Vanhove Egle Therapeutics, Fiorella Kotsias Egle Therapeutics.

© The Authors 2025. This abstract is an open access article published in Annals of Rheumatic Diseases under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). Neither EULAR nor the publisher make any representation as to the accuracy of the content. The authors are solely responsible for the content in their abstract including accuracy of the facts, statements, results, conclusion, citing resources etc.