Autologous polyclonal Treg therapy (T-Rex program)

Regulatory T cells, or Tregs, are the immune system's brakes: they suppress overactive immune responses and help prevent autoimmunity. In type 1 diabetes these brakes appear to fail, letting other T cells destroy insulin-producing cells. Treg therapy aims to fix that directly — isolating a person's own Tregs, expanding them into the billions in a laboratory, and infusing them back to restore immune balance and protect the remaining cells. A landmark phase 1 trial in 14 adults, run by investigators at UCSF and Benaroya, established the approach was feasible and safe: the expanded cells kept their suppressive function, a fraction survived in the bloodstream up to a year, there were no serious infusion reactions, and C-peptide persisted past two years in several participants. That promise led to the randomized, placebo-controlled phase 2 T-Rex trial of CLBS03 in 110 children and adolescents with recent-onset disease. The cells were again safe, and a transient rise in activated Tregs confirmed the infusion engrafted — but a single dose did not preserve the body's own insulin production over one year, regardless of dose or age. Intriguingly, batches that expanded less in the lab were linked to better C-peptide preservation, hinting that Treg quality, not just quantity, matters. The polyclonal one-shot approach has not advanced to a product, but the negative result is reshaping the field toward antigen-specific and engineered Tregs, repeat dosing, and combination strategies. In July 2026 that reframing got a boost from an unexpected direction: researchers pooled pre-treatment samples from 560 people across six immunotherapy trials and found two immune subtypes — unrelated to age — that lose insulin production at different speeds and respond differently to treatment, one subgroup doing better on anti-CD20 and the other on CTLA4-Ig. That study did not test Treg infusions, so whether the same logic explains T-Rex is untested. But it is the most concrete evidence yet that the question in immunotherapy may be who you give a treatment to, not simply whether the mechanism works — and a trial that averages across a mixed population can bury a real effect in a subgroup.