Background: The human regulatory macrophage (Mreg) is already far advanced in its development as a cell therapy for use as an adjunct immunosuppressant in organ transplantation. Preclinical experiments in mice showed that donor-strain Mregs given prior to transplantation prolonged allograft survival beyond the apparent lifespan of the transferred cells. This protective effect depended upon administration of living, iNOS-expressing cells suggesting that, unlike immature DC treatment, allogeneic Mregs might shape recipient T cell responses through direct pathway interactions. To address this hypothesis in humans, characteristics of CD4⁺ T cells exposed to allogeneic Mregs in culture were examined in patients after Mreg treatment.
Methods: Changes in CD4⁺ T cells caused by 5-day coculture with allogeneic Mregs were characterized by flow cytometry, gene expression profiling and suppressor assays. Patients were treated with Mreg_UKR cell products within the ONEmreg12 trial (NCT02085629).
Results: Coculturing naïve human CD4⁺ T cells with allogeneic Mreg led to emergence of IL-10-producing, BTNL8⁺ TIGIT⁺ FoxP3⁺ induced regulatory T cells (iTregs) that suppressed polyclonally-stimulated T cell proliferation and inhibited mo-DC maturation. This induction of Tregs depended upon TCR-, B7-, IDO- and notch- signaling. Administering allogeneic Mregs to NSG mice reconstituted with human naïve CD4⁺ T cells resulted in TIGIT⁺ FoxP3+ Treg generation and systemic human IL-10 production. An acute increase in FoxP3⁺ Tregs was observed after donor-derived Mreg treatment in two prospective living-donor kidney transplant recipients. Investigation of one Mreg-treated kidney transplant recipient with long-term stable graft function revealed an oligoclonal expansion of TCR-Vβ13.1⁺ TIGIT⁺ Tregs at 8-years post-treatment.
Conclusion: This study hints at a feed-forward mechanism by which Mreg treatment could promote allograft acceptance through rapid induction of direct alloreactive Tregs.