Adoptive regulatory T cell (Treg) therapy has emerged as a promising immunotherapeutic approach for treating autoimmune diseases and graft-versus-host disease. However, new research published in Nature Communications reveals an unexpected paradox: Treg therapy can inadvertently promote the generation of pathogenic pro-inflammatory Th17 cells, potentially undermining therapeutic efficacy.
Treg Therapy Drives Pathogenic Th17 Cell Generation
Using classic inflammatory bowel disease (IBD) and experimental autoimmune encephalomyelitis (EAE) models, researchers demonstrated that adoptive Treg cell therapy significantly increased Th17 cell frequencies in target tissues. In the IBD model, single-cell RNA sequencing of 32,898 colonic lamina propria cells revealed that while Treg therapy successfully suppressed overall immune responses, it specifically promoted Th17 cell generation from conventional T effector cells rather than from Treg conversion.
Flow cytometry analysis confirmed these findings, showing significantly higher frequencies of IL-17-producing Th17 cells in the colon, mesenteric lymph nodes, and spleen of Treg-treated mice compared to controls. Similarly, in the EAE model using myelin oligodendrocyte glycoprotein (MOG)-specific Tregs, therapy increased Th17 cell frequencies in the spinal cord and brain while reducing Th1 cells.
Enhanced Pathogenicity of Therapy-Induced Th17 Cells
Single-cell analysis identified two distinct Th17 cell clusters, with the pathogenic subset (C1) showing highest proportions in the Treg therapy group. Gene set enrichment analysis revealed that Treg therapy-induced Th17 cells expressed pathogenic-associated genes including Nfatc1, S100a4, and IL23r at higher levels than conventional inflammatory Th17 cells.
Functional validation confirmed the enhanced pathogenicity of these cells. When Th17 cells sorted from Treg-treated mice were transferred into new recipients, they caused more severe weight loss and colonic inflammation compared to conventional effector T cells. This demonstrates that Treg therapy generates a particularly aggressive subset of pathogenic Th17 cells.
IL-2 Suppression Drives the Paradoxical Effect
The mechanistic investigation revealed that Treg therapy-induced Th17 generation depends on IL-2 suppression combined with TGF-β and IL-6 signaling. Tregs express high-affinity IL-2 receptors and competitively consume IL-2, reducing its availability for conventional T cells. Since IL-2 normally inhibits Th17 differentiation through STAT5 activation, its depletion removes this inhibitory signal.
In vitro co-culture experiments demonstrated that Tregs significantly reduced IL-2 secretion by effector T cells and decreased IL-2 levels in culture medium. Importantly, adding exogenous IL-2 completely reversed Treg-induced Th17 generation, while IL-2 neutralization promoted Th17 differentiation even without Tregs present.
The researchers showed that Tregs alter the phospho-STAT3/phospho-STAT5 ratio in effector T cells by suppressing STAT5 phosphorylation. Since both transcription factors compete for binding sites on the IL-17 promoter, increasing the p-STAT3/p-STAT5 ratio enhances IL-17 transcription and Th17 differentiation.
Cytokine Dependencies Confirmed In Vivo
Using genetically modified mice lacking IL-6 receptor (Il6ra-/-) or TGF-β receptor (Tgfbr1-/-), researchers confirmed that Treg therapy-induced Th17 generation requires both IL-6 and TGF-β signaling pathways. In Il6ra-/- mice, Treg therapy failed to increase Th17 cells, while in Tgfbr1-/- mice, Th17 generation was completely suppressed.
RNA sequencing of IL-2-deficient Th17 cells revealed upregulation of inflammatory pathways and genes associated with chronic inflammatory diseases, including Il23r, Bhlhe40, S100a4, and Cd24a. These cells also showed reduced IL-10 production and enhanced recruitment of neutrophils and macrophages in an acute lung inflammation model, confirming their increased pathogenic potential.
Combination Therapy Improves Outcomes
To address this therapeutic challenge, researchers developed combination strategies targeting the IL-6/STAT3 signaling pathway alongside Treg therapy. In the IBD model, combining Tregs with IL-6 neutralizing antibodies not only prevented pathogenic Th17 generation but also enhanced overall therapeutic efficacy by increasing Treg frequencies and suppressing Th1 responses.
In the EAE model, combining MOG-specific Tregs with Stattic, a STAT3 inhibitor, showed synergistic effects in suppressing disease progression. The combination therapy reversed Treg-induced Th17 generation while maintaining the beneficial immunosuppressive effects of Treg therapy.
Clinical Implications and Future Directions
These findings have significant implications for ongoing clinical trials of Treg therapy. The research provides a mechanistic explanation for suboptimal outcomes observed in some clinical studies and suggests that monitoring Th17 cell responses should be incorporated into future trial designs.
The study identified similar phenomena in clinical data, noting that patients with systemic lupus erythematosus showed concurrent increases in Th17 cells after Treg infusions, and some amyotrophic lateral sclerosis patients exhibited elevated IL-17F levels following Treg therapy.
The research establishes that adoptive Treg therapy can have dual effects: while successfully suppressing certain immune responses, it may simultaneously promote pathogenic inflammation through IL-2-mediated mechanisms. This discovery emphasizes the need for combination therapeutic approaches that harness Treg suppressive functions while preventing unintended pro-inflammatory consequences.
Understanding this IL-2-TGF-β-IL-6 axis opens new avenues for optimizing immunotherapies and developing more precise interventions for autoimmune diseases. The findings underscore the complexity of immune regulation and the importance of comprehensive mechanistic understanding before clinical translation of cell-based therapies.
