Regulatory T (Treg) cells represent an indispensable CD4⁺ T cell subset responsible for immune regulation and inflammation control in diverse diseases. They can maintain immune tolerance by restraining auto- and extended self-reactive effector T cells through non-redundant inhibitory mechanisms. Treg dysfunction has been thought of as being fundamentally quantitative, resulting from a deficit in Treg cell numbers or specificity. According to this canonical view, restoring tolerance thus requires expanding the Treg cell pool.

On July 17, 2026, Dr. Talal A. Chatila, a renowned immunology researcher from Harvard Medical School, delivered an academic presentation entitled "Molecular Programs for Regulatory T Cell Subversion in Tissue Inflammation" at the 2026 CIMR International Symposium for Immunology, arguing for a more expansive view of Treg cell dysfunction, one that results not only from quantitative deficiency but also from the active, programmed subversion of Treg cells by tissue signals in disease. Central to this view is the discovery that Treg cells read their tissue environment through defined molecular codes, and that these same codes can be co-opted in disease to drive Treg cells from a protective into a pathogenic state. Foremost among these is a Notch receptor code, in which individual Notch receptors, engaged by distinct tissue and microbial cues, reprogram Treg cells in a tissue-specific manner.

Talal A. Chatila, is a professor of pediatrics at Harvard Medical School and Director of the Program in Translational Immunology at Boston Children's Hospital. His research focuses on the genetic and cellular mechanisms that regulate immune tolerance, particularly on Treg cells in both monogenic immune disorders and allergic diseases. Dr. Chatila's laboratory has also made seminal contributions to understanding how Notch signaling pathways in Treg cells shape tissue- and pathogen-specific inflammation, with direct relevance to asthma, viral lung infections, and multisystem inflammatory syndrome in children. The talk illustrated how the Notch receptor code subverts Treg cell function across different settings, beginning with Notch3 in the central nervous system.

Notch3 destabilizes regulatory T cells to drive autoimmune neuroinflammation in multiple sclerosis

Background

Multiple sclerosis (MS) is a chronic autoimmune demyelinating disease. Of particular interest in disease pathogenesis is the role of immune regulatory mechanisms normally operative to restrain autoreactivity, key among which are Treg cells. The number and function of Treg cells are impaired in patients with MS, and Treg cells accumulate in CNS tissues but fail to control the autoimmune inflammation in experimental autoimmune encephalomyelitis (EAE, a classic murine model of MS) and eventually lose their suppressive function and Foxp3 expression. Subverted Treg cells may further convert into pathogenic Th cells under the influence of pro-inflammatory cytokines or antigenic stimulation. Whether this Treg cell plasticity plays a role in the pathogenesis of EAE and MS remains unclear. Notch signaling in T cells is implicated in the pathogenesis of MS/EAE. Previous studies from Chatila's team showed that individual Notch receptor signaling in Treg cells licensed inflammation in different tissues by promoting Treg cell destabilization and plasticity.

Main results

To elucidate the role of Notch3 in MS pathogenesis, the team first identified that the expression of Notch3 was elevated in Treg cells of active MS patients, with a higher proportion of Notch3⁺ Treg cells in the CSF than the peripheral blood. The increased levels of IL-17, IL-22 and IFN-γ suggested Treg cells’ transformation into effector T cells. In EAE mice, the expression of Notch3 was elevated in Treg cells of CNS and non-Treg CD4⁺ T cells. The knockout of Notch3 (not other Notch receptor) systemically or specifically in T/Treg cells could alleviate EAE severity, suggesting that Notch3 had a specific pathogenic role in autoimmune neuroinflammation (Fig. 1).

Fig. 1 Increased Notch3 expression on peripheral and CSF Treg cells of MS patients

To answer the question on how Notch3⁺ Treg cells were generated and reached into CNS, the team found that this subset was induced in the gut by gut microbiota in a TLR-MyD88-dependent manner with higher expression of chemokines and homing molecules (α4β7、CCR5、CD49d、CXCR3). In EAE mice, this subset first appeared in the lamina propria and then transferred to CNS. Blocking α4β7 or Vla-4 could mitigate Notch3⁺ Treg cells in CNS and disease severity, suggesting that this subset migrated into CNS and contributed to neuroinflammation (Fig. 2).

Fig. 2 Notch3⁺ Treg cells are induced by the gut microbiota and translocate to the CNS

Through scRNA-seq and transcriptomic analysis, they found that CNS Notch3⁺ Treg cells were likely to lose Foxp3 and transform into Th17. And Notch3⁺ Treg cells were susceptible to become ex-Treg cells, which was reliant on interaction with DLL1 on microglia. These results demonstrated that Notch3 signaling drove Treg cells into pro-inflammatory Th17 cells through DLL1-mediated extracellular signaling (Fig. 3).

Fig. 3 Treg cell Notch3-microglial DLL1 interaction promotes EAE

Furthermore, they discovered two functionally opposite Treg subsets in the CNS: the Notch3⁺ Treg cells and NPY1R⁺ Treg cells. And the latter subset could help maintain immune inhibition in the CNS and the imbalance of these two subsets would impact the disease severity (Fig. 4).

Fig. 4 Notch3 deletion promotes the expansion of a protective NPYR1⁺ Treg cell subpopulation

Highlights

Notch3⁺ Treg cells are induced in the gut in a microbiota and TLR-dependent manner;

They translocate to the CNS in EAE by an integrin-dependent mechanism;

They degenerate in the CNS into Th17 cells by Hippo pathway and DLL1-dependent mechanisms;

They outcompete NPY1R⁺ CNS-resident Treg cells that control IFN-γ and GM-CSF responses. (Fig. 5)

Fig. 5 Graphic abstract

Role of Treg-specific Notch signaling in allergic airway inflammation

Background

In the lung, a different Notch receptor drives Treg subversion. Asthma is a chronic inflammatory airway disease characterized by hyperresponsiveness and tissue remodeling. Its persistence reflects a breakdown of immune tolerance, marked by the subversion of allergen‑specific Treg cells into Th2/Th17 effector cells. Environmental ultrafine particles activate alveolar macrophages via aryl hydrocarbon receptor to induce Jagged1, which engages Notch4 on T cells. Chatila’s study identifies Notch4 as a critical switch that disrupts lung Treg function through Wnt/Hippo pathways, leading to GDF15 expression, which directly activates ILC2 cells and amplifies allergic inflammation. However, the Treg subsets and regulatory signals governing Notch4 induction remain unknown, prompting their investigation into whether targeting Notch4 can restore durable tolerance in asthma.

Main results

Chatila's lab employed CD4^CreNotch4^Δ/Δ and Foxp3^YFPCreNotch4^Δ/Δ mice to delete Notch4 in all T cells or Treg cells, and found that the deletion of Notch4 attenuated allergic airway inflammation. It suppressed lung tissue Th2 and Th17 cell responses and reversed the destabilization of lung tissue Treg cells toward Th2 and Th17 cell-like phenotypes (Fig. 6).

Fig. 6 Notch4 expression on lung Treg cells licenses allergic airway inflammation

Total ILC2 and IL-13-expressing ILC2s were increased in OVA- and OVA'UFP-treated mice, but were reduced on deletion of Notch4 in Treg cells. In vitro studies showed that Notch4^hi Treg cells failed to suppress the upregulation of IL-13 expression in ILC2 derived from the inflamed lungs. Further analysis revealed that Treg cell-specific Ctnnb1, but not Yap and Taz, deletion restored the ILC2-suppressive function of Notch4^hi Treg cells, thus implicating the Wnt pathway in the failure of ILC2 regulation. In vitro co-culture of GDF15-expressing Notch4^hi Treg cells, isolated from lungs of OVA+UFP-treated mice, with naive ILC2s upregulated the expression of IL-13 in the latter, an effect that was reversed by the addition of a GDF15-blocking peptide. Addition of GDF15-blocking peptide restored the ILC2-suppressive function of lung Treg cells of OVA+UFP-treated mice, consistent with the critical role of the Wnt-GDF15 axis in the failure of ILC2 regulation (Fig. 7).

Fig. 7 Notch4 promotes ILC2 activation via a GDF15-dependent mechanism

Subjects with asthma had higher frequencies of circulating Notch4^hi Treg cells, with both the cell frequencies and expression intensity progressively increasing as a function of asthma severity, reaching up to 50% of circulating Treg cells in subjects with severe asthma. Treg cell expression of the Hippo and Wnt pathway effector proteins Yap/Taz and β-catenin, respectively, localized to Notch4⁺ Treg cells and similarly increased as a function of asthma severity. And there were increased concentrations of GDF15 in the sera of subjects with moderate and severe asthma that positively correlated with circulating Treg cell Notch4 expression (Fig. 8).

Fig. 8 Notch4 expression on circulating Treg cells segregates with asthma severity

Highlights

Inhibition of Notch4 expression in Treg cells suppressed airway inflammation and restored immune tolerance;

Notch4 subverted Treg cells into the Th2 and Th17 effector T cells by Wnt and Hippo pathway-dependent mechanisms;

Wnt activation induced GDF15 expression in Treg cells, which activated ILC2s to provide a feed-forward mechanism for aggravated inflammation.

Beyond the Notch Code: Gut epithelial Programming of Treg cell identity

Background

Treg subversion can also be driven from outside the cell, by the epithelial-microbial environment. Type 2 immunity protects against parasites and toxins, but its dysregulation leads to food allergy (FA) and anaphylaxis. IL-4 and IL-13 induce gut epithelial proteins such as RELMβ. In Il4ra^F709 mice with gain-of-function IL-4 receptor signaling, they observed elevated RELMβ, which was further augmented upon FA induction. RELMβ was also highly increased in sera of FA patients. These observations led them to investigate whether RELMβ plays a mechanistic role in the development of FA and how it might regulate epithelial and T-cell type-2 immune programs.

Main results

RORγt⁺ Treg cells play a central role in immune regulation, including in maintenance of tolerance to food allergens. Deletion of Retnlb reversed the RORγt⁺ Treg depletion. Importantly, deletion of Rorc, encoding RORγt, in Treg cells of Il4ra^F709Retnlb^-/- mice abrogated the protection against local and systemic FA provided by RELMβ deficiency. Together, their results demonstrate RELMβ as a central regulator of both type 2 mediators and effector responses in FA through suppression of tolerogenic and protective RORγt+ Treg cells (Fig. 9).

Fig. 9 RELMβ suppresses FA-protective RORγt⁺ Treg cells

They performed fecal microbiome transplantation (FMT) from otherwise unsensitized adult WT, Il4ra^F709, and Il4ra^F709Retnlb^–/– donors into germ-free Il4ra^F709 mice. FMT from Il4ra^F709 mice failed to protect against FA, whereas FMT from Il4ra^F709Retnlb^–/– mice conferred protection, suppressing the OVA-specific IgE response and mast cell release (serum MMCP1) while restoring RORγt⁺ SI-LPL Treg cells. The protective microbiome was enriched in indole-producing bacteria, which had been depleted by dysregulated RELMβ; the resulting indole metabolites promote the RORγt⁺ Treg cells that enforce oral tolerance. Protection against FA conferred by RELMβ deficiency thus reflects a specific restructuring of the gut microbiome toward an indole-producing, tolerogenic community (Fig. 10).

Fig. 10 RELMβ deficiency structures a tolerogenic gut microbiome

Early life RELMβ sets the threshold for the susceptibility to FA

Notably, while Retnlb transcripts increased during the peri-weaning period [week (wk)3 and wk4 of life] and declined thereafter, the magnitude of the increase was much greater in Il4ra^F709 compared with WT mice. The dysregulated Retnlb expression in Il4ra^F709 mice was mirrored by reduced frequencies of RORγt⁺ Treg cells in the SI-LPL at peri-weaning compared with WT mice. Treatment of unsensitized Il4ra^F709 mice in early life with anti-RELMβ mAb induced RORγt⁺ Treg cell differentiation. These data indicate that expression of RELMβ in early life regulates RORγt⁺ Treg cell differentiation (Fig. 11).

Fig. 11 Expression of RELMβ in early life regulates RORγt⁺ Treg cell differentiation

Treatment with anti-RELMβ suppresses established FA

Early life treatment with anti-RELMβ mAb protected mice from FA, suppressing anaphylaxis and mast cell expansion and resulting in increased frequencies of SI-LPL RORγt⁺ Treg cells. Overall, these results indicated that RELMβ activity early in life can influence long-term susceptibility to FA and that targeting RELMβ can prevent the development of FA later in life (Fig. 12).

Fig. 12 Targeting RELMβ prevents and treats FA

Highlights

RELMβ drives food allergy and its deletion prevents IgE and anaphylaxis;

RELMβ depletes indole‑producing microbes, reducing AhR‑dependent protective Tregs and breaking oral tolerance;

Early RELMβ blockade restores tolerance and prevents later food allergy in susceptible offspring.

Concluding Remarks

Toward a different understanding of Treg cell function in tissue inflammation

Tissue-instructed non-canonical programs maintain tissue repair, tolerance and mucosal homeostasis;

These programs are co-opted in disease both intrinsically, through the Notch receptor code and within-lineage antagonism, and extrinsically, through epithelial-microbial dysregulation;

Disease-promoting Treg subsets are not malfunctions of the suppressor program but programmed responses co-opted by pathological cues;

The disruptive idea is not that Tregs can fail, but that their failure is programmed-and therefore so is their rescue;

Treg cells are context-sensitive integrators whose molecular programmability underlies both their non-canonical roles in tissue homeostasis and their capacity to be hijacked into disease-promoting states.

References

1 Benamar, M. et al. Notch3 destabilizes regulatory T cells to drive autoimmune neuroinflammation in multiple sclerosis. Immunity 58, 2753-2768.e2756, doi:10.1016/j.immuni.2025.09.007 (2025).

2 Harb, H. et al. A regulatory T cell Notch4–GDF15 axis licenses tissue inflammation in asthma. Nature Immunology 21, 1359-1370, doi:10.1038/s41590-020-0777-3 (2020).

3 Stephen-Victor, E. et al. RELMβ sets the threshold for microbiome-dependent oral tolerance. Nature 638, 760-768, doi:10.1038/s41586-024-08440-7 (2025).