In a humanized mouse model of spontaneous autoimmune thyroiditis (TAZ10 mice, bearing a thyroiditis-patient-derived T cell receptor specific for thyroid peroxidase), Badami, Cexus and Quaratino (PNAS 2019) showed that self-antigen-specific regulatory T cells (Treg) disappear selectively from thyroid-draining lymph nodes through activation-induced cell death (AICD) triggered by cognate self-antigen, unleashing pathogenic effector T cells. The paper explicitly left open whether Treg and effector lineages bearing the same transgenic TCR are thymically selected on the same or distinct peptides in this model, and whether antigen-specific Treg therapies are undermined by this pathway in inflammatory, antigen-rich tissues. Its findings further raise, without testing, whether this mechanism is general across antigen specificities and human autoimmune disease, and what molecular determinants make self-reactive Treg — but not effector T cells — die upon cognate antigen encounter; for the latter, the paper points only to Treg's reported lower expression of the anti-apoptotic molecule c-FLIP. No later study has resolved these four questions, and the mechanism's generality is even in tension with classical reports that Foxp3 renders Treg resistant to AICD.

Background

Regulatory T cells (Treg) are CD4+ T lymphocytes expressing the transcription factor Foxp3 (forkhead box P3, the master regulator of the Treg lineage) that suppress immune responses and prevent autoimmunity; conventional CD4+ T cells that can react against self-antigens are called self-reactive effector T cells (Teff). Activation-induced cell death (AICD) is a program of apoptosis (most classically via the death receptor Fas/CD95 and its ligand CD95L, triggered by re-stimulation of activated T cells with antigen) that normally deletes over-activated T cells. The TAZ10 model carries a human T cell receptor (TCR, the antigen receptor of T cells) cloned from a thyroiditis patient and specific for thyroid peroxidase (TPO, the key thyroid autoantigen), introduced as a transgene into recombination-activating gene (RAG)-deficient mice (RAG deficiency blocks rearrangement of endogenous TCR genes, so all T cells carry the transgenic receptor); because the human TCR recognizes TPO presented by the mouse MHC class II molecule (the antigen-presenting molecule that displays peptides to CD4+ T cells) A2-K, these mice spontaneously develop autoimmune thyroiditis. Badami et al. (2019) found that the TAZ10 TCR is positively selected in the thymus into both Teff and thymus-derived 'natural' Treg; in the periphery, Treg circulate through all lymphoid tissue except the thyroid-draining lymph nodes, where cognate TPO — aberrantly presented in a highly inflammatory environment, including by MHC class II-expressing thyroid epithelial cells — triggers their AICD, while Teff with the same TCR survive and accumulate. This inverts the classical view: work such as Weiss et al. (2011) showed that Foxp3-mediated suppression of CD95L makes Treg resistant to AICD, and Kaminitz et al. (2011) found no AICD abnormality in Treg of NOD (non-obese diabetic) mice, a strain that spontaneously develops autoimmune diabetes; conversely, the anti-apoptotic molecule c-FLIP (cellular FLICE-inhibitory protein, a regulator of death-receptor signaling; Plaza-Sirvent et al. 2017) is expressed at lower levels in Treg and is required for their survival, and elevated Treg apoptosis is reported in human lupus (Li et al. 2022) and Sjögren's syndrome (Wang et al. 2025). Separately, thymic-selection studies ask which peptides divert thymocytes into the Treg versus conventional lineage: Wojciech et al. (2014) demonstrated in another system that a single self-peptide can select both conventional and regulatory CD4+ T cells carrying identical antigen receptors, and how TCR repertoires are partitioned between lineages remains an open structural question (Lo and Huseby 2024). The finding also bears on the 'buddy hypothesis' (each autoreactive conventional T cell is paired with a same-specificity Treg) and on therapeutic strategies that expand or engineer self-reactive Treg, since self-antigen-driven Treg instability in inflammation is documented (Bailey-Bucktrout et al. 2013), where destabilized Treg can lose Foxp3 and convert toward pro-inflammatory Th17 (T helper 17, interleukin-17-producing) effector cells. Within this landscape, the Badami et al. paper left four questions open, and no later study has closed them.

Problem Statement

The problem, as left open by Badami et al. (PNAS 2019), is to understand antigen-driven activation-induced cell death of self-reactive regulatory T cells as a mechanism of autoimmune tissue attack, comprising four linked questions: (1) Generality — is the selective, cognate-self-antigen-driven death of Treg in the lymphoid tissue draining the target organ (observed in the TAZ10 TCR-transgenic model of autoimmune thyroiditis) reproduced with other antigen specificities, and does it operate in human autoimmune disease? (2) Mechanism — what molecular determinants make self-reactive Treg undergo AICD upon cognate antigen encounter while Teff carrying the same TCR survive and accumulate (the paper itself points only to Treg's reported lower expression of the anti-apoptotic molecule c-FLIP)? (3) Lineage origin — in the thymus, are Treg and conventional CD4+ T cells bearing the same self-reactive TCR selected on identical peptides or on distinct peptides, and does this lineage-choice difference shape their peripheral activation thresholds and susceptibility to AICD? (4) Therapeutic consequence — do ex vivo expanded or engineered self-reactive Treg, upon infusion into inflammatory antigen-rich tissues, get deleted by AICD or convert to non-suppressive phenotypes, and how does the patient-specific, epitope-spreading self-antigen landscape constrain the choice of safe and effective targets for antigen-specific Treg therapy? All four questions are integral to this problem and each must be addressed; an answer that resolves only some of them is a partial answer, not a solution.

References

  1. Badami E, Cexus ONF, Quaratino S. Activation-induced cell death of self-reactive regulatory T cells drives autoimmunity. Proc Natl Acad Sci USA 116(52):26788-26797 (2019). DOI: 10.1073/pnas.1910281116. https://doi.org/10.1073/pnas.1910281116
  2. Quaratino S, Badami E, Pang YY, Bartok I, Dyson J, Kioussis D, Londei M, Maiuri L. Degenerate self-reactive human T-cell receptor causes spontaneous autoimmune disease in mice. Nat Med 10:920-926 (2004). DOI: 10.1038/nm1092. https://doi.org/10.1038/nm1092
  3. Badami E, Maiuri L, Quaratino S. High incidence of spontaneous autoimmune thyroiditis in immunocompetent self-reactive human T cell receptor transgenic mice. J Autoimmun 24:85-91 (2005). DOI: 10.1016/j.jaut.2005.01.003. https://doi.org/10.1016/j.jaut.2005.01.003
  4. Wojciech L, Ignatowicz A, Seweryn M, Rempala G, Pabla SS, McIndoe RA, Kisielow P, Ignatowicz L. The same self-peptide selects conventional and regulatory CD4+ T cells with identical antigen receptors. Nat Commun 5:5061 (2014). DOI: 10.1038/ncomms6061. https://doi.org/10.1038/ncomms6061
  5. Weiss EM, Schmidt A, Vobis D, Garbi N, Lahl K, Mayer CT, Sparwasser T, Ludwig A, Suri-Payer E, Oberle N, Krammer PH. Foxp3-mediated suppression of CD95L expression confers resistance to activation-induced cell death in regulatory T cells. J Immunol 187:1684-1691 (2011). DOI: 10.4049/jimmunol.1002321. https://doi.org/10.4049/jimmunol.1002321
  6. Kaminitz A, Yolcu ES, Askenasy EM, Stein J, Yaniv I, Shirwan H, Askenasy N. Effector and naturally occurring regulatory T cells display no abnormalities in activation induced cell death in NOD mice. PLoS One 6:e21630 (2011). DOI: 10.1371/journal.pone.0021630. https://doi.org/10.1371/journal.pone.0021630
  7. Plaza-Sirvent C, Schuster M, Neumann Y, Heise U, Pils MC, Schulze-Osthoff K, Schmitz I. c-FLIP expression in Foxp3-expressing cells is essential for survival of regulatory T cells and prevention of autoimmunity. Cell Rep 18:12-22 (2017). DOI: 10.1016/j.celrep.2016.12.022. https://doi.org/10.1016/j.celrep.2016.12.022
  8. Bailey-Bucktrout SL, Martinez-Llordella M, Zhou X, Anthony B, Rosenthal W, Luche H, Fehling HJ, Bluestone JA. Self-antigen-driven activation induces instability of regulatory T cells during an inflammatory autoimmune response. Immunity 39:949-962 (2013). DOI: 10.1016/j.immuni.2013.10.016. https://doi.org/10.1016/j.immuni.2013.10.016
  9. Horellou P, de Chalus A, Giorgi L, Leroy C, Chrétien P, Hacein-Bey-Abina S, Bourgeois C, Mariette X, Serguera C, Le Grand R, Deiva K. Regulatory T cells increase after rh-MOG stimulation in non-relapsing but decrease in relapsing MOG antibody-associated disease at onset in children. Front Immunol 12:679770 (2021). DOI: 10.3389/fimmu.2021.679770. https://doi.org/10.3389/fimmu.2021.679770
  10. Li M, Luo L, Wu Y, Song Z, Ni B, Hao F, Luo N. Elevated apoptosis and abnormal apoptosis signaling of regulatory T cells in patients with systemic lupus erythematosus. Lupus 31:1441-1455 (2022). DOI: 10.1177/09612033221119455. https://doi.org/10.1177/09612033221119455
  11. Wang Y, Jia Y, Guo H, Feng M, Qin Y, Liang Z, Zhao Z, Gao C, Luo J. Reduced circulating regulatory T cells in primary Sjögren's syndrome: the contribution of enhanced apoptosis and impaired survival. Front Immunol 16:1603305 (2025). DOI: 10.3389/fimmu.2025.1603305. https://doi.org/10.3389/fimmu.2025.1603305
  12. Huysentruyt J, Steels W, Ruiz Perez M, Verstraeten B, Vadi M, Divert T, Flies K, Takahashi N, Lambrecht BN, Declercq W, Vanden Berghe T, Maelfait J, Vandenabeele P, Tougaard P. RIPK1 protects naive and regulatory T cells from TNFR1-induced apoptosis. Cell Death Differ 31:820-832 (2024). DOI: 10.1038/s41418-024-01301-w. https://doi.org/10.1038/s41418-024-01301-w
  13. Eskandari SK, Allos H, Al Dulaijan BS, Melhem G, Sulkaj I, Alhaddad JB, Saad AJ, Deban C, Chu P, Choi JY, Kollar B, Pomahac B, Riella LV, Berger SP, Sanders JSF, Lieberman J, Li L, Azzi JR. mTORC1 inhibition protects human regulatory T cells from granzyme-B-induced apoptosis. Front Immunol 13:899975 (2022). DOI: 10.3389/fimmu.2022.899975. https://doi.org/10.3389/fimmu.2022.899975
  14. Lo WL, Huseby ES. The partitioning of TCR repertoires by thymic selection. J Exp Med 221:e20230897 (2024). DOI: 10.1084/jem.20230897. https://doi.org/10.1084/jem.20230897
  15. Li L, Guo J, He R, Chai X, Guo C, Xu W, Cao X, Jin Q, Zhang F, Hou B, Zhao F, Zhou X. TCR signal strength determines Treg instability and discrimination of self versus non-self antigens. bioRxiv preprint (2026). DOI: 10.64898/2026.02.12.705537. https://doi.org/10.64898/2026.02.12.705537 (preprint, not peer-reviewed)