Infectious Tolerance

Infectious tolerance is an active, self-propagating peripheral tolerance mechanism in which immunological unresponsiveness established in an initial cohort of T lymphocytes is transmitted to newly recruited, naive, immunocompetent T cells, reprogramming them into secondary regulatory T cells (T_reg) that sustain long-term antigen-specific tolerance even after the original tolerized cohort is depleted or removed. First conceptualized in 1971 by Richard K. Gershon and Kazunari Kondo using sheep red blood cell adoptive transfer experiments and placed on a definitive cellular and molecular foundation in 1993 by Herman Waldmann, Shixin Qin, and Stephen P. Cobbold using non-depleting anti-CD4 and anti-CD8 monoclonal antibodies in murine skin and cardiac allograft models, infectious tolerance explains how a brief, transient pulse of therapeutic co-receptor or costimulation blockade can establish permanent graft acceptance and durable protection against autoimmune tissue destruction without continuous pharmacological immunosuppression.

Gershon and Kondo's Adoptive Transfer Paradigm (1971)

Prior to the 1970s, acquired immunological tolerance—as formulated by Peter Medawar, Rupert Billingham, and Leslie Brent (1953)—was attributed to the passive deletion or intrinsic clonal paralysis of antigen-reactive lymphocytes (central tolerance). If tolerance were merely the absence of reactive clones, infusing normal naive lymphocytes into a tolerized host should restore graft rejection or antibody production, whereas transferring splenocytes from a tolerized donor into a normal recipient should have no suppressive effect.

In 1971, Richard Gershon and Kazunari Kondo demonstrated that thymocytes from mice rendered tolerant to high doses of sheep erythrocytes, when adoptively co-transferred with normal bone marrow and naive T cells into irradiated syngeneic recipients, prevented the naive lymphocytes from mounting an anti-erythrocyte plaque-forming response. Gershon coined the phrase "infectious immunological tolerance" to describe how tolerant T cells actively spread unresponsiveness to otherwise competent lymphocytes, analogous to the transmission of an infectious agent across a host population.

Waldmann and Qin's Non-Depleting Antibody Allograft Model (1993)

Because early suppressor-cell studies lacked lineage-specific molecular markers, the concept remained controversial until Shixin Qin, Stephen Cobbold, Herman Waldmann, and colleagues (1993) demonstrated infectious tolerance in a genetically defined transplantation model using thymectomized CBA/Ca (H-2^k) mice grafted with multiple minor histocompatibility antigen-mismatched B10.BR (H-2^k) skin under a short course of non-depleting anti-CD4 (YTS 177.9) and anti-CD8 (YTS 105.18) monoclonal antibodies:

1. Primary Tolerance Induction: A brief 2-week course of non-depleting anti-CD4 antibody permitted permanent survival (>200 days) of mismatched skin allografts long after the therapeutic antibody had cleared from the circulation.
2. Resistance to Naive Lymphocyte Infusion: Infusing 5 × 10⁷ naive syngeneic CBA splenocytes into tolerized mice failed to trigger graft rejection; instead, the infused naive cohort became functionally silenced within the tolerant host.
3. Serial Passaging Across Cohorts: When Waldmann's group co-transferred CD4⁺ T cells from tolerized mice alongside genetically marked (Thy-1.1 vs. Thy-1.2) naive CD4⁺ T cells into secondary recipients bearing fresh B10.BR skin grafts—and then selectively depleted the original Thy-1.2⁺ tolerant donor T cells after 6 weeks—the formerly naive Thy-1.1⁺ cohort had acquired autonomous regulatory activity and could itself transmit tolerance to a third cohort of naive T cells in tertiary recipients.

The Dendritic Cell Bridge and Shared Microenvironment

Infectious tolerance does not occur through direct T-cell-to-T-cell contact in fluid suspension; rather, it requires a three-cell clustering interaction in which preexisting FOXP3⁺ regulatory T cells and incoming naive CD4⁺Foxp3⁻ T cells simultaneously engage peptide–major histocompatibility complex class II (pMHC-II) complexes presented on the surface of the same dendritic cell across an immunological synapse. Because the two T cells recognize antigens displayed on the same antigen-presenting cell (APC), infectious tolerance operates hand-in-hand with linked suppression, whereby regulatory T cells specific for a single tolerizing epitope A recondition the shared dendritic cell to convert naive T cells specific for either epitope A or a co-presented third-party antigen B into induced Foxp3⁺ regulatory T cells (pT_reg) or IL-35-producing iTr35 cells.

Contact-Dependent TGF-β / GARP Signaling and FOXP3 CNS1 Remodeling

As established by Wanjun Chen, Alexander Rudensky, and Ethan Shevach, the primary molecular engine of infectious tolerance is membrane-bound transforming growth factor-beta 1 (TGF-β1). Activated FOXP3⁺ T_reg cells express the transmembrane anchor GARP (Glycoprotein A Repetitions Predominant / LRRC32), which binds latent TGF-β complexed with its latency-associated peptide (LAP). Upon TCR engagement on a dendritic cell, integrin α_vβ₈ expressed at the T_reg–DC synaptic interface mechanically unfolds LAP, releasing biologically active TGF-β1 across a nanometer-scale pericellular radius directly onto neighboring naive CD4⁺ T cells docked on the same dendritic cell.

When active TGF-β1 ligates the tetrameric TGF-β receptor complex on the naive T cell—concurrently with sub-immunogenic TCR stimulation (low CD80/CD86 costimulation due to CTLA-4 trogocytosis and mTOR inhibition)—phosphorylated SMAD2 and SMAD3 translocate to the nucleus and bind the CNS1 (Conserved Non-coding Sequence 1) enhancer within intron 1 of the FOXP3 locus alongside NFAT, all-trans retinoic acid receptor (RAR/RXR), and FoxO1/FoxO3a. Mice carrying a targeted deletion of the Foxp3 CNS1 element retain normal thymic T_reg development (tT_reg) but lose the ability to generate peripherally induced pT_reg cells, abolishing infectious tolerance at mucosal and maternal-fetal interfaces.

Interleukin-35 and the iTr35 Self-Amplifying Cascade

In addition to generating Foxp3⁺ pT_reg cohorts, Dario Vignali and colleagues (2007–2010) discovered a second, Foxp3-independent arm of infectious tolerance driven by the inhibitory heterodimeric cytokine interleukin-35 (IL-35), composed of the Epstein-Barr virus-induced gene 3 (Ebi3) subunit paired with the IL-12α (p35) subunit. When naive conventional CD4⁺Foxp3⁻ T cells are activated in the presence of T_reg-derived IL-35, signaling through the unconventional IL-12Rβ2:gp130 receptor heterodimer induces a unique STAT1:STAT4 heterodimer that transcribes Ebi3 and Il12a without inducing Foxp3, Il10, or Tgfb1. The resulting induced regulatory population—termed iTr35 cells—secretes high concentrations of IL-35, which in turn converts subsequent waves of naive T cells into second- and third-generation iTr35 cells within allografts and tumor microenvironments.

Metabolic Reprogramming of the Dendritic Cell Microenvironment

Herman Waldmann and Stephen Cobbold demonstrated that T_reg cells also reprogram the antigen-presenting dendritic cell into a metabolically tolerogenic microenvironment by inducing four amino-acid-catabolizing enzymes—indoleamine 2,3-dioxygenase 1 (IDO1) (catabolizing tryptophan), arginase-1 (ARG1) (catabolizing L-arginine), branched-chain amino acid aminotransferase 1 (BCAT1) (depleting leucine/isoleucine/valine), and interleukin-4-induced gene 1 (IL4I1) (catabolizing phenylalanine). Local amino acid depletion activates the uncharged-tRNA sensor GCN2 kinase and suppresses the nutrient sensor mTORC1 in naive T cells docked on that dendritic cell. Because active mTORC1 phosphorylates and excludes FoxO1/3a from the nucleus to favor pro-inflammatory T_H1/T_H17 differentiation, mTORC1 inhibition by T_reg-conditioned dendritic cells forces the naive T cell down the FOXP3⁺ regulatory lineage.

Understanding infectious tolerance has reshaped therapeutic protocols in solid-organ transplantation, hematopoietic stem cell transplantation (alongside donor veto cells), and autoimmune disease:

• Calcineurin-Inhibitor Sparing vs. mTOR Inhibition: Because infectious tolerance requires both TCR-driven calcineurin–NFAT signaling and IL-2–STAT5 signaling to transcribe and epigenetically stabilize FOXP3, high-dose cyclosporine A or tacrolimus (which block calcineurin) can paradoxically abort infectious tolerance induction. Conversely, sirolimus (rapamycin)—which selectively inhibits mTORC1 while sparing calcineurin and STAT5—synergizes with costimulatory blockade (belatacept / CTLA-4–Ig or anti-CD40L) to expand pT_reg cohorts and promote infectious tolerance.
• Adoptive T_reg Cell Therapies (ONE Study): Clinical trials infusing ex vivo-expanded polyclonal or chimeric antigen receptor (CAR-T_reg) cells into kidney and liver transplant recipients rely on infectious tolerance so that the short-lived infused donor-specific T_reg product educates endogenous host naive T cells into a self-renewing, graft-resident regulatory repertoire.

1. Waldmann, H., & Cobbold, S. (1998). "How do monoclonal antibodies induce tolerance? A role for infectious tolerance?" Annual Review of Immunology, 16(1), 619–644. https://doi.org/10.1146/annurev.immunol.16.1.619
2. Qin, S., Cobbold, S. P., Pope, H., Elliott, J., Kioussis, D., Davies, J., & Waldmann, H. (1993). "'Infectious' transplantation tolerance." Science, 259(5097), 974–977. https://doi.org/10.1126/science.8094901
3. Collison, L. W., Chaturvedi, V., Henderson, A. L., Giacomin, P. R., Guy, C., Bankoti, J., Finkelstein, D., Forbes, K., Workman, C. J., Brown, S. A., Rehg, J. E., Jones, M. L., Ni, H. T., Artis, D., Turk, M. J., & Vignali, D. A. A. (2010). "IL-35-mediated induction of a potent regulatory T cell population." Nature Immunology, 11(12), 1093–1101. https://doi.org/10.1038/ni.1952
4. Kendal, A. R., Chen, Y., Regateiro, F. S., Ma, J., Adams, E., Cobbold, S. P., Hori, S., & Waldmann, H. (2011). "Sustained suppression by Foxp3⁺ regulatory T cells is vital for infectious tolerance in control of T cell-mediated colitis." Journal of Experimental Medicine, 208(10), 2043–2053. https://doi.org/10.1084/jem.20110767