Veto Cell

Veto cell activity is a unidirectional, contact-dependent mechanism of peripheral tolerance and transplantation tolerance in which a donor-derived hematopoietic cell—most potently an activated CD8⁺ cytotoxic T lymphocyte (CTL), an interleukin-2-activated natural killer (NK) cell, or a CD34⁺ hematopoietic stem and progenitor cell (HSPC)—selectively deletes or functionally inactivates any host alloreactive T-cell precursor whose T-cell receptor (TCR) recognizes peptide–major histocompatibility complex (pMHC) antigens displayed directly on the plasma membrane of the veto cell itself. Discovered and named in 1980 by Richard G. Miller and Derry C. Roopenian at the Ontario Cancer Institute, the veto phenomenon reverses the polarity of a standard immunological encounter: instead of the recognizing T-cell precursor killing its target cell, the target cell ("veto cell") exploits the incoming TCR engagement to deliver a lethal apoptotic signal back into the recognizing T cell, thereby purging the host repertoire of graft-rejecting clones without harming T cells specific for unrelated pathogens.

Miller's 1980 Paradox: Target Cells That Kill Their Recognizing Effector Precursors

In 1980, Richard G. Miller investigated why sublethally irradiated mice infused with semi-allogeneic or nu/nu bone marrow cells failed to generate cytotoxic T lymphocytes (CTLs) against the donor MHC haplotype in mixed lymphocyte reactions (MLRs), yet mounted normal CTL responses against third-party MHC haplotypes. Miller demonstrated that a specialized subpopulation of donor lymphoid cells acted as "active bait":

1. Unidirectional Recognition: The donor veto cell does not use its own TCR to recognize the host T cell. Indeed, veto cells derived from (Donor × Host) F₁ hybrids, TCR-transgenic mice specific for an irrelevant viral peptide (such as ovalbumin or influenza), or TCR-negative NK cells and CD34⁺ bone marrow progenitors exert full veto suppression.
2. Strict MHC Self-Display Requirement: Conversely, the host alloreactive T-cell precursor (pCTL) must use its own αβ TCR to bind MHC class I (or MHC class II) molecules expressed on the surface of the donor veto cell. When the host pCTL forms an immunological synapse with the donor veto cell, the pCTL is clonally deleted within 24 to 72 hours. Any host T cell sitting in the same culture well whose TCR recognizes a third-party MHC molecule not expressed by the veto cell is completely spared.

Over four decades of experimental and clinical transplantation research—led by Richard G. Miller, Hans-Georg Rammensee, Pamela J. Fink, Michael J. Bevan, and Yair Reisner—have identified a hierarchy of hematopoietic cells capable of mediating veto deletion:

The Dual-Hit Model: Fas–FasL Engagement Coupled to CD8 α3 Domain Ligation

How does a CD8⁺ veto T cell kill an attacking host alloreactive pCTL without being lysed by the pCTL first? Yair Reisner, Esther Bachar-Lustig, and colleagues (1999–2013) resolved this question by demonstrating that maximal veto deletion requires the simultaneous engagement of two distinct molecular axes at the conjugate interface between the recognizing host pCTL and the donor CD8⁺ veto cell:

1. Hit 1 — TCR Activation and Upregulation of Fas (CD95) on the Host Precursor CTL:

When a resting host alloreactive CD8⁺ T-cell precursor recognizes donor peptide–MHC class I on the surface of the veto cell (Signal 1), the host T cell enters early activation and rapidly upregulates the death receptor Fas (CD95 / APO-1) on its plasma membrane. However, during the first 24 to 48 hours after primary antigen encounter, a naive pCTL has not yet synthesized mature cytolytic granules containing perforin and granzyme B, leaving a vulnerability window before it can lyse its target.

1. Hit 2 — Simultaneous FasL Trimerization and Inhibitory CD8 α3 Domain Engagement by the Veto Cell:

During this 24-to-48-hour window, the pre-activated donor CD8⁺ veto cell delivers a coordinated two-pronged counterstrike:

• FasL–Fas Apoptotic Signaling: The donor veto cell expresses high surface levels of Fas Ligand (FasL / CD178), which trimerizes the newly upregulated Fas receptors on the conjugated host pCTL to assemble the Death-Inducing Signaling Complex (DISC: FADD + Procaspase-8).
  • CD8-to-MHC Class I Reverse/Inhibitory Signaling (The c-FLIP Downregulation Gate): Crucially, Fas–FasL engagement alone is insufficient to kill a newly activated T cell because TCR activation simultaneously upregulates high intracellular levels of c-FLIP (Cellular FLICE-Like Inhibitory Protein), which competitively blocks Caspase-8 activation at the DISC. However, the α3 domain of the CD8 molecule expressed on the donor veto cell simultaneously binds to the non-polymorphic α3 domain of MHC class I molecules expressed on the surface of the recognizing host pCTL. As demonstrated by Reisner and colleagues using Fas/Fasl-mutant (lpr/gld) mice and blocking anti-CD8 antibodies, engagement of MHC class I on the host pCTL by the veto cell's CD8 molecule triggers an inhibitory tyrosine phosphorylation cascade (involving MEK/ERK phosphorylation and down-modulation of phosphorylated Akt and XIAP/c-FLIP) inside the host pCTL. Stripped of protective c-FLIP and XIAP, the host pCTL undergoes rapid Caspase-8 and Caspase-3-mediated apoptosis.

Perforin/Granzyme Participation and TNF-α in CD34⁺ Stem Cells

In addition to the FasL + CD8 pathway, early-phase veto killing by memory-phenotype CD8⁺ veto cells utilizes perforin and granzyme B polarized toward the synaptic cleft via LFA-1 / ICAM-1 adhesion—triggered not by the veto cell's TCR, but by reverse signaling through the veto cell's MHC class I and CD8 molecules upon conjugate formation. Conversely, in human CD34⁺ hematopoietic stem and progenitor cells—which lack CD8 and perforin—veto deletion of host anti-donor CTL precursors is mediated primarily by membrane-bound Tumor Necrosis Factor-alpha (mTNF-α) on the CD34⁺ progenitor ligating TNFR1 (p55) and TNFR2 (p75) on the recognizing host T cell alongside paracrine TGF-β1.

1. Megadose CD34⁺ Haploidentical Hematopoietic Stem Cell Transplantation

For leukemia and lymphoma patients who lack an HLA-matched sibling or unrelated registry donor, nearly every patient has a parent, child, or sibling who is HLA-haploidentical (sharing exactly one chromosome 6 haplotype, or 5 of 10 HLA alleles). Historically, haploidentical bone marrow transplantation across a full three-locus HLA barrier failed due to two lethal complications:

• If donor T cells were left in the graft, they caused fatal Graft-Versus-Host Disease (GVHD).
• If donor T cells were stringently depleted to prevent GVHD, residual host radioresistant T cells rejected the graft (Graft Failure).

In 1994–1998, Yair Reisner and Massimo F. Martelli (Perugia University) discovered that escalating the dose of granulocyte colony-stimulating factor (G-CSF)-mobilized, rigorously T-cell-depleted CD34⁺ stem cells by 10-fold—to a "megadose" exceeding 10 × 10⁶ CD34⁺ cells/kg body weight—enabled >95% primary engraftment across full haploidentical barriers without GVHD. Because the infused CD34⁺ progenitors outnumber residual host radioresistant anti-donor pCTLs within bone marrow niches, the veto activity of the CD34⁺ cells (and their rapidly differentiating CD33⁺ immature myeloid progeny) deletes host graft-rejecting T cells in situ.

2. Engineered Anti-Third-Party Central Memory CD8⁺ Veto T Cells (T_cm)

To achieve haploidentical hematopoietic engraftment and mixed hematopoietic chimerism (as a platform for kidney or heart allograft tolerance without lifelong immunosuppression, complementing infectious tolerance and linked suppression) under reduced-intensity conditioning (RIC) without toxic myeloablative radiation, Reisner's group developed a clinical protocol to manufacture non-alloreactive anti-third-party CD8⁺ central memory (CD62L⁺CD44⁺) veto T cells:

1. Donor CD8⁺ T cells are stimulated ex vivo for 6 days against irradiated third-party viral peptides (EBV, CMV, Adenovirus) under IL-2 Deprivation, selectively starving and eliminating any clones reactive against the host's HLA haplotype (thus eliminating GVHD risk).
2. The surviving viral-specific CD8⁺ cells are expanded in IL-15 and IL-7, programming them into CD62L⁺CCR7⁺ central memory T cells (T_cm) that home efficiently through high endothelial venules directly into host lymph nodes and bone marrow.
3. Upon infusion into the recipient, these donor-derived CD8⁺ T_cm veto cells co-localize with host lymph-node T cells, selectively deleting host anti-donor alloreactive T-cell clones via the FasL + CD8 α3 veto mechanism while simultaneously providing anti-viral immunity against EBV and CMV reactivation.

1. Miller, R. G. (1980). "An immunological suppressor cell inactivating cytotoxic T-lymphocyte precursor cells recognizing it." Nature, 287(5782), 544–546. https://doi.org/10.1038/287544a0
2. Fink, P. J., Shimonkevitz, R. P., & Bevan, M. J. (1988). "Veto cells." Annual Review of Immunology, 6(1), 115–137. https://doi.org/10.1146/annurev.iy.06.040188.000555
3. Aversa, F., Tabilio, A., Velardi, A., Cunningham, I., Terenzi, A., Falzetti, F., Ruggeri, L., Barbabietola, G., Aristei, C., Latini, P., Reisner, Y., & Martelli, M. F. (1998). "Treatment of high-risk acute leukemia with T-cell-depleted stem cells from related donors with one fully mismatched HLA haplotype." New England Journal of Medicine, 339(17), 1186–1193. https://doi.org/10.1056/NEJM199810223391702
4. Reich-Zeliger, S., Zhao, Y., Krauthgamer, R., Bachar-Lustig, E., & Reisner, Y. (2000). "Anti-third party CD8⁺ CTLs as potent veto cells: coexpression of CD8 and FasL is a prerequisite." Immunity, 13(4), 507–515. https://doi.org/10.1016/S1074-7613(00)00050-900050-9)