**9.4 Apoptosis of T cells due to persistent activation or neglect**

Apoptosis, or programmed cell death, of lymphocytes is an important mechanism of immune control and homeostasis. Apoptosis contributes to the deletion of clones that are persistently activated and of activated lymphocytes when the immune response is no longer needed (e.g., after an infection clears). Cells that are persistently stimulated undergo activation-induced cell death involving Fas-FasL signaling or tumor necrosis factor. Most T cells that remain after antigen clearance are deprived of the stimuli required to survive and undergo passive cell death. Apoptosis of donor-reactive lymphocytes is also known as the "deletional" method to induce tolerance; in theory, this represents the most fail-safe mechanism of tolerance induction. In the absence of donor-reactive lymphocytes, the response to donor antigens could not be induced no matter what antigens are encountered.

### **9.5 Clonal anergy**

116 Liver Transplantation – Basic Issues

The chief mechanism of T-cell tolerance is the deletion of autoreactive T cells in the thymus, rendering the organism tolerant to "self." Immature T cells migrate from the bone marrow to the thymus, where they encounter peptides derived from endogenous proteins that are bound to major histocompatibility complex (MHC) molecules on thymic epithelial cells.

Double-positive (CD4+ and CD8+) thymocytes initially undergo random generation of different T-cell receptors (TCRs). Positive selection, also called thymic education, ensures that only clones with TCRs that exhibit moderate affinity for self-MHC are allowed to develop. Negative selection by means of apoptosis occurs when T cells do not produce functional TCRs, when TCR rearrangement fails, when T cells have low affinity for MHCself-peptide complexes, or when T cells have extremely high affinity for such complexes. Negative selection also results in the deletion of some thymocytes that interact with autoantigens presented by interdigitating cells and macrophages at the corticomedullary junction. The remaining cells lose either CD4 or CD8 and leave the thymus to function in the

Many potentially reactive T cells escape intrathymic deletion, reflecting the fact that many antigens are absent intrathymically or are present at insufficient levels to induce tolerance in the thymus. Several peripheral, nonthymic mechanisms that prevent autoimmunity by

Some antigens are sequestered into privileged sites away from the immune system because of physical barriers, such as tight junctions, or immunologic barriers, such as the expression of Fas ligand (FasL) or reduced MHC class I expression. Thus, antigen-presenting cells (APCs), and subsequently T lymphocytes, may never encounter these self-antigens. Therefore, immune cells remain ignorant of these antigens. At some of these sites, proinflammatory lymphocytes are controlled by apoptosis due to the expression of FasL or the secretion of cytokines such as transforming growth factor-beta (TGF-β) or interleukin (IL)-10. When T cells enter these sites, their Fas receptors interact with the FasL of these sites, and they undergo apoptosis. Privileged sites include the brain, the testes, and the anterior chamber of the eye. Transplanted tissues are most likely to survive in these

Apoptosis, or programmed cell death, of lymphocytes is an important mechanism of immune control and homeostasis. Apoptosis contributes to the deletion of clones that are persistently activated and of activated lymphocytes when the immune response is no longer needed (e.g., after an infection clears). Cells that are persistently stimulated undergo activation-induced cell death involving Fas-FasL signaling or tumor necrosis factor. Most T cells that remain after antigen clearance are deprived of the stimuli required to survive and undergo passive cell death. Apoptosis of donor-reactive lymphocytes is also known as the "deletional" method to induce tolerance; in theory, this represents the most fail-safe

privileged sites because of the tight control of proinflammatory lymphocytes.

**9.4 Apoptosis of T cells due to persistent activation or neglect** 

*Central and/or Intrathymic Tolerance* 

*Peripheral tolerance* 

periphery as mature, functional CD4+ and CD8+ T cells[31].

rendering peripheral T cell repertoires tolerant also exist[32].

**9.3 Sequestration of antigens into privileged sites** 

T lymphocytes require 2 signals to become activated, proliferate, and differentiate. The first is the recognition of an appropriate MHC-antigen complex by the TCR of the responsive lymphocyte. The second signal is delivered by costimulatory molecules also expressed by APCs; Costimulatory ligands are only able to engage once the first signal is activated. Lack of costimulation causes anergy, when T cells fail to respond to the MHC-peptide complex and remain unresponsive to subsequent challenges.

CD28 is the main costimulatory ligand expressed by naive T cells encountering antigen. CD28 signaling enhances T-cell proliferation by boosting T cell IL-2 production. It also enhances expression of CD40 ligand, which interacts with CD40 on APCs to induce the upregulation of the costimulatory molecules CD80 (B7-1) and CD86 (B7-2) to further enhance costimulatory signaling.

Recently, Rigby et al. used inhibition of T-cell costimulation as an effective means to prevent autoimmunity and allograft rejection in multiple animal models. They studied the effects of anti-CD28 and CTLA4-Ig on diabetes development and the requirements to induce tolerance in nod/scid mice after the transfer of transgenic beta-cell reactive BDC2.5.NOD T-cells. These authors were successful in this set of experiments and have helped to develop the understanding of natural regulatory mechanisms that may have a unique role in establishing targeted, long-standing immune protection and peripheral tolerance.

T lymphocytes also express CD152 (CTLA-4) after CD28 binds to its ligands B7-1 and B7-2 on APCs. The interaction of CTLA-4 and B7 molecules decreases opportunities for B7-CD28 binding and downregulates T-cell IL-2 production, which subsequently reduces T-cell proliferation. CD28 interacts with B7 molecules, first leading to T-cell activation. However, after this effect peaks, upregulation of CTLA-4 with its relatively high affinity for B7 molecules limits the degree of activation. Verbinnen et al. recently studied the involvement of regulatory T cells (Treg) and deletion of alloreactive cells in the induction and maintenance of tolerance after costimulation blockade (CTLA-4) in a mouse model of graftvs.-host disease. The study showed that clonal deletion of host-reactive T cells was a major mechanism responsible for tolerance.
