Induction of Xenoreactive CD4+ T-Cell Anergy by Suppressor CD8+CD28⁻ T Cells. Transplantation 2000; 69: 1304.

Colovai AI, Liu Z, Ciubotariu R, Lederman S, Cortesini R, and Suciu-Foca N.

REGULATION OF XENO-REACTIVE CD4⁺ T CELLS BY CD8⁺CD28⁻ T SUPPRESSOR CELLS

Achievement of antigen-specific tolerance has been the long-term goal in transplantation immunology. The ultimate aim is the establishment of therapeutic modalities that specifically suppress allo- or xenoreactive effector T cells, leaving the rest of the T-cell repertoire unaffected. The induction of antigen-specific tolerance may also solve the problem of chronic rejection, a major hurdle for long-term survival of solid organ allografts.

Antigen-specific tolerance can be induced either in the immature T-cell population through thymic education or in the mature T-cell population through peripheral induction. Exposure of a foreign antigen to T cells undergoing maturation in the thymus results in a deletion of immature T cells with high affinity for that particular antigen. Several studies have attempted to use whole molecules or peptides derived from donor MHC injected into the recipient thymus in an attempt to induce central tolerance to the graft; however, these attempts have had only limited success.

Most of the peripheral tolerance induction strategies can be categorized into clonal deletion, anergy, immune deviation, or suppression. T suppressor (Ts) cells were first described in the 1970s on the basis of their capacity to suppress an immune response against a particular haptenized antigen as well as their capacity to transfer suppressive function to a naïve recipient upon transfer of cells (¹). Thymocytes from DNP-protein hyperimmunized rats could suppress the antibody response to DNP in the same rat upon antigenic challenge. These DNP-specific Ts thymocytes were also able to suppress antibody responses to DNP upon adoptive transfer into a naïve rat. Efforts for defining Ts cell clones have been met with only very limited success. Many of the early experiments can now be categorized as forms of immune deviation, which involves the T helper (Th)1/Th2 dichotomy of cytokine production and mutual regulation. Some antigens can elicit CD4⁺ Th1 responses, which lead to the secretion of cytokines including interleukin (IL)-2, interferon-γ, and IL-12. The interferon-γ and IL-12 secreted by Th1 cells promote Th1 differentiation and suppress Th2 cell proliferation, a possible explanation for many of the early experiments. The adoptively transferred differentiated lymphocyte population containing mostly Th1 cells can also deviate the naïve recipient T cells away from the Th2 response required for antibody production. Recently, the dichotomy function of CD4⁺ Th cells has also been found in CD8⁺ T cells, suggesting that both subpopulations can deviate the immune response toward one direction or another.

It is now clear that not all forms of suppression can be explained by immune deviation. The functional evidence for the existence of antigen-specific Ts cells has been obtained in several transplantation models. The most well-characterized Ts cells are of the CD8⁺ phenotype. It has been shown that some CD8⁺ T cells stimulated by alloantigens are able to specifically suppress T-cell responses directed against the antigen recognized by the CD8⁺ Ts cell. The mechanisms of suppression are still poorly understood, and both T cells and non-T cells have been attributed to have suppressor function. In some instances, this suppressor function may involve transduction of signal through the MHC class I molecule. Other mechanisms of suppression may involve ligand competition, lysis of effector cells (²) as well as negative signaling mediated by CD8⁺ veto cells (³). The veto signal is delivered by the interaction of the CD8 molecule on the veto cell with a MHC class I molecule on the target T cell when the target cell is simultaneously being activated through its T cell receptor by a peptide/MHC complex presented by the veto cell. The final effect of this dual signaling is the deletion by apoptosis of the responding T cells.

In this issue, Colovai et al. report an important study in which they define the functional characteristics of a xenoreactive CD8⁺CD28⁻ Ts cell subpopulation. Along with other studies published by the same group, this report presents a novel pathway of immune suppression mediated by a unique group of Ts cells. The authors show that specific stimulation with xenoantigen-pulsed antigen-presenting cells (APC) leads to the generation of this unique CD8⁺CD28⁻ Ts cell subpopulation, capable of specifically blocking the proliferative response of Th cells to the same xenoantigen. In contrast to the immune deviation, which involves the production of mutually regulating Th1/Th2 cytokines patterns, the ability of these Ts cells to inhibit Th proliferation has been shown to be independent of cytokine production and requires direct cell-to-cell interaction with the stimulating APC. In addition, unlike most of the other proposed mechanisms of suppression, these CD8⁺CD28⁻ Ts cells seem to exert their suppressive function by inhibiting the costimulatory signals by blocking the up-regulation of CD40 ligand (CD40L) expression on the responding T cells and inhibiting the endogenous production of IL-2. Interestingly, the suppression could be reversed by the addition of exogenous IL-2, indicating that the suppressor activity was through the induction of anergy of the responding Th cells.

The interaction between CD40L on T cells and CD40, which is expressed not only on activated B cells but also on a variety of APCs, has been shown to play an important role in providing cognate help for B-cell and APC activation in part by up-regulating costimulatory molecule B7. This interaction also leads to T-cell activation. Therefore, it is not surprising that blocking CD40L/CD40 interactions in vivo by the combination of CTLA4Ig and anti-CD40L antibody could result in marked graft prolongation (⁴). The mechanism that the authors proposed to explain the immunosuppressive effects of the CD8⁺CD28⁻ Ts cells is not mutually exclusive to the deviation model proposed previously. CD40/CD40L blockade has been reported to be associated with a Th2-dominant immune response or a reduction in the production of nitric oxide, an important mediator produced by activated macrophages. The interesting results by Colovai et al. presented in this issue clearly demonstrate that allo- or xenoreactive Ts may be generated in vitro with the capability to prevent T cell-mediated immune responses against the allo- or xenograft. Once again, interest in Ts cells is rekindled, and their possible role in inducing tolerance of allo- and xenografts in vivo awaits further experimentation.