Self determinant selection and acquisition of the autoimmune T cell repertoire

Natural killer cells in rejection and tolerance of solid organ allografts

PURPOSE OF REVIEW

A series of recent studies defy conventional wisdom by showing that natural killer (NK) cells exert a powerful and long-lasting influence on the immune response to whole organ allografts. The early activation of NK cells following transplantation is associated with killing of allogeneic target cells and release of immunomodulatory chemokines and cytokines, which can contribute to either rejection or tolerance. Here, we review findings describing NK cell receptors, potential mediators and mechanisms underlying the dual influence of NK cells in solid organ transplantation.

RECENT FINDINGS

New studies show that NK cells can discriminate between self and foreign tissues and play a key role in the initiation and regulation of adaptive immune responses after solid organ transplantation. Depending upon the types of NK cell receptors engaged and the nature of cytokines released, early NK cell activation can promote either rejection or tolerance.

SUMMARY

Solid organ transplantation is associated with the early activation of NK cells, which are then licensed to kill allogeneic target cells directly or via antibody-dependent cellular cytotoxicity and release various chemokines and immunomodulatory cytokines. Depending upon the nature of NK cell subsets activated and their ability to kill allogeneic target cells and release certain types of cytokines, NK cells can promote the activation/expansion of pro-inflammatory Th1 cells or regulatory Th2/Treg cells thus tilting the balance of alloimmunity towards rejection or tolerance. An in-depth understanding of these mechanisms will be necessary in order to design therapies targeting NK cells in human transplantation.

Functional and phenotypic properties of peripheral T cells anergized by autologous CD3(+) depleted bone marrow cells

We have previously demonstrated that bone marrow cells (BMC) inhibit the generation of autologous Epstein-Barr virus (EBV) -specific cytotoxic T lymphocytes (CTL). It was also observed that CD3(+) cells obtained after 7 days of culture in the presence of autologous BMC could be used as inhibitors of EBV-CTL generation. In the present study, we examined these BMC induced regulator CD3(+) T cells with respect to phenotype, function, and T-cell activation pathways. We also questioned if the CD3(+) regulatory cell function is mediated by their direct effect on peripheral T cells or on the ability of antigen presenting cells (APC) to stimulate peripheral T cells. To answer this, CD3(+) cells from peripheral blood lymphocytes (PBL) were cultured with either CD3-depleted BMC or with CD3-depleted PBL. The CD3(+) cells were then isolated with immunomagnetic beads, designated as T(BM) and T(PBL), and were compared in functional studies. There was an increase in the expression of CD25 on T(BM) cells. The T(BM) cells also expressed less CD122 and a decreased number of CD3 molecules per cell. Both T(BM) and T(PBL) cell populations responded to mitogen (PHA) to the same magnitude. However, when stimulated through the CD3 complex with anti-CD3 monoclonal antibody (mAb), the T(BM) cells had a significantly decreased response than did T(PBL). The addition of IL-2 to these latter cultures augmented, but could not fully restore, the response. Additionally, stimulation of T(BM) cells with allogeneic cells failed to produce cytotoxic T cells. These "anergized" T(BM) and "nonanergized" (control) T(PBL) cells were added as third-party cells to a CTL generating culture of autologous PBL stimulated with allogeneic cells. The T(BM) cells exhibited suppressor function and inhibited the generation of CTL, in contrast with T(PBL). The effect of T(BM) cells on direct and indirect antigen presentation pathways demonstrated that T(BM) primarily effected indirect, but not direct, alloantigen presentation. To further explore the cytoplasmic T-cell activation events that occurred after the coculture of the PBL T cells with BMC, the levels of zeta-associated protein 70 (ZAP70) and extracellular receptor-activated kinase (ERK) were determined. There was a decrease in ZAP70 levels in the T(BM), which correlated with its reduced expression of cell surface CD3 and the attenuated response to anti-CD3 mAb activation. However, the activity of ERK was equally expressed by T(BM) and T(PBL). It, therefore, appears that the culturing of peripheral T cells with (non-T) BMC anergizes these cells (which become refractory to stimulation through the T-cell receptors), and induces immune suppressor function. These in vitro observations may provide a mechanism by which infused donor BMC serve to downregulate T-cell immunity.

Dendritic cells as a tool to induce anergic and regulatory T cells

The induction of antigen-specific T-cell tolerance in the thymus and its maintenance in the periphery is crucial for the prevention of autoimmunity. As well as their stimulatory functions, there is growing evidence that dendritic cells, acting as professional antigen-presenting cells, also maintain and regulate T-cell tolerance in the periphery. This control function is exerted by certain maturation stages and subsets of different ontogeny, and can be influenced by immunomodulatory agents. What is the current state of knowledge of the "immunoregulatory" properties of dendritic cells and how might tolerance-inducing dendritic cells be relevant to therapeutic applications in humans?

Induction of T-cell response to cryptic MHC determinants during allograft rejection

The presentation of MHC peptides by recipient and donor antigen presenting cells is an essential element in allorecognition and allograft rejection. MHC proteins contains two sets of determinants: the dominant determinants that are efficiently processed and presented to T cells, and the cryptic determinants that are not presented sufficiently enough to induce T-cell responses in vivo. In transplanted mice, initial T-cell response to MHC peptides is consistently limited to a single or a few immunodominant determinants on donor MHC molecule. However, in this article we show that under appropriate circumstances the hierarchy of determinants on MHC molecules can be disrupted. First, we observed that gamma IFN can trigger de novo presentation of cryptic self-MHC peptides by spleen cells. Moreover, we showed that allotransplantation is associated with induction of T-cell responses to formerly cryptic determinants on both syngeneic and allogeneic MHC molecules. Our results suggest that cross-reactivity and inflammation are responsible for the initiation of these auto- and alloimmune responses after transplantation.

Activated self-MHC-reactive T cells have the cytokine phenotype of Th3/T regulatory cell 1 T cells

In the present study, we show that human self-MHC-reactive (autoreactive) T cell clones are functionally distinct from Ag-specific T cell clones. Self-MHC-reactive T cells exhibited helper function for B cell Ig production when cultured with non-T cells alone, and they exhibit suppressor function when cultured with PWM- or rCD40 ligand (rCD40L)-activated non-T cells, whereas tetanus toxoid (TT)-specific clones exhibited only helper function in the presence of TT with or without PWM or rCD40L. Addition of neutralizing Abs to the cultures showed that the suppression was mediated by TGF-beta but not by IL-10 or IFN-gamma. The self-MHC-reactive clones also inhibited proliferation of primary CD4+ T cells and TT-specific T cell clones, but in this case the inhibition was mediated by both IL-10 and TGF-beta. In further studies, the interactions between self-MHC-reactive T cell clones and non-T cells that led to suppressor cytokine production have been explored. We found that prestimulation of non-T cells for 8 h with PWM or for 48 h for rCD40L results in non-T cells capable of inducing self-MHC-reactive T cell to produce high levels of TGF-beta and IL-10. In addition, these prestimulation times coincided with peak induction of HLA-DR and costimulatory B7 molecule (especially CD86) expression on B cells. Finally, addition of CTLA-4/Fc or blocking F(ab')2 anti-CTLA-4 mAb, plus optimally stimulated non-T cells, to cultures of self-MHC-reactive clones inhibited the induction of TGF-beta but not IL-10 or IFN-gamma production. In summary, these studies show that activated self-MHC-reactive T cells have the cytokine phenotype of Th3 or T regulatory cell 1 and thus may be important regulatory cells that mediate oral and peripheral tolerance and prevent the development of autoimmunity.

The Self-Directed T Cell Repertoire Against Mouse Lysozyme Reflects the Influence of the Hierarchy of Its Own Determinants and Can Be Engaged by a Foreign Lysozyme

The T cell repertoire is shaped by the processes of positive and negative selection. We have previously shown that mice are tolerant to a native self-Ag, mouse lysozyme (ML), but they respond vigorously when challenged with different ML peptides (“cryptic” self-determinants). In this study, we have addressed the issue of the physiological significance of both the hierarchy (dominance/crypticity) of self-determinants within ML and the anti-cryptic, self (ML)-directed T cell repertoire. Our results demonstrate that there are several ML peptides that bind well to MHC but are totally nonimmunogenic when tested for proliferative T cell response and cytokine secretion: a subset of these peptides presumably represent the originally dominant self-determinants of ML, which have rendered the T cells tolerant during thymic selection. Other ML peptides, which bind well to MHC and are immunogenic, correspond to the cryptic determinants of ML: T cells against cryptic ML determinants escape tolerance induction. Thus, the mature T cell repertoire against ML bears the direct imprint of the hierarchy of self (ML)-determinants. Interestingly, hen egg white lysozyme could prime T cells in vivo that were cross-reactive with certain cryptic ML determinants, and vice versa, without requiring any coimmunization with the foreign lysozyme and ML peptide(s). Moreover, repeated, deliberate priming and expansion of T cells by hen egg white lysozyme immunization concomitantly enhanced T cell response to such cross-reactive ML determinants. This reciprocal self-foreign determinant cross-reactivity may play a previously unrecognized, but crucial, role in the expansion and diversification of self-reactive clones in the autoimmune response.

HIV type 1 envelope glycoprotein 120 carboxy-terminal peptide-induced human T cell lines selectively suppress heterogeneous proliferative T cell responses to soluble antigens

It has been proposed that the highly conserved human immunodeficiency virus type 1 (HIV-1) envelope gp120 carboxy-terminal sequence, TKAKRRVVEREKR (CT120), may represent a functional mimic of the human leukocyte antigen (HLA) class II DR beta-chain third hypervariable region (HVR3) sequence motif located at position 69-81. Presentation of this potentially pathogenic fragment by HLA class I and/or II molecules, in a manner analogous to the indirect pathway of allorecognition, may induce both widespread cellular activation and also break self-tolerance, resulting in the selective and progressive anti-self HLA class II-directed immune suppression, which is a central feature of HIV-1 infection and the associated acquired immune deficiency syndrome (AIDS). To investigate the functional role of the HIV-1 gp120 C-terminal fragment T cell lines (TCLs) were raised from three healthy HIV-1-seronegative subjects at low risk of HIV-1 exposure, by repeated stimulation with a short synthetic 13-mer CT120 peptide in vitro. Graded concentrations (10[3] to 5 x 10[4]) of CT120 TCLs suppressed the primary 6-day proliferation of autologous PBMCs in response to the soluble antigens tetanus toxoid (TT) and purified protein derivative (PPD). In contrast, CT120 TCLs demonstrated no suppressive effect on 3-day phytohemagglutinin (PHA), concanavalin A (ConA), and pokeweed mitogen (PWM) mitogenic responses. Fractionation of CT120 TCLs into highly purified CD4+ and CD8+ T cell subsets demonstrated that the CD8+ T cell fraction mediated the suppressor effector function. HLA restriction analysis revealed a complex pattern as both anti-HLA class II DR and anti-HLA class I (A, B, C) MAbs inhibited proliferation of oligoclonal CD8+ CT120 TCLs. Strategies aimed at specifically inhibiting such putative immunopathogenic HIV-1-encoded T cell epitopes may be an important consideration for development of future HIV-1 immunotherapy.