MHC class II presenting cells are necessary for the induction of intrathymic tolerance

Allopeptides and the alloimmune response

The inherent ability of the host immune system to distinguish between self- and non-self forms the basis of allorecognition. T lymphocytes constitute the most important effector arm of allorecognition. Here we describe the fundamentals of direct and indirect pathways by which allopeptides are presented to effector T cells. The nature of allopeptides presented along with tolerogenic strategies like altered peptide ligands and intra- or extra-thymic allopeptide inoculation are discussed. In addition, we speculate on the potential of regulatory T cells to modulate alloimmune responses.

Two developmentally distinct populations of dendritic cells inhabit the adult mouse thymus: demonstration by differential importation of hematogenous precursors under steady state conditions

Although a variety of lymphoid and myeloid precursors can generate thymic dendritic cells (DCs) under defined experimental conditions, the developmental origin(s) of DCs in the steady state thymus is unknown. Having previously used selective combinations of normal, parabiotic, and radioablated mice to demonstrate that blood-borne prothymocytes are imported in a gated and competitive manner, we used a similar approach in this study to investigate the importation of the hematogenous precursors of thymic DCs. The results indicate that two developmentally distinct populations of DC precursors normally enter the adult mouse thymus. The first population is indistinguishable from prothymocytes according to the following criteria: 1) inefficient (<20%) exchange between parabiotic partners; 2) gated importation by the thymus; 3) competitive antagonism for intrathymic niches; 4) temporally linked generation of thymocytes and CD8alpha(high) DCs; and 5) absence from prothymocyte-poor blood samples. The second population differs diametrically from prothymocytes in each of these properties, and appears to enter the thymus in at least a partially differentiated state. The resulting population of DCs has a CD8alpha(-/low) phenotype, and constitutes approximately 50% of total thymic DCs. The presence of two discrete populations of DCs in the steady state thymus implies functional heterogeneity consistent with evidence implicating lymphoid DCs in the negative selection of effector thymocytes and myeloid DCs in the positive selection of regulatory thymocytes.

Intrathymic alloantigen-mediated, tolerant, completely major histocompatibility complex-mismatched mouse hearts are specifically rejected by adoptively transferred anti-class I L(d+)-specific 2C cells

BACKGROUND

Tolerance to cardiac allografts can be induced in mice and rats by the injection of donor alloantigen into the thymus in combination with a CD4 T-cell-depleting antibody. CD8(+) cells in these animals are hyporesponsive to graft-specific alloantigens. Most of the CD8(+) T cells in the transgenic 2C mouse express a T-cell receptor specific for the class I major histocompatibility complex L(d+) locus. This study was designed to determine whether the adoptive transfer of these 2C T cells could precipitate rejection of a tolerant, completely major histocompatibility complex-mismatched L(d+) or L(d-) heart.

METHODS

C57BL/6 mice (L(d-)) were given 10 x 10(6) cells of BALB/c (L(d+)) or dm2 (BALB/c background lacking L(d) [L(d-)]) splenocytes intrathymically and GK1. 5 (10 mg/kg) intraperitoneally. Twenty-one days later, BALB/c or dm2 hearts were transplanted. On the day of transplantation or after long-term allograft acceptance, recipients received naive 2C cells or 2C cells sensitized by in vitro mixed lymphocyte culture with BALB/c (L(d+)).

RESULTS

Mean survival time of BALB/c cardiac allografts in untreated C57BL/6 mice was 7.3 days, although 73% of the mice that were pretreated with BALB/c splenocytes IT plus GK1.5 accepted the donor antigen-specific heart allografts indefinitely. All recipients that were pretreated with the intrathymic plus GK1.5 and that were injected with naive 2C cells at the time of heart transplantation experienced rejection of the BALB/c (L(d+)), but not the dm2 (L(d-)) hearts. In contrast, naive 2C cells could not reject tolerant (>30 days acceptance) BALB/c (L(d+)) hearts. 2C cells sensitized in vitro against L(d) were able to reject established BALB/c hearts but could not reject the L(d-) dm2 hearts.

CONCLUSIONS

L(d)-specific 2C T-cell receptor transgenic T cells that are adoptively transferred to recipients will precipitate the rejection of accepted hearts that express class I L(d+) in mice rendered tolerant by an intrathymic injection of alloantigen plus anti-CD4 monoclonal antibodies.

Evidence that non-deletional mechanisms are responsible for inducing and maintaining unresponsiveness after intrathymic injection of non-professional antigen presenting cells

INTRODUCTION

Intrathymic inoculation of donor alloantigen and concomitant immunosuppressive treatment can induce immune unresponsiveness to alloantigen. To examine the role of non-deletional mechanisms in the development of unresponsiveness, fractionated splenocytes were injected into only 1 lobe of the thymus.

METHODS AND RESULTS

Untreated CBA (H2(k)) mice or controls pre-treated with anti-CD4 monoclonal antibody alone (on Day -28 and -27 relative to transplantation) acutely rejected C57BL/10 (H2(b)) cardiac allografts. Intrathymic inoculation of unfractionated splenocytes, resting B (rB) cells, or dendritic cells into both thymic lobes with the antibody resulted in indefinite survival of cardiac allografts. In contrast, when donor rB cells or dendritic cells were delivered into a single lobe of the thymus with the antibody, only rB cells induced indefinite prolongation of graft survival; unfractionated splenocytes or dendritic cells were markedly less effective. Mice that had 1 of the 2 thymic lobes removed were able to reject grafts even when treated with the antibody 27 days before transplantation. Therefore, T-cell export from 1 thymic lobe was sufficient to induce graft rejection. Finally, adoptive transfer of splenocytes from mice with long-term surviving primary grafts resulting from the intrathymic injection of rB cells significantly prolonged a graft from the same donor strain in a naive syngeneic recipient.

CONCLUSION

Taken together, these data suggest that regulatory mechanisms generated by intrathymic injection of a non-professional antigen presenting cell, in this study donor rB cells, suppressed the rejection response mediated by T cells exported from the uninjected lobe.

Intrathymic administration of B cells induces prolonged survival of fully allogeneic cardiac grafts without prolonged deletion of donor-specific thymocytes

Intrathymic (IT) injection of alloantigen has been shown to induce unresponsiveness to allografts although the exact mechanisms of tolerance induction remains unclear. C57BL/10 (H2b) cardiac allografts were accepted in C3H/He (H2k) mice pretreated with IT inoculation of donor splenocytes (1 x 10(6)) in combination with a depleting anti-CD4 monoclonal antibody 27 days before cardiac transplantation. To investigate which cell types were responsible for tolerance induction by IT injection of alloantigen, resting B (rB) cells or dendritic cells were used as the thymic inoculum instead of whole splenocytes. IT injection of rB cells induced indefinite graft prolongation in all recipients while only 20% of mice that had received IT injection of dendritic cells accepted grafts for over 100 days. In contrast, IT injection of dendritic cells resulted in significant deletion of donor-specific thymocytes whereas rB cells were relatively ineffective. IT deletion is not essential for the induction of tolerance by IT injection of rB cells; nondeletional mechanisms can be involved.

Deletion of alloantigen-reactive thymocytes as a mechanism of adult tolerance induction following intrathymic antigen administration

Direct injection of foreign antigen into the adult thymus is a potent route of antigen delivery for the induction of tolerance in vivo. In this report, we demonstrate that tolerance to C57BL/10 (H2b/BL10) alloantigens can be induced in CBA/Ca (H2k/CBA) mice by intrathymic (IT) administration of BL10 spleen leukocytes coincident with transient peripheral immunomodulation of CD4+ T cells using a depleting anti-CD4 monoclonal antibody. T cell receptor (TCR) transgenic mice (BM3.6; H2k) expressing a CD8-independent TCR specific for H2Kb were used as recipients to facilitate investigation of the mechanisms responsible for tolerance induction by allowing visualization of events in the thymus following IT injection. IT administration of 5 x 10(7) BL10 spleen leukocytes and concomitant transient peripheral T cell depletion in BM3.6 mice resulted in a substantial H2Kb-specific deletion of transgenic-TCR+ (tg-TCR) thymocytes which was dependent on the level of tg-TCR expression. IT deletion and the failure to export CD8+ T cells to the peripheral lymphoid organs correlated with the induction of tolerance to H2Kb; TCR transgenic mice that had received IT injection of BL10 splenocytes and peripheral T cell depletion accepted a H2Kb+ cardiac allograft indefinitely. Analysis of tolerant BM3.6 mice revealed that there were low numbers of CD8+ T cells in the periphery giving rise to a substantially reduced reactivity in vitro despite the fact that no donor cells or IT deletion were observed in the thymi of the majority of tolerant mice. These results demonstrate for the first time that IT injection of foreign alloantigen into an adult thymus results in the deletion of thymocytes expressing a TCR specific for the injected alloantigen and suggest that this is an important mechanism of tolerance induction following IT injection of alloantigen in vivo. Furthermore, analysis of tolerant TCR-transgenic mice suggests that IT deletion is not required for the maintenance of tolerance, and that peripheral mechanisms enforce continued hyporesponsiveness to H2Kb following transplantation.

FN18-CRM9 immunotoxin promotes tolerance in primate renal allografts

BACKGROUND

Transplant tolerance, rather than immunity, may be favored in the setting of a lower mature lymphoid mass in the recipient induced by anti-T cell agents. A novel immunosuppressive agent, FN18-CRM9, known to specifically kill T cells with great potency, was evaluated in a transplant model.

METHODS

In order to ablate recipient T cells, the immunotoxin FN18-CRM9 was administered to rhesus monkey recipients of MHC-mismatched renal allografts. Donor lymphocytes were injected intrathymically into some animals.

RESULTS

All monkeys with T-cell depletion by immunotoxin had prolonged allograft survival, and tolerance confirmed by skin grafting has been confirmed in five of six long-surviving recipients.

CONCLUSIONS

In this clinically relevant model, profound but transient T-cell depletion by a single agent substantially promotes tolerance.

Induction of tolerance to an experimental cardiac allograft through intrathymic inoculation of class II major histocompatibility complex disparate antigens

Indefinite donor-specific tolerance to a cardiac allograft can be induced through pretransplantation intrathymic injection of donor spleen cells and a single intraperitoneal injection of antilymphocyte serum. This study was designed to determine whether this phenomenon was reproducible with grafts differing in either class I major histocompatibility complex only or class II MHC only. Donors of cells and hearts in all experiments were RP rats. Class I MHC disparate grafts were performed by placing an RP heart into a Lewis recipient, and class II disparate grafts were performed with RP donors and Wistar Furth recipients. Lewis (n = 10) and Wistar Furth (n = 10) recipients underwent intraperitoneal injection of 1 ml antilympocyte serum and intrathymic injection of 5 x 10(7) RP spleen cells. Three weeks later, heterotopic cardiac transplantation was done with a heart from an RP rat. Control rats had no pretreatment or received antilympocyte serum alone. Without pretreatment, RP hearts survived 7 to 9 days (mean 8 days) in Lewis recipients (n = 5) and 9 to 14 days (mean 12 days) in Wistar Furth recipients (n = 5). Antilymphocyte serum alone produced slight prolongation of graft survival. Lewis rats pretreated with class I disparate RP splenocytes and antilympocyte serum had graft survivals of 8 to 27 days (mean 14 days), not significantly different from the results with antilympocyte serum alone. Class II disparate RP grafts placed in pretreated Wistar Furth rats had significant prolongation of graft survival, with four of five grafts surviving longer than 60 days (p < 0.01 vs antilympocyte serum alone). These results suggest that a disparity at the class II locus of the major histocompatibility complex is critical for the induction of cardiac allograft tolerance after intrathymic inoculation of allogeneic cells.