Biology of vascularized organ allograft rejection in sensitized recipients

Alloreactive CD8 T-cell primed/memory responses and accelerated graft rejection in B-cell-deficient sensitized mice

BACKGROUND

The sensitized patients can develop an accelerated form of graft rejection mediated by humoral and T-cell-mediated responses, which are resistant to currently used immunosuppression.

METHODS AND RESULTS

In our model of fulminant cardiac allograft rejection in sensitized hosts, groups of wild-type (WT) and B-cell-deficient (BKO) mice (B6) were challenged with skin grafts (B/c). Alloreactive CD8 T effector (Teff) activation and T memory (Tmem) differentiation during a 60-day follow-up period were reduced in the absence of B-cell help. The expression of interleukin (IL)-2Rα, IL-7Rα, and IL-15Rα, which support/program CD8 Teff/Tmem expansion, differentiation, and survival, were selectively decreased in BKO hosts. Unlike in WT, in vivo cytotoxic activity analysis of alloreactive Tmem recall response has revealed decreased donor-type (B/c) but not third-party (C3H) cell lysis in sensitized BKO hosts. However, such impaired allo-Ag specific Tmem recall function was insufficient to markedly prolong cardiac allograft survival in sensitized BKO recipients. Indeed, despite quantitative and statistically significant differences between both animal groups, the biological impact of decreased CD8 Teff/Tmem activation and function in the sensitization phase was marginal. Indeed, cardiac allografts underwent fulminant rejection in sensitized BKO, albeit with somewhat delayed kinetics. Interestingly, unlike in naïve counterparts, the rejection cascade remained CD154 blockade-resistant, evidenced by comparable kinetics, and intra-graft cytokine gene profiles in MR1 monoclonal antibody-treated sensitized WT and BKO hosts.

CONCLUSION

Although B cells were important for optimal alloreactive CD8 Teff/Tmem function in the sensitization phase, the fulminant rejection of cardiac allografts was B-cell-independent, and CD154 blockade-resistant, as in WT hosts.

Alloimmune activation enhances innate tissue inflammation/injury in a mouse model of liver ischemia/reperfusion injury

The deleterious sensitization to donor MHC Ags represents one of the most challenging problems in clinical organ transplantation. Although the role of effector/memory T cells in the rejection cascade has been extensively studied, it remains unknown whether and how these 'Ag-specific' cells influence host innate immunity, such as tissue inflammation associated with ischemia and reperfusion injury (IRI). In this study, we analyzed how allogeneic skin transplant (Tx) affected the sequel of host's own liver damage induced by partial warm ischemia and reperfusion. Our data clearly showed that allo-Tx recipients had increased inflammatory response against IR insult in their native livers, as evidenced by significantly more severe hepatocelluar damage, compared with syngeneic Tx recipient controls, and determined by serum ALT levels, liver histology (Suzuki's score) and intrahepatic proinflammatory gene inductions (TNF-alpha, IL-1beta and CXCL10). The CD4 T cells, but neither CD8 nor NK cells, mediated the detrimental effect of allo-Ag sensitization in liver IRI. Furthermore, CD154, but not IFN-gamma, was the key mechanism in allo-Tx recipients to facilitate IR-triggered liver damage. These results provide new evidence that alloreactive CD4 T cells are capable of enhancing innate tissue inflammation and organ injury via an Ag-nonspecific CD154-dependent but IFN-gamma independent mechanism.

Critical role of CD4 help in CD154 blockade-resistant memory CD8 T cell activation and allograft rejection in sensitized recipients

Allograft rejection in sensitized recipients remains the major problem in clinical organ transplantation. We have developed a donor-type skin-sensitized mouse cardiac allograft model (BALB/c-->C57BL/6) in which both rejection (<5 days) and alloreactive CD8 activation are resistant to CD154 blockade. First, we attempted to elucidate why CD154 blockade fails to protect cardiac grafts in sensitized recipients. The gene array analysis has revealed that treatment with anti-CD154 mAb (MR1) had distinctive impact on host immunity in naive vs sensitized animals. Unlike in naive counterparts, host sensitization mitigated the impact of CD154 blockade on critical immune signaling pathways. Indeed, we identified 3234 genes in cardiac grafts that were down-regulated by MR1 in naive (at least 5-fold), but remained unaffected in sensitized hosts. Moreover, MR1 treatment failed to prevent accumulation of CD4 T cells in cardiac allografts of sensitized recipients. Then, to determine the role of CD4 help in CD154 blockade-resistant immune response, we used CD4-depleting and CD4-blocking Ab, in conjunction with MR1 treatment. Our data revealed that CD154 blockade-resistant CD8 activation in sensitized mice was dependent on CD4 T cells. In the absence of CD4 help, CD154 blockade prevented differentiation of alloreactive CD8 T cells into CTL effector/memory cells and abrogated acute rejection (cardiac graft survival for >30 days), paralleled by selective target gene depression at the graft site. These results provide the rationale to probe potential synergy of adjunctive therapy targeting CD4 and CD154 to overcome graft rejection in sensitized recipients.

Allograft rejection by primed/memory CD8+ T cells is CD154 blockade resistant: therapeutic implications for sensitized transplant recipients

We have shown that CD8(+) CTLs are the key mediators of accelerated rejection, and that CD8(+) T cells represent the prime targets of CD154 blockade in sensitized mouse recipients of cardiac allografts. However, the current protocols require CD154 blockade at the time of sensitization, whereas delayed treatment fails to affect graft rejection in sensitized recipients. To elucidate the mechanisms of costimulation blockade-resistant rejection and to improve the efficacy of CD154-targeted therapy, we found that alloreactive CD8(+) T cells were activated despite the CD154 blockade in sensitized hosts. Comparative CD8 T cell activation study in naive vs primed hosts has shown that although both naive and primed/memory CD8(+) T cells relied on the CD28 costimulation for their activation, only naive, not primed/memory, CD8(+) T cells depend on CD154 signaling to differentiate into CTL effector cells. Adjunctive therapy was designed accordingly to deplete primed/memory CD8(+) T cells before the CD154 blockade. Indeed, unlike anti-CD154 monotherapy, transient depletion of CD8(+) T cells around the time of cardiac engraftment significantly improved the efficacy of delayed CD154 blockade in sensitized hosts. Hence, this report provides evidence for 1) differential requirement of CD154 costimulation signals for naive vs primed/memory CD8(+) T cells, and 2) successful treatment of clinically relevant sensitized recipients to achieve stable long term graft acceptance.

Immunopathogenesis of accelerated allograft rejection in sensitized recipients: humoral and nonhumoral mechanisms

INTRODUCTION

Accelerated rejection (AccR) in sensitized recipients (second-set rejection) is considered a classic humorally mediated form of allograft rejection, although additional effector mechanisms may be involved.

METHODS

We developed a model of AccR in which C57BL6 mice are sensitized by BALB/c skin grafts, followed 10 days later by transplantation of BALB/c hearts. We undertook analysis of various humoral and cellular components in this model using knockout or monoclonal antibody-treated allograft recipients.

RESULTS

Sensitized mice rejected cardiac allografts in 34+/-7 hr. AccR was accompanied by endothelial deposition of immunoglobulins, complement, and fibrin, but also by dense expression of multiple chemokines and a mixed polymorphonuclear and mononuclear cellular infiltrate. Whereas neutrophil or complement depletion had no significant effect on the tempo of AccR, surprisingly B cell-deficient recipients still underwent AccR (41+/-7 hr) in conjunction with T cell and macrophage recruitment. In contrast, T cell-deficient (nude) mice maintained functioning cardiac allografts for >720 hr despite prior skin engraftment.

CONCLUSIONS

AccR in sensitized experimental recipients involves multiple effector pathways. Although most previous studies have emphasized the key role of humoral pathways in mediating AccR, our data indicate that T cell-dependent mechanisms can also promote AccR, alone or in conjunction with humoral responses.

Importance of T cells to accelerated rejection and acceptance of renal allografts in sensitized rat recipients

BACKGROUND

Sensitized recipients often experience fulminant allograft loss by yet ill-defined cellular and/or humoral immune mechanisms. In this study, we analyzed the contribution of cellular elements, in particular T cells, to the accelerated rejection of renal allografts in sensitized rats.

METHODS AND RESULTS

LEW rats sensitized with BN skin grafts died of uremia in 3.3+/-0.9 days after transplantation of a BN kidney, similarly to bilaterally nephrectomized animals. Adoptive transfer of 10(6) graft-infiltrating mononuclear cells as well as their CD25+ subset into otherwise normal LEW recipients accelerated rejection of BN test cardiac allografts (5.4+/-0.5 days to 6.6+/-0.4 days vs. 7.8+/-0.8 days in controls, P<0.0007), while the CD25- population was ineffective (8.0+/-0.6 days, NS). Furthermore, alpha/beta-T-cell receptor (TCR)-targeted therapy with R73 monoclonal antibody abrogated accelerated rejection, and produced long-term survival in sensitized animals treated before kidney engraftment (day -7 to day -1). Long-term survival was associated with an up-regulation of intragraft interleukin-4 and interleukin-10 expression in conjunction with depressed Th-1-type cytokines. In addition, alpha/beta-TCR-targeted therapy even in low subtherapeutic dose decreased IgM alloantibody levels, and prevented the switch from IgM to IgG alloantibody response.

CONCLUSIONS

This is the first report that documents the striking efficacy of alpha/beta-TCR-targeted therapy in sensitized rat renal transplant recipients. The results provide evidence for a critical role of T cells for both accelerated rejection and long-term graft survival. Up-regulation of Th2-type cytokine profile may, at least in part, contribute to the acquisition of immune unresponsiveness after alpha/beta-TCR-targeted therapy in this well-defined rat renal transplant model.

Sensitization interval and administration method alter the effect of 15-deoxyspergualin on heart transplantation in sensitized recipient rats

We evaluated the effect of 15-deoxyspergualin (DSG) on accelerated rejection. Brown Norway rats (BN) served as organ donors and Lewis rats (LEW) as recipients. In an accelerated rejection model, after a LEW rat was sensitized with BN skin, a BN heart was transplanted. Various intervals between sensitization and heart transplantation were examined. The heart allografts in sensitized recipients were rejected earlier than those in unmodified recipients regardless of the sensitization interval. DSG (2.5 mg/kg per day), given to the recipients during the sensitization phase, significantly prolonged graft survival compared with the untreated hosts when the sensitization interval was short. When the recipients were treated with DSG after heart transplantation, heart graft survival was significantly prolonged regardless of the sensitization interval. Flow cytometric analysis and complement-dependent cytotoxicity tests revealed that DSG suppressed antidonor antibody formation and the postoperative administration of DSG significantly decreased the proliferation of B cells when the sensitization interval was short and the proliferation of class II antigen-positive cells when the sensitization interval was long.

Functional activity in host and graft lymphoid tissue of rats receiving syngeneic heterotopic small bowel transplants with portal or systemic drainage

Adult Lewis rats received syngeneic accessory small bowel transplants (SBT) with venous drainage to the portal vein (PV) or the inferior vena cava (IVC)/hetero-portal (HP) or hetero-systemic (HS) grafts, respectively. At varying times thereafter (5-300 days post-transplantation) animals were killed and cells from different lymphoid organs were tested in vitro for their generation of lymphoproliferative and cytotoxic T-cell responses, as well as their ability to produce a variety of lymphokines after alloantigen or mitogen stimulation. Despite marked decreases in cell recovery in Peyer's Patches of HS rats, no significant loss of cell function (on a per cell basis) was noted in any animal group. Total recovered activity per organ was decreased in small intestinal tissue (host and graft) of HS recipients regardless of the assay under study.

Migration patterns of lymphocytes following syngeneic heterotopic small bowel transplantation in rodents

Adult rats received syngeneic accessory small bowel grafts with venous drainage to either the portal vein (hetero-portal) or the inferior vena cava (hetero-systemic). Lymphoid cell recovery in different lymphoid organs (spleen, pooled peripheral lymph nodes, mesenteric nodes, Peyer's Patches) was evaluated at varying times (days 0-300) post-grafting. While minimal changes were observed for cell recovery in other organ tissues, lymphocyte recovery in Peyer's Patches of both host and graft small intestine of hetero-systemic animals was decreased from 10- to 100-fold with respect to hetero-portal recipients or non-operated controls. These changes were seen throughout the time course of the study. In additional experiments, lymphoid cells from different organs/donors were labelled in vitro with 111In and injected intravenously into normal/transplanted recipients. Recovery of 111In in various organs was assessed at 1 and 6 h postinjection. The major change seen was in the decreased ability of mononuclear cells derived from Peyer's Patches to migrate to small intestinal tissue (host and graft) in hetero-systemic recipients. In addition, Peyer's Patch cells from these animals 'homed' poorly to small intestine in non-operated animals by comparison with cells from normal rats (or hetero-portal donors).