T cell requirements for the rejection of renal allografts bearing an isolated class I MHC disparity

Activation and Regulation of Indirect Alloresponses in Transplanted Patients With Donor Specific Antibodies and Chronic Rejection

Following transplantation, human CD4+T cells can respond to alloantigen using three distinct pathways. Direct and semi-direct responses are considered potent, but brief, so contribute mostly to acute rejection. Indirect responses are persistent and prolonged, involve B cells as critical antigen presenting cells, and are an absolute requirement for development of donor specific antibody, so more often mediate chronic rejection. Novel in vitro techniques have furthered our understanding by mimicking in vivo germinal centre processes, including B cell antigen presentation to CD4+ T cells and effector cytokine responses following challenge with donor specific peptides. In this review we outline recent data detailing the contribution of CD4+ T follicular helper cells and antigen presenting B cells to donor specific antibody formation and antibody mediated rejection. Furthermore, multi-parametric flow cytometry analyses have revealed specific endogenous regulatory T and B subsets each capable of suppressing distinct aspects of the indirect response, including CD4+ T cell cytokine production, B cell maturation into plasmablasts and antibody production, and germinal centre maturation. These data underpin novel opportunities to control these aberrant processes either by targeting molecules critical to indirect alloresponses or potentiating suppression via exogenous regulatory cell therapy.

Predicting Humoral Alloimmunity from Differences in Donor and Recipient HLA Surface Electrostatic Potential

In transplantation, development of humoral alloimmunity against donor HLA is a major cause of organ transplant failure, but our ability to assess the immunological risk associated with a potential donor–recipient HLA combination is limited. We hypothesized that the capacity of donor HLA to induce a specific alloantibody response depends on their structural and physicochemical dissimilarity compared with recipient HLA. To test this hypothesis, we first developed a novel computational scoring system that enables quantitative assessment of surface electrostatic potential differences between donor and recipient HLA molecules at the tertiary structure level [three-dimensional electrostatic mismatch score (EMS-3D)]. We then examined humoral alloimmune responses in healthy females subjected to a standardized injection of donor lymphocytes from their male partner. This analysis showed a strong association between the EMS-3D of donor HLA and donor-specific alloantibody development; this relationship was strongest for HLA-DQ alloantigens. In the clinical transplantation setting, the immunogenic potential of HLA-DRB1 and -DQ mismatches expressed on donor kidneys, as assessed by their EMS-3D, was an independent predictor of development of donor-specific alloantibody after graft failure. Collectively, these findings demonstrate the translational potential of our approach to improve immunological risk assessment and to decrease the burden of humoral alloimmunity in organ transplantation.

Allorecognition pathways in transplant rejection and tolerance

With the advent of cellular therapies, it has become clear that the success of future therapies in prolonging allograft survival will require an intimate understanding of the allorecognition pathways and effector mechanisms that are responsible for chronic rejection and late graft loss.Here, we consider current understanding of T-cell allorecognition pathways and discuss the most likely mechanisms by which these pathways collaborate with other effector mechanisms to cause allograft rejection. We also consider how this knowledge may inform development of future strategies to prevent allograft rejection.Although both direct and indirect pathway CD4 T cells appear active immediately after transplantation, it has emerged that indirect pathway CD4 T cells are likely to be the dominant alloreactive T-cell population late after transplantation. Their ability to provide help for generating long-lived alloantibody is likely one of the main mechanisms responsible for the progression of allograft vasculopathy and chronic rejection.Recent work has suggested that regulatory T cells may be an effective cellular therapy in transplantation. Given the above, adoptive therapy with CD4 regulatory T cells with indirect allospecificity is a rational first choice in attempting to attenuate the development and progression of chronic rejection; those with additional properties that enable inhibition of germinal center alloantibody responses hold particular appeal.

Germinal center alloantibody responses are mediated exclusively by indirect-pathway CD4 T follicular helper cells

The durable alloantibody responses that develop in organ transplant patients indicate long-lived plasma cell output from T-dependent germinal centers (GCs), but which of the two pathways of CD4 T cell allorecognition is responsible for generating allospecific T follicular helper cells remains unclear. This was addressed by reconstituting T cell-deficient mice with monoclonal populations of TCR-transgenic CD4 T cells that recognized alloantigen only as conformationally intact protein (direct pathway) or only as self-restricted allopeptide (indirect pathway) and then assessing the alloantibody response to a heart graft. Recipients reconstituted with indirect-pathway CD4 T cells developed long-lasting IgG alloantibody responses, with splenic GCs and allospecific bone marrow plasma cells readily detectable 50 d after heart transplantation. Differentiation of the transferred CD4 T cells into T follicular helper cells was confirmed by follicular localization and by acquisition of signature phenotype. In contrast, IgG alloantibody was not detectable in recipient mice reconstituted with direct-pathway CD4 T cells. Neither prolongation of the response by preventing NK cell killing of donor dendritic cells nor prior immunization to develop CD4 T cell memory altered the inability of the direct pathway to provide allospecific B cell help. CD4 T cell help for GC alloantibody responses is provided exclusively via the indirect-allorecognition pathway.

Dendritic cells internalise and re-present conformationally intact soluble MHC class I alloantigen for generation of alloantibody

Following organ transplantation soluble MHC class I is released from the graft and may contribute to alloimmunity. We determined in a well-established rat model whether DC are able to internalise soluble MHC class I alloantigen and then re-present intact alloantigen to B cells and T cells for generation of an alloantibody or CD8 T cell response. PVG.RT1(u) BM-derived DC internalised (via an active process) and retained intact a recombinant soluble form of RT1-A(a) (sRT1-A(a)). When PVG.RT1(u) rats were immunised with sRT1-A(a)-pulsed syngeneic DC, they developed a strong anti-sRT1-A(a) alloantibody response and showed accelerated rejection of RT1-A(a)-disparate PVG.R8 heart grafts. Alloantibody production and accelerated heart graft rejection were both dependent on immunisation with viable sRT1-A(a)-pulsed DC. The alloantibody response to sRT1-A(a)-pulsed DC was directed exclusively against conformational epitopes expressed by sRT1-A(a) and not epitopes expressed, for example, by non-conformational sRT1-A(a) heavy chain. Immunisation with sRT1-A(a)-pulsed syngeneic DC did not stimulate a CD8 T cell response. Our findings suggest a novel alloantigen recognition pathway whereby soluble MHC class I alloantigen released from an allograft may be taken up by recipient DC and presented in an intact unprocessed form to B cells for the generation of an alloantibody response.

Chronic allograft nephropathy in athymic nude rats after adoptive transfer of primed T lymphocytes

The impact of presensitized T lymphocytes on the development of chronic allograft nephropathy (CAN) was investigated in nude athymic LEW.RNU recipients of F344 renal allografts. The recipients (n = 8) were reconstituted with 5 x 10(7) T lymphocytes primed against donor skin grafts to induce graft rejection. LEW.RNU (n = 8) and euthymic LEW recipients (n = 6) which underwent no further intervention after transplantation served as control groups. Adoptive transfer of primed T cells induced CAN in LEW.RNU rats. Their kidney function decreased progressively. After 90 days a moderate glomerulopathy, tubular atrophy and interstitial fibrosis were observed, vascular changes were only mild or absent. Cellular infiltrates were predominated by CD4+ T cells and ED1+ macrophages. Deposition of tenascin and laminin was enhanced. Grafts of euthymic recipients displayed only mild signs of CAN according to the Banff criteria. These data implicate an important role for the cellular immune response in the development of CAN.

Critical Role for CD103+CD8+ Effectors in Promoting Tubular Injury following Allogeneic Renal Transplantation

Immune destruction of the graft renal tubules is an important barrier to the long-term function of clinical renal allografts, but the underlying mechanisms remain obscure. CD103-an integrin conferring specificity for the epithelial cell-restricted ligand, E-cadherin-defines a subset of CD8 effectors that infiltrate the graft tubular epithelium during clinical rejection episodes, predicting a causal role for CD103+CD8+ effectors in tubular injury. In the present study, we used rodent transplant models to directly test this hypothesis. Surprisingly, CD8 cells infiltrating renal allografts undergoing unmodified acute rejection did not express significant levels of CD103. However, we demonstrate that a brief course of cyclosporine A to rat renal allograft recipients promotes progressive accumulation of CD103+CD8+ cells within the graft, concomitant with the development of tubular atrophy and interstitial fibrosis. As in the known clinical scenario, graft-associated CD103+CD8+ cells exhibited a T effector phenotype and were intimately associated with the renal tubular epithelium. Treatment with anti-CD103 mAb dramatically attenuated CD8 infiltration into the renal tubules and tubular injury. Mouse studies documented that CD103 expression is required for efficient destruction of the graft renal tubules by CD8 effectors directed to donor MHC I alloantigens. Taken together, these data document a causal role for CD103+CD8+ effectors in promoting tubular injury following allogeneic renal transplantation and identify novel targets for therapeutic intervention in this important clinical problem.

Tolerance to rat heart grafts induced by intrathymic immunomodulation is mediated by indirect recognition primed CD4+CD25+ Treg cells

BACKGROUND

In a rat model (PVG.R8-to-PVG.1U) disparate for one class I antigen, RT.1Aa, we previously demonstrated that intrathymic immunomodulation with donor antigens resulted in prolonged survival of cardiac allografts that underwent chronic rejection. However, long-term survivors developed a regulatory cell population that prevented both acute and chronic rejection when adoptively transferred into secondary graft recipients. The purpose of this study was to characterize these regulatory cells with particular emphasis on CD4+CD25+ Treg cells.

METHODS

Spleens, lymph nodes, and peripheral blood lymphocytes of secondary tolerant recipients were characterized using antibodies to various T cell markers in flow cytometry. In vitro MLR and in vivo adoptive transfer experiments were conducted to investigate the involvement of CD4+CD25+ T cells in the observed tolerance. The presence of various cytokines in the sera of graft recipients and MLR culture supernatants was tested using ELISA.

RESULTS

Tolerant recipients compared with naive rats had substantially higher percentages of CD4+CD25+ T cells in the spleen (28+/-3% vs. 11+/-5%) and blood (23+/-6% vs. 9+/-4%). Tolerant animals also had higher levels of serum IL-10 than naive and rejecting animals. CD4+CD25+ T cells from secondary long-term graft survivors inhibited donor-specific proliferative responses in vitro that was associated with high IL-10 production. Importantly, depletion of CD4+CD25+ T cells from splenocytes of tolerant rats abrogated their ability to transfer tolerance to tertiary graft recipients.

CONCLUSIONS

Our data demonstrate that cardiac allograft tolerance in this model is mediated by CD4+CD25+ Treg cells primed by indirect recognition and is associated with high levels of IL-10.

Is B cell tolerance essential for transplantation tolerance?

In some transplantation settings, achieving B cell as well as T cell tolerance will be essential in order to ensure long-term graft survival. However, in other situations, although B cell tolerance could potentially offer advantages to the long-term function of the graft and may therefore be desirable, there is no evidence that inducing B cell tolerance alongside T cell tolerance is essential. This overview forms part of a debate discussing the potential role of B cell tolerance in transplantation and uses selected examples in the literature to address the question: is B cell tolerance essential for transplantation tolerance?

Donor-Specific Antibodies in Allograft Rejection: Clinical and Experimental Data

The ability of donor-specific major histocompatibility complex alloantibodies to destroy a transplanted organ within minutes, the so-called hyperacute rejection phenomenon, has been known for a long time. It is a clear demonstration of the possible cytotoxic effect of antibodies. Apart from this particular situation, the role of antibodies in inducing acute or chronic allograft rejection remains controversial. Many clinical data have shown that transplant recipients capable of developing class I or class II anti-HLA antibodies experienced shorter survival periods than those who were not. This fact, in accordance with experimental data, only demonstrates that high antibody responders reject a transplant more easily than low responders. More interestingly, there is now increasing evidence that posttransplant appearance of donor-specific alloantibodies, and probably of alloreactive-induced autoantibodies, is strongly correlated with reduced graft survival rate, especially from chronic rejection. We demonstrated that donor-specific HLA antibodies can be found in more than 70% of transplanted kidneys with chronic allograft nephropathy, and that the intragraft presence of such antibodies is significantly correlated with high numbers of plasma cells on early biopsies and C4d deposits, a recognized marker of humoral rejection. It is likely that non-HLA antibodies also play a deleterious role in organ transplant outcome, particularly the heterogeneous group of anti-endothelial cells antibodies, anti-MIC antibodies, autoantibodies and some others with no recognized target. Convincing experimental data, especially using B cell and T cell deficient mice, strongly suggest that B cells and donor-specific antibodies are required for fully developed chronic allograft rejection. The role of antibodies in inducing the cascade of cytokines and growth factors leading to tissue lesions is of increasing interest since it is now possible to control B cell proliferation and antibody production.

Chronic heart allograft rejection in rats demonstrates a dynamic interplay between IFN-γ and IL-10 producing T cells

The relative roles of different types of T cells and their modes of alloantigen recognition and cytokine production in chronic rejection have been controversial. This may be due to the use of models involving various immune interventions. We recently reported a rat heart allograft model (PVG.1U-to-PVG.R8) that consistently develops chronic rejection without any immune manipulations. Using this model, we investigated the frequency of indirectly activated alloreactive CD4+ T cells and directly/indirectly induced CD8+ T cells and characterized their cytokine profiles at various times posttransplantation. In vitro quantitative-proliferative and intracellular cytokine assays were performed using recipient lymphocytes as responders against donor and third party stimulator cells. Intragraft transcript levels for IFN-gamma were measured using competitive RT-PCR. We observed a steady increase in the frequency of donor reactive CD4+ T cells, peaking on day 18-22 posttransplantation, followed by a decline to background levels on day 60. These cells produced high levels of IL-10 that steadily increased throughout the 100-day experimental period. In contrast, the frequency of donor reactive CD8+ T cells was similar to that of naïve cells and remained unchanged throughout the study. CD8+ T cells produced high amounts of IFN-gamma only on days 39-45 posttransplantation when chronic rejection was apparent. There was a sharp increase in intragraft IFN-gamma transcripts on days 8-12 that peaked on days 39-45 followed by a steady decrease thereafter. Our results demonstrate that chronic rejection in this model involves a dynamic interplay between alloreactive CD8+IFN-gamma+ and CD4+IL-10+ T cells, both of which may have differential effects on the pathogenesis of the disease.

Role of integrin CD103 in promoting destruction of renal allografts by CD8 T cells

Infiltration of CD8(+)TCRalphabeta(+) T-effector populations (CD8 effectors) into graft epithelial compartments has long been recognized as a key lesion in progression of clinical renal allograft rejection. While the afferent phase of allograft immunity is increasingly well-defined, the efferent pathways by which donor-reactive CD8-effector populations access and ultimately destroy the graft renal tubules (rejection per se) have received remarkably little attention. This is an important gap in our knowledge of transplantation immunology, because epithelial compartments comprise the functional elements of most commonly transplanted organs including not only kidney, but also liver, lung, pancreas, and intestine. Furthermore, there is increasing evidence that attack of graft epithelial elements by CD8-effector populations not only causes short-term graft dysfunction but is also a major contributor to development of chronic allograft nephropathy and late graft loss, which now represent the salient clinical problems. Recent studies of the T-cell integrin, alpha(E)beta(7) (CD103), have provided insight into the mechanisms that promote interaction of CD8 effectors with graft epithelial compartments. The purpose of this communication is to review the known properties of the CD103 molecule and its postulated role in the efferent phase of renal allograft rejection.

Non-complement- and complement-activating antibodies synergize to cause rejection of cardiac allografts

Alloantibodies (AlloAbs) are a clinically significant component of the immune response to organ transplants. In our experimental model, B10.A (H-2a) cardiac transplants survived significantly longer in C57BL/6 (H-2b) immunoglobulin knock-out (IgKO) recipients than in their wild-type (WT) counterparts. Passive transfer of a single 50-200-microg dose of complement-activating IgG2b AlloAbs to IgKO recipients reconstituted acute rejection of cardiac allografts. Although passive transfer of a subthreshold dose of 25 microg of IgG2b or a single 100-200-microg dose of non-complement-activating IgG1 AlloAbs did not restore acute rejection to IgKO recipients, a combination of these AlloAbs did cause acute graft rejection. Histologically, rejection was accompanied by augmented release of von Willebrand factor from endothelial cells. IgG1 AlloAbs did not activate complement on their own and did not augment complement activation by IgG2b AlloAbs. However, IgG1 AlloAbs stimulated cultured mouse endothelial cells to produce monocyte chemotactic protein 1 (MCP-1) and neutrophil chemoattractant growth-related oncogene alpha (KC). TNF-alpha augmented IgG1 induced secretion of MCP-1 and KC. These findings indicate that non-complement-activating AlloAbs can augment injury to allografts by complement-activating AlloAbs. Non-complement-activating AlloAbs stimulate endothelial cells to produce chemokines and this effect is augmented in the milieu of proinflammatory cytokines.

Multiple pathways to allograft rejection

Allograft rejection results from a complex process involving both the innate and acquired immune systems. The innate immune system predominates in the early phase of the allogeneic response, during which chemokines and cell adhesion play essential roles, not only for leukocyte migration into the graft but also for facilitating dendritic and T-cell trafficking between lymph nodes and the transplant. This results in a specific and acquired alloimmune response mediated by T cells. Subsequently, T cells and cells from innate immune system function synergistically to reject the allograft through nonexclusive pathways, including contact-dependent T cell cytotoxicity, granulocyte activation by either Th1 or Th2 derived cytokines, NK cell activation, alloantibody production, and complement activation. Blockade of individual pathways generally does not prevent allograft rejection, and long-term allograft survival is achieved only after simultaneous blockade of several of them. In this review, we explore each of these pathways and discuss the experimental evidence highlighting their roles in allograft rejection.

Epitope mapping of the indirect T cell response to allogeneic class I MHC: sequences shared by donor and recipient MHC may prime T cells that provide help for alloantibody production

Indirect allorecognition occurs when T cells recognize donor MHC presented as peptide epitopes by recipient APC, but the precise nature of the epitopes involved remains unclear. Rejection of rat MHC class I-disparate PVG.R8 (RT1.A(a)) grafts by PVG.RT1(u) (RT1.A(u)) recipients is mediated by indirectly restricted CD4 T cells that provide help for the generation of alloantibody. In this study, epitope mapping was performed using a functionally relevant readout (alloantibody production) to identify key peptides that prime an indirect alloimmune response, leading to graft rejection. PVG.RT1(u) rats were immunized with a series of overlapping 15-mer peptides (peptides 1-18) that spanned the alpha1 and alpha2 domains of the RT1.A(a) molecule. Several peptides were able to accelerate both the alloantibody response to the intact RT1.A(a) Ag and PVG.R8 heart graft rejection. An immunodominant epitope was identified within the hypervariable region of the alpha1 domain. Fine mapping of this region with a second series of peptides overlapping by single amino acids confirmed the presence of an eight-amino acid core determinant. Additional "subdominant" epitopes were identified, two of which were located within regions of amino acid homology between the RT1.A(a) and RT1.A(u) molecules and not, as had been expected, within other hypervariable regions. The contribution of self-epitopes to indirect allorecognition was emphasized by the demonstration that i.v. administration of a 15-mer peptide encompassing one of the subdominant self-determinants diminished the recipient's ability to mount an alloantibody response on challenge with intact A(a) alloantigen. Our findings suggest that cryptic self-epitopes recognized by autoreactive T cells may contribute to allograft rejection and should be considered when designing novel strategies for inducing tolerance to alloantigen.

TGF-beta1 and donor dendritic cells are common key components in donor-specific blood transfusion and anti-class II heart graft enhancement, whereas tolerance induction also required inflammatory cytokines down-regulation

Heart allograft tolerance in adult recipients can be induced in the LEW.1W to LEW.1A congeneic strain combination by pre-graft donor-specific blood transfusion (DST). Long-term survivors accept LEW.1W graft but reject third party skin grafts. As tolerant recipients of heart allografts showed an increase in anti-donor class II antibodies, we hypothesize that these antibodies could be instrumental in tolerance induction. However, anti-donor MHC class II alone prolonged graft survival but did not induce heart allograft tolerance in this combination. We analyzed the immune response patterns in heart allograft recipients following the injection of anti-donor class II antibodies (prolongation) or DST priming (tolerance). As suggested by the different phenomena, several immunological patterns were strikingly different between the two models. In strong contrast to DST-tolerant recipients, at 5 days after transplantation, neither Th1/Th2 nor inflammatory cytokines were inhibited in recipients treated with anti-donor class II antibodies, in which only prolongation of graft survival was induced. Nevertheless, in both models, depletion of resident dendritic cells (DC) from donor hearts inhibited tolerance induction (DST) or shortened allograft survival (anti-donor class II antibodies). Moreover, TGF-beta1 was not down-regulated and administration of neutralizing anti-TGF-beta1 antibody, which inhibited tolerance induction (DST), also shortened allograft survival (anti-donor class II antibodies). These results suggest that, in these two MHC class II-restricted models, both TGF-beta1 and donor DC have a pivotal role in prolonging graft survival. However, in the days following transplantation, further inhibition of inflammatory cytokine production, particularly Th1 and macrophage-derived cytokines is required for tolerance induction.

Oral exposure to alloantigen generates intragraft CD8+ regulatory cells

We have previously reported that oral administration of allogeneic rat spleen cells before kidney allotransplantation significantly prolongs graft survival. This prolongation was alloantigen specific and was associated with a decrease in graft-infiltrating cells (GIC) and an increase in transcription of IL-4 mRNA in the GIC. In this study increased splenic mixed lymphocyte responses from animals orally exposed to alloantigen before kidney transplantation suggested that the kidney allograft prolongation was not due to a masking of allorecognition, but to an immunomodulation of the immune response. We have assessed GIC T cell subsets on day 5 post-transplant and found decreased numbers of CD4(+) T cells in fed animals compared with controls, but there was no change in CD8(+) T cell numbers. The CD8(+) GIC from fed animals transcribed substantial levels of perforin, granzyme, and Fas ligand mRNA, indicating the presence of active CTL. Direct CTL assays showed that the GIC from fed recipients exhibited higher allo-CTL activity than GIC from control unfed recipients. In addition, the CD8(+) GIC exhibited high levels of IL-4 mRNA, suggesting Tc2-type regulatory cells. Prolonged graft survival in the face of active CTL and Tc2 cells suggests the presence of a CD8(+) regulatory cell population in the allograft. To confirm this, cell transfer experiments were performed. Prolongation of graft survival was transferred from rats orally exposed to alloantigen to naive animals by transfer of CD8(+) GIC. This is the first report that oral exposure to alloantigen prolongs kidney allograft survival by the generation of intragraft CD8(+) regulatory cells.

Passive transfer of alloantibodies restores acute cardiac rejection in IgKO mice

BACKGROUND

Alloantibody is an intrinsic component of the immune response to organ transplants. Although alloantibodies have been correlated with decreased graft survival, the mechanisms of alloantibody-mediated injury remain largely undefined in vivo. In the present study, we have established a model of alloantibody-mediated graft injury using B10.A (H-2a) hearts transplanted to wild type (WT) or immunoglobulin knock out (IgKO) C57BL-Igh-6 (H-2b) mice.

METHODS

Alloantibodies were measured in the circulation and graft by flow cytometry and in immunofluorescence staining, respectively. Intragraft cytokine mRNA expression was evaluated using a competitive template reverse transcriptase polymerase chain reaction (RT-PCR) technique. P-selectin and von Willebrand factor expression were localized by immunoperoxidase staining. The capacity of alloantibodies to restore acute cardiac allograft rejection was tested by passive transfer of monoclonal antibodies (mAbs) against donor major histocompatibility complex (MHC) class I antigens to IgKO recipients.

RESULTS

B10.A cardiac allografts are rejected acutely by WT C57BL/6 recipients, but over 50% of the cardiac allografts survived more than 50 days after transplantation in IgKO mice. Competitive template RT-PCR on the cardiac transplants demonstrated similar levels of IL-1-alpha, IL-12 (p40), TNF-alpha, IL-2, IFN-gamma, IL-4, and IL-10 mRNA in WT and IgKO recipients 8-10 days after transplantation, indicating that macrophage- and T-cell-dependent immune responses were intact in IgKO recipients. The rejection of B10.A hearts in WT recipients was characterized by interstitial and perivascular cellular infiltration; IgG, IgM, and complement (C3) deposition; vascular cell injury and intravascular platelet aggregation; and release of von Willebrand factor and P-selectin. In IgKO recipients the lower degree of vascular injury in the absence of alloantibody responses was reflected by the lack of release of von Willebrand factor and P-selectin, which remained confined to cytoplasmic storage granules of endothelial cells and platelets. Acute rejection of cardiac allografts was restored to IgKO recipients by passive transfer of proinflammatory IgG2b mAbs against donor MHC; recipients injected with isotype-matched control mAbs did not reject. In contrast, passive transfer of IgG1 mAbs against donor MHC failed to restore acute rejection of cardiac allografts to IgKO recipients. Passive transfer of IgG2b, but not IgG1 mAbs was associated with endothelial cell activation and plate. let aggregation together with the release of preformed von Willebrand factor and P-selectin from storage granules.

CONCLUSIONS

Acute rejection of cardiac allografts can be reconstituted in IgKO recipients by passive transfer of IgG2b, but not IgG1 antibody. This model allows the mechanism of alloantibody-mediate graft injury to be dissected in vivo.

Induction of antigraft and antirecipient antibody responses after fully allogeneic and semiallogeneic rat small bowel transplantation

BACKGROUND

Given the potential influence of alloantibodies on organ graft outcome, this study investigated the induction of antigraft and antirecipient antibodies after allogeneic and semiallogeneic rat small bowel transplantation.

METHODS

Fully allogeneic, unidirectional rejection and unidirectional graft-versus-host disease (GvHD) heterotopic small bowel transplantation was performed using DA, PVG, and (PVGxDA)F1 donor-recipient combinations. Serum was obtained before and at time points after transplantation and incubated with blood from untransplanted DA and PVG rats. Antibody binding to T cells was detected by whole blood flow cytometry using FITC-conjugated anti-rat IgM murine monoclonal antibody. Antibody levels were determined by reference to a standard curve of fluorescent intensity generated using a serum sample with known anti-target cell IgM activity. Data are presented as arbitrary units/ml (AU/ml).

RESULTS

In the PVG-->DA combination, five of six DA recipients had detectable anti-graft (PVG) antibodies by day 4 after transplantation (mean 72 AU/ml) and all animals were positive by day 6 (976 AU/ml). Antirecipient (DA) antibodies were also induced, however, they were only apparent after 6 days in five of eight animals (90 AU/ml). Antigraft (DA) antibody responses were also induced in the DA-->PVG combination (day 6-218 AU/ml), however no antirecipient (PVG) response was apparent. Transplantation induced antirecipient (DA) antibodies in the unidirectional GvHD model (day 6-90 AU/ml) and an anti-graft (PVG) response in the unidirectional rejection model (day 6-60 AU/ml). However, the latter was quantitatively lower than that generated in the PVG-->DA combination (day 6-976 AU/ml).

CONCLUSIONS

Antigraft and antirecipient antibody responses are simultaneously induced after fully allogeneic small bowel transplantation, despite rejection being the predominant clinical feature. Further studies are required to elucidate their influence on graft outcome.

Accumulating monocytes in the vasculature of rat renal allografts: phenotype, cytokine, inducible no synthase, and tissue factor mRNA expression

BACKGROUND

Necrotic patches and hemorrhagic lesions develop in the renal tissue between day 4 and day 5 after transplantation of fully allogeneic DA rat kidneys to LEW recipients. These lesions are at least in part due to destruction and obstruction of blood vessels. Damage of graft endothelial cells and blood coagulation are likely to be mediated by intravascular graft leukocytes. However, this cell population has not been thoroughly characterized before.

METHODS

We perfused untreated control kidneys, renal isografts, and allografts on day 4 after transplantation with phosphate-buffered saline/ethylenediaminetetraacetic acid to harvest leukocytes from both the blood stream as well as from the marginal intravascular pool. The mRNA expression of typical products of activated monocytes was analyzed in reverse-transcriptase polymerase chain reaction experiments. Graft monocytes were purified and their immunophenotype was investigated by flow cytometry.

RESULTS

Allograft rejection led to a 10-fold increase in the number of intravascular graft leukocytes compared to isografts. A mean number of about 100x10(6) leukocytes was harvested from a single allogeneic kidney, about 73% of these cells were monocytes and most of them displayed an activated phenotype. Compared to isografts, intravascular allograft leukocytes displayed an increased expression of tumor necrosis factor-alpha, inducible NO synthase and tissue factor.

CONCLUSIONS

Our study shows that large numbers of activated monocytes accumulate inside allograft vessels. As they express genes the products of which might damage the allograft by inducing cell death or thrombosis, we speculate that they directly participate in allograft destruction.

High-risk corneal allografts are capable of stimulating complement dependent cytolytic antibodies

PURPOSE

To prospectively characterize serum antibody responses in high-risk corneal allografts.

METHODS

Orthotopic and heterotopic C3H (H-2k) corneas were transplanted to CB6F1 (H-2b/d) recipient mice. Individual serum samples were collected at days 7, 14, 21, and 28 from orthotopically and heterotopically grafted mice. Samples were tested for anti-C3H antibodies using an enzyme-linked immunosorbent assay (ELISA). Serum samples were also tested for the ability to lyse target cells in the presence of complement.

RESULTS

In high-risk donor-host combinations, 100% corneal allograft rejection occurs in the absence of IgG1 production. Both orthotopic and heterotopic corneal allografts induced cytolytic complement fixing antibody isotypes.

CONCLUSIONS

Allospecific corneal antibody may be involved in corneal graft rejection in high-risk donor-host combinations.

Anti-CD4 induced rat heart tolerance: no presence of primed T cells and regulatory mechanisms for cytotoxic T cells

Treatment with anti-CD4 monoclonal antibody (mAb) (OX38) induces heart, but not skin graft tolerance in WF (RT1u) to Lewis (RT1l) rat strain combinations. We examined differences in cellular responses between heart-bearing and skin-rejected hosts that were both treated with anti-CD4 mAb. In the tolerant LEW rats bearing WF heart transplants, the secondary WF heart but not skin grafts were accepted. On the other hand, in anti-CD4 treated WF skin-rejected hosts, both secondary WF heart and skin grafts were rapidly rejected. Spleen cells from anti-CD4 treated WF skin-rejected LEW rats but not from WF heart-bearing LEW rats received the same treatment generated CTL after in vitro stimulation with paraformaldehyde (PFA) treated donor WF stimulator spleen cells. Adoptive transfer of spleen cells from WF skin-rejected LEW rats with or without anti-CD4 therapy into the tolerant LEW rats at the secondary WF heart transplantation blocked the secondary heart graft acceptance. However, transfer of spleen cells from WF heart-rejected rats without immunosuppression failed to block acceptance of the secondary heart graft. Our results indicated the lack of primed T cells and presence of regulatory mechanisms for tissue specific T cells in anti-CD4 treated heart bearing hosts.

Specific B cell tolerance is induced by cyclosporin A plus donor-specific blood transfusion pretreatment: prolonged survival of MHC class I disparate cardiac allografts

Donor-specific blood transfusion (DST), designed to prolong allograft survival, sensitized recipients of the high-responder PVG-RT1u strain, resulting in accelerated rejection of MHC-class I mismatched (PVG-R8) allografts. Rejection was found to be mediated by anti-MHC class I (Aa) alloantibody. By pretreating recipients 4 wk before grafting with cyclosporin A (CsA) daily (x7), combined with once weekly (x4) DST, rejection was prevented. The investigation explores the mechanism for this induced unresponsiveness. CD4 T cells purified from the thoracic duct of CsA/DST-pretreated RT1u rats induced rejection when transferred to R8 heart-grafted RT1u athymic nude recipients, indicating that CD4 T cells were not tolerized by the pretreatment. To determine whether B cells were affected, nude recipients were pretreated, in the absence of T cells, with CsA/DST (or CsA/third party blood) 4 wk before grafting. The subsequent transfer of normal CD4 T cells induced acute rejection of R8 cardiac allografts in third party- but not DST-pretreated recipients; prolonged allograft survival was reversed by the cotransfer of B cells with the CD4 T cells. Graft survival correlated with reduced production of anti-MHC class I (Aa) cytotoxic alloantibody. The results indicated that the combined pretransplant treatment of CsA and DST induced tolerance in allospecific B cells independently of T cells. The resulting suppression of allospecific cytotoxic Ab correlated with the survival of MHC class I mismatched allografts. The induction of B cell tolerance by CsA has important implications for clinical transplantation.

C6 produced by macrophages contributes to cardiac allograft rejection

The terminal components of complement C5b-C9 can cause significant injury to cardiac allografts. Using C6-deficient rats, we have found that the rejection of major histocompatibility (MHC) class I-incompatible PVG.R8 (RT1.A(a)B(u)) cardiac allografts by PVG.1U (RT1.A(u)B(u)) recipients is particularly dependent on C6. This model was selected to determine whether tissue injury results from C6 produced by macrophages, which are a conspicuous component of infiltrates in rejecting transplants. We demonstrated that high levels of C6 mRNA are expressed in isolated populations of macrophages. The relevance of macrophage-produced C6 to cardiac allograft injury was investigated by transplanting hearts from PVG. R8 (C6-) donors to PVG.1U (C6-) rats which had been reconstituted with bone marrow from PVG.1U (C6+) rats as the sole source of C6. Hearts grafted to hosts after C6 reconstitution by bone marrow transplantation underwent rejection characterized by deposition of IgG and complement on the vascular endothelium together with extensive intravascular aggregates of P-selectin-positive platelets. At the time of acute rejection, the cardiac allografts contained extensive perivascular and interstitial macrophage infiltrates. RT-PCR and in situ hybridization demonstrated high levels of C6 mRNA in the macrophage-laden transplants. C6 protein levels were also increased in the circulation during rejection. To determine the relative contribution to cardiac allograft rejection of the low levels of circulating C6 produced systemically by macrophages, C6 containing serum was passively transferred to PVG.1U (C6-) recipients of PVG.R8 (C6-) hearts. This reconstituted the C6 levels to about 3 to 6% of normal values, but failed to induce allograft rejection. In control PVG.1U (C6-) recipients that were reconstituted with bone marrow from PVG.1U (C6-) donors, C6 levels remained undetectable and PVG.R8 cardiac allografts were not rejected. These results indicate that C6 produced by macrophages can cause significant tissue damage.

Persisting alloantigen prevents primed CD45RC- CD4 T cells from inducing allograft rejection: implications for immunological memory

Antigen stimulation induces specific CD4 T cells to change from a resting phenotype (CD45RC+) to a "memory" phenotype (CD45RC-), an isoform switch that is reversible and regulated by persisting antigen. We show here that CD4 T cells responsible for mediating allograft rejection undergo a similar CD45RC+ to CD45RC- switch irrespective of whether antigen priming results in sensitization or tolerance in vivo. Thus, skin allograft priming, designed to induce second set rejection, and a donor-specific blood transfusion (DST), designed to prolong cardiac allograft survival, will generate CD45RC- CD4 T cells that induce acute rejection when adoptively transferred to T cell-deficient athymic nude recipients. The ability of CD45RC- T cells, obtained from DST donors, to induce graft rejection was prevented by giving nude recipients a DST 14, 28 or even 56 days before grafting and T cell transfer. Thus, prolonged allograft survival in rats after DST was found to be strongly linked with persisting alloantigen from the blood transfusion but was not associated with detectable microchimerism. Importantly, CD45RC- T cells from skin graft-primed animals were similarly prevented from inducing rejection by residual DST-derived alloantigen. The investigation shows (1) that an allogeneic blood transfusion primes (not tolerizes) alloreactive CD4 T cells and (2) that residual DST-derived alloantigen can block the action of specifically primed "memory" CD4 T cells. These findings have implications for understanding immunological memory.

CD4 T cells can reject major histocompatibility complex class I-incompatible skin grafts

We have re-investigated the roles of CD4 and CD8 T cell subsets in skin graft rejection across a single class I MHC disparity. Recipient mice were transplanted with skin from donors transgenic for the class I MHC molecule Kb. As expected, CD8 T cells were sufficient for rapid injection; but surprisingly, CD4 T cells were also competent to do the same. Rejection was dependent on one or the other subset, since elimination of both resulted in indefinite graft survival. The possibility that alloantibody was the downstream effector of CD4 mediated rejection was excluded because CD8-depleted mice rendered B cell deficient still rejected rapidly, but T cell-depleted recipients with pre-existing high titers of alloantibody were unable to do so. In addition, if CD4 cells act to reject by recruiting and/or activating macrophages then this was not dependent on CR3, IFN-gamma or TNF-alpha. Transplantation of skin grafts where the MHC class I disparity was at the level of passenger leukocytes only, demonstrated that transient bystander damage could occur, but that this was insufficient to result in full rejection. We surmise that for CD4 T cells to reject an MHC class I-incompatible graft it is necessary that an appropriate allogeneic peptide is processed and presented in the context of recipient MHC class II. CD4 T cells from B6 mice may fail to reject skin from MHC class I mutants because of the lack of such MHC class II-restricted presentation.

Contrasting outcomes of donor-specific blood transfusion: effectiveness against cell-mediated but not antibody-mediated rejection

BACKGROUND

Giving recipients a prior donor-specific blood transfusion (DST) is effective in prolonging organ allograft survival in some inbred strains but not in others. The present investigation analyzed two such contrasting strains of rats in an attempt to define the basis for this variation.

METHODS AND RESULTS

The survival of fully mismatched Dark Agouti (RT1a) cardiac allografts was significantly prolonged (from 7 to 44 days, median survival times) in PVG (RT1c) rats given a prior (-14 day) DST, whereas it shortened survival in the high-responder PVG-RT1u strain. Injecting PVG recipients with blood from strains bearing defined differences indicated that each disparity contributed to the increased survival time in an incremental way: blood and heart matched at the MHC class I (A) and/or class II (B/D) loci had a major influence on survival; class I-like (C) and non-MHC antigens made only minor contributions. MHC disparities had contrasting effects in RT1u rats. Blood transfusions from Dark Agouti or PVG-R8 (AaB/DuCu) rats induced accelerated rejection and anti-Aa alloantibody formation; transfusing PVG-R23 (AuB/DaCa) blood, a class II and class I-like difference, induced indefinite R23 heart allograft survival. Although produced in high titer, anti-class II antibody was not able to induce rejection in RT1u rats. Specific anti-Aa alloantibody was able, after passive transfer, to destroy class I-disparate allografts in both RT1u nude and PVG nude recipients. However, under normal circumstances, acute rejection in the PVG strain occurred in the absence of anti-Aa antibodies, presumably by a cell-mediated mechanism.

CONCLUSION

Anti-class I alloantibody, when produced, seemed to override the unresponsiveness induced by DST. The results indicated that DST was effective only when rejection was induced by a cell-mediated response. The two contrasting response patterns in animals may reflect the experience of transplant patients who either benefit from DST or become sensitized instead.

Indirect T Cell Allorecognition and Alloantibody-Mediated Rejection of MHC Class I-Disparate Heart Grafts

Recent studies in the rat have identified a role for T cell-dependent alloantibody in rejection of MHC class I-disparate allografts. RT1Aa-disparate PVG.R8 heart grafts are rejected acutely in naive, and hyperacutely in sensitized, PVG.RT1u recipients by CD4 T cell-dependent alloantibody. Here, we examined the T cell Ag recognition pathways responsible and show that direct injection into skeletal muscle of plasmid DNA, encoding a water-soluble form of the RT1Aa MHC class I heavy chain (pcmu-tAa), stimulates IgG2b cytotoxic alloantibody and markedly accelerates rejection of PVG.R8 heart grafts (median survival time 2 days). pcmu-tAa injection did not induce CTL to Aa, arguing against direct allorecognition of soluble Aa. Treatment with mAbs confirmed that the alloimmune response to pcmu-tAa injection depended on CD4, not CD8, T cells. Priming T cells for indirect allorecognition by injection of 15-mer peptides spanning the α1 and α2 domains of Aa failed to stimulate anti-Aa Ab but caused an accelerated Ab response to a PVG.R8 heart and a modest acceleration in graft rejection (median survival time 4 days). These results suggest that both soluble MHC class I and allopeptides prime CD4 T cells by the indirect pathway, but that soluble class I is a more effective immunogen for humoral alloimmunity because its tertiary protein structure provides B cell epitopes. We propose that priming humoral alloimmunity, like CTL priming, requires recognition of intact MHC on donor cells, but essential T cell help can be provided by CD4 T cells recognizing allogeneic class I exclusively by the indirect pathway.

Analysis of primate renal allografts after T-cell depletion with anti-CD3-CRM9

BACKGROUND

FN18-CRM9 is a CD3-specific immunotoxin that is capable of depleting CD3+ T cells. Pretreatment of rhesus monkeys with this agent before transplantation can induce donor-specific tolerance and "split tolerance" to renal allografts.

METHODS

Heterotopic renal transplants were performed on monkeys that received posttransplant FN18-CRM9. Histological and immunohistological staining, as well as analysis of the intragraft cytokine profile by reverse transcriptase polymerase chain reaction, was performed on percutaneous allograft biopsies.

RESULTS

Experimental monkeys had significant prolongation of allograft survival. Although an interstitial, mononuclear cell infiltrate was seen in all of the renal transplants, there was minimal evidence of acute cellular rejection. Histological evidence of alloantibody-mediated damage was detected 3 to 5 months after transplantation in the monkeys treated with FN18-CRM9. Immunohistology demonstrated the reappearance of CD3+ and CD4+ T cells, as well as CD20+ B cells, in the grafts. Cytokine analysis demonstrated expression of interferon-gamma. An intact anti-donor IgG response was seen.

CONCLUSION

Treatment of monkeys with FN18-CRM9 immediately after transplantation significantly prolongs renal allograft survival. Allograft biopsies demonstrate a lack of acute cellular rejection; however, alloantibody-mediated graft damage and rejection occur, with an intact anti-donor IgG response. The intragraft expression of the interferon-gamma may reflect this ongoing humoral rejection. These data suggest that even a brief period of T-cell allosensitization may lead to humorally mediated allograft damage. Efforts to achieve tolerance with posttransplant FN18-CRM9 will require modification of the protocol to deplete T cells before allosensitization exposure or to supplement the posttransplant immunomodification strategy.

Contribution of donor-specific antibodies to acute allograft rejection: evidence from B cell-deficient mice

BACKGROUND

The role of T lymphocytes in acute allograft rejection is well established. The involvement of B lymphocytes in this process, however, is more controversial. A series of reports showed that mice without a functional B-cell compartment rejected allografts with the same kinetics as control animals. In rats, however, alloantibodies were found to play a decisive role in allograft rejection. To provide an explanation for the discrepant results, we readdressed the role of B cells and antibodies in mice with disrupted immunoglobulin mu chain genes. The use of cyclosporine (CsA), which strongly suppresses T cells, allowed us to focus specifically on the function of B cells.

METHODS

C57BL/6 mice rendered B cell deficient by targeted disruption of the immunoglobulin mu chain gene (referred to as microMT/microMT mice) and microMT/+ control mice with one functional mu chain were heterotopically transplanted with fully MHC-disparate BALB/c hearts. CsA was administered subcutaneously by Alzet osmotic pumps. Normal and immune serum specific for donor hearts was given to assess the role of antibodies in the rejection process.

RESULTS

Both B cell-deficient microMT/microMT and heterozygous microMT/+ mice were found to reject transplanted hearts within a similar period of time. In contrast, when T cells were partially suppressed with CsA, graft survival was significantly prolonged in microMT/microMT mice as compared with heterozygous controls. Passive transfer of donor-specific immune serum, obtained from microMT/+ animals rejecting allogeneic hearts, to CsA-treated microMT/microMT mice significantly accelerated allograft rejection as opposed to recipients treated with normal serum.

CONCLUSIONS

B lymphocytes and antibodies play an important role in acute allograft rejection particularly when the dominant T-cell compartment is partially suppressed.

The cellular basis of cardiac allograft rejection: VIII. Mechanisms underlying delayed allograft rejection in PVG C6-deficient rats

BACKGROUND

The delayed allograft rejection in C6-deficient PVG C6- rats compared with normal PVG rats has been attributed to the lack of alloantibody activation of the membrane attack complex of complement. As T cells alone have been shown to effect graft rejection, we examined T-cell responses in PVG C6- rats.

METHODS

The cellular infiltrate and its mRNA for cytokines and effector molecules in DA heart allografts to PVG and PVG C6- rats was compared by immunoperoxidase staining and semiquantitative reverse transcriptase polymerase chain reaction. The ability of pure populations of T cells or alloantibody to mediate DA heart graft rejection in irradiated (750 rads) PVG and PVG C6- rats was also compared.

RESULTS

The median rejection time of DA heart allografts was 8 days in PVG rats and 17.5 days in PVG C6-. PVG C6- rats sensitized to DA by two skin grafts rejected DA heart grafts in 5-6 days. CD3+, CD4+, CD8+, interleukin-2 receptor-positive T cell, macrophage, and natural killer cell infiltration, as well as class II major histocompatibility complex and intercellular adhesion molecule-1 up-regulation, in grafts was similar in naive PVG and PVG C6- rats. mRNA for T helper 1 cytokine interleukin-2, interferon-gamma, tumor necrosis factor-beta, macrophage molecules tumor necrosis factor-alpha, and inducible nitric oxide synthase, as well as cytotoxic T-cell effector molecules perforin and granzyme A and B, were found to be the same in the grafts from both naive PVG and naive PVG C6- rats. Thus, there appeared to be no difference in the T-cell effector response between the PVG and PVG C6- groups. There were higher alloantibody titers in PVG C6- rats than in PVG hosts. Irradiation ablated rejection and alloantibody responses and reconstitution with naive T cells alone restored rejection in both PVG and PVG C6- rats. Irradiated rats given serum from PVG rats that had rejected DA grafts did not effect rejection of DA grafts even if given naive T cells. Sensitized T cells restored second set.

CONCLUSIONS

PVG C6- rats have normal T-cell responses and can mediate allograft rejection in the absence of alloantibody. The failure of PVG C6- to reject allografts rapidly may be a result of the poor clearance of alloantisera leading to enhancement of graft survival rather than a critical role for complement and membrane attack complex in acute rejection.

Induction of allograft nonresponsiveness after intrathymic inoculation with donor class I allopeptides. I. Correlation of graft survival with antidonor IgG antibody subclasses

We have recently demonstrated that cardiac allograft rejection in the PVG.R8-to-PVG.1U rat strain combination involves the recognition of a isolated class I (RT1.Aa) molecules as peptides in the context of the recipient MHC molecules. Three synthetic peptides (P1, P2, and P3) corresponding to the alpha-helices of the RT1.Aa molecule served as T-cell epitopes for graft rejection. In this study, we demonstrate that two of these peptides (P2 and P3) are sufficient to induce immune nonresponsiveness (median survival time >237 days) to cardiac allografts when presented to the recipient immune system in the thymus 7 days before transplantation. This effect was time dependent, as intrathymic inoculation 60 days before transplantation did not prolong graft survival (median survival time=12 days). Previous studies have demonstrated a critical role for alloantibody responses in mediating graft rejection in this rat strain combination. We, therefore, studied the role alloantibody responses may play in the observed immune nonresponsiveness. The titers of alloantibody in serum samples harvested from graft recipients at different times after transplantation were measured. We used recipient primary aortic endothelial cells genetically manipulated to express the donor RT1.Aa molecule as targets in an enzyme-linked immunosorbent assay. High titers of anti-RT1.Aa IgM antibody were detected in unmanipulated controls at the time of graft rejection. The IgM antibody switched to high IgG titers in intrathymically inoculated rats with accelerated or delayed rejection. Graft rejection in intrathymically manipulated recipients that had achieved a transient state of immunological nonresponsiveness correlated with higher titers of the IgG2b alloantibody. In marked contrast, the long-term graft survivors expressed undetectable or low levels of the IgG2b antibody and moderate to high levels of the IgG1 and IgG2a subclasses. These data suggest that the IgG2b alloantibody may contribute to the rejection reaction, whereas IgG1 and IgG2a may be involved in active enhancement of graft survival.

Reduction of alloantibody response to class I major histocompatibility complex by targeting synthetic allopeptides for presentation by B cells

BACKGROUND

PVG.RT1(u) rats develop a strong CD4 T cell-dependent alloantibody response to class I major histocompatibility complex (MHC) A(a) antigen, during which CD4 T helper cells recognize and respond to A(a)-derived peptides presented by recipient class II MHC (indirect allorecognition). On the basis of evidence that CD4 T cells that encounter antigen presented by resting B cells become tolerant, we have targeted synthetic A(a)-derived allopeptides for in vivo presentation to class I MHC-disparate CD4 T cells by resting recipient B cells.

METHODS

PVG.RT1(u) rats were treated with two peptides, P1 and P2, corresponding to the alpha-helical regions of A(a) (residues 57-80 and 143-163), which were conjugated via an N-terminal cysteine residue to monovalent Fab fragments of OX60 monoclonal antibody, which labels membrane IgD-positive B cells.

RESULTS

RT1(u) rats primed with free (nonconjugated) P1 or P2 emulsified in complete Freund's adjuvant produced strong peptide-specific antibody responses and a heightened anti-A(a) antibody response to an A(a)-disparate PVG.R8 heart graft, confirming that each peptide encompasses one or more major T cell determinant for B cell help. Pretreatment of PVG.RT1(u) rats with a mixture of OX60-Fab-P1/P2 conjugates markedly reduced their ability to mount an A(a) antibody response when challenged with either A(a)-disparate blood transfusion or an A(a)-disparate heart graft, although PVG.R8 heart graft survival was not prolonged.

CONCLUSIONS

In this report, we show that synthetic A(a)-derived allopeptides are able, when targeted for in vivo presentation to CD4 T cells by resting B cells, to impair the ability of RT1(u) rats to mount an antibody response to A(a) antigen. All subclasses of IgG anti-A(a) alloantibody were profoundly reduced, suggesting that the responsible mechanism is more likely to be CD4 T helper cell unresponsiveness rather than Th1/Th2 T cell polarization.

Induction of transplantation tolerance by chimeric donor/recipient class I RT1.Aa molecules

Donor-specific transplantation tolerance was induced by administration of chimeric antigens in which four donor immunogenic amino acids (a.a.) were substituted onto the host class I MHC protein. We constructed chimeric rat RT1.Aa cDNA molecules by substituting nucleotides in the alpha1 helical region that encode 10 Lewis (LEW; RT1.A1) a.a., namely Asp58, Arg62, Glu63, Gln65, Lys66, Gly69, Asn70, Asn73, Ser77, and Asn80 ([alpha(1h)1]-RT1.Aa). The chimeric [alpha(1h)1]-RT1.Aa cDNA sequence was verified before transfection into Buffalo (BUF; RT1b) hepatoma cells. Interestingly, the helical regions of LEW rats (alpha(1h)1) and Wistar Furth (WF; RT1u) rats (alpha(1h)u) share four a.a. (Arg62, Glu63, Gln65, and Gly69). Consequently, subcutaneous administration of [alpha(1)1]-RT1.Aa transfectants (20x10(6); day -7) immunized BUF rats to reject in rapid fashion either LEW heart allografts (mean survival time [MST] = 4.2+/-0.4 days vs. 5.6+/-0.5 days in controls; P<0.001) or WF heart allografts (MST=4.4+/-0.6 days vs. 6.0+/-0.0 days in controls; P<0.002). Subcutaneous immunization of ACI (RT1a) rats with [a(1)1]-RT1.Aa transfectants (bearing 10 LEW donor a.a.) accelerated the rejection of LEW hearts (MST=5.0+/-0.8 days vs. 8.2+/-0.4 days in controls; P<0.001). In contrast, the same [a(1)1]-RT1.Aa transfectants (bearing only four WF donor a.a.) injected subcutaneously into ACI rats modestly prolonged the survival of WF hearts to 14.0+/-10.3 days from 5.4+/-0.5 days in controls (P<0.001). Furthermore, ACI recipients were rendered tolerant to WF heart allografts by a single injection via the portal vein of soluble [a(1)1]-RT1.Aa (but not RT1.Aa, RT1.Au, or [a(2)1]-RT1.Aa) antigens in conjunction with brief oral gavage treatment with cyclosporine. Thus, selected donor immunogenic a.a. (Arg62, Glu63, Gln65, and Gly69) of class I MHC antigens become tolerogenic when flanked by host sequences.

Alloantibody and intragraft cellular response to MHC class I-disparate kidney allografts in recipients tolerized by donor-specific transfusion and cyclosporine

Congenic PVG.RT1u rats rapidly reject Aa class I-disparate kidney allografts from recombinant PVG R8 donors and we recently demonstrated that anti-class I MHC alloantibody plays a critical role in effecting acute rejection in this experimental model. In this article, we show that PVG.RT1u recipients can be rendered permanently and specifically tolerant to R8 kidney allografts by administration of four weekly donor-specific transfusions (DST) combined with a 7-day course of cyclosporine given with the first DST. Tolerance induction correlated with abrogation of a cytotoxic alloantibody response by thymus-independent, i.e., peripheral mechanisms; IgM and all IgG subclasses of anti-class I alloantibody were abolished. In contrast, nonrejecting kidney allografts in tolerant rats and rejecting grafts from unmodified recipients were similarly infiltrated by mononuclear cells, and intragraft transcripts for interleukin (IL)-2, interferon-gamma, and IL-13 were readily detected by reverse transcriptase polymerase chain reaction with no apparent quantitative difference between the two groups. Messenger RNA for IL-4 and IL-10 was present in rejecting grafts but barely detectable in grafts from tolerant animals. These results suggest that tolerance induction by DST and cyclosporine is, in this experimental model, associated with a selective impairment in humoral alloimmunity.

Interleukin-12 induces interferon-gamma-dependent switching of IgG alloantibody subclass

Interleukin-12 (IL-12) is a key immunoregulatory cytokine which promotes cell-mediated immunity. Its influence on the humoral immune response is less clearly defined. In this study, the effect of systemic IL-12 treatment on the T cell-dependent humoral immune response in the rat was examined using an experimental system in which PVG-RT1u congenic rats were immunized with class I Aa alloantigen in the form of a blood transfusion from the recombinant PVG.R8 rat strain. Administration of IL-12 following allo-immunization augmented the humoral immune response as determined by increased levels of cytotoxic anticlass I major histocompatibility complex antibodies. However, the effect of IL-12 on individual IgG isotypes was highly selective. Levels of allospecific antibodies of the IgG2b and IgG2c subclasses were markedly increased, whereas IgG1 alloantibody levels were profoundly reduced. The observed alterations in alloantibody response were dependent, in large part, on the stimulatory effect of IL-12 on interferon (IFN)-gamma production by T lymphocytes and natural killer cells, since they were abrogated by co-administration of neutralizing anti-IFN-gamma monoclonal antibody following alloantigen immunization.

Analysis of effector mechanisms in murine cardiac allograft rejection

Multiple effector cells have been implicated in transplant rejection, including cytotoxic T cells, B cells, macrophages and NK cells. The purpose of this study was to examine the effector pathways which are critical to murine cardiac allograft rejection. RT-PCR (reverse transcriptase-polymerase chain reaction) analysis of syngeneic and allogeneic vascularized heterotopic cardiac grafts at 5, 8 and 12 days following transplantation demonstrate constitutive expression of Fas in both the syngeneic and allogeneic grafts as well as in normal heart. However, FasL, granzyme, and perforin expression were shown to be up-regulated on days 5-12 in the allograft with no expression in syngeneic grafts or in normal hearts. We have recently analyzed the functional significance of T cell cytotoxic pathways and found that neither the Fas nor CD8+ cytotoxic pathways are required for murine cardiac allograft rejection. In light of these results, we investigated the functional significance of other effector cells in the rejection process. B cell deficient C57BL/10-IgHtm1Cgn mice rejected cardiac allografts from normal donors at control rate. Finally, RT-PCR was used to analyze the expression of macrophage effector transcripts in allograft rejection. Transcripts for iNOS (inducible nitric oxide synthase) and TNF alpha (tumor necrosis factor-alpha) were up-regulated on days 5-12 in untreated allografts with undetectable expression in normal heart or syngeneic grafts. These results demonstrate that effective allograft rejection can occur in the absence of B cells and T cell cytotoxicity pathways suggesting that other effector pathways, such as delayed-type hypersensitivity responses by macrophages, may be critical for allograft rejection.

Induction of specific allograft immunity by soluble class I MHC heavy chain protein produced in a baculovirus expression system

Spodoptera frugiperda (Sf9) insect cells secreted a class I MHC RT1.Aa heavy chain protein when infected with baculovirus that bore a construct that contained a honeybee melittin secretion (ms) signal attached to RT1.Aa cDNA. The RT1.Aa heavy chain protein in the culture supernatant and cell lysate immunoprecipitated in the presence of 5 individual anti-RT1.Aa-specific mAb. As was revealed by densitometric analysis, the ms signal increased the production (7- to 17-fold) and secretion (20- to 47-fold) of RT1.Aa protein by Sf9 cells (compared with RT1Aa-Sf9 cells without the ms signal). Subcutaneous immunization with secreted RT1.Aa heavy chain protein of Wistar-Furth (WF; RT1u) rats (day -4) accelerated the rejection of ACI (RT1a), but not third-party Brown Norway (BN; RT1n), heart allografts from 5.9 +/- 0.5 days in controls to 4.0 +/- 0.0 days (P < 0.001); cell lysate from RT1.Aa-Sf9 or ms/RT1.Aa-Sf9 cells reduced ACI heart allograft survival to 3.8 +/- 0.4 days or 3.7 +/- 0.5 days, respectively (P < 0.001). Indirect presentation of RT1.Aa heavy chain proteins by syngeneic macrophages shortened the survival of RT1.Aa-disparate PVG.R8 (RT1.AaDuBuCu) heart allografts in PVG.1U (RT1u) hosts from 6.3 +/- 0.5 days in controls to 4.0 +/- 0.0 days (P < 0.01). Finally, RT1.Aa heavy chain proteins injected into the thymus or into the portal vein (day -14) in combination with anti-T cell receptor mAb (days -14 and -13) induced indefinite survival of ACI liver allografts in Lewis (RT1l) recipients ( > 250 days). Thus, indirect presentation of soluble class I MHC heavy chain proteins (produced in a baculovirus/Sf9 cell system) may either sensitize or induce tolerance in the same fashion as native class I MHC alloantigens expressed on donor tissues.

Cytokines and cell surface markers in prediction of cardiac allograft rejection

Endomyocardial biopsy is generally used to quantify heart allograft rejection and guide immunotherapy. Biopsy, however, is invasive, costly, and risky. Since rejection requires lymphocyte activation, the purpose of this study was to assess alternative methods to evaluate rejection dynamics by investigating serum levels of cytokines and cell surface markers after heart transplantation. Interleukin-2-receptor bearing CD4+T (IL-2R/CD4) cell levels were higher in the peripheral blood of human transplant recipients with rejection grade 2 (p < 0.02). HLA-DR/CD3 levels were somewhat higher in rejection grade 2. There was no correlation between biopsy scores and serum levels of tumor necrosis factor (TNF-alpha), IL-2, or percentage of T cell, NK cell, B cell, CD4+T cell, CD8+T cell, HLA-DR/CD4, HLA-DR/CD8, IL-2R/CD3, IL-2R/CD8. Interleukin-1 (IL-1 beta) was not detectable in all of the samples. The current studies suggest that monitoring lymphocyte IL-2R/CD4 and HLA-DR/CD3 levels is useful in predicting cardiac transplant rejection.

Indirect T cell recognition in allograft rejection

Allograft rejection has been attributed predominantly to alloreactive T cells which recognize intact allogeneic MHC on donor antigen presenting cells (APC). Following transplantation, T cells may also recognize donor derived MHC antigens which have been processed and presented as antigenic peptides in the context of self-MHC by recipient APC--so called indirect allorecognition. This article discusses the possible role of indirect T cell recognition in the graft rejection response with particular emphasis on recent experimental data obtained from in vivo transplant models. Although the relative contribution of indirect allorecognition to the rejection process remains unclear the available evidence suggests that the contribution of self-MHC restricted T cells to graft rejection may have been underestimated.