Divergent role of donor dendritic cells in rejection versus tolerance of allografts

Alloreactive memory CD4 T cells promote transplant rejection by engaging DCs to induce innate inflammation and CD8 T cell priming

Alloreactive memory T cells have been implicated as central drivers of transplant rejection. Perplexingly, innate cytokines, such as IL-6, IL-1β, and IL-12, are also associated with rejection of organ transplants. However, the pathways of innate immune activation in allogeneic transplantation are unclear. While the role of microbial and cell death products has been previously described, we identified alloreactive memory CD4 T cells as the primary triggers of innate inflammation. Memory CD4 T cells engaged MHC II-mismatched dendritic cells (DCs), leading to the production of innate inflammatory cytokines. This innate inflammation was independent of several pattern recognition receptors and was primarily driven by TNF superfamily ligands expressed by alloreactive memory CD4 T cells. Blocking of CD40L and TNFα resulted in dampened inflammation, and mice genetically deficient in these molecules exhibited prolonged survival of cardiac allografts. Furthermore, myeloid cell and CD8 T cell infiltration into cardiac transplants was compromised in both CD40L- and TNFα-deficient recipients. Strikingly, we found that priming of naive alloreactive CD8 T cells was dependent on licensing of DCs by memory CD4 T cells. This study unravels the key mechanisms by which alloreactive memory CD4 T cells contribute to destructive pathology and transplant rejection.

Failure of Costimulatory Blockade-induced Regulatory T Cells to Sustain Long-term Survival of High Ischemic Allografts

BACKGROUND

Costimulatory blockade-induced allograft tolerance has been achieved in rodent models, but these strategies do not translate well to nonhuman primate and clinical transplants. One confounder that may underlie this discrepancy is the greater ischemic inflammation imposed on the transplants. In mice, cardiac allografts subjected to prolonged cold ischemic storage (CIS) before transplant have increased ischemia-reperfusion injury, which amplifies infiltrating endogenous memory CD8 T-cell activation within hours after transplantation to mediate acute graft inflammation and cytotoxic lymphocyte-associated molecule-4 immunoglobulin-resistant rejection. This study tested strategies inhibiting memory CD8 T-cell activation within such high ischemic allografts to achieve long-term survival.

METHODS

A/J (H-2 a ) hearts subjected to 0.5 or 8 h of CIS were transplanted to C57BL/6 (H-2 b ) recipients and treatment with peritransplant costimulatory blockade. At 60 d posttransplant, regulatory T cells (Treg) were depleted in recipients of high ischemic allografts with anti-CD25 monoclonal antibody (mAb) or diphtheria toxin.

RESULTS

Whereas peritransplant (days 0 and +1) anti-lymphocyte function-associated antigen-1 mAb and anti-CD154 mAb prolonged survival of >60% allografts subjected to minimal CIS for >100 d, only 20% of allografts subjected to prolonged CIS survived beyond day 80 posttransplant and rejection was accompanied by high titers of donor-specific antibody. Peritransplant anti-lymphocyte function-associated antigen-1, anti-tumor necrosis factor-α, and anti-CD154 mAb plus additional anti-CD154 mAb on days 14 and 16 obviated this donor-specific antibody and promoted Treg-mediated tolerance and survival of 60% of high ischemic allografts beyond day 100 posttransplant, but all allografts failed by day 120.

CONCLUSIONS

These studies indicate a strategy inducing prolonged high ischemic allograft survival through Treg-mediated tolerance that is not sustained indefinitely.

Bone marrow-derived AXL tyrosine kinase promotes mitogenic crosstalk and cardiac allograft vasculopathy

Cardiac Allograft Vasculopathy (CAV) is a leading contributor to late transplant rejection. Although implicated, the mechanisms by which bone marrow-derived cells promote CAV remain unclear. Emerging evidence implicates the cell surface receptor tyrosine kinase AXL to be elevated in rejecting human allografts. AXL protein is found on multiple cell types, including bone marrow-derived myeloid cells. The causal role of AXL from this compartment and during transplant is largely unknown. This is important because AXL is a key regulator of myeloid inflammation. Utilizing experimental chimeras deficient in the bone marrow-derived Axl gene, we report that Axl antagonizes cardiac allograft survival and promotes CAV. Flow cytometric and histologic analyses of Axl-deficient transplant recipients revealed reductions in both allograft immune cell accumulation and vascular intimal thickness. Co-culture experiments designed to identify cell-intrinsic functions of Axl uncovered complementary cell-proliferative pathways by which Axl promotes CAV-associated inflammation. Specifically, Axl-deficient myeloid cells were less efficient at increasing the replication of both antigen-specific T cells and vascular smooth muscle cells (VSMCs), the latter a key hallmark of CAV. For the latter, we discovered that Axl-was required to amass the VSMC mitogen Platelet-Derived Growth Factor. Taken together, our studies reveal a new role for myeloid Axl in the progression of CAV and mitogenic crosstalk. Inhibition of AXL-protein, in combination with current standards of care, is a candidate strategy to prolong cardiac allograft survival.

Acute and chronic phagocyte determinants of cardiac allograft vasculopathy

Post-transplant immunosuppression has reduced the incidence of T cell-mediated acute rejection, yet long-term cardiac graft survival rates remain a challenge. An important determinant of chronic solid organ allograft complication is accelerated vascular disease of the transplanted graft. In the case of cardiac allograft vasculopathy (CAV), the precise cellular etiology remains inadequately understood; however, histologic evidence hints at the accumulation and activation of innate phagocytes as a causal contributing factor. This includes monocytes, macrophages, and immature dendritic cell subsets. In addition to crosstalk with adaptive T and B immune cells, myeloid phagocytes secrete paracrine signals that directly activate fibroblasts and vascular smooth muscle cells, both of which contribute to fibrous intimal thickening. Though maladaptive phagocyte functions may promote CAV, directed modulation of myeloid cell function, at the molecular level, holds promise for tolerance and prolonged cardiac graft function.

The CD8 T-cell response during tolerance induction in liver transplantation

Both experimental and clinical studies have shown that the liver possesses unique tolerogenic properties. Liver allografts can be spontaneously accepted across complete major histocompatibility mismatch in some animal models. In addition, some liver transplant patients can be successfully withdrawn from immunosuppressive medications, developing ‘operational tolerance'. Multiple mechanisms have been shown to be involved in inducing and maintaining alloimmune tolerance associated with liver transplantation. Here, we focus on CD8 T-cell tolerance in this setting. We first discuss how alloreactive cytotoxic T-cell responses are generated against allografts, before reviewing how the liver parenchyma, donor passenger leucocytes and the host immune system function together to attenuate alloreactive CD8 T-cell responses to promote the long-term survival of liver transplants.

Live Images of Donor Dendritic Cells Trafficking via CX3CR1 Pathway

Background

A number of studies have demonstrated the role of CX3CR1 in regulating the migration of monocytes into peripheral tissue and their transformation into dendritic cell (DC). No data are yet available on the importance of chemokine pathways in regulating homeostasis of DC in heart transplants. Recently, we showed that recipients of heart allografts from CX3CR1^−/− donors show longer survival. To assess the trafficking of dDC, we have developed and tested a novel in vivo imaging tool in CX3CR1^GFP/+ DC (B6 background) heart graft into BALB/c recipient model.

Results

Majority of GFP⁺ cells were noted in the middle of cardiac myocyte. However few hours post transplant, they experienced morphological changes including stretching their extensions (3 and 24 h). However, images from 72 h at cardiac graft showed many of GFP⁺ cells moved to vessel areas. GFP⁺ cells were detected in near vessel wall. Only one GFP⁺ cell was observed in three lymph nodes (two mesenteric and one inguinal) (72 h).

Conclusion

Our data indicate that immediately post transplant dDC undergo morphological changes and traffic out of the organs via systemic circulation. While, we still noted presence of dDC in the transplanted organs, their trafficking to lymphoid tissue remains to be fully explored.

The Mononuclear Phagocyte System in Organ Transplantation

The mononuclear phagocyte system (MPS) comprises monocytes, macrophages and dendritic cells (DCs). Over the past few decades, classification of the cells of the MPS has generated considerable controversy. Recent studies into the origin, developmental requirements and function of MPS cells are beginning to solve this problem in an objective manner. Using high-resolution genetic analyses and fate-mapping studies, three main mononuclear phagocyte lineages have been defined, namely, macrophage populations established during embryogenesis, monocyte-derived cells that develop during adult life and DCs. These subsets and their diverse subsets have specialized functions that are largely conserved between species, justifying the introduction of a new, universal scheme of nomenclature and providing the framework for therapeutic manipulation of immune responses in the clinic. In this review, we have commented on the implications of this novel MPS classification in solid organ transplantation.

Cardiac allograft vasculopathy: a donor or recipient induced pathology?

Cardiac allograft vasculopathy (CAV) is one of the main causes of late-stage heart failure after heart transplantation. CAV is characterized by concentric luminal narrowing of the coronary arteries, but the exact pathogenesis of CAV is still not unraveled. Many researchers show evidence of an allogeneic immune response of the recipient, whereas others show contrasting results in which donor-derived cells induce an immune response against the graft. In addition, fibrosis of the neo-intima can be induced by recipient-derived circulating cells or donor-derived cells. In this review, both donor and recipient sides of the story are described to obtain better insight in the pathogenesis of CAV. Dual outcomes were found regarding the contribution of donor and recipient cells in the initiation of the immune response and the development of fibrosis during CAV. Future research could focus more on the potential synergistic interaction of donor and recipient cells leading to CAV.

TIM4 Regulates the Anti-Islet Th2 Alloimmune Response

The role of the novel costimulatory molecule TIM4 in anti-islet response is unknown. We explored TIM4 expression and targeting in Th1 (BALB/c islets into C57BL/6 mice) and Th2 (BALB/c islets into Tbet(-/-) C57BL/6 mice) models of anti-islet alloimmune response and in a model of anti-islet autoimmune response (diabetes onset in NOD mice). The targeting of TIM4, using the monoclonal antibody RMT4-53, promotes islet graft survival in a Th1 model, with 30% of the graft surviving in the long term; islet graft protection appears to be mediated by a Th1 to Th2 skewing of the immune response. Differently, in the Th2 model, TIM4 targeting precipitates graft rejection by further enhancing the Th2 response. The effect of anti-TIM4 treatment in preventing autoimmune diabetes was marginal with only minor Th1 to Th2 skewing. B-Cell depletion abolished the effect of TIM4 targeting. TIM4 is expressed on human B-cells and is upregulated in diabetic and islet-transplanted patients. Our data suggest a model in which TIM4 targeting promotes Th2 response over Th1 via B-cells. The targeting of TIM4 could become a component of an immunoregulatory protocol in clinical islet transplantation, aiming at redirecting the immune system toward a Th2 response.

Delivery of a granzyme B inhibitor gene using carbamate-mannose modified PEI protects against cytotoxic lymphocyte killing

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells protect vertebrates by killing infected or transformed cells using granzyme B (GrB) to induce apoptosis. However, GrB-induced apoptosis of target cells causes inflammatory disease and chronic transplant rejection and so is an important disease target. The aim of this study was to prevent apoptosis of the target cells by delivering a plasmid encoding GrB inhibitor proteinase inhibitor-9 (PI-9) using cationic polymers as a non-viral vector. Polyethyleneimine (PEI, branched, Mn 10 kDa) gives a high degree of gene transfection efficiency in many types of cell lines, but it is highly cytotoxic. To reduce this cytotoxicity, we modified PEI by blocking primary amine groups through nucleophilic addition between primary amine and a protected mannose-functionalized cyclic carbonate (MTC-ipman), generating a carbamate linkage through the ring-opening of the cyclic carbonate. Deprotection of the mannose yielded a PEI polymer that is decorated with the carbohydrate. PEI with 7 or 20 of 67 primary amine groups substituted by the carbohydrate had similar gene binding ability compared to unmodified PEI, leading to almost 100% transfection efficiency of a GFP-reporter plasmid in HEK293T human embryonic kidney cells. Furthermore, modification of PEI resulted in a decrease in the cytotoxicity of PEI/DNA complexes. However, PEI with all primary amine groups blocked was unable to form a complex with DNA, and so reporter transfection was negligible. The PI-9 encoding plasmid was transfected into HEK293T cells effectively using the modified PEIs with the optimal degree of primary amine substitution, protecting up to 80% HEK293T cells from killing by human natural killer-like leukemic YT cells. Therefore, these carbamate-mannose modified PEI/PI-9 encoding plasmid complexes have potential clinical utility in the prevention of chronic transplant rejection and inflammatory disease caused by GrB.

Bidirectional alloreactivity: A proposed microchimerism-based solution to the NIMA paradox

The NIMA paradox is the observation that in transplants of allogeneic kidneys or hematopoietic stem cells, siblings benefit from re-exposure to non-inherited maternal antigens (NIMA), whereas re-exposure to a transplant from mother herself, theoretically the ideal "NIMA" donor, does not yield clinical results superior to a father-donated allograft. Recent observations of bidirectional alloreactivity in kidney and cord blood transplantation offer a possible solution to this paradox. If correct, the proposed solution points the way to clinical applications of microchimerism in solid organ and hematopoetic transplants.

Essential role of PDL1 expression on nonhematopoietic donor cells in acquired tolerance to vascularized cardiac allografts

The PD1:PDL1 pathway is an essential negative costimulatory pathway that plays a key role in regulating the alloimune response. PDL1 is expressed not only on antigen-presenting cells (APCs) but also cardiac endothelium. In this study, we investigated the importance of PDL1 expression on donor cardiac allograft in acquired transplantation tolerance in a fully MHC-mismatched model. We generated PDL1 chimeric mice on B6 background that expressed PDL1 on either hematopoietic cells or nonhematopoietic cells of the heart. Sham animals were used as controls. These hearts were then transplanted into BALB/c recipients and treated with CTLA4-Ig to induce tolerance. Cardiac endothelium showed significant expression of PDL1, which was upregulated upon transplantation. While the absence of PDL1 on hematopoietic cells of the heart resulted in delayed rejection and prevented long-term tolerance in most but not all recipients, we observed an accelerated and early graft rejection of all donor allografts that lacked PDL1 on the endothelium. Moreover, PDL1-deficient endothelium hearts had significant higher frequency of IFN-γ-producing alloreactive cells as well as higher frequency of CD8(+) effector T cells. These findings demonstrate that PDL1 expression mainly on donor endothelium is functionally important in a fully allogeneic mismatched model for the induction of cardiac allograft tolerance.

Donor antioxidant strategy prolongs cardiac allograft survival by attenuating tissue dendritic cell immunogenicity(†)

Ischemic reperfusion injury (IRI) enhances allograft immunogenicity, worsens transplantation outcome, and is the primary cause of activation of the recipient innate immune response, resulting in subsequent amplification of the alloimmune adaptive response. Here, we aimed at demonstrating that the link between innate injury and alloimmunity occurs predominantly through activation of allograft-derived dendritic cells (ADDC). Perfusion of MCI-186, a free radical scavenger, into donor cardiac allografts prior to transplantation resulted in prolongation of complete MHC-mismatched allograft survival in the absence of immunosuppression (MST of 8 vs. 26 days). This prolongation was associated with a reduction in trafficking of ADDC to recipient lymphoid tissue as well as a reduction in T cell priming. Depleting ADDC with diphtheria toxin (using DTR-GFP-DC mice as donors) 24 h prior to transplant resulted in abrogation of the prolongation observed with MCI-186 treatment, demonstrating that the beneficial effect of MCI-186 is mediated by ADDC. This donor-specific anti-ischemic regimen was also shown to reduce chronic rejection, which represents the primary obstacle to long-term allograft acceptance. These data for the first time establish a basis for donor anti-ischemic strategies, which in the ever-expanding marginal donor pools, can be instituted to promote engraftment.

Impact of chemokine receptor CX3CR1 in human renal allograft rejection

Chemokine receptors play pivotal roles for leukocyte recruitment in acute and chronic inflammatory processes. This study was performed to analyze the expression, distribution and cellular localization of CX3CR1 in human renal transplant biopsies and to assess its role as potential diagnostic and prognostic marker. CX3CR1 was prospectively analyzed in 174 renal graft biopsies from patients with normal morphology (n=76), antibody-mediated acute rejection (n=6), acute tubulointerstitial rejection (n=27), acute vascular rejection (n=31), and with acute tubulus necrosis (n=34). Double immunofluorescence was additionally performed for CX3CR1 and CD4, CD8, CD20, CD68, and CD209/DC-SIGN. The number of CX3CR1 positive interstitial cells was significantly higher in the biopsies with acute tubulointerstitial and acute vascular rejection as compared to normal renal allograft biopsies. CX3CR1 positive cells were mainly CD68 positive monocytes/macrophages and CD209/DC-SIGN positive dendritic cells. The percentage of the CX3CR1 positive staining area was a predictor for steroid responsiveness and for worse clinical outcome 3 and 12 months after transplantation. CX3CR1 positive macrophages and/or dendritic cells are significantly elevated in acute renal allograft rejection. As CX3CR1 was associated with outcome parameters, it has to be further evaluated as a prognostic marker in human renal transplantation.

Ischemic injury enhances dendritic cell immunogenicity via TLR4 and NF-kappa B activation

Ischemic (isc) injury during the course of transplantation enhances the immunogenicity of allografts and thus results in poorer graft outcome. Given the central role of dendritic cells (DCs) in mounting alloimmune responses, activation of donor DCs by ischemia may have a primary function in the increased immunogenicity of isc allografts. In this study, we sought to investigate the effect of ischemia on DC activity in vitro. Following induction of ischemia, bone marrow-derived DCs were shown to augment allogeneic T cell proliferation as well as the IFN-gamma response. Isc DCs produced greater levels of IL-6, and isc insult was concurrent with NF-kappaB activation. TLR4 ligation was also shown to occur in isc DCs, most likely in response to the endogenous ligand heat shock protein 70, which was found to be elevated in DCs following isc injury, and lack of TLR4 abrogated the observed effects of isc DCs. As compared with control DCs, isc DCs injected into the footpads of mice demonstrated enhanced migration, which was concomitant with increased recipient T cell activity. Moreover, isc DCs underwent a greater degree of apoptosis in the lymph nodes of injected mice, which may further demonstrate enhanced immunogenicity of isc DCs. We thus show that isc injury of DCs enhances DC function, augments the allogeneic T cell response, and occurs via ligation of TLR4, followed by activation of NF-kappaB. These data may serve to identify novel therapeutic targets to attenuate graft immunogenicity following ischemia.

Tolerogenic dendritic cells: all present and correct?

Although well-recognized for their sentinel role and, when activated, their immunostimulatory function, bone marrow-derived dendritic cells (DC) possess inherent tolerogenic (tol) ability. Under quiescent conditions, these cells maintain central and peripheral self tolerance. When appropriately conditioned, in vitro or in vivo, they inhibit innate and adaptive immunity to foreign antigens, including memory T-cell responses. This suppressive function is mediated by various mechanisms, including the expansion and induction of antigen-specific regulatory T cells. Extensive experience in rodent models and recent work in nonhuman primates, indicate the potential of pharmacologically-modified, tol DC (tolDC) to regulate alloimmunity in vivo and to promote lasting, alloantigen-specific T-cell unresponsiveness and transplant survival. While there are many questions yet to be addressed concerning the functional biology of tolDC in humans, these cells offer considerable potential as natural, safe and antigen-specific regulators for long-term control of the outcome of organ and hematopoietic cell transplantation. This minireview surveys recent findings that enhance understanding of the functional biology and therapeutic application of tolDC, with special reference to transplantation.