Indirect allorecognition is involved in both acute and chronic allograft rejection

Strategies for Cancer Immunotherapy Using Induced Pluripotency Stem Cells-Based Vaccines

Despite improvements in cancer therapy, metastatic solid tumors remain largely incurable. Immunotherapy has emerged as a pioneering and promising approach for cancer therapy and management, and in particular intended for advanced tumors unresponsive to current therapeutics. In cancer immunotherapy, components of the immune system are exploited to eliminate cancer cells and treat patients. The recent clinical successes of immune checkpoint blockade and chimeric antigen receptor T cell therapies represent a turning point in cancer treatment. Despite their potential success, current approaches depend on efficient tumor antigen presentation which are often inaccessible, and most tumors turn refractory to current immunotherapy. Patient-derived induced pluripotent stem cells (iPSCs) have been shown to share several characteristics with cancer (stem) cells (CSCs), eliciting a specific anti-tumoral response when injected in rodent cancer models. Indeed, artificial cellular reprogramming has been widely compared to the biogenesis of CSCs. Here, we will discuss the state-of-the-art on the potential implication of cellular reprogramming and iPSCs for the design of patient-specific immunotherapeutic strategies, debating the similarities between iPSCs and cancer cells and introducing potential strategies that could enhance the efficiency and therapeutic potential of iPSCs-based cancer vaccines.

Costimulation-adhesion blockade is superior to cyclosporine A and prednisone immunosuppressive therapy for preventing rejection of differentiated human embryonic stem cells following transplantation

RATIONALE

Human embryonic stem cell (hESC) derivatives are attractive candidates for therapeutic use. The engraftment and survival of hESC derivatives as xenografts or allografts require effective immunosuppression to prevent immune cell infiltration and graft destruction.

OBJECTIVE

To test the hypothesis that a short-course, dual-agent regimen of two costimulation-adhesion blockade agents can induce better engraftment of hESC derivatives compared to current immunosuppressive agents.

METHODS AND RESULTS

We transduced hESCs with a double fusion reporter gene construct expressing firefly luciferase (Fluc) and enhanced green fluorescent protein, and differentiated these cells to endothelial cells (hESC-ECs). Reporter gene expression enabled longitudinal assessment of cell engraftment by bioluminescence imaging. Costimulation-adhesion therapy resulted in superior hESC-EC and mouse EC engraftment compared to cyclosporine therapy in a hind limb model. Costimulation-adhesion therapy also promoted robust hESC-EC and hESC-derived cardiomyocyte survival in an ischemic myocardial injury model. Improved hESC-EC engraftment had a cardioprotective effect after myocardial injury, as assessed by magnetic resonance imaging. Mechanistically, costimulation-adhesion therapy is associated with systemic and intragraft upregulation of T-cell immunoglobulin and mucin domain 3 (TIM3) and a reduced proinflammatory cytokine profile.

CONCLUSIONS

Costimulation-adhesion therapy is a superior alternative to current clinical immunosuppressive strategies for preventing the post-transplant rejection of hESC derivatives. By extending the window for cellular engraftment, costimulation-adhesion therapy enhances functional preservation following ischemic injury. This regimen may function through a TIM3-dependent mechanism.

Immunogenicity of pluripotent stem cells and their derivatives

The ability of pluripotent stem cells to self-renew and differentiate into all somatic cell types brings great prospects to regenerative medicine and human health. However, before clinical applications, much translational research is necessary to ensure that their therapeutic progenies are functional and nontumorigenic, that they are stable and do not dedifferentiate, and that they do not elicit immune responses that could threaten their survival in vivo. For this, an in-depth understanding of their biology, genetic, and epigenetic make-up and of their antigenic repertoire is critical for predicting their immunogenicity and for developing strategies needed to assure successful long-term engraftment. Recently, the expectation that reprogrammed somatic cells would provide an autologous cell therapy for personalized medicine has been questioned. Induced pluripotent stem cells display several genetic and epigenetic abnormalities that could promote tumorigenicity and immunogenicity in vivo. Understanding the persistence and effects of these abnormalities in induced pluripotent stem cell derivatives is critical to allow clinicians to predict graft fate after transplantation, and to take requisite measures to prevent immune rejection. With clinical trials of pluripotent stem cell therapy on the horizon, the importance of understanding immunologic barriers and devising safe, effective strategies to bypass them is further underscored. This approach to overcome immunologic barriers to stem cell therapy can take advantage of the validated knowledge acquired from decades of hematopoietic stem cell transplantation.

Biological therapy of bone defects: the immunology of bone allo-transplantation

IMPORTANCE OF THE FIELD

Bone is one of the most transplanted tissues worldwide. Autograft is the ideal bone graft but is not widely used because of donor site morbidity and restricted availability. Allograft is easily accessible but can transmit infections and elicit an immune response.

AREAS COVERED IN THIS REVIEW

This review identifies all in vitro and in vivo evidence of immune responses following bone transplantation and highlights methods of improving host tolerance to bone allotransplantation.

WHAT THE READER WILL GAIN

In humans, the presence of anti-HLA specific antibodies against freeze-dried and fresh-frozen bone allografts has been demonstrated. Fresh-frozen bone allograft can still generate immune reactions whilst freeze-dried bone allografts present with less immunogenicity but have less structural integrity. This immune response can have an adverse effect on the graft's incorporation and increase the incidence of rejection. Decreasing the immune reaction against the allograft by lowering the immunogenic load of the graft or lowering the host immune response, would result in improved bone incorporation.

TAKE HOME MESSAGE

It is essential that the complex biological processes related to bone immunogenicity are understood, since this may allow the development of safer and more successful ways of controlling the outcome of bone allografting.

Role of T cells in graft rejection and transplantation tolerance

Transplantation is the most effective treatment for end-stage organ failure, but organ survival is limited by immune rejection and the side effects of immunosuppressive regimens. T cells are central to the process of transplant rejection through allorecognition of foreign antigens leading to their activation, and the orchestration of an effector response that results in organ damage. Long-term transplant acceptance in the absence of immunosuppressive therapy remains the ultimate goal in the field of transplantation and many studies are exploring potential therapies. One promising cellular therapy is the use of regulatory T cells to induce a state of donor-specific tolerance to the transplant. This article first discusses the role of T cells in transplant rejection, with a focus on the mechanisms of allorecognition and the alloresponse. This is followed by a detailed review of the current progress in the field of regulatory T-cell therapy in transplantation and the translation of this therapy to the clinical setting.

Monitoring of indirect allorecognition: wishful thinking or solid data?

Monitoring of T cells involved in the alloimmune response after transplantation requires the availability of reliable in vitro assays for the detection of T cells with both direct and indirect allospecificity. While generally accepted assays exist to measure helper and cytotoxic T cells involved in direct allorecognition, consensus about an assay for monitoring indirect T-cell allorecognition in clinical transplantation is lacking. Many studies claim a relationship between the reactivity of T cells with indirect allospecificity and graft rejection, but different protocols are used and essential controls are often lacking. In this review, the disadvantages and pitfalls of the current approaches are discussed, in some cases supported by the results of our own in vitro experiments. We conclude that an international workshop is necessary to establish and validate a uniform, robust and reliable assay for the monitoring of transplant recipients and to study the actual role of indirect allorecognition in acute and chronic rejection.

A novel pathway of alloantigen presentation by dendritic cells

In the context of transplantation, dendritic cells (DCs) can sensitize alloreactive T cells via two pathways. The direct pathway is initiated by donor DCs presenting intact donor MHC molecules. The indirect pathway results from recipient DCs processing and presenting donor MHC as peptide. This simple dichotomy suggests that T cells with direct and indirect allospecificity cannot cross-regulate each other because distinct APCs are involved. In this study we describe a third, semidirect pathway of MHC alloantigen presentation by DCs that challenges this conclusion. Mouse DCs, when cocultured with allogeneic DCs or endothelial cells, acquired substantial levels of class I and class II MHC:peptide complexes in a temperature- and energy-dependent manner. Most importantly, DCs acquired allogeneic MHC in vivo upon migration to regional lymph nodes. The acquired MHC molecules were detected by Ab staining and induced proliferation of Ag-specific T cells in vitro. These data suggest that recipient DCs, due to acquisition of donor MHC molecules, may link T cells with direct and indirect allospecificity.

CD4+CD25+ regulatory T cells do not significantly contribute to direct pathway hyporesponsiveness in stable renal transplant patients

CD4(+)CD25(+) regulatory T cells have been shown to regulate a variety of autoimmune and allogeneic responses in mice and humans. The role of CD4(+)CD25(+) cells in regulating alloresponses in human transplant recipients remains uncertain. Previous research has demonstrated a reduced frequency of direct pathway donor-specific T cells in renal transplant recipients when compared with the frequency of T cells reactive to an HLA-matched third party. A number of mechanisms have been proposed to account for this finding; the purpose of this study was to determine whether CD4(+)CD25(+) cells play a significant role. Twelve stable renal transplant patients were investigated using limiting dilution assay (LDA) and ELISPOT for interferon-gamma to determine the effect of depleting CD4(+)CD25(+) cells on the direct pathway alloresponse. The percentage of CD4(+)CD25(+) cells in the peripheral blood of the study patients was equivalent to that of healthy controls. Furthermore, in no case did depletion of CD4(+)CD25(+) cells significantly increase the frequency of donor-specific T cells detected by LDA. This was also found with ELISPOT in all except one patient, in whom depletion revealed an increased frequency of alloreactive T cell to both donor and third party. Finally, kinetic analysis of the LDA data did not indicate regulation against donor when compared with third party. It is concluded that the action of CD4(+)CD25(+) regulatory cells is not the main mechanism of donor-specific hyporesponsiveness in the direct pathway of allorecognition.

Pathways of allorecognition: implications for transplantation tolerance

Allorecognition occurs when the host immune system detects same-species, non-self antigens and this is the trigger for allograft rejection. Host T cells detect these 'foreign' antigens which are mostly derived from a highly polymorphic region of the genome called the major histocompatibility complex. Allorecognition can occur by two distinct, but not mutually exclusive pathways: direct and indirect. The direct pathway results from the recognition of foreign major histocompatibility molecules, intact, on the surface of donor cells. Indirect allorecognition occurs when donor histocompatibility molecules are internalised, processed, and presented as peptides by host antigen presenting cells--this is the manner in which the immune system normally sees antigen. However, in addition to antigen recognition, T cell activation requires the provision of costimulatory signals, the prerogative of bone marrow-derived, specialised antigen-presenting cells (APC). Once these have been depleted from a transplanted organ, as occurs within weeks of transplantation, the parenchymal cells of the transplant are incapable of driving direct pathway activation of recipient T cells. Alloantigen recognition on these non-professional APCs may have a tolerising effect and indeed, the frequency of T cells reactive to the direct pathway diminishes with time irrespective of whether or not chronic transplant rejection occurs. This implies that while the direct pathway plays a dominant role in acute rejection, it is unlikely to contribute to chronic rejection. Assays of T cell responses have, however, found an association between the indirect pathway and chronic rejection and animal models support a role for the indirect pathway in both acute and chronic rejection. The indirect pathway is likely to be permanently active due to traffic of recipient APCs through the graft. The challenge that this poses in the pursuit of clinical tolerance is how to induce tolerance in T cells with indirect allospecificity. The answer may lie in manipulation of the environment of the interaction between the T cell and APC. Apart from recognition without costimulation, there are other circumstances when recognition without activation can occur although the in vivo relevance is uncertain. The presence of regulatory cytokines or inhibitory surface molecules either from a distinct regulatory cell, or as a negative feedback loop may prevent activation; this could also happen without sufficient stimulatory support: the final outcome is likely to be decided by the overall balance. Furthermore, some peptides may act as antagonists to T cell activation, usually when the agonist peptide is structurally very similar. It is hoped that the careful study of these mechanisms will reveal ways of ensuring allorecognition without activation and thus donor-specific tolerance.

Role of CD4+ and CD8+ T cells in allorecognition: lessons from corneal transplantation

Corneal transplantation represents an interesting model to investigate the contribution of direct vs indirect Ag recognition pathways to the alloresponse. Corneal allografts are naturally devoid of MHC class II+ APCs. In addition, minor Ag-mismatched corneal grafts are more readily rejected than their MHC-mismatched counterparts. Accordingly, it has been hypothesized that these transplants do not trigger direct T cell alloresponse, but that donor Ags are presented by host APCs, i.e., in an indirect fashion. Here, we have determined the Ag specificity, frequency, and phenotype of T cells activated through direct and indirect pathways in BALB/c mice transplanted orthotopically with fully allogeneic C57BL/6 corneas. In this combination, only 60% of the corneas are rejected, while the remainder enjoy indefinite graft survival. In rejecting mice the T cell response was mediated by two T cell subsets: 1) CD4+ T cells that recognize alloantigens exclusively through indirect pathway and secrete IL-2, and 2) IFN-gamma-producing CD8+ T cells recognizing donor MHC in a direct fashion. Surprisingly, CD8+ T cells activated directly were not required for graft rejection. In nonrejecting mice, no T cell responses were detected. Strikingly, peripheral sensitization to allogeneic MHC molecules in these mice induced acute rejection of corneal grafts. We conclude that only CD4+ T cells activated via indirect allorecognition have the ability to reject allogeneic corneal grafts. Although alloreactive CD8+ T cells are activated via the direct pathway, they are not fully competent and cannot contribute to the rejection unless they receive an additional signal provided by professional APCs in the periphery.

Intragraft cytokine expression in heart transplants with mild or no histological rejection

The study of pro-inflammatory cytokines produced in situ in heart allografts may help to understand the mechanisms of rejection and open new possibilities to control graft rejection.

METHODS

A total of 23 endomyocardial biopsies obtained from 16 transplanted patients treated with triple-drug therapy (azathioprine, prednisone, and cyclosporine) were studied. mRNA expression for tumor necrosis factor (TNF)-alpha, interleukin (IL)-1 beta, IL-6, IL-10, IL-12, IL-15, transforming growth factor (TGF)-beta, and beta-actin was determined by reverse transcription polymerase chain reaction (RT-PCR) and Southern blotting. Semiquantitative analysis was done by establishing the ratio between densitometric integrated value of each cytokine with the beta-actin and correlated with the histopathologic findings.

RESULTS

Three groups of biopsies were determined according to the International Society for Heart and Lung Transplantation criteria: grade 0 (control group, n=12), grade 1A (sub-clinical rejection, n=6) and 'quilty effect' (n=5). An increased expression of mRNA for TNF-alpha and IL-6 (p=0.0091 and 0.0075, respectively) was found associated with rejection grade 1A episodes, mRNA for IL-1 beta was nonspecifically expressed in all the study groups, while IL-10 mRNA was not detected in any of the biopsies studied. mRNA for IL-12 and IL-15 was not associated with rejection. Interestingly, TGF-beta was not detected in any of the biopsies with the 'quilty pattern'.

CONCLUSION

The association of TNF-alpha and IL-6 mRNA in situ expression with mild histologically probed rejection episodes may be used in the monitoring of heart transplants.