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

The T cell allorecognition pathways responsible for the rejection of MHC class I disparate allografts remain poorly defined. The respective contributions of the 'direct' recognition of the allogeneic class I molecule by recipient CD8 T cells, and the 'indirect' recognition of processed allo-class I MHC peptide fragments by recipient CD4 T cells, requires clarification. Recent studies in the rat have established a role for T cell dependent antibody. MHC class I disparate PVG.R8 (RT1.Aa) heart grafts are rejected acutely in naive, and hyperacutely in sensitised, PVG.RTlu recipients by CD4 T cell dependent alloantibody. This thesis explores the T cell allorecognition pathways responsible and demonstrates that direct injection of plasmid DNA encoding a truncated, water-soluble sequence of the RT1.Aa heavy chain (pcmu-tAa) results in accelerated rejection of PVG.R8 heart grafts (MST 2 days). Pcmu-tAa injection did not generate of an allocytotoxic T cell response, but an anti-Aa IgG2b cytotoxic alloantibody response developed. That T cell recognition of the class I MHC alloantigen was restricted to the indirect pathway was confirmed by CD4 T cell depletion, which abrogated the alloantibody response and resulted in prolonged graft survival, rather than accelerated rejection. By comparison CD8 T cell depletion had no discernible effect. Priming CD4 T cells for indirect allorecogntion by the administration of synthetic 15-mer allopeptides spanning the ?1 and ?2 domains of the RT1.Aa antigen did not stimulate an alloantibody response against the intact Aa antigen but the antibody response to a subsequent PVG.R8 heart graft was accelerated. Graft rejection was, however, only modestly accelerated (MST 4 days). These results suggest that soluble class I MHC and allopeptides are equally efficient at priming CD4 T cells by the indirect pathway, but that soluble class I MHC is a more effective immunogen, not because it activates an additional subset of directly restricted T cells, but because its tertiary protein structure provides the appropriate B cell epitopes for the development of humoral immunity against the graft. This is analogous to the provision of cytotoxic T cell help by indirectly restricted T cells and the confirmation that the indirect pathway may be responsible for the development of effector mechanisms directed against intact MHC on donor cells provides the first demonstration of its role in effecting accelerated rejection. These results further imply that the potentially tolerogenic effects conferred by the ability of soluble donor class I MHC to induce apoptosis in the alloreactive CD8 cytotoxic T cell population may be masked by a detrimental indirect CD4 T cell response

Are donor lymphocytes a barrier to transplantation tolerance?

PURPOSE OF REVIEW: Following solid organ transplantation (SOT), populations of donor lymphocytes are frequently found in the recipient circulation. Their impact on host alloimmunity has long been debated but remains unclear, and it has been suggested that transferred donor lymphocytes may either promote tolerance to the graft or hasten its rejection. We discuss possible mechanisms by which the interaction of donor passenger lymphocytes with recipient immune cells may either augment the host alloimmune response or inhibit it. RECENT FINDINGS: Recent work has highlighted that donor T lymphocytes are the most numerous of the donor leukocyte populations within a SOT and that these may be transferred to the recipient after transplantation. Surprisingly, graft-versus-host recognition of major histocompatibility complex class II on host B cells by transferred donor CD4 T cells can result in marked augmentation of host humoral alloimmunity and lead to early graft failure. Killing of donor CD4 T cells by host natural killer cells is critical in preventing this augmentation. SUMMARY: The ability of passenger donor CD4 T cells to effect long-term augmentation of the host humoral alloimmune response raises the possibility that ex-vivo treatment or modification of the donor organ prior to implantation may improve long-term transplant outcomes

T cell Allorecognition Pathways in Solid Organ Transplantation

Transplantation is unusual in that T cells can recognize alloantigen by at least two distinct pathways: as intact MHC alloantigen on the surface of donor cells via the direct pathway; and as self-restricted processed alloantigen via the indirect pathway. Direct pathway responses are viewed as strong but short-lived and hence responsible for acute rejection, whereas indirect pathway responses are typically thought to be much longer lasting and mediate the progression of chronic rejection. However, this is based on surprisingly scant experimental evidence, and the recent demonstration that MHC alloantigen can be re-presented intact on recipient dendritic cells—the semi-direct pathway—suggests that the conventional view may be an oversimplification. We review recent advances in our understanding of how the different T cell allorecognition pathways are triggered, consider how this generates effector alloantibody and cytotoxic CD8 T cell alloresponses and assess how these responses contribute to early and late allograft rejection. We further discuss how this knowledge may inform development of cellular and pharmacological therapies that aim to improve transplant outcomes, with focus on the use of induced regulatory T cells with indirect allospecificity and on the development of immunometabolic strategies.KEY POINTSAcute allograft rejection is likely mediated by indirect and direct pathway CD4 T cell alloresponses.Chronic allograft rejection is largely mediated by indirect pathway CD4 T cell responses. Direct pathway recognition of cross-dressed endothelial derived MHC class II alloantigen may also contribute to chronic rejection, but the extent of this contribution is unknown.Late indirect pathway CD4 T cell responses will be composed of heterogeneous populations of allopeptide specific T helper cell subsets that recognize different alloantigens and are at various stages of effector and memory differentiation.Knowledge of the precise indirect pathway CD4 T cell responses active at late time points in a particular individual will likely inform the development of alloantigen-specific cellular therapies and will guide immunometabolic modulation

Survival advantage for patients accepting the offer of a circulatory death liver transplant.

BACKGROUND & AIMS: Donation after circulatory death (DCD) in the UK has tripled in the last decade. However, outcomes following DCD liver transplantation are worse than for donation after brainstem death (DBD) liver transplants. This study examines whether a recipient should accept a "poorer quality" DCD organ or wait longer for a "better" DBD organ. METHODS: Data were collected on 5,825 patients who were registered on the elective waiting list for a first adult liver-only transplant and 3,949 patients who received a liver-only transplant in the UK between 1 January 2008 and 31 December 2015. Survival following deceased donor liver transplantation performed between 2008 and 2015 was compared by Cox regression modelling to assess the impact on patient survival of accepting a DCD liver compared to deferring for a potential DBD transplant. RESULTS: A total of 953 (23%) of the 3,949 liver transplantations performed utilised DCD donors. Five-year post-transplant survival was worse following DCD than DBD transplantation (69.1% [DCD] vs. 78.3% [DBD]; p <0.0001: adjusted hazard ratio [HR] 1.65; 95% CI 1.40-1.94). Of the 5,798 patients registered on the transplant list, 1,325 (23%) died or were removed from the list without receiving a transplant. Patients who received DCD livers had a lower risk-adjusted hazard of death than those who remained on the waiting list for a potential DBD organ (adjusted HR 0.55; 95% CI 0.47-0.65). The greatest survival benefit was in those with the most advanced liver disease (adjusted HR 0.19; 95% CI 0.07-0.50). CONCLUSIONS: Although DCD liver transplantation leads to worse transplant outcomes than DBD transplantation, the individual's survival is enhanced by accepting a DCD offer, particularly for patients with more severe liver disease. DCD liver transplantation improves overall survival for UK listed patients and should be encouraged. LAY SUMMARY: This study looks at patients who require a liver transplant to save their lives; this liver can be donated by a person who has died either after their heart has stopped (donation after cardiac death [DCD]) or after the brain has been injured and can no longer support life (donation after brainstem death [DBD]). We know that livers donated after brainstem death function better than those after cardiac death, but there are not enough of these livers for everyone, so we wished to help patients decide whether it was better for them to accept an early offer of a DCD liver than waiting longer to receive a "better" liver from a DBD donor. We found that patients were more likely to survive if they accepted the offer of a liver transplant as soon as possible (DCD or DBD), especially if their liver disease was very severe

Prolongation of allograft survival by passenger donor regulatory T cells.

Tissue resident lymphocytes are present within many organs, and are presumably transferred at transplantation, but their impact on host immunity is unclear. Here, we examine whether transferred donor natural regulatory CD4 T cells (nT-regs) inhibit host alloimmunity and prolong allograft survival. Transfer of donor-strain lymphocytes was first assessed by identifying circulating donor-derived CD4 T cells in 21 consecutive human lung transplant recipients, with 3 patterns of chimerism apparent: transient, intermediate, and persistent (detectable for up to 6 weeks, 6 months, and beyond 1 year, respectively). The potential for transfer of donor nT-regs was then confirmed by analysis of leukocyte filters recovered from ex vivo normothermic perfusion circuits of human kidneys retrieved for transplantation. Finally, in a murine model of cardiac allograft vasculopathy, depletion of donor CD4 nT-regs before organ recovery resulted in markedly accelerated heart allograft rejection and augmented host effector antibody responses. Conversely, adoptive transfer or purified donor-strain nT-regs inhibited host humoral immunity and prolonged allograft survival, and more effectively so than following administration of recipient nT-regs. In summary, following transplantation, passenger donor-strain nT-regs can inhibit host adaptive immune responses and prolong allograft survival. Isolated donor-derived nT-regs may hold potential as a cellular therapy to improve transplant outcomes.This work was supported by a British Heart Foundation project grant, the NIHR Cambridge Biomedical Research Centre and the NIHR Blood and Transplant Research Unit in Organ Donation and Transplantation at the University of Cambridge in collaboration with Newcastle University and Royal Papworth Hospital in partnership with NHS Blood and Transplant (NHSBT). The views expressed are those of the authors and not necessarily those of the NHS, the NIHR, the Department of Health or NHSBT. IGH was supported by a Wellcome Trust Clinical Research Training Fellowships and Raymond and Beverly Sackler Scholarships. IGH received additional support from an Addenbrooke’s Charitable Trust Clinical Research Fellowship. RM was supported by a European Society of Organ Transplantation Junior Basic Science Grant. JHS was supported by a Gates PhD Fellowship

Utilization and Outcomes of Single and Dual Kidney Transplants from Older Deceased Donors in the United Kingdom

BACKGROUND AND OBJECTIVES: Kidneys from elderly deceased donors are often discarded after procurement if the expected outcomes from single kidney transplantation are considered unacceptable. An alternative is to consider them for dual kidney transplantation. We aimed to examine the utilization of kidneys from donors aged ≥60 years in the United Kingdom and compare clinical outcomes of dual versus single kidney transplant recipients. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Data from the United Kingdom Transplant Registry from 2005 to 2017 were analyzed. We examined utilization rates of kidneys retrieved from deceased donors aged ≥60 years, and 5-year patient and death-censored graft survival of recipients of dual and single kidney transplants. Secondary outcomes included eGFR. Multivariable analyses and propensity score analysis were used to correct for differences between the groups. RESULTS: During the study period, 7841 kidneys were procured from deceased donors aged ≥60 years, of which 1338 (17%) were discarded; 356 dual and 5032 single kidneys were transplanted. Donors of dual transplants were older (median, 73 versus 66 years; P<0.001) and had higher United States Kidney Donor Risk Indices (2.48 versus 1.98; P<0.001). Recipients of dual transplants were also older (64 versus 61 years; P<0.001) and had less favorable human leukocyte antigen matching (P<0.001). After adjusting for confounders, dual and single transplants had similar 5-year graft survival (hazard ratio, 0.81; 95% CI, 0.59 to 1.12). No difference in patient survival was demonstrated. Similar findings were observed in a matched cohort with a propensity score analysis method. Median 12-month eGFR was significantly higher in the dual kidney transplant group (40 versus 36 ml/min per 1.73 m(2); P<0.001). CONCLUSIONS: Recipients of kidneys from donors aged ≥60 years have similar 5-year graft survival and better graft function at 12 months with dual compared with single deceased donor kidney transplants

Semi-direct allorecognition

Adaptive CD8 T-cell immunity is the principal arm of the cellular alloimmune response, but its development requires help. This can be provided by CD4 T cells that recognize alloantigen "indirectly," as self-restricted allopeptide, but this process remains unexplained, because the target epitopes for CD4 and CD8 T-cell recognition are "unlinked" on different cells (recipient and donor antigen presenting cells (APCs), respectively). Here, we test the hypothesis that the presentation of intact and processed MHC class I alloantigen by recipient dendritic cells (DCs) (the "semidirect" pathway) allows linked help to be delivered by indirect-pathway CD4 T cells for generating destructive cytotoxic CD8 T-cell alloresponses. We show that CD8 T-cell-mediated rejection of murine heart allografts that lack hematopoietic APCs requires host secondary lymphoid tissue (SLT). SLT is necessary because within it, recipient dendritic cells can acquire MHC from graft parenchymal cells and simultaneously present it as intact protein to alloreactive CD8 T cells and as processed peptide alloantigen for recognition by indirect-pathway CD4 T cells. This enables delivery of essential help for generating cytotoxic CD8 T-cell responses that cause rapid allograft rejection. In demonstrating the functional relevance of the semidirect pathway to transplant rejection, our findings provide a solution to a long-standing conundrum as to why SLT is required for CD8 T-cell allorecognition of graft parenchymal cells and suggest a mechanism by which indirect-pathway CD4 T cells provide help for generating effector cytotoxic CD8 T-cell alloresponses at late time points after transplantation.This work was supported by a British Heart Foundation project grant, the National Institute of Health Research Cambridge Biomedical Research Centre and the National Institute of Health Research Blood and Transplant Research Unit. SH was supported by an Academy of Medical Sciences / Wellcome Trust starter grant and the European Society for Organ Transplantation Junior Basic Science Research Grant. JA and IH were supported by Wellcome Trust Clinical Research Training Fellowships and Raymond and Beverly Sackler Scholarships.This is the author accepted manuscript. The final version is available from PNAS via http://dx.doi.org/10.1073/pnas.151353311

Relative Frequencies of Alloantigen-Specific Helper CD4 T Cells and B Cells Determine Mode of Antibody-Mediated Allograft Rejection.

Humoral alloimmunity is now recognized as a major determinant of transplant outcome. MHC glycoprotein is considered a typical T-dependent antigen, but the nature of the T cell alloresponse that underpins alloantibody generation remains poorly understood. Here, we examine how the relative frequencies of alloantigen-specific B cells and helper CD4 T cells influence the humoral alloimmune response and how this relates to antibody-mediated rejection (AMR). An MHC-mismatched murine model of cardiac AMR was developed, in which T cell help for alloantibody responses in T cell deficient (Tcrbd-/-) C57BL/6 recipients against donor H-2Kd MHC class I alloantigen was provided by adoptively transferred "TCR75" CD4 T cells that recognize processed H-2Kd allopeptide via the indirect-pathway. Transfer of large numbers (5 × 105) of TCR75 CD4 T cells was associated with rapid development of robust class-switched anti-H-2Kd humoral alloimmunity and BALB/c heart grafts were rejected promptly (MST 9 days). Grafts were not rejected in T and B cell deficient Rag2-/- recipients that were reconstituted with TCR75 CD4 T cells or in control (non-reconstituted) Tcrbd-/- recipients, suggesting that the transferred TCR75 CD4 T cells were mediating graft rejection principally by providing help for effector alloantibody responses. In support, acutely rejecting BALB/c heart grafts exhibited hallmark features of acute AMR, with widespread complement C4d deposition, whereas cellular rejection was not evident. In addition, passive transfer of immune serum from rejecting mice to Rag2-/- recipients resulted in eventual BALB/c heart allograft rejection (MST 20 days). Despite being long-lived, the alloantibody responses observed at rejection of the BALB/c heart grafts were predominantly generated by extrafollicular foci: splenic germinal center (GC) activity had not yet developed; IgG secreting cells were confined to the splenic red pulp and bridging channels; and, most convincingly, rapid graft rejection still occurred when recipients were reconstituted with similar numbers of Sh2d1a-/- TCR75 CD4 T cells that are genetically incapable of providing T follicular helper cell function for generating GC alloimmunity. Similarly, alloantibody responses generated in Tcrbd-/- recipients reconstituted with smaller number of wild-type TCR75 CD4 T cells (103), although long-lasting, did not have a discernible extrafollicular component, and grafts were rejected much more slowly (MST 50 days). By modeling antibody responses to Hen Egg Lysozyme protein, we confirm that a high ratio of antigen-specific helper T cells to B cells favors development of the extrafollicular response, whereas GC activity is favored by a relatively high ratio of B cells. In summary, a relative abundance of helper CD4 T cells favors development of strong extrafollicular alloantibody responses that mediate acute humoral rejection, without requirement for GC activity. This work is composed of two parts, of which this is Part I. Please read also Part II: Chhabra et al., 2019

The impact of time to death in donors after circulatory death on recipient outcome in simultaneous pancreas-kidney transplantation

Time to arrest in donors after circulatory death is unpredictable and can vary. This leads to variable periods of warm ischaemic damage prior to pancreas transplantation. There is little evidence supporting procurement team stand-down times based on donor time to death (TTD). We examined what impact TTD had on pancreas graft outcomes following DCD SPK transplantation. Data were extracted from the UK transplant registry from 2014 to 2022. Predictors of graft loss were evaluated by a Cox proportional hazards model. Adjusted restricted cubic spline (RCS) models were generated to further delineate the relationship between TTD and outcome. Three-hundred-and-seventy-five DCD simultaneous kidney-pancreas transplant recipients were included. Increasing TTD was not associated with graft survival (aHR 0.98, 95% CI 0.68-1.41, P=0.901). Increasing asystolic time worsened graft survival (aHR 2.51, 95% CI 1.16-5.43, P=0.020). RCS modelling revealed a non-linear relationship was demonstrated between asystolic time and graft survival, and no relationship between TTD and graft survival. We found no evidence that TTD impacts on pancreas graft survival after DCD SPK transplantation, however increasing asystolic time was a significant predictor of graft loss. Procurement teams should attempt to minimise asystolic time to optimize pancreas graft survival rather than focus on the duration of TTD