Donor-reactive CD8 memory T cells infiltrate cardiac allografts within 24-h posttransplant in naive recipients

T-cell receptor sequencing reveals selected donor-reactive CD8+ T cell clones resist antithymocyte globulin depletion after kidney transplantation

High frequencies of donor-reactive memory T cells in the periphery of transplant candidates prior to transplantation are linked to the development of posttransplant acute rejection episodes and reduced allograft function. Rabbit antithymocyte globulin (rATG) effectively depletes naïve CD4+ and CD8+ T cells for >6 months posttransplant, but rATG's effects on human donor-reactive T cells have not been carefully determined. To address this, we performed T cell receptor β-chain sequencing on peripheral blood mononuclear cells aliquots collected pretransplant and serially posttransplant in 7 kidney transplant recipients who received rATG as induction therapy. We tracked the evolution of the donor-reactive CD4+ and CD8+ T cell repertoires and identified stimulated pretransplant, CTV-(surface dye)-labeled, peripheral blood mononuclear cells from each patient with donor cells or third-party cells. Our analyses showed that while rATG depleted CD4+ T cells in all tested subjects, a subset of donor-reactive CD8+ T cells that were present at high frequencies pretransplant, consistent with expanded memory cells, resisted rATG depletion, underwent posttransplant expansion and were functional. Together, our data support the conclusion that a subset of human memory CD8+ T cells specifically reactive to donor antigens expand in vivo despite induction therapy with rATG and thus have the potential to mediate allograft damage.

TCF1highPD-1+Ly108+CD8+ T Cells Are Associated with Graft Preservation in Sensitized Mice Treated with Non-Fc Receptor-Binding CD3 Antibodies

The non-Fc-binding anti-CD3 Ab [anti-CD3F(ab')2] can induce graft acceptance depending on the therapeutic window in a rodent heart transplant model. The delayed protocol allows for early graft infiltration of lymphocytes, which may behave in an inhibitory manner. We investigated the most effective protocol for anti-CD3F(ab')2 in sensitized conditions to confirm the evidence for clinical application. C57BL/6 mice were sensitized with BALB/c tail skin grafts and transplanted with BALB/c heart grafts at 8-12 wk after sensitization. Fifty micrograms of anti-CD3F(ab')2 was administered daily for 5 consecutive days on days 1-5 (day 1 protocol) or days 3-7 (delayed protocol). In nonsensitized mice, the delayed protocol significantly prolonged graft survival after transplantation from BALB/c to naive B6 (median survival time [MST], >100 d). In contrast, the delayed protocol was unable to prevent graft rejection in sensitized mice (MST, 5 d). A significantly increased percentage of granzyme B+ CD8+ T cells was observed in the graft on day 3 posttransplantation in sensitized conditions. Further, the day 1 protocol significantly prolonged graft survival (MST, 18 d), even in sensitized conditions. Day 1 treatment significantly increased the percentage of Foxp3+CD25+CD4+ T cells and phenotypically changed CD8+ T cells in the graft (i.e., caused a significant increase in the proportion of Ly108+TCF1highPD-1+CD8+ T cells). In conclusion, different timings of delayed anti-CD3F(ab')2 treatment promoted allograft preservation in association with phenotypic changes in CD4+ and CD8+ T cells in the graft under sensitized conditions.

p40 homodimers bridge ischemic tissue inflammation and heterologous alloimmunity in mice via IL-15 transpresentation

Virus-induced memory T cells often express functional cross-reactivity, or heterologous immunity, to other viruses and to allogeneic MHC molecules that is an important component of pathogenic responses to allogeneic transplants. During immune responses, antigen-reactive naive and central memory T cells proliferate in secondary lymphoid organs to achieve sufficient cell numbers to effectively respond, whereas effector memory T cell proliferation occurs directly within the peripheral inflammatory microenvironment. Mechanisms driving heterologous memory T cell proliferation and effector function expression within peripheral tissues remain poorly understood. Here, we dissected proliferation of heterologous donor-reactive memory CD8+ T cells and their effector functions following infiltration into heart allografts with low or high intensities of ischemic inflammation. Proliferation within both ischemic conditions required p40 homodimer-induced IL-15 transpresentation by graft DCs, but expression of effector functions mediating acute allograft injury occurred only in high-ischemic allografts. Transcriptional responses of heterologous donor-reactive memory CD8+ T cells were distinct from donor antigen-primed memory CD8+ T cells during early activation in allografts and at graft rejection. Overall, the results provide insights into mechanisms driving heterologous effector memory CD8+ T cell proliferation and the separation between proliferation and effector function that is dependent on the intensity of inflammation within the tissue microenvironment.

The Entangled World of Memory T Cells and Implications in Transplantation

Memory T cells that are specific for alloantigen can arise from a variety of stimuli, ranging from direct allogeneic sensitization from prior transplantation, blood transfusion, or pregnancy to the elicitation of pathogen-specific T cells that are cross-reactive with alloantigen. Regardless of the mechanism by which they arise, alloreactive memory T cells possess key metabolic, phenotypic, and functional properties that render them distinct from naive T cells. These properties affect the immune response to transplantation in 2 important ways: first, they can alter the speed, location, and effector mechanisms with which alloreactive T cells mediate allograft rejection, and second, they can alter T-cell susceptibility to immunosuppression. In this review, we discuss recent developments in understanding these properties of memory T cells and their implications for transplantation.

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.

Animal Models for Heart Transplantation Focusing on the Pathological Conditions

Cardiac transplant recipients face many complications due to transplant rejection. Scientists must conduct animal experiments to study disease onset mechanisms and develop countermeasures. Therefore, many animal models have been developed for research topics including immunopathology of graft rejection, immunosuppressive therapies, anastomotic techniques, and graft preservation techniques. Small experimental animals include rodents, rabbits, and guinea pigs. They have a high metabolic rate, high reproductive rate, small size for easy handling, and low cost. Additionally, they have genetically modified strains for pathological mechanisms research; however, there is a lacuna, as these research results rarely translate directly to clinical applications. Large animals, including canines, pigs, and non-human primates, have anatomical structures and physiological states that are similar to those of humans; therefore, they are often used to validate the results obtained from small animal studies and directly speculate on the feasibility of applying these results in clinical practice. Before 2023, PubMed Central^® at the United States National Institute of Health's National Library of Medicine was used for literature searches on the animal models for heart transplantation focusing on the pathological conditions. Unpublished reports and abstracts from conferences were excluded from this review article. We discussed the applications of small- and large-animal models in heart transplantation-related studies. This review article aimed to provide researchers with a complete understanding of animal models for heart transplantation by focusing on the pathological conditions created by each model.

Novel approaches for long-term lung transplant survival

Allograft failure remains a major barrier in the field of lung transplantation and results primarily from acute and chronic rejection. To date, standard-of-care immunosuppressive regimens have proven unsuccessful in achieving acceptable long-term graft and patient survival. Recent insights into the unique immunologic properties of lung allografts provide an opportunity to develop more effective immunosuppressive strategies. Here we describe advances in our understanding of the mechanisms driving lung allograft rejection and highlight recent progress in the development of novel, lung-specific strategies aimed at promoting long-term allograft survival, including tolerance.

Modulating donor mitochondrial fusion/fission delivers immunoprotective effects in cardiac transplantation

Early insults associated with cardiac transplantation increase the immunogenicity of donor microvascular endothelial cells (ECs), which interact with recipient alloreactive memory T cells and promote responses leading to allograft rejection. Thus, modulating EC immunogenicity could potentially alter T cell responses. Recent studies have shown modulating mitochondrial fusion/fission alters immune cell phenotype. Here, we assess whether modulating mitochondrial fusion/fission reduces EC immunogenicity and alters EC-T cell interactions. By knocking down DRP1, a mitochondrial fission protein, or by using the small molecules M1, a fusion promoter, and Mdivi1, a fission inhibitor, we demonstrate that promoting mitochondrial fusion reduced EC immunogenicity to allogeneic CD8⁺ T cells, shown by decreased T cell cytotoxic proteins, decreased EC VCAM-1, MHC-I expression, and increased PD-L1 expression. Co-cultured T cells also displayed decreased memory frequencies and Ki-67 proliferative index. For in vivo significance, we used a novel murine brain-dead donor transplant model. Balb/c hearts pretreated with M1/Mdivi1 after brain-death induction were heterotopically transplanted into C57BL/6 recipients. We demonstrate that, in line with our in vitro studies, M1/Mdivi1 pretreatment protected cardiac allografts from injury, decreased infiltrating T cell production of cytotoxic proteins, and prolonged allograft survival. Collectively, our data show promoting mitochondrial fusion in donor ECs mitigates recipient T cell responses and leads to significantly improved cardiac transplant survival.

Analysis of T-Cell Receptor Repertoire in Transplantation: Fingerprint of T Cell-mediated Alloresponse

T cells play a key role in determining allograft function by mediating allogeneic immune responses to cause rejection, and recent work pointed their role in mediating tolerance in transplantation. The unique T-cell receptor (TCR) expressed on the surface of each T cell determines the antigen specificity of the cell and can be the specific fingerprint for identifying and monitoring. Next-generation sequencing (NGS) techniques provide powerful tools for deep and high-throughput TCR profiling, and facilitate to depict the entire T cell repertoire profile and trace antigen-specific T cells in circulation and local tissues. Tailing T cell transcriptomes and TCR sequences at the single cell level provides a full landscape of alloreactive T-cell clones development and biofunction in alloresponse. Here, we review the recent advances in TCR sequencing techniques and computational tools, as well as the recent discovery in overall TCR profile and antigen-specific T cells tracking in transplantation. We further discuss the challenges and potential of using TCR sequencing-based assays to profile alloreactive TCR repertoire as the fingerprint for immune monitoring and prediction of rejection and tolerance.

Impact of Graft-Resident Leucocytes on Treg Mediated Skin Graft Survival

The importance and exact role of graft-resident leucocytes (also referred to as passenger leucocytes) in transplantation is controversial as these cells have been reported to either initiate or retard graft rejection. T cell activation to allografts is mediated via recognition of intact or processed donor MHC molecules on antigen-presenting cells (APC) as well as through interaction with donor-derived extracellular vesicles. Reduction of graft-resident leucocytes before transplantation is a well-known approach for prolonging organ survival without interfering with the recipient's immune system. As previously shown by our group, injecting mice with IL-2/anti-IL-2 complexes (IL-2cplx) to augment expansion of CD4 T regulatory cells (Tregs) induces tolerance towards islet allografts, and also to skin allografts when IL-2cplx treatment is supplemented with rapamycin and a short-term treatment of anti-IL-6. In this study, we investigated the mechanisms by which graft-resident leucocytes impact graft survival by studying the combined effects of IL-2cplx-mediated Treg expansion and passenger leucocyte depletion. For the latter, effective depletion of APC and T cells within the graft was induced by prior total body irradiation (TBI) of the graft donor. Surprisingly, substantial depletion of donor-derived leucocytes by TBI did not prolong graft survival in naïve mice, although it did result in augmented recipient leucocyte graft infiltration, presumably through irradiation-induced nonspecific inflammation. Notably, treatment with the IL-2cplx protocol prevented early inflammation of irradiated grafts, which correlated with an influx of Tregs into the grafts. This finding suggested there might be a synergistic effect of Treg expansion and graft-resident leucocyte depletion. In support of this idea, significant prolongation of skin graft survival was achieved if we combined graft-resident leucocyte depletion with the IL-2cplx protocol; this finding correlated along with a progressive shift in the composition of T cells subsets in the grafts towards a more tolerogenic environment. Donor-specific humoral responses remained unchanged, indicating minor importance of graft-resident leucocytes in anti-donor antibody development. These results demonstrate the importance of donor-derived leucocytes as well as Tregs in allograft survival, which might give rise to new clinical approaches.

Functional roles of graft-infiltrating lymphocytes during early-phase post-transplantation in mouse cardiac transplantation models

Immunological behavior of graft-infiltrating lymphocytes (GILs) determines the graft fate (i.e., rejection or acceptance). Nevertheless, the functional alloreactivity and the phenotype of GILs at various times during the early post-transplantation phase have not been fully elucidated. We examined the immunological activities of early-phase GILs using a murine model of cardiac transplantation. GILs from 120-h allografts, but not 72-h allografts, showed robust activation and produced proinflammatory cytokines. In particular, a significant increase in CD69⁺ T-bet⁺ Nur77⁺ T cells was detected in 120-h allografts. Furthermore, isolated GILs were used to reconstitute BALB/c Rag2^-/- γc^-/- (BRG) mice. BRG mice reconstituted with 120-h GILs displayed donor-specific immune reactivity and rejected donor strain cardiac allografts; conversely, 72-h GILs exhibited weak anti-donor reactivity and did not reject allografts. These findings were confirmed by re-transplantation of cardiac allografts into BRG mice at 72-h post-transplantation. Re-transplanted allografts continued to function for >100 days, despite the presence of CD3⁺ GILs. In conclusion, the immunological behavior of GILs considerably differs over time during the early post-transplantation phase. A better understanding of the functional role of early-phase GILs may clarify the fate determination process in the graft-site microenvironment.

Endogenous memory T cells with donor-reactivity: early post-transplant mediators of acute graft injury in unsensitized recipients

The pretransplant presence of endogenous donor-reactive memory T cells is an established risk factor for acute rejection and poorer transplant outcomes. A major source of these memory T cells in unsensitized recipients is heterologously generated memory T cells expressing reactivity to donor allogeneic MHC molecules. Multiple clinical studies have shown that the pretransplant presence of high numbers of circulating endogenous donor-reactive memory T cells correlates with higher incidence of acute rejection and decreased graft function during the first-year post-transplant. These findings have spurred investigation in preclinical models to better understand mechanisms underlying endogenous donor-reactive memory T-cell-mediated allograft injury in unsensitized graft recipients. These studies have led to the identification of unique mechanisms underlying the activation of these memory T cells within allografts at early times after transplant. In particular, optimal activation to mediate acute allograft injury is dependent on the intensity of ischaemia-reperfusion injury. Therapeutic strategies directed at the recruitment and activation of endogenous donor-reactive memory T cells are effective in attenuating acute injury in allografts experiencing increased ischaemia-reperfusion injury in preclinical models and should be translatable to clinical transplantation.

Berberine Promotes Induction of Immunological Tolerance to an Allograft via Downregulating Memory CD8+ T-Cells Through Altering the Gut Microbiota

Emerging evidence has linked the gut microbiota dysbiosis to transplant rejection while memory T-cells pose a threat to long-term transplant survival. However, it's unclear if the gut microbiome alters the formation and function of alloreactive memory T-cells. Here we studied the effects of berberine, a narrow-spectrum antibiotic that is barely absorbed when orally administered, on the gut microbiota, memory T-cells, and allograft survival. In this study, C57BL/6 mice transplanted with islets or a heart from BALB/c mice were treated orally with berberine. Allograft survival was observed, while spleen, and lymph node T-cells from recipient mice were analyzed using a flow cytometer. High-throughput sequencing and qPCR were performed to analyze the gut microbiota. CD8+ T-cells from recipients were cultured with the bacteria to determine potential T-cell memory cross-reactivity to a specific pathogen. We found that berberine suppressed islet allograft rejection, reduced effector CD8+CD44highCD62Llow and central memory CD8+CD44highCD62Lhigh T-cells (TCM), altered the gut microbiota composition and specifically lowered Bacillus cereus abundance. Further, berberine promoted long-term islet allograft survival induced by conventional costimulatory blockade and induced cardiac allograft tolerance as well. Re-colonization of B. cereus upregulated CD8+ TCM cells and reversed long-term islet allograft survival induced by berberine plus the conventional costimulatory blockade. Finally, alloantigen-experienced memory CD8+ T-cells from transplanted recipients rapidly responded to B. cereus in vitro. Thus, berberine prolonged allograft survival by repressing CD8+ TCM through regulating the gut microbiota. We have provided the first evidence that donor-specific memory T-cell generation is linked to a specific microbe and uncovered a novel mechanism underlying the therapeutic effects of berberine. This study may be implicated for suppressing human transplant rejection since berberine is already used in clinic to treat intestinal infections.

BET Protein Inhibition Prolongs Cardiac Transplant Survival via Enhanced Myocardial Autophagy

BACKGROUND

Graft rejection continues to be a major barrier to long-term engraftment after transplantation. Autophagy plays an important role in cardiac injury pathogenesis. The bromodomain and extraterminal protein inhibitor (S)-tert-butyl2-(4-(4-chlorophenyl)-2,3,9-trimethyl-6H-thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4]diazepin-6-yl)acetate (JQ1) inhibits inflammatory responses. However, the beneficial effect of JQ1 on transplant and the potential role of autophagy in the protective effect of graft survival are yet to be investigated.

METHODS

Syngeneic or allogeneic heterotopic heart transplantation was performed using C57BL/6 or BALB/c donors for C57BL/6 recipients through different treatments. Some mice were used to observe the survival of the grafts. The other mice were euthanized on the third, fifth, and seventh days after surgery. The graft samples were taken for cytokines and autophagy pathway analyses.

RESULTS

Our study revealed that JQ1 treatment prolonged cardiac allograft survival. JQ1 increased the expression levels of liver kinase beta 1, autophagy-specific gene 5, and microtubule-associated protein light chain3-II (LC3-II) and potentiated the phosphorylation of AMP-activated protein kinase, unc-51-like kinase 1 (ULK1), and autophagy-specific gene 14 in allografts. A conditional autophagy-specific gene 5 deletion donor was utilized to abrogate the effect induced by JQ1. The combined use of JQ1 with bafilomycin A1 partially reversed the effect of JQ1, suggesting that autophagy is involved in the signaling pathway in graft survival. JQ1 downregulated the expression of inflammatory cytokines, such as interleukin-6, interleukin-1β, tumor necrosis factor-α, and interferon-γ, which was abrogated when autophagy was inhibited.

CONCLUSIONS

JQ1 prolonged cardiac allograft survival by potentiating myocardial autophagy through the liver kinase beta 1-AMP-activated protein kinase-ULK1 signaling pathway and inhibiting the subsequent release of inflammatory cytokines. This result might provide novel insights for extending transplant survival.

Kidney-intrinsic factors determine the severity of ischemia/reperfusion injury in a mouse model of delayed graft function

Delayed graft function due to transplant ischemia/reperfusion injury adversely affects up to 50% of deceased-donor kidney transplant recipients. However, key factors contributing to the severity of ischemia/reperfusion injury remain unclear. Here, using a clinically relevant mouse model of delayed graft function, we demonstrated that donor genetic background and kidney-intrinsic MyD88/Trif-dependent innate immunity were key determinants of delayed graft function. Functional deterioration of kidney grafts directly corresponded with the duration of cold ischemia time. The graft dysfunction became irreversible after cold ischemia time exceeded six hours. When cold ischemia time reached four hours, kidney grafts displayed histological features reflective of delayed graft function seen in clinical kidney transplantation. Notably, kidneys of B6 mice exhibited significantly more severe histological and functional impairment than kidneys of C3H or BALB/c mice, regardless of recipient strains or alloreactivities. Furthermore, allografts of B6 mice also showed an upregulation of IL-6, neutrophil gelatinase-associated lipocalin, and endoplasmic reticulum stress genes, as well as an increased influx of host neutrophils and memory CD8 T-cells. In contrast, donor MyD88/Trif deficiency inhibited neutrophil influx and decreased the expression of IL-6 and endoplasmic reticulum stress genes, along with improved graft function and prolonged allograft survival. Thus, kidney-intrinsic factors involving genetic characteristics and innate immunity serve as critical determinants of the severity of delayed graft function. This preclinical murine model allows for further investigations of the mechanisms underlying delayed graft function.

Thalidomide with blockade of co-stimulatory molecules prolongs the survival of alloantigen-primed mice with cardiac allografts

Background

Miscellaneous memory cell populations that exist before organ transplantation are crucial barriers to transplantation. In the present study, we used a skin-primed heart transplantation model in mouse to evaluate the abilities of Thalidomide (TD), alone or in combination with co-stimulatory blockade, using monoclonal antibodies (mAbs) against memory T cells and alloantibodies to prolong the second cardiac survival.

Results

In the skin-primed heart transplantation model, TD combined with mAbs significantly prolonged the second cardiac survival, accompanied by inhibition of memory CD8⁺ T cells. This combined treatment enhanced the CD4⁺Foxp3⁺ regulatory T cells ratio in the spleen, restrained the infiltration of lymphocytes into the allograft, and suppressed the allo-response of spleen T cells in the recipient. The levels of allo-antibodies also decreased in the recipient serum. In addition, we detected low levels of the constitutions of the lytic machinery of cytotoxic cells, which cause allograft damage.

Conclusions

Our study indicated a potential synergistic action of TD in combination with with mAbs to suppress the function of memory T cells and increase the survival of second allografts in alloantigen-primed mice.

Eosinophils downregulate lung alloimmunity by decreasing TCR signal transduction

Despite the accepted notion that granulocytes play a universally destructive role in organ and tissue grafts, it has been recently described that eosinophils can facilitate immunosuppression-mediated acceptance of murine lung allografts. The mechanism of eosinophil-mediated tolerance, or their role in regulating alloimmune responses in the absence of immunosuppression, remains unknown. Using lung transplants in a fully MHC-mismatched BALB/c (H2d) to C57BL/6 (H2b) strain combination, we demonstrate that eosinophils downregulate T cell-mediated immune responses and play a tolerogenic role even in the absence of immunosuppression. We further show that such downregulation depends on PD-L1/PD-1-mediated synapse formation between eosinophils and T cells. We also demonstrate that eosinophils suppress T lymphocyte responses through the inhibition of T cell receptor/CD3 (TCR/CD3) subunit association and signal transduction in an inducible NOS-dependent manner. Increasing local eosinophil concentration, through administration of intratracheal eotaxin and IL-5, can ameliorate alloimmune responses in the lung allograft. Thus, our data indicate that eosinophil mobilization may be utilized as a novel means of lung allograft-specific immunosuppression.

Peritransplant VLA-4 blockade inhibits endogenous memory CD8 T cell infiltration into high-risk cardiac allografts and CTLA-4Ig resistant rejection

Recipient endogenous memory CD8 T cells expressing reactivity to donor class I MHC infiltrate MHC-mismatched cardiac allografts within 24 hours after reperfusion and express effector functions mediating graft injury. The current study tested the efficacy of Very Late Antigen-4 (VLA-4) blockade to inhibit endogenous memory CD8 T cell infiltration into cardiac allografts and attenuate early posttransplant inflammation. Peritransplant anti-VLA-4 mAb given to C57BL6 (H-2b ) recipients of AJ (H-2a ) heart allografts completely inhibited endogenous memory CD4 and CD8 T cell infiltration with significant decrease in macrophage, but not neutrophil, infiltration into allografts subjected to either minimal or prolonged cold ischemic storage (CIS) prior to transplant, reduced intra-allograft IFN-γ-induced gene expression and prolonged survival of allografts subjected to prolonged CIS in CTLA-4Ig treated recipients. Anti-VLA-4 mAb also inhibited priming of donor-specific T cells producing IFN-γ until at least day 7 posttransplant. Peritransplant anti-VLA plus anti-CD154 mAb treatment similarly prolonged survival of allografts subjected to minimal or increased CIS prior to transplant. Overall, these data indicate that peritransplant anti-VLA-4 mAb inhibits early infiltration memory CD8 T cell infiltration into allografts with a marked reduction in early graft inflammation suggesting an effective strategy to attenuate negative effects of heterologous alloimmunity in recipients of higher risk grafts.

Impact of Mitochondrial Permeability on Endothelial Cell Immunogenicity in Transplantation

BACKGROUND

Microvascular endothelial cells (ECs) are central to an allograft's immunogenicity. Cold ischemia and reperfusion injury associated with static cold storage and warm reperfusion activates ECs and increases the immunogenicity of the allograft. After reperfusion, mitochondrial permeability transition pore (mPTP) opening contributes to mitochondrial dysfunction in the allograft, which correlates to alloimmune rejection. Current understanding of this relationship, however, centers on the whole allograft instead of ECs. This study aimed to elucidate the relationship between EC mPTP opening and their immunophenotype.

METHODS

Mitochondrial metabolic fitness and glycolysis in ECs were assessed in parallel with metabolic gene microarray postreperfusion. NIM811 was used to inhibit mPTP opening to rescue mitochondrial fitness. The immunogenicity of NIM811-treated ECs was determined via levels of EC's proinflammatory cytokines and allogeneic CD8 T cell cocultures. Finally, EC surface expression of adhesion, costimulatory, coinhibitory, MHC-I molecules, and MHC-I machinery protein levels were characterized.

RESULTS

Genes for glycolysis, tricarboxylic acid cycle, fatty acid synthesis, gluconeogenesis were upregulated at 6 hours postreperfusion but either normalized or downregulated at 24 hours postreperfusion. As mitochondrial fitness was reduced, glycolysis increased during the first 6 hours postreperfusion. Endothelial cell treatment with NIM811 during the early postreperfusion period rescued mitochondrial fitness and reduced EC immunogenicity by decreasing CCL2, KC release, and VCAM-1, MHC-I, TAP1 expression.

CONCLUSIONS

Static cold storage and warm reperfusion leads to a reduction in mitochondrial fitness in microvascular ECs due to mPTP opening. Further, mPTP opening promotes increased EC immunogenicity that can be prevented by NIM811 treatment.

Donor IL-6 deficiency evidently reduces memory T cell responses in sensitized transplant recipients

BACKGROUND

Resistance of tolerance induction in sensitized transplantation is mainly caused by generation of memory T cells. It is unknown whether alteration of graft niche such as level of pro-inflammatory cytokines can affect generation of memory T cells.

METHODS

IL-6 deficient or wild-type (WT) C57BL/6 heart grafts were transplanted into pre-sensitized wild-type BALB/c recipients. Frequencies of memory T cells in the peripheral blood, grafts, and spleen were evaluated.

RESULTS

We revealed that deficiency of donor IL-6 could significant prolong sensitized allograft survival. Compared with counterpart of WT group, frequency of effector memory CD4 + T cells (CD4 + CD44 + CD62L-) in the peripheral blood was significantly lower in the IL-6 KO group (p = .026) at day 3 post-transplantation. Frequency of effector memory CD8 + T cells (CD8 + CD44 + CD62L-) in the peripheral blood was significantly lower in the IL-6 KO group (p < .0001) at day 3 post-transplant in comparison to that of WT group. No significant difference of central memory T cells was found between these groups. Histology demonstrated that deficiency of donor pro-inflammatory cytokine IL-6 (IL-6 KO group) preserved cardiac architecture with a mild infiltration of lymphocytes, whereas wild-type donor (control group) caused an evident lymphocytic infiltration within myocardial fibers of grafts and destruction of cardiac structure.

CONCLUSION

Deficiency of pro-inflammatory IL-6 of donor graft could effectively prolong sensitized allograft survival, which was caused by a remarkable decrease of peripheral memory T cells rather than central memory T cells. This unveiled mechanism of targeting IL-6 signaling pathway might provide a novel insight into preventing allograft rejection for sensitized transplant recipients.

Aquaporin 4 blockade improves survival of murine heart allografts subjected to prolonged cold ischemia

Prolonged cold ischemia storage (CIS) is a leading risk factor for poor transplant outcome. Existing strategies strive to minimize ischemia-reperfusion injury in transplanted organs, yet there is a need for novel approaches to improve outcomes of marginal allografts and expand the pool of donor organs suitable for transplantation. Aquaporins (AQPs) are a family of water channels that facilitate homeostasis, tissue injury, and inflammation. We tested whether inhibition of AQP4 improves the survival of fully MHC-mismatched murine cardiac allografts subjected to 8 hours of CIS. Administration of a small molecule AQP4 inhibitor during donor heart collection and storage and for a short-time posttransplantation improves the viability of donor graft cells, diminishes donor-reactive T cell responses, and extends allograft survival in the absence of other immunosuppression. Furthermore, AQP4 inhibition is synergistic with cytotoxic T lymphocyte-associated antigen 4-Ig in prolonging survival of 8-hour CIS heart allografts. AQP4 blockade markedly reduced T cell proliferation and cytokine production in vitro, suggesting that the improved graft survival is at least in part mediated through direct effects on donor-reactive T cells. These results identify AQPs as a promising target for diminishing donor-specific alloreactivity and improving the survival of high-risk organ transplants.

Four-Dimensional Imaging of T Cells in Kidney Transplant Rejection

Kidney transplantation is the treatment of choice for ESRD but is complicated by the response of the recipient's immune system to nonself histocompatibility antigens on the graft, resulting in rejection. Multiphoton intravital microscopy, referred to as four-dimensional imaging because it records dynamic events in three-dimensional tissue volumes, has emerged as a powerful tool to study immunologic processes in living animals. Here, we will review advances in understanding the complex mechanisms of T cell-mediated rejection made possible by four-dimensional imaging of mouse renal allografts. We will summarize recent data showing that activated (effector) T cell migration to the graft is driven by cognate antigen presented by dendritic cells that surround and penetrate peritubular capillaries, and that T cell-dendritic cell interactions persist in the graft over time, maintaining the immune response in the tissue.

High circulating CD4+CD25hiFOXP3+ T-cell sub-population early after lung transplantation is associated with development of bronchiolitis obliterans syndrome

BACKGROUND

Chronic bronchiolitis obliterans syndrome (BOS) remains a major limitation for long-term survival after lung transplantation. The immune mechanisms involved and predictive biomarkers have yet to be identified. The purpose of this study was to determine whether peripheral blood T-lymphocyte profile could predict BOS in lung transplant recipients.

METHODS

An in-depth profiling of CD4⁺ and CD8⁺ T cells was prospectively performed on blood cells from stable (STA) and BOS patients with a longitudinal follow-up. Samples were analyzed at 1 and 6 months after transplantation, at the time of BOS diagnosis, and at an intermediate time-point at 6 to 12 months before BOS diagnosis.

RESULTS

Although no significant difference was found for T-cell compartments at BOS diagnosis or several months beforehand, we identified an increase in the CD4⁺CD25^hiFoxP3⁺ T-cell sub-population in BOS patients at 1 and 6 months after transplantation (3.39 ± 0.40% vs 1.67 ± 0.22% in STA, p < 0.001). A CD4⁺CD25^hiFoxP3⁺ T-cell threshold of 2.4% discriminated BOS and stable patients at 1 month post-transplantation. This was validated on a second set of patients at 6 months post-transplantation. Patients with a proportion of CD4⁺CD25^hiFoxP3⁺ T cells up to 2.4% in the 6 months after transplantation had a 2-fold higher risk of developing BOS.

CONCLUSIONS

This study is the first to report an increased proportion of circulating CD4⁺CD25^hiFoxP3⁺ T cells early post-transplantation in lung recipients who proceed to develop BOS within 3 years, which supports its use as a BOS predictive biomarker.

Immunosuppressive effect of arsenic trioxide on islet xenotransplantation prolongs xenograft survival in mice

The role of arsenic trioxide (As₂O₃) in inhibiting immune rejection and prolonging islet allograft survival has been identified in islet allotransplantation. This study aims to explore the role of As₂O₃ in islet xenotransplantation and the action mechanism. The streptozotocin (STZ) was used in C57BL/6 mice to induce the type 1 diabetes mellitus (T1DM) for xenotransplantation models establishment. Donor islets were isolated by digesting. The flow cytometry (FCM) was used to analyze lymphocyte types. The blood sugar level was detected by using intraperitoneal glucose tolerance test (IPGTT). The serum level of cytokines was determined by the enzyme-linked immunosorbent assay (ELIZA). The cell proliferation was measured by MTT assay. The mRNA levels were quantified with qRT-PCR. As₂O₃ prolonged the survival of the recipient mice but had no influence on body weight. As₂O₃ protected the function of xenograft in insulin secretion and suppressed immune rejection of recipient. As₂O₃ inhibited proliferation of T lymphocyte and increased the proportion of Foxp3⁺ regulatory T cells in recipient mice. As₂O₃ inhibited activation and promoted clonal anergy of T lymphocyte. As₂O₃ decreased total number of B cells and reduced partial antibody levels in recipient mice. As₂O₃ and leflunomide showed a synergistic effect in suppressing islet xenotransplant rejection. As₂O₃ prolongs islet xenograft survival by inhibiting cellular immune response, and increasing Foxp3⁺ regulatory T cells, while decreasing partial antibody levels in serum.

Allograft dendritic cell p40 homodimers activate donor-reactive memory CD8+ T cells

Recipient endogenous memory T cells with donor reactivity pose an important barrier to successful transplantation and costimulatory blockade-induced graft tolerance. Longer ischemic storage times prior to organ transplantation increase early posttransplant inflammation and negatively impact early graft function and long-term graft outcome. Little is known about the mechanisms enhancing endogenous memory T cell activation to mediate tissue injury within the increased inflammatory environment of allografts subjected to prolonged cold ischemic storage (CIS). Endogenous memory CD4+ and CD8+ T cell activation is markedly increased within complete MHC-mismatched cardiac allografts subjected to prolonged versus minimal CIS, and the memory CD8+ T cells directly mediate CTLA-4Ig-resistant allograft rejection. Memory CD8+ T cell activation within allografts subjected to prolonged CIS requires memory CD4+ T cell stimulation of graft DCs to produce p40 homodimers, but not IL-12 p40/p35 heterodimers. Targeting p40 abrogates memory CD8+ T cell proliferation within the allografts and their ability to mediate CTLA-4Ig-resistant allograft rejection. These findings indicate a critical role for memory CD4+ T cell-graft DC interactions to increase the intensity of endogenous memory CD8+ T cell activation needed to mediate rejection of higher-risk allografts subjected to increased CIS.

Ischemia augments alloimmune injury through IL-6-driven CD4+ alloreactivity

Ischemia reperfusion injuries (IRI) are unavoidable in solid organ transplantation. IRI augments alloimmunity but the mechanisms involved are poorly understood. Herein, we examined the effect of IRI on antigen specific alloimmunity. We demonstrate that ischemia promotes alloimmune activation, leading to more severe histological features of rejection, and increased CD4+ and CD8+ T cell graft infiltration, with a predominantly CD8+ IFNγ+ infiltrate. This process is dependent on the presence of alloreactive CD4+ T cells, where depletion prevented infiltration of ischemic grafts by CD8+ IFNγ+ T cells. IL-6 is a known driver of ischemia-induced rejection. Herein, depletion of donor antigen-presenting cells reduced ischemia-induced CD8+ IFNγ+ allograft infiltration, and improved allograft outcomes. Following prolonged ischemia, accelerated rejection was observed despite treatment with CTLA4Ig, indicating that T cell costimulatory blockade failed to overcome the immune activating effect of IRI. However, despite severe ischemic injury, treatment with anti-IL-6 and CTLA4Ig blocked IRI-induced alloimmune injury and markedly improved allograft survival. We describe a novel pathway where IRI activates innate immunity, leading to upregulation of antigen specific alloimmunity, resulting in chronic allograft injury. Based on these findings, we describe a clinically relevant treatment strategy to overcome the deleterious effect of IRI, and provide superior long-term allograft outcomes.

Mechanisms of Graft Rejection and Immune Regulation after Lung Transplant

Outcomes after lung transplant lag behind those of other solid-organ transplants. A better understanding of the pathways that contribute to rejection and tolerance after lung transplant will be required to develop new therapeutic strategies that take into account the unique immunological features of lungs. Mechanistic immunological investigations in an orthotopic transplant model in the mouse have shed new light on immune responses after lung transplant. Here, we highlight that interactions between immune cells within pulmonary grafts shape their fate. These observations set lungs apart from other organs and help provide the conceptual framework for the development of lung-specific immunosuppression.

Selective CD28 blockade attenuates CTLA-4-dependent CD8+ memory T cell effector function and prolongs graft survival

Memory T cells pose a significant problem to successful therapeutic control of unwanted immune responses during autoimmunity and transplantation, as they are differentially controlled by cosignaling receptors such as CD28 and CTLA-4. Treatment with abatacept and belatacept impede CD28 signaling by binding to CD80 and CD86, but they also have the unintended consequence of blocking the ligands for CTLA-4, a process that may inadvertently boost effector responses. Here, we show that a potentially novel anti-CD28 domain antibody (dAb) that selectively blocks CD28 but preserves CTLA-4 coinhibition confers improved allograft survival in sensitized recipients as compared with CTLA-4 Ig. However, both CTLA-4 Ig and anti-CD28 dAb similarly and significantly reduced the accumulation of donor-reactive CD8+ memory T cells, demonstrating that regulation of the expansion of CD8+ memory T cell populations is controlled in part by CD28 signals and is not significantly impacted by CTLA-4. In contrast, selective CD28 blockade was superior to CTLA-4 Ig in inhibiting IFN-γ, TNF, and IL-2 production by CD8+ memory T cells, which in turn resulted in reduced recruitment of innate CD11b+ monocytes into allografts. Importantly, this superiority was CTLA-4 dependent, demonstrating that effector function of CD8+ memory T cells is regulated by the balance of CD28 and CTLA-4 signaling.

Immunomodulatory activity of glycodelin: implications in allograft rejection

Glycodelin is an immunomodulator, indispensable for the maintenance of pregnancy in humans. The glycoprotein induces apoptosis in activated CD4⁺ T cells, monocytes and natural killer (NK) cells, and suppresses the activity of cytotoxic T cells, macrophages and dendritic cells. This study explores the immunosuppressive property of glycodelin for its possible use in preventing graft rejection. Because glycodelin is found only in certain primates, the hypothesis was investigated in an allograft nude mouse model. It is demonstrated that treatment of alloactivated mononuclear cells with glycodelin thwarts graft rejection. Glycodelin decreases the number of activated CD4⁺ and CD8⁺ cells and down-regulates the expression of key proteins known to be involved in graft demise such as granzyme-B, eomesodermin (EOMES), interleukin (IL)-2 and proinflammatory cytokines [tumour necrosis factor (TNF)-α and IL-6], resulting in a weakened cell-mediated immune response. Immunosuppressive drugs for treating allograft rejection are associated with severe side effects. Glycodelin, a natural immunomodulator in humans, would be an ideal alternative candidate.

Inhibition of C-X-C motif chemokine 10 reduces graft loss mediated by memory CD8+ T cells in a rat cardiac re-transplant model

The interaction of chemokine (C-X-C motif) ligand 10 (CXCL10) with its receptor (CXCR3) is a critical process in recruiting donor reactive T cells to a graft and alloantigen-specific memory T (Tm) cells exert a principal function in promoting graft dysfunction during accelerated cardiac rejection. However, whether CXCL10 chemokine exerts any effects on acute accelerated rejection mediated by CD8⁺ Tm cells in a re-transplant model has remained elusive. The present study established a cardiac transplant model by advanced microsurgery technology and improved organ storage. A novel rat model of cardiac re-transplantation was established at 40 days following primary heart transplant. The experiment included two parts, and when models were established, the rats were divided into two groups: Primary cardiac transplant (HTx) and re-transplantation without treatment (HRTx). In part 1, recipients from part 2, including re-transplantation without treatment (HRTx+NS) and re-transplantation treated with anti-CXCL10 antibodies (500 µg every other day by intraperitoneal injection; HRTx+CXCL10 Abs group). The graft survival time was observed and graft infiltration by inflammatory cells was assessed via histology of cardiac graft sections; in addition, the gene expression and the serum concentration of CXCL10 in each group was assessed. Indexes such as rejection-associated cytokines were assayed by reverse-transcription quantitative PCR and ELISA kits, and flow cytometry of splenocytes was used to detect Tm cells in the re-transplantation groups. The results demonstrated that level of CXCL10 was significantly increased and the graft mean survival time was shortened accompanied with aggravated lymphocyte cell infiltration in the HRTx group when compared that in the HTx group; in addition, the serum levels and mRNA expression of interleukin (IL)-2 and interferon (IFN)-γ were increased, while transforming growth factor (TGF)-β was decreased in the HRTx group. Furthermore, neutralization of CXCL10 prolonged the graft mean survival time and delayed accelerated rejection. Compared with that in the HRTx+NS group, serum levels and graft tissue mRNA expression of IFN-γ and IL-2 were decreased in the HRTx+CXCL10 Abs group, while TGF-β mRNA was significantly increased but the serum concentration was not significantly affected. In addition, there was no difference in IL-10 between the two groups, while delayed accelerated rejection paralleled with inflammatory cell infiltration decreased and the proliferation and differentiation of CD8⁺ Tm cells in secondary lymphoid organs were reduced in the HRTx+CXCL10 Abs group vs. those in the HRTx+NS group. The present study demonstrated that CXCL10 had a crucial role in cardiac transplantation and re-transplantation, and that treatment with CXCL10 antibodies delays accelerated acute rejection mediated by Tm cells in a rat model of cardiac re-transplantation.

Alloimmune T cells in transplantation

Alloimmune T cells are central mediators of rejection and graft-versus-host disease in both solid organ and hematopoietic stem cell transplantation. Unique among immune responses in terms of its strength and diversity, the T cell alloresponse reflects extensive genetic polymorphisms between allogeneic donors and recipients, most prominently within the major histocompatibility complex (MHC), which encodes human leukocyte antigens (HLAs) in humans. The repertoire of alloreactive T cell clones is distinct for every donor-recipient pair and includes potentially thousands of unique HLA/peptide specificities. The extraordinary magnitude of the primary alloresponse and diversity of the T cell population mediating it have presented technical challenges to its study in humans. High-throughput T cell receptor sequencing approaches have opened up new possibilities for tackling many fundamental questions about this important immunologic phenomenon.

Implications of Resident Memory T Cells for Transplantation

Recent studies have established resident memory T cells (TRM ) as the dominant memory lymphocyte population surveying most nonlymphoid tissues. Unlike other memory T cell lineages, TRM do not recirculate through blood and are permanently confined to their tissue of residence. TRM orchestrate local immune responses and have been shown to accelerate local pathogen control in many experimental infection models. Here we briefly summarize recent advances in TRM differentiation, maintenance, and their protective function. While little is known, we have speculated on the potential implications of TRM for transplantation biology. Areas of emphasis include the role of passenger TRM in controlling latent viral recrudescence in donor organs, donor TRM as a source of graft-versus-host disease, the ability of TRM to potently induce inflammation through sensing and alarm functions, and differentiation of host T cells into TRM in response to local cues inside the allograft. Further investigation of TRM in the context of transplantation might identify therapeutic targets to prolong graft survival.

Harnessing the properties of dendritic cells in the pursuit of immunological tolerance

The acquisition of self-perpetuating, immunological tolerance specific for graft alloantigens has long been described as the "holy grail" of clinical transplantation. By removing the need for life-long immunosuppression following engraftment, the adverse consequences of immunosuppressive regimens, including chronic infections and malignancy, may be avoided. Furthermore, autoimmune diseases and allergy are, by definition, driven by aberrant immunological responses to ordinarily innocuous antigens. The re-establishment of permanent tolerance towards instigating antigens may, therefore, provide a cure to these common diseases. Whilst various cell types exhibiting a tolerogenic phenotype have been proposed for such a task, tolerogenic dendritic cells (tol-DCs) are exquisitely adapted for antigen presentation and interact with many facets of the immune system: as such, they are attractive candidates for use in strategies for immune intervention. We review here our current understanding of tol-DC mediated induction and maintenance of immunological tolerance. Additionally, we discuss recent in vitro findings from animal models and clinical trials of tol-DC immunotherapy in the setting of transplantation, autoimmunity and allergy which highlight their promising therapeutic potential, and speculate how tol-DC therapy may be developed in the future.

Role of Memory T Cells in Allograft Rejection and Tolerance

Memory T cells are characterized by their low activation threshold, robust effector functions, and resistance to conventional immunosuppression and costimulation blockade. Unlike their naïve counterparts, memory T cells reside in and recirculate through peripheral non-lymphoid tissues. Alloreactive memory T cells are subdivided into different categories based on their origins, phenotypes, and functions. Recipients whose immune systems have been directly exposed to allogeneic major histocompatibility complex (MHC) molecules display high affinity alloreactive memory T cells. In the absence of any prior exposure to allogeneic MHC molecules, endogenous alloreactive memory T cells are regularly generated through microbial infections (heterologous immunity). Regardless of their origin, alloreactive memory T cells represent an essential element of the allograft rejection process and a major barrier to tolerance induction in clinical transplantation. This article describes the different subsets of alloreactive memory T cells involved in transplant rejection and examine their generation, functional properties, and mechanisms of action. In addition, we discuss strategies developed to target deleterious allospecific memory T cells in experimental animal models and clinical settings.

CD8+ effector memory T cells induce acute rejection of allogeneic heart retransplants in mice possibly through activating expression of inflammatory cytokines

BACKGROUND

To investigate the effects of CD8⁺ memory T (Tm) cells and CD8⁺ effector memory T (Tem) cells on the results of allogeneic heart retransplantations performed in mice.

METHODS

A skin transplantation model was used to generate sensitized splenic CD8⁺ Tem cells for infusion into BALB/c mice. One week after infusion, the BALB/c mice underwent allogeneic heart transplantation in the abdominal cavity. Cyclosporin A was administered via intraperitoneal injection starting one day prior to transplantation to arrest immunological rejection of the transplanted heart. The effects of sensitized CD8⁺ T_em cells on allogeneic heart graft rejection were examined by monitoring survival of the transplanted hearts, the infiltration of effector memory CD8⁺ T cells into myocardium, and expressions of inflammatory cytokines in blood serum.

RESULTS

Adoptive transfer of sensitized CD8⁺ Tem cells prior to transplantation induced an acute rejection response which decreased the survival of transplanted hearts. The rejection response was accompanied by an infiltration of CD8⁺ Tem cells into the transplanted myocardial tissue. Additionally, infusion of sensitized CD8⁺ Tem cells induced markedly increased expressions of IL-2 and IFN-γ, and decreased expression of TGF-β in the transplanted hearts, as well as higher levels of IFN-γ and CXCL-9 in blood serum.

CONCLUSIONS

The infusion of sensitized CD8⁺ Tem cells induced an acute graft rejection response and decreased the survival of grafted hearts by regulating the expressions of inflammatory cytokines including CXCL-9, IL-2, and INF-γ. Cyclosporin A had no therapeutic effect on the graft rejection response induced by sensitized CD8⁺ Tem cells.

Effects of Cigarette Smoking on Transplant Survival: Extending or Shortening It?

Cigarette smoking (CS) regulates both innate and adaptive immunity and causes numerous diseases, including cardiovascular, respiratory, and autoimmune diseases, allergies, cancers, and transplant rejection. Therefore, smoking poses a serious challenge to the healthcare system worldwide. Epidemiological studies have always shown that CS is one of the major risk factors for transplant rejection, even though smoking plays redundant roles in regulating immune responses. The complex roles for smoking in immunoregulation are likely due to molecular and functional diversities of cigarette smoke components, including carbon monoxide (CO) and nicotine. Especially, CO has been shown to induce immune tolerance. Although CS has been shown to impact transplantation by causing complications and subsequent rejection, it is overlooked whether CS interferes with transplant tolerance. We have previously demonstrated that cigarette smoke exposure reverses long-term allograft survival induced by costimulatory blockade. Given that CS impacts both adaptive and innate immunity and that it hinders long-term transplant survival, our perspective is that CS impacts transplant tolerance. Here, we review impacts of CS on major immune cells that are critical for transplant outcomes and propose the cellular and molecular mechanisms underlying its effects on alloimmunity and transplant survival. Further investigations are warranted to fully understand why CS exerts deleterious rather than beneficial effects on transplant survival even if some of its components are immunosuppressive.

Mechanisms of graft rejection after lung transplantation

PURPOSE OF REVIEW

To date, outcomes after lung transplantation are far worse than after transplantation of other solid organs. New insights into mechanisms that contribute to graft rejection and tolerance after lung transplantation remain of great interest. This review examines the recent literature on the role of innate and adaptive immunity in shaping the fate of lung grafts.

RECENT FINDINGS

Innate and adaptive immune cells orchestrate allograft rejection after transplantation. Innate immune cells such as neutrophils are recruited to the lung graft early after reperfusion and subsequently promote allograft rejection. Although it is widely recognized that CD4 T lymphocytes in concert with CD8 T cells promote graft rejection, regulatory Foxp3 CD4 T, central memory CD8 T cells, and natural killer cells can facilitate tolerance.

SUMMARY

This review highlights interactions between innate and adaptive immune pathways and how they contribute to lung allograft rejection. These findings lay a foundation for the design of new therapeutic strategies that target both innate and adaptive immune responses.

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.

Premature T Cell Senescence in Pediatric CKD

An individual's immune function, susceptibility to infection, and response to immunosuppressive therapy are influenced in part by his/her T cell maturation state. Although childhood is the most dynamic period of immune maturation, scant information regarding the variability of T cell maturation in children with renal disease is available. In this study, we compared the T cell phenotype in children with renal failure (n=80) with that in healthy children (n=20) using multiparameter flow cytometry to detect markers of T cell maturation, exhaustion, and senescence known to influence immune function. We correlated data with the degree of renal failure (dialysis or nondialysis), prior immunosuppression use, and markers of inflammation (C-reactive protein and inflammatory cytokines) to assess the influence of these factors on T cell phenotype. Children with renal disease had highly variable and often markedly skewed maturation phenotypes, including CD4/CD8 ratio reversal, increased terminal effector differentiation in CD8⁺ T cells, reduction in the proportion of naïve T cells, evidence of T cell exhaustion and senescence, and variable loss of T cell CD28 expression. These findings were most significant in patients who had experienced major immune insults, particularly prior immunosuppressive drug exposure. In conclusion, children with renal disease have exceptional heterogeneity in the T cell repertoire. Cognizance of this heterogeneity might inform risk stratification with regard to the balance between infectious risk and response to immunosuppressive therapy, such as that required for autoimmune disease and transplantation.

IL-1 Receptor Signaling on Graft Parenchymal Cells Regulates Memory and De Novo Donor-Reactive CD8 T Cell Responses to Cardiac Allografts

Reperfusion of organ allografts induces a potent inflammatory response that directs rapid memory T cell, neutrophil, and macrophage graft infiltration and their activation to express functions mediating graft tissue injury. The role of cardiac allograft IL-1 receptor (IL-1R) signaling in this early inflammation and the downstream primary alloimmune response was investigated. When compared with complete MHC-mismatched wild-type cardiac allografts, IL-1R(-/-) allografts had marked decreases in endogenous memory CD8 T cell and neutrophil infiltration and expression of proinflammatory mediators at early times after transplant, whereas endogenous memory CD4 T cell and macrophage infiltration was not decreased. IL-1R(-/-) allograft recipients also had marked decreases in de novo donor-reactive CD8, but not CD4, T cell development to IFN-γ-producing cells. CD8 T cell-mediated rejection of IL-1R(-/-) cardiac allografts took 3 wk longer than wild-type allografts. Cardiac allografts from reciprocal bone marrow reconstituted IL-1R(-/-)/wild-type chimeric donors indicated that IL-1R signaling on graft nonhematopoietic-derived, but not bone marrow-derived, cells is required for the potent donor-reactive memory and primary CD8 T cell alloimmune responses observed in response to wild-type allografts. These studies implicate IL-1R-mediated signals by allograft parenchymal cells in generating the stimuli-provoking development and elicitation of optimal alloimmune responses to the grafts.

The Evolving Roles of Memory Immune Cells in Transplantation

Memory cells are the products of immune responses but also exert significant impact on subsequent immunity and immune tolerance, thus placing them in a unique position in transplant research. Memory cells are heterogeneous, including not only memory T cells but also memory B cells and innate memory cells. Memory cells are a critical component of protective immunity against invading pathogens, especially in immunosuppressed patients, but they also mediate graft loss and tolerance resistance. Recent studies suggest that some memory cells unexpectedly act as regulatory cells, promoting rather than hindering transplant survival. This functional diversity makes therapeutic targeting of memory cells a challenging task in transplantation. In this article, we highlight recent advances in our understanding of memory cells, focusing on diversity of memory cells and mechanisms involved in their induction and functions. We also provide a broad overview on the challenges and opportunities in targeting memory cells in the induction of transplant tolerance.

CD8(+)CD122(+) T-Cells: A Newly Emerging Regulator with Central Memory Cell Phenotypes

CD8(+)CD122(+) T-cells have been traditionally described as antigen-specific memory T-cells that respond to previously encountered antigens more quickly and vigorously than their naïve counterparts. However, mounting evidence has demonstrated that murine CD8(+)CD122(+) T-cells exhibit a central memory phenotype (CD44(high)CD62L(high)), regulate T cell homeostasis, and act as regulatory T-cells (Treg) by suppressing both autoimmune and alloimmune responses. Importantly, naturally occurring murine CD8(+)CD122(+) Tregs are more potent in immunosuppression than their CD4(+)CD25(+) counterparts. They appear to be acting in an antigen-non-specific manner. Human CD8(+)CXCR3(+) T-cells are the equivalent of murine CD8(+)CD122(+) Tregs and also exhibit central memory phenotypes. In this mini-review article, we will summarize recent progresses in their phenotypes, homeostatic expansion, antigen-specificity, roles in the suppression of alloimmune and autoimmune responses, and the mechanisms underlying their inhibitory function.

T-cell alloreactivity and transplantation outcome: a budding role for heterologous immunity?

PURPOSE OF REVIEW

Despite the association between alloreactive T cells and poor graft survival, the mechanisms behind T-cell-mediated rejection are still under investigation. In this review, we will discuss the latest insights into the impact of T-cell alloreactivity on solid organ transplantation and hematopoietic stem cell transplantation (HSCT), with special emphasis on the potential impact of heterologous immunity.

RECENT FINDINGS

A large part of the memory T-cell repertoire is induced upon virus infections, and evidence for a role of T-cell receptor cross-reactivity of virus-induced memory T cells against allogeneic human leukocyte antigen (HLA) is accumulating in experimental and clinical solid organ transplantation studies. In HSCT, strong alloreactive potential of naïve T cells causes concerns for graft-versus-host disease while additional HLA-DP matching is suggested to prevent CD4 alloreactivity. Furthermore, virus-induced memory T cells hamper mixed chimerism induction, pointing once more towards a role for heterologous immunity.

SUMMARY

Both memory and naïve T cells contribute to the alloimmune response after transplantation. Monitoring for T-cell phenotypes could help predict rejection episodes and/or graft-versus-host disease, allowing timely intervention. Tailoring donor lymphocyte infusions and additional HLA matching could prevent strong alloreactivity in HSCT. Furthermore, the potential role of heterologous immunity in T-cell alloreactivity and transplantation is gaining interest.

Role of Memory T Cells and Perspectives for Intervention in Organ Transplantation

Memory T cells are necessary for protective immunity against invading pathogens, especially under conditions of immunosuppression. However, their presence also threatens transplant survival, making transplantation a great challenge. Significant progress has been achieved in recent years in advancing our understanding of the role that memory T cells play in transplantation. This review focuses on the latest advances in our understanding of the involvement of memory T cells in graft rejection and transplant tolerance and discusses potential strategies for targeting memory T cells in order to minimize allograft rejection and optimize clinical outcomes.

Experimental models of cardiac transplantation: design determines relevance

PURPOSE OF REVIEW

Experimental models have contributed enormously to basic immunology. However, the use of reductionist experiments has produced results that are not always successfully translated into the clinic. Recently, incorporation of more realistic clinical parameters in experimental designs has produced new insights relevant to cardiac transplantation.

RECENT FINDINGS

Experiments in mice have provided crucial insights into the concept that T cell responses to pathogens generate memory cells with cross-reactive specificities for histocompatibility antigens. These memory T cells are resistant to current immunosuppressive strategies. Memory T cells infiltrate grafts within hours after transplantation, and grafts subjected to clinically relevant periods of cold ischemia are more susceptible to injury by this cellular infiltrate. Early immune responses now can be investigated with improved 'humanized' mice. Mice with multiple knock-in genes for human cytokines support development of human monocytes, macrophages and natural killer cells in increased numbers and with better function.

SUMMARY

Better and more clinically relevant experimental designs are providing animal models tailored to address clinic exigencies.

Arsenic trioxide inhibits accelerated allograft rejection mediated by alloreactive CD8(+) memory T cells and prolongs allograft survival time

CD8(+) memory T (Tm) cells are a significant barrier to transplant tolerance induction in alloantigen-primed recipients, and are insensitive to existing clinical immunosuppressants. Here, we studied the inhibition of CD8(+) Tm cells by arsenic trioxide (As2O3) for the first time. Alloantigen-primed CD8(+) Tm cells were transferred to T cell immunodeficient nude mice. The mice were subjected to heart allotransplantation, and treated with As2O3. The transplant survival time was determined, and the inhibitory effects of As2O3 on CD8(+) Tm cell-mediated immune rejection were assessed through serological studies and inspection of the transplanted heart and lymphoid organs. We found that As2O3 treatment prolonged the mean survival time of the graft and reduced the number of CD8(+) Tm cells in the spleen and lymph nodes. The expression of the genes encoding interleukin (IL)-2, and IFN-γ was reduced, while expression of IL-10 and transforming growth factor-β was increased in the transplant. Our findings show that As2O3 treatment inhibits allograft rejection mediated by alloreactive CD8(+) Tm cells in the mouse heart transplantation model.

A pilot trial targeting the ICOS-ICOS-L pathway in nonhuman primate kidney transplantation

Costimulation blockade with the B7-CD28 pathway-specific agent belatacept is now used in clinical kidney transplantation, but its efficacy remains imperfect. Numerous alternate costimulatory pathways have been proposed as targets to synergize with belatacept, one of which being the inducible costimulator (ICOS)-ICOS ligand (ICOS-L) pathway. Combined ICOS-ICOS-L and CD28-B7 blockade has been shown to prevent rejection in mice, but has not been studied in primates. We therefore tested a novel ICOS-Ig human Fc-fusion protein in a nonhuman primate (NHP) kidney transplant model alone and in combination with belatacept. ICOS-Ig did not prolong rejection-free survival as a monotherapy or in combination with belatacept. In ICOS-Ig alone treated animals, most graft-infiltrating CD4(+) and CD8(+) T cells expressed ICOS, and ICOS(+) T cells were present in peripheral blood to a lesser degree. Adding belatacept reduced the proportion of graft-infiltrating ICOS(+) T cells and virtually eliminated their presence in peripheral blood. Graft-infiltrating T cells in belatacept-resistant rejection were primarily CD8(+) CD28(-) , but importantly, very few CD8(+) CD28(-) T cells expressed ICOS. We conclude that ICOS-Ig, alone or combined with belatacept, does not prolong renal allograft survival in NHPs. This may relate to selective loss of ICOS with CD28 loss.

De novo alloreactive memory CD8+ T cells develop following allogeneic challenge when CNI immunosuppression is delayed

Allospecific memory T cells are a recognized threat to the maintenance of solid-organ transplants. Limited information exists regarding the development of alloreactive memory T cells when post-transplant immunosuppression is present. The clinical practice of delaying calcineurin inhibitor (CNI) initiation post-transplant may permit the development of a de novo allospecific memory population. We investigated the development of de novo allospecific memory CD8+ T cells following the introduction of CNI immunosuppression in a murine model using allogeneic cell priming. Recipient mice alloprimed with splenocytes from fully mismatched donors received cyclosporine (CyA), initiated at 0, 2, 6, or 10days post-prime. Splenocytes from recipients were analyzed by flow cytometry or enzyme-linked immunosorbent assay for evidence of memory cell formation. Memory and effector CD8+ T cell development was prevented when CyA was initiated at 0day or 2days post-prime (p<0.001), but not 6days post-prime. Following a boost challenge, these memory CD8+ T cells were capable of producing a similarly sized population of secondary effectors as recipients not treated with CyA (p>0.05). Delaying CyA up to 6days or later post-prime permits the development of functional de novo allospecific memory CD8+ T cells. The development of this potentially detrimental T cell population in patients could be prevented by starting CNI immunosuppression early post-transplant.

Effect of cholecalciferol supplementation on inflammation and cellular alloimmunity in hemodialysis patients: data from a randomized controlled pilot trial

Background

Memory T-cells are mediators of transplant injury, and no therapy is known to prevent the development of cross-reactive memory alloimmunity. Activated vitamin D is immunomodulatory, and vitamin D deficiency, common in hemodialysis patients awaiting transplantation, is associated with a heightened alloimmune response. Thus, we tested the hypothesis that vitamin D₃ supplementation would prevent alloreactive T-cell memory formation in vitamin D-deficient hemodialysis patients.

Methods and Findings

We performed a 12-month single-center pilot randomized, controlled trial of 50,000 IU/week of cholecalciferol (D₃) versus no supplementation in 96 hemodialysis patients with serum 25(OH)D<25 ng/mL, measuring effects on serum 25(OH)D and phenotypic and functional properties of T-cells. Participants were randomized 2∶1 to active treatment versus control. D₃ supplementation increased serum 25(OH)D at 6 weeks (13.5 [11.2] ng/mL to 42.5 [18.5] ng/mL, p<0.001) and for the duration of the study. No episodes of sustained hypercalcemia occurred in either group. Results of IFNγ ELISPOT-based panel of reactive T-cell assays (PRT), quantifying alloreactive memory, demonstrated greater increases in the controls over 1 year compared to the treatment group (delta PRT in treatment 104.8+/−330.8 vs 252.9+/−431.3 in control), but these changes in PRT between groups did not reach statistical significance (p = 0.25).

Conclusions

D₃ supplements are safe, effective at treating vitamin D deficiency, and may prevent time-dependent increases in T-cell alloimmunity in hemodialysis patients, but their effects on alloimmunity need to be confirmed in larger studies. These findings support the routine supplementation of vitamin D-deficient transplant candidates on hemodialysis and highlight the need for large-scale prospective studies of vitamin D supplementation in transplant candidates and recipients.

Trial Registration

Clinicaltrials.gov NCT01175798