New Insights into Mechanisms of Spontaneous Liver Transplant Tolerance: The Role of Foxp3-Expressing CD25+CD4+ Regulatory T Cells

Liver allografts in mice are accepted across MHC barriers without requirement for immunosuppressive therapy. The mechanisms underlying this phenomenon remain largely undefined. In this study, we investigated the role of Foxp3-expressing CD25⁺CD4⁺ regulatory T cells (Treg) in the induction of murine liver transplant tolerance. Foxp3⁺CD25⁺CD4⁺ T cells were increased in liver grafts and recipient spleens from day 5 to day 100 posttransplantation, associated with enhanced CTLA4 and TGF-β expression and IL-4 production. Depletion of recipient CD25⁺CD4⁺ T cells using anti-CD25 mAb (250 μg/day) induced acute liver allograft rejection. This was associated with a decreased ratio of Foxp3⁺ Treg: T effector cells, decreased IL-4 and elevated IL-10 and IL-2 production by graft-infiltrating T cells, and reduced apoptotic activity of graft-infiltrating CD4⁺ and CD8⁺ T cells in anti-CD25-mAb-treated recipients. Thus, the data suggest that Foxp3⁺CD25⁺CD4⁺Treg are involved in spontaneous acceptance of liver allografts in mice. The ratio of Treg to T effector cells appears to determine liver transplant outcome. CTLA4, IL-4, TGF-β and apoptosis of graft-infiltrating T cells are also associated with liver transplant tolerance and may contribute, at least in part, to the mechanisms of Treg-mediated immune regulation in this model.     

'Chimeric' Grafts Assembled from Multiple Allodisparate Donors Enjoy Enhanced Transplant Survival

Certain components of a graft that provoke alloimmunity may not be vital for graft function or critical as targets of rejection. Corneal transplantation is an example of this, because graft epithelium plays a role in allosensitization, whereas corneal graft endothelium—which shares the same alloantigens—is the critical target in allorejection. In this study, we found that exploiting this biology by replacing donor epithelium of an allograft with an allodisparate third-party epithelium yields a marked enhancement in transplant survival. Such 'chimeric' allografts consisted of a C3H/He (H-2^k) corneal epithelium over a C57BL/6 (H-2^b) epithelial-denuded cornea (or v.v. ) and orthotopically placed on BALB/c (H-2^d) hosts. Conventional corneal allografts (C3H/He or C57BL/6) or isografts (BALB/c) were also transplanted on BALB/c hosts. Alloreactive T-cell frequencies (CD4⁺ interferon [IFN]-γ⁺) primed to the graft endothelium were strongly diminished in chimeric hosts relative to conventionally allografted hosts. This was corroborated by a decreased T-cell infiltration (p = 0.03) and a marked enhancement of allograft survival (p = 0.001). Our results represent the first successful demonstration of chimeric tissue, epithelial-denuded allograft plus third-party allodisparate epithelium, in the promotion of allograft survival. Moreover, chimeric grafting can be readily performed clinically, whereby corneal allograft rejection remains a significant problem particularly in inflamed graft beds.     

Increased Expression of Senescence-Associated Cell Cycle Inhibitor p16INK4a in Deteriorating Renal Transplants and Diseased Native Kidney

Some features of kidney transplants with dysfunction overlap the lesions of aging, such as tubular atrophy and interstitial fibrosis (TA/IF) without major glomerular abnormalities. Somatic cell limitations could contribute to deterioration in aging and disease states. Since expression of p16(INK4a), a cell cycle inhibitor associated with somatic cell senescence in vitro, is induced in aged kidney, we studied whether kidneys with dysfunction and TA/IF manifested increased p16(INK4a) expression. We performed p16(INK4a) immunostaining on transplanted kidneys and native kidneys with chronic renal diseases. At implantation, transplants manifested little TA/IF, and nuclear p16(INK4a) immunostaining was consistent with age. However, transplants biopsied for abnormal function displaying TA/IF showed strong nuclear and cytoplasmic p16(INK4a) staining, beyond the amount predicted for age. Both atrophic and non-atrophic nephrons displayed increased p16(INK4a), suggesting that it was not simply a feature of atrophy. Epithelial p16(INK4a) staining was not increased in transplants with good function, but was increased in diseased native kidneys. The finding of increased p16(INK4a) expression in renal transplants and diseased kidneys with TA/IF and impaired function supports the concept that some cell senescence changes that accompany aging are also induced by injury and disease stresses. Thus, aging, injury and disease may share common pathways involving somatic cell senescence.     

Cell senescence in rat kidneys in vivo increases with growth and age despite lack of telomere shortening

BACKGROUND: Somatic cells in vitro have a finite life expectancy before entering a state of senescence, but it is unclear whether this state occurs in vivo in kidney development, growth, and aging. We previously showed that human kidney cortex displays telomere shortening with age. In this study, we compared the structural and functional changes in rat kidney with age to phenomena associated with cellular senescence in vitro. METHODS: We assessed the changes in Fischer 344 rat kidneys from age 1 to 9 months to define growth and development and from age 9 to 24 months to define aging. RESULTS: Rat kidney telomeres were approximately 35 to 40 kb long and did not shorten significantly. Expression of mRNA for p16INK4a, a characteristic senescence gene in vitro, was undetectable in most young rats but rose 27 fold during growth and a further 72-fold during aging. p16INK4a protein was localized to the nucleus and increased with age. p16INK4a mRNA also increased in other tissues. Lipofuscin and senescence-associated beta-galactosidase increased in epithelium with growth and aging and their occurrence was significantly associated with each other. Lipofuscin was particularly found in atrophic nephrons. CONCLUSION: We conclude that cell senescence occurs in both growth and aging in rat kidney and may contribute to the age-related pathology. These changes are not due to telomere shortening, but may reflect cumulative environmental stress.     

CCR5 Is Required for Regulation of Alloreactive T-Cell Responses to Single Class II MHC-Mismatched Murine Cardiac Grafts

The effector CD4 T-cell response in wild-type C57BL/6 recipients of single class II MHC-disparate B6.H-2^bm12 cardiac allografts is restricted by CD4⁺CD25⁺ regulatory T cells (Tregs) resulting in long-term allograft survival. To investigate the role chemokine receptors might play in Treg function, this study tested the requirement for CCR5 on Tregs to suppress the alloimmune response in C57BL/6 recipients of B6.H-2^bm12 cardiac allografts. In contrast to the long-term survival of B6.H-2^bm12 allografts in wild-type recipients (>100 days), the allografts were acutely rejected within 25 days in CCR5^−/− recipients with intense infiltration of CD4 T cells. Numbers and duration of donor-reactive CD4 T cells producing IFN-γ and IL-4 were markedly increased in spleens of B6.CCR5^−/− versus wild-type recipients. Wild-type and B6.CCR5^−/− mice had equivalent numbers of splenic FoxP3⁺ Tregs before and following transplantation, and these Tregs were equivalently suppressive in vitro . However, diminished numbers of FoxP3⁺ Tregs infiltrated B6.H-2^bm12 allografts in B6.CCR5^−/− recipients. Adoptive transfer of wild-type, but not CCR5-deficient, CD4⁺CD25⁺ Tregs to CCR5^−/− recipients restored long-term survival of B6.H-2^bm12 cardiac grafts. Collectively, these results indicate that CCR5 expression is required for the regulatory functions of Tregs that restrict alloreactive CD4 T-cell responses to single class II MHC-mismatched cardiac allografts.     

Expression of p16INK4a and other cell cycle regulator and senescence associated genes in aging human kidney

BACKGROUND: Somatic cells in vitro have a finite life expectancy before entering a state of senescence. If this state has an in vivo counterpart, it could contribute to organ aging. We have previously shown that human kidney cortex displays telomere shortening with age. In the present study, we evaluated the relationship between renal age in humans and a number of phenomena associated with cellular senescence in vitro. METHODS: Human kidney specimens were obtained at 8 weeks to 88 years of age and were assessed for changes related to aging. RESULTS: We found that human kidneys expressed relatively constant levels of mRNAs for genes potentially related to senescence. Among the candidate genes surveyed, the cell cycle regulator p16INK4a emerged with the strongest association with renal aging for both mRNA and protein expression. Proliferation as measured by Ki-67 expression was inversely correlated with p16INK4a expression, compatible with a role for p16INK4a as an irreversible cell cycle inhibitor. Cyclooxygenase 1 and 2 (COX-1 and COX-2) mRNA expression was elevated in older kidneys, associated with increased protein expression. Comparison of gene expression with age-related histologic changes revealed that glomerulosclerosis correlated with p16INK4a and p53, whereas interstitial fibrosis and tubular atrophy were associated with p16INK4a, p53, COX-1, transforming growth factor-beta 1 (TGF-beta 1), and heat shock protein A5 (HSPA5). CONCLUSION: We conclude that some changes observed in cellular senescence in vitro do occur in human kidney with age, particularly in the renal cortex, in some cases correlating with histologic features. P16INK4a emerged with the most consistent correlations with age and histologic changes and inversely correlated with cell replication.     

Cellular senescence limits regenerative capacity and allograft survival

Long-term graft survival after kidney transplantation remains unsatisfactory and unpredictable. Interstitial fibrosis and tubular atrophy are major contributors to late graft loss; features of tubular cell senescence, such as increased p16(INK4a) expression, associate with these tubulointerstitial changes, but it is unknown whether the relationship is causal. Here, loss of the INK4a locus in mice, which allows escape from p16(INK4a)-dependent senescence, significantly reduced interstitial fibrosis and tubular atrophy and associated with improved renal function, conservation of nephron mass, and transplant survival. Compared with wild-type controls, kidneys from INK4a(-/-) mice developed significantly less interstitial fibrosis and tubular atrophy after ischemia-reperfusion injury. Consistently, mice that received kidney transplants from INK4a/ARF(-/-) donors had significantly better survival 21 days after life-supporting kidney transplantation and developed less tubulointerstitial changes. This correlated with higher proliferative rates of tubular cells and significantly fewer senescent cells. Taken together, these data suggest a pathogenic role of renal cellular senescence in the development of interstitial fibrosis and tubular atrophy and kidney graft deterioration by preventing the recovery from injury. Inhibiting premature senescence could have therapeutic benefit in kidney transplantation but has to be balanced against the risks of suspending antitumor defenses.     

Alloreactive (CD4-Independent) CD8+ T Cells Jeopardize Long-Term Survival of Intrahepatic Islet Allografts

Despite success of early islet allograft engraftment and survival in humans, late islet allograft loss has emerged as an important clinical problem. CD8⁺ T cells that are independent of CD4⁺ T cell help can damage allograft tissues and are resistant to conventional immunosuppressive therapies. Previous work demonstrates that islet allografts do not primarily initiate rejection by the (CD4-independent) CD8-dependent pathway. This study was performed to determine if activation of alloreactive CD4-independent, CD8⁺ T cells, by exogenous stimuli, can precipitate late loss of islet allografts. Recipients were induced to accept intrahepatic islet allografts (islet 'acceptors') by short-term immunotherapy with donor-specific transfusion (DST) and anti-CD154 mAb. Following the establishment of stable long-term islet allograft function for 60–90 days, recipients were challenged with donor-matched hepatocellular allografts, which are known to activate (CD4-independent) CD8⁺ T cells. Allogeneic islets engrafted long-term were vulnerable to damage when challenged locally with donor-matched hepatocytes. Islet allograft loss was due to allo specific immune damage, which was CD8- but not CD4-dependent. Selection of specific immunotherapy to suppress both CD4- and CD8-dependent immune pathways at the time of transplant protects islet allografts from both early and late immune damage.     

Ex Vivo-Expanded Natural CD4+CD25+ Regulatory T Cells Synergize With Host T-Cell Depletion to Promote Long-Term Survival of Allografts

Foxp3⁺CD4⁺CD25⁺ natural regulatory T (nT_reg) cells have been shown in immunodeficient mice to suppress allograft rejection after adoptive cotransfer. We hypothesized that immunotherapy using ex vivo -expanded nT_reg could suppress allograft rejection in wild-type mice. Donor alloantigen (alloAg) specificity of naive splenic nT_reg was enriched in vitro by culturing with anti-CD3/CD28-coated Dynabeads plus bone marrow-derived dendritic cells (BM-DC) in the presence of interleukin (IL)-2 or IL-2 plus transforming growth factor (TGF)-β. On average, 96.2% fresh CD4⁺CD25⁺ nT_reg were intracellular Foxp3⁺. By d+20 in culture, 6.4% nT_reg were Foxp3⁺ following expansion with IL-2 alone, and 14.4% or 19.7% nT_reg were Foxp3⁺ when expanded with IL-2 plus 0.5 or 2.5 ng/mL TGF-β, respectively. In vitro , alloAg-enriched, TGF-β/IL-2-conditioned nT_reg exerted stronger donor alloAg-specific suppression than cells with IL-2 alone in mixed lymphocyte reaction (MLR) assays. In vivo , alloAg-enriched, TGF-β/IL-2-conditioned nT_reg expressed high-level Foxp3 following infusion, effectively overcame acute rejection and induced long-term survival of donor but not third-party heart allografts in peritransplant host T-cell-depleted mice. Long-term surviving allografts were noted to possess Foxp3⁺ graft-infiltrating cells of exogenous and endogenous origins. In conjunction with transient host T-cell depletion, therapeutic use of ex vivo -expanded nT_reg may be a practical means of preventing acute allograft rejection.     

Airway Epithelial Cell Senescence in the Lung Allograft

Chronic lung allograft dysfunction, manifesting as bronchiolitis obliterans syndrome (BOS), is characterized by airway epithelial injury, impaired epithelial regeneration and subsequent airway remodeling. Increased cellular senescence has been reported in renal and liver allografts affected by chronic allograft dysfunction but the significance of cellular senescence in the airway epithelium of the transplanted lung is unknown. Thirty-four lung transplant recipients, 20 with stable graft function and 14 with BOS, underwent transbronchial lung biopsy and histochemical studies for senescence markers in small airways. Compared to nontransplant control lung tissue (n = 9), lung allografts demonstrate significantly increased airway epithelial staining for senescence-associated beta galactosidase (SA β-gal) (p = 0.0215), p16^ink4a (p = 0.0002) and p21^waf1/cip (p = 0.0138) but there was no difference in expression of these markers between stable and BOS affected recipients (p > 0.05). This preliminary cross-sectional study demonstrates that cellular senescence occurs with increased frequency in the airway epithelium of the lung allograft but does not establish any association between airway epithelial senescence and BOS. A prospective longitudinal study is required to better address any potential causal association between airway epithelial senescence in stable allograft recipients and the subsequent development of BOS.     

Donor-specific Immune Regulation by CD8+ Lymphocytes Expanded from Rejecting Human Cardiac Allografts

To assess whether regulatory T cells are present in rejecting human cardiac allografts, we performed functional analyses of graft lymphocytes (GLs) expanded from endomyocardial biopsies (EMB; n = 5) with histological signs of acute cellular rejection. The GL cultures were tested for their proliferative capacity and regulatory activity on allogeneic-stimulated peripheral blood mononuclear cells (PBMC) of the patient (ratio PBMC:GLs = 5:1). Three of these GL cultures were hyporesponsive to donor antigens and suppressed the antidonor proliferative T-cell response of PBMC, but not the anti-third-party response. Interestingly, it was the CD8⁺ GL subset of these cultures that inhibited the antidonor response (65–91% inhibition of the proportion of proliferating cells); the CD4⁺ GLs of the expanded GL cultures were not suppressive. In conclusion, CD8⁺ GLs expanded from rejecting human cardiac allografts can exhibit donor-specific immune regulatory activities in vitro . We suggest that during acute cellular rejection, GLs may not only consist of graft-destructing effector T cells, but also of cells of the CD8⁺ type with the potential to specifically inhibit antidonor immune reactivity.     

Tumor-infiltrating lymphocytes derived from human renal cell carcinoma: Clonal analysis of its characteristics

Aim:   To assess the characteristics of activated tumor-infiltrating lymphocytes (TIL), we report the isolation, growth response, and functional analysis of a CD4^- CD8⁺ TIL-clone derived from human renal cell carcinoma (RCC).
Methods:   Bulk TILs were expanded from a human RCC and the lymphocytes were separated into a CD8⁺ enriched population. Subsequently, using the limiting dilution technique, a TIL clone was established and its growth response, phenotype and cytotoxic activity were analyzed.
Results:   A clone, T16-13, by day 94 numbering 1 × 10⁷ cells, was harvested and characterized as a CD4^- CD8⁺ clone. On day 144, the cytotoxic activity of this clone against the autologous tumor was relatively high (2.3 ± 0.7 LU₃₀/10⁶ cells). Meanwhile, against allogeneic renal tumors, there was no cytotoxic activity (−0.1 LU₃₀/10⁶ cells).
Conclusions:   A TIL clone possessing modest autologous tumor-specific cytotoxicity can be isolated from human RCC. The characteristics analysis of various TIL clones may provide a better understanding of an RCC tumor microenvironment and may help to establish new modalities for the treatment of patients with metastatic kidney cancer.     

RIP2 Is Required for NOD Signaling But Not for Th1 Cell Differentiation and Cellular Allograft Rejection

Two previous reports that receptor-interacting protein (RIP)-2 knockout (RIP2^–/–) mice had defective nuclear factor-kappa B (NF-κB) signaling and T helper (Th)1 immune responses had led us to believe that this putative serine-threonine kinase might be a possible target for transplant immunosuppression. Thus, we tested whether RIP2^–/– mice were able to reject vascularized allografts. Surprisingly, we found that T cells from RIP2^–/– mice proliferated and produced interferon (IFN)-γ after allostimulation in vitro . Moreover, naïve RIP2^–/– CD4⁺ T cells differentiated normally into Th1 or Th2 cells under appropriate cytokine microenvironments. Consistent with these findings, no difference in allograft survival was observed between wild-type and RIP2^–/– recipient mice, and rejection had similar pathology and cytokine profiles in both types of recipients. RIP2 deficiency was associated with defective NOD signaling, but this did not affect T-cell receptor (TCR)-dependent activation of the canonical NF-κB signaling or expression of NF-κB genes in rejecting allografts. Our data demonstrate that RIP2-deficient mice have intact canonical NF-κB signaling and can mount Th1-mediated alloresponses and reject vascularized allografts as efficiently as wild-type mice, thus arguing against RIP2 as a primary target for immunosuppression.     

Successive emergence of two CD 8 subsets in primary CMV infection of allograft recipients

Abstract Allograft recipients with cytomegalovirus (CMV) infection develop increased proportions of circulating CD8 lymphocytes. A longitudinal study of 11 kidney and 5 liver allograft recipients with primary CMV infection but no other aetiological factor to explain graft dysfunction revealed selective imbalances in peripheral blood CD8' T cell subsets. Initially, CMV viraemia was associated with elevated CD8+bright' T cell numbers and T cell activation. Activation markers fell to normal when viral cultures became negative (before the end of the 1st month). During the 2nd-6th months, most (12/16) patients continued to have high CD8⁺ T cell counts (1050–2900 CD8⁺ cells/mm3), comprising an uncommon CD8⁺ T cell subset, as 45–73% of CD8^{+ bright} lymphocytes were CD3⁺ and TCRαβ⁺ but were not stained by anti-CD28, CD11b, CD16, CD56 and CD57 antibody. Unexpectedly, CD 8⁺ CD 57⁺ T cells, a hallmark of CMV infection, did not appear until the 2nd-6th months of primary CMV infection, and their numbers increased progressively thereafter. They became the predominant CD8⁺ T cell subset after about 6 months of infection and their persistence for several (up to 4) years was strongly correlated ( r = 0.87) with expansion of CD8⁺ cells. Persistence of CD 8 lymphocytosis was, thus, directly related to the rate of expansion of an uncommon CD 8⁺CD 57^- subset and its progressive replacement by CD 8⁺CD 57 ⁺ T cells that were chronically elicited by CMV.     

Homeostatic Repopulation by CD28−CD8+ T Cells in Alemtuzumab-Depleted Kidney Transplant Recipients Treated With Reduced Immunosuppression

Alemtuzumab (CAMPATH-1H) is a depleting agent introduced recently in transplantation and often used with reduced maintenance immunosuppression. In the current study we investigated the immune response of 13 kidney allograft recipients treated with alemtuzumab followed by weaned immunosuppression with reduced dose of mycophenolate mofetil (MMF) and tacrolimus. Tacrolimus was switched to sirolimus at 6 months and MMF withdrawn at 12 months after transplantation.
We found that after alemtuzumab induction the recovery of CD8⁺ T cells was much faster than that of CD4⁺ T cells. It was complete 6 months posttransplant while CD4⁺ T cells did not fully recover even 15 months posttransplant. Repopulating CD8⁺ T cells were mainly of immunosenescent CD28⁻CD8⁺ phenotype. In a series of in vitro experiments we showed that CD28⁻CD8⁺ T cells might suppress proliferation of CD4⁺ T cells. There were three successfully treated acute rejections during the study (first at +70 day, two others +12 months) that occurred in patients with the lowest level of CD28⁻CD8⁺ T cells.
We hypothesize that expanded CD28⁻CD8⁺ T cells might compete for 'immune space' with CD4⁺ T cells suppressing their proliferation and therefore delaying CD4⁺ T-cells recovery. This delay might be associated with the clinical outcome as CD4⁺ T cells, notably CD4⁺ T effector memory cells, were shown to be associated with rejection.     

Aqueous Humor Alloreactive Cell Phenotypes, Cytokines and Chemokines in Corneal Allograft Rejection

As biopsies are not taken at the time of human corneal allograft rejection, most information on the early cellular changes in rejection is from animal models. We examined the phenotype of alloreactive cells present in the human anterior chamber during corneal graft rejection by flow cytometry and quantified aqueous humor levels of cytokines and chemokines using cytometric bead array. Aqueous and peripheral blood samples were taken from patients with graft endothelial rejection (n = 11) and from control patients undergoing cataract surgery (n = 8). CD45⁺CD4⁺, CD45⁺CD8⁺ and CD45⁺CD14⁺ cells were found in aqueous during rejection; no CD45⁺ cells were seen in control samples. Higher proportions of CD45⁺ cells found in aqueous during rejection were CD14⁺, denoting monocyte/macrophage lineage, than were CD4⁺ or CD8⁺. Large elevations were seen in aqueous levels of IL-6, MCP-1 and IP-10 during rejection compared with controls; smaller but still statistically significant increases were seen in MIP-1α and eotaxin. The role of CD14⁺ cells in allorejection is unclear as is the potential of these chemokines and their receptors as therapeutic targets. Aqueous humor samples offer a unique opportunity to analyze components of the allogeneic response in direct contact with donor tissue but without artifacts inherent in examination of tissue.