Cellular immunological barriers in xenotransplantation: current status and prospects for avoidance of graft destruction

Genetic modification of pigs (e.g. transgenic expression of human complement regulatory molecules or inactivation of α1,3galactosyltransferase) enabled the development of promising strategies to overcome hyperacute rejection after pig‐to‐primate xenotransplantation. However, cellular rejection still remains a hurdle for successful xenograft survival. Cellular rejection of porcine cells in xenotransplantation models is mediated by macrophages, T cells and NK cells. Activation of human monocytes by pig cells is partly due to the incapacity of porcine ligands to bind the inhibitory receptor SIRPα (signal regulatory protein α). Thus, one approach to impair the ability of human macrophages to phagocyte porcine cells is the overexpression of the human ligand for SIRPα in porcine cells. To inhibit human NK cell reactivity after xenotranslantation transgenic expression of HLA‐E in pigs has been shown to be a promising concept. Cells from these pigs were partially protected from lysis by human NK cells. Our group focuses on manipulation of human anti‐pig T cell responses by negative costimulatory signals. Thus, we asked whether overexpression of PD‐L1 on porcine cells can (i) downregulate human anti‐pig cellular responses in vitro, and (ii) inhibit rat anti‐pig cellular immune responses in vivo. Pig cells overexpressing PD‐L1 triggered reduced proliferation and low amounts of IL‐2, IFNγ, TNF‐alpha, IL‐4, and IL‐5 in human CD4⁺ T cells compared to control pig cells. The concentration of IL‐10, however, was increased. In long‐term cultures of human CD4⁺ T cells and PD‐L1 transfectants a high frequency of CD4⁺ CD25^high FoxP3⁺ cells showed up which had the capacity to suppress the activation of conventional CD4⁺ T cells. Cytotoxic CD8⁺ T cells and NK cells lysed pig control cells very efficiently. In contrast, PD‐L1 transfected pig cells were partially protected from lysis by human effector cells. Overexpression of PD‐L1 on porcine cells was not sufficient to prevent rejection after transplantation under the rat kidney capsule. However, in rats that had been grafted with PD‐L1 expressing cells we observed reduced cellular infiltrates in the kidneys and lower antibody responses compared to rats grafted with control cells. Together these observations support the assumption that PD‐1/PD‐Ligand pathways are interesting targets to prevent cellular immune responses after xenotransplantation. PD‐L overexpression might not only impede the initiation of an anti‐pig T cell response by suppressing CD4⁺ T cells but may also protect pig cells from destruction by cytotoxic effectors. Supported by the Deutsche Forschungsgemeinschaft (Transregio Forschergruppe “Xenotransplantation”, FOR 535).     

Xenotransplantation: immunological barriers and strategies to overcome them

Hyperacute and acute vascular rejection of xenografts are well defined barriers to clinical pig‐to‐human xenotransplantation. Enormous progress has been made in recent years to overcome these immunological barriers. For example, transgenic expression of human complement regulatory molecules (e.g. CD46, CD55) in pigs has been shown to be an effective strategy to prevent hyperacute rejection in pre‐clinical models of xenotransplantation. Alpha1,3‐galactosyltransferase knock‐out pigs are available and provide a second possibility to avoid hyperacute rejection mediated by pre‐existing antibodies. Furthermore, transfer of protective genes (e.g. A20, HO‐1) to endothelial cells is expected to reduce their susceptibility to effector mechanisms leading to acute vascular rejection. In addition, the efficiency of strategies to avoid coagulation/thrombosis after pig‐to‐human xenotransplantation (e.g. transgenic expression of human thrombomodulin, CD39) is currently tested. Thus, for further development of clinical xenotransplantation immunological concepts are now required facilitating the control of human anti‐pig cellular immune responses. Our group focuses on the inhibition of human anti‐pig T cell responses by targeting “negative” costimulatory pathways. We tested the hypothesis that overexpression of the human negative costimulatory ligands PD‐L1 and PD‐L2 on pig antigen presenting cells will result in reduced human anti‐pig T cell responses. The data so far show that (i) human CD4⁺ T cells respond with reduced proliferation and cytokine synthesis to PD‐L1/PD‐L2 expressing pig cells, (ii) PD‐L1/PD‐L2 pig transfectants induce human regulatory T cells (T_reg) which suppress the activation of conventional T cells, and (iii) PD‐L1/PD‐L2 expressing pig cells are protected from lysis mediated by CD8⁺ human cells. Together these observations support the assumption that transgenic expression of human PD‐L1 and/or PD‐L2 in pig cells and tissues could be an approach to prevent T cell reactivity after pig‐to‐human xenotransplantation. Supported by the Deutsche Forschungsgemeinschaft (Transregio Forschergruppe “Xenotransplantation”, FOR 535).     

Role of Lentivirus‐Mediated Overexpression of Programmed Death‐Ligand 1 on Corneal Allograft Survival

To investigate the role of lentivirus‐mediated overexpression of programmed death‐ligand 1 (PD‐L1) on rat corneal allograft survival. A fully allogeneic rat cornea transplant model was used for in vivo studies. Lentiviral (LV) vectors are efficient tools for ex vivo genetic modification of cultured corneas. LV vector encoding for PD‐L1 (LV.PD‐L1) and LV vector encoding for eGFP (LV.eGFP, as control) were constructed and tested. PD‐L1 or eGFP expression was increased on corneal cells upon LV.PD‐L1 and LV.eGFP transduction, respectively. Both allogeneic controls and allogeneic LV.eGFP transduced corneas were uniformly rejected (MST: 13.8 ± 1.7 days and 12.3 ± 1.9 days, respectively). In contrast, allogeneic LV.PD‐L1 transduced corneas showed a high percentage (83%) of graft survival (MST > 30 days, n = 5, 15 days, n = 1). Graft opacity of PD‐L1 transduced corneas was present but was significantly reduced compared to control or eGFP expressing corneas. Flow cytometric analysis revealed that percentages of CD3⁺CD8⁺CD161⁺ and CD3⁺CD8⁺CD161^– lymphocytes were decreased in animals receiving LV.PD‐L1 transduced corneas compared to animals grafted with LV.eGFP transduced corneas. Moreover, reduced expression of proinflammatory cytokines (IFN‐γ and IL‐6) in PD‐L1 transduced corneas compared to allogeneic controls was also observed. Local PD‐L1 gene transfer in cultured corneas is a promising approach for the prolongation of corneal allograft survival and attenuation of graft rejection.     

Prolonged survival of pig islets xenograft by adenovirus‐mediated expression of either the membrane‐bound human FasL or the human decoy Fas antigen gene

Abstract: Background: Pig islets are considered an attractive alternative treatment for patients with Type 1 diabetes. However, pig islet xenografts, transplanted into non‐human primates, are directly rejected by cell‐mediated processes. We have previously reported that cell‐mediated xenograft‐rejections, and especially human CD8⁺ cytotoxic T lymphocytes (CTL)‐mediated cytotoxicity, are highly detrimental to pig xenograft cells. Moreover, we have explored novel strategies for the prevention of CTL killing by overexpression of either human decoy Fas antigen or membrane‐bound human FasL in pig endothelial cells. In this study, we assessed the cytoprotective effects of these molecules for pig islets both in vitro and in vivo. Materials and methods: Pig islets were freshly isolated by modified Ricordi’s methods. Subsequently, these islets were transfected with an adenoviral expression vector containing the DNA fragments of either membrane‐bound human FasL or human decoy Fas. Transfected islets were transplanted into preimmunized diabetic rats under the kidney capsule. Control pig islets (i.e., MOCK), which were transfected with an adenoviral expression vector containing only the enhanced green fluorescent protein gene, were also transplanted. Results: Efficiency of adenoviral expressions of these molecules in pig islets was approximately 80% at a multiplicity of infection of 100. In an in vitro assay, approximately 80% suppression of cytotoxicity was observed in membrane‐bound human FasL‐expressing pig islets and 60% inhibition of CTL killing was displayed in decoy Fas expression pig islets. In an in vivo transplant model, prolonged survival of pig islets xenografts, expressing either membrane‐bound human FasL or human decoy Fas genes, was elicited in comparison with that of control islets xenografts. Conclusion: The extracellular remodeling of either death receptor or death ligand genes by adenoviral expression was effective for the prevention of CTL‐mediated xenocytotoxicity in pig islets.     

Suppressing memory T cell activation induces islet allograft tolerance in alloantigen‐primed mice

Memory T cells are known to play a key role in prevention of allograft tolerance in alloantigen‐primed mice. Here, we used an adoptively transferred memory T cell model and an alloantigen‐primed model to evaluate the abilities of different combinations of monoclonal antibodies (mAb) to block key signaling pathways involved in activation of effector and memory T cells. In the adoptively transferred model, the use of anti‐CD134L mAb effectively prevented activation of CD4⁺ memory T cells and significantly prolonged islet survival, similar to the action of anti‐CD122 mAb to CD8⁺ memory T cells. In the alloantigen‐primed model, use of anti‐CD134L and anti‐CD122 mAbs in addition to co‐stimulatory blockade with anti‐CD154 and anti‐LFA‐1 prolonged secondary allograft survival and significantly reduced the proportion of memory T cells; meanwhile, this combination therapy increased the proportion of regulatory T cells (Tregs) in the spleen, inhibited lymphocyte infiltration in the graft, and suppressed alloresponse of recipient splenic T cells. However, we also detected high levels of alloantibodies in the serum which caused high levels of damage to the allogeneic spleen cells. Our results suggest that combination of four mAbs can significantly suppress the function of memory T cells and prolong allograft survival in alloantigen primed animals.     

CD154 Blockade Abrogates Allospecific Responses and Enhances CD4+ Regulatory T‐Cells in Mouse Orthotopic Lung Transplant

Acute cellular rejection (ACR) is a common and important clinical complication following lung transplantation. While there is a clinical need for the development of novel therapies to prevent ACR, the regulation of allospecific effector T‐cells in this process remains incompletely understood. Using the MHC‐mismatched mouse orthotopic lung transplant model, we investigated the short‐term role of anti‐CD154 mAb therapy alone on allograft pathology and alloimmune T‐cell effector responses. Untreated C57BL/6 recipients of BALB/c left lung allografts had high‐grade rejection and diminished CD4⁺: CD8⁺ graft ratios, marked by predominantly CD8⁺>CD4⁺ IFN‐γ⁺ allospecific effector responses at day 10, compared to isograft controls. Anti‐CD154 mAb therapy strikingly abrogated both CD8⁺ and CD4⁺ alloeffector responses and significantly increased lung allograft CD4⁺: CD8⁺ ratios. Examination of graft CD4⁺ T‐cells revealed significantly increased frequencies of CD4⁺CD25⁺Foxp3⁺ regulatory T‐cells in the lung allografts of anti‐CD154‐treated mice and was associated with significant attenuation of ACR compared to untreated controls. Together, these data show that CD154/CD40 costimulation blockade alone is sufficient to abrogate allospecific effector T‐cell responses and significantly shifts the lung allograft toward an environment predominated by CD4⁺ T regulatory cells in association with an attenuation of ACR.     

Long‐term survival of pig‐to‐rhesus macaque renal xenografts is dependent on CD4 T cell depletion

The shortage of available organs remains the greatest barrier to expanding access to transplant. Despite advances in genetic editing and immunosuppression, survival in experimental models of kidney xenotransplant has generally been limited to <100 days. We found that pretransplant selection of recipients with low titers of anti‐pig antibodies significantly improved survival in a pig‐to–rhesus macaque kidney transplant model (6 days vs median survival time 235 days). Immunosuppression included transient pan–T cell depletion and an anti‐CD154–based maintenance regimen. Selective depletion of CD4⁺ T cells but not CD8⁺ T cells resulted in long‐term survival (median survival time >400 days vs 6 days). These studies suggested that CD4⁺ T cells may have a more prominent role in xenograft rejection compared with CD8⁺ T cells. Although animals that received selective depletion of CD8⁺ T cells showed signs of early cellular rejection (marked CD4⁺ infiltrates), animals receiving selective CD4⁺ depletion exhibited normal biopsy results until late, when signs of chronic antibody rejection were present. In vitro study results suggested that rhesus CD4⁺ T cells required the presence of SLA class II to mount an effective proliferative response. The combination of low pretransplant anti‐pig antibody and CD4 depletion resulted in consistent, long‐term xenograft survival.     

Flt3L‐mobilized dendritic cells bearing H2‐Kbm1 apoptotic cells do not induce cross‐tolerance to CD8+ T cells across a class I MHC mismatched barrier

Tolerization of allogeneic CD8⁺ T cells is still a pending issue in the field of transplantation research to achieve long‐term survival. To test whether dendritic cells (DC) bearing allogeneic major histocompatibility complex (MHC) class I mismatched apoptotic cells could induce cross‐tolerance to alloreactive CD8⁺ T cells, the following experimental strategy was devised. Rag2/γ_c KO B6 mice were treated with Fms‐like tyrosine kinase 3 ligand (Flt3L)‐transduced B16 melanoma cells to drive a rapid expansion and mobilization of DC in vivo. Of all DC populations expanded, splenic CD11c⁺CD103⁺CD8α⁺ DC were selectively involved in the process of antigen clearance of X‐ray irradiated apoptotic thymocytes in vivo. Considering that CD11c⁺CD103⁺CD8α⁺ DC selectively take up apoptotic cells and that they are highly specialized in cross‐presenting antigen to CD8⁺ T cells, we investigated whether B6 mice adoptively transferred with Flt3L‐derived DC loaded with donor‐derived apoptotic thymocytes could induce tolerance to bm1 skin allografts. Our findings on host anti‐donor alloresponse, as revealed by skin allograft survival and cytotoxic T lymphocyte assays, indicated that the administration of syngeneic DC presenting K^bm1 donor‐derived allopeptides through the indirect pathway of antigen presentation was not sufficient to induce cross‐tolerance to alloreactive CD8⁺ T cells responding to bm1 alloantigens in a murine model of skin allograft transplantation across an MHC class I mismatched barrier.     

Impact of Basiliximab on regulatory T‐cells early after kidney transplantation: down‐regulation of CD25 by receptor modulation

Monoclonal anti‐CD25‐antibodies are successfully applied in organ transplantation to reduce the incidence of acute graft rejection. However, targeting the CD25 molecule might not only affect activated T‐cells but also regulatory T‐cells (T_regs) constitutively expressing the CD4⁺CD25⁺CD127^lowFoxP3⁺ phenotype. In this study, we investigated the influence of the anti‐CD25‐antibody Basiliximab on the frequency of T_regs early after kidney transplantation comparing individuals receiving/not receiving induction therapy (n = 14 and n = 7). Following Basiliximab administration, a distinct loss of CD4⁺CD25^high T‐cells was observed lasting for at least 6 weeks. This was not accompanied by a disappearance of the entire CD4⁺CD25⁺FoxP3⁺ T_regs but rather a decreased expression density of CD25 on the latter. In addition, a transient rise in CD4⁺CD25^?FoxP3⁺ T‐cells was found which expressed the CD127^low phenotype. Thus, a phenotypic shift of T_regs from the CD25⁺ to the CD25^? compartment was suggested. This was supported by in vitro findings showing that the disappearance of CD4⁺CD25^high cells in the presence of Basiliximab was due to down‐regulation of CD25 expression meanwhile the suppressive function of these cells was maintained. In conclusion, Basiliximab therapy directly affects CD4⁺CD25⁺CD127^lowFoxP3⁺ T_regs but does not seem to be associated with functional consequences. Thus, it is unlikely that Basiliximab treatment negatively influences strategies involving T_regs to promote tolerance after organ transplantation.     

Induction of porcine‐specific regulatory T cells with high specificity and expression of IL‐10 and TGF‐β1 using baboon‐derived tolerogenic dendritic cells

Background

Regulatory T cells (Treg) play an important role in maintenance of homeostasis in vivo. Treg application to alleviate allo‐organ rejection is being studied extensively. However, natural Treg (nTreg) expansion in vitro is laborious and expensive. Antigen‐specific Treg are more effective and require lower cell numbers than use of nTreg for immune control. The baboon, as a non‐human primate experimental animal model, is widely used in xenotransplantation research. An effective method to generate baboon xeno‐specific Treg would benefit research on immune tolerance in xenotransplantation using this model system.

Method

Baboon tolerogenic dendritic cells (tolDC) were generated in 3 days from monocytes isolated from baboon peripheral blood mononuclear cells in medium supplemented with anti‐inflammatory cytokines. After loading with porcine‐specific (PS) in vitro‐transcribed RNA (ivtRNA), tolDC were used to induce CD4⁺ T cells to become porcine‐specific Treg (PSTreg) in cocultures supplemented with IL‐2 and rapamycin for 10 days. Anti‐inflammatory and inflammatory cytokine expression was evaluated at the mRNA and protein levels in both baboon tolDC and PSTreg. Functional assays, suppression of activation markers on porcine‐specific effector T cells (PSTeff) and inhibition of PSTeff proliferation, were used to test PSTreg specificity.

Results

TolDC generated with this method exhibited a tolerogenic phenotype, expressed CCR7 and produced high levels of IL‐10 and TGF‐β1, whereas IL‐12p40 and IFN‐γ were not expressed. PSTreg were successfully generated in cocultures of CD4⁺ T cells and PS ivtRNA‐loaded tolDC. They exhibited a CD3⁺ CD4⁺ CD25⁺ CD127^low/? CD45RA^low Foxp3⁺ phenotype and were characterized by high expression of IL‐10 and TGF‐β1 mRNA and protein. They showed upregulated expression of EBI3 and GARP mRNA. PSTreg exhibited highly suppressive effects toward PSTeff, secreting high amounts of IL‐10 and TGF‐β1 cytokine upon interaction with PSTeff and suppressing IFN‐γ expression on PSTeff.

Conclusion

In this study, a fast 3‐day method to generate baboon‐derived tolDC is provided that allows subsequent induction of PSTreg displaying high porcine‐antigen specificity and expression of IL‐10 and TGF‐β1. Porcine‐specific baboon Treg can be used in porcine solid organ or cell xenotransplantation studies through adoptive cell transfer into host baboons.     

The Programmed Death (PD)‐1/PD‐Ligand 1 Pathway Regulates Graft‐Versus‐Host‐Reactive CD8 T Cells After Liver Transplantation

Acute graft‐versus‐host disease (aGVHD) is a life‐threatening complication after solid‐organ transplantation, which is mediated by host‐reactive donor T cells emigrating from the allograft. We report on two liver transplant recipients who developed an almost complete donor chimerism in peripheral blood and bone marrow‐infiltrating T cells during aGVHD. By analyzing these T cells directly ex vivo, we found that they died by apoptosis over time without evidence of rejection by host T cells. The host‐versus‐donor reactivity was selectively impaired, as anti‐third‐party and antiviral T cells were still detectable in the host repertoire. These findings support the acquired donor‐specific allotolerance concept previously established in animal transplantation studies. We also observed that the resolution of aGVHD was not accompanied by an expansion of circulating immunosuppressive CD4/CD25/FoxP3‐positive T cells. In fact, graft‐versus‐host‐reactive T cells were controlled by an alternative negative regulatory pathway, executed by the programmed death (PD)‐1 receptor and its ligand PD‐L1. We found high PD‐1 expression on donor CD4 and CD8 T cells. In addition, blocking PD‐L1 on host‐derived cells significantly enhanced alloreactivity by CD8 T cells in vitro. We suggest the interference with the PD‐1/PD‐L1 pathway as a therapeutic strategy to control graft‐versus‐host‐reactive T cells in allograft recipients.     

PD‐1 expression on CD8+ T cells regulates their differentiation within lung allografts and is critical for tolerance induction

Immunological requirements for rejection and tolerance induction differ between various organs. While memory CD8⁺ T cells are considered a barrier to immunosuppression‐mediated acceptance of most tissues and organs, tolerance induction after lung transplantation is critically dependent on central memory CD8⁺ T lymphocytes. Here we demonstrate that costimulation blockade‐mediated tolerance after lung transplantation is dependent on programmed cell death 1 (PD‐1) expression on CD8⁺ T cells. In the absence of PD‐1 expression, CD8⁺ T cells form prolonged interactions with graft‐infiltrating CD11c⁺ cells; their differentiation is skewed towards an effector memory phenotype and grafts are rejected acutely. These findings extend the notion that requirements for tolerance induction after lung transplantation differ from other organs. Thus, immunosuppressive strategies for lung transplant recipients need to be tailored based on the unique immunological properties of this organ.     

Foxp3 regulates human natural CD4+CD25+ regulatory T‐cell‐mediated suppression of xenogeneic response

Sun L, Yi S, O’Connell PJ. Foxp3 regulates human natural CD4+CD25+ regulatory T‐cell‐mediated suppression of xenogeneic response. Xenotransplantation 2010; 17: 121–130. © 2010 John Wiley & Sons A/S. Abstract: Backgrounds: Cellular rejection of xenografts is predominantly mediated by CD4+ T cells. Foxp3 expressing human naturally occurring CD4+CD25+ regulatory T cells (nTregs) have been shown to suppress pathological and physiological immune responses, including the CD4+ T‐cell‐mediated anti‐pig xenogeneic response in vitro. Although Foxp3 is required for nTreg development and their function, the precise role of Foxp3 in regulating Treg suppressive function in xenoimmune response remains to be identified. Methods: In vitro expanded human nTregs were transfected with fluorescein isothiocyanate ‐conjugated Foxp3 small interfering RNA (siRNA) by Lipofectamine 2000. Transfected nTregs were sorted by fluorescence‐activated cell sorting, and then analyzed for Foxp3 gene and protein expression as well as their phenotypic characteristics. Human CD4+CD25? T cells were stimulated with xenogeneic pig peripheral blood mononuclear cell in the presence or absence of nTregs in a coculture or transwell system for evaluation of nTreg suppressive activity. The production of effector cytokines by xenoreactive CD4+CD25? T cells as well as suppressive cytokine by nTregs in their cocultures was examined by ELISA. Results: The siRNA‐mediated Foxp3 knockdown resulted in impaired nTreg anergic state, downregulated expression of nTreg function associated molecules, and reduced production of suppressive cytokines by nTregs, which together leading to impaired nTreg‐mediated suppression of CD4+CD25? T‐cell proliferation and their effector cytokine production in response to xenogeneic stimulation. Conclusions: This study demonstrates that Foxp3 expression is required for human nTregs to maintain their suppressive function in the xenoimmune response.     

Lymphotoxin‐Beta Receptor Blockade Induces Inflammation and Fibrosis in Tolerized Cardiac Allografts

The lymphotoxin system (LT) regulates interactions between lymphocytes and stromal cells to maintain lymphoid microenvironmental homeostasis. Soluble LT beta‐receptor‐Ig (LTβRIg) blocks lymphocyte LTα1β2‐stromal cell LTβR signaling. In a murine cardiac allograft model, LTbRIg treatment reversed the tolerance induced by anti‐CD40L antibody leading to graft inflammation and fibrosis. LTβRIg treatment decreased PD‐L1 expression by blood endothelial cells, and decreased VCAM‐1 while increasing CXCL1, CXCL2, CXCL12, CCL5, CCL21 and IL‐6 expression in fibroblastic reticular cells. In secondary lymphoid organs these effects caused T‐ and B cell zone disruption, loss of CD35⁺ follicular dendritic cells and abnormal recruitment of CD11b⁺Ly6G⁺ neutrophils. These disruptions correlated with increased numbers of CD8⁺ T cells and CD11b⁺Ly6G⁺ neutrophils, and decreased numbers of CD4⁺ T cells and Foxp3⁺ regulatory T cells in the grafts. Depleting neutrophils or blocking neutrophil‐attracting chemokines restored normal histology in lymph node, spleen and grafts. Taken together, LTβRIg treatment altered stromal subset, particularly fibroblastic reticular cell, production of cytokines and chemokines, resulting in changes in neutrophil recruitment in spleen, lymph node and grafts, and inflammation and fibrosis associated with decreased Foxp3⁺ regulatory T cells and increased CD8⁺ T cell infiltration of grafts.     

CD154 Deficiency Uncouples Allograft CD8+ T‐Cell Effector Function from Proliferation and Inhibits Murine Airway Obliteration

Obliterative bronchiolitis (OB) limits the long‐term success of lung transplantation, while T‐cell effector mechanisms in this process remain incompletely understood. Using the murine heterotopic tracheal transplant model of obliterative airway disease (OAD) to characterize airway allograft rejection, we previously reported an important role for CD8⁺ T cells in OAD. Herein, we studied the role of CD154/CD40 costimulation in the regulation of allospecific CD8⁺ T cells, as airway rejection has been reported to be CD154‐dependent. Airway allografts from CD154^−/− recipients had significantly lower day 28 OAD scores compared to wild‐type (WT) recipients, and adoptive transfer of CD8⁺ T cells from WT recipients, but not CD154^−/− recipients, were capable of airway rejection in fresh CD154^−/− allograft recipients. Intragraft CD8⁺ T cells from CD154^−/− mice showed similar expression of the surface markers CD69, CD62L^low CD44^high and PD‐1, but markedly impaired IFN‐γ and TNF‐α secretion and granzyme B expression versus WT controls. Unexpectedly, intragraft and systemic CD8⁺ T cells from CD154^−/− recipients demonstrated robust in vivo expansion similar to WT recipients, consistent with an uncoupling of proliferation from effector function. Together, these data suggest that a lack of CD154/CD40 costimulation results in ineffective allospecific priming of CD8⁺ T cells required for murine OAD.     

Ex-vivo expanded baboon CD4+ CD25 Hi Treg cells suppress baboon anti-pig T and B cell immune response

Singh AK, Seavey CN, Horvath KA, Mohiuddin MM. Ex‐vivo expanded baboon CD4⁺ CD25^Hi Treg cells suppress baboon anti‐pig T and B cell immune response. Xenotransplantation 2012; 19: 102–111. © 2012 John Wiley & Sons A/S. Abstract: Background: CD4⁺ CD25⁺ FoxP3⁺ regulatory T (Treg) cells play an important role in regulating immune responses. A very small number of Treg cells are present in peripheral blood and lymphoid organs, but due to their ability to suppress the immune response, they have a high potential for immunotherapy in clinics. Successful ex‐vivo expansion of naturally occurring CD4⁺ CD25⁺ T cells has been achieved after TCR stimulation in the presence of T cell growth factors. In this study, we evaluated the role of these Treg cells in suppressing proliferative response of baboon T and B cells to pig xenoantigens. Methods: Naturally occurring baboon CD4⁺ CD25⁺ regulatory T cells (nTreg) were sorted from peripheral blood and expanded in the presence of either anti‐CD3/CD28 beads or irradiated pig peripheral blood mononuclear cells with IL‐2. Treg cells were also enriched directly from CD4⁺ T cells cultured in the presence of rapamycin (0.1–10 nm ). Mixed lymphocyte culture and polyclonal B cell stimulation with ex‐vivo Treg cells were performed to assess the function of ex‐vivo expanded Treg cells. Results: The nTreg cells were expanded to more than 200‐fold in 4 weeks and retained all the nTreg cell phenotypic characteristics, including high levels of FoxP3 expression. 2‐fold increase in enrichment of CD4⁺ CD25⁺ FoxP3⁺ Treg cells from CD4⁺ cells was observed with rapamycin compared to cultures without rapamycin. The ex‐vivo expanded Treg cells obtained from both methods were able to suppress the baboon anti‐porcine xenogeneic T and B cell immune response in‐vitro efficiently (more than 90% suppression at 1 : 1 ratio of T regulatory cells: T effector cells), and their suppression potential was retained even at 1 : 256 ratio. However, freshly isolated nTreg cells had only 70% suppression at 1 : 1 ratio, and their suppressive ability was reduced to ≤50% at 1 : 16 ratio. Furthermore, we have found that ex‐vivo expanded Treg can also suppress the proliferation of B cells after polyclonal stimulation. Forty to 50 percent reduction in B cell proliferation was observed when ex‐vivo expanded Treg cells were added to the culture at a 1 : 1 ratio. The addition of CD4⁺ CD25^Neg cells however induced vigorous proliferation. Conclusion: Ex‐vivo expanded CD4⁺ CD25⁺ FoxP3⁺ Treg cells can be used to efficiently suppress xenogeneic immune responses by inhibiting T and B cell proliferation. These ex‐vivo expanded Treg cells may also be used with other immunosuppressive agents to overcome xenograft rejection in preclinical xenotransplantation models.     

Porcine cells express more than one functional ligand for the human lymphocyte activating receptor NKG2D

Abstract: Background: Xenotransplantation could ameliorate the severe shortage of donor organs. The initial results of transplantation from genetically‐modified pig donors to primate recipients suggest that hyperacute rejection can be overcome, but thrombotic microangiopathy and the human anti‐pig cellular immune response remain as significant impediments to successful clinical xenotransplantation. NKG2D is an activating immunoreceptor found on human natural killer (HuNK) cells, CD8⁺ and γδ T cells. Signaling through NKG2D mediates cytotoxicity and cytokine secretion by NK cells and co‐stimulation of T cells. Methods: Chinese hamster ovary P (CHOP) cells were transfected with human NKG2D and used in cell–cell binding studies with porcine epithelial, and endothelial cell lines. Soluble recombinant NKG2D‐Fc was used to stain various porcine cells and tissues to indicate ligand expression. Porcine cells were used as targets in cytotoxicity assays with the HuNK cell lines NKL and YT, with and without enzymatic removal of pULBP1 and antibody blockade of NKG2D signaling. Results and conclusions: In this study, we demonstrate the expression of ligands for human NKG2D on porcine cell lines of endothelial and epithelial origin, islet cell clusters and rejecting kidney. HuNK cells were activated to kill pig cells expressing NKG2D ligands, and cytotoxicity was inhibited by antibody blockade of NKG2D. A previous study identified pULBP1 as the principal ligand for human NKG2D on pig aortic endothelial cells. In the current study, renal epithelial and intestinal endothelial cells each expressed high surface levels of pULBP1, but binding of soluble recombinant NKG2D and NKG2D‐dependent cytotoxicity against these cells persisted after the enzymatic removal of pULBP1, strongly suggesting the presence of at least one additional functional ligand for human NKG2D in these cell types.     

Human T‐cell proliferation in response to thrombin‐activated GTKO pig endothelial cells

Ezzelarab C, Ayares D, Cooper DKC, Ezzelarab MB. Human T‐cell proliferation in response to thrombin‐activated GTKO pig endothelial cells. Xenotransplantation 2012; 19: 311–316. © 2012 John Wiley & Sons A/S. Abstract: Background: Thrombin formation is a key feature in the activation of coagulation in pig xenograft recipients. As thrombin is known to activate endothelial and immune cells, we explored whether thrombin activation of pig endothelial cells (EC) was associated with an increased human T‐cell response. Methods: α1,3‐galactosyltransferase gene‐knockout (GTKO) pig aortic EC (pAEC) were activated by porcine interferon‐gamma (pIFNγ), human (h)IFN‐γ, or thrombin. Swine leukocyte antigen (SLA) class I and class II expression were measured. Human peripheral blood mononuclear cells (PBMC) and CD4⁺ T‐cell proliferation in response to activated pAEC, the effect of thrombin on pig CD80/CD86 mRNA, and the effect of thrombin inhibition by hirudin were evaluated. Results: After pAEC activation, SLA I expression did not change, and only pIFNγ upregulated SLA II expression. PBMC proliferation to pIFNγ‐ and thrombin‐activated pAEC was significantly higher (P < 0.001 and P < 0.01) than to non‐activated pAEC. CD4⁺ T‐cell proliferation to pIFNγ‐ and thrombin‐activated pAEC was significantly higher (P < 0.001 and P < 0.01) than to non‐activated pAEC. Thrombin inhibition by hirudin reduced thrombin‐induced upregulation of pAEC CD86 mRNA, and significantly reduced human PBMC proliferation to pAEC in comparison with thrombin alone (P < 0.05). Conclusions: Thrombin upregulates CD86 mRNA on pAEC, which is associated with increased human T‐cell proliferation against pAEC. Hirudin reduces CD86 mRNA in thrombin‐activated pAEC and is associated with downregulation of the human T‐cell proliferative response. The transplantation of organs from GTKO pigs transgenic for human thrombomodulin, and/or endothelial protein C receptor, in addition to therapeutic regulation of thrombin activation may reduce the cellular response to a pig xenograft and thus reduce the need for intensive immunosuppressive therapy.     

Selective CD28 Blockade Results in Superior Inhibition of Donor‐Specific T Follicular Helper Cell and Antibody Responses Relative to CTLA4‐Ig

Donor‐specific antibodies (DSAs) are a barrier to improved long‐term outcomes after kidney transplantation. Costimulation blockade with CTLA4‐Ig has shown promise as a potential therapeutic strategy to control DSAs. T follicular helper (Tfh) cells, a subset of CD4⁺ T cells required for optimal antibody production, are reliant on the CD28 costimulatory pathway. We have previously shown that selective CD28 blockade leads to superior allograft survival through improved control of CD8⁺ T cells relative to CTLA4‐Ig, but the impact of CD28‐specific blockade on CD4⁺ Tfh cells is unknown. Thus, we identified and characterized donor‐reactive Tfh cells in a murine skin transplant model and then used this model to evaluate the impact of selective CD28 blockade with an anti‐CD28 domain antibody (dAb) on the donor‐specific Tfh cell–mediated immune response. We observed that the anti‐CD28 dAb led to superior inhibition of donor‐reactive CXCR5⁺PD‐1^high Tfh cells, CD95⁺GL7⁺ germinal center B cells and DSA formation compared with CTLA4‐Ig. Interestingly, donor‐reactive Tfh cells differentially upregulated CTLA4 expression, suggesting an important role for CTLA4 in mediating the superior inhibition observed with the anti‐CD28 dAb. Therefore, selective CD28 blockade as a novel approach to control Tfh cell responses and prevent DSA after kidney transplantation warrants further study.