HLA class I antibody mediated accommodation of endothelial cells via the activation of PI3K/cAMP dependent PKA pathway

Role of HLA-I Structural Variants and the Polyreactive Antibodies They Generate in Immune Homeostasis

Cell-surface HLA-I molecules consisting of β2-microglobulin (β2m) associated heavy chains (HCs), referred to as Face-1, primarily present peptides to CD8+ T-cells. HCs consist of three α-domains, with selected amino acid sequences shared by all alleles of all six isoforms. The cell-surface HLA undergoes changes upon activation by pathological conditions with the expression of β2m-free HCs (Face-2) resulting in exposure of β2m-masked sequences shared by almost all alleles and the generation of HLA-polyreactive antibodies (Abs) against them. Face-2 may homodimerize or heterodimerize with the same (Face-3) or different alleles (Face-4) preventing exposure of shared epitopes. Non-allo immunized males naturally carry HLA-polyreactive Abs. The therapeutic intravenous immunoglobulin (IVIg) purified from plasma of thousands of donors contains HLA-polyreactive Abs, admixed with non-HLA Abs. Purified HLA-polyreactive monoclonal Abs (TFL-006/007) generated in mice after immunizing with Face-2 are documented to be immunoregulatory by suppressing or activating different human lymphocytes, much better than IVIg. Our objectives are (a) to elucidate the complexity of the HLA-I structural variants, and their Abs that bind to both shared and uncommon epitopes on different variants, and (b) to examine the roles of those Abs against HLA-variants in maintaining immune homeostasis. These may enable the development of personalized therapeutic strategies for various pathological conditions.

Protection of transplants against antibody-mediated injuries: from xenotransplantation to allogeneic transplantation, mechanisms and therapeutic insights

Long-term allograft survival in allotransplantation, especially in kidney and heart transplantation, is mainly limited by the occurrence of antibody-mediated rejection due to anti-Human Leukocyte Antigen antibodies. These types of rejection are difficult to handle and chronic endothelial damages are often irreversible. In the settings of ABO-incompatible transplantation and xenotransplantation, the presence of antibodies targeting graft antigens is not always associated with rejection. This resistance to antibodies toxicity seems to associate changes in endothelial cells phenotype and modification of the immune response. We describe here these mechanisms with a special focus on endothelial cells resistance to antibodies. Endothelial protection against anti-HLA antibodies has been described in vitro and in animal models, but do not seem to be a common feature in immunized allograft recipients. Complement regulation and anti-apoptotic molecules expression appear to be common features in all these settings. Lastly, pharmacological interventions that may promote endothelial cell protection against donor specific antibodies will be described.

Reverse Signaling by MHC-I Molecules in Immune and Non-Immune Cell Types

Major histocompatibility complex (MHC) molecules are well-known for their role in antigen (cross-) presentation, thereby functioning as key players in the communication between immune cells, for example dendritic cells (DCs) and T cells, or immune cells and their targets, such as T cells and virus-infected or tumor cells. However, much less appreciated is the fact that MHC molecules can also act as signaling receptors. In this process, here referred to as reverse MHC class I (MHC-I) signaling, ligation of MHC molecules can lead to signal-transduction and cell regulatory effects in the antigen presenting cell. In the case of MHC-I, reverse signaling can have several outcomes, including apoptosis, migration, induced or reduced proliferation and cytotoxicity towards target cells. Here, we provide an overview of studies showing the signaling pathways and cell outcomes upon MHC-I stimulation in various immune and non-immune cells. Signaling molecules like RAC-alpha serine/threonine-protein kinase (Akt1), extracellular signal-regulated kinases 1/2 (ERK1/2), and nuclear factor-κB (NF-κB) were common signaling molecules activated upon MHC-I ligation in multiple cell types. For endothelial and smooth muscle cells, the in vivo relevance of reverse MHC-I signaling has been established, namely in the context of adverse effects after tissue transplantation. For other cell types, the role of reverse MHC-I signaling is less clear, since aspects like the in vivo relevance, natural MHC-I ligands and the extended downstream pathways are not fully known.The existing evidence, however, suggests that reverse MHC-I signaling is involved in the regulation of the defense against bacterial and viral infections and against malignancies. Thereby, reverse MHC-I signaling is a potential target for therapies against viral and bacterial infections, cancer immunotherapies and management of organ transplantation outcomes.

Effect of simultaneous presence of anti-blood group A/B and -HLA antibodies on clinical outcomes in kidney transplantation across positive crossmatch: a nationwide cohort study

ABO-incompatible (ABOi) and positive crossmatch (XM) kidney transplantation (KT) have been considered immunologically challenging. The present study analyzed the clinical outcomes in XM positive KT based on ABO incompatibility. We used data from the Korea Organ Transplantation Registry, a nationwide database, and a single-center registry. A total of 263 patients with positive XM were divided into an ABO compatible (ABOc) & XM positive (ABOc/XM+, n = 176) group and an ABOi & XM positive (ABOi/XM+, n = 87) group. The overall rejection rate one year after KT was significantly higher in the ABOi/XM+ group than in the ABOc/XM+ group (P < 0.01). A total of four mortalities occurred, all in the ABOi/XM+ patients (P < 0.01). There were no differences in surgical complications or the occurrence of infection-related complications, including BK virus nephropathy. Multivariate analysis indicated that female vs. male (odds ratio (OR), 2.27; P = 0.03), DSA class I (MFI/1000) (OR, 1.10; P = 0.03), DSA class II (MFI/1000) (OR, 1.10; P < 0.01), and ABOi & XM+ status (OR, 2.38; P < 0.01) were significant risk factors for acute rejection during the year after transplantation. Overall graft survival was inferior in ABOi/XM+ patients than in ABOc/XM+ patients (P = 0.02). ABO incompatibility in XM-positive KT patients was found to be a significant risk factor for the development of rejection within one year after transplantation as well as for long-term graft survival. The anti-blood group A, B and anti-HLA antibodies may show synergistic activity.

The multifaceted role of complement in kidney transplantation

Increasing evidence indicates an integral role for the complement system in the deleterious inflammatory reactions that occur during critical phases of the transplantation process, such as brain or cardiac death of the donor, surgical trauma, organ preservation and ischaemia-reperfusion injury, as well as in humoral and cellular immune responses to the allograft. Ischaemia is the most common cause of complement activation in kidney transplantation and in combination with reperfusion is a major cause of inflammation and graft damage. Complement also has a prominent role in antibody-mediated rejection (ABMR) owing to ABO and HLA incompatibility, which leads to devastating damage to the transplanted kidney. Emerging drugs and treatment modalities that inhibit complement activation at various stages in the complement cascade are being developed to ameliorate the damage caused by complement activation in transplantation. These promising new therapies have various potential applications at different stages in the process of transplantation, including inhibiting the destructive effects of ischaemia and/or reperfusion injury, treating ABMR, inducing accommodation and modulating the adaptive immune response.

Molecular events contributing to successful pediatric cardiac transplantation in HLA sensitized recipients

Antibodies to HLA resulting in positive cytotoxicity crossmatch are generally considered a contraindication for cardiac transplantation. However, cardiac transplantations have been performed in children by reducing the Abs and modifying immunosuppression. To identify mechanisms leading to allograft acceptance in the presence of Abs to donor HLA, we analyzed priming events in endothelial cells (EC) by incubating with sera containing low levels of anti-HLA followed by saturating concentration of anti-HLA. Pre-transplant sera were obtained from children with low levels of Abs to HLA who underwent transplantation. EC were selected for donor HLA and exposed to sera for 72 h (priming), followed by saturating concentrations of anti-HLA (challenge). Priming of EC with sera induced the phosphatidylinositol 3-kinase/Akt mediated by the BMP4/WNT pathway and subsequent challenge with panel reactive antibody sera increased survival genes Bcl2 and Heme oxygenase-1, decreased adhesion molecules, induced complement inhibitory proteins and reduced pro-inflammatory cytokines. In contrast, EC which did not express donor HLA showed decreased anti-apoptotic genes. Primed EC, upon challenge with anti-HLA, results in increased survival genes, decreased adhesion molecules, induction of complement inhibitory proteins, and downregulation of pro-inflammatory cytokines which may result in accommodation of pediatric cardiac allografts despite HLA sensitization.

Lower incidence of de novo donor-specific antibodies against HLA-DR in ABO-incompatible renal transplantation

Recently, in vitro experiments have demonstrated that anti-blood group A/B antibody binding to endothelial cells induce a protective effect against antibody-mediated injury. This study aimed to clarify the potential clinical benefit of ABO incompatibility in donor-specific HLA antibody (DSA)-induced chronic antibody-mediated rejection (ABMR). We enrolled 215 ABO-incompatible renal transplant (ABO-I) and 467 ABO-identical/compatible renal transplant recipients (ABO-Id/C). The prevalence of de novo DSA production and incidence of biopsy-proven chronic ABMR were compared between the two groups. The incidence of DR-associated de novo DSA was significantly lower in ABO-I than in ABO-Id/C (P = 0.028). Diagnostic biopsy for ABMR was conducted in 54 patients (11 ABO-I and 43 ABO-Id/C). Biopsy-proven chronic ABMR was lower in ABO-I than in ABO-Id/C (27.3% [3/11] vs. 44.2% [19/43]) patients. Our findings suggest that ABO incompatibility may cause low production of DR-associated de novo DSA, possibly resulting in a reduced incidence of chronic ABMR.

Recent advances in allograft vasculopathy

PURPOSE OF REVIEW

Despite considerable advances in controlling acute rejection, the longevity of cardiac and renal allografts remains significantly limited by chronic rejection in the form of allograft vasculopathy. This review discusses recently reported mechanistic insights of allograft vasculopathy pathogenesis as well as recent clinical evaluations of new therapeutic approaches.

RECENT FINDINGS

Although adaptive immunity is the major driver of allograft vasculopathy, natural killer cells mediate vasculopathic changes in a transplanted mouse heart following treatment with donor-specific antibody (DSA). However, natural killer cells may also dampen chronic inflammatory responses by killing donor-derived tissue-resident CD4 T cells that provide help to host B cells, the source of DSA. DSA may directly contribute to vascular inflammation by inducing intracellular signaling cascades that upregulate leukocyte adhesion molecules, facilitating recruitment of neutrophils and monocytes. DSA-mediated complement activation additionally enhances endothelial alloimmunogenicity through activation of noncanonical NF-κB signaling. New clinical studies evaluating mammalian target of rapamycin and proteasome inhibitors to target these pathways have been reported.

SUMMARY

Allograft vasculopathy is a disorder resulting from several innate and adaptive alloimmune responses. Mechanistic insights from preclinical studies have identified agents that are currently being investigated in clinical trials.

Negative regulation of HLA-DR expression on endothelial cells by anti-blood group A/B antibody ligation and mTOR inhibition

Donor-specific antibody (DSA), particularly against HLA class II, is a major cause of chronic antibody-mediated rejection (CAMR) after transplantation, although ABO-incompatible kidney transplantation has recently demonstrated favorable graft outcomes. The condition of no injury even in the presence of anti-donor antibody has been referred to as "accommodation", which would be one of the key factors for successful long-term graft survival. The purpose of this study was to analyze the beneficial effect of anti-blood group A/B antibody ligation on endothelial cells against HLA-DR antibody-mediated, complement-dependent cytotoxicity (CDC). Blood group A/B-expressing endothelial cells EA.hy926 or Human Umbilical Vein Endothelia Cells (HUVEC) were incubated with IFNγ in the presence or absence of anti-blood group A/B antibody or mTOR inhibitor (mTOR-i) for 48h. The effects on signaling pathway, HLA expression, complement regulatory factors, and CDC were investigated. Expression of HLA-DR on EA.hy926 or HUVEC were successfully elicited by IFNγ treatment, although little or no expression was observed in quiescent cells. Pre-incubation with anti-blood group A/B antibody had resistance to HLA-DR antibody-mediated CDC against IFNγ-treated cells in a concentration-dependent manner. This finding was ascribed to decreased expression of HLA-DR by post-translational regulation and increased expression of CD55/59, which was related to ERK and mTOR pathway inhibition. mTOR-i also inhibited HLA-DR expression by itself. Furthermore, the combination of mTOR-I and anti-blood group A/B ligation had an additive effect in preventing HLA-DR antibody-mediated CDC. Anti-blood group A/B antibody might play a preventive role in CAMR. Inhibition of the ERK and mTOR pathways may contribute to the development of a novel treatment in the maintenance period after transplantation.

Heme Oxygenase-1 Inhibits HLA Class I Antibody-Dependent Endothelial Cell Activation

Antibody-mediated rejection (AMR) is a key limiting factor for long-term graft survival in solid organ transplantation. Human leukocyte antigen (HLA) class I (HLA I) antibodies (Abs) play a major role in the pathogenesis of AMR via their interactions with HLA molecules on vascular endothelial cells (ECs). The antioxidant enzyme heme oxygenase (HO)-1 has anti-inflammatory functions in the endothelium. As complement-independent effects of HLA I Abs can activate ECs, it was the goal of the current study to investigate the role of HO-1 on activation of human ECs by HLA I Abs. In cell cultures of various primary human macro- and microvascular ECs treatment with monoclonal pan- and allele-specific HLA I Abs up-regulated the expression of inducible proinflammatory adhesion molecules and chemokines (vascular cell adhesion molecule-1 [VCAM-1], intercellular cell adhesion molecule-1 [ICAM-1], interleukin-8 [IL-8] and monocyte chemotactic protein 1 [MCP-1]). Pharmacological induction of HO-1 with cobalt-protoporphyrin IX reduced, whereas inhibition of HO-1 with either zinc-protoporphyrin IX or siRNA-mediated knockdown increased HLA I Ab-dependent up-regulation of VCAM-1. Treatment with two carbon monoxide (CO)-releasing molecules, which liberate the gaseous HO product CO, blocked HLA I Ab-dependent EC activation. Finally, in an in vitro adhesion assay exposure of ECs to HLA I Abs led to increased monocyte binding, which was counteracted by up-regulation of HO-1. In conclusion, HLA I Ab-dependent EC activation is modulated by endothelial HO-1 and targeted induction of this enzyme may be a novel therapeutic approach for the treatment of AMR in solid organ transplantation.

Antibodies to HLA Molecules Mimic Agonistic Stimulation to Trigger Vascular Cell Changes and Induce Allograft Injury

Human leukocyte antigen (HLA)-induced signaling in endothelial and smooth muscle cells causes dramatic cytoskeletal rearrangement, increased survival, motility, proliferation, adhesion molecule and chemokine expression, and adhesion of leukocytes. These mechanisms are directly related to endothelial activation, neointimal proliferation, and intragraft accumulation of leukocytes during antibody-mediated rejection (AMR) and chronic rejection. Clustering of HLA by ligands in trans, such as in antigen-presenting cells at the immune synapse, triggers physiological functions analogous to HLA antibody-induced signaling in vascular cells. Emerging evidence has revealed previously unknown functions for HLA beyond antigen presentation, including association with coreceptors in cis to permit signal transduction, and modulation of intracellular signaling downstream of other receptors that may be relevant to HLA signaling in the graft vasculature. We discuss the literature regarding HLA-induced signaling in vascular endothelial and smooth muscle cells, as well as under endogenous biological conditions, and how such signaling relates to functional changes and pathological mechanisms during graft injury.

Toward Efficient Enzymes for the Generation of Universal Blood through Structure-Guided Directed Evolution

Blood transfusions are critically important in many medical procedures, but the presence of antigens on red blood cells (RBCs, erythrocytes) means that careful blood-typing must be carried out prior to transfusion to avoid adverse and sometimes fatal reactions following transfusion. Enzymatic removal of the terminal N-acetylgalactosamine or galactose of A- or B-antigens, respectively, yields universal O-type blood, but is inefficient. Starting with the family 98 glycoside hydrolase from Streptococcus pneumoniae SP3-BS71 (Sp3GH98), which cleaves the entire terminal trisaccharide antigenic determinants of both A- and B-antigens from some of the linkages on RBC surface glycans, through several rounds of evolution, we developed variants with vastly improved activity toward some of the linkages that are resistant to cleavage by the wild-type enzyme. The resulting enzyme effects more complete removal of blood group antigens from cell surfaces, demonstrating the potential for engineering enzymes to generate antigen-null blood from donors of various types.

MHC class I signaling: new functional perspectives for an old molecule

Donor-specific antibodies are associated with refractory rejection episodes and poor allograft outcomes in solid organ transplantation. Our understanding of antibody-mediated allograft injury is expanding beyond complement deposition. In fact, unique mechanisms of alloantibodies are advancing our knowledge about transplant vasculopathy and antibody-mediated rejection. These include direct effects on the endothelium, resulting in the recruitment of leukocytes, chemokine and cytokine production, and stimulation of innate and adaptive alloresponses. These effects will be the focus of the following review.

Complement Interception Across Humoral Incompatibility in Solid Organ Transplantation: A Clinical Perspective

The humoral barrier in transplant biology is the result of preformed donor-specific antibodies (DSAs), directed either against human leukocyte antigens (HLA) or non-HLA antigens such as blood group (ABO) molecules. The term "sensitization" applies to patients carrying these antibodies. Transplantation is widely accepted as a life-saving opportunity for patients with terminal end-organ disease. However, in sensitized patients, transplant outcome is hampered by antibody-mediated rejection (AMR) as a consequence of DSA exposure. Furthermore, sensitized patients have limited access to "matched" organs from the both living and deceased donor pool.Considering the crucial role of the complement system in the pathophysiology of AMR and the availability of complement intervention therapeutics, there is a growing interest in complement-targeting strategies. This review highlights the emerging importance of monitoring and modulation of the complement system in the context of enabling transplantation across humoral incompatibility in sensitized recipients with preformed anti-HLA or natural anti-ABO antibodies. It also discusses the significance of the complement system in the induction of accommodation and further emphasizes current and future perspectives of novel complement therapeutics.

Anti-donor HLA class I antibodies: pathways to endothelial cell activation and cell-mediated allograft rejection

BACKGROUND

The development of donor-specific human leukocyte antigen (HLA) class I antibodies after organ transplantation is associated with subsequent acute and chronic rejection. The aim of this study was to examine the role of anti-HLA class I antibody in modulating endothelium-leukocyte interaction.

METHODS

Human microvascular endothelial cells (HMEC-1) stimulated with HLA class I antibody (W6/32) or allospecific antibodies from sensitized patients (n=6) were examined for activation of transcription factor CREB by Western blotting. Up-regulation of endothelial adhesion molecules and chemokines was measured by flow cytometry and quantitative polymerase chain reaction, respectively. Leukocyte adhesion was evaluated by chemotaxis and in vitro flow-based assays.

RESULTS

Treatment of HMEC-1 cells with HLA class I antibody resulted in the phosphorylation of CREB in protein kinase A-dependent pathway. Furthermore, there was a significant increase in the expression of cell surface VCAM-1 (Akt-dependent) and ICAM-1 in Akt-dependent and extracellular signal-regulated kinase-dependent manner (P<0.001). Additionally, exposure to W6/32 antibody induced significant expression of interleukin-6, CXCL8, CXCL10, and CCL5. Knockdown of CREB produced a reduction in W6/32-induced CXCL8 expression (P<0.001). Media from W6/32-treated endothelial cells induced a significant monocyte chemotaxis (P<0.001) and flow-based adhesion assay demonstrated an increase in monocyte adhesion to endothelial cells compared with the control group (P<0.001). Importantly, allospecific antibodies from sensitized patients also activated endothelial CREB and significantly up-regulated VCAM-1, ICAM-1, and CXCL8.

CONCLUSION

These findings suggest that donor-specific HLA class I antibodies directly activate endothelial cells leading to an increase in their potential to recruit and bind recipient leukocytes, thereby increasing the potential for allograft inflammation.

Graft microvascular disease in solid organ transplantation

Alloimmune inflammation damages the microvasculature of solid organ transplants during acute rejection. Although immunosuppressive drugs diminish the inflammatory response, they do not directly promote vascular repair. Repetitive microvascular injury with insufficient regeneration results in prolonged tissue hypoxia and fibrotic remodeling. While clinical studies show that a loss of the microvascular circulation precedes and may act as an initiating factor for the development of chronic rejection, preclinical studies demonstrate that improved microvascular perfusion during acute rejection delays and attenuates tissue fibrosis. Therefore, preservation of a functional microvasculature may represent an effective therapeutic strategy for preventing chronic rejection. Here, we review recent advances in our understanding of the role of the microvasculature in the long-term survival of transplanted solid organs. We also highlight microvessel-centered therapeutic strategies for prolonging the survival of solid organ transplants.

Antibodies in transplantation: the effects of HLA and non-HLA antibody binding and mechanisms of injury

Until recently, allograft rejection was thought to be mediated primarily by alloreactive T cells. Consequently, immunosuppressive approaches focused on inhibition of T cell activation. While short-term graft survival has significantly improved and rejection rates have dropped, acute rejection has not been eliminated and chronic rejection remains the major threat to long-term graft survival. Increased attention to humoral immunity in experimental systems and in the clinic has revealed that donor specific antibodies (DSA) can mediate and promote acute and chronic rejection. Herein, we detail the effects of alloantibody, particularly HLA antibody, binding to graft vascular and other cells, and briefly summarize the experimental methods used to assess such outcomes.

Neither pre-transplant rituximab nor splenectomy affects de novo HLA antibody production after renal transplantation

The long-term effect of rituximab and splenectomy on de novo HLA antibody production and chronic antibody-mediated rejection after renal transplantation is uncertain. In order to gain insight on this, we studied 92 ABO-incompatible and 228 ABO-identical/compatible consecutive renal transplant patients and determined their de novo HLA antibody production and graft outcome. Patients with pretransplant donor-specific antibodies had been excluded. ABO-incompatible transplants included 30 recipients treated with rituximab, 51 by splenectomy, or 11 with neither, due to low anti-A or -B antibody titer. Graft survival in ABO-identical/compatible patients (97.7% at 5 years) was significantly higher than in ABO-incompatible (87.0% at 5 years), rituximab (96.7% at 3 years), or splenectomy (85.7% at 5 years) patients. Only four patients had clinical chronic antibody-mediated rejection (two each identical/compatible and incompatible). There was no significant difference in prevalence of de novo HLA antibody, including donor-specific and nondonor-specific antibodies among ABO-identical/compatible patients (13.9%), patients receiving rituximab (14.3%) or splenectomy (13.2%), or among those receiving cyclosporine, tacrolimus, mycophenolate mofetil, mizoribine, and everolimus. Renal function remained stable in most recipients with de novo HLA antibody. Thus, neither pretransplant splenectomy nor rituximab treatment has an inhibitory effect on de novo HLA antibody production during medium-term follow-up. Further study on long-term effects is needed.

Blockade of p-selectin is sufficient to reduce MHC I antibody-elicited monocyte recruitment in vitro and in vivo

Donor-specific HLA antibodies significantly lower allograft survival, but as yet there are no satisfactory therapies for prevention of antibody-mediated rejection. Intracapillary macrophage infiltration is a hallmark of antibody-mediated rejection, and macrophages are important in both acute and chronic rejection. The purpose of this study was to investigate the Fc-independent effect of HLA I antibodies on endothelial cell activation, leading to monocyte recruitment. We used an in vitro model to assess monocyte binding to endothelial cells in response to HLA I antibodies. We confirmed our results in a mouse model of antibody-mediated rejection, in which B6.RAG1(-/-) recipients of BALB/c cardiac allografts were passively transferred with donor-specific MHC I antibodies. Our findings demonstrate that HLA I antibodies rapidly increase intracellular calcium and endothelial presentation of P-selectin, which supports monocyte binding. In the experimental model, donor-specific MHC I antibodies significantly increased macrophage accumulation in the allograft. Concurrent administration of rPSGL-1-Ig abolished antibody-induced monocyte infiltration in the allograft, but had little effect on antibody-induced endothelial injury. Our data suggest that antagonism of P-selectin may ameliorate accumulation of macrophages in the allograft during antibody-mediated rejection.

HLA class I antibody-mediated endothelial and smooth muscle cell activation

PURPOSE OF REVIEW

Advances in immunosuppression and patient management have successfully improved 1-year transplant outcome. Unfortunately, antibody-mediated rejection is a major barrier to long-term graft survival. This study summarizes the effects of antibodies on endothelial cell and smooth muscle cell (SMC) migration, proliferation and leukocyte recruitment, emphasizing the intracellular signaling pathways that orchestrate these distinct functional outcomes.

RECENT FINDINGS

Several studies have provided further insight into the effects of human leukocyte antigen (HLA) class I antibodies on vascular cells. We found that HLA I molecules partner with integrin β4 to transduce proliferative signaling, and identified proteins that associate with the cytoskeleton after HLA class I crosslinking. Natural killer cells have been strongly implicated in a murine model of donor-specific major histocompatibility complex I antibody-triggered neointimal thickening. A recently developed human arterial graft model revealed the role of matrix metalloproteinases in SMC mitogenesis by HLA class I antibodies. Using a donor transgenic for HLA-A2, Fukami et al. investigated the mechanisms of accommodation induced by low titers of HLA class I antibodies.

SUMMARY

Ligation of HLA class I molecules with antibodies leads to the activation of intracellular signals in endothelial cells and SMCs, which in turn promote actin cytoskeletal remodeling, survival, proliferation, and recruitment of leukocytes.

Mechanism of accommodation in a sensitized human leukocyte antigen transgenic murine cardiac transplant model

BACKGROUND

Presence of donor-specific antibodies (Abs) is detrimental to posttransplant allograft function. Some sensitized recipients have successfully undergone transplantation after pretransplant conditioning regimen using plasmapheresis and/or intravenous immunoglobulin therapy, but underlying mechanisms that confer such allograft protection are undefined.

METHODS

We developed a single human leukocyte antigen (HLA)-mismatched heterotopic murine heart transplant model (HLA-A2 into HLA-A2-sensitized-C57BL/6) to determine whether pretreatment of donors with low concentration of HLA class I (W6/32) or control Ab (C1.18.4) will confer protection. Expression levels of survival genes, Bcl-2 and heme oxygenase-1, were analyzed by gene array analysis and quantitative real-time polymerase chain reaction. Expression levels of cytokine panel were analyzed by Luminex. Role of Bcl-2 in the induction of allograft protection was analyzed by silencing the Bcl-2 expression in the donor hearts using a small hairpin (shRNA) specific for Bcl-2.

RESULTS

Control Ab-pretreated hearts were rejected in less than 5 days demonstrating hemorrhage, Ab, and C4 deposition. In contrast, W6/32-pretreated hearts were rejected at 15 days (P<0.05) that was prolonged to 25 days with antilymphocyte serum treatment. W6/32-pretreated hearts on day 5 exhibited increased expression of Bcl-2 (5.5-folds), Bcl-xl (5.5-folds), and heme oxygenase-1 (4.4-folds); decreased expression of ICAM-1, VCAM-1 (3.2-fold), along with reduced levels of cytokines interleukin (IL)-1β (4.4-folds), tumor necrosis factor α (3.7-folds), IL-6 (7.5-folds), IL-12 (2.3-folds) and chemokines monocyte chemotactic protein 1 (4.5-folds), MIG (4.4-folds), MIP-1α (3.4-folds), and IL-8 (3.1-folds). Silencing of Bcl-2 in accommodated hearts before transplant resulted in loss of protection with rejection (9±3 vs. 15±2days, P<0.05).

CONCLUSION

Pretreatment of hearts with low levels of anti-HLA Abs increases expression of antiapoptotic genes that inhibits caspases, leading to decreased inflammatory cytokines and chemokines, which promote allograft survival.

Comparative study on signal transduction in endothelial cells after anti-a/b and human leukocyte antigen antibody reaction: implication of accommodation

BACKGROUND

Recent development of immunosuppressive therapy has provided a platform for clinical human leukocyte antigen (HLA)- and ABO-incompatible kidney transplantation. However, the prognosis seems to be different between the two. Accommodation, the condition of no injury even in the presence of antidonor antibody, is one of the key factors for successful transplantation with antidonor antibody. The purpose of this study was to compare signal transduction between anti-A/B and anti-HLA antibody reaction and to elucidate the mechanisms underlying accommodation.

METHODS

Blood type A- or B-transferase gene was transfected into human EA.hy926 endothelial cells. After cell sorting, A- or B-expressing cells at high levels were obtained. The effects of anti-HLA and anti-A/B antibody binding on complement-mediated cytotoxicity and signal transduction were examined.

RESULTS

Preincubation with anti-HLA antibodies only at low levels (<10% of saturation level) or anti-A/B antibodies at high levels (even at near saturation levels) for 24 hr resulted in resistance to complement-mediated cytotoxicity. Anti-A/B antibody ligation inactivated ERK1/2 pathway and increased complement regulatory proteins such as CD55 and CD59, whereas anti-HLA ligation activated PI3K/AKT pathway and increased cytoprotective genes such as hemeoxygenase-1 and ferritin H.

CONCLUSION

Complement inhibition by upregulation of CD55 and CD59 through ERK1/2 inactivation might play a substantial role in accommodation after ABO-incompatible transplantation, which could also explain the intriguing finding of C4d deposition in the graft without rejection.

ABO-incompatible hearts for infant transplantation

PURPOSE OF REVIEW

The practice of offering ABO-incompatible (ABOi) heart transplantation during infancy was initiated based on the rationale that infants are at the highest risk of dying while waiting for a transplant, yet are at low risk of hyperacute antibody-mediated rejection due to immunologic immaturity. Since the first report of intentional ABOi heart transplantation a decade ago, its success has been corroborated in numerous reports and the practice has been widely adopted. This review summarizes clinical results in reports of ABOi transplantation and the evolution of ABOi listing strategies, as well as evidence of immune tolerance after ABOi transplantation.

RECENT FINDINGS

Recent reports have documented comparable midterm and long-term clinical outcomes in ABOi and ABO-compatible (ABOc) heart transplant recipients in terms of survival and posttransplant complications. Despite successful outcomes, however, there are obstacles to widespread implementation of ABOi transplantation in the USA and in some European centers. The notable deficiency in development of antibody production to donor A/B antigens following ABOi transplantation described in early reports has been corroborated, with some exceptions. Potential advantages of ABOi transplantation are emerging as well as innovative strategies that may allow ABOi heart transplantation beyond the age of infancy.

SUMMARY

ABOi heart transplantation is one example in which immunologic immaturity has been exploited to the advantage of pediatric transplant recipients. In-depth exploration of transplant-related immunobiology in the young may reveal further opportunities.

Antibody ligation of human leukocyte antigen class I molecules stimulates migration and proliferation of smooth muscle cells in a focal adhesion kinase-dependent manner

Chronic rejection manifests as transplant vasculopathy, which is characterized by intimal thickening of the vessels of the allograft. Intimal thickening is thought to result from the migration and proliferation of vascular smooth muscle cells (SMC) in the vessel media, followed by deposition of extracellular matrix proteins. The development of post-transplantation anti-human leukocyte antigen (HLA) antibodies (Ab) is strongly correlated with the development of transplant vasculopathy and graft loss. Here we demonstrate that cross-linking of HLA class I molecules on the surface of human SMC with anti-HLA class I Ab induced cell proliferation and migration. Class I ligation also increased phosphorylation of focal adhesion kinase (FAK), Akt, and ERK1/2 in SMC. Knockdown of FAK by siRNA attenuated class I-induced phosphorylation of Akt and ERK1/2, as well as cell proliferation and migration. These results indicate that ligation of HLA class I molecules induces SMC migration and proliferation in a FAK-dependent manner, which may be important in promoting transplant vasculopathy.

The link between major histocompatibility complex antibodies and cell proliferation

Experimental evidence indicates that donor-specific antibodies targeting major histocompatibility complex classes I and II molecules can elicit the key features of transplant vasculopathy by acting on the graft vasculature in 3 ways: directly activating proliferative, prosurvival, and migratory signaling in the target endothelial and smooth muscle cells; increasing expression of mitogenic factors in vascular endothelial cells, creating a potential proliferative autocrine loop; and promoting recruitment of inflammatory cells that produce mitogenic factors and elicit chronic inflammation, proliferation, and fibrosis. Here, we review the experimental literature showing the complement and Fc-independent effects of major histocompatibility complex classes I and II antibodies on graft vascular cells that may directly contribute to the proliferative aspect of transplant vasculopathy.

Antibody-mediated allograft rejection: the emerging role of endothelial cell signalling and transcription factors

The presence of antibodies against class I human leukocyte antigens (HLA) can cause the development of chronic allograft rejection. Although endothelial cell activation has been identified as a main effector, the mechanisms mediating this process are not fully understood. Exposure of endothelium to antibodies against HLA antigens induces cell activation which promotes rejection. This activation process can be divided into two phases: an early response in which intracellular signalling proteins and transcription factors are activated, and a later change in protein expression and cell function. In this review, antibody-mediated endothelial signalling and the role of transcription factors in organ transplantation will be described with a particular focus on their potential to mediate some of the graft-damaging effects of anti-HLA class I antibodies.

No association of kidney graft loss with human leukocyte antigen antibodies detected exclusively by sensitive Luminex single-antigen testing: a Collaborative Transplant Study report

BACKGROUND

It is unclear whether kidney transplant recipients with preformed donor-specific human leukocyte antigen (HLA) antibodies (DSA) detectable only in the highly sensitive Luminex single-antigen (LSA) assay are at an increased risk of graft failure.

METHODS

We studied 3148 patients who received a deceased donor kidney graft between 1996 and 2008 and were enrolled in the prospective serum project of the Collaborative Transplant Study. There were 118 patients with graft loss during the first 3 years after transplantation on whom recipient and donor DNA was available for complete HLA typing. We compared the incidence of LSA-detected DSA in these patients with graft failure and matched controls with functioning grafts. All patients were found negative in the less-sensitive complement-dependent lymphocytotoxicity and enzyme-linked immunosorbent assays.

RESULTS

When mean fluorescence intensity (MFI) of greater than or equal to 1000 was used as a cutoff for Luminex positivity, 118 patients with graft loss did not show a higher incidence of DSA against HLA-A, -B, -C, -DRB1/3/4/5, -DQA1, -DQB1, -DPA1, or -DPB1 antigens than 118 matched controls without graft loss (for all loci P not significant). The incidence of strong DSA (MFI ≥2000 or MFI ≥3000) detected only by LSA was low (for all loci between 0% and 5%) and did not identify unacceptable antigens that were relevant for graft loss within the first 3 years after transplantation.

CONCLUSION

We conclude that, given currently practiced crossmatch procedures and immunosuppressive regimens, exclusion of donor organs carrying "unacceptable" HLA based exclusively on sensitive LSA antibody testing is not justified.

C1q-fixing human leukocyte antigen antibodies are specific for predicting transplant glomerulopathy and late graft failure after kidney transplantation

BACKGROUND

Human leukocyte antigen (HLA) antibodies, especially those that fix complement, are associated with antibody-mediated rejection and graft failure. The C1q assay on single antigen beads detects a subset of HLA antibodies that can fix complement and precede C4d deposition. The aim of this study was to determine whether C1q-fixing antibodies distinguish de novo donor-specific antibodies (DSA) that are clinically relevant and harmful.

METHODS

We retrospectively studied 31 of 274 kidney transplant recipients who had pretransplant and concurrent biopsy and serum specimens, 13 with C4d-positive and 18 with C4d-negative staining. We measured IgG and C1q DSA pretransplant and at the time of biopsy using single antigen bead assays. We identified 13 recipients who developed de novo DSA by IgG or C1q and examined associations with C4d deposition, transplant glomerulopathy, and graft failure.

RESULTS

Testing for DSA by IgG is more sensitive for C4d deposition (IgG: 100%, 95% confidence interval [CI] 0.60-1; C1q: 75%, 95% CI 0.36-0.96). Testing for DSA by C1q is more specific for transplant glomerulopathy (C1q: 81%, 95% CI 0.57-0.94; IgG: 67%, 95% CI 0.43-0.85) and graft loss (C1q: 79%, 95% CI 0.54-0.93; IgG: 63%, 95% CI 0.39-0.83). Absence of de novo DSA by IgG and C1q has a high negative predictive value for the absence of C4d deposition (IgG: 100%, 95% CI 0.73-1; C1q: 88%, 95% CI 0.62-0.98), transplant glomerulopathy (IgG: 100%, 95% CI 0.73-1; C1q: 100%, 95% CI 0.77-1), and graft failure (IgG: 86%, 95% CI 0.56-0.97; C1q: 88%, 95% CI 0.62-0.98).

CONCLUSION

Monitoring patients with the C1q assay, which detects antibodies that fix complement, offers a minimally invasive means of identifying patients at risk for transplant glomerulopathy and graft loss.

Mechanisms involved in antibody- and complement-mediated allograft rejection

Antibody-mediated rejection has become critical clinically because this form of rejection is usually unresponsive to conventional anti-rejection therapy, and therefore, it has been recognized as a major cause of allograft loss. Our group developed experimental animal models of vascularized organ transplantation to study pathogenesis of antibody- and complement-mediated endothelial cell injury leading to graft rejection. In this review, we discuss mechanisms of antibody-mediated graft rejection resulting from activation of complement by C1q- and MBL (mannose-binding lectin)-dependent pathways and interactions with a variety of effector cells, including macrophages and monocytes through Fcgamma receptors and complement receptors.

Virtual crossmatch by identification of donor-specific anti-human leukocyte antigen antibodies by solid-phase immunoassay: a 30-month analysis in living donor kidney transplantation

Selection of donors for kidney transplantation depends on accurate prediction of risk factors for immunologic rejection. Historically, cytotoxicity crossmatch (CXM) examining lysis of donor cells by preformed anti-human leukocyte antigen (HLA) antibodies (Abs) has been considered the best predictor of immunologic rejection. However, there is much interest in defining anti-HLA Ab specificity in recipient sera by immunoassay to predict crossmatch results and aid in donor selection. Current immunoassays for anti-HLA Abs are highly sensitive, though correlation between Abs detected by immunoassay and their functional relevance in CXM and subsequent transplantation is not well defined. In this study, we retrospectively examined the predictive value of detection of donor-specific anti-HLA Abs (DSA) by Luminex Single Antigen assay from 149 consecutive living donor kidney transplant recipients. We demonstrate that detection of DSA by immunoassay accurately predicted negative crossmatch and graft survival. However, this approach had limited sensitivity for predicting positive crossmatch, attributable to either limited typing of donor HLA-DQ and -DP alleles or due to non-HLA Abs. False-positive prediction of CXM correlated with detection of "weak" Abs with low mean fluorescence intensity (MFI < 2000). Furthermore, we found that a ratio of the MFI of the DSA bead to the MFI of the positive control bead was a better method for identifying weak DSA that did not result in CXM-positive reactions. Interestingly, patients with weak DSA and negative CXM had equivalent graft survival over an 18 month follow-up period, suggesting that weak DSA may not preclude transplantation.

Significance of HLA class I antibody-induced antioxidant gene expression for endothelial cell protection against complement attack

It has been observed that a graft organ continues to survive and function normally even in the presence of anti-graft antibodies. However, the mechanisms behind acquirement of this condition remain unknown. Here we report that the anti-HLA ligation on endothelial cells induces PI3K/AKT activation followed by antioxidant gene induction through Nrf2-mediated antioxidant-responsive element (ARE) activation. Activation of PI3K/AKT in endothelial cells by a low concentration of anti-HLA ligation enhances protection from complement attack. A real-time quantitative PCR and flow-cytometry experiment showed that ferritin H and HO-1 mRNAs were induced in a PI3K/AKT-dependent manner, while CD55 and CD59 expression were not enhanced by anti-HLA ligation. Anti-HLA ligation on endothelial cells activates ferritin H ARE and induces Nrf2 binding on its enhancer element. Finally, overexpression of Nrf2 in endothelial cells attenuates complement-mediated cytotoxicity. These experiments suggest that induction of PI3K/AKT-dependent cytoprotective genes by Nrf2 is an important mechanism to prevent complement attack. Thus, a protocol to activate this pathway would be a potential strategy for avoidance of graft rejection in transplantation.

Human leukocyte antigen antibodies in chronic transplant vasculopathy-mechanisms and pathways

Transplant recipients exhibiting posttransplant antibodies are at a higher risk for acute and chronic antibody mediated rejection (AMR). The primary alloantigens recognized by antibodies in recipients with AMR are the highly polymorphic HLA class I and class II molecules expressed on the surface of the endothelial cells (ECs) of the graft. Traditionally, anti-HLA antibodies were thought to mediate graft injury through complement-dependent mechanisms. However, recent studies indicate that antibodies can also contribute to alterations in EC function through complement-independent mechanisms by transducing intracellular signals. Anti-HLA antibodies transduce signals that are both pro-inflammatory and pro-proliferative suggesting mechanistic roles in acute and chronic AMR.

Anti-HLA I antibodies induce VEGF production by endothelial cells, which increases proliferation and paracellular permeability

Anti-human leukocyte antigen class I (HLA I) antibodies were shown to activate several protein kinases in endothelial cells (ECs), which induces proliferation and cell survival. An important phenomenon in antibody-mediated rejection is the occurrence of interstitial edema. We investigated the effect of anti-HLA I antibodies on endothelial proliferation and permeability, as one possible underlying mechanism of edema formation. HLA I antibodies increased the permeability of cultured ECs isolated from umbilical veins. Anti-HLA I antibodies induced the production of vascular endothelial growth factor (VEGF) by ECs, which activated VEGF receptor 2 (VEGFR2) in an autocrine manner. Activated VEGFR2 led to a c-Src-dependent phosphorylation of vascular endothelial (VE)-cadherin and its degradation. Aberrant VE-cadherin expression resulted in impaired adherens junctions, which might lead to increased endothelial permeability. This effect was only observed after cross-linking of HLA I molecules by intact antibodies. Furthermore, our results suggest that increased endothelial proliferation following anti-HLA I treatment occurs via autocrine VEGFR2 activation. Our data indicate the ability of anti-HLA I to induce VEGF production in ECs. Transactivation of VEGFR2 leads to increased EC proliferation and paracellular permeability. The autocrine effect of VEGF on endothelial permeability might be an explanation for the formation of interstitial edema after transplantation.

Intentional ABO-incompatible lung transplantation

We report on a case of intentional blood group incompatible lung transplantation. A blood group O cystic fibrosis patient was mechanically ventilated and put on interventional lung assist for severe respiratory decompensation. Since timely allocation of a blood group O donor lung was impossible, an AB deceased donor lung rescue allocation was accepted and the transplant performed using a pre-, peri- and postoperative antibody depletion protocol including plasmapheresis, ivIg administration, rituximab and immunoadsorption. Nine months after the transplant the patient is at home and well.

Differential effects of donor-specific alloantibody

Alloantigen exposure typically provokes an adaptive immune response that can foster rejection of transplanted organs, and these responses present the most formidable biological barrier to kidney transplantation. Although most cellular alloimmune responses can be therapeutically controlled with T-cell-specific immunosuppressants, humoral alloimmune responses remain relatively untamed. Importantly, humoral immunity, typically manifesting as allospecific antibody production, is increasingly recognized for its variable appearance after kidney transplantation. Indeed, the appearance of alloantibody can herald the onset of rapid and destructive antibody-mediated rejection or have no demonstrable acute effects. The factors determining the end result of alloantibody formation remain poorly understood. This review will discuss the breadth of alloantibody responses seen in clinical kidney transplantation and provide an overview of potential factors explaining the phenotypic variability associated with humoral alloimmunity. We propose several avenues ripe for future investigation including the influence of innate immune components and the potential influence of heterologous immune responses in determining the ultimate clinical import of an alloantibody response.

Accommodation in organ transplantation

PURPOSE OF REVIEW

We review recent insights into the mechanisms and prevalence of accommodation. Accommodation refers to an acquired resistance of an organ graft to humoral injury and rejection.

RECENT FINDINGS

Accommodation has been postulated to reflect changes in antibodies, control of complement and/or acquired resistance to injury by antibodies, complement or other factors. We discuss the importance of these mechanisms, highlighting new conclusions.

SUMMARY

Accommodation may be a common, perhaps the most common, outcome of organ transplantation and, in some systems, a predictable outcome of organ xenotransplantation. Further understanding of how accommodation is induced and by what mechanisms it is manifest and maintained could have a profound impact on transplantation in general and perhaps on other fields.

Monitoring of human liver and kidney allograft tolerance: a tissue/histopathology perspective

Several factors acting together have recently enabled clinicians to seriously consider whether chronic immunosuppression is needed in all solid organ allograft recipients. This has prompted a dozen or so centers throughout the world to prospectively wean immunosuppression from conventionally treated liver allograft recipients. The goal is to lessen the impact of chronic immunosuppression and empirically identify occasional recipients who show operational tolerance, defined as gross phenotype of tolerance in the presence of an immune response and/or immune deficit that has little or no significant clinical impact. Rare operationally tolerant kidney allograft recipients have also been identified, usually by single case reports, but only a couple of prospective weaning trials in conventionally treated kidney allograft recipients have been attempted and reported. Pre- and postweaning allograft biopsy monitoring of recipients adds a critical dimension to these trials, not only for patient safety but also for determining whether events in the allografts can contribute to a mechanistic understanding of allograft acceptance. The following is based on a literature review and personal experience regarding the practical and scientific aspects of biopsy monitoring of potential or actual operationally tolerant human liver and kidney allograft recipients where the goal, intended or attained, was complete withdrawal of immunosuppression.

Four stages and lack of stable accommodation in chronic alloantibody-mediated renal allograft rejection in Cynomolgus monkeys

The etiology of immunologically mediated chronic renal allograft failure is unclear. One cause is thought to be alloantibodies. Previously in Cynomolgus monkeys, we observed a relationship among donor-specific alloantibodies (DSA), C4d staining, allograft glomerulopathy, allograft arteriopathy and progressive renal failure. To define the natural history of chronic antibody-mediated rejection and its effect on renal allograft survival, we now extend this report to include 417 specimens from 143 Cynomolgus monkeys with renal allografts. A subset of animals with long-term renal allografts made DSA (48%), were C4d positive (29%), developed transplant glomerulopathy (TG) (22%) and chronic allograft arteriopathy (CAA) (19%). These four features were highly correlated and associated with statistically significant shortened allograft survival. Acute cellular rejection, either Banff type 1 or 2, did not correlate with alloantibodies, C4d deposition or TG. However, endarteritis (Banff type 2) correlated with later CAA. Sequential analysis identified four progressive stages of chronic antibody-mediated rejection: (1) DSA, (2) deposition of C4d, (3) TG and (4) rising creatinine/renal failure. These new findings provide strong evidence that chronic antibody-mediated rejection develops without enduring stable accommodation, progresses through four defined clinical pathological stages and shortens renal allograft survival.

Anti-MHC class I antibody activation of proliferation and survival signaling in murine cardiac allografts

Anti-MHC class I alloantibodies have been implicated in the process of acute and chronic rejection because these Abs can bind to endothelial cells and transduce signals leading to the activation of cell survival and proliferation pathways. To characterize the role of the MHC class I-signaling pathway in the pathogenesis of Ab-mediated rejection, we developed a mouse vascularized heterotopic cardiac allograft model in which B6.RAG1 KO hosts (H-2K(b)/D(b)) received a fully MHC-incompatible BALB/c (H-2K(d)/D(d)) heart transplant and were passively transfused with anti-donor MHC class I Ab. We demonstrate that cardiac allografts of mice treated with anti-MHC class I Abs show characteristic features of Ab-mediated rejection including microvascular changes accompanied by C4d deposition. Phosphoproteomic analysis of signaling molecules involved in the MHC class I cell proliferation and survival pathways were elevated in anti-class I-treated mice compared with the isotype control-treated group. Pairwise correlations, hierarchical clustering, and multidimensional scaling algorithms were used to dissect the class I-signaling pathway in vivo. Treatment with anti-H-2K(d) Ab was highly correlated with the activation of Akt and p70S6Kinase (S6K). When measuring distance as a marker of interrelatedness, multidimensional scaling analysis revealed a close association between members of the mammalian target of rapamycin pathway including mammalian target of rapamycin, S6K, and S6 ribosomal protein. These results provide the first analysis of the interrelationships between these signaling molecules in vivo that reflects our knowledge of the signaling pathway derived from in vitro experiments.

Synergistic deposition of C4d by complement-activating and non-activating antibodies in cardiac transplants

The role of non-complement-activating alloantibodies in humoral graft rejection is unclear. We hypothesized that the non-complement-activating alloantibodies synergistically activate complement in combination with complement-activating antibodies. B10.A hearts were transplanted into immunoglobulin knock out (Ig-KO) mice reconstituted with monoclonal antibodies to MHC class I antigens. In allografts of unreconstituted Ig-KO recipients, no C4d was detected. Similarly, reconstitution with IgG1 or low dose IgG2b alloantibodies did not induce C4d deposition. However, mice administered with a low dose of IgG2b combined with IgG1 had heavy linear deposits of C4d on vascular endothelium. C4d deposits correlated with decreased graft survival. To replicate this synergy in vitro, mononuclear cells from B10.A mice were incubated with antibodies to MHC class I antigens followed by incubation in normal mouse serum. Flow cytometry revealed that both IgG2a and IgG2b synergized with IgG1 to deposit C4d. This synergy was significantly decreased in mouse serum deficient in mannose binding lectin (MBL) and in serum deficient in C1q. Reconstitution of MBL-A/C knock out (MBL-KO) serum with C1q-knock out (C1q-KO) serum reestablished the synergistic activity. This suggests a novel role for non-complement-activating alloantibodies and MBL in humoral rejection.