Contribution of B cells and antibody to cardiac allograft vasculopathy

Decoding immune cell interactions during cardiac allograft vasculopathy: insights derived from bioinformatic strategies

Chronic allograft vasculopathy (CAV) is a major cause of late graft failure in heart transplant recipients, characterized by progressive intimal thickening and diffuse narrowing of the coronary arteries. Unlike atherosclerosis, CAV exhibits a distinct cellular composition and lesion distribution, yet its pathogenesis remains incompletely understood. A major challenge in CAV research has been the limited application of advanced "-omics" technologies, which have revolutionized the study of other vascular diseases. Recent advancements in single-cell and spatial transcriptomics, proteomics, and metabolomics have begun to uncover the complex immune-endothelial-stromal interactions driving CAV progression. Notably, single-cell RNA sequencing has identified previously unrecognized immune cell populations and signaling pathways implicated in endothelial injury and vascular remodeling after heart transplantation. Despite these breakthroughs, studies applying these technologies to CAV remain sparse, limiting the translation of these insights into clinical practice. This review aims to bridge this gap by summarizing recent findings from single-cell and multi-omic approaches, highlighting key discoveries, and discussing their implications for understanding CAV pathogenesis.

Oxymatrine attenuates chronic allograft rejection by modulating immune responses and inhibiting fibrosis

BACKGROUND

Chronic rejection (CR) is a significant obstacle to long-term allograft survival. Oxymatrine (OMT) is a prominent bioactive compound widely utilized in traditional Chinese medicine for the management of inflammatory disorders and it has considerable potential as a therapeutic candidate for the treatment of CR.

METHODS

Well-established major histocompatibility complex (MHC) class II mismatched B6 mice. C-H-2bm12-to-C57BL/6 mouse transplantation was used as a CR model. Hematoxylin and eosin (H&E) staining, immunohistochemistry, and Masson's trichrome staining were used to assess pathological changes in the grafts, and the percentages of immune cells were determined by flow cytometry. The effects of OMT on the regulation of CD4+ T cell differentiation and cytokine secretion were verified in vitro.

RESULTS

OMT effectively alleviated pathological graft damage, characterized by chronic changes in intimal lesions, vasculopathy, and fibrosis and significantly prolonged cardiac allograft survival. OMT exerted its immunomodulatory effects by inhibiting T helper 1 (Th1) and T helper 17 (Th17) cell differentiation while promoting Treg differentiation both in vivo and in vitro. Further studies revealed that OMT inhibited the phosphorylation of mammalian target of rapamycin (mTOR), which is a potential mechanism underlying its immunosuppressive effects. OMT also inhibited the activation of B cells and the production of donor-specific antibody (DSA). In addition, OMT effectively alleviated chronic changes in fibrosis in cardiac allografts, and these changes may be related to the inhibition of the transforming growth factor-β (TGF-β)-Smad 2/3 pathway.

CONCLUSIONS

OMT attenuated CR by modulating the immune response and inhibiting graft fibrosis. Further in-depth investigations of OMT may provide valuable insights into the development of novel therapeutic strategies for CR inhibition.

Failing Heart Transplants and Rejection-A Cellular Perspective

The median survival of patients with heart transplants is relatively limited, implying one of the most relevant questions in the field—how to expand the lifespan of a heart allograft? Despite optimal transplantation conditions, we do not anticipate a rise in long-term patient survival in near future. In order to develop novel strategies for patient monitoring and specific therapies, it is critical to understand the underlying pathological mechanisms at cellular and molecular levels. These events are driven by innate immune response and allorecognition driven inflammation, which controls both tissue damage and repair in a spatiotemporal context. In addition to immune cells, also structural cells of the heart participate in this process. Novel single cell methods have opened new avenues for understanding the dynamics driving the events leading to allograft failure. Here, we review current knowledge on the cellular composition of a normal heart, and cellular mechanisms of ischemia-reperfusion injury (IRI), acute rejection and cardiac allograft vasculopathy (CAV) in the transplanted hearts. We highlight gaps in current knowledge and suggest future directions, in order to improve cellular and molecular understanding of failing heart allografts.

B cell clonal expansion within immune infiltrates in human cardiac allograft vasculopathy

Cardiac allograft vasculopathy (CAV) is associated with intragraft B cell infiltrates. Here, we studied the clonal composition of B cell infiltrates using 4 graft specimens with CAV. Using deep sequencing, we analyzed the immunoglobulin heavy chain variable region repertoire in both graft and blood. Results showed robust B cell clonal expansion in the graft but not in the blood for all cases. Several expanded B cell clones, characterized by their uniquely rearranged complementarity-determining region 3, were detected in different locations in the graft. Sequences from intragraft B cells also showed elevated levels of mutated rearrangements in the graft compared to blood B cells. The number of somatic mutations per rearrangement was also higher in the graft than in the blood, suggesting that B cells continued maturing in situ. Overall, our studies demonstrated B cell clonal expansion in human cardiac allografts with CAV. This local B cell response may contribute to the pathophysiology of CAV through a mechanism that needs to be identified.

Effector B cells in cardiac allograft vasculopathy

PURPOSE OF REVIEW

B cells have recently emerged as important immune players in solid organ rejection, especially in cardiac allograft vasculopathy (CAV), a chronic form of rejection following heart transplantation. B cells can exert either regulatory or effector functions. This review will provide an update on effector B cells in CAV.

RECENT FINDINGS

Independent studies reported the abundance of B cells in graft infiltrates during CAV, especially around coronary arteries. Infiltrates comprise CD20+ CD27+ memory B cells together with differentiated CD20-CD138+ plasma cells, which are almost always associated with T cells and macrophages. The structure of some of these infiltrates evokes that of germinal centers, suggesting the generation of tertiary lymphoid organs in the graft. Remarkably, B-cell infiltrates are most often detected in the absence of circulating donor human leukocyte antigen-specific antibodies, strongly suggesting that the two components are unrelated. Characterization of B-cell clones isolated from explanted human cardiac graft infiltrates revealed the prevalence of polyreactive innate, B1-like B cells. Accumulating evidence suggests that these cells act primarily as antigen-presenting cells in situ. Additional effector functions, such as local antibody secretion and pro-inflammatory cytokine production, promoting T-cell polarization, macrophage activation and fibrosis are also considered.

SUMMARY

Converging observations made through animal and human studies add substantial support for an effector B-cell role in the pathophysiology of CAV. On the basis of these collective findings, a therapeutic strategy targeting B cells could reasonably be envisaged to prevent or treat this complication.

Prevalence of polyreactive innate clones among graft--infiltrating B cells in human cardiac allograft vasculopathy

BACKGROUND

Cardiac allograft vasculopathy (CAV) has been associated with graft-infiltrating B cells, although their characteristics are still unclear. In this study we examined the frequency, localization and reactivity profile of graft-infiltrating B cells to determine their contribution to the pathophysiology of CAV.

METHODS

B cells, plasma cells and macrophages were examined by immunohistochemistry in 56 allografts with CAV, 49 native failed hearts and 25 autopsy specimens. A total of 102 B-cell clones were immortalized directly from the infiltrates of 3 fresh cardiac samples with CAV. Their secreted antibodies were assessed using enzyme-linked immunoassay and flow cytometry.

RESULTS

B-cell infiltration was observed around coronary arteries in 93% of allograft explants with CAV. Comparatively, intragraft B cells were less frequent and less dense in the intraventricular myocardium from where routine biopsies are obtained. Plasma cells and macrophages were also detected in 85% and 95% of explants, respectively. Remarkably, B-cell infiltrates were not associated with circulating donor-specific antibodies (DSA) or prior episodes of antibody-mediated rejection (AMR). Among all B-cell clones generated from 3 explants with CAV, a majority secreted natural antibodies reactive to multiple autoantigens and apoptotic cells, a characteristic of innate B cells.

CONCLUSIONS

Our study reveals a high frequency of infiltrating B cells around the coronary arteries of allografts with CAV, independent of DSA or AMR. These cells are enriched for innate B cells with a polyreactive profile. The findings shift the focus from conventional DSA-producing B cells to the potentially pathogenic polyreactive B cells in the development of clinical CAV.

The immunological function of CD52 and its targeting in organ transplantation

INTRODUCTION

CD52 (Campath-1 antigen), a glycoprotein of 12 amino acids anchored to glycosylphosphatidylinositol, is widely expressed on the cell surface of immune cells, such as mature lymphocytes, natural killer cells (NK), eosinophils, neutrophils, monocytes/macrophages, and dendritic cells (DCs). The anti-CD52 mAb, alemtuzumab, was used widely in clinics for the treatment of patients such as organ transplantation. In the present manuscript, we will briefly summarize the immunological function of CD52 and discuss the application of anti-CD52 mAb in transplantation settings.

FINDINGS

We reviewed studies published until July 2016 to explore the role of CD52 in immune cell function and its implication in organ transplantation. We showed that ligation of cell surface CD52 molecules may offer costimulatory signals for T-cell activation and proliferation. However, soluble CD52 molecules will interact with the inhibitory sialic acid-binding immunoglobulin-like lectin 10 (Siglec10) to significantly inhibit T cell proliferation and activation. Although the physiological and pathological significances of CD52 molecules are still poorly understood, the anti-CD52 mAb, alemtuzumab, was used widely for the treatment of patients with chronic lymphocytic leukemia, autoimmune diseases as well as cell and organ transplantation in clinics.

CONCLUSION

Studies clearly showed that CD52 can modulate T-cell activation either by its intracellular signal pathways or by the interaction of soluble CD52 and Siglec-10 expressing on T cells. However, the regulatory functions of CD52 on other immune cell subpopulations in organ transplantation require to be studied in the near future.

Donor-Specific Antibodies Are Produced Locally in Ectopic Lymphoid Structures in Cardiac Allografts

Cardiac allograft vasculopathy (CAV) is a transplant pathology, limiting graft survival after heart transplantation. CAV arteries are surrounded by ectopic lymphoid structures (ELS) containing B cells and plasma cells. The aim of this study was to characterize the antigenic targets of antibodies produced in ELS. Coronary arteries and surrounding epicardial tissue from 56 transplant recipients were collected during autopsy. Immunofluorescence was used to identify antibody-producing plasma cells. Immunoglobulin levels in tissue lysates were measured by enzyme-linked immunosorbent assay and analyzed for donor-specific HLA antibodies by Luminex assay. Cytokine and receptor expression levels were quantified using quantitative polymerase chain reaction. Plasma cells in ELS were polyclonal and produced IgG and/or IgM antibodies. In epicardial tissue, IgG (p < 0.05) and IgM levels were higher in transplant patients with larger ELS than smaller ELS. In 4 of 21 (19%) patients with ELS, donor-specific HLA type II antibodies were detected locally. Cytokine and receptor expression (CXCR3, interferon γ and TGF-β) was higher in large ELS in the epicardial tissue than in other vessel wall layers, suggesting active recruitment and proliferation of T and B lymphocytes. ELS exhibited active plasma cells producing locally manufactured antibodies that, in some cases, were directed against the donor HLA, potentially mediating rejection with major consequences for the graft.

Immunological and Fibrotic Mechanisms in Cardiac Allograft Vasculopathy

Cardiac allograft vasculopathy (CAV) has a high prevalence among patients that have undergone heart transplantation. Cardiac allograft vasculopathy is a multifactorial process in which the immune system is the driving force. In this review, the data on the immunological and fibrotic processes that are involved in the development of CAV are summarized. Areas where a lack of knowledge exists and possible additional research can be completed are pinpointed. During the pathogenesis of CAV, cells from the innate and the adaptive immune system cooperate to reject the foreign heart. This inflammatory response results in dysfunction of the endothelium and migration and proliferation of smooth muscle cells (SMCs). Apoptosis and factors secreted by both the endothelium as well as the SMCs lead to fibrosis. The migration of SMCs together with fibrosis provoke concentric intimal thickening of the coronary arteries, which is the main characteristic of CAV.

Interleukin-10 From Marginal Zone Precursor B-Cell Subset Is Required for Costimulatory Blockade-Induced Transplantation Tolerance

BACKGROUND

Blocking CD40-CD40L costimulatory signals induces transplantation tolerance. Although B-cell depletion prevents alloantibody formation, nonhumoral functions of B cells in tolerance have not been well characterized. We investigated whether specific subsets of B cell or B cell-derived interleukin (IL)-10 contribute to tolerance.

METHODS

Wild type C57BL/6, or B cell-specific interleukin (IL)-10 (CD19-Cre::IL-10) mice, received vascularized BALB/c cardiac allografts. BALB/c donor-specific splenocyte transfusion and anti-CD40L monoclonal antibody were used as tolerogen. B cells were depleted with antimouse CD20 monoclonal antibody. Various B-cell subsets were purified and characterized by flow cytometry, reverse transcription polymerase chain reaction, and adoptive transfer.

RESULTS

B-cell depletion prevented costimulatory blockade-induced allogeneic tolerance. Costimulatory blockade increased IL-10 in marginal zone precursor (MZP) B cells, but not other subsets. In particular, costimulatory blockade did not change other previously defined regulatory B-cell subsets (Breg), including CD5CD1d Breg or expression of TIM1 or TIM4 on these Breg or other Breg cell subsets. Costimulatory blockade also induced IL-21R expression in MZP B cells, and IL-21R MZP B cells expressed even more IL-10. B-cell depletion or IL-10 deficiency in B cells prevented tolerance in a cardiac allograft model, resulting in rapid acute cardiac allograft rejection. Adoptive transfer of wild type MZP B cells but not other subsets to B cell-specific IL-10 deficient mice prevented graft rejection.

CONCLUSIONS

CD40 costimulatory blockade induces MZP B cell IL-10 which is necessary for tolerance. These observations have implications for understanding tolerance induction and how B cell depletion may prevent tolerance.

Absence of Activation-induced Cytidine Deaminase, a Regulator of Class Switch Recombination and Hypermutation in B Cells, Suppresses Aorta Allograft Vasculopathy in Mice

BACKGROUND

Antibody-mediated rejection is caused in part by increasing circulation/production of donor-specific antibody (DSA). Activation-induced cytidine deaminase (AID) is a key regulator of class switch recombination and somatic hypermutation of immunoglobulin in B cells, yet its role in antibody-mediated transplant rejection remains unclear. We show here that AID deficiency in mice enables suppression of allograft vasculopathy (AV) after aorta transplantation, a DSA-mediated process.

METHODS

Splenocytes from C57BL/6 J (B6) AID(−/−) mice were used for determining in vitro proliferation responses, alloreactivity, cell surface marker expression, and antibody production. BALB/c mouse aortas were transplanted into B6 AID(−/−) mice with or without FK506 treatment. Blood and aorta grafts were harvested on day 30 after transplantation and were subjected to DSA, histological, and immunohistological analyses.

RESULTS

The AID(−/−) splenocytes were comparable to wild type splenocytes in proliferation responses, alloreactivity, and expression of cell surface markers in vitro. However, they completely failed to produce immunoglobulin G, although they were not impaired in immunoglobulin M production relative to controls. Furthermore, BALB/c aorta grafts from B6 AID(−/−) recipient mice on day 30 after transplantation showed reduced signs of AV compared to the grafts from B6 wild type recipient mice which had severe vascular intimal hyperplasia, interstitial fibrosis, and inflammation. Treatment with FK506 produced a synergistic effect in the grafts from AID(−/−) recipients with further reduction of intimal hyperplasia and fibrosis scores.

CONCLUSIONS

The AID deficiency inhibits DSA-mediated AV after aorta transplantation in mice. We propose that AID could be a novel molecular target for controlling antibody-mediated rejection in organ transplantation.

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

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

Novel functions of B cells in transplantation

PURPOSE OF REVIEW

This manuscript reviews current knowledge and recent findings regarding antibody-independent functions of B cells in transplantation.

RECENT FINDINGS

Until recently the functions of B cells in transplantation have been attributed almost entirely to the antibodies they produce. However, the results of recent trials of B-cell-depleting agents for treatment of antibody-mediated rejection as well as auto-immune disease raised awareness that B cells mediate functions independent of antibody synthesis.

SUMMARY

These 'nonclassical' functions place B cells at the center of immune regulation with the power to enhance or inhibit immunity.

Patterns of de novo allo B cells and antibody formation in chronic cardiac allograft rejection after alemtuzumab treatment

Even though the etiology of chronic rejection (CR) is multifactorial, donor specific antibody (DSA) is considered to have a causal effect on CR development. Currently the antibody-mediated mechanisms during CR are poorly understood due to lack of proper animal models and tools. In a clinical setting, we previously demonstrated that induction therapy by lymphocyte depletion, using alemtuzumab (anti-human CD52), is associated with an increased incidence of serum alloantibody, C4d deposition and antibody-mediated rejection in human patients. In this study, the effects of T cell depletion in the development of antibody-mediated rejection were examined using human CD52 transgenic (CD52Tg) mice treated with alemtuzumab. Fully mismatched cardiac allografts were transplanted into alemtuzumab treated CD52Tg mice and showed no acute rejection while untreated recipients acutely rejected their grafts. However, approximately half of long-term recipients showed increased degree of vasculopathy, fibrosis and perivascular C3d depositions at posttransplant day 100. The development of CR correlated with DSA and C3d deposition in the graft. Using novel tracking tools to monitor donor-specific B cells, alloreactive B cells were shown to increase in accordance with DSA detection. The current animal model could provide a means of testing strategies to understand mechanisms and developing therapeutic approaches to prevent chronic rejection.

Cyclosporine immunosuppression does not prevent the production of donor-specific antibody capable of mediating allograft vasculopathy

BACKGROUND

Late cardiac graft rejection, primarily mediated by allograft vasculopathy (AV), remains a major limitation to cardiac transplantation, even in the face of significant calcineurin inhibitor (CNI) immunosuppression. The role played by alloantibody in AV is unclear. Evidence that CNI immunosuppression suppresses CD4(+) T-cell function would suggest that antibody production and effector function would be severely limited in CNI-treated patients. In this study we examine the capacity of CNI-treated animals to develop effective alloantibody that can mediate AV.

METHODS

Wild-type (WT) B6 mice were alloimmunized using donor splenocytes or a fully major histocompatibility complex-mismatched allogeneic abdominal aortic graft in the presence of CNI immunosuppression (30 or 50 mg/kg/day cyclosporine A). Anti-serum was harvested and tested using complement-dependent in vitro cytotoxicity assays. Anti-serum was passively transferred to immunodeficient RAG1(-/-) recipients of allogeneic grafts. C4d deposition was quantified in the allografts from WT recipients.

RESULTS

CNI immunosuppression did not prevent the development of alloantibody in response to either immunization method (p < 0.05). Passive transfer of anti-serum generated AV lesions in immunodeficient graft recipients and mediated complement-dependent destruction of donor cells (p < 0.05). C4d deposition was localized to the media of grafts of CNI treated animals.

CONCLUSIONS

CNI therapy does not prevent the production of alloantibody with the capacity to mediate AV. C4d deposition in the media suggests a role for medial smooth muscle cell loss in antibody-mediated AV lesion development in our model.

Role of Mitogen-Activated Protein Kinases in Myocardial Ischemia-Reperfusion Injury during Heart Transplantation

In solid organ transplantation, ischemia/reperfusion (IR) injury during organ procurement, storage and reperfusion is an unavoidable detrimental event for the graft, as it amplifies graft inflammation and rejection. Intracellular mitogen-activated protein kinase (MAPK) signaling pathways regulate inflammation and cell survival during IR injury. The four best-characterized MAPK subfamilies are the c-Jun NH2-terminal kinase (JNK), extracellular signal- regulated kinase-1/2 (ERK1/2), p38 MAPK, and big MAPK-1 (BMK1/ERK5). Here, we review the role of MAPK activation during myocardial IR injury as it occurs during heart transplantation. Most of our current knowledge regarding MAPK activation and cardioprotection comes from studies of preconditioning and postconditioning in nontransplanted hearts. JNK and p38 MAPK activation contributes to myocardial IR injury after prolonged hypothermic storage. p38 MAPK inhibition improves cardiac function after cold storage, rewarming and reperfusion. Small-molecule p38 MAPK inhibitors have been tested clinically in patients with chronic inflammatory diseases, but not in transplanted patients, so far. Organ transplantation offers the opportunity of starting a preconditioning treatment before organ procurement or during cold storage, thus modulating early events in IR injury. Future studies will need to evaluate combined strategies including p38 MAPK and/or JNK inhibition, ERK1/2 activation, pre- or postconditioning protocols, new storage solutions, and gentle reperfusion.

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.

B lymphocytes differentially influence acute and chronic allograft rejection in mice

The relative contributions of B lymphocytes and plasma cells during allograft rejection remain unclear. Therefore, the effects of B cell depletion on acute cardiac rejection, chronic renal rejection, and skin graft rejection were compared using CD20 or CD19 mAbs. Both CD20 and CD19 mAbs effectively depleted mature B cells, and CD19 mAb treatment depleted plasmablasts and some plasma cells. B cell depletion did not affect acute cardiac allograft rejection, although CD19 mAb treatment prevented allograft-specific IgG production. Strikingly, CD19 mAb treatment significantly reduced renal allograft rejection and abrogated allograft-specific IgG development, whereas CD20 mAb treatment did not. By contrast, B cell depletion exacerbated skin allograft rejection and augmented the proliferation of adoptively transferred alloantigen-specific CD4(+) T cells, demonstrating that B cells can also negatively regulate allograft rejection. Thereby, B cells can either positively or negatively regulate allograft rejection depending on the nature of the allograft and the intensity of the rejection response. Moreover, CD19 mAb may represent a new approach for depleting both B cells and plasma cells to concomitantly impair T cell activation, inhibit the generation of new allograft-specific Abs, or reduce preexisting allograft-specific Ab levels in transplant patients.

Early innate immune events induced by prolonged cold ischemia exacerbate allograft vasculopathy

BACKGROUND

Ischemia/reperfusion induced innate immune injury is inescapable in solid organ transplantation. Prolonged cold ischemia exacerbates the primary manifestation of late graft rejection, allograft vasculopathy (AV). The relationship between prolonged cold ischemia and late graft events is unclear and the subject of this study.

METHODS

Aortic interposition transplants were performed between fully disparate mice treated with CyclosporineA. Allografts were exposed to 20 min or 60 min of cold ischemia and harvested between 1 d-6 wk. Lesion size, smooth muscle cells (SMC), neutrophils (NØ), and CD8+ T cells were quantified.

RESULTS

Early SMC loss was identical in both groups. When compared to 20 min cold ischemia, grafts exposed to 60 min exhibited greater early NØ influx, greater SMC proliferation but fewer medial SMC at 1 wk and 2 wk. Subsequently, earlier and greater CD8+ T cell infiltration were seen in the 60 min group with larger lesions at every time point.

CONCLUSIONS

These data suggest that the larger neointimal lesions in grafts exposed to 60 min cold ischemia result from enhanced early innate immune events resulting in impaired SMC recovery and subsequent increased adaptive immune response.

Neutrophil mediated smooth muscle cell loss precedes allograft vasculopathy

BACKGROUND

Cardiac allograft vasculopathy (AV) is a pathological process of vascular remodeling leading to late graft loss following cardiac transplantation. While there is consensus that AV is alloimmune mediated, and evidence that the most important alloimmune target is medial smooth muscle cells (SMC), the role of the innate immune response in the initiation of this disease is still being elucidated. As ischemia reperfusion (IR) injury plays a pivotal role in the initiation of AV, we hypothesize that IR enhances the early innate response to cardiac allografts.

METHODS

Aortic transplants were performed between fully disparate mouse strains (C3H/HeJ and C57BL/6), in the presence of therapeutic levels of Cyclosporine A, as a model for cardiac AV. Neutrophils were depleted from some recipients using anti-PMN serum. Grafts were harvested at 1,2,3,5d and 1,2wk post-transplant. Ultrastructural integrity was examined by transmission electron microscopy. SMC and neutrophils were quantified from histological sections in a blinded manner.

RESULTS

Grafts exposed to cold ischemia, but not transplanted, showed no medial SMC loss and normal ultrastructural integrity. In comparison, allografts harvested 1d post-transplant exhibited > 90% loss of SMC (p < 0.0001). SMC partially recovered by 5d but a second loss of SMC was observed at 1wk. SMC loss at 1d and 1wk post-transplant correlated with neutrophil influx. SMC loss was significantly reduced in neutrophil depleted recipients (p < 0.01).

CONCLUSIONS

These novel data show that there is extensive damage to medial SMC at 1d post-transplant. By depleting neutrophils from recipients it was demonstrated that a portion of the SMC loss was mediated by neutrophils. These results provide evidence that IR activation of early innate events contributes to the etiology of AV.

An unexpected counter-regulatory role of IL-10 in B-lymphocyte-mediated transplantation tolerance

Monoclonal antibody against the CD45RB protein induces stable transplantation tolerance to multiple types of allograft. We have previously established that this tolerance protocol relies on the regulatory function of B lymphocytes for its effect. B lymphocytes have also been reported to participate in immune regulation in several other settings. In most of these systems, the regulatory function of B lymphocytes depends on the production of IL-10. Therefore, we investigated the role of IL-10 in the anti-CD45RB model of B-cell-mediated transplantation tolerance. Surprisingly, using antibody-mediated neutralization of IL-10, IL-10-deficient recipients and adoptive transfer of IL-10-deficient B lymphocytes, we found that IL-10 actually counter-regulates tolerance induced by anti-CD45RB. Furthermore, neutralization of IL-10 reduced the development of chronic allograft vasculopathy compared to anti-CD45RB alone and reduced the production of graft reactive alloantibodies. These data suggest that the participation of regulatory B lymphocytes in transplantation tolerance may be distinct from how they operate in other systems. Identifying the specific B lymphocytes that mediate transplantation tolerance and defining their mechanism of action may yield new insights into the complex cellular network through which antigen-specific tolerance is established and maintained.