Antibody-mediated rejection of single class I MHC-disparate cardiac allografts

Failure of Costimulatory Blockade-induced Regulatory T Cells to Sustain Long-term Survival of High Ischemic Allografts

BACKGROUND

Costimulatory blockade-induced allograft tolerance has been achieved in rodent models, but these strategies do not translate well to nonhuman primate and clinical transplants. One confounder that may underlie this discrepancy is the greater ischemic inflammation imposed on the transplants. In mice, cardiac allografts subjected to prolonged cold ischemic storage (CIS) before transplant have increased ischemia-reperfusion injury, which amplifies infiltrating endogenous memory CD8 T-cell activation within hours after transplantation to mediate acute graft inflammation and cytotoxic lymphocyte-associated molecule-4 immunoglobulin-resistant rejection. This study tested strategies inhibiting memory CD8 T-cell activation within such high ischemic allografts to achieve long-term survival.

METHODS

A/J (H-2 a ) hearts subjected to 0.5 or 8 h of CIS were transplanted to C57BL/6 (H-2 b ) recipients and treatment with peritransplant costimulatory blockade. At 60 d posttransplant, regulatory T cells (Treg) were depleted in recipients of high ischemic allografts with anti-CD25 monoclonal antibody (mAb) or diphtheria toxin.

RESULTS

Whereas peritransplant (days 0 and +1) anti-lymphocyte function-associated antigen-1 mAb and anti-CD154 mAb prolonged survival of >60% allografts subjected to minimal CIS for >100 d, only 20% of allografts subjected to prolonged CIS survived beyond day 80 posttransplant and rejection was accompanied by high titers of donor-specific antibody. Peritransplant anti-lymphocyte function-associated antigen-1, anti-tumor necrosis factor-α, and anti-CD154 mAb plus additional anti-CD154 mAb on days 14 and 16 obviated this donor-specific antibody and promoted Treg-mediated tolerance and survival of 60% of high ischemic allografts beyond day 100 posttransplant, but all allografts failed by day 120.

CONCLUSIONS

These studies indicate a strategy inducing prolonged high ischemic allograft survival through Treg-mediated tolerance that is not sustained indefinitely.

Neutrophil diversity and plasticity: Implications for organ transplantation

Neutrophils, as the first defenders against external microbes and stimuli, are highly active and finely regulated innate immune cells. Emerging evidence has challenged the conventional dogma that neutrophils are a homogeneous population with a short lifespan that promotes tissue damage. Recent findings on neutrophil diversity and plasticity in homeostatic and disease states have centered on neutrophils in the circulation. In contrast, a comprehensive understanding of tissue-specialized neutrophils in health and disease is still lacking. This article will first discuss how multiomics advances have contributed to our understanding of neutrophil heterogeneity and diversification in resting and pathological settings. This discussion will be followed by a focus on the heterogeneity and role of neutrophils in solid organ transplantation and how neutrophils may contribute to transplant-related complications. The goal of this article is to provide an overview of the research on the involvement of neutrophils in transplantation, with the aim that this may draw attention to an underappreciated area of neutrophil research.

Animal Models for Heart Transplantation Focusing on the Pathological Conditions

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

The clinical impact of donor-specific antibodies on antibody-mediated rejection and long-term prognosis after heart transplantation

PURPOSE OF REVIEW

Outcomes after cardiac transplantation have improved over past decades, but long-term graft survival remains limited in part because of uncertainty regarding clinical implications of donor-specific antibodies (DSAs). The purpose of this review is to consolidate recent advances in knowledge on the topic of DSA and their potential to impact long-term prognosis after heart transplantation.

RECENT FINDINGS

The presence of persistent DSA increases the risk of poor outcome after heart transplantation, including development of antibody-mediated rejection (AMR), graft failure, cardiac allograft vasculopathy, and mortality. Importantly, different DSA vary in clinical significance. DSA capable of activating the complement cascade portend a higher risk of developing AMR. human leukocyte antigen class I and class II antigens are expressed differently within the heart, and so, clinical manifestations of class I and class II DSA vary accordingly. Further, compared with class I, class II DSA carry an increased risk of graft loss and mortality. When comparing preexisting DSA with formation of de-novo DSA, de-novo DSA are associated with worse outcome.

SUMMARY

DSAs are generally associated worse long-term prognosis after heart transplantation but vary in their clinical significance. Recognition of specific risk profiles is essential for guiding posttransplant antibody management.

The Role of Neutrophils in Transplanted Organs

Neutrophils are often viewed as nonspecialized effector cells whose presence is a simple indicator of tissue inflammation. There is new evidence that neutrophils exist in subsets and have specialized effector functions that include extracellular trap generation and the stimulation of angiogenesis. The application of intravital imaging to transplanted organs has revealed novel requirements for neutrophil trafficking into graft tissue and has illuminated direct interactions between neutrophils and other leukocytes that promote alloimmunity. Paradoxically, retaining some neutrophilia may be important to induce or maintain tolerance. Neutrophils can stimulate anti-inflammatory signals in other phagocytes and release molecules that inhibit T cell activation. In this article, we will review the available evidence of how neutrophils regulate acute and chronic inflammation in transplanted organs and discuss the possibility of targeting these cells to promote tolerance.

Natural killer cells play a critical role in mediating inflammation and graft failure during antibody-mediated rejection of kidney allografts

While the incidence of antibody-mediated kidney graft rejection has increased, the key cellular and molecular participants underlying this graft injury remain unclear. Rejection of kidney allografts in mice lacking the chemokine receptor CCR5 is dependent on production of donor-specific antibody. Here we determine if cells expressing cytotoxic function contributed to antibody-mediated kidney allograft rejection in these recipients. Wild-type C57BL/6, B6.CCR5(-/-), and B6.CD8(-/-)/CCR5(-/-) mice were transplanted with complete MHC-mismatched A/J kidney grafts, and intragraft inflammatory components were followed to rejection. B6.CCR5(-/-) and B6.CD8(-/-)/CCR5(-/-) recipients rejected kidney allografts by day 35, whereas 65% of allografts in wild-type recipients survived past day 80 post-transplant. Rejected allografts in wild-type C57BL/6, B6.CCR5(-/-), and B6.CD8(-/-)/CCR5(-/-) recipients expressed high levels of VCAM-1 and MMP7 mRNA that was associated with high serum titers of donor-specific antibody. High levels of perforin and granzyme B mRNA expression peaked on day 6 post-transplant in allografts in all recipients, but were absent in isografts. Depletion of natural killer cells in B6.CD8(-/-)/CCR5(-/-) recipients reduced this expression to background levels and promoted the long-term survival of 40% of the kidney allografts. Thus, natural killer cells have a role in increased inflammation during antibody-mediated kidney allograft injury and in rejection of the grafts.

Anti-huCD20 antibody therapy for antibody-mediated rejection of renal allografts in a mouse model

We have reported that B6.CCR5(-/-) mice reject renal allografts with high serum donor-specific antibody (DSA) titers and marked C4d deposition in grafts, features consistent with antibody-mediated rejection (AMR). B6.huCD20/CCR5(-/-) mice, where human CD20 expression is restricted to B cells, rejected A/J renal allografts by day 26 posttransplant with DSA first detected in serum on day 5 posttransplant and increased thereafter. Recipient treatment with anti-huCD20 mAb prior to the transplant and weekly up to 7 weeks posttransplant promoted long-term allograft survival (>100 days) with low DSA titers. To investigate the effect of B cell depletion at the time serum DSA was first detected, recipients were treated with anti-huCD20 mAb on days 5, 8, and 12 posttransplant. This regimen significantly reduced DSA titers and graft inflammation on day 15 posttransplant and prolonged allograft survival >60 days. However, DSA returned to the titers observed in control treated recipients by day 30 posttransplant and histological analyses on day 60 posttransplant indicated severe interstitial fibrosis. These results indicate that anti-huCD20 mAb had the greatest effect as a prophylactic treatment and that the distinct kinetics of DSA responses accounts for acute renal allograft failure versus the development of fibrosis.

Graft-derived CCL2 increases graft injury during antibody-mediated rejection of cardiac allografts

The pathogenic role of macrophages in antibody-mediated rejection (AMR) remains unclear. Monocyte chemoattractant protein-1 (MCP-1/CCL2) is a potent chemotactic factor for monocytes and macrophages. The current studies used a murine model of AMR to investigate the role of graft-derived CCL2 in AMR and how macrophages may participate in antibody-mediated allograft injury. B6.CCR5−/−/CD8−/− recipients rejected MHC-mismatched WT A/J allografts with high donor-reactive antibody titers and diffuse C4d deposition in the large vessels and myocardial capillaries, features consistent with AMR. In contrast, A/J.CCL2−/− allografts induced low donor-reactive antibody titers and C4d deposition at Day 7 posttransplant. Decreased donor-reactive CD4 T cells producing interferon gamma were induced in response to A/J.CCL2−/− versus WT allografts. Consequently, A/J.CCL2−/− allograft survival was modestly but significantly longer than A/J allografts. Macrophages purified from WT allografts expressed high levels of IL-1β and IL-12p40 and this expression and the numbers of classically activated macrophages were markedly reduced in CCL2-deficient allografts on Day 7. The results indicate that allograft-derived CCL2 plays an important role in directing classically activated macrophages into allografts during AMR and that macrophages are important contributors to the inflammatory environment mediating graft tissue injury in this pathology, suggesting CCL2 as a therapeutic target for AMR.

Characterization of transfusion-elicited acute antibody-mediated rejection in a rat model of kidney transplantation

Animal models of antibody-mediated rejection (ABMR) may provide important evidence supporting proof of concept. We elicited donor-specific antibodies (DSA) by transfusion of donor blood (Brown Norway RT1(n) ) into a complete mismatch recipient (Lewis RT1(l) ) 3 weeks prior to kidney transplantation. Sensitized recipients had increased anti-donor splenocyte IgG1, IgG2b and IgG2c DSA 1 week after transplantation. Histopathology was consistent with ABMR characterized by diffuse peritubular capillary C4d and moderate microvascular inflammation with peritubular capillaritis + glomerulitis > 2. Immunofluorescence studies of kidney allograft tissue demonstrated a greater CD68/CD3 ratio in sensitized animals, primarily of the M1 (pro-inflammatory) phenotype, consistent with cytokine gene analyses that demonstrated a predominant T helper (TH )1 (interferon-γ, IL-2) profile. Immunoblot analyses confirmed the activation of the M1 macrophage phenotype as interferon regulatory factor 5, inducible nitric oxide synthase and phagocytic NADPH oxidase 2 were significantly up-regulated. Clinical biopsy samples in sensitized patients with acute ABMR confirmed the dominance of M1 macrophage phenotype in humans. Despite the absence of tubulitis, we were unable to exclude the effects of T cell-mediated rejection. These studies suggest that M1 macrophages and TH 1 cytokines play an important role in the pathogenesis of acute mixed rejection in sensitized allograft recipients.

Increased macrophage density of cardiac allograft biopsies is associated with antibody-mediated rejection and alloantibodies to HLA antigens

BACKGROUND

Antibody-mediated rejection (AMR) is characterized histologically by intracapillary macrophages. Macrophage density may be an alternative method of determining inflammatory changes in AMR.

METHODS

We identified 118 heart transplant patients with serologic testing for HLA alloantibodies. Macrophage density was graded as 1+ (<45/mm(2)), 2+ (46-90/mm(2)), and 3+ (>90/mm(2)). Maximal macrophage density and complement staining over multiple biopsies were correlated with peak panel reactive antibodies (PRA), donor-specific antibodies (DSA), and the clinical diagnosis of AMR.

RESULTS

The presence of PRA correlated with macrophage score (p = 0.001). Macrophage density correlated with any DSA (p < 0.0001), class I DSA (p < 0.0001), class II DSA (p < 0.0001), and class II DQ (p < 0.0001). Nine patients had clinical AMR. Among patients with AMR, 89% had a biopsy over the period of AMR with ≥3+ macrophage density (89% sensitivity); among patients without AMR, 93% of patients had no biopsy at any time with ≥3+ macrophage density (specificity). There was perfect concordance between the scores of C4d positivity and macrophage density in 61% and only partial concordance in 20%, with complete discordance in 19% in biopsies taken during clinical episodes of AMR.

CONCLUSIONS

Macrophage density in allograft endomyocardial biopsies is frequently elevated during clinical episodes of AMR and correlates well with alloantibodies.

Lessons and limits of mouse models

Seminal studies in rabbits and rodent transplantation models by Peter Medawar revealed that cellular processes, rather than humoral antibodies, are central to the acute rejection of transplanted organs, and much of basic transplantation research continues to be focused on the biology and control of these cells, which were subsequently shown to be T cells. However, the success of current immunosuppression at controlling T-cell-mediated rejection has resulted in an increasing awareness of antibody-mediated rejection in the clinic. This, in turn, has fueled an emerging interest in the biology of allospecific antibodies, the B cells that produce these antibodies, and the development of mouse models that allow their investigation. Here we summarize some of the more widely used mouse models that have been developed to study the immunobiology of alloreactivity, transplantation rejection and tolerance, and used to identify therapeutic strategies that modulate these events.