Donor-specific B-cell tolerance after ABO-incompatible infant heart transplantation

Neonatal heart transplantation in the United States: Trends and outcomes

BACKGROUND

Heart transplantation in the neonatal period is associated with excellent survival. However, outcomes data are scant and have been obtained primarily from two single-center reports within the United States. We sought to analyze the outcomes of all neonatal heart transplants performed in the United States using the United Network for Organ Sharing (UNOS) dataset.

METHODS

The UNOS dataset was queried for patients who underwent infant heart transplantation from 1987 to 2021. Patients were divided into two groups based on age - neonates (<=31 days), and older infants (32 days-365 days). Demographic and clinical characteristics were analyzed and compared, along with follow up survival data.

RESULTS

Overall, 474 newborns have undergone heart transplantation in the United States since 1987. Freedom from death or re-transplantation for neonates was 63.5%, 58.8% and 51.6% at 5, 10, and 20 years, respectively. Patients in the newborn group had lower unadjusted survival compared to older infants (p < .001), but conditional 1-year survival was higher in neonates (p = .03). On multivariable analysis, there was no significant difference in survival between the two age groups (p = .43). Black race, congenital heart disease diagnosis, earlier surgical era, and preoperative mechanical circulatory support use were associated with lower survival among infant transplants (p < .05).

CONCLUSIONS

Neonatal heart transplantation is associated with favorable long-term clinical outcomes. Neonates do not have a significant survival advantage over older infants. Widespread applicability is limited by the small number of available donors. Efforts to expand the donor pool to include non-standard donor populations ought to be considered.

ABO-incompatible heart transplantation-evolution of a revolution

In the 1990s, neonates born with severe congenital heart disease faced more than 50% mortality awaiting an ABO-compatible (ABOc) transplant donor. This desperate situation, together with knowledge of gaps in the adaptive immune system in early childhood, led to the clinical exploration of intentional ABO-incompatible (ABOi) heart transplantation. In 2001, West et al. reported the first series of 10 infants in Canada. Since then, consideration of ABOi heart donors has become the standard of care for children awaiting transplantation in the first few years of life, resulting in reduced wait times and better organ utilization with noninferior post-transplant outcomes compared to ABOc recipients. This state-of-the-art review discusses the clinical development and evolution, underlying and resulting immunological aspects, current challenges, and future directions of ABOi heart transplantation.

Paediatric heart transplantation: life-saving but not yet a cure

In the 1980s, heart transplantation was the first successful treatment for infants born with hypoplastic left heart syndrome. Infants who have required heart transplantation benefit from immunologic "advantages," including long-term survival free from cardiac allograft vasculopathy. Currently ∼ 90% of children undergoing a heart transplant are reaching their first-year anniversary and the clinical practices of paediatric heart transplantation have dramatically improved. These successes are largely attributed to research sponsored by the Pediatric Heart Transplant Study Group, the International Society of Heart and Lung Transplantation and, more recently, the Non-profits Enduring Hearts and Additional Ventures. Despite these successes, the field is challenged to increase progress to achieve long-term survival into adulthood. The wait-list mortality, especially among infants, is unacceptably high often leading to palliative measures that can increase post-transplant mortality. Cardiac allograft vasculopathy remains a major cause for progressive graft loss of function and sudden death. The relative tolerance seen in immature recipients has not been translated to modifying older recipients' post-transplant outcomes. The modifiable cause(s) for the increased risks of transplantation in children of different ethnicities and races require definition. Addressing these challenges faces the reality that for-profit research favours funding adult recipients, with ∼ 10-fold greater numbers, and their more modest longevity goals. Advocacy for funding "incentives" such as the Orphan Drug rules in the United States and upholding principles of equity and inclusion are critical to addressing the challenges of paediatric heart transplant recipients worldwide.

Sex, T Cells, and the Microbiome in Natural ABO Antibody Production in Mice

BACKGROUND

"Natural" ABO antibodies (Abs) are produced without known exposure to A/B carbohydrate antigens, posing significant risks for hyperacute rejection during ABO-incompatible transplantation. We investigated anti-A "natural" ABO antibodies versus intentionally induced Abs with regard to the need for T-cell help, the impact of sex, and stimulation by the microbiome.

METHODS

Anti-A was measured by hemagglutination assay of sera from untreated C57BL/6 wild-type (WT) or T cell-deficient mice of both sexes. Human ABO-A reagent blood cell membranes were injected intraperitoneally to induce anti-A Abs. The gut microbiome was eliminated by maintenance of mice in germ-free housing.

RESULTS

Compared with WT mice, CD4 + T-cell knockout (KO), major histocompability complex-II KO, and αβ/γδ T-cell receptor KO mice produced much higher levels of anti-A nAbs; females produced dramatically more anti-A nAbs than males, rising substantially with puberty. Sensitization with human ABO-A reagent blood cell membranes did not induce additional anti-A in KO mice, unlike WT. Sex-matched CD4 + T-cell transfer significantly suppressed anti-A nAbs in KO mice and rendered mice responsive to A-sensitization. Even under germ-free conditions, WT mice of several strains produced anti-A nAbs, with significantly higher anti-A nAbs levels in females than males.

CONCLUSIONS

Anti-A nAbs were produced without T-cell help, without microbiome stimulation, in a sex- and age-dependent manner, suggestive of a role for sex hormones in regulating anti-A nAbs. Although CD4 + T cells were not required for anti-A nAbs, our findings indicate that T cells regulate anti-A nAb production. In contrast to anti-A nAbs, induced anti-A production was T-cell dependent without a sex bias.

Xenotransplantation in transition

The application of xenotransplantation of porcine organs and tissues for treatment of disease, sought for more than a century, might soon be realized. Until now, the immune response of recipients against xenogeneic organs and tissues posed the main obstacle to clinical application. However, decades of research into this immune response and identification of other molecular barriers together with advances in genetic engineering and cloning of large animals and immune therapeutics coalesced to support prolonged survival and function of porcine organ grafts in nonhuman primates. This experimental progress in turn sparks consideration of clinical trials. The papers in this special section provide authoritative views concerning the immune hurdles that still limit and potentially still preclude clinical application of xenotransplantation. Xenoreactive antibodies elicited in T cell-dependent B cell-responses constitute the most important hurdle and control of these responses impels use of intense regimens of immunosuppression. These antibodies pose a danger to xenografts and potentially compromise subsequent allografts. However, new insights into the specificity of these antibodies, the pathways and kinetics of production and genetic determinants of pathogenicity offer novel opportunities for intervention. Likewise, the rapid ability to propose and test new strategies in nonhuman primate models hastens needed advances. However further progress will depend on development and validation of laboratory methods for identification and assay of pathogenic immune responses and evaluation of the response to therapy.

Accommodation in allogeneic and xenogeneic organ transplantation: Prevalence, impact, and implications for monitoring and for therapeutics

Accommodation refers to acquired resistance of organs or tissues to immune or inflammatory reactions that might otherwise cause severe injury or rejection. As first observed in ABO-incompatible kidney transplants and heterotopic cardiac xenografts, accommodation was identified when organ transplants continued to function despite the presence of anti-graft antibodies and/or other reactants in the blood of recipients. Recent evidence suggests many and perhaps most organ transplants have accommodation, as most recipients mount B cell responses specific for the graft. Wide interest in the impact of graft-specific antibodies on the outcomes of transplants prompts questions about which mechanisms confer protection against such antibodies, how accommodation might be detected and whether and how rejection could be superimposed on accommodation. Xenotransplantation offers a unique opportunity to address these questions because immune responses to xenografts are easily detected and the pathogenic impact of immune responses is so severe. Xenotransplantation also provides a compelling need to apply these and other insights to decrease the intensity and toxicity of immunosuppression that otherwise could limit clinical application.

Case Report: Successful ABO-Incompatible Deceased Donor Kidney Transplantation in an Infant Without Pre-transplant Immunological Treatment

ABO blood group antibodies have not been generated or are at low titer during early infancy. Therefore, in theory, ABO-incompatible kidney transplantation (ABOi KT) may be successfully achieved in small infants without any pre-transplant treatment. We report here the first ABO-incompatible deceased donor kidney transplantation (ABOi DDKT) in an infant. The recipient infant was ABO blood group O, and the donor group A. The recipient was diagnosed with a Wilms tumor gene 1 (WT1) mutation and had received peritoneal dialysis for 4 months prior to transplant. At 7 months and 27 days of age, the infant underwent bilateral native nephrectomy and single-kidney transplantation from a 3-year-old brain-dead donor. No pre- or post-transplantation antibody removal treatment was performed, since the recipient's anti-iso-hemagglutinin-A Ig-M/G antibody titers were both low (1:2) before transplantation and have remained at low levels or undetectable to date. At 11 months post-transplant, the recipient is at home, thriving, with normal development and graft function. This outcome suggests that ABOi DDKT without antibody removal preparatory treatment is feasible in small infants, providing a new option for kidney transplantation in this age range.

Characterization of ABH-subtype donor-specific antibodies in ABO-A-incompatible kidney transplantation

ABO-incompatible (ABOi) transplantation requires preemptive antibody reduction; however, the relationship between antibody-mediated rejection (AMR) and ABO-antibodies, quantified by hemagglutination (HA), is inconsistent, possibly reflecting variable graft resistance to AMR or HA assay limitations. Using an ABH-glycan microarray, we quantified ABO-A antigen-subtype (A-subtype)-specific IgM and IgG in 53 ABO-O recipients of ABO-A kidneys, before and after antibody removal (therapeutic plasma exchange [TPE] or ABO-A-trisaccharide immunoadsorption [IA]) and 1-year posttransplant. IgM binding to all A-subtypes correlated highly (R²  ≥ .90) and A-subtype antibody specificities was reduced equally by IA versus TPE. IgG binding to the A-subtypes (II-IV) expressed in kidney correlated poorly (.27 ≤ R²  ≤ .69). Reduction of IgG specific to A-subtype-II was equivalent for IA and TPE, whereas IgG specific to A-subtypes-III/IV was not as greatly reduced by IA (p < .005). One-year posttransplant, IgG specific to A-II remained the most reduced antibody. Immunostaining revealed only A-II on vascular endothelium but A-subtypes II-III/IV on tubular epithelium. These results show that ABO-A-trisaccharide is sufficient for IgM binding to all A-subtypes; this is true for IgG binding to A-II, but not subtypes-III/IV, which exhibits varying degrees of specificity. We identify A-II as the major, but importantly not the sole, antigen relevant to treatment and immune modulation in adult ABO-A-incompatible kidney transplantation.

Current opinion in pediatric heart transplantation

PURPOSE OF REVIEW

Cardiac transplant remains the most effective therapy for children with end-stage heart disease. Outcomes remain better than any alternative therapy for this condition, but its use is limited by donor organ availability. As a result, waitlist times and mortality on the waiting list remain unacceptably high. Novel approaches are necessary to address this problem.

RECENT FINDINGS

Organ Procurement and Transplant Network/United Network for Organ Sharing readjusted the pediatric heart allocation system in 2016 to prioritize children at highest risk of mortality, encourage judicious listing, and improve appropriate donor organ utilization. Subsequent studies have aligned with these priorities to help risk-stratify patients at the time of listing and identify the importance that should be assigned to donor-specific factors. In addition, many authors are advocating for increased utilization of hearts donated after cardiac death. Pediatric Ventricular Assist Device (VAD) application has also been increasing to help decrease waitlist mortality. Although results have significantly improved, there remain important limitations to widespread use of VADs in the pediatric population. This has prompted novel techniques such as pulmonary artery banding to improve cardiac function and, in some cases, promote recovery. The demand for cardiac replacement continues to increase with an ageing population of patients with congenital heart disease, presenting new challenges and stressors to the system.

SUMMARY

Pediatric cardiac transplant outcomes are excellent but remain plagued by the limited supply of donor organs. Recent strategies to combat this problem have focused on judicious listing, maximal utilization of available donor organs, and safely extending the lives of patients on the waitlist. New demands on the organ supply chain will continue to stress the system, making these efforts of the highest importance.Clinical Trial Registry Number not applicable.

Clinical outcomes of children receiving ABO-incompatible versus ABO-compatible heart transplantation: a multicentre cohort study

BACKGROUND

ABO-incompatible heart transplantation increases donor availability in young children and is evolving into standard of care in children younger than 2 years. Previous smaller studies suggest similar outcomes to ABO-compatible heart transplantation, but persisting alterations of the immune system in ABO-incompatible recipients might increase the risk of some infections or benefit the graft owing to reduced HLA reactivity. We aimed to assess long-term outcomes in young children after they received ABO-incompatible or ABO-compatible heart transplantation.

METHODS

In this multicentre, prospective cohort study, we analysed data from the Pediatric Heart Transplant Society registry to compare children who received ABO-incompatible or ABO-compatible heart transplantation before age 2 years between Jan 1, 1999, and June 30, 2018. Given significantly different clinical demographics between the two groups, we also matched each ABO-incompatible recipient to two ABO-compatible recipients using propensity score matching. We assessed patient and graft survival, coronary allograft vasculopathy, malignancy, acute rejection (any episode resulting in augmentation of immunosuppression), and infections (requiring intravenous antibiotic or antiviral therapy or life-threatening infections treated with oral therapy).

FINDINGS

We included 2206 children who received a heart transplant before age 2 years, with 11 332·6 patient-years of cumulative observation time. Children who received an ABO-incompatible transplant (n=364) were younger and a larger proportion had congenital heart disease and ventilator and mechanical circulatory support than the ABO-compatible recipients (n=1842). After matching, only differences in blood group (more O in ABO-incompatible and more AB in ABO-compatible groups) and use of polyclonal induction therapy with anti-thymocyte globulins persisted. The two matched groups had similar post-transplantation graft survival (p=0·74), freedom from coronary allograft vasculopathy (p=0·75), and malignancy (p=0·51). ABO-incompatible recipients showed longer freedom from rejection (p=0·0021) in the overall cohort, but not after matching (p=0·48). Severe infections (p=0·0007), bacterial infections (p=0·0005), and infections with polysaccharide encapsulated bacteria (p=0·0005) that share immunological properties with blood group antigens occurred less frequently after ABO-incompatible heart transplantation.

INTERPRETATION

ABO-incompatible heart transplantation for children younger than 2 years is a clinically safe approach, with similar survival and incidences of rejection, coronary allograft vasculopathy, and malignancy to ABO-compatible recipients, despite higher-risk pre-transplant profiles. ABO-incompatible transplantation was associated with less bacterial infection, particularly encapsulated bacteria, suggesting that the acquired immunological changes accompanying ABO tolerance might benefit rather than jeopardise transplanted children.

FUNDING

Pediatric Heart Transplant Society.

The immune system in infants: Relevance to xenotransplantation

Despite the improvement in surgical interventions in the treatment of congenital heart disease, many life-threatening lesions (eg, hypoplastic left heart syndrome) ultimately require transplantation. However, there is a great limitation in the availability of deceased human cardiac donors of a suitable size. Hearts from genetically engineered pigs may provide an alternative source. The relatively immature immune system in infants (eg, absence of anti-carbohydrate antibodies, reduced complement activation, reduced innate immune cell activity) should minimize the risk of early antibody-mediated rejection of a pig graft. Additionally, recipient thymectomy, performed almost routinely as a preliminary to orthotopic heart transplantation in this age-group, impairs the T-cell response. Because of the increasing availability of genetically engineered pigs (eg, triple-knockout pigs that do not express any of the three known carbohydrate antigens against which humans have natural antibodies) and the ability to diagnose congenital heart disease during fetal life, cardiac xenotransplantation could be preplanned to be carried out soon after birth. Because of these several advantages, prolonged graft survival and even the induction of tolerance, for example, following donor-specific pig thymus transplantation, are more likely to be achieved in infants than in adults. In this review, we summarize the factors in the infant immune system that would be advantageous in the success of cardiac xenotransplantation in this age-group.

ABO-incompatible heart transplantation in children-a systematic review of current practice

Pediatric heart transplantation has significantly improved in the survival of children with cardiomyopathy and/or complex congenital heart defects. With the increasing number of children needing transplantation, there is a growing demand for the organ, making it harder to cope with the increasing number of children on the waiting list. One of the advances that helped reduce the waiting list mortality significantly is the ability to transplant children from donors with ABO incompatibility. Modification of perfusion abilities and management of donor organ improves outcome in this select population, making ABO-incompatible transplantation an attractive option in the wider armamentarium available for pushing boundaries in these children without impacting on outcomes.

The five dimensions of B cell tolerance

B cell tolerance has been generally understood to be an acquired property of the immune system that governs antibody specificity in ways that avoid auto-toxicity. As useful as this understanding has proved, it fails to fully explain the existence of auto-reactive specificities in healthy individuals and contribution these may have to health. Mechanisms underlying B cell tolerance are considered to select a clonal repertoire that generates a collection of antibodies that do not bind self, ie tolerance operates more or less in three dimensions that largely spare autologous cells and antigens. Yet, most B lymphocytes in humans and probably in other vertebrates are auto-reactive and absence of these auto-reactive B cells is associated with disease. We suggest that auto-reactivity can be embodied by extending the concept of tolerance by two further dimensions, one of time and circumstance and one that allows healthy cells to actively resist injury. In this novel concept, macromolecular recognition by the B cell receptor leading to deletion, anergy, receptor editing or B cell activation is extended by taking account of the time of development of normal immune responses (4th dimension) and the accommodation (or tolerance) of normal cells to bound antibody, activation of complement, and interaction with inflammatory cells (fifth dimension). We discuss how these dimensions contribute to understanding B cell biology in health or disease.

Donor-specific anti-HLA antibody production following pediatric ABO-incompatible heart transplantation

ABO-i heart transplantation can be performed in infants with end-stage heart failure to increase organ availability. The development of newly detected DSAs is associated with decreased cardiac graft survival, and the effect of ABO-i transplantation on DSA production is unknown. We examined DSA production and rejection frequency in infant recipients of ABO-i and ABO-c heart transplants via a retrospective cohort study of infant heart transplant recipients transplanted at a single pediatric center between January 2004 and November 2014. Patients were included if they were less than 1 year of age at transplant and had a minimum of 6 months follow-up. DSA positivity was examined under two categories, either the lowest level detectable (MFI > 500) or a level presumed to have clinical relevance in our immunogenetics laboratory (MFI > 5000). Of 52 patients, 36 received ABO-c transplants and 16 received ABO-i transplants. Compared to ABO-c recipients, the ABO-i group showed a consistent but statistically non-significant finding of less frequent ndDSA positivity (69.4% ABO-c vs 43.8% ABO-i with MFI >500, P = 0.122; 41.7% ABO-c vs 25% ABO-i with MFI >5000, P = 0.353). Additionally, ABO-i patients were less likely to have any form of rejection (12.5% vs 47.2%, P = 0.027) or acute cellular rejection (6.3% vs 38.9%, P = 0.021). Our data suggest that infants receiving ABO-i heart transplants may be less likely to develop ndDSAs or have rejection compared to same age ABO-c recipients. Larger multicenter studies are needed to confirm results from this single center study.

Low Titers of Antidonor ABO Antibodies After ABO-Incompatible Living Donor Liver Transplantation: A Long-Term Follow-Up Study

Background

The ABO blood-type barrier in kidney and liver transplantation has been overcome by aggressive treatments such as B cell depletion using rituximab. However, the long-term effects of ABO-incompatible liver transplantation (ABO-I LTx) on immunological status have not previously been studied. Here, we assessed whether long-term immune hyporesponsiveness against ABO blood-group antigens was retained.

Methods

We recruited 81 patients, 75 patients who had survived ABO-I LTx without retransplantation and 6 patients who had survived after retransplantation using blood type-compatible grafts. The time between ABO-I LTx and outpatient visits for blood sampling for this study ranged from 1.1 to 16.8 years. We also evaluated patients' backgrounds and postoperative therapies.

Results

Overall, antidonor ABO antibody titers in the 75 patients without retransplantation decreased during long-term follow-up. In the subset of 40 patients with blood type O, anti-nondonor ABO antibody titers did not decrease and were significantly higher than antidonor ABO antibody titers. In addition, long-term antidonor ABO antibody titers were significantly lower in pediatric patients than in adult patients. In the 6 patients who were retransplanted with blood type–compatible grafts, antidonor ABO antibody immunoglobulin G titers remained low, but IgM titers increased slightly long after removal of the ABO-incompatible graft.

Conclusions

These findings suggest that donor-specific hyporesponsiveness remains after ABO-I LTx, particularly in pediatric patients. Long-term persistence of blood antigens may contribute to this donor-specific hyporesponsiveness.

Toward a solution for cardiac failure in the newborn

The newborn infant with severe cardiac failure owed to congenital structural heart disease or cardiomyopathy poses a daunting therapeutic challenge. The ideal solution for both might be cardiac transplantation if availability of hearts was not limiting and if tolerance could be induced, obviating toxicity of immunosuppressive therapy. If one could safely and effectively exploit neonatal tolerance for successful xenotransplantation of the heart, the challenge of severe cardiac failure in the newborn infant might be met. We discuss the need, the potential for applying neonatal tolerance in the setting of xenotransplantation and the possibility that other approaches to this problem might emerge.

Pediatric Outcomes in Transplant: PersOnaliSing Immunosuppression To ImproVe Efficacy (POSITIVE Study): The Collaboration and Design of a National Transplant Precision Medicine Program

Background

Despite age-related differences in biology, physiology, and behavior, transplant immunosuppression is not tailored by age. This likely contributes to high graft failure and posttransplant complications. We present the aims, design, and methods of the Pediatric Outcomes in Transplant: PersOnaliSing Immunosuppression To ImproVe Efficacy Study aimed at personalizing posttransplant immunosuppression in children and young adults.

Methods

In this prospective observational cohort study, we recruited pediatric and young adult solid organ transplant, pediatric allogeneic hematopoietic stem cell transplant recipients, and matched living and deceased organ donors from 14 transplant centers across Canada. Clinical data, questionnaires, biospecimens, and pharmacy records were collected at serial time points: (1) to identify genetic and host immune factors that influence immunosuppression dose requirements across different ages and transplant types, (2) to identify viral-host interactions that increase susceptibility to Epstein-Barr virus infection, and (3) to define care processes and structures associated with medication adherence in adolescents and young adults.

Results

From 2015 to 2018, 1662 new and prevalent transplant recipients were screened, 1166 were recruited for the various aims, including 370 liver, 445 kidney, 277 heart, 19 lung, 19 multiple, and 36 hematopoietic stem cell transplant transplants. Twelve percent were younger than 2 years, 30% were 2 to 10 years, 42% were 10 to 18 years, and 16% were 18 to 24 years at enrollment. Nine hundred thirty-one consented to participation in aims 1 and 2 (90% consent rate), 287 to aim 3 (82% consent rate). Biospecimens collected included 898 for DNA, 276 for immunoassays, and 717 for biomarker studies. Seventy percent participants have completed follow-up; 30% are pending study completion.

Conclusions

The design of this national multicenter cross-organ network helped maximize recruitment of a large patient cohort for studying age and organ-related differences in immunosuppression needs that would not otherwise be feasible. Leveraging the unique clinical, biological, environmental, and behavioral characteristics of this cohort will help develop precision medicine strategies for individualizing posttransplant immunosuppression.

Allergies and autoimmune disorders in children after heart transplantation

Pediatric heart transplantation requires lifelong immune suppression and may require thymectomy, both of which alter T-cell repertoires. We hypothesized that atopic and autoimmune diseases are more common in pediatric heart transplant patients than the general population, and that transplantation in early childhood increases the risk of development or worsening of atopic or autoimmune disease. A cross-sectional single-center study including 21 heart transplant patients aged ≤18 years was conducted. Data collected included age at transplant, induction, thymectomy, and development and severity of atopic or autoimmune disease. A majority (67%) reported having any atopic disease post-transplant, all of whom reported onset or worsening post-transplantation. Thymectomized patients were significantly more likely to have asthma (P = 0.018) and report asthma worsening post-transplant (P = 0.045). Patients with worsening of asthma post-transplant were transplanted at a significantly younger age (P = 0.040). ABO incompatible and ABO compatible recipients presented similarly. Anemia was common (38%) but not always clearly of autoimmune origin. Atopic diseases are common in children following heart transplantation: Compared to the general population, there is a higher prevalence of eczema (43% vs 11%) and asthma (33% vs 9%). Both thymectomy and younger age at transplant are associated with atopic disorders, possibly due to altered T-cell repertoires.

An approach to induction of tolerance to pig cardiac xenografts in neonates

There is a continuing need for donor hearts for infants with complex congenital heart defects. The transplantation of hearts from neonatal pigs would be an alternative to human organs, particularly if donor-specific immunological tolerance could be achieved. The great majority of infant humans do not make natural (preformed) antibodies against triple-knockout (TKO) pigs (that do not express any of the three known pig antigens against which humans have natural anti-pig antibodies). The transplantation of a heart from a TKO pig into an infant would therefore minimize any risk of early antibody-mediated rejection, and, with adequate immunosuppressive therapy, prolonged graft survival may well be achieved. Total host thymectomy (commonly carried out at the time of orthotopic heart transplantation in this age group) ± residual T-cell depletion and donor-specific pig thymus tissue transplantation might induce T-cell tolerance and allow immunosuppressive therapy to be discontinued (if there is in vitro evidence of T-cell and B-cell nonresponsiveness to donor-specific pig cells). Even if tolerance were not achieved, with continuing immunosuppressive therapy, the graft would likely "bridge" the patient until a suitable allograft became available or be associated with prolonged xenograft function.

The Risk of Failure With HLA Mismatch and Recipient Age in First Pediatric (<18 years) Kidney Transplants

BACKGROUND

Even in the modern era of kidney transplantation with improved surgical techniques, immunosuppression, and clinical care, HLA matching has been shown to be important in allograft survival in adults who receive an organ from either a deceased or living donor. We now explore the impact of genetic matching in pediatric first-kidney transplants.

METHODS

Using the United Network for Organ Sharing data, we identified 18 602 first pediatric (<18 years) kidney transplants between October 1, 1987, and December 31, 2016. Recipients were classified by number of HLA mismatches and donor origin. Cox proportional hazard analyses, adjusting for recipient and donor transplant covariates, were performed to study the impact of HLA on kidney allograft survival.

RESULTS

For the fully adjusted Cox model there was a 30% increase in the hazard of allograft failure for 1 HLA mismatch, when compared with 0 mismatched recipients, and a 92% increase in risk for 6 mismatches. Although pediatric allografts from living donors survive as long or longer than those from deceased persons, they have a higher hazard of failure as a function of HLA mismatch. Kidney allografts from deceased donors HLA mismatched 0 to 3 were found to survive as long as organs from living donors HLA mismatched 4 to 6. In the full Cox model, there was a strong, linear effect on the hazard of allograft failure with quartile of age such that the youngest patients at age of transplant had the longest surviving grafts.

CONCLUSIONS

HLA plays an important role in the survival of first pediatric kidney transplants. The better the match, and the earlier the transplant is performed in the child's life, the lower is the risk that the organ will fail.

Accommodation and related conditions in vascularized composite allografts

PURPOSE OF REVIEW

The outcome of vascularized composite allografts (VCA) often appear unrelated to the presence of donor-specific antibodies (DSA) in blood of the recipient or deposition of complement in the graft. The attenuation of injury and the absence of rejection in other types of grafts despite manifest donor-specific immunity have been explained by accommodation (acquired resistance to immune-mediated injury), adaptation (loss of graft antigen) and/or enhancement (antibody-mediated antigen blockade). Whether and how accommodation, adaptation and/or enhancement impact on the outcome of VCA is unknown. Here we consider how recent observations concerning accommodation in organ transplants might advance understanding and resolve uncertainties about the clinical course of VCA.

RECENT FINDINGS

Investigation of the mechanisms through which kidney allografts avert antibody-mediated injury and rejection provide insights potentially applicable to VCA. Interaction of DSA can facilitate replacement of donor by recipient endothelial cells, modulate or decrease synthesis of antigen, mobilize antigen that in turn blocks further immune recognition and limit the amount of bound antibody, allowing accommodation to ensue. These processes also can explain the apparent dissociation between the presence and levels of DSA in blood, deposition of C4d in grafts and antibody-mediated rejection. Over time the processes might also explain the inception of chronic graft changes.

SUMMARY

The disrupted tissue in VCA and potential for repopulation by endothelial cells of the recipient establish conditions that potentially decrease susceptibility to acute antibody-mediated rejection. These conditions include clonal suppression of donor-specific B cells, and adaptation, enhancement and accommodation. This setting also potentially highlights heretofore unrecognized interactions between these 'protective' processes.

Construction of Multivalent Homo- and Heterofunctional ABO Blood Group Glycoconjugates Using a Trifunctional Linker Strategy

The design and synthesis of multivalent ligands displaying complex oligosaccharides is necessary for the development of therapeutics, diagnostics, and research tools. Here, we report an efficient conjugation strategy to prepare complex glycoconjugates with 4 copies of 1 or 2 separate glycan epitopes, providing 4-8 carbohydrate residues on a tetravalent poly(ethylene glycol) scaffold. This strategy provides complex glycoconjugates that approach the size of glycoproteins (15-18 kDa) while remaining well-defined. The synthetic strategy makes use of three orthogonal functional groups, including a reactive N-hydroxysuccinimide (NHS)-ester moiety on the linker to install the first carbohydrate epitope via reaction with an amine. A masked amine functionality on the linker is revealed after the removal of a fluorenylmethyloxycarbonyl (Fmoc)-protecting group, allowing the attachment to the NHS-activated poly(ethylene glycol) (PEG) scaffold. An azide group in the linker was then used to incorporate the second carbohydrate epitope via catalyzed alkyne-azide cycloaddition. Using a known tetravalent PEG scaffold (PDI, 1.025), we prepared homofunctional glycoconjugates that display four copies of lactose and the A-type II or the B-type II human blood group antigens. Using our trifunctional linker, we expanded this strategy to produce heterofunctional conjugates with four copies of two separate glycan epitopes. These heterofunctional conjugates included Neu5Ac, 3'-sialyllactose, or 6'-sialyllactose as a second antigen. Using an alternative strategy, we generated heterofunctional conjugates with three copies of the glycan epitope and one fluorescent group (on average) using a sequential dual-amine coupling strategy. These conjugation strategies should be easily generalized for conjugation of other complex glycans. We demonstrate that the glycan epitopes of heterofunctional conjugates engage and cluster target B-cell receptors and CD22 receptors on B cells, supporting the application of these reagents for investigating cellular response to carbohydrate antigens of the ABO blood group system.

Liver-inclusive intestinal transplantation results in decreased alloimmune-mediated rejection but increased infection

Background and aims

A co-transplanted liver allograft has been thought to protect other organs from rejection-mediated injury; however, detailed analyses of co-transplanted liver on intestinal allograft outcomes have not been conducted to date. The aim of the study was to compare immune-mediated injury, causes of graft failure and clinical outcomes between recipients who underwent either a liver-inclusive intestinal transplant (LITx) or liver-exclusive intestinal transplant (LETx).

Methods

Between May 2000 and May 2010, 212 adult patients undergoing LITx (n =76) and LETx (n =136) were included. LITx underwent either liver combined intestinal or full multivisceral transplantation. LETx underwent either isolated intestinal or modified multivisceral transplantation.

Results

During 44.9 ± 31.4 months of follow-up, death-censored intestinal graft survival was significantly higher for LITx than LETx (96.9%, 93.2% and 89.9% vs 91.4%, 69.3% and 60.0% at 1, 3 and 5 years; p =0.0001). Incidence of graft loss due to rejection was higher in LETx than in LITx (30.9% vs 6.6%; p <0.0001), while infection was the leading cause of graft loss due to patient death in LITx (25.0% vs 5.1%; p <0.0001). Despite similar immunosuppression, the average number (0.87 vs 1.42, p =0.02) and severity of acute cellular rejection episode (severe grade: 7.9% vs 21.3%; p =0.01) were lower in LITx than in LETx. Incidence of acute antibody-mediated rejection was also significantly lower in LITx than in LETx (3.6% vs 15.2%; p =0.03). Incidence of chronic rejection was reduced in LITx (3.9% vs 24.3%; p =0.0002).

Conclusions

Intestinal allografts with a liver component appear to decrease risk of rejection but increase risk of infection. Our findings emphasize that LITx has characteristic immunologic and clinical features. Lower immunosuppression may need to be considered for patients who undergo LITx to attenuate increased risk of infection.

Human Bocavirus Type-1 Capsid Facilitates the Transduction of Ferret Airways by Adeno-Associated Virus Genomes

Human bocavirus type-1 (HBoV1) has a high tropism for the apical membrane of human airway epithelia. The packaging of a recombinant adeno-associated virus 2 (rAAV2) genome into HBoV1 capsid produces a chimeric vector (rAAV2/HBoV1) that also efficiently transduces human airway epithelia. As such, this vector is attractive for use in gene therapies to treat lung diseases such as cystic fibrosis. However, preclinical development of rAAV2/HBoV1 vectors has been hindered by the fact that humans are the only known host for HBoV1 infection. This study reports that rAAV2/HBoV1 vector is capable of efficiently transducing the lungs of both newborn (3- to 7-day-old) and juvenile (29-day-old) ferrets, predominantly in the distal airways. Analyses of in vivo, ex vivo, and in vitro models of the ferret proximal airway demonstrate that infection of this particular region is less effective than it is in humans. Studies of vector binding and endocytosis in polarized ferret proximal airway epithelial cultures revealed that a lack of effective vector endocytosis is the main cause of inefficient transduction in vitro. While transgene expression declined proportionally with growth of the ferrets following infection at 7 days of age, reinfection of ferrets with rAAV2/HBoV1 at 29 days gave rise to approximately 5-fold higher levels of transduction than observed in naive infected 29-day-old animals. The findings presented here lay the foundation for clinical development of HBoV1 capsid-based vectors for lung gene therapy in cystic fibrosis using ferret models.

Acquired Downregulation of Donor-Specific Antibody Production After ABO-Incompatible Kidney Transplantation

The mechanism of long-term B cell immunity against donor blood group antigens in recipients who undergo ABO-incompatible (ABOi) living-donor kidney transplantation (LKTx) is unknown. To address this question, we evaluated serial anti-A and anti-B antibody titers in 50 adult recipients. Donor-specific antibody titers remained low (≤1:4) in 42 recipients (84%). However, antibodies against nondonor blood group antigens were continuously produced in recipients with blood type O. We stimulated recipients' peripheral blood mononuclear cells in vitro to investigate whether B cells produced antibodies against donor blood group antigens in the absence of graft adsorption in vivo. Antibodies in cell culture supernatant were measured using specific enzyme-linked immunosorbent assays (ELISAs). Thirty-five healthy volunteers and 57 recipients who underwent ABO-compatible LKTx served as controls. Antibody production in vitro against donor blood group antigens by cells from ABOi LKTx patients was lower than in the control groups. Immunoglobulin deposits were undetectable in biopsies of grafts of eight recipients with low antibody titers (≤1:4) after ABOi LKTx. One patient with blood type A1 who received a second ABOi LKTx from a type B donor did not produce B-specific antibodies. These findings suggest diminished donor-specific antibody production function in the setting of adult ABOi LKTx.

ABO-incompatible heart transplantation

PURPOSE OF REVIEW

ABO-incompatible (ABOi) heart transplantation (HTx) in young children has evolved from an experimental approach to a standard allocation option in many countries. Clinical and immunological research in ABOi transplantation has revealed insight into the immature immune system and its role in superior graft acceptance in childhood and antigen-specific tolerance.

RECENT FINDINGS

Multicenter experience has confirmed equal actuarial survival, freedom from rejection, and graft vasculopathy comparing ABOi with ABO-compatible HTx. Observations of reduced antibody production and B-cell immunity toward the donor blood group have been confirmed in long-term follow-up. Mechanisms contributing to tolerance in this setting involve the interplay between B-cells and the complement system and the development of B-cell memory. Better characterization of the ABH polysaccharide antigens has improved diagnostic methods and clinical assessment of blood group antibodies. Boundaries regarding age, immune maturity, and therapeutic interventions to extend the applicability of ABOi HTx have been explored and resulted in data that may be useful for HTx patients beyond infancy and ABOi transplantation of other organs. Tolerance of ABH antigens possibly extends to HLA response.

SUMMARY

The review provides insight into the clinical evolution of ABOi HTx and associated immunologic discoveries. Current experiences and boundaries are discussed together with recent and potential future developments for utilization in other patient and age groups.

Pediatric Heart Transplantation across ABO Blood Type Barriers: A Case Study

Heart transplantation with ABO blood type–incompatible donors has historically been contraindicated because of the high risk of an immediate hyperacute humoral graft rejection. The immature neonatal immune system presents an immunologic window that allows for breaching the ABO barrier before the natural development of anti-ABO antibodies. Information from a small series of neonates has demonstrated similar survival rates and posttransplant outcomes compared to ABO-compatible transplantations. In the posttransplant period, particular attention is placed on the surveillance of graft-specific antibody production and monitoring for immunologic signs and symptoms of early graft vasculopathy. This article presents a case study of a neonate with congenital heart disease who underwent one of the first successful ABO-incompatible heart transplantations in the United States.

Neonatal tolerance: applicability to solid organ transplantation

PURPOSE OF REVIEW

The phenomenon of tolerance induced during immunologic immaturity has been explored for more than 60 years. Although direct application of neonatal tolerance to organ transplantation in human newborns is limited, exploiting discrete components of neonatal immaturity is proving fruitful.

RECENT FINDINGS

Two reviews comprehensively considered features and impact of neonatal tolerance as described in the 1950s. Recent imaging studies in mice demonstrated complex functional interactions especially of donor regulatory T cells with emerging neonatal immune components. The propensity of the developing immune system toward tolerance rather than immunity to non-self carbohydrates in ABO-incompatible transplantation was shown using glyconanotechnology tools to have exquisite specificity, and is associated with age-related changes in the B-cell compartment and complement components. Discarded infant thymus was found to be a source of abundant therapeutic regulatory T cells. Erythroid precursors transiently present in newborn mice and humans were shown to have immunosuppressive properties that may contribute to a tolerogenic environment.

SUMMARY

Neonatal tolerance has profound impact on immunology well beyond transplantation. Continued exploration of mechanisms underlying the malleability of the developing immune system and exploitation of particular components are leading to tools for immune manipulation beyond infancy.

Monitoring alloimmune response in kidney transplantation

Currently, immunosuppressive therapy in kidney transplant recipients is generally performed by protocols and adjusted according to functional or histological evaluation of the allograft and/or signs of drug toxicity or infection. As a result, a large fraction of patients are likely to receive too much or too little immunosuppression, exposing them to higher rates of infection, malignancy and drug toxicity, or increased risk of acute and chronic graft injury from rejection, respectively. Developing reliable biomarkers is crucial for individualizing therapy aimed at extending allograft survival. Emerging data indicate that many assays, likely used in panels rather than single assays, have potential to be diagnostic and predictive of short and also long-term outcome. While numerous cross-sectional studies have found associations between the results of these assays and the presence of clinically relevant post-transplantation outcomes, data from prospective studies are still scanty, thereby preventing widespread implementation in the clinic. Of note, some prospective, randomized, multicenter biomarker-driven studies are currently on-going aiming at confirming such preliminary data. These works as well as other future studies are highly warranted to test the hypothesis that tailoring immunosuppression on the basis of results offered by these biomarkers leads to better outcomes than current standard clinical practice.

ABH-Glycan Microarray Characterizes ABO Subtype Antibodies: Fine Specificity of Immune Tolerance After ABO-Incompatible Transplantation

Organ transplantation from ABO blood group-incompatible (ABOi) donors requires accurate detection, effective removal and subsequent surveillance of antidonor antibodies. Because ABH antigen subtypes are expressed differently in various cells and organs, measurement of antibodies specific for the antigen subtypes in the graft is essential. Erythrocyte agglutination, the century-old assay used clinically, does not discriminate subtype-specific ABO antibodies and provides limited information on antibody isotypes. We designed and created an ABO-glycan microarray and demonstrated the precise assessment of both the presence and, importantly, the absence of donor-specific antibodies in an international study of pediatric heart transplant patients. Specific IgM, IgG, and IgA isotype antibodies to nonself ABH subtypes were detected in control participants and recipients of ABO-compatible transplants. Conversely, in children who received ABOi transplants, antibodies specific for A subtype II and/or B subtype II antigens-the only ABH antigen subtypes expressed in heart tissue-were absent, demonstrating the fine specificity of B cell tolerance to donor/graft blood group antigens. In contrast to the hemagglutination assay, the ABO-glycan microarray allows detailed characterization of donor-specific antibodies necessary for effective transplant management, representing a major step forward in precise ABO antibody detection.

Some considerations on the current debate about typing resolution in solid organ transplantation

Background

The shortage of suitable organs and achieved tolerance are uncontested main concerns in transplantation. Long waiting lists for deceased donors and limited numbers of living donors are the current scenarios. Kidney grafts from living donors have better overall survival compared to cadaveric and require less aggressive immunosuppressive regimens. The human leukocyte antigen (HLA) labs have the key role to test the recipient and donors compatibility based on typing and antibody profile. The current standard molecular procedure in solid organ transplantation is low-resolution typing, at the antigen level.

Main text

In this commentary, the merits of high versus low degree of typing resolution in solid organ transplantation are discussed. Critical questions and reasons to bring high-resolution typing as a routine test in health system are considered. Specifically, with the introduction of the next-generation sequencing (NGS) in HLA, the pros and cons in living donation and benefits after deceased donation are critically evaluated.

Conclusion

NGS has the potential to improve the transplant rates and the overall graft survival. Alternative strategies to increase in demanding the number of transplants are briefly highlighted.

B cells in transplantation

B cell responses underlie the most vexing immunological barriers to organ transplantation. Much has been learned about the molecular mechanisms of B cell responses to antigen and new therapeutic agents that specifically target B cells or suppress their functions are available. Yet, despite recent advances, there remains an incomplete understanding about how B cell functions determine the fate of organ transplants and how, whether or when potent new therapeutics should optimally be used. This gap in understanding reflects in part the realization that besides producing antibodies, B cells can also regulate cellular immunity, contribute to the genesis of tolerance and induce accommodation. Whether non-specific depletion of B cells, their progeny or suppression of their functions would undermine these non-cognate functions and whether graft outcome would suffer as a result is unknown. These questions were discussed at a symposium on "B cells in transplantation" at the 2015 ISHLT annual meeting. Those discussions are summarized here and a new perspective is offered.

Conjugation of A and B Blood Group Structures to Silica Microparticles for the Detection of Antigen-Specific B Cells

Silica microparticles were functionalized with A and B blood group carbohydrate antigens (A type I, A type II, B type I, and B type II) to enable the detection and monitoring of ABO antigen-specific B cells. Microparticles were prepared via the Stöber synthesis, labeled with an Alexafluor fluorescent dye, and characterized via TEM and fluorescence microscopy. The silica microparticles were functionalized with (3-aminopropyl)trimethoxysilane (APTMS), followed by the use of an established fluorenylmethyloxycarbonyl (Fmoc)-protected PEG-based linker. The terminal Fmoc moiety of the PEG-based linker was then deprotected, yielding free amino groups, to which the A and B antigens were coupled. The carbohydrate antigens were synthesized with a p-nitrophenol ester to enable conjugation to the functionalized silica microparticles via an amide bond. The number of free amine groups available for coupling for a given mass of PEG-functionalized silica microparticles was quantified via reaction with Fmoc-glycine. The antigen-functionalized microparticles were then evaluated for their specificity in binding to A and B antigen-reactive B-cells via flow cytometry, and for blocking of naturally occurring antibodies in human serum. Selective binding of the functionalized microparticles to blood group-reactive B cells was observed by flow cytometry and fluorescence microscopy. The modular approach outlined here is applicable to the preparation of silica microparticles containing any carbohydrate antigen and alternative fluorophores or labels. This approach therefore comprises a novel, general platform for screening B cell populations for binding to carbohydrate antigens, including, in this case, the human A and B blood group antigens.

Microbiota Control of Malaria Transmission

Stable mutualistic interactions between multicellular organisms and microbes are an evolutionarily conserved process with a major impact on host physiology and fitness. Humans establish such interactions with a consortium of microorganisms known as the microbiota. Despite the mutualistic nature of these interactions, some bacterial components of the human microbiota express immunogenic glycans that elicit glycan-specific antibody (Ab) responses. The ensuing circulating Abs are protective against infections by pathogens that express those glycans, as demonstrated for Plasmodium, the causative agent of malaria. Presumably, a similar protective Ab response acts against other vector-borne diseases.

Development of B-cell memory in early childhood and the impact on antigen-specific tolerance after heart transplantation

BACKGROUND

Young children show better survival after heart transplant compared with older individuals and can receive heart transplants safely from ABO-incompatible donors. Children develop immunologic tolerance to donor ABH antigens reflected in persistent absence of specific antibodies. We hypothesized that immature T-independent B-cell response and lack of B-cell memory play a crucial role in tolerance of ABH antigens after ABOi transplants.

METHODS

We determined phenotypes of splenic lymphocytes from adults and children and peripheral blood from ABO-incompatible or ABO-compatible heart transplant recipients and control subjects by flow cytometry. In vitro immune response to T-independent stimulation, erythrocytes, and ABH antigens was assessed using proliferation assays.

RESULTS

A predominant role for CD27(+) B cells in T-independent activation was demonstrated; these cells were significantly less frequent in infants than older subjects. Only IgM(+)CD27(+) B cells proliferated in response to non-self erythrocytes. In blood, IgM(+) and switched IgM(-) memory B cells were rare in infants, increasing to near-adult levels in children 5 years old. IgM(+)CD27(+) B cells were significantly fewer in ABO-incompatible transplant recipients than in ABO-compatible recipients.

CONCLUSIONS

CD27(+) cells play a key role in T-independent B-cell activation. Response to ABH antigens is mediated by IgM(+)CD27(+) B cells, and donor ABO-specific tolerance after ABO-incompatible transplantation in children is facilitated by low prevalence of these cells. The pattern of B-cell memory development is altered after ABO-incompatible transplant. Memory B cells may be quantified to assess eligibility for ABO-incompatible transplant and represent a potential therapeutic target to extend the benefits of the immature immune system to older age groups.

B Cells, Antibodies, and More

B cells play a central role in the immunopathogenesis of glomerulonephritides and transplant rejection. B cells secrete antibodies that contribute to tissue injury via multiple mechanisms. In addition, B cells contribute to disease pathogenesis in autoimmunity and alloimmunity by presenting antigens as well as providing costimulation and cytokines to T cells. B cells also play an immunomodulatory role in regulating the immune response by secreting cytokines that inhibit disease onset and/or progression. B cell-targeted approaches for treating immune diseases of the kidney and other organs have gained significant momentum. However, much remains to be understood about B-cell biology in order to determine the timing, duration, and context of optimal therapeutic response to B cell-targeted approaches. In this review, we discuss the multifaceted roles of B cells as enhancers and regulators of immunity with relevance to kidney disease and transplantation.

Chemical Basis for Qualitative and Quantitative Differences Between ABO Blood Groups and Subgroups: Implications for Organ Transplantation

Blood group ABH(O) carbohydrate antigens are carried by precursor structures denoted type I-IV chains, creating unique antigen epitopes that may differ in expression between circulating erythrocytes and vascular endothelial cells. Characterization of such differences is invaluable in many clinical settings including transplantation. Monoclonal antibodies were generated and epitope specificities were characterized against chemically synthesized type I-IV ABH and related glycans. Antigen expression was detected on endomyocardial biopsies (n = 50) and spleen (n = 11) by immunohistochemical staining and on erythrocytes by flow cytometry. On vascular endothelial cells of heart and spleen, only type II-based ABH antigens were expressed; type III/IV structures were not detected. Type II-based ABH were expressed on erythrocytes of all blood groups. Group A1 and A2 erythrocytes additionally expressed type III/IV precursors, whereas group B and O erythrocytes did not. Intensity of A/B antigen expression differed among group A1 , A2 , A1 B, A2 B and B erythrocytes. On group A2 erythrocytes, type III H structures were largely un-glycosylated with the terminal "A" sugar α-GalNAc. Together, these studies define qualitative and quantitative differences in ABH antigen expression between erythrocytes and vascular tissues. These expression profiles have important implications that must be considered in clinical settings of ABO-incompatible transplantation when interpreting anti-ABO antibodies measured by hemagglutination assays with reagent erythrocytes.

Strategies to overcome the ABO barrier in kidney transplantation

Kidney transplantation across the ABO blood group barrier was long considered a contraindication for transplantation, but in an effort to increase donor pools, specific regimens for ABO-incompatible (ABOi) transplantation have been developed. These regimens are now widely used as an integral part of the available treatment options. Various desensitization protocols, commonly based on transient depletion of preformed anti-A and/or anti-B antibodies and modulation of B-cell immunity, enable excellent transplant outcomes, even in the long-term. Nevertheless, the molecular mechanisms behind transplant acceptance facilitated by a short course of anti-humoral treatment are still incompletely understood. With the evolution of efficient clinical programmes, tailoring of recipient preconditioning based on individual donor-recipient blood type combinations and the levels of pretransplant anti-A/B antibodies has become possible. In the context of low antibody titres and/or donor A2 phenotype, immunomodulation and/or apheresis might be dispensable. A concern still exists, however, that ABOi kidney transplantation is associated with an increased risk of surgical and infectious complications, partly owing to the effects of extracorporeal treatment and intensified immunosuppression. Nevertheless, a continuous improvement in desensitization strategies, with the aim of minimizing the immunosuppressive burden, might pave the way to clinical outcomes that are comparable to those achieved in ABO-compatible transplantation.

Detecting the humoral alloimmune response: we need more than serum antibody screening

Whereas many techniques exist to detect HLA antibodies in the sera of immunized individuals, assays to detect and quantify HLA-specific B cells are only just emerging. The need for such assays is becoming clear, as in some patients, HLA-specific memory B cells have been shown to be present in the absence of the accompanying serum HLA antibodies. Because HLA-specific B cells in the peripheral blood of immunized individuals are present at only a very low frequency, assays with high sensitivity are required. In this review, we discuss the currently available methods to detect and/or quantify HLA-specific B cells, as well as their promises and limitations. We also discuss scenarios in which quantification of HLA-specific B cells may be of additional value, besides classical serum HLA antibody detection.

The Role of Liver Sinusoidal Endothelial Cells in Induction of Carbohydrate Reactive B Cells Tolerance Through the Programmed Death 1/Programmed Death Ligand 1 Pathway

BACKGROUND

A spontaneous tolerance of B cells responding to blood group antigens frequently develops in ABO-incompatible pediatric liver transplantation (LT). Liver sinusoidal endothelial cells (LSECs), which exclusively express blood group antigens in the liver, possess a capacity to induce alloantigen-specific tolerance. In this study, we elucidated the role of LSECs in the tolerance induction of blood group antigen-reactive B cells after ABO-incompatible LT using mice that lack galactose-α(1,3)galactose (Gal) epitopes resembling blood group carbohydrate antigens.

METHODS

Using adoptive transfer of LSECs from wild type (WT) C57BL/6J mice to congenic α1,3-galactosyltransferase gene knockout (GalT) mice, we established orthotropic GalT → GalT LSEC chimera mice. Anti-Gal Ab (antibody) production was evaluated after immunization of GalT → GalT LSEC chimera mice with Gal rabbit RBC.

RESULTS

Adoptive transfer of LSECs isolated from WT GalT mice via the portal vein resulted in persistent engraftment of Gal LSECs in congenic GalT mouse livers. Only when GalT mice were splenectomized before LSEC inoculation, the GalT → GalT LSEC chimera lost the ability to produce anti-Gal Abs. The administration of blocking monoclonal Abs (mAbs) against programmed death ligand 1 to the splenectomized GalT → GalT LSEC chimera resulted in the recovery of anti-Gal Ab production.

CONCLUSIONS

These findings suggest that LSECs take a part in tolerization of immature but not mature B cells specifically for Gal. Furthermore, the programmed death 1/programmed death ligand 1 pathway likely plays a crucial role in the mechanisms underlying spontaneous tolerization of B cells responding to ABO-blood group antigens in LT.

Neonatal Systemic AAV Induces Tolerance to CNS Gene Therapy in MPS I Dogs and Nonhuman Primates

The potential host immune response to a nonself protein poses a fundamental challenge for gene therapies targeting recessive diseases. We demonstrate in both dogs and nonhuman primates that liver-directed gene transfer using an adeno-associated virus (AAV) vector in neonates induces a persistent state of immunological tolerance to the transgene product, substantially improving the efficacy of subsequent vector administration targeting the central nervous system (CNS). We applied this approach to a canine model of mucopolysaccharidosis type I (MPS I), a progressive neuropathic lysosomal storage disease caused by deficient activity of the enzyme α-l-iduronidase (IDUA). MPS I dogs treated systemically in the first week of life with a vector expressing canine IDUA did not develop antibodies against the enzyme and exhibited robust expression in the CNS upon intrathecal AAV delivery at 1 month of age, resulting in complete correction of brain storage lesions. Newborn rhesus monkeys treated systemically with AAV vector expressing human IDUA developed tolerance to the transgene, resulting in high cerebrospinal fluid (CSF) IDUA expression and no antibody induction after subsequent CNS gene therapy. These findings suggest that inducing tolerance to the transgene product during a critical period in immunological development can improve the efficacy and safety of gene therapy.

Achieving incompatible transplantation through desensitization: current perspectives and future directions

The application of life-saving transplantation is severely limited by the shortage of organs, and histoincompatibility. To increase transplant rates in sensitized patients, new protocols for HLA and blood type incompatible (ABOi) desensitization have emerged. These approaches require significant desensitization using intravenous immunoglobulin, rituximab and plasma exchange. In addition, the development of donor-specific antibody responses post transplant is the major cause of allograft failure with return to dialysis. This increases patient morbidity/mortality and cost. Immunotherapeutic agents used for desensitization evolved from drug development in oncology and autoimmune diseases. Currently, there is a renaissance in development of novel drugs likely to improve antibody reduction in transplantation. These include agents that inactivate IgG molecules, anticytokine antibodies, costimulatory molecule blockade, anticomplement agents and therapies aimed at the plasma cell.

Meeting report: 3rd international transplant conference: how much risk can you take?

The 3rd International Transplant Conference took place on 31st October and 1st November 2014 at the University of Warwick, Coventry, UK. Key focal points of the meeting were the exploration of the molecular basis of antibody-antigen interactions and their relation to clinical practice and to share experiences and knowledge regarding strategies to transplant the 'high-risk' patient. In addition, lively debate sessions were hosted where controversial clinical and immunological themes were discussed by leading experts in the field.

Induction of immune tolerance to RBC, platelet, and neutrophil antigens and IgA

Antibodies to red blood cell (RBC), platelet, and neutrophil antigens, and IgA may cause serious clinical problems. With a few exceptions, preventing these conditions is a matter of limiting exposure to the foreign antigen while treatment consists of managing the consequences. Might immune tolerance induction (ITI) be possible and beneficial in these situations? Neonatal exposure to antigens is known to induce central tolerance. However central tolerance may not be absolute. Factors that determine whether an antibody will be produced in response to an antigen are not well understood but include the appropriate expression of major histocompatibility complex-class II and/or co-stimulatory molecules on dendritic cells, the presence or absence of adjuvants and whether or not the antigen is presented together with agonists for the toll-like receptor. Modifying these may prevent alloimmunization. Peripheral tolerance, in sensitized individuals, as routinely used in patients with allergic/anaphylactic reactions, those with haemophilia A or B with inhibitors and acquired haemophilia, may also be possible. Briefly, monitored, graded, increasing exposure to the antigen of interest with or without additional immunosuppression is used. Neither central nor peripheral ITI has been tried or suggested for individuals sensitizable or sensitised to RBC, platelet, and neutrophil antigens, or IgA. Theoretically, this is possible and may be of benefit.