Somatic Cell Fusion in Host Defense and Adaptation

Evidence of fusion of somatic cells has been noted in health and in disease for more than a century. The most obvious but uncertain hallmark has been the presence of multiple nuclei in cells. Although multinucleated cells are found in normal and diseased tissues, the benefit or harm of such cells can be difficult to elucidate. Still more difficult however is the identification of mononuclear cells previously formed by fusion of somatic cells with one or more nuclei disposed. The later process can introduce mutations that promote viral diversification, cancer, and tissue senescence. Less obvious the potential benefits of cell fusion. Recent work in cell biology, immunology, and genomic analysis however makes it possible to postulate benefits and potentially arrive at novel therapeutic agents and approaches that replicate or enhance these benefits.

Ixazomib for Desensitization (IXADES) in Highly Sensitized Kidney Transplant Candidates: A Phase II Clinical Trial

Key Points

Ixazomib treatment resulted in decreases in B-cell subsets and bone marrow lymphocytes. Ixazomib treatment resulted in modest decreases in certain anti-HLA antibody specificities. Ixazomib treatment was tolerated, with modest adverse events.

Background

Ixazomib is a second-generation oral proteasome inhibitor approved for treatment of refractory multiple myeloma. We conducted an open-label phase II trial, IXAzomib for DESensitization (IXADES), testing the safety of ixazomib treatment as an approach to decreasing the level and diversity of specificities of anti-HLA antibodies in subjects awaiting kidney transplantation. The trial (NCT03213158 ) enrolled highly sensitized kidney transplant candidates, defined as subjects with calculated panel reactive antibodies (cPRA) >80%, awaiting kidney transplantation >24 months. The subjects were treated with 12 monthly cycles of ixazomib 3 mg+dexamethasone 20 mg. Efficacy was defined as a decrease of cPRA >20% or kidney transplantation. The safety end point was tolerability.

Methods

In ten enrolled subjects, no grade IV, five grade III, 11 grade II, and 43 grade I adverse events were noted. The adverse events included infection, transient paresthesia, nausea, vomiting, and diarrhea. The IXADES regimen was not associated with significant change in levels or diversity of anti-HLA antibodies (cPRA).

Results

Although the IXADES regimen did not exhibit a clear impact on levels and diversity of anti-HLA antibodies in this small cohort, the prolonged half-life of IgG could necessitate a longer duration of treatment for accurate evaluation of efficacy.

Conclusions

In conclusion, treatment with ixazomib/dexamethasone engendered mild-to-moderate toxicity. The impact on anti-HLA was modest and paradoxical in the case of anti-HLA-DR. Clinical trials combining ixazomib with other immunosuppressive agents may be more effective in addressing antibody-mediated processes in kidney transplantation.

Metabolic reprogramming by immune-responsive gene 1 up-regulation improves donor heart preservation and function

Preservation quality of donor hearts is a key determinant of transplant success. Preservation duration beyond 4 hours is associated with primary graft dysfunction (PGD). Given transport time constraints, geographical limitations exist for donor-recipient matching, leading to donor heart underutilization. Here, we showed that metabolic reprogramming through up-regulation of the enzyme immune response gene 1 (IRG1) and its product itaconate improved heart function after prolonged preservation. Irg1 transcript induction was achieved by adding the histone deacetylase (HDAC) inhibitor valproic acid (VPA) to a histidine-tryptophan-ketoglutarate solution used for donor heart preservation. VPA increased acetylated H3K27 occupancy at the IRG1 enhancer and IRG1 transcript expression in human donor hearts. IRG1 converts aconitate to itaconate, which has both anti-inflammatory and antioxidant properties. Accordingly, our studies showed that Irg1 transcript up-regulation by VPA treatment increased nuclear translocation of nuclear factor erythroid 2-related factor 2 (Nrf2) in mice, which was accompanied by increased antioxidant protein expression [hemeoxygenase 1 (HO1) and superoxide dismutase 1 (SOD1)]. Deletion of Irg1 in mice (Irg1^-/-) negated the antioxidant and cardioprotective effects of VPA. Consistent with itaconate's ability to inhibit succinate dehydrogenase, VPA treatment of human hearts increased itaconate availability and reduced succinate accumulation during preservation. VPA similarly increased IRG1 expression in pig donor hearts and improved its function in an ex vivo cardiac perfusion system both at the clinical 4-hour preservation threshold and at 10 hours. These results suggest that augmentation of cardioprotective immune-metabolomic pathways may be a promising therapeutic strategy for improving donor heart function in transplantation.

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.

TNFRSF13B in B cell responses to organ transplantation

Antibodies directed against organ transplants are thought to pose the most vexing hurdle to enduring function and survival of the transplants, particularly organ xenotransplants, and accordingly basic and clinical investigation has focused on elucidating the specificity and pathogenicity of graft-specific antibodies. While much has been learned about these matters, far less is known about the B cells producing graft-specific antibodies and why these antibodies appear to injure some grafts but not others. With the goal of addressing those questions, we have investigated the properties of tumor necrosis factor receptor super family-13B (TNFRSF13B), which regulates various aspects of B cell responses. A full understanding of the functions of TNFRSF13B however is hindered by extreme polymorphism and by diversity of interactions of the protein. Nevertheless, TNFRSF13B variants have been found to exert distinct impact on natural and elicited antibody responses and host defense and mutations of TNFRSF13B have been found to influence the propensity for development of antibody-mediated rejection of organ transplants. Because B cell responses potentially limit application of xenotransplantation, understanding how TNFRSF13B diversity and TNFRSF13B variants govern immunity in xenotransplantation could inspire development of novel therapeutics that could in turn accelerate clinical implementation of xenotransplantation.

Allogeneic and xenogeneic lymphoid reconstitution in a RAG2 -/- IL2RG y/- severe combined immunodeficient pig: A preclinical model for intrauterine hematopoietic transplantation

Mice with severe combined immunodeficiency are commonly used as hosts of human cells. Size, longevity, and physiology, however, limit the extent to which immunodeficient mice can model human systems. To address these limitations, we generated RAG2^−/−IL2RG^y/− immunodeficient pigs and demonstrate successful engraftment of SLA mismatched allogeneic D42 fetal liver cells, tagged with pH2B-eGFP, and human CD34⁺ hematopoietic stem cells after in utero cell transplantation. Following intrauterine injection at day 42–45 of gestation, fetuses were allowed to gestate to term and analyzed postnatally for the presence of pig (allogeneic) and human (xenogeneic) B cells, T-cells and NK cells in peripheral blood and other lymphoid tissues. Engraftment of allogeneic hematopoietic cells was detected based on co-expression of pH2B-eGFP and various markers of differentiation. Analysis of spleen revealed robust generation and engraftment of pH2B-eGFP mature B cells (and IgH recombination) and mature T-cells (and TCR-β recombination), T helper (CD3⁺CD4⁺) and T cytotoxic (CD3⁺CD8⁺) cells. The thymus revealed engraftment of pH2B-eGFP double negative precursors (CD4⁻CD8⁻) as well as double positive (CD4⁺, CD8⁺) precursors and single positive T-cells. After intrauterine administration of human CD34⁺ hematopoietic stem cells, analysis of peripheral blood and lymphoid tissues revealed the presence of human T-cells (CD3⁺CD4⁺ and CD3⁺CD8⁺) but no detectable B cells or NK cells. The frequency of human CD45⁺ cells in the circulation decreased rapidly and were undetectable within 2 weeks of age. The frequency of human CD45⁺ cells in the spleen also decreased rapidly, becoming undetectable at 3 weeks. In contrast, human CD45⁺CD3⁺T-cells comprised >70% of cells in the pig thymus at birth and persisted at the same frequency at 3 weeks. Most human CD3⁺ cells in the pig's thymus expressed CD4 or CD8, but few cells were double positive (CD4⁺ CD8⁺). In addition, human CD3⁺ cells in the pig thymus contained human T-cell excision circles (TREC), suggesting de novo development. Our data shows that the pig thymus provides a microenvironment conducive to engraftment, survival and development of human T-cells and provide evidence that the developing T-cell compartment can be populated to a significant extent by human cells in large animals.

A non-human primate model of acute liver failure suitable for testing liver support systems

Acute hepatic failure is associated with high morbidity and mortality for which the only definitive therapy is liver transplantation. Some fraction of those who undergo emergency transplantation have been shown to recover native liver function when transplanted with an auxiliary hepatic graft that leaves part of the native liver intact. Thus, transplantation could have been averted with the development and use of some form of hepatic support. The costs of developing and testing liver support systems could be dramatically reduced by the availability of a reliable large animal model of hepatic failure with a large therapeutic window that allows the assessment of efficacy and timing of intervention. Non-lethal forms of hepatic injury were examined in combination with liver-directed radiation in non-human primates (NHPs) to develop a model of acute hepatic failure that mimics the human condition. Porcine hepatocyte transplantation was then tested as a potential therapy for acute hepatic failure. After liver-directed radiation therapy, delivery of a non-lethal hepatic ischemia-reperfusion injury reliably and rapidly generated liver failure providing conditions that can enable pre-clinical testing of liver support or replacement therapies. Unfortunately, in preliminary studies, low hepatocyte engraftment and over-immune suppression interfered with the ability to assess the efficacy of transplanted porcine hepatocytes in the model. A model of acute liver failure in NHPs was created that recapitulates the pathophysiology and pathology of the clinical condition, does so with reasonably predictable kinetics, and results in 100% mortality. The model allowed preliminary testing of xenogeneic hepatocyte transplantation as a potential therapy.

He reshaped the forefront of xenotransplantation: Agustin Pasqual Dalmasso (1933-2021)

Agustin Pasqual Dalmasso died in December 2021. He was 88 years old and an Honorary Member of the International Xenotransplantation Association. Gus made seminal contributions to understanding and overcoming the barrier complement system poses to xenotransplantation. Those endeavoring to advance xenotransplantation to clinical application and those seeking the most topics in which to devote their life's work could do no better than examining how Gus approached the subjects of his life's work.

Limited Expansion of Human Hepatocytes in FAH/RAG2-Deficient Swine

BACKGROUND

The mammalian liver's regenerative ability has led researchers to engineer animals as incubators for expansion of human hepatocytes. The expansion properties of human hepatocytes in immunodeficient mice are well known. However, little has been reported about larger animals that are more scalable and practical for clinical purposes. Therefore, we engineered immunodeficient swine to support expansion of human hepatocytes and identify barriers to their clinical application.

METHODS

Immunodeficient swine were engineered by knockout of recombinase activating gene 2 (RAG2) and fumarylacetoacetate hydrolase (FAH). Immature human hepatocytes (ihHCs) were injected into fetal swine by intrauterine cell transplantation (IUCT) at day 40 of gestation. Human albumin was measured as a marker of engraftment. Cytotoxicity against ihHCs was measured in transplanted piglets and control swine.

RESULTS

Higher levels of human albumin were detected in cord blood of newborn FAH/RAG2-deficient (FR) pigs compared to immunocompetent controls (196.26 ng/dL vs 39.29 ng/dL, p = 0.008), indicating successful engraftment of ihHC after IUCT and adaptive immunity in the fetus. Although rare hepatocytes staining positively for human albumin were observed, levels of human albumin did not rise after birth but declined suggesting rejection of xenografted ihHCs. Cytotoxicity against ihHCs increased after birth 3.8% (95% CI: [2.1%, 5.4%], p < 0.001) and correlated inversely to declining levels of human albumin (p = 2.1 x 10-5, R2 = 0.17). Circulating numbers of T-cells and B-cells were negligible in FR pigs. However, circulating natural killer (NK) cells exerted cytotoxicity against ihHCs. NK cell activity was lower in immunodeficient piglets after IUCT than naive controls (30.4% vs 40.1% (p = 0.011, 95% CI for difference [2.7%, 16.7%]).

CONCLUSION

Immature human hepatocytes successfully engrafted in FR swine after IUCT. NK cells were a significant barrier to expansion of hepatocytes. New approaches are needed to overcome this hurdle and allow large scale expansion of human hepatocytes in immunodeficient swine.

TNFRSF13B polymorphisms counter microbial adaptation to enteric IgA

TNFRSF13B encodes the transmembrane activator and CAML interactor (TACI) receptor, which drives plasma cell differentiation. Although TNFRSF13B supports host defense, dominant-negative TNFRSF13B alleles are common in humans and other species and only rarely associate with disease. We reasoned that the high frequency of disruptive TNFRSF13B alleles reflects balancing selection, the loss of function conferring advantage in some settings. Testing that concept, we investigated how a common human dominant-negative variant, TNFRSF13B A181E, imparts resistance to enteric pathogens. Mice engineered to express mono- or biallelic A144E variants of tnrsf13B, corresponding to A181E, exhibited a striking resistance to pathogenicity and transmission of Citrobacter rodentium, a murine pathogen that models enterohemorrhagic Escherichiacoli, and resistance was principally owed to natural IgA deficiency in the intestine. In WT mice with gut IgA and in mutant mice reconstituted with enteric IgA obtained from WT mice, IgA induces LEE expression of encoded virulence genes, which confer pathogenicity and transmission. Taken together, our results show that C. rodentium and most likely other enteric organisms appropriated binding of otherwise protective antibodies to signal induction of the virulence program. Additionally, the high prevalence of TNFRSF13B dominant-negative variants reflects balancing selection.

TNFRSF13B genotypes control immune-mediated pathology by regulating the functions of innate B cells

Host genes define the severity of inflammation and immunity but specific loci doing so are unknown. Here we show that TNF receptor superfamily member 13B (TNFRSF13B) variants, which enhance defense against certain pathogens, also control immune-mediated injury of transplants, by regulating innate B cells’ functions. Analysis of TNFRSF13B in human kidney transplant recipients revealed that 33% of those with antibody-mediated rejection (AMR) but fewer than 6% of those with stable graft function had TNFRSF13B missense mutations. To explore mechanisms underlying aggressive immune responses, we investigated alloimmunity and rejection in mice. Cardiac allografts in Tnfrsf13b-mutant mice underwent early and severe AMR. The dominance and precocity of AMR in Tnfrsf13b-deficient mice were not caused by increased alloantibodies. Rather, Tnfrsf13b mutations decreased “natural” IgM and compromised complement regulation, leading to complement deposition in allografted hearts and autogenous kidneys. Thus, WT TNFRSF13B and Tnfrsf13b support innate B cell functions that limit complement-associated inflammation; in contrast, common variants of these genes intensify inflammatory responses that help clear microbial infections but allow inadvertent tissue injury to ensue. The wide variation in inflammatory reactions associated with TNFRSF13B diversity suggests polymorphisms could underlie variation in host defense and explosive inflammatory responses that sometimes enhance morbidity associated with immune responses.

IgV somatic mutation of human anti-SARS-CoV-2 monoclonal antibodies governs neutralization and breadth of reactivity

Abs that neutralize SARS-CoV-2 are thought to provide the most immediate and effective treatment for those severely afflicted by this virus. Because coronavirus potentially diversifies by mutation, broadly neutralizing Abs are especially sought. Here, we report a possibly novel approach to rapid generation of potent broadly neutralizing human anti-SARS-CoV-2 Abs. We isolated SARS-CoV-2 spike protein-specific memory B cells by panning from the blood of convalescent subjects after infection with SARS-CoV-2 and sequenced and expressed Ig genes from individual B cells as human mAbs. All of 43 human mAbs generated in this way neutralized SARS-CoV-2. Eighteen of the forty-three human mAbs exhibited half-maximal inhibitory concentrations (IC50) of 6.7 × 10-12 M to 6.7 × 10-15 M for spike-pseudotyped virus. Seven of the human mAbs also neutralized (with IC50 < 6.7 × 10-12 M) viruses pseudotyped with mutant spike proteins (including receptor-binding domain mutants and the S1 C-terminal D614G mutant). Neutralization of the Wuhan Hu-1 founder strain and of some variants decreased when coding sequences were reverted to germline, suggesting that potency of neutralization was acquired by somatic hypermutation and selection of B cells. These results indicate that infection with SARS-CoV-2 evokes high-affinity B cell responses, some products of which are broadly neutralizing and others highly strain specific. We also identify variants that would potentially resist immunity evoked by infection with the Wuhan Hu-1 founder strain or by vaccines developed with products of that strain, suggesting evolutionary courses that SARS-CoV-2 could take.

TNFRSF13B Diversification Fueled by B Cell Responses to Environmental Challenges-A Hypothesis

B cell differentiation and memory are controlled by the transmembrane activator and CAML interactor (TACI), a receptor encoded by TNFRSF13B. TNFRSF13B mutations are frequently found in common variable immunodeficiency (CVID) and in IgA -deficiency; yet, ~98% of those with mutant TNFRSF13B are healthy. Indeed, TNFRSF13B is among the 5% most polymorphic genes in man. Other mammals evidence polymorphism at comparable loci. We hypothesize that TNFRSF13B diversity might promote rather than detract from well-being by controlling key elements of innate immunity. We shall discuss how extraordinary diversity of TNFRSF13B could have evolved and persisted across diverse species of mammals by controlling innate and adaptive B cell responses in apparently paradoxical ways.

Xenotransplantation: Progress Along Paths Uncertain from Models to Application

For more than a century, transplantation of tissues and organs from animals into man, xenotransplantation, has been viewed as a potential way to treat disease. Ironically, interest in xenotransplantation was fueled especially by successful application of allotransplantation, that is, transplantation of human tissue and organs, as a treatment for a variety of diseases, especially organ failure because scarcity of human tissues limited allotransplantation to a fraction of those who could benefit. In principle, use of animals such as pigs as a source of transplants would allow transplantation to exert a vastly greater impact than allotransplantation on medicine and public health. However, biological barriers to xenotransplantation, including immunity of the recipient, incompatibility of biological systems, and transmission of novel infectious agents, are believed to exceed the barriers to allotransplantation and presently to hinder clinical applications. One way potentially to address the barriers to xenotransplantation is by genetic engineering animal sources. The last 2 decades have brought progressive advances in approaches that can be applied to genetic modification of large animals. Application of these approaches to genetic engineering of pigs has contributed to dramatic improvement in the outcome of experimental xenografts in nonhuman primates and have encouraged the development of a new type of xenograft, a reverse xenograft, in which human stem cells are introduced into pigs under conditions that support differentiation and expansion into functional tissues and potentially organs. These advances make it appropriate to consider the potential limitation of genetic engineering and of current models for advancing the clinical applications of xenotransplantation and reverse xenotransplantation.

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.

Non-canonical B cell functions in transplantation

B cells are differentiated to recognize antigen and respond by producing antibodies. These activities, governed by recognition of ancillary signals, defend the individual against microorganisms and the products of microorganisms and constitute the canonical function of B cells. Despite the unique differentiation (e.g. recombination and mutation of immunoglobulin gene segments) toward this canonical function, B cells can provide other, "non-canonical" functions, such as facilitating of lymphoid organogenesis and remodeling and fashioning T cell repertoires and modifying T cell responses. Some non-canonical functions are exerted by antibodies, but most are mediated by other products and/or direct actions of B cells. The diverse set of non-canonical functions makes the B cell as much as any cell a central organizer of innate and adaptive immunity. However, the diverse products and actions also confound efforts to weigh the importance of individual non-canonical B cell functions. Here we shall describe the non-canonical functions of B cells and offer our perspective on how those functions converge in the development and governance of immunity, particularly immunity to transplants, and hurdles to advancing understanding of B cell functions in transplantation.

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.

Accommodation in ABO-incompatible organ transplants

Accommodation refers to a condition in which a transplant (or any tissue) appears to resist immune-mediated injury and loss of function. Accommodation was discovered and has been explored most thoroughly in ABO-incompatible kidney transplantation. In this setting, kidney transplants bearing blood group A or B antigens often are found to function normally in recipients who lack and hence produce antibodies directed against the corresponding antigens. Whether accommodation is owed to changes in anti-blood group antibodies, changes in antigen or a change in the response of the transplant to antibody binding are critically reviewed and a new working model that allows for the kinetics of development of accommodation is put forth. Regardless of how accommodation develops, observations on the fate of ABO-incompatible transplants offer lessons applicable more broadly in transplantation and in other fields.

The mutable vaccine for mutable viruses

Mutable viruses, such as HIV, pose difficult obstacles to prevention and/or control by vaccination. Mutable viruses rapidly diversify in populations and in individuals, impeding development of effective vaccines. We devised the 'mutable vaccine' to appropriate the properties of mutable viruses that undermine conventional strategies. The vaccine consists of a DNA construct encoding viral antigen and regulatory sequences that upon delivery to B cells target the enzymatic apparatus of 'somatic hypermutation' causing the construct to mutate one million-times baseline rates and allowing production and presentation of antigen variants. We postulate the mutable vaccine might thus anticipate diversification of mutable viruses, allowing direct control or slowing of evolution. Initial work presented here should encourage consideration of this novel approach.

C3d regulates immune checkpoint blockade and enhances antitumor immunity

Despite expression of immunogenic polypeptides, tumors escape immune surveillance by engaging T cell checkpoint regulators and expanding Tregs, among other mechanisms. What orchestrates these controls is unknown. We report that free C3d, a fragment of the third component of complement, inside tumor cells - or associated with irradiated tumor cells and unattached to antigen - recruits, accelerates, and amplifies antitumor T cell responses, allowing immunity to reverse or even to prevent tumor growth. C3d enhances antitumor immunity independently of B cells, NK cells, or antibodies, but it does so by increasing tumor infiltrating CD8+ lymphocytes, by depleting Tregs, and by suppressing expression of programmed cell death protein 1 (PD-1) by T cells. These properties of C3d appear specific for the tumor and dependent on complement receptor 2, and they incur no obvious systemic toxicity. The heretofore unrecognized properties of free C3d suggest that protein might determine the effectiveness of immune surveillance and that increasing availability of the protein might prove advantageous in the treatment or prevention of cancer and premalignant conditions.

Host conditioning and rejection monitoring in hepatocyte transplantation in humans

BACKGROUND & AIMS

Hepatocyte transplantation partially corrects genetic disorders and has been associated anecdotally with reversal of acute liver failure. Monitoring for graft function and rejection has been difficult, and has contributed to limited graft survival. Here we aimed to use preparative liver-directed radiation therapy, and continuous monitoring for possible rejection in an attempt to overcome these limitations.

METHODS

Preparative hepatic irradiation was examined in non-human primates as a strategy to improve engraftment of donor hepatocytes, and was then applied in human subjects. T cell immune monitoring was also examined in human subjects to assess adequacy of immunosuppression.

RESULTS

Porcine hepatocyte transplants engrafted and expanded to comprise up to 15% of irradiated segments in immunosuppressed monkeys preconditioned with 10Gy liver-directed irradiation. Two patients with urea cycle deficiencies had early graft loss following hepatocyte transplantation; retrospective immune monitoring suggested the need for additional immunosuppression. Preparative radiation, anti-lymphocyte induction, and frequent immune monitoring were instituted for hepatocyte transplantation in a 27year old female with classical phenylketonuria. Post-transplant liver biopsies demonstrated multiple small clusters of transplanted cells, multiple mitoses, and Ki67⁺ hepatocytes. Mean peripheral blood phenylalanine (PHE) level fell from pre-transplant levels of 1343±48μM (normal 30-119μM) to 854±25μM (treatment goal ≤360μM) after transplant (36% decrease; p<0.0001), despite transplantation of only half the target number of donor hepatocytes. PHE levels remained below 900μM during supervised follow-up, but graft loss occurred after follow-up became inconsistent.

CONCLUSIONS

Radiation preconditioning and serial rejection risk assessment may produce better engraftment and long-term survival of transplanted hepatocytes. Hepatocyte xenografts engraft for a period of months in non-human primates and may provide effective therapy for patients with acute liver failure.

LAY SUMMARY

Hepatocyte transplantation can potentially be used to treat genetic liver disorders but its application in clinical practice has been impeded by inefficient hepatocyte engraftment and the inability to monitor rejection of transplanted liver cells. In this study, we first show in non-human primates that pretreatment of the host liver with radiation improves the engraftment of transplanted liver cells. We then used this knowledge in a series of clinical hepatocyte transplants in patients with genetic liver disorders to show that radiation pretreatment and rejection risk monitoring are safe and, if optimized, could improve engraftment and long-term survival of transplanted hepatocytes in patients.

Cell Fusion in the War on Cancer: A Perspective on the Inception of Malignancy

Cell fusion occurs in development and in physiology and rarely in those settings is it associated with malignancy. However, deliberate fusion of cells and possibly untoward fusion of cells not suitably poised can eventuate in aneuploidy, DNA damage and malignant transformation. How often cell fusion may initiate malignancy is unknown. However, cell fusion could explain the high frequency of cancers in tissues with low underlying rates of cell proliferation and mutation. On the other hand, cell fusion might also engage innate and adaptive immune surveillance, thus helping to eliminate or retard malignancies. Here we consider whether and how cell fusion might weigh on the overall burden of cancer in modern societies.

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.

Cell Fusion Connects Oncogenesis with Tumor Evolution

Cell fusion likely drives tumor evolution by undermining chromosomal and DNA stability and/or by generating phenotypic diversity; however, whether a cell fusion event can initiate malignancy and direct tumor evolution is unknown. We report that a fusion event involving normal, nontransformed, cytogenetically stable epithelial cells can initiate chromosomal instability, DNA damage, cell transformation, and malignancy. Clonal analysis of fused cells reveals that the karyotypic and phenotypic potential of tumors formed by cell fusion is established immediately or within a few cell divisions after the fusion event, without further ongoing genetic and phenotypic plasticity, and that subsequent evolution of such tumors reflects selection from the initial diverse population rather than ongoing plasticity of the progeny. Thus, one cell fusion event can both initiate malignancy and fuel evolution of the tumor that ensues.

Heparan Sulfate Proteoglycan Metabolism and the Fate of Grafted Tissues

Tissue and organ transplants between genetically distinct individuals are always or nearly always rejected. The universality and speed of transplant rejection distinguishes this immune response from all others. Although this distinction is incompletely understood, some efforts to shed light on transplant rejection have revealed broader insights, including a relationship between activation of complement in grafted tissues, the metabolism of heparan sulfate proteoglycan and the nature of immune and inflammatory responses that ensue. Complement activation on cell surfaces, especially on endothelial cell surfaces, causes the shedding heparan sulfate, an acidic saccharide, from the cell surface and neighboring extracellular matrix. Solubilized in this way, heparan sulfate can activate leukocytes via toll like receptor-4, triggering inflammatory responses and activating dendritic cells, which migrate to regional lymphoid organs where they spark and to some extent govern cellular immune responses. In this way local ischemia, tissue injury and infection, exert systemic impact on immunity. Whether or in what circumstances this series of events explains the distinct characteristics of the immune response to transplants is still unclear but the events offer insight into the inception of immunity under the sub-optimal conditions accompanying infection and mechanisms by which infection and tissue injury engender systemic inflammation.

TACI deficiency enhances antibody avidity and clearance of an intestinal pathogen

The transmembrane activator and calcium-modulating cyclophilin ligand interactor (TACI) controls differentiation of long-lived plasma cells, and almost 10% of individuals with common variable immunodeficiency (CVID) express either the C104R or A181E variants of TACI. These variants impair TACI function, and TACI-deficient mice exhibit a CVID-like disease. However, 1%-2% of normal individuals harbor the C140R or A181E TACI variants and have no outward signs of CVID, and it is not clear why TACI deficiency in this group does not cause disease. Here, we determined that TACI-deficient mice have low baseline levels of Ig in the blood but retain the ability to mutate Ig-associated genes that encode antigen-specific antibodies. The antigen-specific antibodies in TACI-deficient mice were produced in bursts and had higher avidity than those of WT animals. Moreover, mice lacking TACI were able to clear Citrobacter rodentium, a model pathogen for severe human enteritis, more rapidly than did WT mice. These findings suggest that the high prevalence of TACI deficiency in humans might reflect enhanced host defense against enteritis, which is more severe in those with acquired or inherited immunodeficiencies.

The paradoxical functions of B cells and antibodies in transplantation

Scarcely anyone would dispute that donor-specific B cells and the Abs that they produce can cause rejection of transplants. Less clear and more controversial, however, is the possibility that donor-specific B cells and the Abs that they produce are one or more means by which transplants can be protected from injury. In this article, we review and discuss this possibility and consider how less well-known functions of B cells and Abs might impact on the design of therapeutics and the management of transplant recipients.

Affinity maturation of antibodies requires integrity of the adult thymus

The generation of B-cell responses to proteins requires a functional thymus to produce CD4(+) T cells which helps in the activation and differentiation of B cells. Because the mature T-cell repertoire has abundant cells with the helper phenotype, one might predict that in mature individuals, the generation of B-cell memory would proceed independently of the thymus. Contrary to that prediction, we show here that the removal of the thymus after the establishment of the T-cell compartment or sham surgery without removal of the thymus impairs the affinity maturation of antibodies. Because removal or manipulation of the thymus did not decrease the frequency of mutation of the Ig variable heavy chain exons encoding antigen-specific antibodies, we conclude that the thymus controls affinity maturation of antibodies in the mature individual by facilitating the selection of B cells with high-affinity antibodies.

LPS-induced murine systemic inflammation is driven by parenchymal cell activation and exclusively predicted by early MCP-1 plasma levels

Systemic inflammation remains a major cause of morbidity and mortality in the United States, across many disease processes. One classic murine model to study this syndrome is lipopolysaccharide (LPS)-induced Toll-like receptor 4 (TLR4)-dependent systemic inflammation. Although most studies have focused on inflammatory cell TLR4 responses, parenchymal cells also express TLR4. Our objective was to define the in vivo role of parenchymal- versus marrow-derived cell activation via TLR4 during LPS-induced inflammation. Mice bearing TLR4 on parenchymal cells only, marrow-derived cells only, both, or neither were generated using bone marrow transplantation. Mortality occurred only in mice that had TLR4 expression on their parenchymal cells. Before onset, virtually all major plasma cytokines and blood neutrophil responses were related to marrow-derived cell activation via TLR4. The only cytokine predictive of oncoming systemic inflammation was the chemokine monocyte chemoattractant protein-1. Late blood neutrophil responses were related to the presence of TLR4 on either parenchymal or marrow cells, whereas plasma cytokine elevations late in LPS-induced systemic inflammation were dependent on mice having TLR4 in both cell compartments. Parenchymal cell activation via TLR4 is a key component of LPS-induced systemic inflammation and mortality, although most plasma cytokine levels and blood neutrophil responses were not key components. Given its unique role, future studies into monocyte chemoattractant protein-1's exact role during systemic inflammation are warranted.

Donor specific antibodies after transplantation

The detection of donor-specific antibodies after organ transplantation might provide an incisive way to monitor allo-specific immunity and predict graft outcome. Still, the availability of new assays for these antibodies prompts us to pose some questions about results that might be observed. These questions include whether the antibodies detected in the blood are a sensitive measure of alloimmunity, whether the detected antibodies are truly specific for the donor and whether they are noxious for the graft. Here, we explain why answers to these questions might interest the basic scientist and clinician.

Cleavage of PGRP-LC receptor in the Drosophila IMD pathway in response to live bacterial infection in S2 cells

Drosophila responds to Gram-negative bacterial infection by activating the immune deficiency (IMD) pathway, leading to production of antimicrobial peptides (AMPs). As a receptor for the IMD pathway, peptidoglycan-recognition protein (PGRP), PGRP-LC is known to recognize and bind monomeric peptidoglycan (DAP-type PGN) through its PGRP ectodomain and in turn activate the IMD pathway. The questions remain how PGRP-LC is activated in response to pathogen infection to initiate the IMD signal transduction in Drosophila. Here we present evidence to show that proteases such as elastase and Mmp2 can also activate the IMD pathway but not the TOLL pathway. The elastase-dependent IMD activation requires the receptor PGRP-LC. Importantly, we find that live Salmonella/E. coli infection modulates PGRP-LC expression/receptor integrity and activates the IMD pathway while dead Salmonella/E. coli or protease-deficient E. coli do neither. Our results suggest an interesting possibility that Gram-negative pathogen infection may be partially monitored through the structural integrity of the receptor PGRP-LC via an infection-induced enzyme-based cleavage-mediated activation mechanism.

Report from a consensus conference on antibody-mediated rejection in heart transplantation

BACKGROUND

The problem of AMR remains unsolved because standardized schemes for diagnosis and treatment remains contentious. Therefore, a consensus conference was organized to discuss the current status of antibody-mediated rejection (AMR) in heart transplantation.

METHODS

The conference included 83 participants (transplant cardiologists, surgeons, immunologists and pathologists) representing 67 heart transplant centers from North America, Europe, and Asia who all participated in smaller break-out sessions to discuss the various topics of AMR and attempt to achieve consensus.

RESULTS

A tentative pathology diagnosis of AMR was established, however, the pathologist felt that further discussion was needed prior to a formal recommendation for AMR diagnosis. One of the most important outcomes of this conference was that a clinical definition for AMR (cardiac dysfunction and/or circulating donor-specific antibody) was no longer believed to be required due to recent publications demonstrating that asymptomatic (no cardiac dysfunction) biopsy-proven AMR is associated with subsequent greater mortality and greater development of cardiac allograft vasculopathy. It was also noted that donor-specific antibody is not always detected during AMR episodes as the antibody may be adhered to the donor heart. Finally, recommendations were made for the timing for specific staining of endomyocardial biopsy specimens and the frequency by which circulating antibodies should be assessed. Recommendations for management and future clinical trials were also provided.

CONCLUSIONS

The AMR Consensus Conference brought together clinicians, pathologists and immunologists to further the understanding of AMR. Progress was made toward a pathology AMR grading scale and consensus was accomplished regarding several clinical issues.

In utero cell transfer between porcine littermates

Trafficking of cells between mother and fetus during the course of normal pregnancy is well documented. Similarly, cells are known to travel between twins that share either a placenta (i.e. monozygotic) or associated chorion (i.e. monochorionic). Transferred cells are thought to be channelled via the vessels of the placenta or vascular connections established via the chorion and the long-term presence of these cells (i.e. microchimerism) can have important consequences for immune system function and reparative capacity of the host. Whether cells can be transferred between twins with separate placentas and separate chorions (i.e. no vascular connections between placentas) has not been investigated nor have the biological consequences of such a transfer. In the present study, we tested the possibility of this type of cell transfer by injecting human cord blood-derived cells into a portion of the littermates of swine and probing for human cells in the blood and tissues of unmanipulated littermates. Human cells were detected in the blood of 78% of unmanipulated littermates. Human cells were also detected in various tissues of the unmanipulated littermates, including kidney (56%), spleen (33%), thymus (11%) and heart (22%). Human cells were maintained in the blood until the piglets were sacrificed (8 months after birth), suggesting the establishment of long-term microchimerism. Our findings show that the transfer of cells between fetuses with separate placentas and separate chorions is significant and thus such twins may be subject to the same consequences of microchimerism as monozygotic or monochorionic counterparts.

Barriers to the successful treatment of liver disease by hepatocyte transplantation

Management of patients with hepatic failure and liver-based metabolic disorders is complex and expensive. Hepatic failure results in impaired coagulation, altered consciousness and cerebral function, a heightened risk of multiple organ system failure, and sepsis [1]. Such manifold problems are only treatable today and for the foreseeable future by transplantation. In fact, whole or auxiliary partial liver transplantation is often the only available treatment option for severe, even if transient, hepatic failure. Patients with life-threatening liver-based metabolic disorders similarly require organ transplantation even though their metabolic diseases are typically the result of a single enzyme deficiency, and the liver otherwise functions normally. For all of the benefits it may confer, liver transplantation is not an ideal therapy, even for severe hepatic failure. More than 17,000 patients currently await liver transplantation in the United States, a number that seriously underestimates the number of patients that need treatment [2], as it has been estimated that more than a million patients could benefit from transplantation [3]. Unfortunately, use of whole liver transplantation to treat these disorders is limited by a severe shortage of donors and by the risks to the recipient associated with major surgery [4].

Antibodies in transplantation

Transplantation of cells, tissues, and organs from one individual to another can incite the production of antibodies specific for foreign antigens, especially major histocompatibility antigens, in the graft. Antibodies specific for a graft provide an index of immunity and a potential trigger for injury and rejection. However, the index of immunity can sometimes miss antibody-mediated rejection and besides causing injury the antibodies against a graft can also protect a graft from injury by blocking immune recognition, called enhancement, regulating activation of complement, and inducing changes in the graft that resist damage. Reviewed here are potential limitations in the use of antibodies as an index of immunity and the ways antibodies cause and/or prevent injury.

Induction of human blood group a antigen expression on mouse cells, using lentiviral gene transduction

The ABO histo-blood group system is the most important antigen system in transplantation medicine, yet no small animal model of the ABO system exists. To determine the feasibility of developing a murine model, we previously subcloned the human alpha-1,2-fucosyltransferase (H-transferase, EC 2.4.1.69) cDNA and the human alpha-1,3-N-acetylgalactosaminyltransferase (A-transferase, EC 2.4.1.40) cDNA into lentiviral vectors to study their ability to induce human histo-blood group A antigen expression on mouse cells. Herein we investigated the optimal conditions for human A and H antigen expression in murine cells. We determined that transduction of a bicistronic lentiviral vector (LvEF1-AH-trs) resulted in the expression of A antigen in a mouse endothelial cell line. We also studied the in vivo utility of this vector to induce human A antigen expression in mouse liver. After intrahepatic injection of LvEF1-AH-trs, A antigen expression was observed on hepatocytes as detected by immunohistochemistry and real-time RT-PCR. In human group A erythrocyte-sensitized mice, A antigen expression in the liver was associated with tissue damage, and deposition of antibody and complement. These results suggest that this gene transfer strategy can be used to simulate the human ABO blood group system in a murine model. This model will facilitate progress in the development of interventions for ABO-incompatible transplantation and transfusion scenarios, which are difficult to develop in clinical or large animal settings.