Genetic control of the humoral responses to xenografts. III. Identification of the immunoglobulin V(H) genes responsible for encoding rat immunoglobin G xenoantibodies to hamster heart grafts

Xenogeneic bone matrix immune risk assessment using GGTA1 knockout mice

Homeotransplantation of bones for replacement therapy have been demonstrated reliably in clinical data. However, human donor bones applicable for homeotransplantation are in short supply, which facilitates the search for suitable alternatives, such as xenografts grafts. The α-Gal antigen-related immune risk of xenografts directly affects the safety and effectiveness of the biomaterials and limits their applications in the clinic. The immune risk can be prevented by depletion or breaking anti-Gal antibody prior to transplant. Therefore, how to assess the immune risk of the bone substitutes and select the reliable animal research model become extremely important. In this study, we prepared lyophilized bone substitutes (T1) and Guanghao Biotech bone substitutes (T2, animal-derived biomaterials with α-Gal antigen decreased), aimed to assess the immune risk of xenografts bone substitutes on GGTA1 knockout mice. The α-Gal antigen contents of T1 and T2 were firstly detected by ELISA method in vitro. The bone substitutes were then implanted subcutaneously into GGTA1 knockout mice for 2, 4 and 12 weeks, respectively. The total serum antibody levels, anti-α-Gal antibody levels, inflammatory cytokine and splenic lymphocyte surface molecules were detected and histology analysis of skin and thymus were performed to systematically evaluate the immune response caused by the T1 and T2 bone substitutes in mice. In vitro results showed that the amount of α-Gal epitopes in T1 bone substitutes was significantly higher than T2 bone substitutes, and the clearance rate of α-Gal antigen in T2 bone substitutes achieved about 55.6%. Results of antibody level in vivo showed that the T1 bone substitutes group possessed significantly higher total IgG, IgM, IgA and anti-α-Gal IgG levels than T2 and control group, while T2 group showed no significant changes of these indexes compared with control. In terms of inflammatory cytokines, T1 bone substitutes showed evidently higher levels of IL-4, IL-12P70 and IL-10 than T2 and control, while T2 group was comparable to control. No changes in the levels of splenic lymphocyte surface molecules were found in the three groups (T1, T2 and control group) during the experimental periods. The pathological results demonstrated that the inflammatory response in T2 group was lighter than the T1 group, which was in accordance with the inflammatory cytokines levels. The above results indicated that the process of antigen removal effectively reduced the α-Gal antigens content in T2 bone substitutes, which caused little immune response in vivo and could be used as bone healing materials. This study also demonstrated that GGTA1 knockout mice can be used as a routine tool to assess the immune risk of animal-derived biomaterials.

The immune effect of intrathymic inoculation and whole body irradiation on production of xenoantibody in a pig-to-monkey heart transplantation model

BACKGROUND

This study was designed to investigate the immune effect of intrathymic inoculation and whole body irradiation on the production of monkey anti-pig xenoantibodies in a pig-to-monkey model.

METHOD

Donor (Meishan pig) and recipient (Rhesus monkey) animals were divided randomly into three groups: 1) the control group (group A), 2) the whole body irradiation (WBI) group (group B), and 3) the irradiation and intrathymic inoculation group (group C). The change in the percentage of antibody-positive (IgM and IgG) cells after pretreatment and xenografting were detected by flow cytometry.

RESULTS

After WBI, the level of natural IgM antibody decreased significantly (p<0.01), but the level of natural IgG antibody did not change (p>0.05). After xenografting, the levels of elicited xenoreactive IgM and IgG antibodies (EXA) in group C rose slower than in groups A and B (p<0.01).

CONCLUSIONS

Irradiation and intrathymic inoculation dampened the production of elicited xenoreactive IgM and IgG antibodies in a pig-to-monkey model.

Immunosuppression and xenotransplantation of cells for cardiac repair

The death of highly vulnerable cardiomyocytes during ischemia leads to cardiac dysfunction, including heart failure. Due to limited proliferation of adult mammalian cardiomyocytes, the dead myocardium is replaced by noncontractile fibrotic tissue. Introducing exogenous cells to participate in the regeneration of infarcted myocardium has thus been proposed as a novel therapeutic approach. In view of the availability of various xenogeneic cells and fewer ethical and political concerns that surround human embryonic stem cells and fetal cardiomyocytes, cellular xenotransplantation may be a potential alternative approach for cardiac repair in humans. However, one of the most daunting challenges of xenotransplantation is immunorejection. This article summarizes the progress in cellular xenotransplantation for cardiac repair in experimental settings and the current understanding of possible immune responses following the engraftment of xenogeneic cells. The public attitude towards xenotransplantation is reportedly more favorable to receiving cells or tissues than a whole organ, but many scientific obstacles need to be overcome before the utilization of xenogeneic cells for cardiac repair in patients with heart disease becomes applicable to clinical practice.

Maturation of xenoantibody gene expression during the humoral immune response of rats to hamster xenografts

Immunoglobulin isotype switching represents an important component of antibody maturation in the development of humoral immune responses. We have recently conducted a series of studies in a nonimmunosuppressed rodent model to define the kinetics of xenoantibody production and seek evidence for the maturation of xenoantibody Ig gene expression by xenograft recipients. LEW rats were transplanted with hamster cardiac xenografts and the grafts were allowed to remain in situ for prolonged immune stimulation of the host. Anti-hamster antibodies were examined at days 4, 8, 21, 28 and 40 post-transplantation. cDNA libraries specific for rat mu or gamma heavy chains were constructed from B lymphocytes of the xenograft recipients at day 4 and day 21 post-transplantation. Selected cDNA clones encoding the Ig V(H)HAR family of genes from each group were sequenced and analyzed for the presence of somatic mutations. We found that the reactivity of xenoantibodies examined with flow cytometry underwent sequential changes in which IgM titers peaked at day 8 post-transplantation (PTx) and returned to low levels after 21 days. IgG titers started to increase at about one week PTx and peaked at 21-28 days. All the IgG isotypes (IgG1, 2a, 2b and 2c) were differentially involved in the IgG responses. Serum passive transfer experiments demonstrated that IgM antibody fractions separated from sera at day 4 post-transplantation were capable of causing hyperacute rejection (HAR) of hamster xenografts, whereas IgM fractions from days 21-40 failed to cause HAR (N = 7, MST = 4 days), a pattern that was consistent with a rise in total xenoreactive IgM levels at days 4-8 and a fall to low levels at 21 days post-transplantation. IgG-containing fractions separated from day 21-40 antisera caused HAR (N = 7, MST = 36 min) whereas IgG fractions from day 8 sera failed to induce graft rejection. Genetic analysis of the rearranged VH genes from 10 cDNA clones demonstrated that the Ig mu (n = 5) and gamma (n = 5) chain clones used the same family of VH genes (V(H)HAR family) to encode their antibody binding activity. The majority (80%) of the IgM clones were present in their original germline configuration. In contrast, the nucleotide sequences from IgG clones manifested an increase in the numbers of replacement mutations in the CDR region of the Ig heavy chain genes, providing evidence for a potential role for somatic mutation in the maturation of IgG xenoantibody responses as the humoral response matures with time post-transplantation.

Natural antibodies and the host immune responses to xenografts

Natural antibodies are present in the serum of individuals in the absence of known antigenic stimulation. These antibodies are primarily IgM, polyreactive, and encoded by immunoglobulin V genes in germline configuration. Natural antibodies are produced by B-1 lymphocytes, cells that form the primary cell of the fetal and newborn B cell repertoire and may represent the basic foundation upon which the adult repertoire of B cell antibodies is based. Natural antibodies react with a variety of endogenous and exogenous antigens, including xenoantigens expressed by tissues between unrelated species. These antibodies are capable of causing the immediate rejection of grafts exchanged across species barriers. One of the central issues related to our understanding of the immunopathologic mechanisms responsible for rejection of xenografts is whether pre-formed natural antibodies and new antibodies induced following xenotransplantation are produced by the same pathways of B cell antibody production. We have established in studies conducted in rodents and humans that the initial phases of antibody production xenogeneic tissues involves the use of a restricted population of Ig germline genes to encode xenoantibody binding. As the humoral xenoantibody response matures, the same closely-related groups of Ig V genes are used to encode antibody binding and there is evidence for an isotype switch to IgG antibody production and the appearance of somatic mutations consistent with antigen-driven affinity maturation. Our findings in both rodent and human studies form the basis for our proposal that the xenograft response reflects the use of B cell natural antibody repertoires originally intended to provide protection against infection. The host humoral response is inadvertently recruited to mount antibody responses against foreign grafts because they display carbohydrate antigens that are shared by common environmental microbes. This model of xenoantibody responses is being tested in our laboratory through the analysis of the binding of xenoantibodies in their original non-mutated configuration, and the examination of the effect of specific point mutations and gene shuffling have on xenoantibody binding activity. Establishment of the relationships between Ig structural changes and subsequent changes in binding affinity should provide important insights into the role that, natural antibodies and the cells that produce them play in the evolution of the host's humoral responses to xenografts.

The Human Antibody Response to Porcine Xenoantigens Is Encoded by IGHV3-11 and IGHV3-74 IgVH Germline Progenitors

Preformed and induced Ab responses present a major immunological barrier to the use of pig organs for human xenotransplantation. We generated IgM and IgG gene libraries established from lymphocytes of patients treated with a bioartificial liver (BAL) containing pig hepatocytes and used these libraries to identify IgVH genes that encode human Ab responses to pig xenoantigens. Genes encoded by the VH3 family are increased in expression in patients following BAL treatment. cDNA libraries representing the VH3 gene family were generated, and the relative frequency of expression of genes used to encode the Ab response was determined at days 0, 10, and 21. Ig genes derived from the IGHV3-11 and IGHV3-74 germline progenitors increase in frequency post-BAL. The IGHV3-11 gene encodes 12% of VH3 cDNA clones expressed as IgM Abs at day 0 and 32.4–39.0% of cDNA clones encoding IgM Abs in two patients at day 10. IGHV3-11 and IGHV3-74 genes encoding IgM Abs in these patients are expressed without evidence of somatic mutation. By day 21, an isotype switch occurs and IGHV3-11 IgVH progenitors encode IgG Abs that demonstrate somatic mutation. We cloned these genes into a phagemid vector, expressed these clones as single-chain Abs, and demonstrated that the IGHV3-11 gene encodes Abs with the ability to bind to the gal α (1,3) gal epitope. Our results demonstrate that the xenoantibody response in humans is encoded by IgVH genes restricted to IGHV3-11 and IGHV3-74 germline progenitors. IgM Abs are expressed in germline configuration and IgG Abs demonstrate somatic mutations by day 21.