Revisiting a challenge in transplantation: discordant xenografting

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.

Protection of xeno-hepatocytes from complement-mediated cytolysis by transduction with homologous restriction factor 20 gene using retroviral vector

Xeno-hepatocyte transplantation has the possibility to substitute for clinical liver transplantation in certain restricted hepatic diseases such as inherited metabolic disorders. To overcome human complement-dependent cytotoxicity on xeno-hepatocytes, the effectiveness of ex vivo transfer with the homologous restriction factor 20 (HRF20, CD59) gene was examined on primary-cultured xeno-hepatocytes using a retroviral vector. Primary-cultured rat hepatocytes transduced with HRF20 cDNA expressed HRF20 antigen by flow cytometric analysis and showed the integration of HRF20 cDNA to the genomic DNA by the polymerase chain reaction. The viability of rat hepatocytes incubated with 50% human serum was decreased due to complement-dependent cytotoxicity, whereas that of the transfectant was significantly protected (77.2+/-9.4 vs. 97.8+/-5.2%, p < 0.01). It was concluded that primary-cultured xeno-hepatocytes transduced with HRF20 cDNA using a retroviral vector could escape complement-dependent cytolysis by human serum.

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.

Induction of T-independent xenotolerance in a semi-discordant hamster-to-presensitized, nude rat model

BACKGROUND

We have previously demonstrated that in a concordant hamster-to-nude rat cardiac transplant model, T-independent specific B-lymphocyte and natural killer (NK)-cell tolerance could be induced, leading to long-term xenograft (Xg) survival. Here, we investigated whether the same could be achieved in a clinically more relevant semi-discordant model involving hamster hearts transplanted into pre-sensitized, nude rats.

METHODS

Sensitized, nude rats with high titers of anti-hamster immunoglobulin (Ig)M xenoantibodies (XAbs) were prepared by transplanting a first hamster heart without treatment. One week after rejection, a complete tolerizing regimen was given, including the following: a) an i.v. injection of hamster heart antigens; b) a 4-week administration of malononitriloamide; and c) a single injection of an anti-NK antiserum. Two weeks later, a second hamster heart was grafted. The isotype and level of XAb were examined by fluorescence-activated cell sorting. NK cytotoxicity was evaluated by a standard 4-hr 51Cr release assay. Hamster heart Xgs were examined by conventional histologic and immunohistochemical analysis.

RESULTS

Untreated, presensitized, nude rats developing high titers of IgM XAb underwent hyperacute rejection within 4 hr (n=4) after transplantation of the second hamster heart. Immunohistochemical analysis showed intensive staining for IgM and C3 along the vascular endothelia in the rejected Xgs. In contrast, presensitized, nude rats receiving the complete tolerizing regimen had a rapid decrease in anti-hamster IgM XAb. The second hamster hearts were not rejected and showed long-term survival even after withdrawal of malononitriloamide (n=6). Moreover, tolerant rats showed specific B-lymphocyte tolerance and a specific continuous absence of anti-hamster NK-cell reactivity.

CONCLUSION

T-independent B-lymphocyte and NK-cell xenotolerance can also be achieved in recipients with pre-existing IgM XAb.

Suppression of T-independent IgM xenoantibody formation by leflunomide during xenografting of hamster hearts in rats

BACKGROUND

It was recently shown that leflunomide (LF) delayed xenoantibody (XAb) formation and xenograft (Xg) rejection in a hamster-to-rat heart transplantation model. Our aim in this study was further investigation of the mechanism of LF-mediated suppression of XAb formation.

METHODS

Hamster hearts were heterotopically transplanted to euthymic or nude rats receiving LF and/or cyclosporine (CsA). Second hamster hearts were transplanted at the time of first Xg rejection. Serum from rejecting rats was transferred to naive rats receiving a hamster heart Xg. The isotype of XAbs was examined by fluorescence-activated cell sorting. Tissue deposition of XAbs and complement was determined by immunofluorescence. XAb formation and its response to LF were also investigated in severe combined immunodeficient mice reconstituted with purified CD5+ or CD5- rat B cells.

RESULTS

After xenografting, untreated PVG rats developed high titers of anti-hamster IgM XAbs that appeared T-independent (T-I) as they could not be suppressed by CsA and also occurred in athymic nude rats. A second Xg transplanted in control or CsA-treated rats rejecting a first Xg was subject to hyperacute rejection. Hyperacute rejection also occurred in naive rats after adoptive transfer of serum from rejecting rats. Monotherapy with LF resulted in a suppression of early IgM XAb formation and in a delay of Xg rejection, which was associated with predominantly IgG anti-hamster XAbs. These XAbs were T-dependent, as they did not occur in nude rats and were suppressed by CsA. CD5+ B lymphocytes appeared to contribute to T-I IgM XAb formation, as LF reduced the percentage of peripheral blood CD5+ B lymphocytes and severe combined immunodeficient mice reconstituted with purified CD5+ B cells, but not with CD5- B cells, produced anti-hamster IgM which were suppressed by LF but not CsA.

CONCLUSIONS

In rats, T-I XAb formation is a first step leading to hamster Xg rejection and is suppressed by LF leading to prolonged Xg survival.

In vitro accommodation of porcine endothelial cells by low dose human anti-pig antibody: reduced binding of human lymphocytes by accommodated cells associated with increased nitric oxide production

Transplanted xenografts, protected from rejection by depletion of xenoreactive natural antibodies (XNA) and complement, can sometimes survive when complement levels and titres of anti-graft antibodies return to baseline; this phenomenon is called accommodation. We have previously reported that low concentrations of human IgG induce a change in the phenotype of immortalised porcine endothelial cells (IPEC) consistent with the development of accommodation. The cells acquired a resistance to lysis by human complement and showed a reduced expression of VCAM. In this study, we extend these findings by showing that VCAM expression falls on several IPEC clones and on primary porcine endothelial cells. Moreover, we show that these accommodated cells bind fewer human lymphocytes compared to controls, implying that leukocyte traffic through accommodated endothelium may be altered compared to that through normal endothelium. Finally we show that during the induction of accommodation, porcine endothelial cells produce greater amounts of nitric oxide than controls, due to the expression of inducible nitric oxide synthase (iNOS). We speculate that nitric oxide may be an important mediator in accommodation.

Fetal inoculation with donor cells in cardiac xenotransplantation

BACKGROUND

In utero fetal inoculation with allogeneic cells has produced subsequent tolerance to experimental cardiac allografts. We attempted to extend this observation to a model of xenogeneic cardiac transplantation.

METHODS

Lewis rat fetuses were inoculated with Golden Syrian hamster thymocytes (n = 5) or whole spleen cells (n = 5) on the tenth day of intrauterine life. Six weeks after the birth of pretreated fetuses, heterotopic cardiac transplantation using a hamster donor was performed. Three to 4 weeks after parturition, we performed heterotopic cardiac transplantation using hamster donors in the female Lewis rats whose fetuses had been treated in utero.

RESULTS

Animals treated in utero with either thymocytes or whole spleen cells had graft survival of 3 days, not different from that in untreated Lewis rats (n = 5) (p = not significant). Maternal Lewis rats whose fetuses were treated with thymocytes (n = 5) or whole spleen cells (in = 4) had markedly reduced survival of xenogeneic cardiac grafts (range, 3 to 20 hours; mean, 15 hours; p < 0.01; and range, 5 to 15 minutes; mean, 10 minutes; p < 0.01, respectively). Female Lewis rats without intrauterine inoculation (n = 5) had expected xenograft survival time (3 days) (p = not significant). Immunohistochemical staining of hyperacutely rejected grafts showed deposits of immunoglobulin M as well as immunoglobulin G and complement. In normally rejected xenografts, no immunoglobulin M was detected.

CONCLUSIONS

These studies reveal the surprising observation that fetal exposure to xenogeneic cells sensitizes the maternal rat without tolerizing the fetal rat as observed in an allograft model. In addition, whole spleen cells produce a more vigorous hyperacute rejection than thymocytes, suggesting that B cells or macrophages may be the sensitizing agents. The accelerated rejection observed has the characteristics of an immunoglobulin M antibody-mediated hyperacute rejection response with deposition of complement.

Different membrane cofactor protein (CD46) isoforms protect transfected cells against antibody and complement mediated lysis

The need for organ transplantation, especially of kidneys, exceeds the availability of human donors and the possibility of xenotransplantation from suitable animals is now being addressed. The immediate barrier to success is hyperacute graft rejection, resulting from naturally occurring xenoreactive antibodies and the activation of complement. It is proposed that the intensity of the hyperacute response can be reduced by providing additional regulatory molecules to limit activation of the complement cascade, initially as transfected gene products in cultured cells as an in vitro model and eventually as a transgene in potential donor animals, such as pigs. Limiting the activity of C3b reduces the production of the C3a, C4a and C5a anaphylotoxins, thus curtailing not only the immediate C3b-mediated lytic pathway but also the later effects of a cellular inflammatory response including endothelial and platelet cell activation. To develop and assess the first part of this strategy, we have transfected several cDNA's encoding isoforms of CD46 (membrane cofactor protein). At least four different CD46 isoforms are commonly expressed in almost all human cells, and we have compared two of these and a third form to determine if they mediate different functions. After transfection, CD46-expressing CHO-K1 cells were selected with methionine sulphoximine and identified using monoclonal antibodies. Transfectants with suitable CD46 expression were assayed for primary CD46 function using a lysis assay dependent on the reaction of antibody and complement. In this in vitro model of hyperacute rejection, normal human sera containing natural xenoreactive antibodies were shown to lyse CHO cells, but only in the presence of complement.(ABSTRACT TRUNCATED AT 250 WORDS)

Presence of human chromosome 1 with expression of human decay-accelerating factor (DAF) prevents lysis of mouse/human hybrid cells by human complement

Xenogeneic organs transplanted to phylogenetically distant species are subject to rapid destruction mediated by complement. In humans, the complement activation is regulated by several proteins encoded by a series of closely linked genes (RCA locus) located on chromosome 1. The mouse/human hybrid cell line B10 was found to have retained human chromosome 1. FACS analysis confirmed that RCA products such as decay-accelerating factor (DAF) were expressed on the membrane surface of B10 cells. When exposed to human or rabbit complement in the presence of 'naturally occurring' human anti-mouse antibodies these cells were not lysed by human complement but were killed by rabbit complement. This effect could be abrogated by addition of anti-DAF monoclonal antibody (IC6). The results offer potential for genetic manipulation of the human complement regulatory products in animals to overcome xenograft hyperacute rejection.