Anti-non-Gal porcine endothelial cell antibodies in acute humoral xenograft rejection of hDAF-transgenic porcine hearts in cynomolgus monkeys

BACKGROUND

Anti-Gal alpha 1-3Gal (Gal) antibodies play a major role in hyperacute rejection and acute humoral xenograft rejection (AHXR) in porcine-to-nonhuman primate transplantation. The role of anti-non-Gal antibodies in AHXR is less well defined.

METHODS

Eleven cynomolgus monkeys received a heterotopic heart transplant from a human decay-accelerating factor transgenic pig, and maintenance immunosuppression with cyclosporin A or tacrolimus, steroids, mycophenolate sodium or mycophenolate mofetil, and in 10 animals the Gal-containing soluble glycoconjugate GAS914. Six ended with AHXR (6 to 78 day survival) and five did not show AHXR (9 to 36 day survival). Anti-Gal antibodies were depleted in vivo with GAS914, or in vitro with Gal-coated Sepharose beads. IgM- and IgG-class anti-non-Gal antibodies in serum depleted of anti-Gal antibodies were measured by flow cytometry using porcine endothelial target cells.

RESULTS

Compared with pre-transplant values, all six recipients with AHXR showed a substantially higher level of anti-non-Gal IgM antibodies at rejection; in five animals there was also an increase in IgG-class antibodies. There was no relevant change in recipients without AHXR. AHXR at time of cessation of heart contraction could be preceeded by a steady increase in antibody level starting 2 to 3 weeks earlier.

CONCLUSIONS

AHXR is invariably associated with increased circulating anti-non-Gal antibodies. These antibodies are not observed in recipients without AHXR, and five of six recipients with AHXR were adequately depleted of anti-Gal antibodies by maintenance GAS914. This indicates that anti-non-Gal antibodies play a significant role in the pathogenesis of AHXR. Also, the assessment of these antibodies could be used as an early monitor of AHXR.

FDA/PI flow cytometry assay of complement-mediated cytotoxicity of antibodies generated during xenotransplantation

BACKGROUND

A flow cytometry complement-mediated cytotoxicity assay (FCCA) using fluorescein diacetate (FDA) and propidium iodide (PI) to measure antibody-dependent toxicity is useful to determine the success of xenotransplant organs. We evaluated the validity of different mathematical models as a measure of cytotoxicity in FCCA.

METHODS

Sera from untreated baboons (n = 7) and from immunosuppressed animals (n = 5) undergoing different xenotransplantation protocols with pig organs were tested by endogenous FCCA and a similar assay also using exogenous complement, and the results were compared with those of a complement-dependent hemolytic assay to detect anti-pig antibodies (APHA). The influence of PI/FDA staining and the use of several mathematical models were analyzed.

RESULTS

For both groups of animals, we observed high correlations between the endogenous and exogenous FCCA pathways and between calculations based on PI and FDA staining. Of the four mathematical models tested--the Von Krogh equation, two exponential models, and area under the curve--the Von Krogh equation was the most appropriate in terms of goodness of fit and concordance with APHA.

CONCLUSIONS

FDA/PI FCCA is useful to measure endogenous and exogenous complement-mediated cytotoxicities, and it has advantages related to identification of potential new xenoantibodies. Although all four mathematical models produced acceptable solutions, the Von Krogh equation was the best option.

Heterophils isolated from chickens resistant to extra-intestinal Salmonella enteritidis infection express higher levels of pro-inflammatory cytokine mRNA following infection than heterophils from susceptible chickens

Previous studies showed differences in in vitro heterophil function between parental (A > B) broilers and F1 reciprocal crosses (D > C). Our objectives were to (1) determine if in vitro variations translate to differences in resistance to Salmonella enteritidis (SE) and (2) quantitate cytokine mRNA in heterophils from SE-infected chicks. One-day-old chicks were challenged and organs were cultured for SE. Chicks with efficient heterophils (A and D) were less susceptible to SE compared to chicks with inefficient heterophils (B and C). Heterophils were isolated from SE-infected chicks and cytokine mRNA expression was evaluated using quantitative real-time RT-PCR. Pro-inflammatory cytokine mRNA was up-regulated in heterophils from SE-resistant chicks compared to susceptible chicks. This is the first report to quantitate cytokine mRNA in heterophils from SE-infected chicks. These data show a relationship between in vitro heterophil function, increased pro-inflammatory cytokine mRNA expression, and increased resistance to SE in 1-day-old chicks.

Xenogeneic immunization in mice using HER2 DNA delivered by an adenoviral vector

The protective efficacy of xenogeneic vaccination with DNA encoding the HER2 oncogene was evaluated in BALB/c mice transgenic for the transforming form of the neu oncogene, which spontaneously develops carcinomas in all mammary glands. Intramuscular injection of either plasmid DNA followed by electrical stimulation (pVIJ-HER2 with ES) or an adenoviral vector (Ad5-HER2), both expressing the HER2 oncogene, was tested. Immunization using pVIJ-HER2 with ES elicited a cell-mediated response that was much lower than that elicited by the immunization with Ad5-HER2, as measured by the frequency of IFN-gamma-secreting spleen cells. The dominant T-cell epitope of the HER2 protein product (p185) in the BALB/c (H-2(d)) genetic background was identified. While the T-cell response elicited was only partially crossreactive with the corresponding rat epitopes because of sequence variations (89% similarity), a cytotoxic T-lymphocyte activity against the rat immunodominant epitope was also evident. The Ad5-HER2 vaccination induced also antibodies against p185, which crossreacted with the rat protein homolog. Both T- and B-cell responses slowly declined with time. Vaccination with Ad5-HER2 at 6 and 9 weeks of age delayed incidence and reduced multiplicity of tumors in neu transgenic mice.

Anti-pig antibody adsorption efficacy of {alpha}-Gal carrying recombinant P-selectin glycoprotein ligand-1/immunoglobulin chimeras increases with core 2 {beta}1, 6-N-acetylglucosaminyltransferase expression

We have previously described the construction of a P-selectin glycoprotein ligand-1-mouse immunoglobulin Fc fusion protein, which when transiently coexpressed with the porcine alpha1,3 galactosyltransferase in COS cells becomes a very efficient adsorber of xenoreactive, anti-pig antibodies. To relate the adsorption capacity with the glycan expression of individual fusion proteins produced in different cell lines, stable CHO-K1, COS, and 293T cells producing this fusion protein have been engineered. On alpha1,3 galactosyltransferase coexpression, high-affinity adsorbers were produced by both COS and 293T cells, whereas an adsorber of lower affinity was derived from CHO-K1 cells. Stable coexpression of a core 2 beta1,6 N-acetylglucosaminyltransferase in CHO-K1 cells led to increased alpha-Gal epitope density and improved anti-pig antibody adsorption efficacy. ESI-MS/MS of O-glycans released from PSGL-1/mIgG(2b) produced in an alpha1,3 galactosyl- and core 2 beta1,6 N-acetylglucosaminyltransferase expressing CHO-K1 cell clone revealed a number of structures with carbohydrate sequences consistent with terminal Gal-Gal. In contrast, no O-glycan structures with terminal Gal-Gal were identified on the fusion protein when expressed alone or in combination with the alpha1,3 galactosyltransferase in CHO-K1 cells. In conclusion, the density of alpha-Gal epitopes on PSGL-1/mIgG(2b) was dependent on the expression of O-linked glycans with core 2 structures and lactosamine extensions. The structural complexity of the terminal Gal-Gal expressing O-glycans with both neutral as well as sialic acid-containing structures is likely to contribute to the high adsorption efficacy.

Does exposure to swine leukocyte antigens after pig-to-nonhuman primate xenotransplantation provoke antibodies that cross-react with human leukocyte antigens?

BACKGROUND

A potential concern of using pig kidney xenografts for human transplantation is that antibodies produced to swine leukocyte antigens (SLA) may cross-react with human leukocyte antigens (HLA) and thereby limit the scope for a subsequent human organ donor transplant. We therefore investigated whether exposure to SLA after pig-to-nonhuman primate kidney xenotransplantation gives rise to HLA cross-reactive antibodies.

METHODS

Serum samples were obtained from 52 cynomolgus monkeys that received kidney transplants from human decay-accelerating factor (hDAF) transgenic pigs. Samples were collected pre-transplant and at time of autopsy (mean 20 days post-transplantation, range 1 to 53 days) and analyzed for IgG HLA class I and HLA class II specific antibodies by enzyme-linked immunosorbent assay (ELISA) against pooled purified HLA antigens. To ensure the ability of the HLA ELISA to detect cynomolgus monkey IgG binding, parallel experiments were performed to detect IgG Gal-alpha-1,3-Gal-specific antibodies known to be present in cynomolgus monkey serum.

RESULTS

Analysis of both pre- and post-transplantation serum samples by ELISA demonstrated no detectable IgG antibody binding to HLA class I or class II antigens. Using the same ELISA antibody detection reagents, IgG Gal-alpha-1,3-Gal-specific antibodies were identified in 13 of 38 (34%) sera obtained before transplantation and 21 of 52 (40%) sera collected post-transplantation, confirming that the negativeHLA ELISA results were not due to a technical aspect of the assay.

CONCLUSION

This study suggests that exposure to SLA following transplantation of porcine kidneys in nonhuman primates does not give rise to antibodies that cross-react with HLA.

Flow cytometry complement-mediated cytotoxicity assay detects baboon xenoantibodies directed to porcine epitopes undetected by hemolytic assay

The pig-to-primate model is increasingly being utilized as the final preclinical means of assessing therapeutic strategies aimed at allowing discordant xenotransplantation. To obtain information about the nature of cytotoxic response in pig-to-baboon xenotransplants, we sought to determine if serum cytotoxicity in this model was assay dependent. Sera from nine kidney or heart xenotransplanted baboons were obtained before transplantation and at the time of acute humoral xenograft rejection (AHXR). Cytotoxicity was measured by an anti-pig haemolytic assay (APHA) and by a flow cytometry complement-dependent assay (FCCA), using pig blood lymphocytes (PBLs). Serum samples showing inter-assay differences were absorbed with pig erythrocytes and assayed by APHA and FCCA, as well as by measuring anti-alphaGal and total anti-pig xenoantibodies. The results showed that in four AHXR samples, FCCA cytotoxicity was higher than APHA cytotoxicity. Absorption with pig erythrocytes diminished FCCA and removed APHA cytotoxicity. Residual FCCA activity was due to total anti-pig and IgM anti-alphaGal and non-Gal antibodies. Our results indicate that some cytotoxic antibodies present in the sera of xenotransplanted baboons at time of AHXR are IgM antibodies directed against pig PBL antigens not detected by APHA.

Gene Therapy-Mediated CD40L and CD28 Co-stimulatory Signaling Blockade plus Transient Anti-xenograft Antibody Suppression Induces Long-Term Acceptance of Cardiac Xenografts

BACKGROUND

The authors have previously demonstrated that inhibition of CD28 and CD40 ligand (CD40L) co-stimulatory signals by adenovirus-mediated cytotoxic T-lymphocyte-associated (CTL) antigen 4 (A4) immunoglobulin (Ig) and CD40Ig gene therapies induces tolerance or long-term acceptance in rat liver and heart allograft transplantation. In this study, the authors examined whether co-stimulation blockade with a brief course treatment of FK779, a novel leflunomide derivative, would be an ideal strategy for controlling xenograft rejection.

METHODS

Hamster hearts were transplanted into Lewis rats. Adenovirus vector coding (Ad) CD40Ig, CTLA4Ig, or LacZ gene (1 x 10(9) plaque-forming units) was administered intravenously to recipient rats 2 days before or immediately after xenografting. FK779 (10 mg/kg/day) was administered orally to recipients for 7 days beginning on day -1. Graft survival, graft histology, and xenoreactive antibodies were examined.

RESULTS

: Both untreated and AdLacZ-treated control rats rejected cardiac xenografts, with a median survival time (MST) of 3 days. Co-stimulatory blockade alone by AdCTLA4Ig, AdCD40Ig, or both could not overcome such delayed xenograft rejection (DXR) (MST, 3-4 days). Under a short-course FK779 treatment that suppressed T-cell-independent xenoreactive antibodies, administration of AdCD40Ig (MST, 30.5 days) but not AdCTLA4Ig (MST, 9 days) significantly prolonged xenograft survival as compared with the FK779 monotherapy (MST, 7 days). In contrast, DXR and cellular rejection were controlled successfully and all xenografts were accepted for over 100 days when AdCTLA4Ig and AdCD40Ig were administered under FK779 induction therapy. However, chronic rejection was present in all long-term surviving xenografts.

CONCLUSIONS

: Gene therapy-based co-stimulation blockade with FK779 induction treatment seems to be an attractive strategy with which to control xenograft rejection.

Hyperacute rejection of hDAF-transgenic pig organ xenografts in cynomolgus monkeys: influence of pre-existing anti-pig antibodies and prevention by the alphaGAL glycoconjugate GAS914

BACKGROUND

Our introductory pig-to-cynomolgus monkey heart or kidney transplantation using organs from pigs transgenic for human decay-accelerating factor (hDAF), showed a high incidence of hyperacute rejection (HAR), which was ascribed to extraordinary high levels of anti-pig antibodies. We evaluated the efficacy of GAS914, a Gal alpha 1-3Gal trisaccharide linked to a poly-l-lysine backbone, in inhibition of HAR.

METHODS

hDAF transgenic heterotopic heart (n = 15) or life-supporting kidney (n = 8) transplantation included induction with cyclophosphamide or anti-thymocyte globulin, and maintenance with cyclosporine or tacrolimus, steroids and mycophenolate sodium/mofetil. Four doses of GAS914 were given before transplantation. Rejection was confirmed by graft histology, and anti-pig antibody levels were determined in various assays.

RESULTS

Four of six heart transplants without GAS914 treatment showed HAR. Nine subsequent transplants with GAS914 pre-treatment, did not show HAR (chi-square, P < 0.05). Two of four kidney transplants without GAS914 treatment ended with HAR. Four subsequent transplants with GAS914 did not show HAR. Animals with HAR showed extremely high antibody levels. Samples just before transplantation showed significantly higher antibody levels in recipients presenting with HAR. In all assays antibody levels were significantly lowered by GAS914 pre-treatment.

CONCLUSIONS

HAR of hDAF solid organs could be ascribed to high levels of anti-pig antibodies. It is hypothesized that the hDAF transgene shows a threshold in efficacy, above which an overwhelming attack by antibodies and complement activation cannot be modulated to prevent HAR. HAR does not occur when animals with lower levels are used, or when antibodies are effectively depleted from the circulation by GAS914 treatment.

A normal organ antigen and its corresponding antibodies in pathological human sera

Previous studies on the appearance in pathological human sera of antibodies to an antigen of normal mammalian organs were continued. In gel precipitation reactions, antibodies combining with saline extracts of mammalian organs were found in 20 of 63 cancer sera and in 4 of 15 syphilis sera, but only in 3 of 56 other pathological sera. Furthermore, an identical antigen was demonstrated in 5 of 58 pathological sera. The antigen under study did not belong to any group of known heterophile antigens and it was devoid of organ and tissue specificity.

Cross-reactivity between swine leukocyte antigen and human anti-HLA-specific antibodies in sensitized patients awaiting renal transplantation

Xenotransplantation is increasingly viewed as a promising way to alleviate the problem of patients who have alloreactive lymphocytotoxic antibodies and therefore tend to accumulate on the waiting list for renal transplantation. One barrier to xenotransplantation in these patients could be the hyperacute or acute vascular rejection as a result of preexisting anti-HLA antibodies that recognize swine leukocyte antigens. The cross-reactivity of sera from 98 patients with pig lymphocytes was studied by flow cytometry. After absorption of xenoreactive natural antibodies (XNA), isotype, class, and antibody specificity causing a positive cross-match (XM) were determined. For nonsensitized patients, all of the antibody binding to pig lymphocytes was due to XNA, which were removed by pig red blood cells absorption. In contrast, in sensitized patients, after removal of XNA, pig lymphocyte XM remained positive. There was no correlation between antibody binding to pig lymphocytes and Ig isotype (IgG or IgM) or HLA class-specific antibodies. For testing evidence that class II-specific antibodies were responsible for antibody binding to pig lymphocytes, HLA class I-specific antibodies were absorbed with pooled human platelets. It was confirmed that HLA class II-specific antibodies were responsible for the positive pig XM, but the strength of the positive XM was weaker than the strength caused by HLA class I-specific antibodies. Sera with multiple specificities (plurispecific sera) displayed a greater frequency of cross-reactivity with swine leukocyte antigens (P < 0.05). Seven of 11 highly immunized patients without cross-reactivity IgG with porcine lymphocytes showed positive XM before an IgM was used. The results demonstrate the cross-reactive nature of HLA antibodies and therefore point out the need to perform a prospective XM after absorption of XNA in presensitized individuals.

Tacrolimus inhibits discordant islet xenograft rejection: a study in the pig-to-rat model

The aim of the present study was to evaluate the immunosuppressive effect of tacrolimus (TAC) in discordant islet xenotransplantation. Fetal porcine islet-like cell clusters (ICCs) were transplanted under the kidney capsule in normoglycemic rats treated with TAC monotherapy, TAC plus other immunosuppressive drugs or cyclosporin A (CsA) monotherapy. Twelve or 24 days after transplantation, the extent of a cellular infiltration in the xenografts was evaluated using immunohistochemistry. In some animals, the grafts were examined for antibody and complement deposition and the levels of xenoreactive antibodies in serum were determined. In untreated rats, the xenografts were completely rejected after 12 days and no intact ICCs remained. TAC monotherapy (at 0.5 and 1.0 mg/kg b.w.) almost completely inhibited rejection for up to 12 days. In animals treated with TAC monotherapy (at 0.5 mg/kg b.w.), rejection was markedly inhibited for up to 24 days. However, the effect after 24 days was not consistent and in some grafts there were signs of rejection. The protective effect of TAC observed in this study is in contrast to the findings in rats given CsA monotherapy in which no or only a marginal effect on islet xenograft rejection was observed. Only when CsA was given at 20 mg/kg b.w., an inhibitory effect could be observed. Immunosuppression with TAC at a suboptimal dose (0.3 mg/kg b.w.) plus 15-deoxyspergualin or brequinar also had an inhibitory effect on the rejection. In animals given TAC plus mycophenolate mofetil, a protective effect was observed as well; however, this effect was not consistent.

IgM-enriched human introvenous immunoglobulin strongly inhibits complement-dependent porcine cell cytotoxicity mediated by human xenoreactive antibodies

Treatment with intravenous immunoglobulin preparations consisting of human IgG (IVIgG) prevents hyperacute rejection of pig xenografts transplanted into primates by inhibition of the classical complement pathway. Recent studies indicate that IVIg preparations mainly consisting of human IgM (IVIgM) have a stronger capacity than IVIgG to inhibit the complement system. IVIg preparations also contain xenoreactive antibodies (XAb) binding to pig cells. In the present study, we compared IVIgG and IVIgM for their capacity to inhibit xenogeneic complement activation, with special reference to the roles of IgG and IgM XAb present in these preparations. Xenogeneic complement activation was studied by exposure of pig cells (PK15) to human serum. For some experiments, IVIgG and IVIgM were depleted from XAb by immune absorption. Exposure of PK15 cells to human serum induced surface deposition of C4 and C3 and cytotoxicity, which could be inhibited in a dose-dependent fashion by both IVIgM and IVIgG. The efficacy of IVIgM was more than 10 times higher than that of IVIgG. IgG XAb were detected IVIgG and IVIgM whereas IgM XAb were only present in IVIgM. Depletion of XAb from the IVIg preparations did not modify the protective properties of IVIgG against cytotoxicity induced by human serum, whereas the IVIgM-mediated protection against xenogeneic cytotoxicity was only slightly improved. IgM-enriched IVIg is a potent inhibitor of xenogeneic complement activation and complement-dependent cytotoxicity of human serum to pig cells, irrespective of the presence of cytotoxic xenoreactive IgM antibodies in this preparation. Therefore, IVIgM has a promising therapeutic significance for the treatment of (hyper)acute xenograft rejection.

Definition of the immunogenic forms of modified human LDL recognized by human autoantibodies and by rabbit hyperimmune antibodies

Humans and laboratory animals recognize human modified LDL as immunogenic. Immune complexes (ICs) isolated from human sera contain malondialdehyde-modified LDL (MDA-LDL) and N (epsilon)(carboxymethyl)lysine-modified LDL (CML-LDL) as well as antibodies reacting with MDA-LDL, copper-oxidized LDL (OxLDL), CML-LDL, and advanced glycosylation end product (AGE)-modified LDL. OxLDL and AGE-LDL antibodies isolated from human sera recognize the same LDL modifications and do not react with modified non-LDL proteins. Rabbit antibodies have different reactivity patterns: MDA-LDL antibodies react strongly with MDA-LDL and MDA-BSA but weakly with OxLDL; OxLDL antibodies react strongly with OxLDL and weakly with MDA-LDL; CML-LDL antibodies react with CML-LDL > CML-BSA > AGE-LDL > OxLDL; AGE-LDL antibodies react strongly with AGE-LDL, react weakly with OxLDL, and do not react with CML-LDL. Thus, human and rabbit antibodies seem to recognize different epitopes. Capture assays carried out with all rabbit antibodies showed binding of apolipoprotein B-rich lipoproteins isolated from ICs, suggesting that laboratory-generated epitopes are expressed by in vivo-modified LDL, although they are not necessarily recognized by the human immune system. Thus, the definition of immunogenic forms of modified LDL eliciting human autoimmune responses requires the isolation and characterization of autoantibodies and modified LDL from human samples, whereas rabbit antibodies can be used to detect in vivo-modified human LDL.

Acute vascular rejection of xenografts: roles of natural and elicited xenoreactive antibodies in activation of vascular endothelial cells and induction of procoagulant activity

BACKGROUND

Hyperacute rejection of vascularized discordant xenografts can now be effectively managed. However, acute vascular rejection (AVR) then ensues, resulting in graft destruction, coagulopathy, or both within weeks. The aim of this study was to determine associations between humoral responses to the xenograft and the induction of AVR, coagulopathy, or both.

METHODS

In vitro, heat-inactivated, naive or sensitized baboon sera containing xenoreactive natural or elicited antibodies were used to activate porcine aortic endothelial cells (PAEC) in vitro. Tissue factor expression on PAEC was determined as an index of heightened procoagulant activity. In vivo, porcine renal xenografts were transplanted into immunosuppressed baboons, and at the time of rejection or the development of a consumptive coagulopathy, biopsy specimens were obtained for studies of xenoreactive antibody binding and tissue factor expression.

RESULTS

In vitro, incubation of PAEC with naive baboon sera containing natural anti-Galalpha1,3Gal (Gal) antibodies resulted in minimal tissue factor induction; the addition of complement boosted procoagulant responses. Elicited xenoreactive antibodies, and to non-Gal epitopes alone, induced high amounts of procoagulant activity on PAEC; the addition of complement resulted in overt cytotoxicity. In vivo, AVR was associated with xenoreactive antibody deposition in the graft. When vascular endothelial binding of xenoreactive antibody was combined with the expression of tissue factor, consumptive coagulopathy developed irrespective of histopathologic features of AVR.

CONCLUSIONS

Our in vitro results indicate that elicited antibodies, potentially to non-Gal epitopes, induce endothelial cell activation and tissue factor expression; in vivo, a consumptive coagulopathy occurred when there was xenoreactive antibody deposition and increase of tissue factor.

Retransplantation of discordant xenogeneic islets with costimulatory blockade

BACKGROUND

The aim of the study was to analyze the possibility of xenogeneic islet retransplantation using costimulatory blockade.

METHODS

Streptozotocin-induced diabetic mice were transplanted under the kidney capsule with human islets. Mice were nephrectomized and retransplanted with 1000 human islets under the contralateral kidney capsule 14 days later. Four groups were performed group I, first and second Tx without MR1; group II, first Tx without MR1, second Tx with MR1; group III, first Tx with MR1, second Tx without MR1; group IV, first and second Tx with MR1. A control group was transplanted only once without MR1 with human islets. After second Tx, cross-matches between recipient, serum and human lymphocyte were done for detection of antidonor antibodies.

RESULTS

In the control group, mean graft survival was 13 (+/-7) days. In group I, mean graft survival was 5 +/- 3 days. In group II, mean graft survival was 16 +/- 13 days. In group III, mean graft survival was 81 +/- 22 days. In group IV, no rejection were recorded and all graft survived more than 120 days. Pretransplant cross-matches were negative. In groups I and II all cross-matches were positive, while none were positive in group IV.

CONCLUSION

Retransplantation of xenogeneic islets was associated with accelerated rejection. After presensitization, MR1 was unable to induce tolerance to a second Tx. MR1 given at the first Tx only allowed prolonged survival of the second Tx, but rejection still occurred. MR1 given at first and second Tx allowed long-term survival of retransplanted xenoislets and prevented occurrence of antidonor antibodies.

Anti-Gal IgG potentiates natural killer cell migration across porcine endothelium via endothelial cell activation and increased natural killer cell motility triggered by CD16 cross-linking

Xenoreactive antibodies (Ab) are important for the development of acute vascular rejection (AVR) of xenografts characterized by monocytes, natural killer (NK) cells and neutrophils infiltrating the graft. The mechanisms by which anti-galactose alpha 1,3galactose (alpha-Gal) IgG influence NK cell migration across porcine aortic endothelium (PAEC) were investigated. NK cell migration across PAEC increased in the presence of anti-alpha-Gal IgG. Anti-alpha-Gal IgG exposure activated PAEC as shown by an increased expression of CD62E and CD106. NK cells adhered, spread and showed motile forms on plastic surfaces coated with human IgG, IgG Fc and on mAb against CD16, but not on mouse IgG or BSA, suggesting that CD16 cross-linking can mediate increased adhesiveness. Increased NK cell motility was observed on Boyden filters coated with human IgG, IgG Fc, and mAb against CD16 and the alpha 4, alpha 5, alpha L, beta 1 and beta 2 integrin chains. No motile response was seen on mouse IgGor CD7, CD56 and alpha 6 integrin mAb. NK cell migration on human IgG and anti-CD16 Ab was blocked by anti-CD16 or anti-beta 2, but not anti-beta 1 Ab, implying that the motile response triggered by CD16 cross-linking is mediated via beta 2 integrins. Preformed or induced anti-alpha-Gal IgG may therefore contribute to AVR by stimulating innate immune cell infiltration of the graft.

Alloantibody and xenoantibody cross-reactivity in transplantation

The recent availability of pigs homozygous for alpha1,3-galactosyltransferase gene knockout, and improved immunosuppressive regimens that prevent an elicited antibody response, are expected to contribute to significantly increased survival of pig organs transplanted into primates, bringing clinical trials of xenotransplantation closer. Patients highly sensitized to human leukocyte antigens, who may be precluded from obtaining a human donor organ, would be one group that might benefit from xenotransplantation. However, there have been few studies on whether there is cross-reactivity of anti-human leukocyte antigen antibodies with pig antigens. What data there are suggest that such cross-reactivity exists and that this may be detrimental to the outcome after transplantation of a pig organ. Neither is it known whether sensitization after a pig xenograft would preclude subsequent allotransplantation, although the data available suggest that this will not be the case. Further investigation on allo- and xenoantibody cross-reactivity is required.

Testing and validating a homogeneous immunometric assay for interference

We analyzed 95 sera, demonstrating interference in a previous study, with the Kryptor homogeneous time-resolved fluorescence resonance energy transfer carcinoembryonic antigen (CEA) immunoassay (Brahms AG, Berlin, Germany). Only one serum differed, i.e., 6.0 μg/l for Kryptor vs. 13.3 μg/l for a microtiter plate in-house immunofluorometric assay (IFMA), using both aggregated mouse immunoglobulins as blocker and capture monoclonal antibody (Mab) F(ab′)

Kryptor-CEA assay results thus agreed with our inhouse CEA assay results, showing no interference. As the Kryptor-CEA assay antibodies were sensitive to interference and the Kryptor-CEA assay buffer did not reduce interference as efficiently as our in-house assay buffer, the Kryptor-CEA assay format was crucial for the absence of interference.

Reduced bleeding events with subcutaneous administration of recombinant human factor IX in immune-tolerant hemophilia B dogs

Intravenous administration of recombinant human factor IX (rhFIX) acutely corrects the coagulopathy in hemophilia B dogs. To date, 20 of 20 dogs developed inhibitory antibodies to the xenoprotein, making it impossible to determine if new human FIX products, formulations, or methods of chronic administration can reduce bleeding frequency. Our goal was to determine whether hemophilia B dogs rendered tolerant to rhFIX would have reduced bleeding episodes while on sustained prophylactic rhFIX administered subcutaneously. Reproducible methods were developed for inducing tolerance to rhFIX in this strain of hemophilia B dogs, resulting in a significant reduction in the development of inhibitors relative to historical controls (5 of 12 versus 20 or 20, P <.001). The 7 of 12 tolerized hemophilia B dogs exhibited shortened whole blood clotting times (WBCTs), sustained detectable FIX antigen, undetectable Bethesda inhibitors, transient or no detectable antihuman FIX antibody titers by enzyme-linked immunosorbent assay (ELISA), and normal clearance of infused rhFIX. Tolerized hemophilia B dogs had 69% reduction in bleeding frequency in year 1 compared with nontolerized hemophilia B dogs (P =.0007). If proven safe in human clinical trials, subcutaneous rhFIX may provide an alternate approach to prophylactic therapy in selected patients with hemophilia B.

Hyperacute rejection of mouse lung by human blood: characterization of the model and the role of complement

BACKGROUND

The pathophysiology of hyperacute lung rejection (HALR) is not fully understood. A mouse model of HALR by human blood would be valuable to efficiently dissect the molecular mechanisms underlying this complex process, but it has not been described.

METHODS

We developed a xenogenic mouse lung-perfusion model. Perfusion with heparinized autologous blood (n=3) was compared with human blood unmodified (n=7) or pretreated with C1 inhibitor (n=5) or soluble complement receptor type 1 (n=6) at unchanged flow conditions.

RESULTS

Perfusion with autologous blood was associated with stable physiologic parameters and no overt evidence of lung injury for up to 2 hr. Pulmonary artery perfusion pressure increased rapidly after introduction of unmodified human blood, plasma anti-Gal(alpha)1,3Gal antibodies declined (90% immunoglobulin [Ig]M, 80% IgG), and lungs reliably met survival endpoints within 11 min (median 10 min, confidence interval [CI]: 9-11). Human Ig and neutrophils were rapidly sequestered in the lung. Survival was significantly prolonged in the soluble complement receptor type 1 group (36 min, CI: 26-46) (P<0.01) and in the C1 inhibitor group (23 min, CI: 21-25) (P<0.05), and pulmonary vascular resistance elevation and complement activation were significantly attenuated but not prevented.

CONCLUSIONS

Hyperacute rejection of mouse lung by human blood occurs with kinetics, physiology, and histology closely analogous to the pig-to-human model. In addition, as in that model, neither of two potent soluble-phase complement inhibitors prevented complement activation or HALR. We conclude that the mouse lung model is relevant to dissect the cellular and molecular mechanisms governing HALR.

CD46 protects pig islets from antibody but not cell-mediated destruction in the mouse

We have previously reported that islets present in cultured fetal pig pancreas (FPP) are resistant to destruction by Galalpha(1,3)Gal antibodies and compliment, but are susceptible to the 'secondary' antibody response which occurs on transplanting pig islet tissue to Galo/o murine recipients. In a model of antibody-mediated graft rejection, we tested the resistance of porcine islets to antibody. Using FPP from human CD46 transgenic pigs, we now report that the complement regulator, CD46, affords protection from antibody-mediated rejection when mouse anti-pig serum (MAPS) was administered to scid mice bearing PFF grafts from either CD46 transgenic or normal pigs. Indeed, whereas normal pig islets were destroyed by an intraperitoneal (i.p.) injection of 0.1 to 0.2 ml of MAPS antibody, destruction of CD46-expressing transgenic islets required 0.5 ml, i.e. up to five times the amount. In contrast, there was no prolongation of the survival of CD46 transgenic mouse skin or heart major histocompatibility complex-compatible or -incompatible allografts--rejected by predominantly cellular immune mechanisms, as opposed to xenograft rejection. Although complement regulators have been examined for their protective role in hyperacute rejection of vascularized xenografts, it is clear that they also have protective effects in the later, antibody-mediated responses, but are unlikely to effect the inflammatory response in cell-mediated rejection.

Cellular participation in delayed xenograft rejection of hCD55 transgenic pig hearts by baboons

Delayed xenograft rejection (DXR) of pig organs by baboons currently represents the major obstacle to successful xenotransplantation. Although antibodies (Abs) are believed to play a fundamental role in this form of rejection, so far little is known concerning the potential cellular component. Biopsies taken during DXR of human CD55 transgenic pig hearts by non-treated (n = 2), alpha-Gal immunoadsorbed (n = 2), or immunosuppressed (n = 9) baboons were studied. The cellular element was explored by determining not only its phenotype by classical immunohistochemical techniques but also its activity in terms of cytokines, cytolytic enzymes and other mediators using quantitative reverse transcription polymerase chain reaction. All porcine xenografts underwent DXR; within 5 days in non-treated and immunoadsorbed animals but significantly delayed (6 to 29 days) in immunosuppressed animals. Cellular infiltration in non-immunosuppressed grafts consisted predominantly of monocytes/macrophages, CD8 cells and a few CD4 T-cells. The predominant baboon transcripts detectable were the proinflammatory cytokines interleukin1-alpha and tumor necrosis factor-alpha, the lymphokine interferon-gamma and the cytotoxic enzyme granzyme B. However, these cellular components were lacking in the immunosuppressed animals. Despite these differences, strong immunoglobulin M (IgM) and C5b-9 complement deposition was observed in all animals at rejection. Together our findings suggest that although the humoral response plays a predominant role in DXR through IgM Abs and complement activation, there is a clear cellular infiltrate in DXR in this model that is likely to contribute to rejection through a strong pro-inflammatory and cytotoxic environment, necessitating substantial immunosuppression for a prolonged graft survival.

The role of antibodies and von Willebrand factor in discordant pulmonary xenotransplantation

Pulmonary xenotransplantation is one potential solution to the paucity of donors but is currently limited by rapid failure of the graft. Unlike cardiac and renal xenotransplants, pulmonary xenografts release large quantities of swine von Willebrand factor (vWF). Swine vWF binds xenoreactive antibodies and is capable of activating primate platelets. The contribution of swine vWF to lung xenograft dysfunction is not entirely clear. To probe the role vWF plays in xenograft dysfunction, we traced the fate of xenoantibodies in vWF+ and von Willebrand factor-deficient (vWFD) swine lungs. These studies showed that the vast majority of xenoantibodies bind the vWF released from the vWF+ swine lung, and thus do not remain bound on lung endothelium. The vWF complexed to xenoantibody remained capable of aggregating primate platelets. With this information, we performed swine-to-baboon lung transplants using vWF+ and vWFD donors. Without vWF present to complex xenoantibodies, a picture of hyperacute rejection more typical of heart and kidney xenografts, with antibody deposition along the graft endothelium, interstitial hemorrhage, and edema occurred. These findings suggest that porcine vWF plays a major role in the pathogenesis of pulmonary xenograft dysfunction, and suggests promising strategies to treat lung xenograft dysfunction.

Characteristics of CD59 up-regulation induced in porcine endothelial cells by alphaGal ligation and its association with protection from complement

BACKGROUND

Activation of endothelial cells may result in proinflammatory and procoagulant changes, or in changes that protect the endothelial cells (EC) from injurious insults. Stimulation of porcine EC with human anti-porcine antibodies, or lectins from Bandeiraea simplicifolia that bind terminal Galalpha(1-3)Gal (abbreviated alphaGal), can induce EC protection from cytotoxicity by human complement. These EC also exhibit up-regulation of CD59 protein and mRNA expression. Porcine CD59 has been reported to protect porcine cells from human complement. Therefore we investigated the specificity requirements and other characteristics of the induced CD59 up-regulation, as well as the role of up-regulated CD59 in lectin-induced protection of EC from human complement.

METHODS

Aortic EC were incubated in vitro with alphaGal-binding lectins B. simplicifolia lectin I isolectin B4 (IB4) and B. simplicifolia lectin I (BS-I) and CD59 expression was assessed by flow cytometry and enzyme linked immunosorbent assay (ELISA). Binding requirement was studied using disaccharides containing either alphagalactosyl or betagalactosyl moieties to inhibit CD59 up-regulation. Protection from complement killing was assessed after incubation of EC with human serum as a source of anti-porcine antibodies and complement. The role of CD59 in lectin-induced protection was studied in the presence of an anti-pig CD59 antibody and after removal of CD59 using phosphatidylinositol (PI)-specific phospholipase C (PI-PLC).

RESULTS

We found that induction of CD59 up-regulation required specific binding of the lectin to terminal alphaGal and was not induced either by soluble factors that may be released from EC by stimulation with the lectin or by TNF-alpha, IFN-gamma, or IL-1alpha. Unstimulated or BS-I-treated EC showed little or no expression of decay accelerator factor (DAF). Removal of membrane-associated CD59 (and other proteins that are associated with the membrane through PI linkage) with PI-PLC from EC that had been exposed to lectin restored their complement sensitivity to various degrees, depending on the extent of lectin-induced protection. Cytotoxicity was completely restored in cells that exhibited partial protection induced with lectin at low doses or for a short period of time. However, EC that were fully resistant to complement did not regain sensitivity to complement after removal of CD59. Changes in CD59 expression did not modify the degree of C9 binding.

CONCLUSIONS

Induction of CD59 expression required specific binding of the lectin to terminal alphaGal and was not induced by soluble factors that may be released from EC by lectin stimulation. Increased CD59 expression may contribute to this form of protection from complement; however, mechanisms other than CD59 up-regulation appear to be essential for the development of full protection.

Elicited non-anti-alphaGAL antibodies may cause acute humoral rejection of hDAF pig organs transplanted in baboons

The combination of immunosuppression and GAS 914, a polylysine containing alphaGal trisaccharide type 2 (TRI 2), has been associated with the prevention of acute humoral xenograft rejection (AHXR) in human decay accelerating factor (hDAF) pig-to-baboon xenotransplants. The aim of this study was to investigate the role of immunosuppression and GAS 914 to neutralize xenoantibodies before and after xenotransplantation. Eight baboons underwent heteropic heart xenotransplantation with hDAF transgenic pig organs, receiving GAS 914 before and after transplantation. Six baboons (Group A) were treated with an immunosuppression protocol that included cyclophosphamide (CyP), Neoral, ERL, and steroids. The other 2 baboons (Group B) were treated with the same immunosuppression but with a 50% reduction in the doses of CyP. No xenograft from Group A underwent acute humoral xenograft (median survival, 27 days), whereas the 2 in Group B experienced rejection (median survival, 6 days). GAS 914 depleted both immunoglobulin (Ig)M and IgG anti-alphaGAL disaccharide (DI), trisaccharide type 2 (TRI 2), and trisaccharide type 6 (TRI 6), before and after transplantation in Groups A and B. However, cytotoxic antibodies with other anti-pig specificities were elicited by the xenografts in Group B leading to AHXR.

Depletion of anti-Gal antibodies by the intravenous infusion of Gal type 2 and 6 glycoconjugates in baboons

BACKGROUND

Natural anti-Gal antibodies (NAb) to Gal epitopes play a key role in the rejection of pig cells or organs transplanted into primates. We have investigated the effect on NAb return after extracorporeal immunoadsorption (EIA) of the continuous intravenous (i.v.) infusion of (i) bovine serum albumin conjugated to Gal type 6 oligosaccharides (BSA-Gal) or (ii) a poly l-lysine backbone conjugated to Gal type 2 or 6 oligosaccharides (PLL-Gal).

METHODS

Porcine mobilized peripheral blood progenitor cells (PBPC) obtained by leukapheresis from MHC-inbred miniature swine (n = 9) were infused intravenously (i.v.) into baboons: Group 1 baboons (n = 4) received whole body and thymic irradiation, splenectomy, antithymocyte globulin, cobra venom factor, cyclosporine, mycophenolate mofetil, anti-CD154mAb, porcine hematopoietic growth factors, and EIA before transplantation of high doses (2 to 4 x 1010 cells/kg) of PBPC; Group 2 baboons (n = 3) received the Group 1 regimen plus a continuous i.v. infusion of BSA-Gal for up to 30 days; Group 3 baboons (n = 5) received the Group 1 regimen plus a continuous i.v. infusion of PLL-Gal type 2 (n = 2) or both PLL-Gal types 2 and 6 (n = 3) for up to 30 days.

RESULTS

Group 1: NAb returned to pre-PBPC levels within 20-30 days, but there was no induction of antibody to Gal or non-Gal determinants; Group 2: NAb was undetectable or at very low level during BSA-Gal therapy. In one baboon, however, IgG to Gal type 2, but not to type 6, returned during BSA-Gal therapy; Group 3: NAb was undetectable or at very low level during PLL-Gal therapy. In two baboons that received PLL-Gal type 2, NAb to Gal type 6, but not to type 2, returned during PLL-Gal treatment. Two of five baboons, however, developed systemic infection. Four of five baboons died within 14 days; autopsy revealed focal hemorrhagic injury to their hearts, lungs, and small intestines, with histologic abnormalities that varied between animals from hemorrhage and/or thrombosis in some organs (heart, lungs, or intestine) to signs of infections (bacteria in intestine, cytomegalovirus in liver).

CONCLUSIONS

(i) BSA-Gal and PLL-Gal therapy maintained depletion of NAb. (ii) Some heterogeneity in specificity of NAb was identified, indicating that the infusion of a combination of Gal type 2 and 6 glycoconjugates may be required. (iii) The addition of PLL-Gal to the immunosuppressive regimen was associated with a high incidence of morbidity and mortality without a clear histopathologic entity underlying the cause of death.

Ultrastructural and immunologic characteristics of mouse x cattle xenogeneic hybridomas originating from bovine leukemia virus-infected cattle

Nine percent of xenogeneic hybridomas originating from a bovine leukemia virus (BLV)-infected cow secreted monoclonal IgM antibodies with multispecific reactivity. Similar reactivity was evident in some antibodies with an unusually long (> 50 amino acids) third complementarity-determining region of the heavy chain. Electron microscopy of hybridomas demonstrated the presence of c-type virus particles consistent with polymerase chain reaction detection of BLV env gene. Some hybridomas contained dilated rough endoplasmic reticulum and cisternae filled with moderately electron-dense granular substance compatible with plasma cells at presecretory stage. The number of chromosomes in xenogeneic hybridomas corresponded to the sum total of mouse and bovine chromosomes. None of the hybridomas showed polyploidy. The immunochemical and genetic analysis of stable bovine immunoglobulin-secreting xenogeneic hybridomas confirms that BLV infection causes polyclonal B cell activation regardless of antigen specificity. Presence of c-type particles in hybridomas suggests that T cell-derived cytokines are not required for sustained BLV expression.

Heterophile antibody blocking agent to confirm false positive serum human chorionic gonadotropin assay

BACKGROUND

Persistently false positive human chorionic gonadotropin (hCG) test results in the absence of an intrauterine pregnancy may lead to a diagnosis of ectopic pregnancy or gestational trophoblastic neoplasia and unnecessary testing and therapy.

CASE

A 38-year-old woman on oral contraceptives and a 53-year-old perimenopausal woman presented with persistently elevated serum beta-hCG levels from 30 to 225 mIU/mL. Both women had a history of working with mice. Urine hCG testing was negative. Serum beta-hCG levels for both patients were negative after pretreatment of their serum with a heterophile antibody blocking agent.

CONCLUSION

These cases illustrate that serum heterophile antibodies can interfere with the hCG enzyme immunoassay and result in false positive values. The addition of heterophile blocking agent to the serum can exclude false positives, thereby preventing unnecessary evaluation and treatment.

Potential implications of ABO blood group for vascular rejection in pig to human kidney xenotransplantation

BACKGROUND

A substantial hurdle for successful xenotransplantation is to negate the effect of xenoreactive natural antibodies [mainly Galalpha1-3Galbeta1-4GlcNAc (alpha-Gal) specific] that cause hyperacute xenograft rejection. Galalpha1-3Gal molecules (alpha-Gal) have close structural homology with human ABO blood groups and therefore an individual's blood group might influence the formation of alpha-Gal specific antibodies. Genetic heterogeneity controlling alpha-Gal specific antibody formation could have important implications for future pig to human xenotransplantation clinical trials. We have investigated the relationship between ABO blood group and immunoglobulin M (IgM) and immunoglobulin G (IgG) alpha-Gal specific antibody titres in sera obtained from renal dialysis patients and healthy blood donors.

METHODS

Serially diluted sera (n = 166) obtained from renal dialysis patients awaiting kidney transplantation (n = 116) and healthy blood donors (n = 50) were tested for IgM and IgG alpha-Gal antibodies using an enzyme-linked immunosorbent assay (ELISA) specific for alpha-Gal. The study cohort comprised 62, 48, 36 and 20 sera obtained from blood group O, A, B and AB individuals, respectively. Reciprocal alpha-Gal specific antibody titres were calculated from ELISA titration curves and stratified by individual blood group.

RESULTS

No significant heterogeneity was found in IgM alpha-Gal specific antibody titres across ABO blood groups. In contrast, marked heterogeneity was observed in IgG alpha-Gal specific antibody titres when stratified by blood group. IgG alpha-Gal specific antibody titres were higher in sera obtained from blood group O renal dialysis patients [median titre 40, interquartile range (IQR) 14 to 72], compared with blood group A (median titre 18, IQR 7 to 54, P = 0.05), blood group B (median titre 6, IQR 0 to 15, P < 0.001) and blood group AB patients (median titre 3.5, IQR 0 to 16, P = 0.002). A similar correlation was found for IgG alpha-Gal specific antibody titres in sera obtained from healthy blood donors with median titres of 20 (IQR 12 to 34), 37 (10 to 91), 9 (0 to 20), and 5.5 (0 to 12) in blood groups O, A, B and AB individuals, respectively. There was a strong interrelationship between alpha-Gal specific antibody class and blood group, with both IgM and IgG alpha-Gal specific antibodies found in 84% of the blood group O sera, 73% of blood group A sera, 50% of blood group B sera and 40% of blood group AB sera (P < 0.001). In a subgroup of 39 renal dialysis patients, IgM and IgG alpha-Gal specific antibody titres were measured in two serum samples obtained at different time-points (median time interval 581 days, range 42 to 4414), and showed a high degree of stability (correlation coefficient 0.88 and 0.90 for IgM and IgG, respectively).

CONCLUSION

IgG alpha-Gal specific antibody titres are significantly higher in the sera of blood group O and A renal dialysis patients and healthy individuals compared with blood groups B and AB. These data indicate that future clinical trials of pig to human xenotransplantation may be more problematic for non-blood group B patients who are likely to have high levels of IgG alpha-Gal specific antibodies that are associated with acute vascular rejection.

Multivalent Galalpha1,3Gal-substitution makes recombinant mucin-immunoglobulins efficient absorbers of anti-pig antibodies

Hyperacute organ xenograft rejection can be prevented by removing anti-pig antibodies by extracorporeal absorption prior to transplantation. A novel recombinant absorber of anti-pig antibodies was developed by fusing the cDNA encoding the extracellular part of a mucin-type protein, P-selectin glycoprotein ligand-1, with an antibody Fc fragment cDNA, which upon coexpression with the porcine alpha1,3 galactosyltransferase carried the xenogeneic epitope, Galalpha1,3Gal (Liu J., Qian Y., Holgersson J., Transplantation 1997, 63, 1673-1682). The biochemical characterization of the mucin/Ig and its absorption efficacy compared with that of porcine thyroglobulin and Galalpha1,3Gal-conjugated beads are reported. The carbohydrate portion of the mucin/Ig constituted 43% of its molecular weight and the majority of the Galalpha1,3Gal epitopes were O-linked as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting following N-glycosidase F digestion. Gas chromatography-mass spectrometry of reduced and acetylated saccharides released by alpha-galactosidase treatment revealed that the fusion protein carried approximately 140 mol of terminal, alpha-linked galactose per mole protein. Based on the reduction in pig aortic endothelial cell cytotoxicity, Galalpha1,3Gal-substituted mucin/Igs on agarose beads were, on a carbohydrate molar basis, shown to be approximately 20 times more efficient than agarose-conjugated pig thyroglobulin, and approximately 5000 and 30,000 times more efficient than Galalpha1,3Gal-substituted agarose and macroporous glass beads, respectively. Structural features of the mucin backbone and its carbohydrate core saccharide chains determine the structural context, spatial orientation and spacing of Galalpha1,3Gal epitopes and are likely to explain the superior absorption efficacy of the recombinant mucin-type chimera.

Therapeutic factor VIII levels and negligible toxicity in mouse and dog models of hemophilia A following gene therapy with high-capacity adenoviral vectors

High-capacity adenoviral (HC-Ad) vectors expressing B-domain-deleted human or canine factor VIII from different liver-specific promoters were evaluated for gene therapy of hemophilia A. Intravenous administration of these vectors into hemophilic FVIII-deficient immunodeficient SCID mice (FVIIIKO-SCID) at a dose of 5 x 10(9) infectious units (IU) resulted in efficient hepatic gene delivery and long-term expression of supraphysiologic FVIII levels (exceeding 15 000 mU/mL), correcting the bleeding diathesis. Injection of only 5 x 10(7) IU still resulted in therapeutic FVIII levels. In immunocompetent hemophilic FVIII-deficient mice (FVIIIKO), FVIII expression levels peaked at 75 000 mU/mL but declined thereafter because of neutralizing anti-FVIII antibodies and a cellular immune response. Vector administration did not result in thrombocytopenia, anemia, or elevation of the proinflammatory cytokine interleukin-6 (IL-6) and caused no or only transient elevations in serum transaminases. Following transient in vivo depletion of macrophages before gene transfer, significantly higher and stable FVIII expression levels were observed. Injection of only 5 x 10(6) HC-Ad vectors after macrophage depletion resulted in long-term therapeutic FVIII levels in the FVIIIKO and FVIIIKO-SCID mice. Intravenous injection of an HC-Ad vector into a hemophilia A dog at a dose of 4.3 x 10(9) IU/kg led to transient therapeutic canine FVIII levels that partially corrected whole-blood clotting time. Inhibitory antibodies to canine FVIII could not be detected, and there were no signs of hepatotoxicity or of hematologic abnormalities. These results contribute to a better understanding of the safety and efficacy of HC-Ad vectors and suggest that the therapeutic window of HC-Ad vectors could be improved by minimizing the interaction between HC-Ad vectors and the innate immune system.

The footprint of antibody bound to pig cells: evidence of complex surface topology

The disaccharide Gal(alpha)1-3Gal is found on more than 45 different molecules on the endothelium of porcine cells and has recently attracted considerable interest, being the major target recognized by xenoreactive antibodies. In this study, the distribution and topology of Gal(alpha)1-3Gal on porcine endothelial cells was examined to access whether some Gal(alpha)1-3Gal-containing molecules might be preferentially recognized by antibodies binding to Gal(alpha)1-3Gal. Thirteen percent of the Gal(alpha)1-3Gal was found on glycolipid and 87% on glycoproteins. Of all the glycoproteins and glycolipids containing Gal(alpha)1-3Gal, two molecules, fibronectin and the integrin beta1 subunit, were most intensely labeled by galactose oxidase, suggesting that these molecules may be preferentially exposed on the apical surface of the endothelium. Binding of anti-Gal(alpha)1-3Gal antibodies to endothelial cell surfaces significantly diminished labeling of fibronectin and the integrin beta1 subunit by galactose oxidase, indicating that these glycoproteins are targets for the antibodies when binding to intact porcine cells.

An investigation of the specificity of induced anti-pig antibodies in baboons

AIM

To provide information on the specificity of induced anti-pig antibodies (Abs) in baboons after exposure and sensitization to pig antigens.

MATERIALS AND METHODS

Baboons (n=7) received either porcine mobilized peripheral blood progenitor cells (n=3), kidney (n=3) or heart (n=1) transplants. After rejection of these cells or organs, pre- and post-rejection sera were analyzed by enzyme-linked immunosorbent assay (ELISA) and flow cytometry to detect and measure anti-Galactosealpha1,3Galactose (Gal) and anti-non-Gal Abs. To study the anti-non-Gal carbohydrate response, the sera were incubated with pig red blood cells pretreated with alpha-galactosidase (to remove Gal) and three other exoglycosidases to remove other potential oligosaccharide epitopes, and studied by flow cytometry. To study the anti-swine leukocyte antigen (SLA) response, non-Gal Abs from two baboons sensitized with kidneys from inbred miniature swine of dd or aa haplotype, respectively, were adsorbed on cells of aa, cc, or dd haplotypes, and binding to aa, cc or dd cells was measured by flow cytometry. Cytotoxicity of anti-non-Gal Abs was tested in vitro by a complement-mediated cytotoxicity assay, using pig cells as targets.

RESULTS

In pre-transplant and pre-rejection sera, anti-Gal Abs were detected, but anti-non-Gal Abs were either absent or at minimal levels. After exposure to pig antigens, baboons developed induced anti-Gal and anti-non-Gal Abs. No anti-non-Gal Abs directed to the tested carbohydrate epitopes could be detected. Anti-non-Gal Abs showed minor evidence of specific SLA haplotype reactivity, suggesting that the major Ab response was to pan-pig determinants. Anti-non-Gal Abs showed a low level of complement-mediated lysis of pig cells in vitro.

CONCLUSIONS

In this limited study, no Ab response to non-Gal carbohydrates was observed, and anti-SLA specificity was minor, indicating that most induced anti-non-Gal Ab was directed against non-specific pig proteins, including SLA-epitopes.

Pre-transplant analysis of accommodation in donor pigs

Accommodation could lead to xenograft acceptance without the need for severe immune suppression. Generally graft accommodation is appreciated in the sensitized recipient, after transplantation. By inducing accommodation in chimeric donors, however, the risk and cost of inducing accommodation in the recipient would be reduced. An indirect assay of accommodation in the donor pig is needed for screening donors prior to procurement of the xenograft. The resistance of peripheral blood lymphocytes to cytolysis by antibody and complement was assessed in chimeric pigs and compared with control pigs. Peripheral blood lymphocytes (PBL) from chimeric pigs demonstrated a wide range of cytolysis (0 to 85%, median 13%) whereas PBL from control pigs were consistently lysed with these conditions (86 to 99%, median 96.5%, P < 0.0001). Accommodation or reduction in cytolysis did not correlate with the amount of chimerism. A longitudinal study demonstrated persistent accommodation of the PBL for as long as 15 weeks, when the donors averaged 68 kg in weight. Accommodation has been induced by low levels of antibodies interacting with the target tissue. An ELISA for sheep IgG was developed and the serum from newborn pigs assessed. Sheep IgG (up to 4.6 microg/ml) was detected in four of seven piglets with chimerism detectable by flow cytometry and in one of four piglets with minimal chimerism, detectable only by PCR. Lymphocyte accommodation was observed in all pigs with detectable sheep IgG. Of four pigs without accommodation, none had sheep IgG. Three pigs without detectable sheep IgG also had accommodation, suggesting that factors other than sheep IgG may induce accommodation. Acute vascular rejection was not observed in the heterotopic heart transplants from six donors with PBL accommodation. Only one incident of moderate diffuse cellular rejection (grade 3) was observed.

Are anti-endothelial cell antibodies a pre-requisite for the acute vascular rejection of xenografts?

BACKGROUND

Vascular rejection occurring within the first few weeks after transplantation is still the major immunological barrier to the long term survival of xenografts. Currently there is no consensus about what to call this type of rejection (acute vascular rejection, delayed xenograft rejection or acute humoral xenograft rejection), nor about how to prevent or treat it.

METHODS

A review of published evidence to define the heterogeneity of this phase of rejection and examine the role of antibodies, complement and graft-infiltrating inflammatory cells.

RESULTS

i) antibodies are always involved in acute vascular rejection; ii) this antibody-mediated rejection may be complement-dependent or -independent; iii) inflammatory cells may mediate an antibody- and complement-independent phase of rejection in some small animal models (which, in its pure form cannot be called 'vascular rejection') iv) there remain significant questions about the relevance of 'accommodation' and the importance of coagulation abnormalities.

CONCLUSIONS

Without doubt, future research would be helped by distinguishing between these different forms of delayed xenograft rejection, using terminology to reflect the involvement of specific pathophysiological mechanisms. An updated classification of the stages of xenograft rejection is proposed here.

Modulation of human anti-pig T cell responses by monoclonal antibodies directed to porcine CD45 molecules

Antibody-mediated targeting of pig costimulatory molecules is assumed to be a possible strategy to achieve donor-specific tolerance after xenotransplantation. However, porcine molecules of the B7 family (e.g. CD86) are expressed on typical antigen presenting cells (APC) and also on vascular endothelial cells. Thus, in vascularized porcine xenografts the usage of therapeutic anti-B7 monoclonal antibodies (mAb) might be associated with damage of the endothelium.

OBJECTIVE

In the present study we asked whether modulation of human T cell reactivity can be obtained by targeting molecules selectively expressed on pig leucocytes.

METHODS

MAb directed to pig CD45 were tested for their capacity to modulate the in vitro activation of human T cells induced by porcine peripheral blood mononuclear cells.

RESULTS

Porcine stimulatory cells induced significant proliferation of human T cells. In the presence of porcine CD45 mAb human T cell responses were reduced by 30-40%. The inhibitory effects were most pronounced when CD45RA mAb were used whereas mAb directed to CD45RC isoforms only moderately inhibited human T cell activation. The tested antibodies had no effects on human T cell activation induced by mitogens or by alloantigen.

CONCLUSION

Manipulation of CD45 molecules on pig leucocytes may reduce their potential to stimulate human T cells. In recipients of vascularized porcine xenografts the usage of anti-pig CD45 mAb could be an approach to block the direct pathway of T cell activation initiated by porcine APC without affecting the endothelium of the graft.

Paul-Bunnell antigen and a possible mechanism of formation of heterophile antibodies in patients with infectious mononucleosis

Sera of patients with infectious mononucleosis contain heterophile anti-Paul- Bunnell (PB) antibodies to erythrocytes of numerous mammalian species. Evidence is presented that the corresponding antigen of bovine erythrocytes is not, as previously described, a single molecule, but a series of glycoproteins with glycans terminated with N-glycolylneuraminic acid (Neu5Gc). The latter compound should be an important part of the PB epitope because, in agreement with the results of others, we found that desialylation of the PB antigen abolishes almost completely its activity. We examined three different preparations of GM3 ganglioside for their capacity to bind anti-PB and found that only GM3 from horse erythrocytes containing Neu5Gc exhibited a low although ELISA measurable PB activity. The other two GM3 preparations, from bovine milk and dog erythrocytes, containing N-acetylneuraminic acid (Neu5Ac) bound little if any anti-PB antibodies. This finding confirms a previous report that human erythrocyte Neu5Ac containing sialoglycoprotein with similar O-linked glycans as the PB-antigen of bovine erythrocytes exhibits only very low PB activity (Patarca & Fletcher, 1995, Crit Rev Oncogen., 6: 305). In conclusion, we present a hypothesis that anti-PB antibodies in patients with infectious mononucleosis are formed against infection-induced cell membrane glycoconjugates containing highly immunogenic Neu5Gc.

Removal of anti-Galalpha1,3Gal xenoantibodies with an injectable polymer

Preformed and elicited Ab's against the Galalpha1,3Gal terminating carbohydrate chains (alphaGal Ab's) are the primary cause of hyperacute and acute vascular xenograft rejection in pig-to-primate transplantation. alphaGal Ab's are produced by long-lived Ab-producing cells that are not susceptible to pharmacological immunosuppression. We reasoned that antigen-specific elimination of alphaGal Ab's might be achieved in vivo by systemic administration of nonimmunogenic polyvalent alphaGal structures with high avidity for alphaGal Ab's. We devised GAS914, a soluble trisaccharide-polylysine conjugate of approximately 500 kDa that effectively competes for alphaGal binding by alphaGal IgM (IC(50), 43 nM) and IgG (IC(50), 28 nM) in vitro. Injections of GAS914 in cynomolgus monkeys, at the dose of 1 mg/kg, resulted in the immediate decrease of more than 90% of circulating alphaGal Ab's and serum anti-pig cytotoxicity. In baboons, repeated injections of GAS914 effectively reduced both circulating alphaGal Ab's and cytotoxicity over several months. Studies with [(14)C]GAS914 in rhesus monkeys and Gal(-/-) mice indicate that GAS914 binds to circulating alphaGal Ab's and that the complex is quickly metabolized by the liver and excreted by the kidney. Remarkably, posttreatment alphaGal Ab titers never exceeded pretreatment levels and no sensitization to either alphaGal or the polylysine backbone has been observed. Furthermore there was no apparent acute or chronic toxicity associated with GAS914 treatment in primates. We conclude that GAS914 may be used therapeutically for the specific removal of alphaGal Ab's.

Transcriptional up-regulation of the signaling regulatory protein LNK in activated endothelial cells

BACKGROUND

A better understanding of inflammatory processes in endothelial cells (ECs) might reveal new ways of controlling inflammation and graft rejection. This study investigates EC genes regulated in response to human tumor necrosis factor (TNF)-alpha and xenogeneic natural antibodies (XNAs) that contribute to endothelial activation during transplantation.

METHODS

Gene expression between resting and activated ECs was investigated by RNA differential display reverse-transcriptase polymerase chain reaction and confirmed by reverse-Northern blot.

RESULTS

Forty-five cDNA fragments corresponding to genes up-regulated in activated ECs were identified. Our findings show that TNF-alpha-mediated EC activation was associated with increased levels of mRNA for the adaptor protein Lnk, the nuclear protein RED, and the initiation factor eIF4G. We further show that Lnk and eIF4G were also up-regulated in response to XNA binding to ECs.

CONCLUSION

Our data suggest that TNF-alpha and XNAs could share common signaling pathways involving Lnk and eIF4G but may also drive specific transcriptional events.

High avidity antibodies to fetal pig pancreas endocrine cells transfer rejection but are not normally generated to fetal pig pancreas xenografts

Previous studies on the contribution of T cell-dependent antibody (Ab) to non-vascular xenograft rejection have yielded conflicting results, being confounded by the presence of recipient T cells and the use of different tissues and immunizing regimens to generate Ab. In the present study, the effect of adoptive transfer of Ab on fetal pig pancreas (FPP) and pig PK15 cell xenografts was examined in T cell-deficient severe combined immune deficiency (SCID) mice. T cell-dependent Abs raised by hyperimmunization with different cell types and by FPP transplantation were compared. Ab raised by hyperimmunization with pig thymocytes exhibited strong binding to pig thymocytes and PK15 cells but did not transfer FPP rejection. IgG1 and IgM, but not IgG3, Abs bound strongly to FPP exocrine and connective tissue, whereas binding to endocrine cells in vitro and in vivo was weak or absent. This pattern of Ab binding was similar to that observed after transplanting FPP into BALB/c mice. Furthermore, serum recovered from BALB/c mice 20 days after FPP transplantation bound strongly to non-endocrine but not endocrine cells and did not transfer FPP rejection. In contrast, serum from mice hyperimmunized with PK15 cells bound strongly to PK15 cells and transferred rejection of intraperitoneal PK15 cells. Furthermore, this serum contained IgG1 and IgM Abs that bound strongly, and IgG3 Abs that bound weakly, to endocrine cells in FPP, and also transferred rejection of FPP in SCID mice. These results indicate that endocrine cells express low concentrations of xenoreactive Ab epitopes and that high Ab concentrations and/or high avidity Abs are required for sufficient endocrine cell binding to cause damage and rejection in the immunodeficient mouse model. Such Abs are not elicited by transplanting FPP into immunocompetent mice. Nevertheless, a contribution of Ab to rejection in immunocompetent mice cannot be excluded.

[Neoglycoconjugates based on dendrimeric poly(aminoamides)]

Neoglycoconjugates containing 4, 8, 16, 32, and 64 terminal residues of B-disaccharide (BDI) or N-acetylneuraminic acid (Neu5Ac) attached to poly(aminoamide)-type dendrimers (PAMAMs) were synthesized. The ability of BDI conjugates to bind natural xenoantibodies (anti-BDI antibodies) and the ability of Neu5Ac conjugates to inhibit the hemagglutinin-mediated adhesion of influenza virus were studied. The biological activity of PAMAM conjugates turned out to be higher than that of free carbohydrate ligands, but less than that of multivalent glycoconjugates based on other types of synthetic polymeric carriers. A conformational analysis of PAMAM matrices and resulting conjugates was performed to determine the statistical distances between carbohydrate ligands. The computations revealed the tendency of the PAMAM chains toward compaction and formation of dense globules. The process results in a decrease in the distances between the carbohydrate ligands in the conjugates and, hence, could affect the ability of glycoconjugates to efficiently bind the polyvalent carbohydrate-recognizing proteins. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2002, vol. 28, no. 6; see also http://www.maik.ru.

Rejection of porcine islet xenografts mediated by CD4+ T cells activated through the indirect antigen recognition pathway

We have previously demonstrated that human T cells responding to porcine islets are primarily CD4+ and recognized porcine major histocompatibility complex class I molecules through the indirect pathway of antigen presentation. To determine whether this mechanism is responsible for rejection of adult porcine islets xenografts, porcine islets from adult pigs were transplanted under the kidney capsule of streptozotocin-treated CD4-knockout (KO), CD8-KO, Ig-KO and normal C57BL/6 mice. Islet xenografts were acutely rejected with similar kinetics when transplanted into normal C57BL/6 (MST=17.6 +/- 3.5 days) and Ig-KO (MST=19.0 +/- 1.7 days) mice. Interestingly, islet xenografts were rejected significantly earlier when transplanted into CD8-KO mice as compared with normal C57BL/6 (MST=7.0 +/- 0.01 days, P=2 x 10-4). Histopathological analysis revealed classical acute cellular rejection with severe diffuse interstitial cellular infiltrates in all rejected islet xenografts. In contrast, islet xenografts were not rejected when transplanted into CD4-KO mice (MST >/= 100 days, P=1 x 10-9). Histopathological analysis revealed no cellular infiltrates and intact islet xenografts. CD4+ T cells from both normal C57BL/6 and CD8-KO xenograft recipients showed detectable proliferative responses to porcine islets in the presence but not in the absence of syngeneic antigen-presenting cells. In addition, the anti-islet proliferative responses observed in normal C57BL/6 mice were significantly lower than those observed in CD8-KO mice. IgG anti-porcine antibodies were readily detected in C57BL/6 and CD8-KO xenograft recipients but not in Ig-KO or CD4-KO recipients. These results indicate that indirectly activated CD4+ T cells mediate acute rejection of adult porcine islet xenografts and that xenoreactive CD8+ T cells and antibodies are not necessary in this process.

An ELISA avoiding interference by heterophilic antibodies in the measurement of components of the plasminogen activation system in blood

Endogenous heterophilic antibodies in blood are known to interfere with two-site enzyme-linked immunosorbent assays (ELISAs) evoking false positive signals. In the present study, we describe an assay for the assessment of components of the plasminogen activation system (uPA, tPA and PAI-1, and their complexes) in blood which is not susceptible to interference by heterophilic antibodies. In the ELISA format, two avian (duck, chicken) antibodies are employed in the pre-analyte and two mammalian (rabbit, goat) antibodies in the post-analyte stage. The assay is compared to our earlier reported ELISA for measuring uPA, tPA and PAI-1 components in tumor tissue extracts. Applying the so-called "nonsense formats", designed against non-existent components, to the NIBSC reference preparation of rheumatoid factor (RF), no response was found with the new assay, whereas a clear RF dose-dependent interfering signal was observed with the original assay designed for tumor tissue extracts. Analysis of tumor-tissue based international reference preparations (RBG EORTC 101094 and 040297), human anti-mouse antibodies (HAMA) containing sera, and sera from patients with rheumatoid arthritis (RA), also displayed no false positive signals. In conclusion, we have developed an ELISA that permits the determination of blood levels of components in the urokinase system, free from disturbance by endogenous heterophilic antibodies.

Purification of salmon thrombin and its potential as an alternative to mammalian thrombins in fibrin sealants

A method to produce highly purified thrombin from salmon blood is described, and a series of biochemical, cell biologic, and biophysical assays demonstrate the functional similarities and some differences between salmon and human thrombins. Salmon thrombin with specific activity greater than 1000 units/mg total protein can be prepared by modifications of the methods used for purification of human thrombin. Using a synthetic substrate based on the human fibrinogen A-alpha polypeptide sequence as an indicator of enzymatic activity, salmon and human thrombin preparations contain similar specific activities per mass of purified protein. Salmon thrombin activates human fibrinogen and initiates the formation of fibrin clots whose structure and rheologic properties are indistinguishable from those of human fibrin clotted by human thrombin. Salmon thrombin also activates human platelets. Approximately 10 times higher activities are needed for the same rate of platelet aggregation compared to human thrombin, and some aspects of platelet activation, most notably phosphatidylserine exposure, are diminished relative to the effects of human thrombin. This latter finding suggests that salmon thrombin may not activate all of the receptors that are targets of human thrombin, although it does appear to activate signals that are sufficient to produce normal rates of activation and aggregation as measured by conventional aggregometry. Together with the recent purification of salmon fibrinogen and its application in mammalian wound healing, the availability of salmon thrombin allows the formulation of biological sealants devoid of any exogenous mammalian proteins and so may aid the design of materials with increased safety from infectious disease transmission.