Epitope mapping of 10 monoclonal antibodies against the pig analogue of human membrane cofactor protein (MCP)

Development of Improved Double-Nanobody Sandwich ELISAs for Human Soluble Epoxide Hydrolase Detection in Peripheral Blood Mononuclear Cells of Diabetic Patients and the Prefrontal Cortex of Multiple Sclerosis Patients

Nanobodies have been progressively replacing traditional antibodies in various immunological methods. However, the use of nanobodies as capture antibodies is greatly hampered by their poor performance after passive adsorption to polystyrene microplates, and this restricts the full use of double nanobodies in sandwich enzyme-linked immunosorbent assays (ELISAs). Herein, using the human soluble epoxide hydrolase (sEH) as a model analyte, we found that both the immobilization format and the blocking agent have a significant influence on the performance of capture nanobodies immobilized on polystyrene and the subsequent development of double-nanobody sandwich ELISAs. We first conducted epitope mapping for pairing nanobodies and then prepared a horseradish-peroxidase-labeled nanobody using a mild conjugation procedure as a detection antibody throughout the work. The resulting sandwich ELISA using a capture nanobody (A9, 1.25 μg/mL) after passive adsorption and bovine serum albumin (BSA) as a blocking agent generated a moderate sensitivity of 0.0164 OD·mL/ng and a limit of detection (LOD) of 0.74 ng/mL. However, the introduction of streptavidin as a linker to the capture nanobody at the same working concentration demonstrated a dramatic 16-fold increase in sensitivity (0.262 OD·mL/ng) and a 25-fold decrease in the LOD for sEH (0.03 ng/mL). The streptavidin-bridged double-nanobody ELISA was then successfully applied to tests for recovery, cross-reactivity, and real samples. Meanwhile, we accidentally found that blocking with skim milk could severely damage the performance of the capture nanobody by an order of magnitude compared with BSA. This work provides guidelines to retain the high effectiveness of the capture nanobody and thus to further develop the double-nanobody ELISA for various analytes.

Development and Characterization of New Species Cross-Reactive Anti-Sialoadhesin Monoclonal Antibodies

Sialoadhesin (Sn) is a surface receptor expressed on a subset of macrophages in steady state conditions. During inflammation and diseases, Sn is highly upregulated on macrophages and blood monocytes. Therefore, therapies using monoclonal antibodies (mAbs) to target Sn-positive (Sn⁺) cells are a potential strategy for targeted treatment. It has been shown that Sn internalizes after binding with a mAb, though it is not clear whether this is species-specific. In this study, new Sn-specific mAbs were developed and analyzed for cross-reactivity between species. In addition, the newly developed mAbs were compared to mAbs used in previous research for their epitope recognition and other Sn-specific characteristics. Both species-specific and cross-reactive antibodies could be identified. Furthermore, sialic acid-binding of red blood cells (RBC) could be inhibited with mAbs recognizing different epitopes and all mAb showed internalization of Sn. The newly developed mAbs can be used as novel tools for Sn research and further analysis of Sn internalization in different species.

Deciphering IgM interference in IgG anti-HLA antibody detection with flow beads assays

In flow beads assays, the interference of IgM for IgG anti-HLA antibodies detection is not precisely understood. Using the screening flow beads assay for class I HLA antibodies, we analyzed the binding of two IgG mAbs, the anti-class I HLA W6/32 and an anti-beta-2-microglobulin, in the presence of an anti-beta-2-microglobulin IgM mAb. In neat serum, the IgM mAb impaired the detection of both IgG. In EDTA-treated serum, the interference was stronger for the anti-beta-2-microglobulin IgG than for W6/32, in agreement with the finding in surface plasmon resonance that this IgM competed with the anti-beta-2-microglobulin IgG but not with W6/32. The IgM interference was higher in neat than in EDTA-treated serum for both IgG mAbs. The IgM interference was also analyzed with class II single antigen flow beads and sera from two kidney recipients containing IgG and IgM donor specific antibodies. Anti-HLA IgG detection was partially corrected by EDTA, and restored by IgM inactivation with DTT, confirming the results observed with the mAbs. Therefore, three mechanisms can explain the IgM interference for IgG anti-HLA antibodies in flow beads assays: direct competition for antigen, steric hindrance and complement activation.

Design and synthesis of an antigenic mimic of the Ebola glycoprotein

An antigenic mimic of the Ebola glycoprotein was synthesized and tested for its ability to be recognized by an anti-Ebola glycoprotein antibody. Epitope-mapping procedures yielded a suitable epitope that, when presented on the surface of a nanoparticle, forms a structure that is recognized by an antibody specific for the native protein. This mimic-antibody interaction has been quantitated through ELISA and QCM-based methods and yielded an affinity (K(d) = 12 × 10(-6) M) within two orders of magnitude of the reported affinity of the native Ebola glycoprotein for the same antibody. These results suggest that the rational design approach described herein is a suitable method for the further development of protein-based antigenic mimics with potential applications in vaccine development and sensor technology.

Porcine complement regulators protect aortic smooth muscle cells poorly against human complement-induced lysis and proliferation: consequences for xenotransplantation

BACKGROUND

Accelerated atherosclerosis after transplantation has been observed and is characterized by smooth muscle cell proliferation in the graft. Porcine cells are frequently used in models of atherosclerosis and porcine organs are considered for use in transplantation. Complement (C) activation is known to play a major role in rejection of xenografts and is also considered to play a role in the development of atherosclerosis. The aim of this study was to investigate the expression and function of membrane bound regulators of complement (CReg) on porcine aortic smooth muscle cells (PASMC).

METHODS

The PASMC were assessed for expression of CReg and susceptibility to lysis by human C by flow-cytometry. The effect of various cytokines on CReg expression and C-susceptibility was investigated. The ability of human C to induce cell proliferation was assessed using the Alamar blue assay.

RESULTS

The PASMC only express the CReg membrane cofactor protein (MCP) and CD59 on their cell surface. MCP expression was increased by interleukin (IL)-4. In contrast to porcine aortic endothelial cells (PAEC), PASMC were found to be surprisingly sensitive to C-mediated lysis, mainly due to a low level of expression of CD59. Human C-induced proliferation of PASMC, which was dependent on complete membrane attack complex (MAC) formation.

CONCLUSIONS

Endogenously expressed CReg on PASMC poorly protect these cells to human C. Human C can induce proliferation of PASMC. In order to prevent accelerated atherosclerosis in porcine xenografts, increased levels of CReg not only have to be obtained on the endothelial cells but also on the smooth muscle cells.

''Homologous restriction'' in complement lysis: roles of membrane complement regulators

The complement system is a powerful bactericidal immune defence with the potential to damage self cells. Protection of self is provided by expression on cells of a battery of membrane regulators that inhibit activation of complement. Roles of complement in the rejection of transplanted organs have long been recognized, and are particularly relevant in xenotransplantation, where hyperacute rejection is complement-driven. Inhibiting complement was therefore considered early in the history of xenografting, and the use of membrane complement regulators to this end was proposed more than two decades ago. For each of the membrane regulators in humans, early studies implied a species-specificity of action, inhibiting human complement but not that from other species. The dogma of species-specificity dictated strategies for inhibiting complement in xenografts and drove the creation of donor transgenic pigs expressing human regulators. Here we critically evaluate the evidence for species-specificity in membrane complement regulators from humans and other animals. We challenge the dogma and show that there is considerable cross-species activity for each of the membrane regulators of complement. Acceptance of the fact that species selectivity is not a limitation will open new avenues for protection of the xenograft from complement damage.

Isolation and characterization of immortalized porcine aortic endothelial cell lines

Primary porcine endothelial cells have a limited life span in culture. After four to five passages, they tend to de-differentiate and eventually reach senescence. The aim of this work was to establish immortalized porcine aortic endothelial cell lines (AOCs) to facilitate in vitro studies of different pathological process involving the endothelium. Primary porcine aortic endothelial cells (PAECs) were transfected with a plasmid containing the SV40 genome and selected on the basis of morphological and phenotypical features. Flow cytometry analysis demonstrated uptake of acetylated low density lipoproteins (Ac-LDL) and constitutive expression of SLA class I, CD29, CD31, CD41/61, CD80/86, CD46, SWC3, and LAMP-1 antigens by all analyzed lines and showed little differences to primary cells. The functional similarity between primary and immortalized endothelial cells was demonstrated in a cytotoxicity assay using a human natural killer cell line (NKL) as effector. The AOCs cell lines should be valuable tools for in vitro study of the human immune response against pig endothelial cells. In addition, they would be very useful to gain insight in the pathogenesis of some viral haemorrhagic diseases of pig such as African swine fever (ASF) or classical swine fever (CSF).

Swine platelet antigens: section report

A total of 14 antibodies were found to label resting and/or activated swine platelets. Six recognized CD previously characterized for swine (CD29, CD41/61 and CD46). One had been characterized for human cells (CD47). Two antibodies with CD14 and SLA class I specificity suggested by the donor as well as five blind antibodies were also positive on platelets. One antibody appeared to recognize the swine homologue to human CD47, and four remained unclustered.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

Role and regulation of pig CD59 and membrane cofactor protein/CD46 expressed on pig aortic endothelial cells

BACKGROUND

Hyperacute rejection in xenotransplantation is caused by activation of complement (C) on endothelium. We have previously shown that purified C-regulators of the pig (CD59 and membrane cofactor protein [MCP]) are efficient regulators of human C (HuC). The aim of this study was to clarify the role of endogenously expressed C-regulatory molecules on pig endothelium in the protection against hyperacute rejection.

METHODS

Porcine aortic endothelial cells (PAEC) were harvested and cultured for various passages. PAEC were examined for the expression of endogenous pig CD59 and MCP by flow cytometry. PAEC were assessed for their susceptibility to lysis by HuC. The effect of phorbol 12-myristate 13-acetate and various cytokines on the expression of MCP and CD59 and C-susceptibility was assessed.

RESULTS

Primary PAEC showed an initial high level of expression of pig CD59, however, upon culturing, CD59 levels decreased dramatically to about 20% after five passages. In contrast, levels of MCP doubled upon culturing of PAEC to confluency and remained stable during at least five passages. Primary cells and cells in the early passages were more resistant to HuC than cells that were cultured for longer. Blocking the function of CD59 but not of MCP using monoclonal antibody increased the susceptibility to HuC. Purified human CD59 incorporated to a level of expression similar to that of pig CD59 reversed the increased C-susceptibility, suggesting that pig and human CD59 are similarly protective against HuC. Increase of C-resistance and of expression of pig MCP, but not of CD59, was achieved upon incubation with phorbol 12-myristate 13-acetate. Tumor necrosis factor-alpha, interleukin-1beta, interleukin-4, or interferon-gamma had no effect on C-regulator expression or C-susceptibility.

CONCLUSIONS

These data demonstrate the importance of using primary PAEC or cells in the first passages of culturing in in vitro models of xenotransplantation and show that pig MCP and, in particular, pig CD59 play an important role in protection of PAEC from HuC.

Molecular remodelling of human CD46 for xenotransplantation: designing a potent complement regulator without measles virus receptor activity

In pig-to-human discordant xenotransplantation, human complement (C) is a major barrier to long survival of xenografts. The current idea on how to cope with this barrier is that human complement regulatory proteins are forcibly expressed on xenografts to serve as safeguards against host C-induced hyperacute rejection of xenografts. Co-expression of decay-accelerating factor (DAF) (CD55) and membrane cofactor protein (MCP) (CD46) would be the first choice for this trial, because most of the human cells are protected from C-mediated damage by two different modes with these two kinds of C-regulators. Many problems have arisen, however, for MCP expression on grafts. (i) MCP acts as a measles virus receptor, which may function to render donor pigs measles virus (MV) sensitive. (ii) MCP signals immune suppression which causes devastation of the recipient's immune responses. (iii) MCP exerts relatively low self-protective activity against C compared with other cofactors; development of more efficient forms is desirable. (iv) Grafts with a high expression level of MCP are difficult to produce. In this study, we made a number of cDNA constructs of MCP, expressed them on swine endothelial cell lines, and tested cell-protective potency and MV susceptibility. The short consensus repeat 1 (SCR1)-deleted MCP with glycosyl phosphatidylinositol (GPI)-anchored form (Delta1MCP-PI) of MCP was found to be most suitable for the purpose of overcoming these problems. However, it was also found that MV induces two modes of cytopathic effect (CPE) on swine endothelial cells, either MCP-dependent or -independent. Here, we discuss these two points which will be raised through study of MCP-transgenic animals.

Induction of aggregation in porcine lymphoid cells by antibodies to CD46

CD46 is a major transmembrane glycoprotein that belongs to the regulator of complement activation (RCA) family. Recently, mAbs to human CD46 were shown to suppress IL-12 production. Here, we describe that mAbs against different porcine CD46 epitopes induced a marked adhesion of normal lymphocytes. Addition of low amounts of antibody to freshly isolated lymphocytes or thymocytes resulted in the clustering of the cells. Cross-linking of CD46 molecules seems essential since Fab fragments failed to induce aggregation. This aggregation was dependent on active cell metabolism and on the presence of divalent cations and required a functional cytoskeleton. It was not inhibited by antibodies to CD18, CD29, CD2, CD11a and CD11b. Staurosporine, an inhibitor of protein kinases, partially blocked the aggregation. This finding is indicative of a role of protein kinases in the transduction of the signal generated by CD46 engagement.

Distribution of membrane cofactor protein (MCP/CD46) on pig tissues. Relevance To xenotransplantation

Membrane cofactor protein (MCP; CD46) is a 50-60 000 MW glycoprotein, expressed on a wide variety of cells and tissues in man, which plays an important role in regulating complement activation. Human MCP has also been shown to be the receptor for measles virus. We have recently identified the pig analogue of MCP and demonstrated that pig MCP has cofactor activity for factor I-mediated cleavage of C3b when these components are derived either from pig or human. As a consequence, pig MCP is an efficient regulator of the classic and alternative pathways of human and pig complement. In order to define the potential importance of MCP in protecting against complement activation in the pig, we have conducted a comprehensive survey of its distribution in pig cells and organs. As in humans, MCP in the pig is broadly and abundantly distributed. Pig MCP is highly expressed on all circulating cells, including erythrocytes, in contrast to its absence on human erythrocytes. Multiple isoforms of MCP are found on cells and in tissues, probably representing products of alternative splicing analogous to those found in man. MCP is abundantly expressed throughout all tissues examined with particularly strong staining on the vascular endothelium. Connective tissue elements within liver and testis are also strongly stained by anti-pig MCP antibodies. Pig MCP is expressed only weakly on skeletal muscle cells and expression is absent from smooth muscle cells in the lung and vessel walls, sites at which human MCP is expressed. Of particular note, MCP is not expressed in B-cell areas of the germinal centres of lymph nodes.