A monoclonal antibody against human decay-accelerating factor (DAF, CD55), D17, which lacks reactivity with semen-DAF

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins I: Localization at Plasma Membranes and Extracellular Compartments

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of plasma membranes (PMs) of all eukaryotic organisms studied so far by covalent linkage to a highly conserved glycolipid rather than a transmembrane domain. Since their first description, experimental data have been accumulating for the capability of GPI-APs to be released from PMs into the surrounding milieu. It became evident that this release results in distinct arrangements of GPI-APs which are compatible with the aqueous milieu upon loss of their GPI anchor by (proteolytic or lipolytic) cleavage or in the course of shielding of the full-length GPI anchor by incorporation into extracellular vesicles, lipoprotein-like particles and (lyso)phospholipid- and cholesterol-harboring micelle-like complexes or by association with GPI-binding proteins or/and other full-length GPI-APs. In mammalian organisms, the (patho)physiological roles of the released GPI-APs in the extracellular environment, such as blood and tissue cells, depend on the molecular mechanisms of their release as well as the cell types and tissues involved, and are controlled by their removal from circulation. This is accomplished by endocytic uptake by liver cells and/or degradation by GPI-specific phospholipase D in order to bypass potential unwanted effects of the released GPI-APs or their transfer from the releasing donor to acceptor cells (which will be reviewed in a forthcoming manuscript).

Natural human immunoglobulins in cancer immunotherapy

The natural or innate immunity is the first-line defense against transformed cells. It guarantees the recognition and removal of malignant cells at an early stage and makes manifest cancers an exceptional event. Natural antibodies, which are predominantly IgM molecules, play a major role in these defense mechanisms and they have some typical features in common. They are coded by specific germline families and equipped mainly with lambda-chains, in contrast to the majority of circulating antibodies. The targets that are recognized by these antibodies are not newly synthesized proteins, but instead post-translationally modified carbohydrate structures on membrane-bound glycoproteins and glycolipids. Another typical feature of these natural IgM antibodies is their ability to induce apoptosis in vitro and in vivo in a death domain-independent manner. These results show that natural IgM antibodies represent a huge reservoir of therapeutic antibodies.

Mapping of binding epitopes of a human decay-accelerating factor monoclonal antibody capable of enhancing rituximab-mediated complement-dependent cytotoxicity

Complement-dependent cytotoxicity (CDC) is thought to be one of the most important mechanisms of action of therapeutic monoclonal antibodies (mAbs). The decay-accelerating factor (DAF) overexpressed in certain tumors limits the CDC effect of the therapeutic anticancer antibodies. The use of DAF blocking antibodies targeted specifically at cancer cells in combination with immunotherapeutic mAbs of cancer may improve the therapeutic effect in cancer patients. In this study, the lysis of Raji cells mediated by CDC was determined after blocking DAF function by anti-DAF polyclonal antibody and 3 mAbs (DG3, DG9, DA11) prepared in our laboratory, respectively, in the presence of the anti-CD20 chimeric mAb rituximab. The binding domains of the three anti-DAF mAbs were identified using yeast surface display technique, and the mimic epitopes of mAb DG3 were screened from a random phage-display nonapeptide library. The results showed that blocking DAF function by anti-DAF polyclonal antibody enhanced complement-mediated killing of Raji cells. Among the 3 mAbs against DAF, only DG3 was found to be able to remarkably enhance the CDC effect of the therapeutic mAb rituximab. DG3 bound to the third short consensus repeat (SCR) of DAF. Binding of DG3 to immobilized DAF was inhibited by mimic epitope peptides screened from the peptide library. Our results suggest that a higher level of DAF expressed by certain tumor cells is significant to abolish the CDC effect of therapeutic anticancer antibodies, and mAbs binding to SCR3 can enhance the complement-mediated killing of Raji cells. It is of significance to identify the DAF epitopes required in inhibiting CDC not only for better understanding of the relationship between the structure and function of DAF, but also for designing and developing anti-DAF mAbs capable of enhancing CDC.

Regulation of the new coexpressed CD55 (decay-accelerating factor) receptor on stomach carcinoma cells involved in antibody SC-1-induced apoptosis

The human monoclonal antibody SC-1 was isolated from a patient with a diffuse-type adenocarcinoma of the stomach using somatic cell hybridization. The immunoglobulin (Ig)M antibody reacts specifically with diffuse- (70%) and intestinal-type (25%) gastric adenocarcinoma and induces apoptosis in vitro and in vivo. When used in clinical trials with stomach carcinoma patients, significant apoptotic and regressive effects in primary tumors have been observed with the antibody SC-1. The SC-1 receptor is a new 82 kd membrane-bound isoform of glycosylphosphatidylinositol (GPI)-linked CD55 (decay-accelerating factor, DAF). CD55 is known to protect cells from lysis through autologous complement and is coexpressed with the ubiquitously distributed 70 kd isoform. The SC-1-specific CD55 isoform is up-regulated shortly after antibody binding, followed by an internalization of the antibody/receptor-complex, whereas the membranous expression of wild-type CD55 remains unchanged. The apoptotic process is marked by cleavage of cytokeratin 18, indicating the involvement of caspase-6 in the apoptotic process. In contrast to other apoptotic pathways, a cleavage of poly(ADP-ribose)polymerase (PARP) is not observed. The expression of the cell-cycle regulator c-myc becomes up-regulated, whereas expression of topoisomerase IIalpha is down-regulated. Induction of apoptosis leads to an increase in the internal Ca(2+) concentration, which is not necessary for the apoptotic process but for the transport of newly synthesized SC-1-specific CD55 isoform to the membrane.

Antibody-independent classical complement pathway activation and homologous C3 deposition in xeroderma pigmentosum cell lines

Of human malignantly transformed cell lines, xeroderma pigmentosum (XP) cell lines were found to be highly susceptible to homologous complement (C): cells were opsonized by C3 fragments on incubation with diluted normal human serum. C3 fragment deposition on XP cells was Ca2+-dependent and occurred on live cells but not UV-irradiated apoptotic cells. (Ca2+ is required for activation of the classical C pathway via C1q and the lactin pathway via mannose binding lectin (MBL), and the surface of apoptotic cells usually activates the alternative C pathway.) In this study we tested which of the pathways participates in XP cell C3 deposition. In seven cell lines that allowed C3 deposition (i), Clq was shown to be essential but MBL played no role in C activation, (ii) Cls but not MASP bound XP cells for activation, (iii) no antibodies recognizing XP cells were required for homologous C3 deposition, and (iv) the alternative pathway barely participated in C3 deposition. Furthermore, the levels of C-regulatory proteins for host cell protection against C, decay-accelerating factor (DAF, CD55) and membrane cofactor protein (MCP, CD46), were found to be relatively low in almost all XP cell lines compared with normal cells. These results indicate that XP cells activate the classical C pathway in an antibody-independent manner through the expression of a molecule which directly attracts C1q in a C-activating form, and that relatively low levels of DAF and MCP on XP cells facilitate effective C3 deposition. The possible relationship between the pathogenesis of XP and our findings is discussed.

The regulation of membrane cofactor protein (CD46) expression by the 3' untranslated region in transgenic mice

Regulation of the membrane cofactor protein (MCP: CD46) was examined. While the expression of MCP in mice carrying MCP(BC2) cDNA with 125 bp of 3' untranslated region (3'UT) was minimal, that in mice carrying MCP cDNA without total 3' UT was evident in many organs. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis clearly showed the presence of mRNA even in transgenic mice with 3' UT, suggesting that the expression was regulated at the post-transcriptional stage. The in vitro expression data of MCP molecules on the stable Chinese hamster ovary (CHO) cell clone corresponded to that in transgenic mice. The first 125 bp downregulated the expression of MCP molecules in combination with not only beta-actin, but also SR alpha, promoter. Also, this region inhibited expression of decay accelerating factor (DAF: CD55) molecules when it was inserted into cDNA of DAF. Furthermore, the first 32 bp of the 3' UT revealed the same downregulation effect as 125 bp on MCP molecules. These findings indicated that the first 125 bp (and the first 32 bp in particular) of 3' UT regulate the expression of MCP molecules in transgenic mice.

The CD46 transmembrane domain is required for efficient formation of measles-virus-mediated syncytium

Two phosphatidylinositol (PI)-anchored versions of a measles virus (MV) receptor membrane cofactor protein (MCP; CD46) were generated by fusing the extracellular domain of MCP to the decay-accelerating factor (DAF; CD55) or its PI anchor. The PI-anchored forms of MCP expressed on Chinese hamster ovary cells, otherwise non-permissive to MV, conferred a smaller MV cytopathic effect than a wild-type MCP, a Ser/Thr-rich domain-deletion mutant and a cytoplasmic tail-deletion mutant of MCP. Therefore the differences in MV receptor properties between the two PI-anchored and three transmembrane forms were investigated. The PI-anchored forms were predominantly expressed on microvilli as in DAF, whereas the other transmembrane forms were found on intracellular membranes. The PI-anchored forms conferred high MV-binding capacity compared with the transmembrane versions. MV replication was, however, severely suppressed in cells expressing the PI-anchored forms, resulting in ineffective syncytium formation. In contrast, cell-to-cell fusion occurred efficiently after co-transfection of cDNA species encoding MV-H. MV-F and any version of MCP. Thus the PI-anchored forms, despite showing sufficient MV binding and cell-to-cell fusion competence together with MV-H and MV-F, mediate inefficient MV entry or replication, which causes severe suppression of the MV cytopathic effect. A biased receptor distribution on microvilli might participate in the selection of a low MV uptake pathway in the PI-anchored forms of MCP. Taken together, the transmembrane portion of MCP is a critical factor for effective virus-cell fusion and the subsequent MV replication.

GPI-anchored complement regulatory proteins in seminal plasma. An analysis of their physical condition and the mechanisms of their binding to exogenous cells

We analyzed and compared the properties of three glycosylphosphatidylinositol (GPI)-anchored proteins. CD59, CD55 (both C regulators), and CDw52, and of the transmembrane C regulator CD46 in seminal plasma (SP). We demonstrated previously that anchor-intact SP CD59 is present on the membranes of vesicles (prostasomes) and that cells acquire this protein during incubation with SP. We now report that this acquisition is due partly to adherence of prostasomes to cells and partly to a second mechanism which may involve micellar intermediates. Using fluorescent labeling, ultracentrifugation, and density gradient centrifugation, virtually all CD46 was present on prostasomes whereas CD59, CD55, AND CDw52 were also detected in a form which remained in the 200,000 g supernatant and equilibrated at higher density than prostasomes in gradients. All three GPI-linked proteins eluted at high molecular mass during size exclusion chromatography of this nonprostasome fraction. As documented by videomicroscopy and biochemical analysis, cells acquired new copies of the GPI-linked proteins during incubation with the nonprostasome fraction as well as with prostasomes. These data demonstrate the presence in SP of a stable population of membrane-free, GPI-linked proteins available for transfer to cells. Binding of these proteins to spermatozoa and pathogens in SP may confer new properties on their membranes including increased resistance to C attack. Finally, our data raise the possibility that lipid-associated GPI-linked proteins may be suitable for therapeutic applications.

Homologous complement activation on drug-induced apoptotic cells from a human lung adenocarcinoma cell line

Activation of the alternative pathway of homologous complement (C) was observed in a human lung adenocarcinoma cell line, CADO 43, after the cells had become apoptotic following treatment in vitro with vincristine and predonisolone. Deposition of C3b and C3bi on the serum-treated apoptotic cells was revealed by flow cytometry with anti-C3b and -C3bi-specific antibodies and immunoblotting with anti-C3 antibody immunoprecipitates extracted from solubilized fractions of serum-treated apoptotic cells. Two molecular mechanisms were found to be responsible for this post-apoptotic C-activation. Firstly, all C regulators, decay accelerating factor (DAF), membrane cofactor protein (MCP) and C3b/C4b receptor (CR1), were diminished on the cell surface concomitantly with the apoptotic process. Secondly, unidentified molecules which potentially activate homologous C and accept C3b/C3bi fragments became expressed on the cell surface during the apoptotic process. These findings may explain the mechanism whereby tumor cells are efficiently eliminated through chemotherapy.

Levels of complement regulatory molecules in lung cancer: disappearance of the D17 epitope of CD55 in small-cell carcinoma

The levels of complement-regulatory molecules (complement receptor type one [CR1], decay-accelerating factor [DAF], membrane cofactor protein [MCP], and an inhibitor of membrane attack complex [CD59]) in lung cancer cells were analyzed to investigate the relation between their expression and histological subtypes, and the possibility of homologous complement deposition on cancer cells. In 25 cell lines (10 adenocarcinoma, 3 large-cell carcinoma, 7 small-cell lung cancer [SCLC], and 5 squamous cell carcinoma), flow cytometric analysis revealed that MCP was expressed in all cell lines, whereas none of the cell lines was CR1-positive. CD59 was detected in all cells. The DAF epitope defined by IA10 was expressed in all cells except one large cell carcinoma cell line. However, another epitope for anti-DAF monoclonal antibody, D17, was not detected in 5 (71.4%) SCLC and in 4 (22.2%) non-small-cell lung cancer. This disparity was seen in most cell lines, irrespective of histological subtypes. The loss of D17 reactivity seemed to be pertinent to malignant phenotype, because most of the normal pulmonary cells possessed the D17 epitope. Furthermore, a cell line lacking DAF (IA10-/D17-) allowed alternative pathway-mediated homologous complement (C3) deposition after pretreatment with anti-MCP antibody. This raises a new possibility for immunotargeting of cancer. These cell lines should be useful in studying the biology of lung cancer.