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1.
In pursuing studies on the early events in the infection of human B cells by Epstein-Barr virus (EBV), we examined the host cell attachment phase with a panel of B-cell-specific monoclonal antibodies. One of the monoclonal antibodies, OKB7, directly blocked the attachment of purified EBV to B lymphocytes in the absence of a second anti-immunoglobulin antibody and thereby prevented EBV infection of tonsil and peripheral blood B cells. Although earlier studies have shown a close association of the EBV and complement receptor (CR2), an anti-CR2 monoclonal antibody, anti-B2, did not directly block the binding of EBV to B cells. A comparison of the structures recognized by these monoclonal antibodies on various cell types and their functional and physiochemical properties was undertaken. Flow cytometric analysis revealed that the molecules detected by OKB7 and anti-B2 were coexpressed to the same extent on B cells but were not expressed on T-cell lines. OKB7 and anti-B2 both immunoprecipitated a 145,000-molecular-weight membrane protein with an isoelectric point of 8.2 from membrane extracts of Raji lymphoblastoid cells. OKB7 and, to a lesser extent, anti-B2 directly blocked the attachment of C3d,g-coated fluorescent microspheres and sheep erythrocytes bearing C3d to B cells, indicating that these antibodies also react with CR2. These studies indicate that the EBV-CR2 receptor is a single membrane glycoprotein which possesses multiple antigenic and functional epitopes.  相似文献   

2.
Epstein-Barr virus (EBV) was purified and biotinylated without significant loss of its cell-transforming activity. The use of biotinylated virus in conjunction with antibodies specific for selected cell surface molecules and flow cytometric analysis allowed for the positive identification of the virus-binding lymphocytes among a heterogeneous mononuclear cell population. Biotinylated EBV efficiently bound to all B lymphocytes, including those bearing surface mu, delta, gamma, and alpha immunoglobulin heavy chains or the surface CD5 (Leu-1) marker, but not to T lymphocytes, natural killer cells, or monocytes. By using biotinylated EBV and specific monoclonal antibodies in competitive inhibition experiments, it was also found that the virus attaches to an epitope on the CR2 molecule (the receptor for C3d and EBV), which is close to or identical with the one recognized by OKB7 monoclonal antibody, and that cell surface structures other than CR2 cannot mediate attachment of EBV. Moreover, studies on the binding of the virus to induced B lymphocytes (cells in S through G2 phase), and this was associated with the disappearance of the surface CR2 molecule and the inability of the virus to attach to these cells. The approach described here should be useful in studying the attachment of other viruses, identifying the specific cell types involved, and analyzing the effect of the cell cycle on virus binding.  相似文献   

3.
A number of studies have indicated that the complement receptor type 2 (CR2), which is the receptor for C3d, a degradation fragment of the complement component C3, regulates B lymphocyte activation and growth. Early reports have described that C3 regulates T cell-dependent (TD) antibody responses. The involvement of CR2 in the antibody response to T cell-independent type 2(TI-2) antigens was investigated because neonatal B cells, which are unresponsive to TI-2 antigens both in vivo and in vitro, express a significantly decreased level of CR2 as compared to B cells of adult donors. We utilized type 4 pneumococcal polysaccharide (PS4) as a model TI-2 antigen. In order to study the relationship between CR2 and the response to PS4, B cells were costimulated with PS4 and monoclonal antibodies (MAb) to CR2. HB5 and OKB7 anti-CR2 monoclonal antibodies enhanced the in vitro response of adult B cells to PS4, as measured in a PS4-specific spot-forming cell assay. Neonatal B cells could only be induced to respond to PS4 using high concentrations of OKB7 anti-CR2 MAb. The 8-mercaptoguanosine (8MGuo), an agent that can overcome the in vitro unresponsiveness to PS4 of neonatal B cells, increased CR2 expression on adult and neonatal B cells. Furthermore, 8MGuo synergizes strongly with anti-CR2 antibodies in augmenting the anti-PS4 antibody response. Data presented in this report provide evidence of CR2 involvement in the antibody response to PS4 and that the neonatal B cell unresponsiveness to TI-2 antigens may be due to the decreased expression of CR2.  相似文献   

4.
OKB7, an IgG2 mouse monoclonal antibody, binds to the C3d receptor (CR2) of human B lymphocytes in or near the active site of the receptor. OKB7 precipitates the same molecule as monoclonal antibodies B2 and HB-5 which have previously been reported to react with the C3d receptor at epitopes distant from the active site. The epitope recognized by OKB7 is distinct from those bound by B2 and HB-5 as shown in competitive binding experiments and is near the complement binding site since OKB7 blocks EAC3d rosetting at concentrations as low as 1 microgram/ml in the absence of cross-linking antibodies.  相似文献   

5.
The 145-kDa molecule that has been identified as the C3d receptor CR2 was isolated from lysates of Raji cells by affinity chromatography by using the monoclonal antibody (MoAb)HB-5. The purified protein was incorporated into 14C-phosphatidylcholine liposomes by deoxycholate dialysis followed by flotation on discontinuous sucrose gradients. Incorporation of the receptor was verified by testing the gradient fractions for CR2 by an enzyme-linked immunosorbent assay. Liposomes were shown to be unilamellar vesicles ranging in diameter from 25 to 100 nm by electron microscopy. The external orientation of CR2 in the membranes was demonstrated by immunoelectron microscopy. The functional activities of liposomes containing CR2 and liposomes without protein were compared. CR2 liposomes bound to EC3d, but not to E, and this binding was inhibited by the anti-CR2 MoAb OKB7 and by a MoAb specific for C3d. Control liposomes failed to bind to either E or EC3d. The ability of CR2 to function as a receptor for Epstein Barr virus (EBV) was tested in two ways. First, CR2 liposomes bound to B95-8, a cell line expressing EBV membrane antigens, but not to B95-8 cells treated with the viral DNA polymerase inhibitor phosphonoformic acid. Second, liposomes containing CR2 were shown by ultracentrifugal analyses to bind directly to purified EBV, and this binding was also inhibited by OKB7. Control liposomes did not bind to B95-8 cells or to EBV. These findings show that CR2 purified from detergent extracts of Raji cells can be reconstituted into lipid membranes with maintenance of its dual functions as a receptor for C3d and EBV.  相似文献   

6.
We transfected human complement receptor 2 (CR2/CD21) cDNA containing eukaryotic expression constructs into CR2-negative mouse L cells and human K562 erythroleukemia cells. We subsequently selected stably transformed cells that expressed human CR2, as assessed by flow microfluorimetry analysis and immunoprecipitation of 125I-labeled surface membranes using the monoclonal anti-CR2 antibody, HB5. Utilizing flow microfluorimetry analysis, epitopes recognized by anti-CR2 mAb HB5, OKB7, B2, and four other anti-CR2 antibodies were detected on CR2 expressing transfectants but not parental cells. In addition, CR2 expressing transfected cells efficiently formed rosettes with sheep erythrocyte intermediates bearing human C3bi and C3d, but not C4b or C3b, consistent with the known ligand specificity of CR2. CR2 containing transfectants were also demonstrated to specifically bind EBV. Infection with EBV of CR2 expressing L cells and K562 cells resulted in mean expression of Epstein-Barr nuclear Ag (EBNA) at 48 h in 0.35% of CR2 expressing L cells and 3.7% of CR2 expressing K562 cells. Parental L cells and K562 cells did not express EBNA after EBV infection. These results indicate that CR2 alone is sufficient to transfer both C and EBV receptor functions to heterologous cells. In addition, expression of EBNA was found to be significantly higher in human K562 than mouse L cells, both expressing the same recombinant receptor. These results suggest that mechanisms other than CR2 binding lead to inefficient EBV infection and/or EBNA synthesis in mouse fibroblasts.  相似文献   

7.
The major Epstein-Barr virus (EBV) envelope glycoprotein, gp350, was purified from the B95-8 cell line and analyzed for its ability to mediate virus attachment to the isolated EBV/C3d receptor (CR2) of human B lymphocytes. Purified gp350 and EBV, but not cytomegalovirus, exhibited dose-dependent binding to purified CR2 in dot blot immunoassays. Binding was inhibited by certain monoclonal antibodies to CR2 and to gp350. Liposomes bearing incorporated gp350 bound to CR2-positive B-cell lines but not to CR2-negative lines. Liposome binding was also inhibited by the OKB7 anti-CR2 monoclonal antibody. A computer-generated comparison of the deduced gp350 amino acid sequence with that of the human C3d complement fragment revealed two regions of significant primary sequence homology, a finding which suggests that a common region on these two unrelated proteins may be involved in CR2 binding.  相似文献   

8.
The effect of ligand interactions with the C3d/C3dg complement receptor (CR2) on proliferation of human B lymphoblastoid cells was investigated by using cell cultures performed at low density (1 to 1.5 x 10(3) cells/ml) in a serum-free defined medium to which only transferrin had been added. This medium does not allow proliferation of Raji cells which die within 48 hr with formation of polykaryons. Addition of purified human C3 to the cultures resulted in a dose-dependent proliferation of the cells. A steady growth of Raji cells with a doubling time of 36 hr was observed in cultures containing 10 micrograms/ml of C3. A growth rate similar to that observed in the presence of native C3 was found in the presence of equimolar concentrations of purified C3dg but not of C3c. F(ab')2 anti-C3d but not F(ab')2 anti-C3c antibodies inhibited the mitogenic effect of C3. Preincubation of Raji cells with monoclonal antibody OKB7 which directly inhibits the binding of C3dg to CR2, totally suppressed C3-induced growth of the cells. C3 did not enhance growth of the T lymphoma-derived cell line JM and monocytic cell line U937 which do not express CR2. These results provide direct evidence that the interaction between CR2 and C3 fragments stimulates proliferation of human cells of the B lineage. Because CR2 also acts as a receptor for Epstein-Barr virus on B cells, our results may pertain to the B cell mitogenic properties of the virus.  相似文献   

9.
We have investigated the expression, molecular association, ligand binding properties, and ability to transduce intracellular signals of CR1 and CR2 C3 receptors on cells of the human HPB-ALL T cell line. CR1 and CR2 on HPB-ALL cells bound polymeric C3b and C3dg and several anti-CR1 and anti-CR2 mAb recognizing different epitopes of the receptors on normal peripheral blood cells. Immunoprecipitated CR1 and CR2 exhibited similar m.w. to those of the receptors on normal peripheral blood T and B lymphocytes. CR1 and CR2 were partially associated in the form of CR1/CR2 complexes in the cell membrane as assessed by the ability of the receptors to cocap and cointernalize and to form a detergent-sensitive complex upon immunoprecipitation analysis. Triggering of CR2 with mAb OKB7 that recognizes an epitope associated with the ligand binding site of the receptor induced an increase in intracellular free calcium concentration in HPB-ALL cells. The signal provided by mAb OKB7 did not synergize with that triggered by anti-CD3 mAb UCHT1. Triggering of CR1 did not result in changes in intracellular free calcium concentration. Our observations have significance for the biology of normal human T cells because the majority of peripheral blood T cells that express CR1 also expressed CR2 and because a change in (Ca2+)i was induced by mAb OKB7 in purified normal T cells. These functions may be relevant for the regulatory role of C3 fragments on the immune response to T-dependent Ag and for the penetration into T cells of lymphocytotropic viruses.  相似文献   

10.
Four monoclonal antibodies (OKB7, HB-5, AB-1, and anti-B2) that recognize a 145-kDa B cell-specific membrane structure have markedly different abilities to 1) inhibit C3d and EBV binding to B cells, 2) immunoprecipitate a 145-kDa B cell protein, and 3) stimulate B cell proliferation and differentiation into Ig-secreting cells. This study was initiated to determine whether these four monoclonal antibodies (MoAb) react with the same protein; a related goal was to determine whether the structure(s) recognized by these antibodies constitutes an antigenically related family of structurally distinct molecules. In the studies presented here, the four MoAb were found to fully immunoprecipitate the purified 145-kDa B cell molecule isolated by immunoaffinity chromatography on either OKB7, HB-5, or AB-1 columns, findings that show conclusively that the antibodies all react with the same B cell protein. The variable ability to immunoprecipitate this B cell membrane protein was found to result from differences in exposure or accessibility of the relevant antigenic epitopes in the detergent extract. The 145-kDa molecule immunoprecipitated with the four MoAb was equivalently sensitive to endoglycosidase F and yielded the same banding pattern after digestion with endoglycosidase F and after partial digestion with either S. aureus V8 protease or with trypsin. Within the limits of the sensitivity of these techniques, therefore, there is no evidence for carbohydrate or protein differences in the EBV/C3d receptor (CR2) molecule recognized by the four MoAb. Additional studies showed that the four MoAb react with distinct and nonoverlapping antigenic epitopes on the 145-kDa molecule. The variable abilities of the four MoAb to inhibit CR2 function and EBV binding and to trigger B cell activation, together with the other findings noted above, indicates that the 145-kDa EBV/C3d receptor possesses discretely localized functional domains.  相似文献   

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