Tissue-specific phosphorylation of complement receptors CR1 and CR2

Possible involvement of a 95-kDa protein phosphorylation in phorbol 12-myristate 13-acetate-induced suppression of zymosan phagocytosis in guinea pig macrophages

Recently, we characterized a surface antigen (Z-1) of guinea pig macrophages by monoclonal anti-Z-1 antibody. The Z-1 antigen consists of two different polypeptide chains; alpha (140 kDa) and beta (95 kDa). This antigen is closely correlated with the phagocytic activity of the cells for zymosan and presumably functions as a receptor for zymosan. In the present study, the effect of phorbol 12-myristate 13-acetate (PMA) on the function of Z-1 was examined. Incubation of ortho-[32P]phosphate-labeled macrophages with PMA greatly increased the phosphorylation of the beta subunit of Z-1 but not that of the alpha subunit. Optimal phosphorylation was observed when cells were incubated with 300 ng/ml of PMA for 60-120 min. The PMA-induced phosphorylation was markedly suppressed by treatment of the macrophages with H-7, an inhibitor of protein kinase C. A chemotactic peptide, N-formyl-Met-Leu-Phe (fMLP) also caused phosphorylation of the beta subunit. Unlike PMA, fMLP maximized the phosphorylation within 30 s. Purified Z-1 was an excellent substrate for the exogenously added protein kinase C only in the presence of both Ca2+ and phosphatidylserine. H-7 completely inhibited the in vitro phosphorylation. These data suggest that the beta subunit of Z-1 is phosphorylated by protein kinase C. The phosphorylation of Z-1 by PMA and fMLP coincided with inhibition of zymosan phagocytosis. A linear relationship was obtained between the level of phosphorylation of Z-1 and the degree of inhibition of zymosan phagocytosis induced by PMA. Thus, the results suggest that zymosan uptake is negatively regulated by protein kinase C-mediated phosphorylation of the beta subunit of Z-1.

Enhancement of Epstein-Barr virus/C3d receptor (EBV/C3dR or CR2) and nuclear p120 ribonucleoprotein phosphorylation by specific EBV/C3dR ligands in subcellular fractions of the human B lymphoma cell line, Raji

We present herein the first evidence that interaction of specific EBV/C3dR ligands, as human C3bi/C3d and anti-EBV/C3dR MoAb, with EBV/C3dR enhanced significantly, in a dose dependent process, phosphorylation of EBV/C3dR and p120 RNP present in subcellular fractions, as purified plasma membranes and nuclei, of the human B lymphoma cell line, Raji. The use of kinase effectors allowed to detect some of the kinases involved in these phosphorylations. Pp60src-like phosphotyrosine kinase and protein kinase C were involved in the phosphorylation of plasma membrane or nuclear EBV/C3dR. An additional calcium/calmodulin-dependent kinase was also involved in nuclear EBV/C3dR phosphorylation. P120 RNP phosphorylation was under the control of protein kinase C and of CaCl2/Calmodulin-dependent kinase but also of casein kinase II.

Identification of an epitope in the major envelope protein of Epstein-Barr virus that mediates viral binding to the B lymphocyte EBV receptor (CR2)

The Epstein-Barr virus gp350/220 envelope protein mediates virus attachment to the EBV/C3dg receptor (CR2) of human B lymphocytes. Synthetic peptides corresponding to two regions in gp350/220, which have a similar amino acid sequence with the complement C3dg protein, were used to identify a receptor binding epitope. A peptide corresponding to the N terminus of gp350/220, EDPGFFNVE, bound to purified CR2 and to CR2 positive but not CR2 negative B and T lymphoblastoid cell lines. Soluble monomeric gp350/220 peptide blocked CR2 binding to immobilized EBV, while multimeric forms of the N-terminal gp350/220 peptide conjugated to albumin efficiently blocked recombinant gp350/220 and C3dg binding to B cells as well as EBV-induced B cell proliferation and transformation. These studies indicate that the N-terminal region of gp350/220 plays a crucial role in mediating the earliest stages of EBV infection of B cells and provides a molecular basis for the restricted host cell EBV tropism.

Posttranscriptional stabilization of c-fms mRNA by a labile protein during human monocytic differentiation

The c-fms proto-oncogene encodes a transmembrane glycoprotein that is closely related or identical to the receptor for the monocyte colony-stimulating factor CSF-1. The present studies examined the mechanisms responsible for the regulation of c-fms gene expression during human monocytic differentiation. Levels of c-fms mRNA were undetectable in HL-60 promyelocytic leukemia cells, while 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced monocytic differentiation of these cells was associated with the appearance of these transcripts. Run-on transcription assays demonstrated that the c-fms gene was transcriptionally active in uninduced HL-60 cells and that the rate of transcription was unchanged after TPA treatment. These findings suggested that c-fms mRNA levels in HL-60 cells are controlled by posttranscriptional mechanisms. The half-life of c-fms transcripts in TPA-induced HL-60 cells was found to be at least 6 h, while inhibition of protein synthesis with cycloheximide (CHX) decreased this half-life to 4 h. Moreover, inhibition of protein synthesis was associated with decreases in c-fms mRNA levels and a block in the induction of c-fms transcripts by TPA. These findings indicated that the c-fms transcript is stabilized by a labile protein. In contrast to HL-60 cells, c-fms mRNA is constitutively expressed in resting human monocytes and is down-regulated by treatment of these cells with TPA. Run-on assays demonstrated that TPA-induced downregulation of c-fms mRNA levels in monocytes occurred at the posttranscriptional level. Moreover, the results demonstrate that levels of c-fms mRNA are regulated posttranscriptionally by a labile protein. In this regard, the half-life of the c-fms transcript was 6.1 h in monocytes, while treatment of these cells with CHX decreased the half-life to 30 min. Furthermore, this effect of CHX occurred in the absence of changes in the rate of c-fms gene transcription. Together, these findings indicate that c-fms gene expression is regulated at a posttranscriptional level both in HL-60 cells induced to differentiate along the monocytic lineage and in human monocytes. The findings also indicate that levels of c-fms mRNA are regulated by the synthesis of a labile protein which is involved in stabilization of the c-fms transcript.

Identification of murine complement receptor type 2

A rabbit antiserum reactive with the human complement component C3d/Epstein-Barr virus receptor (complement receptor type 2, CR2) immunoprecipitates a Mr 155,000 murine B-cell surface antigen. The apparent molecular weight and cellular distribution of this murine antigen are similar to those of human CR2. Cells expressing the murine protein bind sheep erythrocytes coated with antibody and murine C1-C3d but do not bind Epstein-Barr virus at all. The monospecific antiserum to human CR2 together with goat F(ab')2 anti-rabbit IgG blocks attachment of the C3d-coated erythrocytes to receptor-bearing murine B lymphocytes. To further characterize murine CR2, a lambda gt11 library from the murine late pre-B-cell line 70Z/3 was screened with human CR2 cDNA. A partial cDNA clone of 3.5 kilobases with 79% amino acid sequence identity to human CR2 in the unique intracytoplasmic region and 63% identity to the sixth human CR2 repeat was obtained. Blot hybridization with the murine cDNA clone identified an RNA species of approximately 4.7 kilobases, similar in size to human CR2 mRNA, from a murine B-cell line but not from a murine T-cell line.

Heterogeneity in the B1 (CD20) cell surface molecule expressed by human B-lymphocytes

The B1 molecule (CD20) is a phosphoprotein expressed only by B-lymphocytes. In this study, analysis of B1 immunoprecipitated from surface iodinated B-cell lines and B-lymphocytes has shown that there are several expressed forms of B1. A predominant species of Mr 33,000 represents 75-80% of the iodinated cell surface B1 and a Mr 35,000 species represents 20-25%. Limited proteinase digestion of these two species generated similar peptide maps demonstrating that the different forms of B1 shared common peptides. Biosynthetic labeling with [35S]methionine revealed that the Mr 35,000 B1 species may actually represent two bands of Mr 34,500 and 36,000. Endoglycosidase digestion studies and metabolic labeling in the presence of tunicamycin indicated that neither the Mr 33,000 or 34,500-36,000 forms of B1 were glycosylated. The Mr 33,000 and 34,500-36,000 forms of B1 were constitutively phosphorylated in B-cell lines. However, exposure of B-cells to PMA resulted in a significant increase in the phosphorylation of the Mr 34,500-36,000 form. Exposure to PMA also resulted in an increase in the amount of Mr 34,500-36,000 protein immunoprecipitated from 35S labeled cells. These results suggest that there are multiple forms of the B1 molecule expressed by B-lymphocytes and that this heterogeneity may result from phosphorylation of the B1 protein.

C1q acts synergistically with phorbol dibutyrate to activate CR1-mediated phagocytosis by human mononuclear phagocytes

The adherence of human monocytes and culture-derived macrophages to surfaces coated with complement subcomponent C1q has been previously shown to enhance Fc receptor (FcR)-mediated phagocytosis by these cells. We examined the effects of C1q on C3b/C4b receptor (CR1)-mediated phagocytosis by mononuclear phagocytes. A small percentage of human monocytes cultured in the presence of serum became competent to ingest sheep erythrocytes bearing IgM and C4b (EAC4b). This phagocytic activity was enhanced when these cultured-derived macrophages were adhered to C1q-coated surfaces. However, when cultured in a defined serum-free medium, these cells did not ingest EAC4b, even in the presence of C1q. To investigate this differential responsiveness, we studied the effects of C1q in conjunction with cell-activating agents on CR1 activation. Treatment of serum-free cultured monocytes with phorbol dibutyrate (PDBu), prior to addition of the targets, induced these cells to ingest EAC4b. In addition, when exposed to C1q, both the percentage of these PDBu mononuclear phagocytes ingesting EAC4b and the number of targets ingested increased threefold over the level achieved by macrophages treated with PDBu alone. The chemoattractant N-formyl-methionyl-leucyl-phenylalanine did not activate CR1-mediated phagocytosis and did not substitute for PDBu in causing synergy with C1q. Freshly isolated monocytes adhered to human serum albumin-coated glass slides in the absence or presence of PDBu did not phagocytose EAC4b. Also C1q did not stimulate monocyte CR1-mediated phagocytosis. However, addition of PDBu to cells adherent to the C1q surface triggered phagocytosis of EAC4b. The concentration of PDBu and the time of addition of PDBu relative to addition of the EAC4b targets were found to be important parameters for the achievement of maximal synergy in both the freshly isolated and cultured cell systems. This enhanced phagocytic activity was also seen with cells adhered to the purified collagen-like, pepsin-resistant, fragment of C1q. Since this region was previously shown to interact with C1q surface receptors, it appears that occupancy of this receptor is triggering events contributing to the enhanced cellular function. These experiments suggest that C1q and PDBu promote ingestion via CR1 by different but synergistic mechanisms. These data also demonstrate that the CR1-mediated enhancement of phagocytosis is not specific for FcR-mediated ingestion, but also applies to phagocytosis via CR1.

Up-regulation of the iC3b receptor (CR3) is neither necessary nor sufficient to promote neutrophil aggregation

The iC3b receptor (CR3) is required for neutrophil adhesive functions, including homotypic aggregation. Because stimuli that enhance neutrophil adhesion also induce up-regulation of surface CR3, it is widely held that these two responses are causally related. We have dissociated CR3 display (immunofluorescence) from CR3 function (aggregation). Neutrophils isolated at 4 degrees C and rewarmed to 37 degrees C up-regulated surface CR3 twofold, but did not aggregate. The kinetics of FMLP-induced CR3 up-regulation were discordant with those of aggregation. In the absence of extracellular divalent cations, CR3 expression increased twofold after exposure to FMLP, but neutrophils did not aggregate. FMLP elicited 3.5-fold more aggregation than the ionophore A23187, yet less than one-half as much CR3 up-regulation. 3 mM sodium salicylate inhibited aggregation 55 +/- 4%, but had no effect on CR3 up-regulation. Conversely, 1 mM tetracaine completely inhibited CR3 up-regulation, while significantly enhancing aggregation. Neutroplasts expressed CR3, but did not up-regulate the receptor; in contrast, FMLP induced CR3-dependent aggregation of neutroplasts. We conclude that, although constitutive surface CR3 is required for neutrophil aggregation, the up-regulation of CR3 is neither necessary nor sufficient to promote cell-cell adhesion.

Recombinant human granulocyte-macrophage colony-stimulating factor (rH GM-CSF) regulates f Met-Leu-Phe receptors on human neutrophils

The regulation of mature human neutrophil function by recombinant human granulocyte-macrophage colony-stimulating factor (rH GM-CSF) was studied. Preincubation of neutrophils with this CSF did not stimulate superoxide anion directly but enhanced the subsequent release of superoxide anion in response to stimulation with the bacterial product formylmethionylleucyl-phenylalanine (f Met-Leu-Phe). Enhanced superoxide anion production was evident by 5 min and reached a plateau at 30 min. In contrast, neutrophils preincubated with rH GM-CSF exhibited reduced chemotaxis under agarose in response to a gradient of f Met-Leu-Phe. The inhibition of neutrophil migration was dependent on the dose of rH GM-CSF and exhibited a time-course similar to the effect on superoxide production. Binding studies of f Met-Leu-[3H]Phe to purified human neutrophils revealed heterogeneous binding to unstimulated cells. Two affinity components were identified. The high-affinity component consisted of approximately 2000 sites/cell and had an average Kd of 4 +/- 2 nM (n = 6). The low-affinity component consisted of approximately 40,000 sites/cell and had an average Kd of 220 +/- 130 nM (n = 6). rH GM-CSF caused conversion to a linear Scatchard plot showing no significant change in total binding sites but a single Kd of 30 +/- 10 nM. These data indicate that rH GM-CSF may influence neutrophil responses to f Met-Leu-Phe by regulating the affinity of f Met-Leu-Phe receptors.

Evidence for ecto-protein kinase activity on the surface of human neutrophils

Recent studies have suggested a possible role for extracellular ATP in neutrophil function. This report provides evidence for the existence of an ecto-protein kinase activity on the surface of human neutrophils capable of phosphorylating intrinsic cell membrane proteins as well as exogenous proteins. Addition of extracellular [gamma-32P]ATP to neutrophils resulted in rapid incorporation of 32P into cellular proteins that were sensitive to trypsin. The ability of adherent cells to phosphorylate the exogenous substrate casein, while no protein kinase activity was released into the supernate, provided further evidence for a cell surface associated protein kinase activity.

Transcriptional and posttranscriptional regulation of tumor necrosis factor gene expression in human monocytes

Regulation of tumor necrosis factor (TNF) gene expression was investigated in resting human monocytes and in 12-O-tetradecanoylphorbol-13-acetate (TPA) activated monocytes. TNF transcripts were undetectable in resting monocytes. However, in TPA-activated monocytes, TNF mRNA was first detectable by 3 h and reached maximal levels by 12 h of drug exposure. Using run-on transcription assays, the TNF gene was transcriptionally inactive in resting monocytes, but was rapidly activated after TPA exposure. The protein synthesis inhibitor, cycloheximide (CHX), had no detectable effect on levels of TNF transcripts in resting monocytes, while this agent superinduced the level of TNF mRNA by 50-fold in TPA-activated cells. TPA activated monocytes were also exposed to actinomycin D and/or CHX to determine whether transcriptional or posttranscriptional control of TNF gene expression was responsible for the induction of TNF transcripts. After 1 h of actinomycin D treatment, the amount of TNF transcripts was reduced by 75%. In contrast, no difference in TNF mRNA levels was observed in TPA-activated monocytes exposed to CHX alone or CHX in combination with actinomycin D. These findings indicated that CHX prevented the degradation of TNF mRNA by inhibiting the synthesis of a labile protein. Run-on transcription assays performed on cells exposed to either TPA or the combination of TPA and CHX further indicated that CHX treatment increased transcription of the TNF gene. Thus, TNF gene expression is controlled at the transcriptional level in resting human monocytes, while both transcriptional and posttranscriptional events regulate the level of TNF transcripts in TPA-activated cells.

Structure of the human B lymphocyte receptor for C3d and the Epstein-Barr virus and relatedness to other members of the family of C3/C4 binding proteins

Human complement receptor type 2 (CR2) is the B lymphocyte receptor for C3d and the Epstein-Barr virus. This protein is also a member of a family of C3b/C4b binding proteins that regulate complement activation, comprise tandemly repeated 60-75 amino acid sequences, and whose genes map to band q32 on chromosome 1. Overlapping cDNA clones encoding the entire human CR2 protein have been isolated from a human tonsillar cDNA library. The derived amino acid sequence of 1,032 residues encodes a peptide of 112,716 mol wt. A signal peptide was identified, followed by 15 copies of the short consensus repeat (SCR) structure common to the C3/C4 binding protein family. The entire extracellular portion of the protein comprised SCRs, thus, the ligand binding sites both for C3d and the EBV protein gp350/220 are positioned within this structure. Immediately following the final SCR was a transmembrane sequence of 24 amino acids and a cytoplasmic region of 34 amino acids. One of five cDNA clones isolated contained an additional SCR, providing evidence for alternative mRNA splicing or gene products of different human alleles. The CR2 cDNAs were used to isolate CR2-specific genomic phage. The entire CR2 coding sequences were found within 20 kb of human DNA. Analysis of the CR2 cDNA sequence indicated that CR2 contained internally homologous regions and suggested that CR2 arose by duplication of a primordial gene sequence encoding four SCRs. Comparison of the CR2 peptide sequence with those of other members of the gene family has identified many regions highly homologous with human CR1, fewer with C4bp and decay accelerating factor, and very few with factor H, and suggested that CR2 and CR1 arose by duplication of the same ancestral gene sequence. The homology between CR2 and CR1 extended to the transmembrane and cytoplasmic regions, suggesting that these sequences were derived from a common membrane-bound precursor.

Characterization of the human monocyte high affinity Fc receptor (hu FcRI)

The high affinity Fc receptor (FcRI) of a human monocytic cell line, U937, was further characterized using a previously described murine monoclonal antibody, FcRmAb32. This antibody immunoprecipitated a 70 K cell surface glycoprotein. A solid phase ligand binding assay and a solid phase immunoprecipitation assay were combined to confirm that the 70 K cell surface glycoprotein immunoprecipitated by FcRmAb32 is an IgG binding protein. N-glycanase digestion shows that at least 20% of the relative mobility of the 70 K FcRI glycoprotein is due to N-linked carbohydrate. FcRmAb32 immunoprecipitated a 70 K glycoprotein from biosynthetically labelled U937 cells that co-migrated with the surface iodinated glycoprotein on 2-dimensional gel electrophoresis. A 50 K protein, that is biosynthetically labelled but not accessible to surface iodination, which, bound to control antibodies was also present in FcRmAb32 immunoprecipitates. FcRmAb32 only bound the mature fully glycosylated form of FcRI. The 70 K FcRI was not phosphorylated constitutively nor when U937 cells were stimulated by PMA.

Transcriptional and posttranscriptional control of c-fos gene expression in human monocytes

We examined the mechanisms that are responsible for the regulation of c-fos gene expression in human monocytes. Levels of c-fos mRNA were low or undetectable in resting monocytes. Results of run-on transcription assays, however, demonstrated that both the first two and last two exons of the c-fos gene were transcribed at similar rates, and that only the sense strand of this gene was transcribed. These findings suggest that the level of c-fos transcripts in resting human monocytes is controlled at a posttranscriptional level. Activation of resting monocytes with phorbol ester was associated with a rapid and transient increase in c-fos mRNA levels. This increase in c-fos transcripts was related to an enhanced rate of c-fos transcription. Moreover, exposure of resting monocytes to inhibitors of protein synthesis induced (i) a rapid and marked (300-fold) increase in c-fos mRNA levels, despite only a 9-fold increase in c-fos transcription, and (ii) a prolongation of the half-life of c-fos mRNA. Thus, while posttranscriptional control was responsible for the down-regulation of c-fos transcripts in both resting and activated human monocytes, transcriptional mechanisms were responsible for the transient increase in c-fos expression induced by phorbol ester. Furthermore, the marked increases in c-fos mRNA associated with inhibition of protein synthesis were regulated by both transcriptional and posttranscriptional mechanisms. These findings may be related to recent observations which indicate that both positive and negative factors transcriptionally regulate c-fos gene expression and that sequences found in the 3'-untranslated region of the c-fos mRNA are responsible for the stability of this transcript.

Transcriptional and posttranscriptional control of c-fos gene expression in human monocytes

We examined the mechanisms that are responsible for the regulation of c-fos gene expression in human monocytes. Levels of c-fos mRNA were low or undetectable in resting monocytes. Results of run-on transcription assays, however, demonstrated that both the first two and last two exons of the c-fos gene were transcribed at similar rates, and that only the sense strand of this gene was transcribed. These findings suggest that the level of c-fos transcripts in resting human monocytes is controlled at a posttranscriptional level. Activation of resting monocytes with phorbol ester was associated with a rapid and transient increase in c-fos mRNA levels. This increase in c-fos transcripts was related to an enhanced rate of c-fos transcription. Moreover, exposure of resting monocytes to inhibitors of protein synthesis induced (i) a rapid and marked (300-fold) increase in c-fos mRNA levels, despite only a 9-fold increase in c-fos transcription, and (ii) a prolongation of the half-life of c-fos mRNA. Thus, while posttranscriptional control was responsible for the down-regulation of c-fos transcripts in both resting and activated human monocytes, transcriptional mechanisms were responsible for the transient increase in c-fos expression induced by phorbol ester. Furthermore, the marked increases in c-fos mRNA associated with inhibition of protein synthesis were regulated by both transcriptional and posttranscriptional mechanisms. These findings may be related to recent observations which indicate that both positive and negative factors transcriptionally regulate c-fos gene expression and that sequences found in the 3'-untranslated region of the c-fos mRNA are responsible for the stability of this transcript.

gp140, the EBV/C3d receptor (CR2) of human B lymphocytes, is involved in cell-free phosphorylation of p120, a nuclear ribonucleoprotein

gp140, the EB/C3d receptor (EBV/C3dR; CR2), is a membrane site involved in human B cell regulation. Cross-linking of this receptor on the cell surface by its specific ligands led to the enhancement of B cell proliferation in synergy with T cell factors. In vitro activation of human peripheral B lymphocytes by cross-linking membrane immunoglobulins with anti-mu antibody induced EBV/C3dR phosphorylation. These studies were pursued by analyzing cell-free phosphorylation of EBV/C3dR isolated from Raji cell fractions, and immobilized on OKB7, a monoclonal anti-EBV/C3dR antibody. Three EBV/C3dR-related antigens which could be cell-free phosphorylated were detected: gp140, the EBV/C3dR, p130 and p120. gp140, the mature form of EBV/C3dR, was isolated from plasma membrane and from purified nuclei. p130 was identified as an intracellular intermediate of EBV/C3dR glycosylation, localized in low-density microsomes. Phosphoamino acid analysis of EBV/C3dR allowed the detection of phosphotyrosine and phosphoserine residues. These data suggest that EBV/C3dR could carry an autophosphorylation activity and could be associated to serine kinases. Using polyclonal anti-p120 antibody and anti-120 kDa nuclear ribonucleoprotein monoclonal antibody (mAb), p120 was identified as a nuclear ribonucleoprotein antigenically not related to EBV/C3dR. Detection of p120 on EBV/C3dR, immobilized on OKB7, was due to interactions between both antigens, instead of anti-EBV/C3dR mAb cross-reactivity with p120. Cell-free phosphorylation of p120 was under the control of EBV/C3dR. However, it is not yet established whether other nuclear or membrane components were involved in the control of p120 cell-free phosphorylation by EBV/C3dR. From the data presented herein, we propose that phosphorylation of a 120-kDa nuclear ribonucleoprotein by EBV/C3dR-associated kinases could represent a crucial step in in vivo regulation of human B cell activation.

Mitogen stimulation promotes human T lymphocyte adhesion to fibronectin

Purified human peripheral blood T lymphocytes were shown to adhere to growth substrata coated with purified human plasma fibronectin (pFn) and its Mr 120,000-140,000 proteolytic fragments containing the cell-binding site. In contrast, significant binding to laminin- or type I collagen-coated surfaces could not be demonstrated. Binding of T cells to pFn could be inhibited by the synthetic peptide Arg-Gly-Asp-Ser. Activation of T lymphocytes with concanavalin A (Con A) and a phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), led to a higher adherence to pFn than in unstimulated, resting T cells. Activation with only Con A in the presence of accessory cells also promoted binding. Increased adherence of T cells to pFn could be demonstrated as early as 2 h after the onset of stimulation and reached its maximum in 2-3 days. Furthermore, activated but not resting T cells actively spread on pFn-coated surfaces and displayed an altered F-actin organization. In an overlay assay of electrophoretically separated polypeptides of activated T lymphocytes, pFn bound to a high molecular weight polypeptide of Mr 190,000, suggesting that the cells bind to pFn via a receptor-like molecule. Thus, adhesion ot pFn may be a two-stage process. At the first stage cells bind to Fn. Activated T cells proceed to the second stage, where cells begin to spread on pFn. This may be due to an altered relationship between Fn receptors and microfilaments.

Molecular cloning of the cDNA encoding the Epstein-Barr virus/C3d receptor (complement receptor type 2) of human B lymphocytes

Complementary DNA clones for complement receptor type 2 (CR2), the B-lymphocyte membrane protein that serves as the receptor for Epstein-Barr virus and the C3d complement fragment, were obtained by screening a lambda gt11 library generated from Raji B lymphoblastoid cell mRNA. A 4.2-kilobase (kb) clone, representing the entire coding sequence of the protein plus untranslated 5' and 3' nucleotide sequences was obtained and sequenced. The 4.2-kb clone, which contains all but about 500 base pairs (bp) of the 5' untranslated region of the full-length CR2 mRNA, consists of 63 bp of 5' untranslated nucleotide sequence followed successively by a start codon, a 20-amino acid hydrophobic signal peptide, 1005 amino acids having a repeating motif, a 28-amino acid probable transmembrane domain, and a 34-amino acid cytoplasmic tail. The deduced amino acid sequence of the protein indicates that the extracellular domain consists entirely of 16 tandemly arranged repeating elements, each 60-75 amino acids in length, which are identified by multiple conserved residues. This repeating motif also occurs in the C3b/C4b receptor, several complement proteins, and a number of noncomplement proteins. In CR2, the 16 repeats occur in four clusters of four repeats each. Approximately 10% of the deduced amino acid sequence, including the amino and carboxyl termini, was confirmed by amino acid sequencing of tryptic peptides derived from purified CR2. The nucleotide and derived amino acid sequence of CR2 and related studies are presented here.

Isolation of decay-accelerating factor (DAF) from rabbit erythrocyte membranes

A regulatory protein purified from rabbit erythrocyte stroma had decay-accelerating activity to C5 convertases on hemolytic intermediate cells (EAC1-3b, P). Its molecular weight was estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under non-reducing conditions to be 66,000, slightly smaller than that of human decay-accelerating factor (DAF). The amino acid composition of the protein resembled that of human DAF. The protein was incorporated into sheep erythrocytes. The incorporation and the decay-accelerating activity were inhibited by low density lipoprotein. It did not bind to rabbit C3-Sepharose in a column under conditions in which human CR1 bound, but its interaction with rabbit C3 was evident from the retardation of its elution from the column. These results indicate that the protein purified from rabbit erythrocyte stroma was DAF.

Expression of the human B-cell surface protein CD20: alteration by phorbol 12-myristate 13-acetate

The monoclonal antibody 1F5 recognizes human B-cell surface protein CD20 and can activate resting B cells; with this antibody we found CD20 to be a 35/37-kDa non-disulfide-linked protein. The protein has a pI of 7.5-8.0 and is phosphorylated in B-cell lines, tonsillar B cells, and peripheral blood B cells. Both CD20 surface expression and phosphorylation are increased on buoyant tonsillar B cells activated in vivo. Because phorbol 12-myristate 13-acetate (PMA) supports the activation signal initiated by monoclonal antibody 1F5, we studied the effect of PMA on CD20 expression. After brief incubation with mitogenic levels of PMA, the number of dense tonsillar B cells positive for CD20 protein transiently decreased. Paradoxically, the cells remaining positive had more surface CD20 than did control cells, and these remaining surface CD20 molecules were hyperphosphorylated. Furthermore, PMA not only induced phosphorylation of CD20 protein on Raji cells but also increased the internalization of CD20 molecules; both phosphorylation and internalization of CD20 molecules were decreased with the protein kinase C inhibitor palmitoyl carnitine. Conditions that increase CD20 phosphorylation are shown also to increase surface mobility of the molecule, suggesting that CD20 protein internalization may be a critical early event for B-cell entry into the G1 phase of the cell cycle.

Modulation of human neutrophil LTA hydrolase activity by phorbol myristate acetate

The phorbol ester, phorbol 12-myristate 13-acetate enhanced leukotriene B4 production stimulated by formyl-methionyl-leucyl-phenylalanine and arachidonic acid and reduced the production of the all-trans isomers of LTB4 by human neutrophils. Production of 5-hydroxyeicosatetraenoic acid was unaffected. These observations are consistent with a stimulatory effect of phorbol ester on LTA hydrolase, the enzyme which catalyses the conversion of LTA4 to LTB4. We demonstrate that a protein of the same molecular weight as LTA hydrolase is phosphorylated upon stimulation of neutrophils with PMA. These data suggest that the activity of LTA hydrolase may be regulated by protein kinase C-dependent phosphorylation.

Epstein-Barr virus gp350/220 binding to the B lymphocyte C3d receptor mediates adsorption, capping, and endocytosis

The type 2 complement receptor, CR2, a B lymphocyte surface glycoprotein, is known to be a component of the EBV receptor. We now demonstrate that the major EBV outer membrane glycoprotein, gp350/220, is a highly specific ligand for CR2. EBV or beads coated with purified recombinant gp350/220 adsorb to normal B lymphocytes, cap with CR2, become endocytosed into vesicles, and are released into the cytoplasm. This is the first demonstration of herpesvirus glycoprotein-cell glycoprotein receptor interaction in viral adsorption and penetration. The capping of CR2 in response to virus, gp350/220-coated beads, or anti-CR2 monoclonal antibodies is associated with cocapping of surface immunoglobulin. Interaction between CR2 and surface immunoglobulin may be important in modulating the B cell activation that normally follows EBV infection or exposure to antigen.

Human C3b/C4b receptor (CR1). Demonstration of long homologous repeating domains that are composed of the short consensus repeats characteristics of C3/C4 binding proteins

10 overlapping CR1 cDNA clones that span 5.5 kb were isolated from a tonsillar library and sequenced in whole or in part. A single long open reading frame beginning at the 5' end of the clones and extending 4.7 kb downstream to a stop codon was identified. This sequence represents approximately 80% of the estimated 6 kb of coding sequence for the F allotype of CR1. Three tandem, direct, long homologous repeats (LHRs) of 450 amino acids were identified. Analysis of the sequences of tryptic peptides provided evidence for a fourth LHR in the F allotype of CR1. Amino acid identity between the LHRs ranged from 70% between the first and third repeats to 99% between the NH2-terminal 250 amino acids of the first and second repeats. Each LHR comprises seven short consensus repeats (SCRs) of 60-70 amino acids that resemble the SCRs of other C3/C4 binding proteins, such as complement receptor type 2, factors B and H, C4 binding protein, and C2. Two additional SCRs join the LHRs to a single membrane-spanning domain of 25 amino acids; thus, the F allotype of CR1 probably contains at least 30 SCRs, 23 of which have been sequenced. Each SCR is predicted to form a triple loop structure in which the four conserved half-cystines form disulfide linkages. The linear alignment of 30 SCRs as a semi-rigid structure would extend 1,140A from the plasma membrane and might facilitate the interaction of CR1 with C3b and C4b located within the interstices of immune complexes and microbial cell walls. The COOH-terminal cytoplasmic domain of 43 residues contains a six-amino-acid sequence that is homologous to the sequence in the epidermal growth factor receptor that is phosphorylated by protein kinase C.

Activation of human platelets through gp140, the C3d/EBV receptor (CR2)

gp140, the C3d/EBV receptor (CR2), previously isolated and characterized from human B lymphocytes, was identified on human platelets: by measuring the specific binding of either polyclonal anti-gp140 IgG and monoclonal anti-C3d/EBVR antibodies, as OKB-7 and HB-5, or human C3d; by isolating gp140 from solubilized platelet components with polyclonal anti-gp140 IgG or monoclonal OKB-7, using immunoprecipitation and electro-immunoblotting assays; by inducing specific activation of human platelets. Cross-linking of this receptor by polyclonal anti-gp140 IgG induced aggregation of human platelets and stimulated ATP release. Absence of lactate dehydrogenase release and inhibition by EDTA and prostacyclin of anti-gp140-induced aggregation, support strongly active aggregation and absence of lysis. Platelet aggregation by anti-gp140 required metabolic activities and was modulated by fibrinogen, paf-acether or thrombin. OKB-7 triggered human platelet aggregation when cross-linked by anti-mouse second-step antibodies. In the same way, platelet activation by C3d fragment was detected, in presence of fibrinogen, only when C3d was cross-linked on the cell surface by anti-C3d F(ab')2 fragments.

Autoantibodies against gp140, the Epstein-Barr virus and C3d receptor in sera from rheumatoid arthritis patients

gp140 is the Epstein-Barr virus receptor and the C3d receptor (EBVR/C3dR) of human B lymphocytes. Recently, we have shown that cross-linking of EBVR/C3dR on cell surface by polyclonal anti-gp140 induced B cell activation, in presence of T cell factors. Immunoregulatory abnormalities of EBV-induced B cell activation have been demonstrated in rheumatoid arthritis (RA) patients. These data prompted us to analyze the putative presence of anti-EBVR/C3dR autoantibodies in human sera. The IgG fractions from eleven RA and 10 normal sera were tested for their ability to: (a) bind to Raji cell surface; (b) inhibit the binding to cell surface of 3 anti-EBVR/C3dR monoclonal antibodies (mAb), which recognized different epitopes on gp140; (c) inhibit the binding of particle-bound C3d and (d) react with 1% Nonidet-P40-solubilized gp140 from Raji cell membranes, in immunoblotting assays. Three RA sera carry anti-EBVR/C3dR autoantibodies which react with gp140 expressed on Raji cell surface or its solubilized form. The purification of monomeric IgG fraction of selected RA sera ruled out involvement of immune complexes carrying C3 molecules, which could interfere in these assays. One of these 3 RA sera was able to inhibit the binding to cell surface of anti-EBVR/C3dR mAb and particle-bound C3d. However, the 2 other RA sera, found positive by immunoblotting, did not inhibit particle-bound C3d and presented differences in their inhibitory effect on anti-EBVR/C3dR mAb binding to Raji cell surface. These data allow us to demonstrate differences which exist in the properties of anti-EBVR/C3dR autoantibodies. These autoantibodies were not detected in all the normal and other RA sera. Anti-EBVR/C3dR autoantibodies could play a role "in vivo" in B lymphocyte activation of RA patients.

Purification of the B lymphocyte receptor for the C3d fragment of complement and the Epstein-Barr virus by monoclonal antibody affinity chromatography, and assessment of its functional capacities

The human C3d receptor (complement receptor type 2, CR2), that also serves as the B lymphocyte receptor for the Epstein-Barr virus, was purified from detergent lysates from the B lymphoblastoid cell lines, SB and Raji, by monoclonal antibody affinity chromatography using the anti-CR2 monoclonal antibody, HB-5. Relative to the concentration of cellular protein and receptor that was initially solubilized by detergent, the procedure provided a 37,000-fold purification with a 40-50% recovery of CR2. The purified receptor presented a single Coomassie blue-stained band when analyzed by SDS-PAGE, and it retained its function of binding to C3-Sepharose. The N-terminus of CR2 was blocked. The amino acid composition was significantly similar to that of the C3b/C4b receptor, factor H and C4 binding protein, suggesting that CR2 may be a member of this newly defined protein family. However, CR2 did not exhibit the regulatory functions of these proteins, namely, the decay dissociation of the classical or alternative pathway C3 convertases and serving as a cofactor for the cleavage of C3b.

Phosphatidylinositol liposomes opsonized by concanavalin A stimulate phosphatidylinositol turnover in macrophages

We have previously found that concanavalin A binds specifically to inositol and phosphatidylinositol. In the present study we demonstrate that binding of concanavalin A to liposomes containing phosphatidylinositol influences the uptake of such liposomes by macrophages. Although resident mouse peritoneal macrophages normally ingest liposomes only to a slight extent, attachment of concanavalin A to the liposomes caused a marked enhancement of phagocytosis. Furthermore induction of phagocytosis of concanavalin A-opsonized liposomes was associated with increased phosphatidylinositol turnover. We conclude that (a) concanavalin A can serve as an opsonizing agent for liposomes; (b) opsonization results from binding of concanavalin A to phosphatidylinositol on the liposomal surface; and (c) concanavalin A-induced phagocytosis is associated with increased phosphatidylinositol turnover in the macrophages.

Identification of a partial cDNA clone for the C3d/Epstein-Barr virus receptor of human B lymphocytes: homology with the receptor for fragments C3b and C4b of the third and fourth components of complement

Human complement receptor type 2 (CR2) is the B-lymphocyte receptor both for the C3d fragment of the third component of complement and for the Epstein-Barr virus. Amino acid sequence analysis of tryptic peptides of CR2 revealed a strong degree of homology with the human C3b/C4b receptor, CR1. This homology suggested that CR1 gene sequences could be used to detect the CR2 sequences at conditions of low-stringency hybridization. Upon screening a human tonsillar cDNA library with CR1 cDNA sequences, two clones were identified that hybridized at low, but not at high, stringency. Redundant oligonucleotides specific for CR2 sequences were synthesized and used to establish that the two cDNA clones weakly hybridizing with the CR1 cDNA contained CR2 sequences. One of these CR2 cDNA clones hybridized to oligonucleotides derived from two distinct CR2 tryptic peptides, whereas the other, smaller cDNA clone hybridized to oligonucleotides derived from only one of the CR2 peptides. Nucleotide sequence analysis of the CR2 cDNA confirmed that the site of oligonucleotide hybridization was identical to that predicted from the peptide sequence, including flanking sequences not included within the oligonucleotide probes. The CR2-specific cDNA sequences identified a poly(A)+ RNA species of 5 kilobases in RNA extracted from human B cells but did not hybridize to any RNA obtained from the CR2-negative T-cell line HSB-2, thus confirming the appropriate size and tissue-specific distribution for the CR2 mRNA. The striking peptide sequence homology between CR2 and CR1 and the cross-hybridization of the CR2 cDNA with the CR1-specific sequences allow the placement of CR2 in a recently defined gene family of C3- and C4-binding proteins consisting of CR1, C4-binding protein, factor H, and now, CR2.