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PubMed1996

Markedly impaired humoral immune response in mice deficient in complement receptors 1 and 2.

Complement receptor 1 (CR1, CD35) and complement receptor 2 (CR2, CD21) have been implicated as regulators of B-cell activation. We explored the role of these receptors in the development of humoral immunity by generating CR1- and CR2-deficient mice using gene-targeting techniques. These mice have normal basal levels of IgM and of IgG isotypes. B- and T-cell development are overtly normal. Nevertheless, B-cell responses to low and high doses of a T-cell-dependent antigen are impaired with decreased titers of antigen-specific IgM and IgG isotypes. This defect is not complete because there is still partial activation of B lymphocytes during the primary immune response, with generation of splenic germinal centers and a detectable, although reduced, secondary antibody response. These data suggest that certain T-dependent antigens manifest an absolute dependence on complement receptors for the initiation of a normally robust immune response.

Animals↗

PubMed2003

Extracellular signal-regulated kinase 1/2 is involved in the activation of NADPH oxidase induced by FMLP receptor but not by complement receptor 3 in rat neutrophils.

This experiment was performed to clarify the role of extracellular signal-regulated kinase, ERK1/2, in NADPH oxidase-dependent O2- production in rat peritoneal neutrophils. When neutrophils were exposed to N-formyl-methionyl-leucyl-phenylalanine (fMLP) to stimulate an N-formyl peptide receptor, not only the production of O2- but also the activation of ERK1/2 was observed. The translocation of an NADPH oxidase component, p47(phox), from cytosol to membrane also occurred in neutrophils stimulated with fMLP. U0126, an ERK1/2 kinase inhibitor, inhibited both the production of O2- and the translocation of p47(phox) elicited by fMLP. On the other hand, when complement receptor 3 of neutrophils was stimulated with opsonized zymosan (OZ), weaker activation of ERK1/2 than that by fMLP was observed. In this case, U0126 showed no inhibition against the production of O2- and slight inhibition against the translocation of p47(phox). Large inhibition against the OZ-induced production of O2- was only observed in neutrophils treated with GF109203X, a PKC inhibitor. The present study indicates that receptor dependence exists in the ERK1/2 signaling pathway leading to the activation of NADPH oxidase.

Animals↗

PubMed1993

Macrophage phagocytosis of virulent but not attenuated strains of Mycobacterium tuberculosis is mediated by mannose receptors in addition to complement receptors.

We have examined macrophage receptors that mediate phagocytosis of virulent strains (Erdman and H37Rv) and an attenuated strain (H37Ra) of the intracellular pathogen, Mycobacterium tuberculosis. Adherence of the three strains to monocyte-derived macrophages (MDM) is markedly enhanced (>threefold) in the presence of low levels of fresh serum and requires heat-labile serum components because heat inactivation of serum reduces adherence by 65 +/- 5 to 71 +/- 2%. In the presence and absence of serum, adherence of the three strains to MDM is comparable. By electron microscopy, all bacteria are ingested and reside in phagosomes. C receptors (CR) play an important role in adherence of the three strains to MDM in the presence and absence of serum. mAb against CR1, CR3, and CR4 inhibit adherence of Erdman M. tuberculosis in fresh serum by 75 +/- 3% and inhibit the low level of adherence of Erdman (71 +/- 13%), H37Rv (72 +/- 1%), and H37Ra (64 +/- 14%) M. tuberculosis in the absence of serum. Mannose receptors (MR) play an important role in mediating macrophage adherence of the virulent strains but not the attenuated strain of M. tuberculosis. Preincubation of MDM with soluble mannan or mannose-BSA consistently and significantly inhibits adherence of Erdman and H37Rv (up to 60 +/- 7%) but not H37Ra (0 +/- 1 to 5 +/- 5% enhancement of adherence) in the absence of serum. Down-modulation of macrophage MR on mannan substrates inhibits adherence of Erdman (52 +/- 8%) and H37Rv (55 +/- 6%) but not H37Ra (2 +/- 2% enhancement of adherence). Preincubation of MDM with soluble N-acetylglucosamine-BSA also significantly inhibits adherence of the virulent strains (42 +/- 3%). Preincubation of MDM with glucose-BSA minimally inhibits adherence of the three strains (2 +/- 4 to 12 +/- 5%). Anti-MR antibody inhibits adherence of Erdman (57 +/- 2%) and H37Rv (44 +/- 4%) but not H37Ra (4 +/- 5% enhancement of adherence). Inhibition of adherence of zymosan was comparable with that seen with virulent strains of M. tuberculosis in these studies. Down-modulation of macrophage MR also inhibits adherence of Erdman (48 +/- 9%) and H37Rv (20 +/- 2%) in the presence of serum. Simultaneous blockade of MR and CR does not further inhibit adherence of the virulent M. tuberculosis strains over that seen with blocking CR alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1996

Nonopsonic binding of Mycobacterium tuberculosis to human complement receptor type 3 expressed in Chinese hamster ovary cells.

Nonopsonic invasion of mononuclear phagocytes by Mycobacterium tuberculosis is likely important in the establishment of a primary infection in the lung. M. tuberculosis binds to a variety of phagocyte receptors, of which the mannose receptor and complement receptor type 3 (CR3) may support nonopsonic binding. CR3, a beta2 integrin, is a target for diverse intracellular pathogens, but its role in nonopsonic binding remains uncertain. We have examined the binding of M. tuberculosis H37Rv to human CR3 heterologously expressed in Chinese hamster ovary (CHO) cells, thereby circumventing the problems of competing receptors and endogenously synthesized complement, which are inherent in studies with mononuclear phagocytes. The surface expression of CD11b and CD18 was assessed by immunofluorescence, immunobead binding, flow cytometry, and immunoprecipitation with anti-CD11b and anti-CD18 monoclonal antibodies (MAbs). The functional activity of the surface-expressed CD11b/CD18 (CR3) heterodimer was confirmed by rosetting with C3bi-coated microspheres. We found that M. tuberculosis bound four- to fivefold more avidly to CR3-expressing CHO cells than to wild-type cells and, importantly, that this binding was at similar levels in the presence of fresh or heat-inactivated human or bovine serum or no serum. In contrast, Mycobacterium smegmatis bound poorly to CR3-expressing CHO cells in the absence of serum, but after opsonization in serum, binding was comparable to that of M. tuberculosis. The binding of M. tuberculosis to the transfected CHO cells was CR3 specific, as it was inhibited by anti-CR3 MAbs, particularly the anti-CD11b MAbs LM2/1 (I domain epitope) and OKM1 (C-terminal epitope), neither of which inhibit C3bi binding. MAb 2LPM19c, which recognizes the C3bi-binding site on CD11b, had little or no effect on M. tuberculosis binding. The converse was found for the binding of opsonized M. smegmatis, which was strongly inhibited by 2LPM19c but unaffected by LM2/1 or OKM1. CR3-specific binding was also evidenced by the failure of M. tuberculosis to bind to CHO cells transfected with an irrelevant surface protein (angiotensin-converting enzyme) in the presence or absence of serum. We conclude that the binding of M. tuberculosis H37Rv to CR3 expressed in CHO cells is predominantly nonopsonic and that the organism likely expresses a ligand that binds directly to CR3.

Animals↗

PubMed1989

Mapping of the Epstein-Barr virus and C3dg binding sites to a common domain on complement receptor type 2.

Complement receptor type 2 (CR2;CD21), a member of the superfamily of proteins containing short consensus repeats (SCRs), is the B cell receptor for both the gp350/220 envelope protein of Epstein-Barr virus (EBV), and for the C3dg protein of complement. By analysis of CR2 deletion mutants and chimeras formed with CR1 (CD35) we determined that of the 15 SCRs in CR2, the NH2-terminal two SCRs are necessary and sufficient to bind both gp350/220 and C3dg with affinities equivalent to those of the wild-type receptor. The epitope for OKB-7, a mAb that blocks binding of both EBV and C3dg and shares with these ligands B cell-activating capabilities, also requires both SCR-1 and SCR-2, whereas mAbs lacking these functions bind to other SCRs. Thus, EBV, a polyclonal activator of B cells, has selected a site that is proximate or identical to the natural ligand binding site in CR2, perhaps reflecting the relative immutability of that site as well as its signal transducing function.

Animals↗

PubMed2001

Multiple transcription factors regulate the inducible expression of the human complement receptor 2 promoter.

Complement receptor 2 (CR2) is regulated at the transcriptional level, but the promoter elements and the transcription factors that bind to them and contribute to its regulation are unknown. After documenting that PMA and cAMP induced the activity of the CR2 promoter by 10-fold, we conducted promoter truncation and mutagenesis experiments, in conjunction with shift assays, to determine the functionally important regions of the promoter and the proteins that bind to them. We identified two regions, separated by approximately 900 nucleotides, which together were responsible for inducible promoter activity. Mutagenesis of single promoter elements demonstrated a functional upstream stimulatory factor/E box in the TATA box-proximal region and three equally important, closely spaced, CREB/AP-1 half-sites in the upstream promoter region. The cAMP response element-binding protein (CREB)/AP-1 half-sites bound in vitro Jun and CREB that are induced by protein kinases A and/or C. The 900-nucleotide segment stretching between the above two regions had no functional impact on the induced transcription, and its deletion increased the promoter activity. Finally, a region upstream of the distal site had a repressor activity on CR2 transcription. Moreover, IL-4 induced binding of CREB and AP-1 to the upstream promoter elements and resulted in increased CR2 surface protein expression. These studies have characterized regions of the CR2 promoter and the transcription factors that bind to them and are crucial to induced CR2 expression. Our studies may provide insights to novel approaches to modulate B cell function by regulating CR2 gene transcription.

5' Untranslated Regions↗

PubMed1988

The role of C3 and its fragments in the control of S phase entry of activated mouse B lymphocytes via the complement receptor type 2.

Complement component C3 and receptors for its activated fragments have been recognized as regulators of humoral immune responses. Cross-linked C3b and C3d will substitute for alpha B cell growth factors (alpha BCGF) produced by macrophages by stimulating activated murine B lymphocytes to enter S phase. In contrast, soluble C3d and synthetic oligopeptides--containing the amino acid sequence of the binding site of C3d to its receptor, the complement receptor type 2--inhibit the entry into S phase of alpha BCGF-stimulated B cells. There is evidence supporting the hypothesis that a portion of the C3 gene at the 3' end, encoding parts of the alpha chain, encodes an alternate C3 product with alpha BCGF activity.

Amino Acid Sequence↗

PubMed1991

Modulation of mouse complement receptors 1 and 2 suppresses antibody responses in vivo.

A mAb, 7G6, that binds to mouse CR1 and CR2 and down-modulates their expression on splenic B cells in vivo, was used to determine whether a decrease in CR1 and CR2 expression affects antibody responses to different T-dependent and T-independent Ag. Injection of mice with the mAb 7G6 prior to immunization with FITC haptenated Salmonella typhimurium (SH5771), Salmonella montevideo (SH5770), SRBC, or Ficoll dramatically decreased subsequent antibody responses to FITC. Although both IgM and IgG primary antibody responses were affected similarly, the antibody levels were most inhibited during early phases of the response. In contrast, down-modulation of the CR did not affect memory antibody responses, because injection of mice with 7G6 before a second immunization with FITC-SH5771 had no effect on subsequent anti-FITC antibody production. Moreover, polyclonal in vivo activation of the mouse immune system by anti-mouse IgD antibodies was not affected by previous administration of 7G6, because anti-IgD-induced increases in Ia expression and serum IgG1 levels were not affected. Taken together, these observations suggest that CR1 and CR2 may play an important role in enhancing primary antibody responses to many T-dependent and T-independent Ag and may contribute to a host's response to naturally occurring antigens such as bacteria.

Animals↗

PubMed1977

Complement receptors and cell associated complement components.

Membrane receptors for activated complement components are widely distributed amongst tissue cells of most mammalian species. Common amongst these are receptors for C3b which mediate many of the biological functions of C3. In addition, the genetic control of certain complement components is linked to the genes which code for the major histocompatibility complex. Many of these components are also present on cell surfaces. This suggests that the function of the complement system and the major histocompatibility complex may be related.

Animals↗

PubMed2005

Synergy between two active sites of human complement receptor type 1 (CD35) in complement regulation: implications for the structure of the classical pathway C3 convertase and generation of more potent inhibitors.

The extracellular domain of the complement receptor type 1 (CR1; CD35) consists entirely of 30 complement control protein repeats (CCPs). CR1 has two distinct functional sites, site 1 (CCPs 1-3) and two copies of site 2 (CCPs 8-10 and CCPs 15-17). In this report we further define the structural requirements for decay-accelerating activity (DAA) for the classical pathway (CP) C3 and C5 convertases and, using these results, generate more potent decay accelerators. Previously, we demonstrated that both sites 1 and 2, tandemly arranged, are required for efficient DAA for C5 convertases. We show that site 1 dissociates the CP C5 convertase, whereas the role of site 2 is to bind the C3b subunit. The intervening CCPs between two functional sites are required for optimal DAA, suggesting that a spatial orientation of the two sites is important. DAA for the CP C3 convertase is increased synergistically if two copies of site 1, particularly those carrying DAA-increasing mutations, are contained within one protein. DAA in such constructs may exceed that of long homologous repeat A (CCPs 1-7) by up to 58-fold. To explain this synergy, we propose a dimeric structure for the CP C3 convertase on cell surfaces. We also extended our previous studies of the amino acid requirements for DAA of site 1 and found that the CCP 1/CCP 2 junction is critical and that Phe82 may contact the C3 convertases. These observations increase our understanding of the mechanism of DAA. In addition, a more potent decay-accelerating form of CR1 was generated.

Binding Sites↗

PubMed1994

Soluble human complement receptor type 1 inhibits complement-mediated host defense.

Soluble complement receptor type 1 (sCR1) is a powerful inhibitor of complement activation. Because of this ability, sCR1 may prove to be an important therapeutic agent that can be used to block the immunopathologic effects of uncontrolled complement activation in a variety of clinically significant disorders. Although several previous studies have examined the ability of sCR1 to inhibit complemented-mediated immunopathologic damage, there is no information on its ability to interfere with the host's defense against infection. In the current experiments sCR1 exerted a concentration-dependent inhibitory effect on the phagocytosis of Streptococcus pneumoniae by human polymorphonuclear leukocytes in vitro. Not only di sCR1 inhibit complement-dependent opsonization of the pneumococcus but at higher concentrations it also inhibited the ingestion of bacteria which had been previously opsonized. Furthermore, when rats were injected with sCR1, it inhibited both their serum hemolytic activity and serum opsonic activity in a dose-dependent fashion. Finally, for rats treated with sCR1, the 50% lethal dose was S. pneumoniae and Pseudomonas aeruginosa. These data demonstrate that sCR1 significantly inhibits complement-mediated host against bacterial infection.

Animals↗

PubMed1993

Receptor-receptor interactions of complement receptor type 3 in neutrophil membranes.

The leukocyte integrin CR3 (CD11b/CD18) is known to participate in a variety of cell functions. Recent studies have indicated that CR3 may communicate with other plasma membrane receptors to carry out several cell functions. In this review we discuss these potential receptor-receptor interactions of CR3 and present a unifying model of CR3's diverse functions.

Animals↗

PubMed1985

Human complement receptors for C3b (CR1) and C3d (CR2).

The human C3b receptor (CR1) is a polymorphic glycoprotein comprised of a single polypeptide chain. Of the 4 allotype forms of CR1 that have been described, the 2 most common have Mr's of 250,000 and 260,000, and are regulated by alleles having frequencies in a Caucasian population of 81.5% and 18.5%, respectively. CR1 is present on erythrocytes, neutrophils, eosinophils, monocytes, macrophages, B lymphocytes, some T lymphocytes, mast cells, and glomerular podocytes. CR1 number on erythrocytes is genetically regulated, and ranges from less than 100 sites per cell to greater than 1000 sites per cell, the average in the normal population being 500-600 sites per cell. A model accounting for this wide distribution proposes the existence of 2 codominant alleles determining low and high receptor number respectively; CR1 number is not affected by the structural polymorphism, so that the loci for these two phenotypic characteristics are distinct. The function of CR1 on erythrocytes may be to promote the clearance of immune complexes from the circulation. CR1 number on myelomonocytic cells is regulated by chemotactic factors which can rapidly transfer CR1 sites from a latent, presumably intracellular, site to the plasma membrane of these cells, thereby enhancing their ability to interact with opsonized foreign material. The receptor is involved in the endocytic reactions of these cells, and recent findings have demonstrated that this function can be modulated by T cell-derived factors, fibronectin, and phorbol esters. The role of CR1 on lymphocytes remains to be fully explored, although the receptor may enhance the differentiation of B cells into antibody-secreting cells.

Alleles↗

PubMed1992

Expression, molecular association, and functions of C3 complement receptors CR1 (CD35) and CR2 (CD21) on the human T cell line HPB-ALL.

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.

Antigens, Differentiation, B-Lymphocyte↗

PubMed1991

Differential effects of the stimulation of complement receptors CR1 (CD35) and CR2 (CD21) on cell proliferation and intracellular Ca2+ mobilization of chronic lymphocytic leukemia B cells.

The regulatory role of CR1 and CR2 on B cell activation and proliferation has been investigated by using B cells from patients with chronic lymphocytic leukemia. The chronic lymphocytic leukemia B cells are clonal expansions of B lymphocytes frozen at specific stages of activation. They displayed two patterns of response upon surface Ig (sIg) cross-linking in terms of in vitro proliferation and intracellular free Ca2+ mobilization: cells from patient F (first pattern) proliferated in the presence of mitogenic anti-mu antibodies, whereas cells from patient A (second pattern) did not respond to sIg cross-linking but proliferated in the presence of low m.w. B cell growth factor and IL-2. Coculture of A or F cells with C3b-bearing SRBC led to a two- to four-fold increase in thymidine incorporation in cultures containing low m.w. B cell growth factor but not in cultures containing rIL-2. This enhanced proliferation was inhibited by F(ab')2 polyclonal rabbit antihuman CR1 antibodies. Only cells which proliferated in the presence of anti-mu (cells F) responded to cross-linking of sIg with a rise in intracellular Ca2+. No increase in calcium mobilization was observed after co-cross-linking of CR1 and sIg on A and F cells with mAb or polyclonal anti-CR1 antibodies. Co-cross-linking of CR2 with sIg only led to an enhanced intracellular Ca2+ rise in F cells but not in A cells. The lack of CR2-mediated synergy in Ca2+ rise in A cells indicates that the synergy occurs only if there is a proper coupling of sIg to phospholipase C. CR1-induced proliferation of B cells does not involve the signaling pathways of sIg. These results provide additional evidences for the role of C3 fragments in modulation of human B cell activation.

Antigens, CD↗

PubMed1990

A molecular and immunochemical characterization of mouse CR2. Evidence for a single gene model of mouse complement receptors 1 and 2.

The relationships between functional, biochemical, and genetic homologues of human and mouse C receptors 1 (CR1) and 2 (CR2) are incompletely understood. We have isolated and characterized a partial mouse CR2 cDNA clone and determined the exon-intron organization of the gene encoding it. Together they predict a form of mouse CR2 highly identical to the 15 short consensus repeat form of human CR2. Strong similarities in genomic organization and exon-intron junctions indicate that this mouse gene and human CR2 are evolutionary homologues. A polyclonal rabbit anti-mouse CR2 fusion protein, BRN-1, was prepared. BRN-1 immunoprecipitates bands of 155 to 160 kDa under nonreducing conditions in mouse CR2 expressing B cell lines. In mouse spleen a doublet of 155 kDa and 190 kDa under nonreducing and 165 and 205 kDa under reducing conditions is recognized by immunoprecipitation and Western blot analysis. Staphylococcus aureus V8 protease maps of these two proteins show many shared bands. Crossed immunoprecipitation using BRN-1 and 7E9, a previously described mAb reported to identify the 190-kDa mouse CR1 and a smaller 150-kDa protein, indicates that both antibodies react with the same proteins. Therefore, by using BRN-1 we have now linked the genetic mouse CR2 to its functional, biochemically characterized gene product. The observation that BRN-1 also recognizes a second 190-kDa mouse protein defined functionally as a homologue of human CR1, and that these proteins have very similar peptide maps, provides strong evidence that these two proteins are expressed by a single mouse CR2/CR1 transcription unit.

Amino Acid Sequence↗

PubMed1990

Deficiencies of human C3 complement receptors type 1 (CR1, CD35) and type 2 (CR2, CD21).

CR1 (CD35) and CR2 (CD21) are structurally related integral transmembrane glycoproteins that function as cellular receptors for human C3b and C3dg, respectively. The primary sequence of the most common structural allotype of CR1 and that of CR2 have been established, and ligand binding on the molecules has been mapped. CR1 and CR2 genes are located in close vicinity in the RCA locus of chromosome 1. CR1 has a wide cellular/tissular distribution and mediates a variety of biologic functions, including the transport of C3-bearing immune complexes on erythrocytes, enhancement of phagocytosis, induction of IL-1 secretion and enhancement of B-cell differentiation. Expression of CR2 is restricted to B lymphocytes and follicular dendritic cells. The receptor modulates B-cell growth. CR2 also serves as the receptor for EBV and determines the cellular tropism of the virus. This review discusses the molecular biology and functional characteristics of CR1 and CR2. It focuses on alterations of expression of the receptors in disease, with particular emphasis on the genetic and acquired factors that contribute to the defective expression of CR1 in patients with systemic lupus erythematosus.

Amino Acid Sequence↗

PubMed1995

Characterization of rat complement receptors and regulatory proteins. CR2 and Crry are conserved, and the C3b receptor of neutrophils and platelets is distinct from CR1.

In the mouse, CR1 and CR2 on B lymphocytes are encoded by alternatively spliced Cr2 gene transcripts. Immune adherence receptors that bind C3b are present on mouse platelets and unstimulated neutrophils, but they are not CR1. In this study, rabbit anti-mouse CR1/CR2 Ab immunoprecipitated a 145- to 150-kDa CR2 protein from rat platelets, neutrophils, and splenocytes, but not a approximately 200-kDa CR1 protein. By Northern analysis, cDNA for mouse CR2 hybridized to mRNA of 3.7 and 5.2 kb from both mouse and rat splenocytes. The murine decay-accelerating factor and membrane cofactor protein analogue Crry was present in rat platelets, neutrophils, E, and splenocytes as two distinct proteins of 65 to 70 kDa and 75 to 85 kDa. Rat platelets, neutrophils, and splenocytes contained a novel 200-kDa cell membrane protein that specifically bound to a rat C3b-Sepharose column. We have named this protein C3bR-200. C3bR-200 was not identified by anti-mouse CR1/CR2 or anti-human CR1 Ab. Rat E lacked C3bR-200. Rat neutrophils and splenocytes also contained an 80-kDa C3b-binding protein that was distinct from Crry, which we have named C3bR-80. Therefore, CR2 and Crry are present in the rat, and have similar qualities to those from the mouse, except that CR2 is located on rat platelets and neutrophils, which is not the case in the mouse. Rat platelets, neutrophils, and splenocytes have a 200-kDa C3b-binding protein, C3bR-200, that is likely to be the rodent immune adherence receptor.

Animals↗