CD19 is essential for B cell activation by promoting B cell receptor-antigen microcluster formation in response to membrane-bound ligand

Here we describe the spatiotemporal architecture, at high molecular resolution, of receptors and signaling molecules during the early events of mouse B cell activation. In response to membrane-bound ligand stimulation, antigen aggregation occurs in B cell antigen receptor (BCR) microclusters containing immunoglobulin (Ig) M and IgD that recruit the kinase Syk and transiently associate with the coreceptor CD19. Unexpectedly, CD19-deficient B cells were significantly defective in initiation of BCR-dependent signaling, accumulation of downstream effectors and cell spreading, defects that culminated in reduced microcluster formation. Hence, we have defined the dynamics of assembly of the main constituents of the BCR 'signalosome' and revealed an essential role for CD19, independent of the costimulatory molecule CD21, in amplifying early B cell activation events in response to membrane-bound ligand stimulation.

Role for CD21 in the establishment of an extracellular HIV reservoir in lymphoid tissues

Follicular dendritic cells (FDC) represent a major extracellular reservoir for HIV. A better understanding of the mechanisms of virion attachment to FDC may offer new avenues for reducing viral burdens in infected individuals. We used a murine model to investigate the establishment of extracellular HIV reservoirs in lymph nodes (LN). Consistent with findings in human tissues, CD21 was required for trapping of HIV to LN cells, as evidenced by significantly reduced virion binding when mice were pretreated with a C3 ligand-blocking anti-CD21 mAb and absence of virion trapping in CD21 knockout mice. Also consistent with findings in human tissues, the majority of HIV virions were associated with the FDC-enriched fraction of LN cell preparations. Somewhat surprisingly, HIV-specific Abs were not essential for HIV binding to LN cells, indicating that seeding of the FDC reservoir may begin shortly after infection and before the development of HIV-specific Abs. Finally, the virion-displacing potential for anti-CD21 mAbs was investigated. Treatment of mice with anti-CD21 mAbs several days after injection of HIV significantly reduced HIV bound to LN cells. Our findings demonstrate a critical role for CD21 in HIV trapping by LN cells and suggest a new therapeutic avenue for reducing HIV reservoirs.

Complement-Dependent P-Selectin Expression and Injury following Ischemic Stroke

The mechanisms that contribute to inflammatory damage following ischemic stroke are poorly characterized, but studies indicate a role for both complement and P-selectin. In this study, we show that compared with wild-type mice, C3-deficient mice showed significant improvement in survival, neurological deficit, and infarct size at 24 h after middle cerebral artery occlusion and reperfusion. Furthermore, P-selectin protein expression was undetectable in the cerebral microvasculature of C3-deficient mice following reperfusion, and there was reduced neutrophil influx, reduced microthrombus formation, and increased blood flow postreperfusion in C3-deficient mice. We further investigated the use of a novel complement inhibitory protein in a therapeutic paradigm. Complement receptor 2 (CR2)-Crry inhibits complement activation at the C3 stage and targets to sites of complement activation. Treatment of normal mice with CR2-Crry at 30 min postreperfusion resulted in a similar level of protection to that seen in C3-deficient mice in all of the above-measured parameters. The data demonstrate an important role for complement in cerebrovascular thrombosis, inflammation, and injury following ischemic stroke. P-selectin expression in the cerebrovasculature, which is also implicated in cerebral ischemia and reperfusion injury, was shown to be distal to and dependent on complement activation. Data also show that a CR2-targeted approach of complement inhibition provides appropriate bioavailability in cerebral injury to enable complement inhibition at a dose that does not significantly affect systemic levels of serum complement activity, a potential benefit for stroke patients where immunosuppression would be undesirable due to significantly increased susceptibility to lung infection.

B-cell extrinsic CR1/CR2 promotes natural antibody production and tolerance induction of anti-alphaGAL-producing B-1 cells

B-1b cells produce IgM natural antibodies against alpha1-3Galbeta1-4GlcNAc (alphaGal). These can be tolerized by nonmyeloablative induction of mixed chimerism using alphaGal-positive (alphaGal+) donor marrow. We assessed the role of CR1/2 in this model for induction of tolerance of B-1b cells. Mixed hematopoietic chimerism was induced in alpha1-3galactosyltransferase (GalT-/-) and GalT-/-Cr2-/- mice with alphaGal+ BALB/c marrow donors. Anti-alphaGal Ab and anti-alphaGal Ab-producing B cells became undetectable in GalT-/- chimeras, whereas they persisted in chimeric GalT-/-Cr2-/- mice. To determine whether CR1/2 expression on stromal cells and/or hematopoietic cells was critical for B-1-cell tolerance, we generated GalT-/- radiation chimeras in which CR1/CR2 was expressed on either stromal cells, hematopoietic cells, neither, or both. After induction of mixed chimerism from alphaGal+ allogeneic bone marrow (BM) donors, anti-alphaGal-producing B cells were rendered tolerant in reconstituted recipients expressing only stromal CR1/CR2. Our results suggest a possible role for follicular dendritic cells that pick up immune complexes via CR1/CR2 receptors in the tolerization of B-1b cells.

Characterization of human complement receptor type 2 (CR2/CD21) as a receptor for IFN-alpha: a potential role in systemic lupus erythematosus

Human complement receptor type 2 (CR2/CD21) is a B lymphocyte membrane glycoprotein that plays a central role in the immune responses to foreign Ags as well as the development of autoimmunity to nuclear Ags in systemic lupus erythematosus. In addition to these three well-characterized ligands, C3d/iC3b, EBV-gp350, and CD23, a previous study has identified CR2 as a potential receptor for IFN-alpha. IFN-alpha, a multifunctional cytokine important in the innate immune system, has recently been proposed to play a major pathogenic role in the development of systemic lupus erythematosus in humans and mice. In this study, we have shown using surface plasmon resonance and ELISA approaches that CR2 will bind IFN-alpha in the same affinity range as the other three well-characterized ligands studied in parallel. In addition, we show that IFN-alpha interacts with short consensus repeat domains 1 and 2 in a region that serves as the ligand binding site for C3d/iC3b, EBV-gp350, and CD23. Finally, we show that treatment of purified human peripheral blood B cells with the inhibitory anti-CR2 mAb 171 diminishes the induction of IFN-alpha-responsive genes. Thus, IFN-alpha represents a fourth class of extracellular ligands for CR2 and interacts with the same domain as the other three ligands. Defining the role of CR2 as compared with the well-characterized type 1 IFN-alpha receptor 1 and 2 in mediating innate immune and autoimmune roles of this cytokine should provide additional insights into the biologic roles of this interaction.

Complement receptor 2, natural antibodies and innate immunity: Inter-relationships in B cell selection and activation

Complement receptor type 2 (CR2) is a receptor that serves as an important interface between the complement system and adaptive immunity. Recent studies have shown that CR2 is also centrally involved in innate immunity, and one key area is the development of potentially pathogenic natural antibodies that target neo-epitopes revealed in ischemic tissue undergoing reperfusion. Mice lacking either total immunoglobulins or CR2 alone are protected from the development of ischemia-reperfusion injury, and this effect can be reversed by introducing CR2-sufficient B-1 cells or by transferring polyclonal natural IgM antibody from wild type mice as well as monoclonal antibodies that recognize phospholipids, DNA or non-muscle myosin. We will report at the XXI ICW an additional membrane-associated protein to which pathogenic IgM antibodies are directed. Whether B cells producing these natural antibodies are differentially selected in CR2-deficient mice is as yet not well understood, and the complement-related mechanism(s) whereby this differential repertoire selection process could occur have yet to be explored in any detail. In addition to this important role in innate immunity, CR2 can also act as a receptor for other components or activators of innate immunity. One such component is interferon-alpha, an anti-viral cytokine that binds CR2 and induces a component of its mRNA signature in B cells through this receptor. Other potential CR2 ligands are DNA and DNA-containing complexes such as chromatin. The biologic role of these CR2 interactions with interferon-alpha and DNA-containing complexes is not well understood, but may be important in the development of the autoimmune disease systemic lupus erythematosus that is characterized by enhanced interferon-alpha levels and loss of self tolerance to DNA-containing self antigens.

Signalling pathways in B cells: implications for autoimmunity

Following investigations of the pathogenic role of autoantibodies in rheumatic diseases, preclinical and clinical studies suggest a more central role of B cells in the maintenance of the disease process beyond just being precursors of (auto)antibody-producing plasma cells. Detailed analyses have implicated a number of surface molecules and subsequent downstream signalling pathways in the regulation of the events induced by BCR engagement. In this review, we discuss the potential role of molecules involved in altered B cell longevity, especially molecules involved in apoptosis (bcl-2, bcl-x, mutations in the Fas/Fas-L pathway), as well as molecules that might alter activation thresholds of B cells (CD19, CD21, CD22, lyn, SHP, SHIP-1) in the development of autoimmunity. Although focused on intrinsic B cell abnormalities, the complexity of interactions of B cells with other immune cells also makes it possible that increased B cell activation can be induced by distortions in the interaction with other cells. Further delineation of these alterations of B cell function in autoimmune conditions will allow development of more precise B cell-directed therapies beyond drastic B cell depletion, with the potential to improve the risk-benefit ratio of the treatments of autoimmune diseases.

A critical role for complement C3d and the B cell coreceptor (CD19/CD21) complex in the initiation of inflammatory arthritis

Complement C3 cleavage products mediate the recognition and clearance of toxic or infectious agents. In addition, binding of the C3d fragment to Ag promotes B lymphocyte activation through coengagment of the BCR and complement receptor 2 (CD21). Signal augmentation is thought to be achieved through enhanced recruitment and activation of CD21-associated CD19. In this study we show, using the DBA/1 collagen-induced arthritis (CIA) model, that conjugation of C3d to heterologous type II collagen is sufficient to cause disease in the absence of the mycobacterial components of CFA. Transient depletion of C3 during the inductive phase of CIA delays and lessens the severity of disease, and DBA/1 mice deficient for coreceptor components CD19 or CD21 are not susceptible to CIA. Adoptive transfer experiments revealed that CD21 expression on either B cells or follicular dendritic cells is sufficient to acquire disease susceptibility. Although CD19(-/-) and CD21(-/-) mice produce primary Ab responses to heterologous and autologous type II collagen, they are impaired in the ability to activate T cells, form germinal centers, and produce secondary autoantibody responses. These findings indicate that binding of C3d to self-Ags can promote autoimmunity through enhanced Ag retention and presentation by follicular dendritic cells and B cells, respectively.

Innate immunity and human B cell clonal expansion: effects on the recirculating B2 subpopulation

Foci of autoantigen-specific B lymphocytes in nonlymphoid tissues have been associated with development of autoimmune disease. To better understand the genesis of such ectopic lymphoid tissue, this study investigated whether several B cell-tropic innate immune system molecules, known to be elevated in response to inflammatory stimuli, can cooperate in fostering the T cell-independent clonal expansion of mature human B2 cells under conditions of limiting BCR engagement. Notable synergy was observed between BCR coligation with the C3dg-binding CD21/CD19 costimulatory complex, B cell-activating factor belonging to the TNF family (BAFF), and IL-4 in generating B cell progeny with sustained CD86 and DR expression. The synergy was observed over a wide range of BCR:ligand affinities and involved: 1) cooperative effects at promoting early cell cycle progression and viability; 2) BCR:CD21 coligation-promoted increases in BAFF receptors that were highly regulated by IL-4; 3) reciprocal effects of IL-4 and BAFF at dampening daughter cell apoptosis typical of stimulation by BCR:CD21 and either cytokine alone; and 4) BAFF-sustained expression of antiapoptotic Mcl-1 within replicating lymphoblasts. The results suggest that significant clonal proliferation of recirculating B2 cells occurs upon limited binding to C3dg-coated Ag in an inflammatory in vivo milieu containing both BAFF and IL-4. When rare autoantigen-presenting B cells undergo such expansions, both B cell and T cell autoimmunity may be promoted.

CD21/CD19 Coreceptor Signaling Promotes B Cell Survival during Primary Immune Responses

The adaptive immune response is tightly regulated to limit responding cells in an Ag-specific manner. On B cells, coreceptors CD21/CD19 modulate the strength of BCR signals, potentially influencing cell fate. The importance of the CD95 pathway was examined in response of B cells to moderate affinity Ag using an adoptive transfer model of lysozyme-specific Ig transgenic (HEL immunoglobulin transgene (MD4) strain) B cells. Although adoptively transferred Cr2+/+ MD4 B cells are activated and persist within splenic follicles of duck egg lysozyme-immunized mice, Cr2-/- MD4 B cells do not. In contrast, Cr2-/- MD4 lpr B cells persist after transfer, suggesting that lack of CD21/CD35 signaling results in CD95-mediated elimination. Cr2 deficiency did not affect CD95 levels, but cellular FLIP (c-FLIP) protein and mRNA levels were reduced 2-fold compared with levels in Cr2+/+ MD4 B cells. In vitro culture with Cr2+/+ MD4 B cells demonstrated that equimolar amounts of rHEL-C3d3 were more effective than hen egg lysozyme alone in up-regulating c-FLIP levels and for protection against CD95-mediated apoptosis. Collectively, this study implies a mechanism for regulating B cell survival in vivo whereby the strength of BCR signaling (including coreceptor) determines c-FLIP levels and protection from CD95-induced death.

The influence of immune complex-bearing follicular dendritic cells on the IgM response, Ig class switching, and production of high affinity IgG

It is believed that Ag in immune complexes (ICs) on follicular dendritic cells (FDCs) selects high affinity B cells and promotes affinity maturation. However, selection has been documented in the absence of readily detectable ICs on FDCs, suggesting that FDC-ICs may not be important. These results prompted experiments to test the hypothesis that IC-bearing murine FDCs can promote high affinity IgG responses by selecting B cells after stimulating naive IgM(+) cells to mature and class switch. Coculturing naive lambda(+) B cells, FDCs, (4-hydroxy-3-nitrophenyl)acetyl-chicken gamma-globulin (CGG) + anti-CGG ICs, and CGG-primed T cells resulted in FDC-lymphocyte clusters and production of anti-4-hydroxy-5-iodo-3-nitrophenyl acetyl. Class switching was indicated by a shift from IgM to IgG, and affinity maturation was indicated by a change from mostly low affinity IgM and IgG in the first week to virtually all high affinity IgG anti-4-hydroxy-5-iodo-3-nitrophenyl acetyl in the second week. Class switching and affinity maturation were easily detectable in the presence of FDCs bearing appropriate ICs, but not in the absence of FDCs. Free Ag plus FDCs resulted in low affinity IgG, but affinity maturation was only apparent when FDCs bore ICs. Class switching is activation-induced cytidine deaminase (AID) dependent, and blocking FDC-CD21 ligand-B cell CD21 interactions inhibited FDC-IC-mediated enhancement of AID production and the IgG response. In short, these data support the concept that ICs on FDCs can promote AID production, class switching, and maturation of naive IgM(+) B cells, and further suggest that the IC-bearing FDCs help select high affinity B cells that produce high affinity IgG.

Anti-phospholipid antibodies restore mesenteric ischemia/reperfusion-induced injury in complement receptor 2/complement receptor 1-deficient mice

Complement receptor 2-deficient (Cr2(-/-)) mice are resistant to mesenteric ischemia/reperfusion (I/R) injury because they lack a component of the natural Ab repertoire. Neither the nature of the Abs that are involved in I/R injury nor the composition of the target Ag, to which recognition is lacking in Cr2(-/-) mice, is known. Because anti-phospholipid Abs have been shown to mediate fetal growth retardation and loss when injected into pregnant mice, we performed experiments to determine whether anti-phospholipid Abs can also reconstitute I/R injury and, therefore, represent members of the injury-inducing repertoire that is missing in Cr2(-/-) mice. We demonstrate that both murine and human monoclonal and polyclonal Abs against negatively charged phospholipids can reconstitute mesenteric I/R-induced intestinal and lung tissue damage in Cr2(-/-) mice. In addition, Abs against beta2 glycoprotein I restore local and remote tissue damage in the Cr2(-/-) mice. Unlike Cr2(-/-) mice, reconstitution of I/R tissue damage in the injury-resistant Rag-1(-/-) mouse required the infusion of both anti-beta2-glycoprotein I and anti-phospholipid Ab. We conclude that anti-phospholipid Abs can bind to tissues subjected to I/R insult and mediate tissue damage.

Activation of mitogen activated protein kinases via complement receptor type 2

BACKGROUND

Complement receptor type 2 (CR2) is the receptor for C3d and C3dg and for Epstein-Barr virus. The aim of our study was to explore whether CR2 can independently mediate the activation of mitogen-activated protein kinases (MAPKs, including ERK, JNK, and p38MAPK), and to highlight the molecular mechanism of CD4+ cell deletion in AIDS.

METHODS

HOS cells (HOS-CR2) and HOS-CD4 cells (HOS-CD4CR2) stably expressing CR2 were established and then identified by FACS and Western blotting. Activation and blocking tests of MAPKs were assessed by Western blot. Cell proliferation was determined using Cell Titer 96((R)) Aqueous One Solution Reagent.

RESULTS

FACS results showed that the positive rates of HOS-CR2 and HOS-CD4CR2 cells were greater than 96%, and Western blot showed that the CR2 expression levels on HOS-CR2 and HOS-CD4CR2 cells were high. Activation and blocking tests of MAPKs (ERK, JNK, and p38MAPK) were carried out in HOS-CR2, HOS-CD4, and HOS-CD4CR2 cells. The activation of MAPKs in HOS-CR2 cells stimulated with PMA (100 ng/ml) and NHS (10%) was identical. The activation of MAPKs increased at 5 minutes, reached a peak at 10 minutes, and decreased to baseline within 30 minutes, all in a time-dependent manner; the activation of MAPKs was blocked by anti-CR2 McAb, PD98059 (inhibitor of ERK), and Wortmanin (inhibitor of PI-3K), respectively. In HOS-CD4 cells, MAPKs were activated by HIV-gp160. In HOS-CD4CR2 cells, MAPK activation was induced by HIV-gp160, 10% NHS, and HIV-gp160 + 10% NHS; phosphorylation of p38MAPK was dramatically induced by HIV-gp160 + NHS, and lasted for 1 hour. The cell proliferation results showed that HIV-gp160 inhibited the proliferation of HOS-CD4 and HOS-CD4CR2 cells (P < 0.01) and that NHS enhanced the effect of HIV-gp160 (P < 0.01).

CONCLUSIONS

The activation of MAPKs is independently mediated by CR2 and that anti-CR2 McAb, PD98059, and Wortmanin block the activation of MAPKs, respectively. The results of the signal transduction and cell proliferation assays of HOS-CD4CR2 cells show that CR2 plays a role in the pathogenesis of HIV infection, especially in the inhibition of CD4+ cell proliferation.

Murine hindlimb reperfusion injury can be initiated by a self-reactive monoclonal IgM

BACKGROUND

Murine hindlimb reperfusion injury (I/R), is initiated by activation of the classical pathway of complement. Complement receptor-2 knockout mice (Cr2-/-) are protected from I/R injury due to defective B-1 cells with a resulting deficient natural immunoglobulin M (IgM) repertoire. Cr2-/- and wild type (WT) mice were studied to isolate the antibody or antibodies responsible for initiation of I/R.

METHODS

IgM-secreting B-1 cell clones were produced with hybridoma technology from WT cells. Of 21 clones tested in murine I/R models, only 1 clone, CM22, was found to restore injury in protected mice. Cr2-/- mice reconstituted with IgM from individual clones, WT serum, or saline were subjected to 2 hours hindlimb ischemia and 3 hours reperfusion and compared with WT.

RESULTS

Muscle injury in Cr2-/- mice reconstituted with CM22 was similar to injury in WT mice reconstituted with saline and Cr2-/- mice reconstituted with WT serum. This injury was 137% greater (P < .05) than in both Cr2-/- mice reconstituted with saline and those reconstituted with a different IgM clone, CM31. IgM and C3 deposition was found only on injured muscle of WT mice or Cr2-/- mice reconstituted with CM22 or WT serum.

CONCLUSION

A single clone of self-reactive IgM, CM22, can initiate complement-dependent I/R injury.

Distinct sequences in the cytoplasmic domain of complement receptor 2 are involved in antigen internalization and presentation

B cells express randomly rearranged surface Ig that forms part of a multiprotein complex known as the B cell receptor (BCR). Recognition of Ag via this receptor results in its capture, internalization, proteolysis and presentation to CD4+ T cells. The recognition of Ag by CD4+ T cells is critical for the selection of individual B cells, leading to the eventual secretion of a high affinity version of the BCR as an effective circulating Ab. B cells also express other receptors that recognize Ags associated with components of innate immunity. One of these receptors, CR2, binds Ags coated with activated complement components. Studies have shown that cross-linking CR2 and the BCR with complement-tagged Ags leads to enhanced Ag presentation by B cells. In addition, Ags targeted to B cell CR2 in the absence of BCR coligation are also efficiently presented to T cells. In this report, we identify several distinct sequences within the cytoplasmic domain of mouse CR2 (mCR2) that are essential for mCR2-mediated Ag presentation in both the presence and the absence of BCR cross-linking. The finding that distinct sequences in the cytoplasmic domain of mCR2 are essential for BCR-independent Ag presentation leads us to propose a novel role for CR2.

Use of gHgL for attachment of Epstein-Barr virus to epithelial cells compromises infection

Epstein-Barr virus (EBV) is a lymphotropic herpesvirus. However, access to B lymphocytes during primary infection may be facilitated by replication in mucosal epithelial cells. Attachment and penetration of EBV into these two cell types are fundamentally different. Both the distribution of receptors and the cellular origin of the virus impact the efficiency of infection. Epithelial cells potentially offer a wide range of receptors with which virus can interact. We report here on analyses of epithelial cells expressing different combinations of receptors. We find that the stoichiometry of the virus glycoprotein complex that includes gHgL and gp42 affects the use of gHgL not just for entry into epithelial cells but also for attachment. Penetration can be mediated efficiently with either a coreceptor for gp42 or gHgL, but the use of gHgL for attachment as well as penetration greatly compromises its ability to mediate entry.

Cutting Edge: C3d Functions as a Molecular Adjuvant in the Absence of CD21/35 Expression

Complement component C3 covalently attaches to Ags following activation, where the C3d cleavage fragment can function as a molecular adjuvant to augment humoral immune responses. C3d is proposed to exert its adjuvant-like activities by targeting Ags to the C3d receptor (CD21/35) expressed by B cells and follicular dendritic cells. To directly assess the importance of CD21/35 in mediating the immunostimulatory effects of C3d, CD21/35-deficient (CD21/35(-/-)) mice were immunized with streptavidin (SA), SA-C3dg tetramers, recombinant HIV gp120 (gp120), or gp120 fused with linear multimers of C3d. Remarkably, SA- and gp120-specific Ab responses were significantly augmented in CD21/35(-/-) mice when these Ags were complexed with C3d in comparison to Ag alone. In fact, primary and secondary Ab responses and Ab-forming cell responses of CD21/35(-/-) mice approached those of wild-type mice immunized with SA-C3dg and gp120-C3d. Thus, C3d can function as a molecular adjuvant in the absence of CD21/35 expression.

The tetraspanin CD81 is necessary for partitioning of coligated CD19/CD21-B cell antigen receptor complexes into signaling-active lipid rafts

Tetraspanins have been hypothesized to facilitate the organization of functional multimolecular membrane complexes. In B cells the tetraspanin CD81 is a component of the CD19/CD21 complex. When coligated to the B cell Ag receptor (BCR), the CD19/CD21 complex significantly enhances BCR signaling in part by prolonging the association of the BCR with signaling-active lipid rafts. In this study CD81 is shown to associate with lipid rafts upon coligation of the BCR and the CD19/CD21 complex. Using B cells from CD81-deficient mice we demonstrate that in the absence of CD81, coligated BCR and CD19/CD21 complexes fail to partition into lipid rafts and enhance BCR signaling from rafts. Furthermore, a chimeric CD19 protein that associates only weakly if at all with CD81 fails to promote the association of coligated BCR with lipid rafts. The requirement for CD81 to promote lipid raft association may define a novel mechanism by which tetraspanins function as molecular facilitators of signaling receptors.

[CD21-independent infection of a human signet ring cell gastric carcinoma cell line by Epstein-Barr virus]

OBJECTIVE

To understand Epstein-Barr virus (EBV) infection of gastric carcinoma cells.

METHODS

The authors tested the infection of a signet ring cell line HSC-39 derived from human gastric carcinoma with Akata and P3HR-1 strains of EBV. Akata and P3HR-1 infected of EBV cell clones were isolated by a limiting dilution method.

RESULTS

EBV-encoded small RNAs (EBERs) were expressed in the infected cells with each EBV strain by in situ hybridization. The EBV infected parental cells and most clones expressed EBNA1, but not EBNA2, latent membrane protein (LMP) 1 and LMP2A. Both EBV strains infected parental cells and clones presented type I latency. The uninfected HSC-39 cells were negative for CD21 expression; however, the Akata but not P3HR-1-infected clones were positive for CD21 expression at mRNA level.

CONCLUSION

These results demonstrated that EBV infecting HSC-39 by CD21-independent pathway. This study also defined a signet ring cell line as a new target for EBV.

Complement receptor 2 and autoimmunity

Complement receptor 2 (CR2/CD21) plays a major role in the immune response by linking innate and adaptive immunity to foreign pathogens and proteins. In addition, several lines of evidence strongly support a role for CR2 in the maintenance of tolerance to self-antigens. Both the absence of CR2 expression (along with the alternatively spliced gene product CR1) and the presence of a dysfunctional CR2 protein are tightly associated with the development of autoreactivity to nuclear antigens. Altered levels of expression of CR2 in patients with systemic lupus erythematosus support a clinically relevant role for this phenotype. Several possible mechanisms could underlie the loss of self-tolerance related to CR2, but the effect is most likely related to the failure of one or more specific checkpoints that limit autoreactivity during B cell development and immune reactions.

Complement receptors type 1 (CR1, CD35) and 2 (CR2, CD21) cooperate in the binding of hydrolyzed complement factor 3 (C3i) to human B lymphocytes

The C3b-binding receptor, CR1/CD35, supports CR2/CD21-mediated activation of complement by human B lymphocytes, possibly by associating with CR2 to promote or stabilize the binding of hydrolyzed C3 (C3i), the primary component of the AP convertase, C3i-Bb. To evaluate this hypothesis, we examined the uptake kinetics and binding equilibria for C3i dimer interaction with human blood cells in the absence and presence of CR1- and CR2-blocking mAb. C3i displayed dual uptake kinetics to B lymphocytes, comprising of rapid binding to CR1 and slower binding to CR2. The forward rate constants (k(1)) for CR1 and CR2, operating independently, differed ca. 9-fold (k(1)=193+/-9.4 and 22.2+/-6.0 x 10(3) M(-1)s(-1), respectively). Equilibrium binding of C3i to B lymphocytes was also complex, varying in strength by ca. 13-fold over the C3i concentration range examined. The maximum association constant (K(a, max)=109+/-27.2 x 10(7) l/mole) was ca. 9- and 6-fold greater, respectively, than those for CR1 or CR2 acting alone (K(a)=13.2+/-5.3 and 18.5+/-3.5 x 10(7) l/mole). The high avidity of the CR1-CR2 complex for C3i is consistent with its rates of C3i uptake and release being determined by CR1 and CR2, respectively.

Role of Complement-Binding CD21/CD19/CD81 in Enhancing Human B Cell Protection from Fas-Mediated Apoptosis

Defective expression of Fas leads to B cell autoimmunity, indicating the importance of this apoptotic pathway in eliminating autoreactive B cells. However, B cells with anti-self specificities occasionally escape such regulation in individuals with intact Fas, suggesting ways of precluding this apoptosis. Here, we examine whether coligation of the B cell Ag receptor (BCR) with the complement (C3)-binding CD21/CD19/CD81 costimulatory complex can enhance the escape of human B cells from Fas-induced death. This was warranted given that BCR-initiated signals induce resistance to Fas apoptosis, some (albeit not all) BCR-triggered events are amplified by coligation of BCR and the co-stimulatory complex, and several self Ags targeted in autoimmune diseases effectively activate complement. Using a set of affinity-diverse surrogate Ags (receptor-specific mAb:dextran conjugates) with varying capacity to engage CD21, it was established that BCR:CD21 coligation lowers the BCR engagement necessary for inducing protection from Fas apoptosis. Enhanced protection was associated with altered expression of several molecules known to regulate Fas apoptosis, suggesting a unique molecular model for how BCR:CD21 coligation augments protection. BCR:CD21 coligation impairs the generation of active fragments of caspase-8 via dampened expression of membrane Fas and augmented expression of FLIP(L). This, in turn, diminishes the generation of cells that would be directly triggered to apoptosis via caspase-8 cleavage of caspase 3 (type I cells). Any attempt to use the mitochondrial apoptotic protease-activating factor 1 (Apaf-1)-dependent pathway for apoptosis (as type II cells) is further blocked because BCR:CD21 coligation promotes up-regulation of the mitochondrial antiapoptotic molecule, Bcl-2.

Regulation of B-cell activation by complement receptors CD21 and CD35

The role of complement in the development and regulation of antibody responses under both healthy and pathological conditions is known for long. Unraveling the elements involved and the molecular mechanisms underlying the events however is still in progress. This review focuses on the role of complement receptors CR1 (CD35) and CR2 (CD21) expressed on B lymphocytes, which interact with ligands generated upon activation of component C3, the major protein of the complement cascade. The binding and possible effects of immune complexes comprising antigen, antibody and complement on B-cell activation are discussed. Results of clinical studies of autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis and conclusions drawn from animal models used to investigate various aspects of human diseases are also debated. We discuss similarities regarding the overall structure and certain functions of complement and complement receptors in mice and men however, call the attention to major differences regarding tissue distribution and their role in B-cell functions.

Was it there all the time?

The complement system is old, yet it may still have something new to teach us. For many years, research has existed which shows that C3d, in addition to its established role as an adjuvant, could have an immunosuppressive activity. Being true, it suggests that a common mechanism may be used both by organisms and by their pathogens to prevent unwanted immune responses.

Epstein-Barr virus infection of rat lymphocytes expressing human CD21 results in restricted latent viral gene expression and not in immunoblastic transformation

Transgenic rats expressing human CD21 gene (hCD21) driven by the mouse immunoglobulin enhancer were generated. hCD21 was expressed in lymphoid tissues, especially in the spleen and in the brain. Flow cytometric analysis indicated that about 20% of spleen cells, most having a B-lymphocyte marker, expressed hCD21. After Epstein-Barr virus (EBV) infection of spleen cells, EBV-determined nuclear antigen (EBNA) was first detected on Day 4 and reached a maximum of 0.3% on Day 5, but the infection was abortive and was not followed by blastogenesis, cellular DNA synthesis or proliferation. Reverse transcription-polymerase chain reaction (RT-PCR) analyses demonstrated that EBV-infected spleen cells expressed EBNA1 and EBV-encoded small RNA (EBER), but not other latent EBV products. EBNA promoter analysis by RT-PCR indicated that the Q promoter was active, whereas C and W promoters were not active. The present findings indicate that human and rat lymphocytes respond to EBV infection differently in vitro.

Impaired antibody response to group B streptococcal type III capsular polysaccharide in C3- and complement receptor 2-deficient mice

Group B Streptococcus (GBS) is the foremost bacterial cause of serious neonatal infections. Protective immunity to GBS is mediated by specific Abs to the organism's capsular polysaccharide Ags. To examine the role of complement in the humoral immune response to type III GBS capsular polysaccharide (III-PS), mice deficient in C3 or in CD21/CD35 (i.e., complement receptors 1 and 2; CR1/CR2) were immunized with III-PS. Mice deficient in C3 or Cr2 had an impaired primary immune response to III-PS. The defective response was characterized by low IgM levels and the lack of an isotype switch from IgM to IgG Ab production. Compared with wild-type mice, C3- and Cr2-deficient mice exhibited decreased uptake of III-PS by follicular dendritic cells within the germinal centers and impaired localization of III-PS to the marginal zone B cells. Complement-dependent uptake of capsular polysaccharide by marginal zone B cells appears necessary for an effective immune response to III-PS. The normal immune response in wild-type mice may require localization of polysaccharide to marginal zone B cells with subsequent transfer of the Ag to follicular dendritic cells.

Signaling by the CD19/CD21 complex on B cells

Early studies, largely based on in vitro models, revealed potential functional roles for the components of the CD19/21/81 complex in B cell proliferation and antibody production. These studies also identified signal transduction pathways linked to these receptors. Over the last decade, studies on knockout mice defined the biologic functions of CD19, CD21 and CD81. This review focuses on current attempts to use these receptors as tools to understand how the immune system regulates responsiveness and tolerance, while correlating specific biochemical pathways with biologic function.

Autoimmunity, complement activation, tissue injury and reciprocal effects

Proteins of the complement system limit the expression of systemic autoimmunity by raising the threshold for negative selection and, in their absence, autoreactive lymphocytes seem to enter the periphery. On the other hand, complement activation in the course of systemic autoimmunity leads to tissue injury in a number of ways including direct lysis of cells, modification of cell function and by contributing to the formation of immune complexes. Excessive complement activation as a result of a regulator component deficiency leads to tissue injury that mimics that seen in autoimmune disease. Complement activation occurs during tissue injury and contributes in a major way to the expression of pathology. It appears that natural antibodies represent an early culprit in tissue injury following ischemia/reperfusion injury. Natural antibodies and probably autoantibodies present in sera of patients with systemic autoimmune disease bind to tissues already exposed to a damaging insult, activate complement and produce pathology.

Engagement of complement receptor 2 on the surface of B cells from patients with systemic lupus erythematosus contributes to the increased responsiveness to antigen stimulation

B cells from patients with systemic lupus erythematosus (SLE) display increased responses following cross-linking of the surface antigen receptor. We explored the possibility that the increased responses are at least partially due to simultaneous cross-linking of the complement receptor 2 (CR2). To this end, we stimulated fresh B cells from SLE patients with an anti-IgD antibody conjugated to the Epstein-Barr virus gp350 protein, which binds to CR2, and recorded the free intracytoplasmic calcium response during the first 10 min. Despite the fact that SLE B cells were found to express half as many surface CR2 as normal B cells, both peak responses and the percentage of responding cells were significantly increased in the former. These observations suggest that regulatory molecules such as CR2 are involved in the increased B cell responses in SLE patients. We propose that certain immune complexes that circulate in the sera of SLE patients that have anti-surface immunoglobulin specificities and are decorated with natural ligands of CR2, such as C3d, elicit and promote B cell overactivity.

Regulation of IgG antibody responses by epitope density and CD21-mediated costimulation

Epitope density and organization have been shown to be important factors for B cell activation in many animal model systems. However, it has been difficult to separate the role of antigen organization from the role of local antigen concentrations because highly organized antigens are usually particulate whereas non-organized antigens are more soluble. Hence, highly organized and non-organized antigens may interact with different cell types and in different locations within lymphoid organs. In order to assess the role of antigen organization in regulating B cell responses, we immunized mice with highly repetitive virus-like particles, which exhibit different epitope densities covalently attached to them. Therefore, the same particulate structure was used to present identical epitopes that differed in their degree of organization. Induction of epitope-specific IgM titers, reflecting early B cell activation, were unaffected by the degree of epitope density. Furthermore, the absence of Th cells or CD21/CD35 did not reduce the IgM response. In contrast, the degree of organization was a critical factor influencing the magnitude of the epitope-specific IgG response. Moreover, the threshold for IgG responses was shifted in the absence of CD21/CD35, resulting in the requirement for higher epitope densities to allow efficient IgG responses. Thus, IgG but not IgM responses are regulated by epitope density and B cell costimulatory thresholds.

Infection of immortalized human epithelial cell line Hacat with high-concentration Epstein-Barr virus

OBJECTIVE

To study the mechanism by which Epstein-Barr (EB) virus infects human epithelial cells.

METHODS

Large-scale culture of marmoset lymphocytes B958 was carried out to extract and condense EB virus therein. Titrated by lymphocytes from fetal umbilical blood, the EB virus of high concentration was used to infect immortalized human epithelial cell line Hacat, followed by genomic DNA extraction from the Hacat cells and amplification of the special DNA sequence Bam HIw fragments of EBV genomic DNA by PCR. Southern blotting and in situ hybridization were employed to confirm the result of PCR.

RESULTS

The results of PCR, Southern blotting and in situ hybridization all indicated that high-concentration EBV could infect Hacat cell line, but the rate of infection was rather low.

CONCLUSION

There is an unknown pathway of infection for EBV entry into the epithelial cells other than the mediation by CR2 or pIgR.

Age-related depression of FDC accessory functions and CD21 ligand-mediated repair of co-stimulation

Morphological and kinetic studies of immune complex (IC) trapping by follicular dendritic cells (FDC) show marked age-related deficits. We postulated that a reduction in trapped IC, which generate CD21 ligands (L) on FDC, would lead to inadequate FDC-Ag-B cell interactions resulting in depressed Ab responses. To determine whether the age-related defect was the result of the aging of FDC or changes in the in vivo microenvironment of FDC (i.e. aging B and T cells), FDC-B cell-T cell-Ag interactions were studied in in vitro germinal centers where various combinations of old and young cells could be compared. Since we reasoned that reduced IC on FDC would generate less CD21L needed to stimulate the B cell co-receptor via CD21, we also examined the role of complement (C'). The hypothesis that aging reduces the accessory activity of FDC was tested with increasing numbers of FDC from young (12 weeks) or old (20 months) mice in the presence of young (12 weeks) B and T lymphocytes. The Ag-specific stimulatory activity of FDC was studied using the OVA-specific Ab response which was reduced by 40-50% in the presence of old FDC. Antigen-independent FDC-mediated co-stimulation was studied by using LPS to stimulate B-lymphocytes to produce immunoglobulin (Ig). In the presence of old FDC, co-stimulation was decreased by 70-80% in the LPS system. Incubation of aged FDC with IC and C' to provide FDC with CD21L restored co-stimulatory activity to near normal levels. In marked contrast, no defects in old B and T cells were apparent. The data suggest that the Ag handling capacity and co-stimulatory activity of old FDC become defective with aging and this appears to be a consequence of reduced trapping and presentation FDC-Ag and CD21L to B cells.

Functional analysis of the human complement receptor 2 (CR2/CD21) promoter: characterization of basal transcriptional mechanisms

Human complement receptor (CR) type 2 (CR2/CD21) is a 145-kDa membrane protein encoded within the regulators of complement activation gene cluster localized on human chromosome 1q32. Understanding the mechanisms that regulate CR2 expression is important because CR2 is expressed during specific stages of B cell development, and several lines of evidence suggest a role for altered CR2 function or expression in a number of autoimmune diseases. Additionally, even modest changes in CR2 expression are likely to affect relative B cell responses. In this study we have delineated the transcriptional requirements of the human CR2 gene. We have studied the human CR2 proximal promoter and identified sites important for controlling the level of transcription in CR2-expressing cells. We have determined that four functionally relevant sites lie within very close proximity to the transcriptional initiation site. These sites bind the transcription factors USF1, an AP-2-like transcription factor, and Sp1.

Mucosal type mast cells express complement receptor type 2 (CD21)

Fragments of complement component C3 generated upon activation of the cascade play an important role in the induction and regulation of immune responses. Receptors interacting with various fragments of this versatile complement protein are expressed on a wide variety of cell types, including lymphocytes, macrophages, dendritic cells, follicular dendritic cells, granulocytes, erythrocytes and consequently, C3-products may influence several biological functions at different sites of the body, where complement activation occurs. Regarding the expression of various C3-receptors on mast cells, mainly rodent serosal type mastocytes have been investigated so far. It has been known for a long time that C3a triggers the release of mediators of immediate type hypersensitivity via binding to serosal-type cells. Complement receptor type 1 (CR1/CD35) and type 2 (CR2/CD21) interacting with the larger activation products, such as C3b and C3d, have so far been shown on serosal type mast cells only. In this study, the expression of CR1/2 on mucosal type mast cells is demonstrated. Using mouse CR1/2 specific single chain antibodies and the natural ligand C3d in cytofluorimetric measurements, we show that the rat mucosal mast cell line RBL-2H3 and mouse bone marrow-derived mast cells (BMMC) express CD21. RT-PCR experiments carried out with mouse CR1 and CR2 specific primers show CD21, but not CD35 specific products in BMMC. It is also demonstrated that, in contrast to serosal type mast cells, mucosal mastocytes do not express CD19. In an attempt to reveal the possible function of CR2 on mucosal type mast cells, the effect of receptor-clustering was tested regarding degranulation, Ca-response and IL-6 production, but no CR2-mediated change was detected in any of these processes.

Modulation of mouse endotoxic fever by complement

It was recently reported that the complement system may be critically involved in the febrile response of guinea pigs to systemic, particularly intraperitoneally (i.p.) injected, lipopolysaccharides (LPS). The present study was designed to identify which component(s) of the complement cascade may be specifically critical. To this end, we used mice with C3, C5, and CR2 gene deletions. To assess preliminarily the suitability of mice for such a study, we replicated our earlier studies with guinea pigs. Thus, to verify initially whether complement is similarly involved in the febrile response of wild-type (C57BL/6J) mice to i.p. LPS (Escherichia coli, 1 microg/mouse), we depleted complement with cobra venom factor (CVF; 7 U/mouse, intravenously [i.v.]). These animals did not develop fever, whereas the core temperature (T(c)) of CVF vehicle-treated controls rose approximately 1 degrees C by 80 min postinjection and then gradually abated over the following 2.5 h, confirming the involvement of complement in fever production after i.p. LPS injection and the suitability of this species for these studies. C3- and C5-sufficient (C3(+/+) and C5(+/+)) mice also developed 1 degrees C fevers within 80 min after i.p. LPS (1 or 2 microg/mouse) injection. These fevers were totally prevented by CVF (10 U/mouse, i.v.) pretreatment. C3- and C5-deficient (C3(-/-) and C5(-/-)) mice were also unable to develop T(c) rises after i.p. LPS. Both CR2(+/+) and CR2(-/-) mice responded normally to i.p. LPS (1 microg/mouse). These data indicate that C5, but not C3d acting through CR2, may play a critical role in the febrile response of mice to i.p. LPS.

Complement component C3 promotes T-cell priming and lung migration to control acute influenza virus infection

The complement cascade defines an important link between the innate and the specific immune system. Here we show that mice deficient for the third component of complement (C3-/- mice) are highly susceptible to primary infection with influenza virus. C3-/- mice showed delayed viral clearance and increased viral titers in lung, whereas mice deficient for complement receptors CR1 and CR2 (Cr2-/- mice) cleared the infection normally. Priming of T-helper cells and cytotoxic T cells (CTLs) in lung-draining lymph nodes was reduced, and the recruitment into the lung of virus-specific CD4+ and CD8+ effector T cells producing interferon-gamma was severely impaired in C3-/- but not in Cr2-/- mice. Consequently, T-helper cell-dependent IgG responses were reduced in C3-/- mice but remained intact in Cr2-/- mice. These results demonstrate that complement induces specific immunity by promoting T-cell responses.

Antigen receptor triggered upregulation of CD86 and CD80 in human B cells: augmenting role of the CD21/CD19 co-stimulatory complex and IL-4

The impact of BCR:CD21 co-engagement on B cell expression of molecules critical for T cell activation was investigated with receptor-specific mAbs conjugated to high MW dextran as stimulatory ligands. In the absence of IL-4, BCR:CD21 co-ligation augmented BCR-triggered CD86 only under conditions of very low BCR ligand dose or affinity, and CD80 was minimally induced by BCR and/or CD21 crosslinking. In the presence of IL-4, BCR:CD21 co-ligation augmented CD86 and CD80 expression under conditions of greater BCR engagement. However, with very high level BCR engagement, no bonus effect of BCR:CD21 crosslinking was observed. Co-ligation-promoted CD86 and CD80 expression was associated with heightened B cell activation of resting allogeneic T cells. The data suggest that co-clustering of BCR and the CD21/CD19 co-stimulatory complex following B cell engagement with C3d-bound microbial or self-antigens will enhance B cell recruitment of T cell help only when IL-4 is present and/or BCR engagement is very limiting.

Transmission of antibody-induced arthritis is independent of complement component 4 (C4) and the complement receptors 1 and 2 (CD21/35)

The K/BxN murine model of rheumatoid arthritis (RA) is dependent on the specificity of the KRN alpha beta-TCR, to recognize glucose-6-phosphate-isomerase (GPI) on the NOD MHC class II A(g7) allele and production of GPI-specific autoantibodies. Transfer of K/BxN serum into MHC-unrelated and lymphocyte-deficient mice induces RA. To investigate whether K/BxN serum-induced RA involves complement activation and/or the complement receptors (CR) 1 and 2, we analyzed the role of complement C4 and of CR1 and CR2. For this purpose we used C4(-/-) mice impaired in the classical and the lectin complement pathways; Cr2(-/-) mice lacking CR1 and CR2 and, as control strains, BALB/c, C57BL/6, KRN and NOD. RA was assessed by caliper measurement of ankle thickness, clinical index and joint histology. We found that all mouse strains except NOD developed RA. The lack of protection in C4(-/-) mice suggests that antibody-mediated RA is independent of the classical as well as the lectin complement pathways and the split complement product C4b. The lack of protection in Cr2(-/-) mice suggests that absence of CR1 had no significant affect, considering its role in immune complex clearance, inhibition of C3 and C5 convertase and as receptor for C3b/C4b. Also, CR2 lacks a role in disease as analyzed here, in its possible functions as receptor for C3dg, germinal center reaction and activation of alternative pathway on binding iC3. Hence we conclude that the transmission of K/BxN serum-induced RA is independent of the classical and the lectin complement pathways and CR1 and CR2. The crucial role of complement C5, while neither classical nor lectin pathway is necessary, indicates that the alternative complement pathway may have a role in the K/BxN serum-induced RA model.

Modeling the presentation of C3d-coated antigen by B lymphocytes: enhancement by CR1/2-BCR co-ligation is selective for the co-ligating antigen

We have used a set of single-chain variable fragment antibodies (sc) genetically fused with an influenza hemagglutinin-derived peptide as a means to investigate the role of CR1 and CR2 in antigen presentation by B cells. When incubated with the B cell lymphoma 2PK3, peptide-containing sc specific for either CR1 or CR1/2 mediated activation of the hemagglutinin peptide-specific T cell line IP-12-7, as assessed by IL-2 production. Efficient presentation was dependent on the binding of the constructs to CR1/2, implying that receptor-mediated endocytosis is responsible for the effect. Cross-linkage of CR1/2 or CD19 by mAb did not increase the extent of T cell activation. However, when CR1/2 was co-ligated with the BCR--using either polyclonal goat anti-mouse IgG or recombinant protein LA--the antigen concentration required to activate T cells decreased by two orders of magnitude. Moreover, this enhancement was selective for the antigen included in these complexes and did not affect the presentation of a free peptide or of antigen bound to CR1/2 excluded from the complexes. These results suggest that B cells may bind various C3d-coated antigens at a time, but only the one which reacts with the BCR will be processed with high efficiency. This mechanism may ensure the specificity of cognate T cell help.

Depletion of Lyn kinase from the BCR complex and inhibition of B cell activation by excess CD21 ligation

The human and murine CD21 gene products have been functionally linked to B cell activation by the co-ligation of the BCR and the CD21/CD19/CD81 complexes. Binding of low levels of antigen complexed to the complement ligand(s) for CD21 enhances B cell activation compared to the stimulation caused by antigen alone. Mice lacking functional CD21 predispose to autoimmune responses suggesting that this receptor may also play a negative role: thus in the presence of excess complement-bearing immune complexes, B cell antigen-specific activation may be inhibited. This possibility was investigated using intracellular calcium elicitation analyses to follow BCR-mediated activation. Ligation of the BCR and limiting quantities of the CD21 receptor demonstrated the expected enhanced cellular response compared to BCR ligation alone: CD21 ligation alone demonstrated no alteration in calcium flux. However, co-ligation of the BCR with excess CD21 binding resulted in the elimination of the calcium response, suggesting that CD21 ligation was down-modulating the BCR response. Immunoprecipitation of kinases associated with the BCR and CD21/CD19/CD81 complexes demonstrated that Lyn is preferentially depleted from the BCR complex following excess binding of CD21. Localization of other kinases integral for B cell activation is not altered. These data suggest that excess CD21 ligand binding can negatively impact B cell activation by sequestering Lyn kinase away from the BCR complex.

The role of the CD19/CD21 complex in B cell processing and presentation of complement-tagged antigens

The CD19/CD21 complex is an essential B cell coreceptor that functions synergistically to enhance signaling through the B cell Ag receptor in response to T cell-dependent, complement-tagged Ags. In this study, we use a recombinant protein containing three tandemly arranged copies of C3d and the Ag hen egg lysozyme, shown to be a highly effective immunogen in vivo, to evaluate the role of the CD19/CD21 complex in Ag processing in B cells. Evidence is provided that coengagement of the CD19/CD21 complex results in more rapid and efficient production of antigenic peptide/class II complexes as compared with B cell Ag receptor-mediated processing alone. The CD19/CD21 complex does not itself target complement-tagged Ags for processing, but rather appears to influence B cell Ag processing through its signaling function. The ability of the CD19/CD21 complex to augment processing may be an important element of the mechanism by which the CD19/CD21 complex functions to promote B cell responses to T cell-dependent complement-tagged Ags in vivo.

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.

The Follicular versus Marginal Zone B Lymphocyte Cell Fate Decision Is Regulated by Aiolos, Btk, and CD21

Most splenic B cells in mice that lack Aiolos are mature IgD(hi)IgM(lo) follicular lymphocytes, suggesting that maturation signals delivered via the BCR are enhanced in the absence of Aiolos. The enhanced maturation of follicular B cells is accompanied by the absence of MZ B lymphocytes and the downregulation of CD21 expression in follicular B cells, all of which depend on the generation of signals via Btk, which is in epistasis to Aiolos. The inverse relationship between the strength of BCR signaling and MZ B cell development is supported by an examination of MZ B cells in CD21 null mice. These data support the view that antigens (in contrast to "tonic" signals) drive the development of naive B cells.

Structure and flexibility of the multiple domain proteins that regulate complement activation

In this review we summarise more than 10 years of biophysical exploration into the structural biology of the regulators of complement activation (RCA). The five human proteins responsible for regulation of the early events of complement are homologous and are composed largely from building blocks called "complement control protein (CCP) modules". Unlike most multiple domain proteins they do not contain any of the other widely occurring module types. This apparent simplicity of RCA structure, however, is belied by their sophistication of function. In fact, the structures of the individual CCP modules exhibit wide variations on a common theme while the extent and nature of intermodular connections is diverse. Some neighbouring modules within a protein stabilise each other and some co-operate to form specific binding surfaces. The degree of true "modularity" of CCPs is open to debate. The study of RCA proteins clearly illustrates the value of combining complementary structural biology techniques. The results could have implications for folding, evolution, flexibility and structure-function relationships of other molecules in the large, diverse and little understood category of multiple domain proteins.

Activation of the EBV/C3d receptor (CR2, CD21) on human B lymphocyte surface triggers tyrosine phosphorylation of the 95-kDa nucleolin and its interaction with phosphatidylinositol 3 kinase

We previously demonstrated that CR2 activation on human B lymphocyte surface triggered tyrosine phosphorylation of a p95 component and its interaction with p85 subunit of phosphatidylinositol 3' (PI 3) kinase. Despite identical molecular mass of 95 kDa, this tyrosine phosphorylated p95 molecule was not CD19, the proto-oncogene Vav, or the adaptator Gab1. To identify this tyrosine phosphorylated p95 component, we first purified it by affinity chromatography on anti-phosphotyrosine mAb covalently linked to Sepharose 4B, followed by polyacrylamide gel electrophoresis. Then, the isolated 95-kDa tyrosine phosphorylated band was submitted to amino acid analysis by mass spectrometry; the two different isolated peptides were characterized by amino acid sequences 100% identical with two different domains of nucleolin, localized between aa 411--420 and 611--624. Anti-nucleolin mAb was used to confirm the antigenic properties of this p95 component. Functional studies demonstrated that CR2 activation induced, within a brief span of 2 min, tyrosine phosphorylation of nucleolin and its interaction with Src homology 2 domains of the p85 subunit of PI 3 kinase and of 3BP2 and Grb2, but not with Src homology 2 domains of Fyn and Gap. These properties of nucleolin were identical with those of the p95 previously described and induced by CR2 activation. Furthermore, tyrosine phosphorylation of nucleolin was also induced in normal B lymphocytes by CR2 activation but neither by CD19 nor BCR activation. These data support that tyrosine phosphorylation of nucleolin and its interaction with PI 3 kinase p85 subunit constitute one of the earlier steps in the specific intracellular signaling pathway of CR2.

Cytokine dependency of human B cell cycle progression elicited by ligands which coengage BCR and the CD21/CD19/CD81 costimulatory complex

Coengagement of BCR and the C3dg binding CD21/CD19/CD81 costimulatory complex can profoundly reduce the BCR binding threshold for eliciting B cell S phase entry, provided cytokine is present. IL-4 is substantially better than IL-2, IL-13, and TNF-alpha at exhibiting synergy with BCR:CD21 coengaging ligand (anti-IgM:anti-CD21:dextran) in promoting B cell DNA synthesis. Synergy between IL-4 and anti-IgM:anti-CD21:dextran (a) is not explained by the viability-promoting function of IL-4, (b) occurs when the anti-CD21 moiety engages either C3dg binding or non-C3dg binding domains, (c) does not reflect reversal of FcgammaRII-mediated negative regulation, and (d) involves differing temporal requirements for BCR and IL-4R signal transduction during the activation process. The IL-4R signaling pathway appears to synergize directly with the BCR:CD21 signaling pathway(s) in promoting the progression of resting B cells past an early G1 checkpoint, as well as to promote independently the progression of activated B cells past a later G1 to S checkpoint.

Follicular dendritic cell accessory activity crosses MHC and species barriers

Productive follicular dendritic cell (FDC)-B cell interactions appear to involve critical ligand-receptor interactions. Immune complexes (IC) on FDC activate complement and provide FDC with a complement-derived CD21 ligand (CD21L), which bind CD21, while antigen in the IC binds on the B cell-BCR. Further, FDC-FcgammaRIIB binds Fc regions of antibodies in IC and reduces coligation of BCR and FcgammaRIIB minimizing an inhibitor of B cell activation. Given that Fc receptors and complement receptors bind immunoglobulins and complement fragments of other species, we reasoned that FDC accessory activity should cross MHC and species barriers. This prediction was tested using memory lymphocytes from OVA-immune mice and TT-immune humans in combination with FDC from murine lymph nodes and human tonsils. Human and murine FDC converted IC into potent immunogens (specific antibody increased from background to thousands of ng / ml). MHC barriers did not restrict this activity and human FDC worked with murine lymphocytes and murine FDC worked with human lymphocytes. Furthermore, stimulation via MHC-dependent allogeneic or zenogeneic mechanisms did not promote antibody production by FDC. Polyclonal responses stimulated by lipopolysaccharide and pokeweed mitogen were also promoted (10 - 100-fold) and anti-CD21 blocked FDC activity. These results substantiate the hypothesis that FDC are necessary for strong recall responses and that FDC-CD21L is critical.

A role for CD21/CD35 and CD19 in responses to acute septic peritonitis: a potential mechanism for mast cell activation

Although it is now appreciated that mast cell-mediated release of TNF-alpha is critical for resolution of acute septic peritonitis, questions remain as to how mast cells are activated upon peritoneal bacterial infection. Clues to how this may occur have been derived from earlier studies by Prodeus et al. in which complement proteins C3 and C4 were shown to be required for survival following cecal ligation and puncture (CLP), a model for acute septic peritonitis. To evaluate the mechanism for mast cell activation in the CLP model, complement receptor CD21/CD35-deficient mice (Cr2(null)) were examined in the present study. Along with CD19-deficient (CD19(null)) mice, these animals exhibit decreased survival following CLP compared with wild-type littermates. Injection of IgM before CLP does not change survival rates for Cr2(null) mice and only partially improves survival of CD19(null) mice, implicating CD21/CD35 and CD19 in mast cell activation. Interestingly, early TNF-alpha release is also impaired in Cr2(null) and CD19(null) animals, suggesting that these molecules directly affect mast cell activation. Cr2(null) and CD19(null) mice demonstrate an impairment in neutrophil recruitment and a corresponding increase in bacterial load. Examination of peritoneal mast cells by flow cytometry and confocal microscopy reveals the expression and colocalization of CD21/CD35 and CD19. Taken together, these findings suggest that the engagement of complement receptors CD21/CD35 along with CD19 on the mast cell surface by C3 fragments may be necessary for the full expression of mast cell activation in the CLP model.

CD21-mediated entry and stable infection by Epstein-Barr virus in canine and rat cells

We developed an adenovirus vector for transduction of the human CD21 gene (Adv-CD21), the Epstein-Barr virus (EBV)-specific receptor on human B lymphocytes, to overcome the initial barrier of EBV infection in nonprimate mammalian cells. Inoculation of Adv-CD21 followed by exposure to recombinant EBV carrying a selectable marker resulted in the successful entry of EBV into three of seven nonprimate mammalian cell lines as evidenced by expression of EBV-determined nuclear antigen (EBNA). The EBV-susceptible cell lines included rat glioma-derived 9L, rat mammary carcinoma-derived c-SST-2, and canine kidney-derived MDCK. Subsequent selection culture with G418 yielded drug-resistant cell clones. In these cell clones, EBV existed as an episomal form, as evidenced through the Gardella gel technique. Among the known EBV latency-associated gene products, EBV-encoded small RNAs, EBNA1 and transcripts from the BamHI-A rightward reading frame (BARF0), and latent membrane protein 2A were expressed in all EBV-infected cell clones. The viral lytic events could be induced in these cell clones by simultaneous treatment with 12-O-tetradecanoylphorbol-13-acetate and n-butyric acid, but they were abortive, and infectious virus was not produced. These results indicate that once the initial barrier for attachment is overcome artificially, EBV can establish a stable infection in some nonprimate mammalian cells, and they raise the possibility that transgenic animals with the human CD21 gene could provide an animal model for EBV infection.