Disruption of the Cr2 locus results in a reduction in B-1a cells and in an impaired B cell response to T-dependent antigen

Cutting edge commentary: origins of B-1 cells

The origin of B-1a cells, a minority population of B cells that express CD5, are abundant in coelomic cavities, and often produce autoantibodies, has been the subject of study for many years. Accumulating evidence demonstrates that the hypothesis that only B cells arising in fetal or neonatal tissues have the potential to become B-1a cells cannot be true. Rather, B cell receptor-mediated signaling initiated by ligation of autoantigen has now been shown to be required for induction of the B-1a phenotype. Furthermore, cells with a functional B-1a phenotype can be induced from adult precursors by appropriate Ag. At the same time, microenvironment-specific events may determine the likelihood that a given B cell, either adult or fetal derived, enters this pathway. CD5 expression and possibly localization to the peritoneum appear to provide some protection to autoreactive cells otherwise slated for elimination.

The CD19/CD21 complex functions to prolong B cell antigen receptor signaling from lipid rafts

The CD19/CD21 complex functions to significantly enhance B cell antigen receptor (BCR) signaling in response to complement-tagged antigens. Recent studies showed that following antigen binding the BCR translocates into plasma membrane lipid rafts that serve as platforms for BCR signaling. Here, we show that the binding of complement-tagged antigens stimulates the translocation of both the BCR and the CD19/CD21 complex into lipid rafts, resulting in prolonged residency in and signaling from the rafts, as compared to BCR cross-linking alone. When coligated to the BCR, the CD19/CD21 complex retards the internalization and degradation of the BCR. The colocalization and stabilization of the BCR and the CD19/CD21 complex in plasma membrane lipid rafts represents a novel mechanism by which a coreceptor enhances BCR signaling.

Neutralizing antiviral antibody responses

Neutralizing antibodies are evolutionarily important effectors of immunity against viruses. Their evaluation has revealed a number of basic insights into specificity, rules of reactivity (tolerance), and memory—namely, (1) Specificity of neutralizing antibodies is defined by their capacity to distinguish between virus serotypes; (2) B cell reactivity is determined by antigen structure, concentration, and time of availability in secondary lymphoid organs; and (3) B cell memory is provided by elevated protective antibody titers in serum that are depending on antigen stimulation. These perhaps slightly overstated rules are simple, correlate with in vivo evidence as well as clinical observations, and appear to largely demystify many speculations about antibodies and B cell physiology. The chapter also considers successful vaccines and compares them with those infectious diseases where efficient protective vaccines are lacking, it is striking to note that all successful vaccines induce high levels of neutralizing antibodies (nAbs) that are both necessary and sufficient to protect the host from disease. Successful vaccination against infectious diseases such as tuberculosis, leprosy, or HIV would require induction of additional long-lasting T cell responses to control infection.

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.

Role of natural and immune IgM antibodies in immune responses

IgM antibodies constitute the major component of the natural antibodies and is also the first class of antibodies produced during a primary antibody response. The IgM-type antibodies differ from other classes of antibodies in that they are predominantly produced by B1 cells, in the absence of apparent stimulation by specific antigens. In addition, IgM antibodies are mostly encoded by germline V gene segments and have low affinities but broad specificites to both foreign and self structures. New developments regarding the function of both immune IgM antibodies and natural IgM antibodies will be examined here.

Complement C4 inhibits systemic autoimmunity through a mechanism independent of complement receptors CR1 and CR2

The complement system enhances antibody responses to T-dependent antigens, but paradoxically, deficiencies in C1 and C4 are strongly linked to autoantibody production in humans. In mice, disruption of the C1qa gene also results in spontaneous autoimmunity. Moreover, deficiencies in C4 or complement receptors 1 and 2 (CR1/CR2) lead to reduced selection against autoreactive B cells and impaired humoral responses. These observations suggest that C1 and C4 act through CR1/CR2 to enhance humoral immunity and somehow suppress autoimmunity. Here we report high titers of spontaneous antinuclear antibody (ANA) in C4(-/)- mice. This systemic lupus erythematosus-like autoimmunity is highly penetrant; by 10 mo of age, all C4(-)(/)- females and most males produced ANA. In contrast, titers and frequencies of ANA in Cr2(-)(/)- mice, which are deficient in CR1 and CR2, never rose significantly above those in normal controls. Glomerular deposition of immune complexes (ICs), glomerulonephritis, and splenomegaly were observed in C4(-)(/)- but not Cr2(-)(/)- mice. C4(-)(/)-, but not Cr2(-)(/)-, mice accumulate activated T and B cells. Clearance of circulating ICs is impaired in preautoimmune C4(-)(/)-, but not Cr2(-)(/)-, mice. C4 deficiency causes spontaneous, lupus-like autoimmunity through a mechanism that is independent of CR1/CR2.

Antibody regulation of B cell development

Antibodies on the surface of B lymphocytes trigger adaptive immune responses and control a series of antigen-independent checkpoints during B cell development. These physiologic processes are regulated by a complex of membrane immunoglobulin and two signal transducing proteins known as Ig alpha and Ig beta. Here we focus on the role of antibodies in governing the maturation of B cells from early antigen-independent through the final antigen-dependent stages.

Reduced expression of the complement receptor type 2 (CR2, CD21) by synovial fluid B and T lymphocytes

The expression of CR2 (CD21) by synovial B and T lymphocytes of patients suffering from various forms of arthritis was analysed with cytofluorometry and with reverse transcriptase-polymerase chain reaction. CR2 (CD21) cell surface protein was detected in normal quantities on peripheral B cells, but was almost absent on synovial B lymphocytes of the same patients. This reduction was most severe in patients with rheumatoid arthritis, but also observed in all other cases. CR2 (CD21) did not reappear after in vitro culture. CR2 (CD21) mRNA was also strongly reduced in synovial B and T lymphocytes. Synovial fluid B lymphocytes were larger than peripheral blood B lymphocytes, while T cells from the same patients showed no size differences. We conclude that synovial fluid B lymphocytes have undergone an irreversible step towards terminal differentiation. The presence or absence of CR2 (CD21) mRNA in peripheral versus synovial T cells indicates that CR2 (CD21) is also differentially expressed by T lymphocytes.

Germinal center initiation, variable gene region hypermutation, and mutant B cell selection without detectable immune complexes on follicular dendritic cells

Serum antibody (Ab) can play several roles during B cell immune responses. Among these is to promote the deposition of immune complexes (ICs) on follicular dendritic cells (FDCs). ICs on FDCs are generally thought to be critical for normal germinal center (GC) formation and the development and selection of memory B cells. However, it has been very difficult to test these ideas. To determine directly whether FDC-bound complexes do indeed function in these roles, we have developed a transgenic (Tg) mouse in which all B lymphocytes produce only the membrane-bound form of immunoglobulin M. Immune Tg mice have 10,000-fold less specific Ab than wild-type mice and lack detectable ICs on FDCs. Nonetheless, primary immune responses and the GC reaction in these mice are robust, suggesting that ICs on FDCs do not play critical roles in immune response initiation and GC formation. Moreover, as indicated by the presence and pattern of somatic mutations, memory cell formation and selection appear normal in these IC-deficient GCs.

Enhanced immunogenicity of aldehyde-bearing antigens: a possible link between innate and adaptive immunity

Innate immunity directs the adaptive immune response by identifying antigens that are associated with infectious agents. Although some microbial antigens can be recognized by innate immune receptors, most cannot, and these require identification by some other means. The introduction of aldehydes into antigens by glycolaldehyde, which can be produced by activated neutrophils reacting with serine, or by the oxidation of an N-linked oligosaccharide with NaIO4, enhances by several orders of magnitude their immunogenicity in mice. The augmented immunogenicity requires the presence of an aldehyde on the antigen, and is not dependent on protein aggregation. An in vitro correlate of augmented immunogenicity is the enhanced presentation of glycolaldehyde-modified antigen to T cells by macrophages and bone marrow-derived dendritic cells. The potential clinical importance of this form of antigen modification is twofold: glycolaldehyde renders a model self antigen immunogenic, and it converts a relatively non-immunogenic malaria antigen, merozoite surface protein-1, into an effective immunogen. Thus, the tagging of antigens by the addition of aldehydes, which may be an innate immune mechanism to facilitate their recognition by the adaptive immune system, may have a role in the genesis of autoimmunity and the development of vaccines.

Structure-guided identification of C3d residues essential for its binding to complement receptor 2 (CD21)

A vital role for complement in adaptive humoral immunity is now beyond dispute. The crucial interaction is that between B cell and follicular dendritic cell-resident complement receptor 2 (CR2, CD21) and its Ag-associated ligands iC3b and C3dg, where the latter have been deposited as a result of classical pathway activation. Despite the obvious importance of this interaction, the location of a CR2 binding site within C3d, a proteolytic limit fragment of C3dg retaining CR2 binding activity, has not been firmly established. The recently determined x-ray structure of human C3d suggested a candidate site that was remote from the site of covalent attachment to Ag and consisted of an acidic residue-lined depression, which accordingly displays a significant electronegative surface potential. These attributes were consistent with the known ionic strength dependence of the CR2-C3d interaction and with the fact that a significant electropositive surface was apparent in a modeled structure of the C3d-binding domains of CR2. Therefore, we have performed an alanine scan of all of the residues within and immediately adjacent to the acidic pocket in C3d. By testing the mutant iC3b molecules for their ability to bind CR2, we have identified two separate clusters of residues on opposite sides of the acidic pocket, specifically E37/E39 and E160/D163/I164/E166, as being important CR2-contacting residues in C3d. Within the second cluster even single mutations cause near total loss of CR2 binding activity. Consistent with the proposed oppositely charged nature of the interface, we have also found that removal of a positive charge immediately adjacent to the acidic pocket (mutant K162A) results in a 2-fold enhancement in CR2 binding activity.

Impaired affinity maturation in Cr2-/- mice is rescued by adjuvants without improvement in germinal center development

Cr2-/- mice have an impairment in humoral immunity, as shown by the decrease in the Ab titers against T cell-dependent Ags and abnormalities in germinal center formation. Germinal centers are present, but they are decreased in size and number, indicating problems in their development. In this study, we investigated whether this abnormality in germinal center development is associated with problems in the establishment of optimal affinity maturation and the generation of memory B cells, processes closely related to the germinal center reaction. We immunized the Cr2-/- animals with different Ags with or without adjuvants. We showed that, when immunized without adjuvants, complement receptors are absolutely required for optimal affinity maturation. Although limited affinity maturation is elicited in the Cr2-/- Ab response, it is decreased as compared with normal animals. Memory B cell generation is also impaired. In the presence of adjuvants, germinal center development in the Cr2-/- mice is still abnormal, as demonstrated by their decreased size and number. Surprisingly, adjuvants establish optimal affinity maturation and partially restore the amount of Ab produced during the primary response and memory B cell generation. However, adjuvants cannot improve the ability of follicular dendritic cells to retain Ags in the form of immune complexes. These observations indicate that immunization with inflammatory Ags offset some of the immunological abnormalities found in the Cr2-/- mice and show that optimal affinity maturation in the Cr2-/- mice can be achieved in the absence of normal germinal centers.

Antibody production in mice deficient for complement receptors 1 and 2 can be induced by IgG/Ag and IgE/Ag, but not IgM/Ag complexes

Deficiencies in C factors C2, C3, or C4 as well as lack of C receptors 1 and 2 (CR1/2) lead to impaired Ab production. Classical pathway activation plays a major role, as mice deficient in factor B, a key factor in the alternative pathway, have normal Ab production. Abs in complex with their specific Ag are known to feedback regulate the Ab response, and enhanced responses are initiated by IgM, IgE, and IgG. IgM acts via the C system, whereas IgE and IgG can operate independently of C via Fc receptors. Here we have investigated whether these isotypes are able to enhance Ab responses in mice lacking CR1/2. SRBC-specific IgM, administered with SRBC, does not enhance Ab responses in these animals. In contrast, 2,4, 6-trinitrophenyl-specific IgE and IgG2a, administered with BSA-2,4, 6-trinitrophenyl, induce potent Ab responses in CR1/2-deficient mice. Additionally, BSA administered with CFA or alum induced strong Ab responses in the absence of CR1/2. These results indicate that CR1/2 is needed to promote IgM-mediated induction of primary Ab responses. The data also show that the need for CR1/2 can be circumvented by Abs typical of a secondary immune response forming complexes with Ag or by conventional adjuvants, presumably mimicking physiological inflammatory reactions.

Expression of human complement receptor 2 (CR2, CD21) in Cr2-/- mice restores humoral immune function

Complement receptor type 2 (CR2, CD21) is expressed by both human and murine B cells and has been demonstrated to play a pivotal role in the humoral immune response. We have reconstituted Cr2-/- mice with an 80-kb human genomic fragment (designated P1-5) containing the full-length human CR2 (hCR2) gene. Transfection of P1-5 into the mouse A20 B cell line confirmed that it would direct expression of the hCR2 protein in mouse B cells. Immunoprecipitation analysis in these cells revealed that hCR2 coassociates with mouse CD19. After creation of transgenic mice using P1-5, we found significant expression of hCR2 on peripheral blood and splenic B cells by flow cytometric analysis. RT-PCR analysis of tissues and purified cell populations from transgene-positive mice revealed that hCR2 expression was restricted to B cells and the spleen in a pattern that matches mouse CR2. To rigorously assess the functional capabilities of hCR2, the transgene was bred onto Cr2-/- mice, which have a notable defect in response to SRBC Ag. We found that Cr2-/- mice expressing hCR2 had a substantial restoration of the humoral immune response to SRBC as compared with nontransgenic Cr2-/- littermate controls. Overall, this study suggests that hCR2 is able to substitute for mouse CR2 in the murine immune system. Therefore, hCR2-transgenic mice offer a valuable model system to further examine immunologic roles as well as structure-function relationships important for hCR2 function in primary cells in vivo.

C3d enhancement of antibodies to hemagglutinin accelerates protection against influenza virus challenge

The ability of the C3d component of complement to enhance antibody responses and protective immunity to influenza virus challenges was evaluated using a DNA vaccine encoding a C3d fusion of the hemagglutinin (HA) from influenza virus. Plasmids were generated that encoded a transmembrane HA (tmHA), a secreted form of HA (sHA), or a sHA fused to three tandem copies of the murine homologue of the C3d (sHA-3C3d). Analysis of the titers, avidity maturation, and hemagglutinin-inhibition activity of raised antibody revealed that immunizations with sHA-3C3d DNA accelerated both the avidity maturation of antibody to HA and the appearance of hemagglutinin-inhibition activity. These accelerated antibody responses correlated to a more rapid appearance of protective immunity. They also correlated to complete protection from live virus challenge by a single vaccination at a dose ten times lower than the protective dose for non-C3d forms of HA.

Phenotypes of complement knockouts

Although complete and partial complement deficiencies are well described in humans and several spontaneous animal models, many questions have remained regarding the exact role that these deficiency states play in the observed clinical manifestations. Likewise, many important mechanistic questions cannot be addressed using patients or spontaneously arising animal models of deficiency states. To provide additional insights and create readily manipulable experimental systems, over the last 5 years mice have been created by several groups in which specifically targeted insertional mutagenesis has resulted in complete deficiencies of complement activation proteins, receptors or regulatory proteins. Many surprising findings have already been made using mice derived from these strategies, and clinically relevant studies have begun to provide great insights into human deficiency states. This review includes an overview of these complement deficient mice and highlights some of the important findings that have resulted from their creation. A discussion of future experimental directions thought to be important by this author then follows and concludes the review.

Genome-wide, large-scale production of mutant mice by ENU mutagenesis

In the post-genome era, the mouse will have a major role as a model system for functional genome analysis. This requires a large number of mutants similar to the collections available from other model organisms such as Drosophila melanogaster and Caenorhabditis elegans. Here we report on a systematic, genome-wide, mutagenesis screen in mice. As part of the German Human Genome Project, we have undertaken a large-scale ENU-mutagenesis screen for dominant mutations and a limited screen for recessive mutations. In screening over 14,000 mice for a large number of clinically relevant parameters, we recovered 182 mouse mutants for a variety of phenotypes. In addition, 247 variant mouse mutants are currently in genetic confirmation testing and will result in additional new mutant lines. This mutagenesis screen, along with the screen described in the accompanying paper, leads to a significant increase in the number of mouse models available to the scientific community. Our mutant lines are freely accessible to non-commercial users (for information, see http://www.gsf.de/ieg/groups/enu-mouse.html).

Immunomodulatory functions of murine CR1/2

C3-fragments generated upon complement activation play an important role in the formation and regulation of immune responses. Receptors interacting with various activation fragments of this versatile complement component are expressed on a wide variety of cell types, such as lymphocytes, macrophages, dendritic cells, follicular dendritic cells, granulocytes, erythrocytes, consequently C3-products may influence several biological functions at different sites of the body, where complement activation takes place. In the last decade, genes, protein structure and functions played by murine complement receptors CR1 and CR2 (mCR1/2) have been deciphered. In this review, we wish to relate these properties, and fit it into the context of events following in vivo complement activation. We separately address the roles played by murine mCR1/2 as BCR coreceptor and as BCR independent structure, and propose a mchanism for the utilization of antigen-C3d conjugates bound on B cells. Finally, we raise some of the questions that remain to be elucidated in order to get a more precise picture of the functions of mCR1/2.

Functional domains, structural variations and pathogen interactions of MCP, DAF and CR1

The Regulators of Complement Activation (RCA) are a fascinating group of proteins that play important roles in innate and acquired immunity. In this review, we examine structure-function aspects of three membrane-bound RCA proteins and discuss the unique impact of their genetic organization on their evolution.

Regulation of antibody responses via antibodies, complement, and Fc receptors

Antibodies can completely suppress or enhance the antibody response to their specific antigen by several hundredfold. Immunoglobulin M (IgM) enhances antibody responses via the complement system, and complement activation by IgM probably starts the chain of events leading to antibody responses to suboptimal antigen doses. IgG can enhance primary antibody responses in the absence of the complement system and seems to be dependent on Fc receptors for IgG (FcgammaRs). IgE enhances antibody responses via the low-affinity receptor for IgE (FcepsilonRII/CD23). The precise effector mechanisms that cause enhancement are not known, but direct B-cell signaling, antigen presentation, and increased follicular localization are all possibilities. IgG, IgE, and IgM may also suppress antibody responses when used in certain immunization regimes, and it seems reasonable that an important mechanism behind suppression is the masking of antigenic epitopes by antibodies. In addition, FcgammaRIIB, which contains a cytoplasmic inhibitory motif, acts as a negative regulator of antibody responses. This receptor, however, may prevent the antibody responses from exceeding a certain level rather than causing complete suppression.

Regulation of B lymphocyte responses to foreign and self-antigens by the CD19/CD21 complex

The membrane protein complex CD19/CD21 couples the innate immune recognition of microbial antigens by the complement system to the activation of B cells. CD21 binds the C3d fragment of activated C3 that becomes covalently attached to targets of complement activation, and CD19 co-stimulates signaling through the antigen receptor, membrane immunoglobulin. CD21 is also expressed by follicular dendritic cells and mediates the long-term retention of antigen that is required for the maintenance of memory B cells. Understanding of the biology of this receptor complex has been enriched by analyses of genetically modified mice; these analyses have uncovered roles not only in positive responses to foreign antigens, but also in the development of tolerance to self-antigens. Studies of signal transduction have begun to determine the basis for the coreceptor activities of CD19. The integration of innate and adaptive immune recognition at this molecular site on the B cell guides the appropriate selection of antigen by adaptive immunity and emphasizes the importance of this coreceptor complex.

Fc gamma receptor IIB on follicular dendritic cells regulates the B cell recall response

Generation of the B cell recall response appears to involve interaction of Ag, in the form of an immune complex (IC) trapped on follicular dendritic cells (FDCs), with germinal center (GC) B cells. Thus, the expression of receptors on FDC and B cells that interact with ICs could be critical to the induction of an optimal recall response. FDCs in GCs, but not in primary follicles, express high levels of the IgG Fc receptor Fc gamma RIIB. This regulated expression of Fc gamma RIIB on FDC and its relation to recall Ab responses were examined both in vitro and in vivo. Trapping of IC in spleen and lymph nodes of Fc gamma RII-/- mice was significantly reduced compared with that in wild-type controls. Addition of ICs to cultures of Ag-specific T and B cells elicited pronounced Ab responses only in the presence of FDCs. However, FDCs derived from Fc gamma RIIB-/- mice supported only low level Ab production in this situation. Similarly, when Fc gamma RIIB-/- mice were transplanted with wild-type Ag-specific T and B cells and challenged with specific Ag, the recall responses were significantly depressed compared with those of controls with wild-type FDC. These results substantiate the hypothesis that FcgammaRIIB expression on FDCs in GCs is important for FDCs to retain ICs and to mediate the conversion of ICs to a highly immunogenic form and for the generation of strong recall responses.

Humoral immune responses in Cr2-/- mice: enhanced affinity maturation but impaired antibody persistence

Deficiency in CD21/CD35 by disruption of the Cr2 loci leads to impaired humoral immune responses. In this study, we detail the role of CD21/CD35 on Ab responses to the hapten (4-hydroxy-3-nitrophenyl)acetyl conjugated to chicken gamma-globulin. Surprisingly, Cr2-/- mice generate significant Ab responses and germinal center (GC) reactions to low doses of this Ag in alum, although the magnitude of their responses is much reduced in comparison with those of Cr2+/- and C57BL/6 controls. Increasing Ag dose partially corrected this deficit. In situ study of the somatic genetics of GC B cells demonstrated that VDJ hypermutation does not require CD21/CD35, and Cr2-/- mice exhibited enhanced affinity maturation of serum Ab in the post-GC phase of the primary response. On the other hand, Cr2-/- mice displayed accelerated loss of serum Ab and long-lived Ab-forming cells. These observations suggest that B cell activation/survival signals mediated by CD21 and/or the retention of Ag by CD21/CD35 play important roles in the generation, quality, and maintenance of serum Ab.

A VH11V kappa 9 B cell antigen receptor drives generation of CD5+ B cells both in vivo and in vitro

B lymphocytes can be divided into different subpopulations, some with distinctive activation requirements and probably mediating specialized functions, based on surface phenotype and/or anatomical location, but the origins of most of these populations remain poorly understood. B cells constrained by transgenesis to produce an Ag receptor derived from a conventional (B-2) type cell develop a B-2 phenotype, whereas cells from mice carrying a B-1-derived receptor acquire the B-1 phenotype. In this study transgenic enforced expression of a B cell receptor (mu/kappa) originally isolated from a CD5+ (B-1a) B cell generates B-1 phenotype cells in bone marrow cultures that show a distinctive B-1 function, survival in culture. Despite their autoreactivity, we find no evidence for receptor editing or that the paucity of B-2 cells is the result of tolerance-induced selection. Finally, Ca2+ mobilization studies reveal a difference between transgenic B-1 cells in spleen and peritoneal cavity, with cells in spleen much more responsive to anti-B cell receptor cross-linking. We discuss these results in terms of specificity vs lineage models for generation of distinctive B cell subpopulations.

Crry/p65, a membrane complement regulatory protein, has costimulatory properties on mouse T cells

It is known that certain type I membrane molecules (complement receptors type 1 and 2) belonging to the regulators of complement activation (RCA) family are involved in the regulation of B lymphocyte activation. In contrast, only GPI-anchored RCA molecules (CD55) have been described to be involved in T lymphocyte activation. In this study, we describe a novel function for the mouse RCA type I membrane protein Crry/p65 as a costimulatory molecule in CD4+ T cell activation. This is shown by increased anti-CD3-induced proliferation of CD4+ spleen T lymphocytes in the presence of the Crry/p65-specific mAb P3D2. Furthermore, Ab-induced coligation of Crry/p65 and CD3 favors IL-4 rather than IFN-gamma secretion in these cells. Crry/p65 signaling was also observed regardless of additional Ca2+, protein kinase C, or CD28-mediated costimuli. Analysis of intracellular intermediaries shows that Crry/p65-CD3 coligation enhances certain TCR/CD3-mediated signals, producing increased early tyrosine phosphorylation of many substrates and enhanced activation of the mitogen-activated protein kinase, extracellular signal-related kinase. These data fit well with the association of Crry/p65 with the tyrosine kinase Lck found in T cell lysates. The epitope recognized by the mAb P3D2 interferes with the protective role of Crry/p65 on C3 deposition. The relationship between protective function and costimulation by Crry/p65 is discussed. Our results support a multifunctional role for Crry/p65 in T cells and suggest new links between the natural and adaptive immune responses.

The role of complement in the acquired immune response

Studies over the past three decades have clearly established a central role for complement in the promotion of a humoral immune response. The primary function of complement, in this regard, is to opsonize antigen or immune complexes for uptake by complement receptor type 2 (CR2, CD21) expressed on B cells, follicular dendritic cells (FDC) and some T cells. A variety of mechanisms appear to be involved in complement-mediated promotion of the humoral response. These include: enhancement of antigen (Ag) uptake and processing by both Ag-specific and non-specific B cells for presentation to specific T cells; the activation of a CD21/CD19 complex-mediated signalling pathway in B cells, which provides a stimulus synergistic to that induced by antigen interaction with the B-cell receptor (BCR); and promotion of the interaction between B cells and FDC, where C3d-bearing immune complexes participate in intercellular bridging. Finally, current studies suggest that CR2 may also play a role in the determination of B-cell tolerance towards self-antigens and thereby hold the key to the previously observed correlation between deficiencies of the early complement components and autoimmune disease.

Serum regulates the expression of complement receptor 2 on human B cell lines

Complement receptor 2 (CR2) participates in the regulation of B cell responses to antigen. In this study we report that treatment of IM-9 B lymphoblastoid cells or Raji Burkitt's lymphoma cells with 10% heat-inactivated fetal bovine serum for 24 hr increased both the CR2 mRNA level and CR2 surface protein expression more than two-fold. No change in the CR2 expression level was observed if cells were cultured in serum-free medium. The CD19 mRNA level decreased after 24 hr independently of the presence of serum. The serum-stimulated increase in CR2 expression was not due to changes in the proliferative capacity of the cells and could not be mimicked by various cytokines. However, IFN-gamma as well as OKB7, a CR2-specific monoclonal antibody, blocked the serum-induced increase in CR2 expression at the mRNA level. Our data show for the first time that factors in serum induce the expression of the CR2 gene and that signals initiated by IFN-gamma and OKB7 interfere with the serum-induced changes. Because stimuli that alter CR2 expression can influence the extent of the B cell response to antigen-C3d complexes, serum factors may play a role in regulating the responsiveness of B cells.

A protective role for innate immunity in autoimmune disease

Systemic lupus erythematosus is an incurable autoimmune disease characterized by autoantibodies directed against highly conserved nuclear proteins and DNA. While the cause of lupus is not known, B-lymphocytes are essential for disease. Most striking is that deficiency in components of the innate immune system such as complement is a major risk factor in disease. One explanation for the involvement of complement and innate immunity is that they are important in the induction and maintenance of B cell tolerance to lupus antigens.

Identification of a precursor to phosphatidyl choline-specific B-1 cells suggesting that B-1 cells differentiate from splenic conventional B cells in vivo: cyclosporin A blocks differentiation to B-1

The origin of B-1 cells is controversial. The initial paradigm posited that B-1 and B-2 cells derive from separate lineages. More recently it has been argued that B-1 cells derive from conventional B cells as a result of T-independent Ag activation. To understand B-1 cell differentiation, we have generated Ig transgenic (Tg) mice using the H and L chain genes (VH12 and Vkappa4) of anti-phosphatidyl choline (anti-PtC) B cells. In normal mice anti-PtC B cells segregate to B-1. Segregation is intact in VH12 (6-1) and VH12/Vkappa4 (double) Tg mice that develop large numbers of PtC-specific B cells. However, if B-1 cell differentiation is blocked, anti-PtC B cells in these Tg mice are B-2-like in phenotype, suggesting the existence of an Ag-driven differentiative pathway from B-2 to B-1. In this study, we show that double Tg mice have a population of anti-PtC B cells that have the phenotypic characteristics of both B-2 and B-1 cells and that have the potential to differentiate to B-1 (B-1a and B-1b). Cyclosporin A blocks this differentiation and induces a more B-2-like phenotype in these cells. These findings indicate that these cells are intermediate between B-2 and B-1, further evidence of a B-2 to B-1 differentiative pathway.

Accelerated development of IgG autoantibodies and autoimmune disease in the absence of secreted IgM

Individuals with systemic lupus erythematosus and rheumatoid arthritis are characterized by the presence of high levels of circulating IgM and IgG autoantibodies. Although IgG autoantibodies often are pathogenic, the role of IgM autoantibodies in autoimmune disease is not clear. Using mice that are unable to secrete IgM but are able to express surface IgM and IgD and to secrete other classes of immunoglobulins, we examined the effect of the absence of secreted IgM in the development of IgG autoantibodies and autoimmune disease in lupus-prone lymphoproliferative (lpr) mice. Compared with regular lpr mice, lpr mice that lack secreted IgM developed elevated levels of IgG autoantibodies to double-stranded DNA and histones and had more abundant deposits of immune complexes in the glomeruli; they also suffered more severe glomerulonephritis and succumbed to the disease at an earlier age. Similarly, the absence of secreted IgM also resulted in an accelerated development of IgG autoantibodies in normal mice. These findings suggest that secreted IgM, including IgM autoantibodies produced naturally or as part of an autoimmune response, may lessen the severity of autoimmune pathology associated with IgG autoantibodies.

The role of complement in B cell activation and tolerance

It is becoming well accepted that innate immunity serves as a natural adjuvant in enhancing and directing the adaptive immune response. In this review, I have discussed how the complement system, a major mediator of innate immunity, links the two systems. The recent availability of knockout mice bearing selective deficiencies in the critical complement proteins and receptors has allowed formal demonstration of the importance of complement in enhancement of humoral immunity. Characterization of the mice has also uncovered mechanisms for maintaining survival of activated B cells within the lymphoid compartment. For example, co-ligation of the CD21/CD19/Tapa-1 receptor with the BCR not only reduces the threshold for B cell follicular survival but provides a unique signal for survival in the germinal centers. In addition complement receptors are critical for localization of antigen and C3d ligand to FDCs for maintenance of long-term B cell memory. A surprise that has come from analysis of the deficient mice is that complement is also important in negative selection of B lymphocytes. This observation provides new insight to a long-standing enigma that the major predisposing factor in lupus is deficiency in complement C1q or C4. The seeming contradiction of dual role for complement in both B cell activation and tolerance is reconciled by the hypothesis that natural IgM provides a mechanism to selectively identify self-antigens that are highly conserved and cross-react with microbial ones such as DNA and nuclear proteins. Thus, the importance of complement in tolerance to self-antigens is restricted to those self-antigens that are evolutionary conserved, and they are identified by natural antibody. The future should hold further surprises as to the intricate interactions between the complement system and acquired immunity.

Induction of a germinal center phenotype in B cells in vitro by a Th2 cell line

We have investigated the contribution of various stimuli for generating in vitro the changes in surface phenotype characteristic of B cells responding to a T-dependent antigen in a germinal center (GC). We show that, unlike many other stimuli such as B cell mitogens, cytokines, and surrogate antigen, alone or in combination, an alloreactive Th2 clonal line induces splenic B cells to become cell surface peanut agglutinin (PNA)(hi), Ig(lo), CD62L(lo), and CD44(hi) to produce mRNA for M17 and to express a GC-specific transgene even without B cell receptor ligation. Neither proliferation nor prior activation of responding B cells is needed, but B cells from CD45-null mice show reduced efficiency of this induction. These findings open up possibilities for separation and dissection of the various components of the GC response.

Decay accelerating activity of complement receptor type 1 (CD35). Two active sites are required for dissociating C5 convertases

The goal of this study was to identify the site(s) in CR1 that mediate the dissociation of the C3 and C5 convertases. To that end, truncated derivatives of CR1 whose extracellular part is composed of 30 tandem repeating modules, termed complement control protein repeats (CCPs), were generated. Site 1 (CCPs 1-3) alone mediated the decay acceleration of the classical and alternative pathway C3 convertases. Site 2 (CCPs 8-10 or the nearly identical CCPs 15-17) had one-fifth the activity of site 1. In contrast, for the C5 convertase, site 1 had only 0.5% of the decay accelerating activity, while site 2 had no detectable activity. Efficient C5 decay accelerating activity was detected in recombinants that carried both site 1 and site 2. The activity was reduced if the intervening repeats between site 1 and site 2 were deleted. The results indicate that, for the C5 convertases, decay accelerating activity is mediated primarily by site 1. A properly spaced site 2 has an important auxiliary role, which may involve its C3b binding capacity. Moreover, using homologous substitution mutagenesis, residues important in site 1 for dissociating activity were identified. Based on these results, we generated proteins one-fourth the size of CR1 but with enhanced decay accelerating activity for the C3 convertases.

Humoral response to herpes simplex virus is complement-dependent

The complement system represents a cascade of serum proteins, which provide a major effector function in innate immunity. Recent studies have revealed that complement links innate and adaptive immunity via complement receptors CD21/CD35 in that it enhances the B cell memory response to noninfectious protein antigens introduced i.v. To examine the importance of complement for immune responses to virus infection in a peripheral tissue, we compared the B cell memory response of mice deficient in complement C3, C4, or CD21/CD35 with wild-type controls. We found that the deficient mice failed to generate a normal memory response, which is characterized by a reduction in IgG antibody and germinal centers. Thus, complement is important not only in the effector function of innate immunity but also in the stimulation of memory B cell responses to viral-infected cell antigens in both blood and peripheral tissues.

Protective T cell-independent antiviral antibody responses are dependent on complement

Complement is part of the innate immune system and one of the first lines of host defense against infections. Its importance was evaluated in this study in virus infections in mice deficient either in soluble complement factors (C3(-/-), C4(-/-)) or in the complement signaling complex (complement receptor [CR]2(-/-), CD19(-/-)). The induction of the initial T cell-independent neutralizing immunoglobulin (Ig)M antibody response to vesicular stomatitis virus (VSV), poliomyelitis virus, and recombinant vaccinia virus depended on efficient antigen trapping by CR3 and -4-expressing macrophages of the splenic marginal zone. Neutralizing IgM and IgG antibody responses were largely independent of CR2-mediated stimulation of B cells when mice were infected with live virus. In contrast, immunizations with nonreplicating antigens revealed an important role of B cell stimulation via CR2 in the switch to IgG. The complement cascade was activated after infection with VSV via the classical pathway, and active complement cleavage products augmented the effector function of neutralizing IgM and IgG antibodies to VSV by a factor of 10-100. Absence of the early neutralizing antibody responses, together with the reduced efficiency of neutralizing IgM in C3(-/-) mice, led to a drastically enhanced susceptibility to disease after infection with VSV.

CD72-deficient mice reveal nonredundant roles of CD72 in B cell development and activation

CD72, a B cell surface protein of the C-type lectin superfamily, recruits the tyrosine phosphatase SHP-1 through its ITIM motif(s). Using CD72-deficient (CD72-/-) mice, we demonstrate that CD72 is a nonredundant regulator of B cell development. In the bone marrow of CD72-/- mice, there was a reduction in the number of mature recirculating B cells and an accumulation of pre-B cells. In the periphery of CD72-/- mice, there were fewer mature B-2 cells and more B-1 cells. In addition, CD72 is a negative regulator of B cell activation, as CD72-/- B cells were hyperproliferative in response to various stimuli and showed enhanced kinetics in their intracellular Ca2+ response following IgM cross-linking.

Co-operation between human CR1 (CD35) and CR2 (CD21) in internalization of their C3b and iC3b ligands by murine-transfected fibroblasts

CR1 and CR2 are expressed as associated proteins on the B-lymphocyte surface. To investigate their respective contributions to the internalization of C3 fragments, transfected murine fibroblasts expressing human CR1, CR2, or both CR1 and CR2 were produced. CR1- and CR1-CR2-expressing cells bound C3b and C3b-dimer whereas CR2- and CR1-CR2-expressing cells bound iC3b and C3de. In all cases, maximum binding was achieved at low ionic strength. CR1-CR2-positive cells internalized two- to threefold more C3b and 1.5-fold more iC3b than CR1- and CR2-single-positive cells, respectively. Internalization of the anti-CR1 antibody J3D3, or C3de was at the same level, in both double-transfected and single-transfected cells. Furthermore, the internalization of C3b dimer by CR1-CR2 cells was impaired in the presence of OKB7, an anti-CR2-blocking antibody, but it was not altered in CR1 cells. Taken together, these findings suggest that CR1 and CR2 collaborate to internalize C3b and iC3b proteins. We suggest that the induction of conformational changes of the ligands enhances their binding to both receptors.

Interaction with complement receptor 1 (CD35) leads to amelioration of sepsis-triggered mortality but aggravation of arthritis during Staphylococcus aureus infection

The aim of this study was to assess the importance of complement receptor 1 (CR1, CD35) in Staphylococcus aureus arthritis and sepsis. The murine model of haematogenously acquired septic arthritis was used, injecting toxic shock syndrome toxin 1 (TSST-1)-producing S. aureus LS-1 intravenously. CR1 was blocked using immunoglobulin G (IgG) rat antimouse CR1 monoclonal antibody (MoAb) (8C12). Evaluation of arthritis was performed clinically and histopathologically. In addition, the effect of blocking CR1 was assessed on the phagocytic activity of leucocytes and on T-cell dependent and independent inflammation. Seven days after inoculation with bacteria, 96% of CR1 MoAb-treated mice had clinical symptoms of arthritis compared with 58% of the control animals (P < 0.01). The severity of arthritis, expressed as mean arthritic index, was 2.9 +/- 0.5 and 1.4 +/- 0.5, respectively (P = 0.004). Fifteen days after bacterial inoculation, all CR1 MoAb-treated mice had severe arthritis (mean arthritic index 6.3 +/- 0.6), while only 77% of controls were affected (mean arthritic index 2.9 +/- 0.6; P = 0.002). The potential explanation of these findings is that treatment with CR1 MoAb significantly increases the polymorphonuclear cell-dependent inflammatory response as a result of enhanced vasodilatation in treated animals. We conclude that treatment with CR1 MoAb leads to amelioration of sepsis-induced mortality during S. aureus infection, possibly as a result of the increased phagocytic activity of peripheral phagocytes.

Enhancement of lectin pathway haemolysis by immunoglobulins

We recently reported that indicator sheep erythrocytes (E) coated with mannan and sensitized with mannan-binding lectin (MBL) (E-M-MBL) are lysed by human serum in the absence of calcium via the lectin pathway of complement activation by a process which requires alternative pathway amplification and is associated with increased binding of and control by complement regulatory proteins C4 bp and factor H. In the present study, we investigated the effect of immunoglobulin (Ig) on this haemolysis. Co-sensitization of indicator E with anti-E haemolysin led to threefold enhancement of lectin pathway haemolysis in the absence of calcium, associated with increased binding of C3 and C5. Lysis was enhanced approximately twofold when E-M-MBL were chemically or immunologically coated with IgM or IgA, and fourfold when coated with IgG, prior to lysis in human serum-Mg-ethyleneglycol tetraacetic acid. The presence of haemolysin did not reduce the binding or inhibitory activity of C4 bp, and the enhancing activity of haemolysin was retained in serum depleted of C4 bp. By contrast, binding of factor H was greatly reduced in the presence of haemolysin, which had no enhancing effect in serum depleted of factor H. These experiments demonstrate the ability of IgG, IgM and IgA to enhance lectin pathway cytolysis, and that this enhancement occurs by neutralization of the inhibitory activity of factor H. Immunoglobulin enhancement of lectin pathway cytolysis represents another interaction between the innate and adaptive systems of immunity.

Role of complement receptors CD21/CD35 in B lymphocyte activation and survival

In summary, the complement system has evolved an important function in regulation of humoral immunity to T-dependent antigens. Covalent attachment of activated C3 to antigen alters its fate by enhancing uptake on the surface of FDC via CD21/CD35; and by enhancing signal transduction via the B cell coreceptor CD21/CD19/Tapa-1. In the absence of complement receptors CD21/CD35 or C3 ligand, naive B cells bearing low affinity BCR fail to effectively survive within the lymphoid follicle following contact with antigen and death is mediated by a Fas-dependent mechanism. Alternatively, B cells sufficiently activated to initiate a GC reaction fail to survive in the absence of CD21-CD21L interaction.

Humoral Response Suppression Observed with CD23 Transgenics

CD23, also known as the low affinity IgE receptor (FcεRII), has been hypothesized to have a role in IgE regulation. A new CD23 transgenic mouse was generated using the MHC class I promoter and IgH enhancer to further test the hypothesis that CD23 plays a role in the down-regulation of IgE. Study of three founder lines by FACS showed overexpression to varying extents on both B and T lymphocytes. No alterations in lymphocyte populations was observed. All three founder lines exhibited strong suppression of IgE in response to DNP-keyhole limpet hemocyanin/alum and Nippostrongylus brasiliensis infection compared with that in parental or littermate controls. The founder line exhibiting the highest level of suppression also was less susceptible to Ag-induced systemic anaphylactic shock. Overall, the data support the concept that enhancing CD23 levels can be used to suppress IgE-mediated disease. The mechanism involves decreased IgE synthesis, because the serum half-life of IgE was not altered in transgenics, and enzyme-linked immunospot analysis demonstrated lower IgE-producing cells stimulated by injection of anti-IgD. Transgenics also exhibited significantly decreased IgG1 responses and exhibited lower levels of all Ig isotypes, although this was more variable in different founder lines.