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Follicular dendritic cells (FDCs) are rare and enigmatic cells that mainly reside in germinal centers (GCs). They are capable of capturing immune complexes, via their Fc (FcRs) and complement receptors (CRs) and storing them for long periods in non-degradative vesicles. Presentation of ICs on FDCs to B cells is believed to drive affinity maturation. CR1 and CR2 are expressed on B cells and FDCs. Cr2 knock out (KO) mice, lacking both receptors, have impaired antibody and GC responses. Utilizing a novel ImageJ macro to analyze confocal fluorescence microscopy images of spleen sections, we here investigate how FDCs in wild type (WT) and Cr2 KO mice behave during the first two weeks after immunization with sheep red blood cells (SRBC). Mice were immunized with SRBC i.v. and spleen and serum samples harvested at various time points. As expected, antibody and GC responses in Cr2 KO mice were impaired in comparison to WT mice. Fewer FDCs were identified in Cr2 KO mice, and these exhibited differential localization and organization in comparison to WT mice. WT FDCs were primarily located within GCs at the light zone/dark zone border. FDCs from WT but not Cr2 KO mice were actively dispersed in GCs, i.e. tended to move away from each other, presumably to increase their surface area for B cell interaction. FDCs from Cr2 KO mice were more often found on follicles outside of the GCs and those within the GCs were closer to the periphery in comparison to WT FDCs. Expression of CR1 and CR2, FcγRIIB, and FcµR increased in FDCs from WT mice during the course of immunization. The results suggest that decreased ability to capture ICs by FDCs lacking CR1 and CR2 may not be the only explanation for the impaired GC and antibody responses in Cr2 KO mice. Poor FDC organization in GCs and failure to increase receptor expression after immunization may further contribute to the inefficient immune responses observed.

Immunoglobulin (Ig) M is the first antibody isotype produced during an immune response and is critical for host defense against infections. Recent studies have revealed that IgM also plays an important role in immune regulation and immunological tolerance. Mice lacking secretory IgM not only exhibit impaired production of antigen-specific IgG and are more susceptible to bacterial and viral infections, but also produce autoantibodies and are prone to develop autoimmune diseases. For many years, IgM has been thought to function predominantly by binding to antigen and activating complement (C') system. It is now clear that IgM can also elicit its function through the IgM Fc receptor (FcμR). In this chapter, we will review the role of FcμR in B cell development, maturation, survival and activation, antibody production, host defense against bacterial and viral infections, and B cell tolerance. We will also discuss the relative contribution of IgM-C' and IgM-FcμR pathways in humoral immune responses. Finally, we will discuss the possible involvement of FcμR in human chronic lymphocytic leukemia.

Antibodies forming a complex with antigen in vivo can dramatically change the antibody response to this antigen. In some situations, the response will be a 100-fold stronger than in animals immunized with antigen alone, and in other situations, the response will be completely suppressed. IgG is known to suppress the antibody response, for example to erythrocytes, and this is used clinically in Rhesus prophylaxis. The mechanism behind IgG-mediated immune suppression is still not understood. Here, we will review studies performed in experimental animal models and discuss the various hypotheses put forward to explain the profound suppressive effect of IgG. We conclude that an exclusive role for negative regulation of B cells through Fc gamma RIIB, increased clearance of erythrocytes from the circulation or complement-mediated lysis is unlikely. Epitope masking, where IgG hides the epitope from B cells, or trogocytosis, where IgG removes the epitope from the erythrocyte, is compatible with many observations. These two mechanisms are not mutually exclusive. Moreover, it cannot be ruled out that clearance, in combination with other mechanisms, plays a role.

Immunoglobulin (Ig) M is the first antibody isotype produced during an immune response and is critical for host defense against infections. Recent studies have revealed that IgM also plays an important role in immune regulation and immunological tolerance. Mice lacking secretory IgM not only exhibit impaired production of antigen-specific IgG and are more susceptible to bacterial and viral infections, but also produce autoantibodies and are prone to develop autoimmune diseases. For many years, IgM has been thought to function predominantly by binding to antigen and activating complement (C′) system. It is now clear that IgM can also elicit its function through the IgM Fc receptor (FcμR). In this chapter, we will review the role of FcμR in B cell development, maturation, survival and activation, antibody production, host defense against bacterial and viral infections, and B cell tolerance. We will also discuss the relative contribution of IgM-C′ and IgM-FcμR pathways in humoral immune responses. Finally, we will discuss the possible involvement of FcμR in human chronic lymphocytic leukemia.

This work elucidates a novel pain mechanism generated by rheumatoid arthritis–associated autoantibodies, uncoupled from inflammation, that is dependent on immune complex formation and activation of neuronal FcγRI., Rheumatoid arthritis–associated joint pain is frequently observed independent of disease activity, suggesting unidentified pain mechanisms. We demonstrate that antibodies binding to cartilage, specific for collagen type II (CII) or cartilage oligomeric matrix protein (COMP), elicit mechanical hypersensitivity in mice, uncoupled from visual, histological and molecular indications of inflammation. Cartilage antibody–induced pain-like behavior does not depend on complement activation or joint inflammation, but instead on tissue antigen recognition and local immune complex (IC) formation. smFISH and IHC suggest that neuronal Fcgr1 and Fcgr2b mRNA are transported to peripheral ends of primary afferents. CII-ICs directly activate cultured WT but not FcRγ chain–deficient DRG neurons. In line with this observation, CII-IC does not induce mechanical hypersensitivity in FcRγ chain–deficient mice. Furthermore, injection of CII antibodies does not generate pain-like behavior in FcRγ chain–deficient mice or mice lacking activating FcγRs in neurons. In summary, this study defines functional coupling between autoantibodies and pain transmission that may facilitate the development of new disease-relevant pain therapeutics.

Specific IgM, administered together with the antigen it recognizes, enhances primary antibody responses, formation of germinal centers, and priming for secondary antibody responses. The response to all epitopes on the antigen to which IgM binds is usually enhanced. IgM preferentially enhances responses to large antigens such as erythrocytes, malaria parasites, and keyhole limpet hemocyanine. In order for an effect to be seen, antigens must be administered in suboptimal concentrations and in close temporal relationship to the IgM. Enhancement is dependent on the ability of IgM to activate complement, but the lytic pathway is not required. Enhancement does not take place in mice lacking complement receptors 1 and 2 (CR1/2) suggesting that the role of IgM is to generate C3 split products, i.e., the ligands for CR1/2. In mice, these receptors are expressed on follicular dendritic cells (FDCs) and B cells. Optimal IgM-mediated enhancement requires that both cell types express CR1/2, but intermediate enhancement is seen when only FDCs express the receptors and low enhancement when only B cells express them. These observations imply that IgM-mediated enhancement works through several, non-mutually exclusive, pathways. Marginal zone B cells can transport IgM-antigen-complement complexes, bound to CR1/2, from the marginal zone and deposit them onto FDCs. In addition, co-crosslinking of the BCR and the CR2/CD19/CD81 co-receptor complex may enhance signaling to specific B cells, a mechanism likely to be involved in induction of early extrafollicular antibody responses.

Specific IgM, administered together with the antigen it recognizes, enhances primary antibody responses, formation of germinal centers, and priming for secondary antibody responses. The response to all epitopes on the antigen to which IgM binds is usually enhanced. IgM preferentially enhances responses to large antigens such as erythrocytes, malaria parasites, and keyhole limpet hemocyanine. In order for an effect to be seen, antigens must be administered in suboptimal concentrations and in close temporal relationship to the IgM. Enhancement is dependent on the ability of IgM to activate complement, but the lytic pathway is not required. Enhancement does not take place in mice lacking complement receptors 1 and 2 (CR1/2) suggesting that the role of IgM is to generate C3 split products, i.e., the ligands for CR1/2. In mice, these receptors are expressed on follicular dendritic cells (FDCs) and B cells. Optimal IgM-mediated enhancement requires that both cell types express CR1/2, but intermediate enhancement is seen when only FDCs express the receptors and low enhancement when only B cells express them. These observations imply that IgM-mediated enhancement works through several, non-mutually exclusive, pathways. Marginal zone B cells can transport IgM-antigen-complement complexes, bound to CR1/2, from the marginal zone and deposit them onto FDCs. In addition, co-crosslinking of the BCR and the CR2/CD19/CD81 co-receptor complex may enhance signaling to specific B cells, a mechanism likely to be involved in induction of early extrafollicular antibody responses.

IgE-antigen complexes, administered intravenously to mice, induce a several 100-fold higher specific antibody response than antigen alone. Additionally, in vivo activation and proliferation of specific CD4(+) T cells is enhanced. The mechanism behind these effects is thought to be that peripheral B cells capture IgE-antigen complexes via their low-affinity receptor for IgE, CD23, and rapidly transport them to splenic B cell follicles where an immune response is initiated. Here, we demonstrate that ovalbumin, covalently coupled to anti-CD23 antibodies and administered intravenously to mice, is also transported to splenic follicles and induces an enhanced primary antibody response. These effects are absent in CD23-deficient mice. No enhanced induction of immunological memory was observed. These findings extend previous observations regarding the in vivo role of CD23 and emphasize that recirculating B cells play an important role in antigen transport to the spleen.

Antibodies, forming immune complexes with their specific antigen, can cause complete suppression or several 100-fold enhancement of the antibody response. Immune complexes containing IgG and IgM may activate complement and in such situations also complement components will be part of the immune complex. Here, we review experimental data on how antibodies via the complement system upregulate specific antibody responses. Current data suggest that murine IgG1, IgG2a, and IgG2b upregulate antibody responses primarily via Fc-receptors and not via complement. In contrast, IgM and IgG3 act via complement and require the presence of complement receptors 1 and 2 (CR1/2) expressed on both B cells and follicular dendritic cells. Complement plays a crucial role for antibody responses not only to antigen complexed to antibodies, but also to antigen administered alone. Lack of C1q, but not of Factor B or MBL, severely impairs antibody responses suggesting involvement of the classical pathway. In spite of this, normal antibody responses are found in mice lacking several activators of the classical pathway (complement activating natural IgM, serum amyloid P component (SAP), specific intracellular adhesion molecule-grabbing non-integrin R1 (SIGN-R1) or C-reactive protein. Possible explanations to these observations will be discussed.

Ag administered together with specific IgG3 induces a higher Ab response than Ag administered alone, an effect requiring the presence of complement receptors 1 and 2 (CR1/2). In this study, we have investigated the fate of Ag, the development of germinal centers (GCs), and the Ab response after i.v. administration of IgG3 anti-trinitrophenyl (TNP) in complex with OVA-TNP. After 2 h, OVA-TNP was detected on marginal zone (MZ) B cells, and a substantial amount of Ag was detected in splenic follicles and colocalized with follicular dendritic cells (FDCs). After 10 d, the percentage of GCs and the IgG responses were markedly higher than in mice immunized with uncomplexed OVA-TNP. The effects of IgG3 were dependent on CR1/2 known to be expressed on B cells and FDCs. Using bone marrow chimeric mice, we demonstrate that an optimal response to IgG3-Ag complexes requires that CR1/2 is expressed on both cell types. These data suggest that CR1/2+ MZ B cells transport IgG3-Ag-C complexes from the MZ to the follicles, where they are captured by FDCs and induce GCs and IgG production. This pathway for initiating the transport of Ags into splenic follicles complements previously known B-cell dependent pathways where Ag is transported by 1) MZ B cells, binding large Ags-IgM-C complexes via CR1/2; 2) recirculating B cells, binding Ag via BCR; or 3) recirculating B cells, binding IgE-Ag complexes via the low-affinity receptor for IgE, CD23.

Mast cell progenitors (MCp) leave the bone marrow and migrate to peripheral tissues where they mature. Although the existence of committed MCp in adult mouse and human blood has been postulated, they have never been found. We have isolated a rare population of cells in adult mouse blood, committed to the mast cell lineage. These were identified as lineage- c-kit(hi) ST2+ integrin β7(hi) CD16/32(hi) cells. Moreover, a major difference in maturity of these cells based on FcεRI expression was observed between the Th2-prone BALB/c strain and the Th1-prone C57BL/6 strain (66% vs. 25% FcεRI+, respectively). Therefore, the choice of mouse strain is critical when studying disease models such as experimental asthma where mast cells and their progenitors are involved.

Specific IgG, passively administered together with particulate antigen, can completely prevent induction of antibody responses to this antigen. The ability of IgG to suppress antibody responses to sheep red blood cells (SRBCs) is intact in mice lacking FcγRs, complement factor 1q, C3, or complement receptors 1 and 2, suggesting that Fc-dependent effector functions are not involved. Two of the most widely discussed explanations for the suppressive effect are increased clearance of IgG–antigen complexes and/or that IgG “hides” the antigen from recognition by specific B cells, so-called epitope masking. The majority of data on how IgG induces suppression was obtained through studies of the effects on IgM-secreting single spleen cells during the first week after immunization. Here, we show that IgG also suppresses antigen-specific extrafollicular antibody-secreting cells, germinal center B-cells, long-lived plasma cells, long-term IgG responses, and induction of memory antibody responses. IgG anti-SRBC reduced the amount of SRBC in the spleens of wild-type, but not of FcγR-deficient mice. However, no correlation between suppression and the amount of SRBC in the spleen was observed, suggesting that increased clearance does not explain IgG-mediated suppression. Instead, we found compelling evidence for epitope masking because IgG anti-NP administered with NP-SRBC suppressed the IgG anti-NP, but not the IgG anti-SRBC response. Vice versa, IgG anti-SRBC administered with NP-SRBC, suppressed only the IgG anti-SRBC response. In conclusion, passively transferred IgG suppressed all measured parameters of an antigen-specific antibody/B cell response and an important mechanism of action is likely to be epitope masking.

IgM is the first antibody produced after initial antigen exposure. Its high avidity enables it to bind antigen effectively, even with relatively low affinity binding sites, and to mobilize other effector arms of the immune system, namely complement and various Fc receptors. This article focuses on the structural features of IgM and the importance of IgM in combating infection. IgM is traditionally represented as a single isoform – a ‘pentamer’ composed of 10 μ-heavy chains, 10 light chains, and a single J chain linked by inter-μ chain disulfide bonds in a well-defined quaternary arrangement. However, IgM is also made as a hexamer that lacks J chain; the J-chain stoichiometry is uncertain; and the disulfide bonds form in multiple arrangements. Analyses of IgM-bearing-mutated μ chains or hybrid μ/γ heavy chains have identified molecular requirements for IgM assembly and for its activation of complement. This article also reviews the cellular basis of IgM production and the dual roles of antigen-specific IgM in the immune response – its direct role whereby activation of complement results in antigen destruction and its regulatory role whereby activation of complement enhances specific antibody production. The physiological significance of the different IgM isoforms is also briefly considered.

IgE, forming an immune complex with small proteins, can enhance the specific antibody and CD4+ T cell responses in vivo. The effects require the presence of CD23 (Fcε-receptor II)+ B cells, which capture IgE-complexed antigens (Ag) in the circulation and transport them to splenic B cell follicles. In addition, also CD11c+ cells, which do not express CD23, are required for IgE-mediated enhancement of T cell responses. This suggests that some type of dendritic cell obtains IgE-Ag complexes from B cells and presents antigenic peptides to T cells. To elucidate the nature of this dendritic cell, mice were immunized with ovalbumin (OVA)-specific IgE and OVA and different populations of CD11c+ cells, obtained from the spleens four hours after immunization, were tested for their ability to present OVA. CD8α− conventional dendritic cells (cDCs) were much more efficient in inducing specific CD4+ T cell proliferation ex vivo than were CD8α+ cDCs or plasmacytoid dendritic cells. Thus, IgE-Ag complexes administered intravenously are rapidly transported to the spleen by recirculating B cells where they are delivered to CD8α− cDCs which induce proliferation of CD4+ T cells.

Patients with allergic asthma have more lung mast cells, which likely worsens the symptoms. In experimental asthma, CD11c+ cells have to be present during the challenge phase for several features of allergic inflammation to occur. Whether CD11c+ cells play a role for Ag-induced increases of lung mast cells is unknown. In this study, we used diphtheria toxin treatment of sensitized CD11c-diphtheria toxin receptor transgenic mice to deplete CD11c+ cells. We demonstrate that recruitment of mast cell progenitors to the lung is substantially reduced when CD11c+ cells are depleted during the challenge phase. This correlated with an impaired induction of endothelial VCAM-1 and led to a significantly reduced number of mature mast cells 1 wk after challenge. Collectively, these data suggest that Ag challenge stimulates CD11c+ cells to produce cytokines and/or chemokines required for VCAM-1 upregulation on the lung endothelium, which in turn is crucial for the Ag-induced mast cell progenitor recruitment and the increase in mast cell numbers.

Lack of complement factors C1q, C2, C3, or C4, but not of factor B in the alternative pathway or of mannane binding lectin (MBL) in the lectin pathway, leads to severe impairment of primary antibody responses (reviewed in. Because C1q is only used in classical pathway activation, these observations suggest that only this pathway is involved in antibody responses. An efficient classical pathway activator is IgM. Specific IgM, passively administered together with suboptimal doses of the antigen it recognizes, induces a stronger antibody response than does the antigen alone. This article is protected by copyright. All rights reserved.

Activation of serum complement triggers Th17 cell–dependent spontaneous autoimmune disease in an animal model. In genetically autoimmune-prone SKG mice, administration of mannan or β-glucan, both of which activate serum complement, evoked Th17 cell–mediated chronic autoimmune arthritis. C5a, a chief component of complement activation produced via all three complement pathways (i.e., lectin, classical, and alternative), stimulated tissue-resident macrophages, but not dendritic cells, to produce inflammatory cytokines including IL-6, in synergy with Toll-like receptor signaling or, notably, granulocyte/macrophage colony-stimulating factor (GM-CSF). GM-CSF secreted by activated T cells indeed enhanced in vitro IL-6 production by C5a-stimulated macrophages. In vivo, C5a receptor (C5aR) deficiency in SKG mice inhibited the differentiation/expansion of Th17 cells after mannan or β-glucan treatment, and consequently suppressed the development of arthritis. Transfer of SKG T cells induced Th17 cell differentiation/expansion and produced arthritis in C5aR-sufficient recombination activating gene (RAG)−/− mice but not in C5aR-deficient RAG−/− recipients. In vivo macrophage depletion also inhibited disease development in SKG mice. Collectively, the data suggest that complement activation by exogenous or endogenous stimulation can initiate Th17 cell differentiation and expansion in certain autoimmune diseases and presumably in microbial infections. Blockade of C5aR may thus be beneficial for controlling Th17-mediated inflammation and autoimmune disease.

Severely impaired Ab responses are seen in animals lacking C (complement) factors C2, C3 or C4 as well as CR1/2 (C receptors 1 and 2). The molecular mechanism behind this phenomenon is not understood. One possibility is that C-containing immune complexes are endocytosed via CR2 on B cells and presented to specific CD4+ T cells, which would then proliferate and provide efficient help to specific B cells. In vitro, B cells can endocytose immune complexes via CR1/2 and present the Ag to T cells. Whether absence of this Ag presenting function in Cr2(-/-) mice (mice lacking CR1/2) explains their low Ab response is unclear. To address this question, Cr2(-/-) and wild type mice were transferred with OVA-specific T cells, obtained from the DO11.10 strain which has a transgenic TCR recognizing an OVA peptide. The animals were subsequently immunized with sheep red blood cells (SRBC) conjugated to OVA. Interestingly, proliferation of the OVA-specific T cells was normal in Cr2(-/-) mice, although their Ab response to both SRBC and OVA was severely impaired. These observations suggest that the impaired Ab response in Cr2(-/-) mice cannot be explained by a lack of appropriate induction of T cell help.

IgE administered with its specific antigen in vivo induces enhanced proliferation of specific T cells as well as enhanced production of specific antibodies. Both effects are dependent on the low-affinity receptor for IgE (CD23) and the underlying mechanism is thought to be increased antigen presentation following uptake of IgE/antigen complexes via CD23(+) B cells. By contrast, CD23 negatively regulates antibody responses to antigens administered with alum, i.e. without IgE. This effect has been observed as low IgG1 and IgE responses in transgenic mice overexpressing CD23 (CD23Tg). The present study was designed to test whether IgE could enhance antibody and T-cell responses in CD23Tg animals or whether CD23's downregulatory effect precludes IgE-mediated enhancement. IgE-anti-TNP administered with OVA-TNP enhances the OVA-specific antibody responses in wild-type (wt) and CD23Tg mice equally well. Interestingly, the total magnitude of antibody responses to IgE + OVA-TNP and to uncomplexed OVA-TNP, as well as to sheep erythrocytes and keyhole limpet haemocyanine, were lower in the CD23Tg mice. IgE induced proliferation of OVA-specific CD4(+) T cells to the same degree in wt and CD23Tg mice. The effect on T cells was dependent on CD23(+) B cells as demonstrated in in vitro proliferation assays. In conclusion, CD23 does indeed have dual immunoregulatory effects in the same animal. The receptor mediates enhancement of antibody and T-cell responses to IgE-complexed antigen, most likely via increased presentation of complexed antigen, while it negatively regulates the total antibody response to a variety of antigens.

Antigen-specific IgG antibodies, passively administered to mice or humans together with large particulate antigens like erythrocytes, can completely suppress the antibody response against the antigen. This is used clinically in Rhesus prophylaxis, where administration of IgG anti-RhD prevents RhD-negative women from becoming immunized against RhD-positive fetal erythrocytes aquired transplacentally. The mechanisms by which IgG suppresses antibody responses are poorly understood. We have here addressed whether complement or Fc-receptors for IgG (FcγRs) are required for IgG-mediated suppression. IgG, specific for sheep red blood cells (SRBC), was administered to mice together with SRBC and the antibody responses analyzed. IgG was able to suppress early IgM- as well as longterm IgG-responses in wildtype mice equally well as in mice lacking FcγRIIB (FcγRIIB knockout mice) or FcγRI, III, and IV (FcRγ knockout mice). Moreover, IgG was able to suppress early IgM responses equally well in mice lacking C1q (C1qA knockout mice), C3 (C3 knockout mice), or complement receptors 1 and 2 (Cr2 knockout mice) as in wildtype mice. Owing to the previously described severely impaired IgG responses in the complement deficient mice, it was difficult to assess whether passively administered IgG further decreased their IgG response. In conclusion, Fc-receptor binding or complement-activation by IgG does not seem to be required for its ability to suppress antibody responses to xenogeneic erythrocytes.

Antibodies, administered together with their specific antigen, can feedback-regulate antibody responses to this antigen. IgG1, IgG2a and IgG2b enhance antibody responses to soluble protein antigens. This effect is primarily mediated by FcRs as enhancement is impaired in FcR gamma-/- mice, reported to lack Fc gammaRI and Fc gammaRIII because of deletion of the common FcR gamma chain. Also IgG3 can enhance antibody responses. However, this effect is unperturbed in FcR gamma-/- mice but severely impaired in complement-depleted animals and in animals lacking complement receptor 1 and 2. Although this argues against involvement of Fc gammaRs, FcR gamma-/- mice may express one-fifth of the normal levels of Fc gammaRI and, in addition, Fc gammaRI has been suggested to bind IgG3. We re-investigated the dependence of IgG3-mediated enhancement on Fc gammaRs using a mouse strain selectively lacking Fc gammaRI and found that IgG3-mediated enhancement is completely normal. Unlike IgE and IgG2a, which are both thought to enhance T-cell proliferation via FcR-mediated antigen presentation, IgG3 was a poor enhancer of T-cell proliferation both in vivo and in vitro. These findings argue against a significant involvement of Fc gammaRs in IgG3-mediated enhancement of antibody responses and support our previous conclusion that complement plays a major role.

IgE Abs, passively administered together with their specific Ag, can enhance the production of Abs recognizing this Ag by >100-fold. IgE-mediated feedback enhancement requires the low affinity receptor for IgE, CD23. One possible mechanism is that B cells take up IgE-Ag via CD23 and efficiently present Ag to Th cells, resulting in better Ab responses. To test whether IgE Abs have an effect on Th cells in vivo, mice were adoptively transferred with CD4+ T cells expressing a transgenic OVA-specific TCR, before immunization with IgE anti-TNP (2,4,6-trinitrophenyl) plus OVA-TNP or with OVA-TNP alone. IgE induced a 6- to 21-fold increase in the number of OVA-specific T cells. These cells acquired an activated phenotype and were visible in splenic T cell zones. The T cell response peaked 3 days after immunization and preceded the OVA-specific Ab response by a few days. Transfer of CD23+ B cells to CD23-deficient mice rescued their ability to respond to IgE-Ag. Interestingly, in this situation also CD23-negative B cells produce enhanced levels of OVA-specific Abs. The data are compatible with the Ag presentation model and suggest that B cells can take up Ag via “unspecific” receptors and activate naive T cells in vivo.

Antibodies passively administered together with their specific antigen are known to influence the antibody response to this antigen. Antibody feedback regulation can either enhance or suppress anti

Antibodies of all classes and subclasses except IgD have been shown to have the capacity to feedback regulate the production of themselves. This phenomenon has been known for over a century and was originally described by the first Nobel laureate in physiology, Emil von Behring. When an animal is immunized with antibodies together with an antigen they recognize, the antibody response to this very antigen is often dramatically modulated. Sometimes feedback regulation results in complete suppression and sometimes in several 100-fold enhancement of the specific response. An immune complex contains the antigen, the specific antibodies bound to it and, when the antibodies can activate complement, complement factors, and can, therefore, bind to antigen-specific receptors on B cells (BCR), various Fc-receptors (FcRs) as well as to complement receptors. This gives the immune complex many possibilities to regulate the immune response via e.g. receptor cocrosslinking, leading to changes in signal transduction, or by increased antigen uptake and processing by antigen-presenting cells. This review will focus on the role of IgG as a feedback regulator. Three different pathways will be discussed: (i) the ability of IgG to induce complete suppression of erythrocyte responses, which takes place equally efficiently in the absence as in the presence of FcgammaRs, (ii) the ability of IgG to enhance responses to soluble protein antigens, a phenomenon severely impaired in FcRgamma-chain-deficient mice (with non-functional FcgammaRI and FcgammaRIII), and (iii) the ability of IgG to, via FcgammaRIIB, downregulate the response to IgG-complexed soluble antigens.

IgG1, IgG2a, and IgG2b, passively administered with soluble Ags, enhance specific Ab responses. The effect of IgG3 in this type of feedback regulation has not been studied previously. We immunized mice with trinitrophenyl (TNP)-coupled carrier proteins (bovine serum albumin [BSA] or ovalbumin [OVA]) alone or complexed to monoclonal TNP-specific IgG3. The carrier-specific Ab responses were enhanced by several hundred-fold by IgG3. Enhancement was significantly impaired in mice depleted of complement factor C3 and in mice lacking complement receptors 1 and 2 (Cr2-/-). In contrast, mice lacking the common Fc-receptor gamma chain (FcR gamma -/-), resulting in reduced expression of Fc gamma RI and lack of Fc gamma RIII, and mice lacking Fc gamma RIIB (Fc gamma RIIB-/-), responded equally well to immunization with IgG3-complexed Ag as wild-type controls. These findings demonstrate that IgG3 can induce feedback enhancement and that IgG3, in analogy with IgM, uses the complement system for this function.

Antibodies in complex with specific antigen can dramatically change the antibody response to this antigen. Depending on antibody class and type of antigen,99 % suppression or100-fold enhancement of the response can take place. IgM and IgG3 are efficient enhancers and operate via the complement system. In contrast, IgG1, IgG2a, and IgG2b enhance antibody and CD4(+) T cell responses to protein antigens via activating Fcγ-receptors. IgE also enhances antibody and CD4(+) T cell responses to small proteins but uses the low-affinity receptor for IgE, CD23. Most likely, IgM and IgG3 work by increasing the effective concentration of antigen on follicular dendritic cells in splenic follicles. IgG1, IgG2a, IgG2b, and IgE probably enhance antibody responses by increasing antigen presentation by dendritic cells to T helper cells. IgG antibodies of all subclasses have a dual effect, and suppress antibody responses to particulate antigens such as erythrocytes. This capacity is used in the clinic to prevent immunization of Rhesus-negative women to Rhesus-positive fetal erythrocytes acquired via transplacental hemorrage. IgG-mediated suppression in mouse models can take place in the absence of Fcγ-receptors and complement and to date no knock-out mouse strain has been found where suppression is abrogated.

Antibodies in complex with specific antigen can dramatically change the antibody response to this antigen. Depending on antibody class and type of antigen, >99 % suppression or >100-fold enhancement of the response can take place. IgM and IgG3 are efficient enhancers and operate via the complement system. In contrast, IgG1, IgG2a, and IgG2b enhance antibody and CD4+ T cell responses to protein antigens via activating Fcγ-receptors. IgE also enhances antibody and CD4+ T cell responses to small proteins but uses the low-affinity receptor for IgE, CD23. Most likely, IgM and IgG3 work by increasing the effective concentration of antigen on follicular dendritic cells in splenic follicles. IgG1, IgG2a, IgG2b, and IgE probably enhance antibody responses by increasing antigen presentation by dendritic cells to T helper cells. IgG antibodies of all subclasses have a dual effect, and suppress antibody responses to particulate antigens such as erythrocytes. This capacity is used in the clinic to prevent immunization of Rhesus-negative women to Rhesus-positive fetal erythrocytes acquired via transplacental hemorrage. IgG-mediated suppression in mouse models can take place in the absence of Fcγ-receptors and complement and to date no knock-out mouse strain has been found where suppression is abrogated.

The suppressive effect of IgG on Ab responses to particulate Ags such as erythrocytes is well documented. IgG-mediated suppression is used clinically in rhesus prophylaxis to prevent RhD-negative mothers from becoming immunized against their Rh D-positive fetuses. We have recently shown that IgG anti-SRBC, passively administered together with SRBC, can induce efficient suppression of primary Ab responses to SRBC in mice lacking the known FcRs for IgG (FcγRI, FcγIII, and FcγRIIB or the neonatal FcR). The lack of a demonstrable effect of the inhibitory FcγRIIB was particularly surprising, and, in this study, the involvement of this receptor is further investigated during broader experimental conditions. The data show that SRBC-specific IgG administered up to 5 days after SRBC can induce suppression both in wild-type and FcγRIIB-deficient mice. Suppression of secondary Ab responses to SRBC in vivo was similar in the two strains. In contrast, IgG-mediated suppression of Ab responses in vitro was impaired in cultures with primed FcγRIIB-deficient spleen cells. In conclusion, inhibition of in vivo Ab responses to SRBC by passively administered IgG can take place via an FcγRIIB-independent pathway. This pathway causes >99% suppression and operates during all experimental conditions studied so far. The nature of the mechanism can at present only be hypothesized. Masking of epitopes and/or rapid elimination of IgG-Ag complexes would both be compatible with the observations.

Antibodies (Ab) administered in complex with antigens (Ag) have the capacity to regulate the out-coming specific immune response. Primary immunization with complexes of bovine serum albumin-2,4,6-trinitrophenyl (BSA–TNP) and immunoglobulin (Ig)G2a anti-TNP induced a significant enhancement of IgG1 and IgG2a BSA-specific Ab response compared to immunization with the Ag alone. Enhancement was absent in nude mice, demonstrating the requirement of T cells for this regulation. Secondary immunization with BSA alone in mice previously primed with BSA-TNP/IgG2a led to a dramatic increase of Ab production, showing that immune complexes are efficient inducers of immunological memory. IgG-mediated enhancement of Ab responses has previously been shown to be impaired in mice lacking FcγRI, FcγRIII and FceRI owing to gene targeting of the common FcRγ subunit (FcRγ−/–). Here we show that enhancement after immunization with BSA–TNP/IgG2a complexes is restored in irradiated FcRγ−/– recipients transferred with wild-type (FcRγ+/+) bone marrow (BM) cells. In contrast, no enhancement is seen in FcRγ+/+ irradiated animals reconstituted with FcRγ−/– BM cells. We conclude that IgG2a-mediated enhancement of Ab responses is dependent on the presence of FcγRI and/or FcγRIII on BM-derived cells and that the presence of these receptors on the radioresistant follicular dendritic cell is not essential.

SETTING THE STAGEIn a paper in this issue of Scandinavian Journal ofImmunology, Sinclair addresses the role of Fc-signalling inthe modulation of immune responses by passive antibody [1].This is a broad field, encompassing aspects of immuneresponses from autoimmune disease models to cell signallingin vitro. A substantial part of the paper deals in a critical waywith recent results from our laboratory concerning the abilityof immunoglobulin (Ig)G to feedback-regulate in vivo anti-body responses in mice [2,3] and we will limit our reply tothis area. In a field as complicated as antibody feedbackregulation, it is important to define exactly what is discussed.Antibodies of many classes have the capacity to modulateantibody responses, either positively or negatively [4]. IgGhas a dual role, suppressing the response to particulateantigens such as erythrocytes [5–9], and enhancing theresponse to soluble antigens such as keyhole limpethemocyanine (KLH), bovine serum albumin (BSA), andovalbumin (OVA) [3,9–11]. The same monoclonal IgG anti-trinitriphenyl (anti-TNP) can suppress or enhance, dependingon whether TNP is conjugated to an erythrocyte or a solubleantigen [9,11]. Considering the strikingly different structure ofthese two types of immune complexes this is not surprising.While an IgG molecule is much smaller than an erythrocyte(10 nm versus 7 mm in diameter), IgG is on average twice aslarge as BSA. When discussing IgG-mediated immunoregula-tion, it is therefore vital to keep apart studies using particulateversus soluble antigens. Major differences between in vitroand in vivo systems for studying antibody responses exist(e.g., lack of lympoid organ structure in vitro) and unlessspecifically stated, only in vivo systems will be consideredhere.NO EVIDENCE FOR A ROLE OF FCgRIIB INSUPPRESSION OF IN VIVO ANTIBODYRESPONSES TO SRBC BY PASSIVELYADMINISTERED IgGWhy IgG has the capacity to completely inhibit a response toSRBC is a question that has been debated extensively over theyears. Hypotheses put forward are: (i) epitope masking by IgG,preventing recognition of the antigen by B cells, (ii) effectiveelimination of IgG-coated antigen via Fc-dependent phago-cytosis, and (iii) central inhibition of B-cell activation viacocrosslinking of the B-cell receptor and the inhibitoryFcgRIIB. Sinclair argues that the mechanism behind the IgG-mediated suppression of erythrocyte-specific responses wasresolved 30 years ago and that his so called TripartiteInactivation Model ((iii) see above) explains the phenomenon[12]. Some textbooks and reviews also state for a fact that themechanism behind the IgG-mediated suppression has beenproven to be FcgRIIB-mediated negative signalling to B cells.We do not share this opinion. Although this receptorundoubtedly plays an inhibitory role in many experimentalsystems, there are many observations calling for scepticismabout its role in inhibition of erythrocyte responses by passivelyadministered IgG. Firstly, data regarding the Fc-dependence ofsuppression are conflicting. The efficiency of F(ab

The ability of immunoglobulin (Ig)G to feedback suppress antibody (Ab) responses is a well known property clinically used to prevent haemolytic disease of newborns. We recently found that IgG was able to suppress the primary Ab response to sheep red blood cells (SRBC) in mice lacking the known Fc-receptors for IgG. In addition, IgE and F(ab′)2 fragments of IgG were able to suppress the response to SRBC in wild-type mice. These results suggested that the IgG-mediated suppression can take place independently of the IgG (Fc) portion and that masking of the epitopes is an important mechanism. In the present report we investigated whether the suppression caused by IgE is Fc-dependent. Monoclonal IgE anti-2,4,6-trinitrophenyl (TNP), administered with TNP-coupled SRBC (SRBC–TNP), can induce an efficient suppression in mice lacking FcγRI + RIII + FceRI (owing to the lack of the common γ chain, FcRγ), FcγRIIB or FceRII (CD23). Because the known IgE-binding receptors are FceRI, CD23, FcγRIIB and FcγRIII, the results suggest that also the IgE-mediated suppression can take place independently of the Fc-receptors. A slightly less efficient suppression in CD23-deficient animals, suggests a minor involvement of this receptor.

It is well known that the transfer of immunoglobulins (Igs) from mother to young via milk contributes to the offspring's immune defense. The present study suggests that not only is IgG transmitted to progeny, but that functional maternal Ig-secreting cells (or B cells) can also be transferred to the neonate. We have used B cell-deficient (micro(-/-)) mice and found that a high proportion of them obtain long-lasting, partial reconstitution of their serum Ig levels if born to micro(+/-) mothers. In some of these serum IgG-positive micro(-/-) mice, Ig-secreting cells were detected in spleen and bone marrow. To ensure that cells of maternal origin were present in the progeny, micro(-/-) offspring born to micro(+/-) dams transgenic for green fluorescent protein (GFP) were used. In spleens and bone marrow from some of these micro(-/-)GFP(-/-) offspring, GFP-positive cells were detected, which demonstrated that cells of maternal origin could infiltrate the progeny. In addition, splenic Ig-secreting cells were detected in micro(-/-) mice that were born to micro(-/-) dams and transferred to a lactating micro(+/+) foster dam at birth. This indicates that maternal Ig-secreting cells can be transferred postnatally via milk.

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 (FcγRs). IgE enhances antibody responses via the low-affinity receptor for IgE (FcεRII/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, FcγRIIB, 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.

Receptors for immunoglobulin (Ig)G (FcgammaRs) are important for the antibody-mediated effector functions of the immune system. FcgammaRI and FcgammaRIII trigger cell activation through a common gamma chain, whereas FcgammaRII acts as a negative regulator of antibody production and immune complex-triggered activation. Here we describe the in vivo consequences of FcgammaR deficiency in a mouse model of human rheumatoid arthritis. FcRgamma chain-deficient mice on arthritis-susceptible DBA/1 background were immunized with collagen for induction of collagen-induced arthritis. The DBA/1 mice lacking FcRgamma chain were protected from collagen-induced arthritis in contrast to wild-type mice, although both groups produced similar levels of IgG anticollagen antibodies. In comparison, DBA/1 mice lacking FcgammaRII developed an augmented IgG anticollagen response and arthritis. These observations suggest a crucial role of FcgammaRI and FcgammaRIII in triggering autoimmune arthritis.

In murine in vivo systems, Ags administered in physiologic solutions together with specific IgE induce a significantly higher Ab response than Ags administered alone. In vitro, IgE in complex with Ag enhances B cell-mediated presentation of the Ag to T cells. Both phenomena require an intact low affinity receptor for IgE (FcεRII/CD23), suggesting that the effect on in vivo Ab responses is caused by increased Ag presentation. We here show that mice carrying the MHC class II Ab molecule (e.g., C57BL/6 and 129/Sv) do not produce Abs to BSA when immunized with BSA-2,4,6-trinitrophenyl (TNP) in complex with monoclonal IgE anti-TNP. In contrast, strains of all other MHC haplotypes tested (H-2d, H-2k, H-2p, H-2q, and H-2s) respond vigorously to IgE/BSA-TNP complexes, with Ab responses several hundred-fold higher than the responses in H-2b mice. C57BL/6 mice were unable to produce a carrier-specific response also after immunization with IgE/OVA-TNP, IgE/diphtheria toxoid-TNP, or IgE/tetanus toxoid-TNP. Although the low responsiveness mapped to the Ab region, responsiveness was not restored in C57BL/6 mice carrying transgenic Ak, suggesting that a nonclassical A-region-encoded gene product is involved. Most importantly, our data call attention to the fact that the C57BL/6 and 129 mouse strains, which are widely used for producing transgenic animals, have defective immune responses.

IgM antibodies specific for a certain antigen can enhance antibody responses when administered together with this antigen, a process believed to require complement activation by IgM. However, recent data show that a knock-in mouse strain, C mu 13, which only produces IgM unable to activate complement, has normal antibody responses. Moreover, the recently discovered murine IgM Fc receptor (Fc mu R or TOSO/FAIM3) was shown to affect antibody responses. This prompted the re-investigation of whether complement activation by specific IgM is indeed required for enhancement of antibody responses and whether the mutation in C mu 13 IgM also caused impaired binding to Fc mu R. The results show that IgM from C mu 13 and wildtype mice bound equally well to the murine Fc mu R. In spite of this, specific C mu 13 IgM administered together with sheep red blood cells or keyhole limpet hemocyanine was a very poor enhancer of the antibody and germinal center responses as compared with wildtype IgM. Within seconds after immunization, wildtype IgM induced deposition of C3 on sheep red blood cells in the blood. IgM which efficiently enhanced the T-dependent humoral immune response had no effect on activation of specific CD4+ T cells as measured by cell numbers, cell division, blast transformation, or expression of the activation markers LFA-1 and CD44 in vivo. These observations confirm the importance of complement for the ability of specific IgM to enhance antibody responses and suggest that there is a divergence between the regulation of T-and B-cell responses by IgM.