Fc alpha/mu receptor mediates endocytosis of IgM-coated microbes

The human IgM pentamer is a mushroom-shaped molecule with a flexural bias

The textbook planar model of pentameric IgM, a potent activator of complement C1q, is based upon the crystallographic structure of IgG. Although widely accepted, key predictions of this model have not yet been directly confirmed, which is particularly important since IgG lacks a major Ig fold domain in its Fc region that is present in IgM. Here, we construct a homology-based structural model of the IgM pentamer using the recently obtained crystallographic structure of IgE Fc, which has this additional Ig domain, under the constraint that all of the cysteine residues known to form disulfide bridges both within each monomer and between monomers are bonded together. In contrast to the planar model, this model predicts a non-planar, mushroom-shaped complex, with the central portion formed by the C-terminal domains protruding out of the plane formed by the Fab domains. This unexpected conformation of IgM is, however, directly confirmed by cryo-atomic force microscopy of individual human IgM molecules. Further analysis of this model with free energy calculations of out-of-plane Fab domain rotations reveals a pronounced asymmetry favoring flexions toward the central protrusion. This bias, together with polyvalent attachment to cell surface antigen, would ensure that the IgM pentamer is oriented on the cell membrane with its C1q binding sites fully exposed to the solution, and thus provides a mechanistic explanation for the first steps of C1q activation by IgM.

The chicken leukocyte receptor complex encodes a family of different affinity FcY receptors

Chicken Ig-like receptors (CHIR) form a large family in the leukocyte receptor complex on microchromosome 31 with inhibitory, activating, and bifunctional receptors. Recently, we characterized CHIR-AB1 as a high-affinity, primordial FcY receptor. Given that the CHIR family represents a multigene family, it is plausible that more than a single receptor binds to IgY. Therefore, after comparing CHIR-AB1-like sequences in databases, we cloned CHIR-AB1 homologues from two individual chickens representing the lines M11 and R11 with primers binding to highly conserved regions. In both lines this approach yielded 18 different CHIR-AB amino acid versions, with one sequence out of each line that was identical with the previously characterized B19 CHIR-AB1 Ig domain and two additional R11-M11 identical sequence pairs. All M11-derived CHIR-AB homologues were then expressed as soluble human Ig fusion proteins. Following standardization of the fusion protein concentration with an ELISA, the IgY, IgM, and IgA binding activities were determined by ELISA. Six fusion proteins recognized IgY, whereas none bound to IgM and IgA. The affinities of selected fusion proteins were determined using surface plasmon resonance yielding an equilibrium binding constant between 25 nM for high binders and 260 nM for low binders. Sequence comparisons and subsequent mutational analysis of selected residues identified five amino acids that are potentially involved in IgY binding. These results imply that multiple FcY receptors of variable affinity are encoded by the CHIR locus and that different chicken lines may express both unique as well as highly conserved FcY receptors.

Enhanced humoral immune responses against T-independent antigens in Fc alpha/muR-deficient mice

IgM is an antibody class common to all vertebrates that plays a primary role in host defenses against infection. Binding of IgM with an antigen initiates the complement cascade, accelerating cellular and humoral immune responses. However, the functional role of the Fc receptor for IgM in such immune responses remains obscure. Here we show that mice deficient in Fc alpha/muR, an Fc receptor for IgM expressed on B cells and follicular dendritic cells (FDCs), have enhanced germinal center formation and affinity maturation and memory induction of IgG3(+) B cells after immunization with T-independent (TI) antigens. Moreover, Fc alpha/muR-deficient mice show prolonged antigen retention by marginal zone B (MZB) cells and FDCs. In vitro studies demonstrate that interaction of the IgM immune complex with Fc alpha/muR partly suppress TI antigen retention by MZB cells. We further show that downregulation of complement receptor (CR)1 and CR2 or complement deprivation by in vivo injection with anti-CR1/2 antibody or cobra venom factor attenuates antigen retention by MZB cells and germinal center formation after immunization with TI antigens in Fc alpha/muR(-/-) mice. Taken together, these results suggest that Fc alpha/muR negatively regulates TI antigen retention by MZB cells and FDCs, leading to suppression of humoral immune responses against T-independent antigens.

Roles for phagocytic cells and complement in controlling relapsing fever infection

Relapsing fever spirochetes, such as Borrelia hermsii, proliferate to high levels in their hosts' bloodstream until production of IgM against borrelial surface proteins promotes bacterial clearance. The mechanisms by which B. hermsii survives in host blood, as well as the immune mediators that control this infection, remain largely unknown. It has been hypothesized that B. hermsii is naturally resistant to killing by the alternative pathway of complement activation as a result of its ability to bind factor H, a host complement regulator. However, we found that Cfh(-/-) mice were infected to levels identical to those seen in wild-type mice. Moreover, only a small minority of B. hermsii in the blood of wild-type mice had detectable levels of factor H adhered to their outer surfaces. In vitro, complement was found to play a statistically significant role in antibody-mediated inactivation of B. hermsii, although in vivo studies indicated that complement is not essential for host control of B. hermsii. Depletion of mphi and DC from mice had significant impacts on B. hermsii infection, and depleted mice were unable to control bloodstream infections, leading to death. Infection studies using muMT indicated a significant antibody-independent role for mphi and/or DC in host control of relapsing fever infection. Together, these findings indicate mphi and/or DC play a critical role in the production of B. hermsii-specific IgM and for antibody-independent control of spirochete levels.

Impaired antibody response causes persistence of prototypic T cell-contained virus

CD8 T cells are recognized key players in control of persistent virus infections, but increasing evidence suggests that assistance from other immune mediators is also needed. Here, we investigated whether specific antibody responses contribute to control of lymphocytic choriomeningitis virus (LCMV), a prototypic mouse model of systemic persistent infection. Mice expressing transgenic B cell receptors of LCMV-unrelated specificity, and mice unable to produce soluble immunoglobulin M (IgM) exhibited protracted viremia or failed to resolve LCMV. Virus control depended on immunoglobulin class switch, but neither on complement cascades nor on Fc receptor gamma chain or Fc gamma receptor IIB. Cessation of viremia concurred with the emergence of viral envelope-specific antibodies, rather than with neutralizing serum activity, and even early nonneutralizing IgM impeded viral persistence. This important role for virus-specific antibodies may be similarly underappreciated in other primarily T cell-controlled infections such as HIV and hepatitis C virus, and we suggest this contribution of antibodies be given consideration in future strategies for vaccination and immunotherapy.

Cytoplasmic domain of human Fcalpha/mu receptor is required for ligand internalization

The Fcalpha/mu receptor (Fcalpha/microR), a type I transmembrane protein, is an immunoglobulin Fc receptor for both IgA and IgM. Its functions in immune defense are not clear at present. In this work, human Fcalpha/microR was expressed in CHO, 293T, and COS-7 cells to study its biochemical functions. Fcalpha/microR expressed by CHO and 293T was only in monomer form in cytoplasma and the monomeric receptor could not bind IgA or IgM. In comparison, Fcalpha/microR expressed by COS-7 cells had both monomer and dimer forms. The binding assay showed that Fcalpha/microR expressed by COS-7 cells could bind IgM strongly and IgA weakly, implying that dimeric receptor could be expressed on cell membrane and functioned. The bound IgM could be internalized and the internalization was abolished when the cytoplasmic domain of Fcalpha/microR was truncated. Therefore, the cytoplasmic portion of human Fcalpha/microR is required in the internalization.

Specificity and affinity of human Fcγ receptors and their polymorphic variants for human IgG subclasses

Distinct genes encode 6 human receptors for IgG (hFcγRs), 3 of which have 2 or 3 polymorphic variants. The specificity and affinity of individual hFcγRs for the 4 human IgG subclasses is unknown. This information is critical for antibody-based immunotherapy which has been increasingly used in the clinics. We investigated the binding of polyclonal and monoclonal IgG1, IgG2, IgG3, and IgG4 to FcγRI; FcγRIIA, IIB, and IIC; FcγRIIIA and IIIB; and all known polymorphic variants. Wild-type and low-fucosylated IgG1 anti-CD20 and anti-RhD mAbs were also examined. We found that (1) IgG1 and IgG3 bind to all hFcγRs; (2) IgG2 bind not only to FcγRIIAH131, but also, with a lower affinity, to FcγRIIAR131 and FcγRIIIAV158; (3) IgG4 bind to FcγRI, FcγRIIA, IIB and IIC and FcγRIIIAV158; and (4) the inhibitory receptor FcγRIIB has a lower affinity for IgG1, IgG2, and IgG3 than all other hFcγRs. We also identified parameters that determine the specificity and affinity of hFcγRs for IgG subclasses. These results document how hFcγR specificity and affinity may account for the biological activities of antibodies. They therefore highlight the role of specific hFcγRs in the therapeutic and pathogenic effects of antibodies in disease.

Structural requirements for the interaction of human IgM and IgA with the human Fcalpha/mu receptor

Here we unravel the structural features of human IgM and IgA that govern their interaction with the human Fcalpha/mu receptor (hFcalpha/muR). Ligand polymerization status was crucial for the interaction, because hFcalpha/muR binding did not occur with monomeric Ab of either class. hFcalpha/muR bound IgM with an affinity in the nanomolar range, whereas the affinity for dimeric IgA (dIgA) was tenfold lower. Panels of mutant IgM and dIgA were used to identify regions critical for hFcalpha/muR binding. IgM binding required contributions from both Cmu3 and Cmu4 Fc domains, whereas for dIgA, an exposed loop in the Calpha3 domain was crucial. This loop, comprising residues Pro440-Phe443, lies at the Fc domain interface and has been implicated in the binding of host receptors FcalphaRI and polymeric Ig receptor (pIgR), as well as IgA-binding proteins produced by certain pathogenic bacteria. Substitutions within the Pro440-Phe443 loop resulted in loss of hFcalpha/muR binding. Furthermore, secretory component (SC, the extracellular portion of pIgR) and bacterial IgA-binding proteins were shown to inhibit the dIgA-hFcalpha/muR interaction. Therefore, we have identified a motif in the IgA-Fc inter-domain region critical for hFcalpha/muR interaction, and highlighted the multi-functional nature of a key site for protein-protein interaction at the IgA Fc domain interface.

The tetraspanin protein CD37 regulates IgA responses and anti-fungal immunity

Immunoglobulin A (IgA) secretion by plasma cells in the immune system is critical for protecting the host from environmental and microbial infections. However, the molecular mechanisms underlying the generation of IgA⁺ plasma cells remain poorly understood. Here, we report that the B cell–expressed tetraspanin CD37 inhibits IgA immune responses in vivo. CD37-deficient (CD37−/−) mice exhibit a 15-fold increased level of IgA in serum and significantly elevated numbers of IgA⁺ plasma cells in spleen, mucosal-associated lymphoid tissue, as well as bone marrow. Analyses of bone marrow chimeric mice revealed that CD37–deficiency on B cells was directly responsible for the increased IgA production. We identified high local interleukin-6 (IL-6) production in germinal centers of CD37−/− mice after immunization. Notably, neutralizing IL-6 in vivo reversed the increased IgA response in CD37−/− mice. To demonstrate the importance of CD37—which can associate with the pattern-recognition receptor dectin-1—in immunity to infection, CD37−/− mice were exposed to Candida albicans. We report that CD37−/− mice are evidently better protected from infection than wild-type (WT) mice, which was accompanied by increased IL-6 levels and C. albicans–specific IgA antibodies. Importantly, adoptive transfer of CD37−/− serum mediated protection in WT mice and the underlying mechanism involved direct neutralization of fungal cells by IgA. Taken together, tetraspanin protein CD37 inhibits IgA responses and regulates the anti-fungal immune response.

Antibody, or immunoglobulin (Ig), production by plasma cells in the immune system is important for protecting the host from microbial infections. IgA is the most abundant antibody isotype produced in the body. However, the molecular mechanisms underlying the generation of IgA–producing plasma cells remain poorly understood. We now report that the B cell–expressed protein CD37 regulates IgA immune responses, both in steady-state conditions and during infection. We found highly increased levels of IgA in serum and elevated numbers of IgA⁺ plasma cells in lymphoid tissue of mice that are deficient for CD37 (CD37−/− mice). To demonstrate the importance of CD37 in immunity to infection, CD37−/− mice were exposed to the fungus Candida albicans. C. albicans can cause systemic infection with high mortality in immunocompromised patients. We demonstrate that CD37−/− mice are evidently better protected from infection than wild-type mice, which was dependent on C. albicans–specific IgA antibodies. The underlying mechanism involved direct neutralization of fungal cells by IgA. In summary, the B cell protein CD37 inhibits IgA responses and anti-fungal immunity. This study may contribute to the development of novel immunotherapeutic approaches for invasive fungal disease.

Hatching time for monotreme immunology

The sequencing of the platypus genome has spurred investigations into the characterisation of the monotreme immune response. As the most divergent of extant mammals, the characterisation of the monotreme immune repertoire allows us to trace the evolutionary history of immunity in mammals and provide insights into the immune gene complement of ancestral mammals. The immune system of monotremes has remained largely uncharacterised due to the lack of specific immunological reagents and limited access to animals for experimentation. Early immunological studies focussed on the anatomy and physiology of the lymphoid system in the platypus. More recent molecular studies have focussed on characterisation of individual immunoglobulin, T-cell receptor and MHC genes in both the platypus and short-beaked echidna. Here, we review the published literature on the monotreme immune gene repertoire and provide new data generated from genome analysis on cytokines, Fc receptors and immunoglobulins. We present an overview of key gene families responsible for innate and adaptive immunity including the cathelicidins, defensins, T-cell receptors and the major histocompatibility complex (MHC) Class I and Class II antigens. We comment on the usefulness of these sequences for future studies into immunity, health and disease in monotremes.

Clearance of adenovirus by Kupffer cells is mediated by scavenger receptors, natural antibodies, and complement

Kupffer cells (KCs) rapidly remove intravenously injected adenovirus (Ad) vectors from the circulation. A better understanding of the mechanisms involved could suggest strategies to improve Ad gene delivery by suppressing or evading KC uptake. We recently showed that clearance of Ad type 5 vectors by KCs does not involve the interaction of Ad with the well-established Ad receptors, namely, integrins or the coxsackievirus and Ad receptor (J. S. Smith, Z. Xu, J. Tian, S. C. Stevenson, and A. P. Byrnes, Hum. Gene Ther. 19:547-554, 2008). In the current study, we systematically quantified the contributions of various receptors and plasma proteins to the clearance of Ad by KCs. We found that scavenger receptors are a predominant mechanism for the clearance of Ad by KCs. In addition, we found that Ad is opsonized by natural immunoglobulin M antibodies and complement and that these opsonins play a contributory role in the clearance of Ad by KCs. We also examined additional mechanisms that have been postulated to be involved in the clearance of Ad, including the binding of Ad to platelets and vitamin K-dependent coagulation factors, but we found that neither of these were required for the clearance of Ad by KCs.

Identification of residues in the Cmu4 domain of polymeric IgM essential for interaction with Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1)

The binding of nonspecific human IgM to the surface of infected erythrocytes is important in rosetting, a major virulence factor in the pathogenesis of severe malaria due to Plasmodium falciparum, and IgM binding has also been implicated in placental malaria. Herein we have identified the IgM-binding parasite ligand from a virulent P. falciparum strain as PfEMP1 (TM284var1 variant), and localized the region within this PfEMP1 variant that binds IgM (DBL4beta domain). We have used this parasite IgM-binding protein to investigate the interaction with human IgM. Interaction studies with domain-swapped Abs, IgM mutants, and anti-IgM mAbs showed that PfEMP1 binds to the Fc portion of the human IgM H chain and requires the IgM Cmu4 domain. Polymerization of IgM was shown to be crucial for the interaction because PfEMP1 binding did not occur with mutant monomeric IgM molecules. These results with PfEMP1 protein have physiological relevance because infected erythrocytes from strain TM284 and four other IgM-binding P. falciparum strains showed analogous results to those seen with the DBL4beta domain. Detailed investigation of the PfEMP1 binding site on IgM showed that some of the critical amino acids in the IgM Cmu4 domain are equivalent to those regions of IgG and IgA recognized by Fc-binding proteins from bacteria, suggesting that this region of Ig molecules may be of major functional significance in host-microbe interactions. We have therefore shown that PfEMP1 is an Fc-binding protein of malaria parasites specific for polymeric human IgM, and that it shows functional similarities with Fc-binding proteins from pathogenic bacteria.

Inhibition of Salmonella enterica serovar typhimurium motility and entry into epithelial cells by a protective antilipopolysaccharide monoclonal immunoglobulin A antibody

Secretory immunoglobulin A (SIgA) antibodies directed against the O antigen of lipopolysaccharide (LPS) are the primary determinants of mucosal immunity to gram-negative enteric pathogens. However, the underlying mechanisms by which these antibodies interfere with bacterial colonization and invasion of intestinal epithelial cells are not well understood. In this study, we report that Sal4, a protective, anti-O5-specific monoclonal IgA, is a potent inhibitor of Salmonella enterica serovar Typhimurium flagellum-based motility. Using video light microscopy, we observed that Sal4 completely and virtually instantaneously "paralyzed" laboratory and clinical strains of serovar Typhimurium. Sal4-mediated motility arrest preceded and occurred independently of agglutination. Polyclonal anti-LPS IgG antibodies and F(ab)(2) fragments were as potent as was Sal4 at impeding bacterial motility, whereas monovalent Fab fragments were 5- to 10-fold less effective. To determine whether motility arrest can fully account for Sal4's protective capacity in vitro, we performed epithelial cell infection assays in which the requirement for flagellar motility in adherence and invasion was bypassed by centrifugation. Under these conditions, Sal4-treated serovar Typhimurium cells remained noninvasive, revealing that the monoclonal IgA, in addition to interfering with motility, has an effect on bacterial uptake into epithelial cells. Sal4 did not, however, inhibit bacterial uptake into mouse macrophages, indicating that the antibody interferes specifically with Salmonella pathogenicity island 1 (SPI-1)-dependent, but not SPI-1-independent, entry into host cells. These results reveal a previously unrecognized capacity of SIgA to "disarm" microbial pathogens on mucosal surfaces and prevent colonization and invasion of the intestinal epithelium.

Complement factor H-binding protein, a putative virulence determinant of Borrelia hermsii, is an antigenic target for protective B1b lymphocytes

Vaccination is the most effective way to control infectious diseases. A variety of microbial pathogens use antigenic variation, an immune evasion strategy that poses a challenge for vaccine development. To understand protective immune responses against such pathogens, we have been studying Borrelia hermsii, a bacterium that causes recurrent bacteremia due to antigenic variation. An IgM response is necessary and sufficient to control B. hermsii infection. We have recently found a selective expansion of B1b cells concurrent with the resolution of B. hermsii bacteremia. B1b cells from convalescent but not naive mice confer long-lasting immunity, but the Ag(s) driving the protective IgM responses is unknown. Herein we demonstrate that convalescent B1b cell-derived IgM recognizes complement factor H-binding protein (FhbA), a B. hermsii outer-surface protein and putative virulence factor that does not undergo antigenic variation and is expressed by all clinical isolates. A progressive increase in the IgM response to FhbA correlated with the kinetics of B1b cell expansion, diminished the severity of bacteremic episodes, and led to the eventual resolution of the infection. These data indicate that FhbA is a specific target for protective B1b cell responses. Ags recognized by B1b cells may be considered as an important component in vaccination strategies.

Catalytic antibodies to amyloid beta peptide in defense against Alzheimer disease

Immunoglobulins (Igs) that bind amyloid beta peptide (Abeta) are under clinical trials for immunotherapy of Alzheimer disease (AD). We have identified IgMs and recombinant Ig fragments that hydrolyze Abeta. Hydrolysis of peripheral Abeta by the IgMs may induce increased Abeta release from the brain. The catalytic IgMs are increased in AD patients, presumably reflecting a protective autoimmune response. Reduced Abeta aggregation and neurotoxicity attributable to the catalytic function were evident. These findings provide a foundation for development of catalytic Igs for AD immunotherapy.

Fc receptor genes and the systemic lupus erythematosus diathesis

Fc receptors represent a distinct group of hematopoeitic cell surface glycoproteins that have a characterized role in affecting the efficiency of the mononuclear phagocyte system to clear IgG immune complexes. Functional genetic variations in this family of receptors have been identified as heritable susceptibility factors for SLE and lupus nephritis across diverse populations. In this review, we describe the roles of the classical Fc receptors for IgG (Fc gamma) and non-classical Fc-like receptors (FCR1-FCRL6L), Fc receptors for IgE (Fc epsilon RI) and IgA and IgM (Fc alpha/mu R) in SLE diathesis. The combined effects of these genes on SLE pathogenesis, either via linkage disequilibrium or epistasis with additional genetic or environmental factors, provide a challenge for future investigations. The pursuit of a polygenic SLE-profile that includes longitudinal evaluations of SLE and markers involved in the protean clinical manifestations associated with SLE will facilitate our understanding of the cascade of inflammatory events associated with the diathesis.

The glomerular response to IgA deposition in IgA nephropathy

Compelling evidence points to a role for IgA receptors in the pathogenesis of IgA nephropathy. The soluble form of the type I IgA receptor (FcalphaRI or CD89) forms complexes with IgA that can be found in patients' serum and that initiate the disease in CD89 transgenic mice. A nonclassic IgA receptor, identified as the transferrin receptor (TfR), is highly expressed in patients' mesangium and colocalizes with IgA deposits. TfR preferentially binds polymeric IgA1 complexes, but not monomeric IgA1 or IgA2. The TfR-IgA1 interaction is dependent on carbohydrate moieties because hypoglycosylated IgA1 has superior binding to TfR than normally glycosylated IgA1. Polymeric IgA1 binding enhances mesangial cell TfR expression and results in cell proliferation and inflammatory and profibrogenic cytokine and chemokine production, suggesting a pivotal role in mesangial cell proliferation, matrix expansion, and recruitment of inflammatory cells. We propose that, as a second event, activation of the classic, FcRgamma-associated transmembrane FcalphaRI expressed on circulating myeloid leukocytes takes place. FcalphaRI/gamma2 cross-linking in human FcalphaRI transgenic animals promotes disease progression by enhancing leukocyte chemotaxis and cytokine production, and IgA immune complexes from IgA nephropathy patients induce FcalphaRI-dependent cell activation. This review therefore details the functional consequences of IgA/receptor interactions and discusses proposed mechanisms to explain the development and chronicity of the disease.

Unusual biochemical features and follicular dendritic cell expression of human Fcalpha/mu receptor

The Fc receptor for IgA and IgM (Fcalpha/muR) is of particular interest because it can bind antibodies of both IgM and IgA isotypes and thus may play a pivotal role in systemic and mucosal immunity. Using IgM and IgA ligands and newly generated Fcalpha/muR specific monoclonal antibodies we have defined biochemical features and cellular distribution of the human Fcalpha/muR. Both recombinant and native forms of human Fcalpha/muR are expressed on the cell surface as remarkably stable homodimeric transmembrane glycoproteins that can bind specifically polymeric IgM or IgA. The only human B cells to express Fcalpha/muR, albeit at very low levels, are found in the pre-germinal center subpopulation defined by the IgD+/CD38+ phenotype. Hence the expression pattern differs from that of the mouse wherein Fcalpha/muR is expressed by both circulating and resident B cell populations. Significantly, the predominant cell type expressing the Fcalpha/muR in humans is the follicular dendritic cell of germinal centers. The Fcalpha/muR may thus function in antigen presentation and B cell selection in the germinal center response.

Secretory IgA mediates bacterial translocation to dendritic cells in mouse Peyer's patches with restriction to mucosal compartment

In addition to fulfilling its function of immune exclusion at mucosal surfaces, secretory IgA (SIgA) Ab exhibits the striking feature to adhere selectively to M cells in the mouse and human intestinal Peyer's patches (PPs). Subsequent uptake drives the SIgA Ab to dendritic cells (DCs), which become partially activated. Using freshly isolated mouse DCs, we found that the interaction with SIgA was tissue and DC subtype dependent. Only DCs isolated from PPs and mesenteric lymph nodes interacted with the Ab. CD11c(+)CD11b(+) DCs internalized SIgA, while CD11c(+)CD19(+) DCs only bound SIgA on their surface, and no interaction occurred with CD11c(+)CD8alpha(+) DCs. We next examined whether SIgA could deliver a sizeable cargo to PP DCs in vivo by administering SIgA-Shigella flexneri immune complexes into a mouse ligated intestinal loop containing a PP. We found that such immune complexes entered the PPs and were internalized by subepithelial dome PP DCs, in contrast to S. flexneri alone that did not penetrate the intestinal epithelium in mice. Dissemination of intraepithelial S. flexneri delivered as immune complexes was limited to PPs and mesenteric lymph nodes. We propose that preexisting SIgA Abs associated with microbes contribute to mucosal defense by eliciting responses that prevent overreaction while maintaining productive immunity.

Protective immune mechanisms in helminth infection

Important insights have recently been gained in our understanding of how host immune responses mediate resistance to parasitic helminths and control associated pathological responses. Although similar cells and cytokines are evoked in response to infection by helminths as diverse as nematodes and schistosomes, the components of the response that mediate protection are dependent on the particular parasite. In this Review, we examine recent findings regarding the mechanisms of protection in helminth infections that have been elucidated in murine models and discuss the implications of these findings in terms of future therapies.

Elevated levels of endogenous apoptotic DNA and IFN-alpha in complement C4-deficient mice: implications for induction of systemic lupus erythematosus

Systemic lupus erythematosus (SLE), an autoimmune disease characterized by chronic nephritis, arthritis and dermatitis, and the presence of antinuclear autoantibodies, is associated with complement factor deficiencies in the classical activation pathway. In addition, IFN-alpha seems to be a key cytokine in SLE as an activated IFN-alpha system is regularly observed in patients with SLE. Here, we demonstrate that in lupus-susceptible, complement C4-deficient mice the lack of complement results in elevated intravascular levels of apoptotic DNA. The apoptotic DNA is targeted to the splenic marginal zone where it accumulates and induces IFN-alpha. As such, we present here a unifying hypothesis for the induction of SLE that incorporates the role of complement deficiency and elevated levels of IFN-alpha.

Molecular genetic analysis of two loci (Ity2 and Ity3) involved in the host response to infection with Salmonella typhimurium using congenic mice and expression profiling

Numerous genes have been identified to date that contribute to the host response to systemic Salmonella Typhimurium infection in mice. We have previously identified two loci, Ity2 and Ity3, that control survival to Salmonella infection in the wild-derived inbred MOLF/Ei mouse using a (C57BL/6J x MOLF/Ei)F(2)cross. We validated the existence of these two loci by creating congenic mice carrying each quantitative trait locus (QTL) in isolation. Subcongenic mice generated for each locus allowed us to define the critical intervals underlying Ity2 and Ity3. Furthermore, expression profiling was carried out with the aim of identifying differentially expressed genes within the critical intervals as potential candidate genes. Genomewide expression arrays were used to interrogate expression differences in the Ity2 congenics, leading to the identification of a new candidate gene (Havcr2, hepatitis A virus cellular receptor 2). Interval-specific oligonucleotide arrays were created for Ity3, identifying one potential candidate gene (Chi3l1, chitinase 3-like 1) to be pursued further. The combination of the use of congenics in QTL confirmation and fine mapping and in the identification of candidate genes by expression profiling has been successful and represents a step toward quantitative gene(s) identification.

IgEb immune complexes activate macrophages through FcgammaRIV binding

Because functional analysis of Fc receptors (FcRs) relies heavily on mouse models, the identification of another Fcgamma receptor is particularly noteworthy. We demonstrate that FcgammaRIV, identified here as the mouse ortholog of primate FcgammaRIII, required association of the FcR gamma-chain for optimal expression and function on myeloid cells; its signaling potential was also enhanced by a cytoplasmic 'YEEP' motif that was able to recruit the adaptor molecule Crk-L and phosphatidylinositol-3-OH kinase. Unexpectedly, FcgammaRIV 'preferentially' bound immunoglobulin E antibodies of the 'b' allotype (IgE(b)) as well as IgG2a and IgG2b antibodies. Ligation of FcgammaRIV by antigen-IgE(b) immune complexes promoted macrophage-mediated phagocytosis, presentation of antigen to T cells, production of proinflammatory cytokines and the late phase of cutaneous allergic reactions. IgE(b) antibody-mediated modification of macrophage responses may therefore influence mouse asthma models and strain-dependent differences in parasite susceptibility.

Roundtrip ticket for secretory IgA: role in mucosal homeostasis?

An important activity of mucosal surfaces is the production of Ab referred to as secretory IgA (SIgA). SIgA serves as the first line of defense against microorganisms through a mechanism called immune exclusion. In addition, SIgA adheres selectively to M cells in intestinal Peyer's patches, thus mediating the transepithelial transport of the Ab molecule from the intestinal lumen to underlying gut-associated organized lymphoid tissue. In Peyer's patches, SIgA binds and is internalized by dendritic cells in the subepithelial dome region. When used as carrier for Ags in oral immunization, SIgA induces mucosal and systemic responses associated with production of anti-inflammatory cytokines and limits activation of dendritic cells. In terms of humoral immunity at mucosal surfaces, SIgA appears thus to combine properties of a neutralizing agent (immune exclusion) and of a mucosal immunopotentiator inducing effector immune responses in a noninflammatory context favorable to preserve local homeostasis of the gastrointestinal tract.

IgM and IgA anti-erythrocyte autoantibodies induce anemia in a mouse model through multivalency-dependent hemagglutination but not through complement activation

By generating IgM and IgA switch variants of the 34-3C IgG2a anti-red blood cell (RBC) autoantibody, we evaluated the pathogenic activity of these 2 isotypes in view of the Fc-associated effector functions (ie, complement activation and polyvalency-dependent agglutination). We found that polymeric forms of 34-3C IgM and IgA anti-RBC autoantibody were as pathogenic as IgG2a, which was the most pathogenic among 4 different IgG subclasses, whereas their monomeric variants completely lacked pathogenic effects. Histological examination showed that 34-3C IgM and IgA autoantibodies caused anemia as a result of multivalency-dependent hemaggultination and subsequent sequestration of RBC in the spleen, in contrast to Fc receptor- and complement receptor-mediated erythrophagocytosis by Kupffer cells with IgG isotypes. In addition, the development of anemia induced by IgM and IgA isotypes of 34-3C antibody and by 2 additional IgM anti-RBC monoclonal autoantibodies was not inhibited at all in C3-deficient mice, indicating the lack of involvement of complement activation in the pathogenesis of IgM- and IgA-induced anemia. Our data demonstrate a remarkably high pathogenic potential of polymeric forms of IgM and IgA anti-RBC autoantibodies due to their ability to induce hemagglutination but completely independent of complement activation.

Role of CD7 expressed in lung microvascular endothelial cells as Fc receptor for immunoglobulin M

Immunoglobulins (Igs) against anti-human white blood cells are putative contributors to the development of transfusion-related adverse effects, particularly transfusion-related acute lung injury (TRALI). Studies of Igs that are considered to be implicated in transfusion-related adverse effects have mainly focused on immunoglobulin G (IgG) class antibodies (Abs). In the authors' previous in vitro study, the association of polymorphonuclear neutrophils (PMNs) and lung microvascular endothelial (LME) cells was up-regulated in the presence of normal human serum-derived IgMs, when F(ab')2 fragments of IgMs were specific to low-affinity Fc receptors (FcR) for IgG, namely, Fcgamma R III (CD16) and Fcgamma RII (CD32). In this study, the authors found that CD7 antigen notably expresses in LME cells and that it acts as an Fc receptor for IgM in LME cells.

Identification of poly-reactive natural IgM antibody that recognizes late apoptotic cells and promotes phagocytosis of the cells

Natural IgM can recognize apoptotic cells, but the molecular structure and the role in macrophage phagocytosis of apoptotic cells remain unclear.

OBJECTIVES

(1) To examine the binding of previously isolated natural IgM (3B4) to apoptotic cells and its effects on phagocytosis of apoptotic cells. (2) To characterize the molecular structure of 3B4.

METHODS

3B4 binding to apoptotic thymocytes was examined by flow cytometry. Polyreactivity of 3B4 was assayed by ELISA. PKH26-labeled Macrophages were incubated with PKH67-stained apoptotic cells in the presence of 3B4. Macrophages phagocytosis of apoptotic cell was evaluated by flow cytometry. The DNA segments of 3B V(H) and V(K) were sequenced and analyzed.

RESULTS

3B4 IgM recognized late apoptotic cells. Polyreactive-recognitions of lysophosphatidylcholine (LPC) as well as some autoantigens were observed in 3B4. Phagocytosis of late apoptotic cells was increased in the presence of 3B4. The V(H) and V(K) genes of 3B4 showed a germline gene context, while N-sequences and nucleotide loss were observed in CDR3.

CONCLUSION

3B4 promotes macrophage phagocytosis of late apoptotic cells in a complement-independent process. 3B4 has a germline configuration and is possibly ligand-selected. Out experiments suggest an independent role of natural IgM as opsonin in clearance of late apoptotic cells.

Channel catfish leukocyte immune-type receptors contain a putative MHC class I binding site

The recent identification of a large and diverse family of leukocyte immune-type receptors (IpLITRs) in channel catfish (Ictalurus punctatus) indicates that immunoglobulin superfamily (IgSF) members related to both mammalian Fc receptors (FcRs) and leukocyte receptor complex (LRC)-encoded proteins exist in fish. In the present study, it was found that IpLITR messages were preferentially up regulated in catfish peripheral blood leukocytes (PBL) and clonal cytotoxic T cells (CTL) after alloantigen stimulation. Detailed sequence analyses of the expressed IpLITR cDNAs from two clonal CTL lines indicated an unexpectedly large array of putative activatory and inhibitory IpLITR-types containing variable numbers of extracellular immunoglobulin (Ig)-like domains. Importantly, all expressed IpLITRs shared similar membrane distal Ig domains (i.e., D1 and D2), suggesting that they may bind a common type of ligand. Sequence alignments and comparative homology modeling revealed that IpLITR domains, D1 and D2, have similar predicted 3-D structural properties with the corresponding domains of the human LRC-encoded leukocyte Ig-like receptor (LILR) family. Furthermore, conservation of key major histocompatibility class I (MHC I)-binding residues were located at similar positions within the membrane distal tip of D1 between representative IpLITRs and group 1 LILRs. Taken together, these results suggest that fish LITRs have an orthologous relationship to LRC-encoded receptors such as the human LILRs and could potentially function as a diverse family of MHC class I-binding receptors.

Molecular and functional characteristics of the Fcα/μR, a novel Fc receptor for IgM and IgA

IgM is the first antibody to be produced in a humoral immune response and is a major isotope of natural antibodies and may play an important role in innate immunity. On the other hand, IgA is a secreted antibody at the mucosal membrane such as the gastrointestinal and respiratory tracts and protects from initial invasion of microbes. However, how these antibodies are involved in immunity has been poorly elucidated. We previously identified a novel Fc receptor for IgA and IgM, designated Fcalpha/mu receptor (Fcalpha/muR), whose gene is closely located at the polymeric immunoglobulin receptor (poly-IgR), also a receptor for IgA and IgM, in the Fc receptor gene cluster on the chromosome 1. In contrast to the the poly-IgR that is expressed on epithelial, but not hematopoietic, cells, Fcalpha/muR is constitutively expressed on the majority of B lymphocytes and macrophages in the spleen and at the center of the secondary lymphoid follicles. The Fcalpha/muR mediates endocytosis Staphylococcus aureus /anti-S. aureus IgM antibody immune complexes by B lymphocytes, for which the dileucine motif in the cytoplasmic tail of the mouse Fcalpha/muR is responsible. These results reveal a new mechanism in the primary stage of immune defense against microbes.

IgA and IgA-specific receptors in human disease: structural and functional insights into pathogenesis and therapeutic potential

IgA antibodies play an important role in humoral immunity. IgA is the predominant antibody in mucosal secretions and the second most prevalent in the serum. It occupies a unique position among human antibodies in that it can both trigger and suppress inflammatory responses, depending on the situation. Recent structural and functional studies have revealed details of the structure of IgA and its interaction with key cell-surface receptors. We look at the role IgA and IgA receptors (particularly FcalphaRI) play in the pathogenesis of diseases such as IgA nephropathy and other autoimmune conditions. Finally, we address the potential of IgA as a therapeutic tool to either trigger specific inflammatory responses to destroy target cells or suppress inflammatory responses in the case of autoimmune diseases, and the promise of mucosal vaccines for eliciting specific IgA responses to pathogens in mucosal environments.

Structural insights into antibody-mediated mucosal immunity

The mucosal regions of the body are responsible for defense against environmental pathogens. Particularly in the lumen of the gut, antibody-mediated immune responses are critical for preventing invasion by pathogens. In this chapter, we review structural studies that have illuminated various aspects of mucosal immunity. Crystal structures of IgA1-Fc and IgA-binding fragments of the polymeric immunoglobulin receptor and Fc alphaRI, combined with models of intact IgA and IgM from solution scattering studies, reveal potential mechanisms for immune exclusion and induction of inflammatory responses. Other recent structures yield insights into bacterial mechanisms for evasion of the host immune response.

Host defence againstC. albicansinfections in IgH transgenic mice with VHderived from a natural anti-keratin antibody

Fungal infections have been increasing and life-threatening in recent years, but host immune responses, especially the humoral immunity, to fungi have not been fully understood. In the present study, we report that natural antibodies from unimmunized mice bind to Candida albicans. We established a monoclonal natural antibody, 3B4, which recognized a surface antigen located at germ tubes of C. albicans. The 3B4 antibody protected mice from C. albicans-induced death in passive immunization, by mechanisms involving suppressing germ tube formation and modulating phagocytosis. Interestingly, 3B4 also bound to a self-antigen keratin. To further study the generation and anti-C. albicans activities of natural antibodies in vivo, we constructed a mu chain transgenic mouse (TgV(H)3B4) using the V(H) gene from 3B4. TgV(H)3B4 had elevated serum anti-keratin/C. albicans IgM, and were resistant to C. albicans infections. Analyses of B cell development showed that in TgV(H)3B4, B cells secreting the anti-keratin/C. albicans antibodies were enriched in the B1 B cell compartment. Our findings reveal an important role of keratin-reactive natural antibodies in anti-C. albicans immune responses, and suggest that keratin may function in selecting B cells into the B1 B cell compartment, where natural antibodies are made to fight fungal infections.

IgA receptors in health and disease

The varied interaction of the Fc region of IgA with receptors confers this antibody class with many of its unique properties. The epithelial polymeric Ig receptor on mucosal epithelial cells transports polymeric immunoglobulin A (pIgA) produced by mucosal B cells to the mucosal surface where, in complex with the secretory component (SC), this secretory immunoglobulin A (SIgA) excludes the multitude of dietary, environmental, and microbial antigens that continuously bombard the mucosae. In health, this IgA-mediated exclusion not only forms the initial defence against infection, it also spares the systemic immune system from potentially deleterious responses to innocuous antigens which can otherwise culminate in inflammatory bowel disease or asthma. Beyond antigen exclusion, in closer encounters with antigens, IgA receptors play roles in protective immunity and disease. FcaRI is the principal myeloid IgA receptor and is responsible for differing IgA-mediated effector responses such as respiratory burst, degranulation, and phagocytosis variously by granulyoctes, monocytes, and macrophages. Furthermore an unknown IgA receptor specific for the secretory component (SC) elicits powerful effector responses from eosinophils. On dendritic cells, FcaRI participates in antigen presentation while on microfold cells, key cells in mucosal antigen presentation, another unknown IgA receptor functions in the transport of antigens across the mucosal epithelial barrier. The activity of another uncharacterized IgA1/IgD receptor on T cells may affect autoimmune disorders. The interplay of different IgA receptors affects immune complex deposition in the common renal disease immunoglobulin A nephropathy (IgAN). Finally, the therapeutic application of various IgA receptors has been sought in the areas of infectious disease, vaccines, and cancer.

IgM-enriched human intravenous immunoglobulin suppresses T lymphocyte functions in vitro and delays the activation of T lymphocytes in hu-SCID mice

Previous studies of an experimental human immunoglobulin preparation for intravenous use, containing normal pooled IgM (IVIgM), have shown its beneficial therapeutic effect in experimental autoimmune diseases. The mechanisms of its immunomodulatory activity remain however, poorly understood. In the experiments reported here, IVIgM inhibited the proliferation of various autonomously growing human lymphoid cell lines in vitro, as well as of MLR- and of PHA-stimulated human T-lymphocytes. These effects of IVIgM were observed at non-apoptotic concentrations and were stronger on a molar basis than those of normal pooled IgG for intravenous use (IVIg). Both preparations, when administered to SCID mice, repopulated with human peripheral blood mononuclear cells, delayed the expression of the early activation marker CD69 on both human CD4+ and CD8+ T-lymphocytes, activated by the mouse antigenic environment. The data obtained show that normal pooled human IgM exerts a powerful antiproliferative effect on T-cells that is qualitatively similar but quantitatively superior to that of therapeutic IVIg. Our results suggest that infusions with IVIgM might have a significant beneficial immunomodulating activity in patients with selected autoimmune diseases.

Identification and Characterization of a FcR Homolog in an Ectothermic Vertebrate, the Channel Catfish (Ictalurus punctatus)

An FcR homolog (IpFcRI), representing the first such receptor from an ectothermic vertebrate, has been identified in the channel catfish (Ictalurus punctatus). Mining of the catfish expressed sequence tag databases using mammalian FcR sequences for CD16, CD32, and CD64 resulted in the identification of a teleost Ig-binding receptor. IpFcRI is encoded by a single-copy gene containing three Ig C2-like domains, but lacking a transmembrane segment and cytoplasmic tail. The encoded Ig domains of IpFcRI are phylogenetically and structurally related to mammalian FcR and the presence of a putative Fc-binding region appears to be conserved. IpFcRI-related genomic sequences are also present in both pufferfish and rainbow trout, indicating the likely presence of a soluble FcR in other fish species. Northern blot and qualitative PCR analyses demonstrated that IpFcRI is primarily expressed in IgM-negative leukocytes derived from the lymphoid kidney tissues and PBL. Significantly lower levels of IpFcRI expression were detected in catfish clonal leukocyte cell lines. Using the native leader, IpFcRI was secreted when transfected into insect cells and importantly the native IpFcRI glycoprotein was detected in catfish plasma using a polyclonal Ab. Recombinant IpFcRI binds catfish IgM as assessed by both coimmunoprecipation and cell transfection studies and it is presumed that it functions as a secreted FcR akin to the soluble FcR found in mammals. The identification of an FcR homolog in an ectothermic vertebrate is an important first step toward understanding the evolutionary history and functional importance of vertebrate Ig-binding receptors.

Activated microglia stimulate transcriptional changes in primary oligodendrocytes via IL-1beta

No therapy currently exists to repair demyelinated lesions in multiple sclerosis. However, the use of IgM antibodies may provide a valuable therapeutic avenue for evoking such repair. Unfortunately, the mechanism of immunoglobulin action in CNS repair is currently unknown but may depend upon complex interactions between multiple cell types rather than upon direct activation of a single cell type. Using rat mixed glial cultures containing oligodendrocytes, microglia, and astrocytes, we found that the Fc portion of human IgM shifts microglia to an activated phenotype, reduces glial proliferation, upregulates a variety of immediate early genes, including JunB, Egr-1, and c-Fos, and stimulates microglial production and release of IL-1beta. Microglia-derived IL-1beta consequently triggers transcriptional upregulation of immediate early genes such as c-Jun, Egr-1, and c-Fos in the mixed glial cultures, and stimulates the upregulation of late response genes such as lipocalin in purified oligodendrocytes. Treatment with an IL-1beta receptor antagonist abrogates the effects of Fcmu on glial proliferation and prevents the upregulation of lipocalin in response to Fcmu, but does not prevent Fcmu-mediated upregulation of IL-1beta, suggesting that IL-1beta mediates at least some of the downstream effects of Fcmu in mixed glial cultures. We hypothesize that Fcmu-stimulated IL-1beta-induced upregulation of immediate early and late response genes in oligodendrocytes may promote CNS repair.

CD300 antigen like family member G: A novel Ig receptor like protein exclusively expressed on capillary endothelium

We report the characteristics of CD300LG, a member of the CD300 antigen like family. Its genomic structure is similar in both mouse and human, and at least four isoforms exist in both species. The amino acid sequence of the immunoglobulin (Ig) V like domain of CD300LG showed approximately 35% identity to those of the polymeric Ig receptor (pIgR) and Fcalpha/muR. Interestingly, mouse CD300LG proteins were uniquely expressed on capillary endothelium. Immunoelectron microscopy revealed that mouse CD300LG is localized on both apical and basolateral plasma membranes, as well as on intracellular vesicular structures, in the capillary endothelium. Transcytosis assays using polarized MDCK epithelial cells showed that CD300LG could be transcytosed bidirectionally. Furthermore, CD300LG exogenously expressed on HeLa cells could take up IgA2 and IgM, but not IgG. These results suggest that CD300LG might play an important role in molecular traffic across the capillary endothelium.

In search of the elusive mouse macrophage Fc-alpha receptor

Human macrophages express an Fc receptor for IgA (FcalphaR, CD89) but so far no mouse counterpart or an alternative IgA receptor has been found. Given the biological importance of IgA in countering infections, and the extensive use of mouse experimental models for passive and active prophylactic strategies, it is somewhat surprising that this subject has received relatively little attention. So, what do we know so far?

Molecular characteristics of IgA and IgM Fc binding to the Fcalpha/muR

Fcalpha/mu receptor (Fcalpha/muR), a novel Fc receptor for IgA and IgM, is a type I transmembrane protein with an immunoglobulin (Ig)-like domain in the extracellular portion. Although IgA and IgM bind to Fcalpha/muR, the molecular and structural characteristics of the ligand-receptor interactions have been undetermined. Here, we developed twelve monoclonal antibodies (mAbs) against murine Fcalpha/muR by immunizing mice deficient in Fcalpha/muR gene. Eight mAbs totally or partially blocked IgA and IgM bindings to Fcalpha/muR. These blocking mAbs bound to a peptide derived from the Ig-like domain of murine Fcalpha/muR, which is conserved not only in human and rat Fcalpha/muR but also in polymeric Ig receptor (poly-IgR), another Fc receptor for IgA and IgM. These results suggest that IgA and IgM bind to an epitope in the conserved amino acids in the Ig-like domain of Fcalpha/muR as well as poly-IgR.

Ancient divergence of a complex family of immune-type receptor genes

Multigene families of activating/inhibitory receptors belonging to the immunoglobulin superfamily (IgSF) regulate immunological and other cell-cell interactions. A new family of such genes, termed modular domain immune-type receptors (MDIRs), has been identified in the clearnose skate (Raja eglanteria), a phylogenetically ancient vertebrate. At least five different major forms of predicted MDIR proteins are comprised of four different subfamilies of IgSF ectodomains of the intermediate (I)- or C2-set. The predicted number of individual IgSF ectodomains in MDIRs varies from one to six. MDIR1 contains a positively charged transmembrane residue and MDIR2 and MDIR3 each possesses at least one immunoreceptor tyrosine-based inhibitory motif in their cytoplasmic regions. MDIR4 and MDIR5 lack characteristic activating/inhibitory signalling motifs. MDIRs are encoded in a particularly large and complex multigene family. MDIR domains exhibit distant sequence similarity to mammalian CMRF-35-like molecules, polymeric immunoglobulin receptors, triggering receptors expressed on myeloid cells (TREMs), TREM-like transcripts, NKp44 and FcR homologs, as well as to sequences identified in several different vertebrate genomes. Phylogenetic analyses suggest that MDIRs are representative members of an extended family of IgSF genes that diverged before or very early in evolution of the vertebrates and subsequently came to occupy multiple, fully independent distributions in the present day.

CR3 (CD11b/CD18) is the major macrophage receptor for IgM antibody-mediated phagocytosis of African trypanosomes: Diverse effect on subsequent synthesis of tumor necrosis factor α and nitric oxide

Immunoglobulin M (IgM) antibodies to the variant surface glycoproteins (VSG) of African trypanosomes are the first and predominant class of anti-trypanosomal antibodies in the infected host. They are a major factor in controlling waves of parasitemia, but not in long-term survival. The macrophage receptor(s) that enables phagocytosis of IgM anti-VSG-coated African trypanosomes is unknown. We assessed whether complement receptor CR3 (CD11b/CD18) might be involved in mediating phagocytosis of Trypanosoma congolense. We show that murine complement C3 fragments are deposited onto T. congolense when the trypanosomes are incubated with IgM anti-VSG and fresh mouse serum. In the presence of fresh mouse serum, there is significantly and markedly less phagocytosis of IgM-opsonized T. congolense by CD11b-deficient macrophages compared to phagocytosis by wild-type macrophages (78% fewer T. congolense are ingested per macrophage). Significantly less tumor necrosis factor (TNF)-alpha (38% less), but significantly more nitric oxide (NO) (63% more) are released by CD11b-deficient macrophages that have engulfed trypanosomes than by equally treated wild-type macrophages. We conclude that CR3 is the major, but not the only, receptor involved in IgM anti-VSG-mediated phagocytosis of T. congolense by macrophages. We further conclude that IgM anti-VSG-mediated phagocytosis of T. congolense enhances synthesis of disease-producing TNF-alpha and inhibits synthesis of parasite-controlling NO. We suggest that signaling of inhibition of NO synthesis is mediated via CR3.

Antiviral antibody responses: the two extremes of a wide spectrum

Viruses elicit a diverse spectrum of antiviral antibody responses. In this review, we discuss two widely used experimental model systems for viral infections - non-cytopathic lymphocytic choriomeningitis virus (LCMV) and acutely cytopathic vesicular stomatitis virus (VSV) - to analyse two fundamentally different types of antiviral antibody response. The basic principles found in these model infections are discussed in the context of other viral infections, and with regard to protective neutralizing versus non-protective enzyme-linked immunosorbent assay (ELISA)-detected antibody responses. Issues of antibody specificity, affinity and avidity, maturation and escape are discussed in the context of co-evolution of the host and viruses.

Aggregated IgG inhibits the differentiation of human fibrocytes

Fibrocytes are fibroblast-like cells, which appear to participate in wound healing and are present in pathological lesions associated with asthma, pulmonary fibrosis, and scleroderma. Fibrocytes differentiate from CD14+ peripheral blood monocytes, and the presence of serum delays this process dramatically. We previously purified the factor in serum, which inhibits fibrocyte differentiation, and identified it as serum amyloid P (SAP). As SAP binds to Fc receptors for immunoglobulin G (IgG; Fc gammaRs), Fc gammaR activation may be an inhibitory signal for fibrocyte differentiation. Fc gammaR are activated by aggregated IgG, and we find aggregated but not monomeric, human IgG inhibits human fibrocyte differentiation. Monoclonal antibodies that bind to Fc gammaRI (CD64) or Fc gammaRII (CD32) also inhibit fibrocyte differentiation. Aggregated IgG lacking Fc domains or aggregated IgA, IgE, or IgM do not inhibit fibrocyte differentiation. Incubation of monocytes with SAP or aggregated IgG inhibited fibrocyte differentiation. Using inhibitors of protein kinase enzymes, we show that Syk- and Src-related tyrosine kinases participate in the inhibition of fibrocyte differentiation. These observations suggest that fibrocyte differentiation can occur in situations where SAP and aggregated IgG levels are low, such as the resolution phase of inflammation.

Antibody- and Fc-receptor-based therapeutics for malaria

Abs (antibodies) are complex glycoproteins that play a crucial role in protective immunity to malaria, but their effectiveness in mediating resistance can be enhanced by genetically engineered modifications that improve on nature. These Abs also aid investigation of immune mechanisms operating to control the disease and are valuable tools in developing neutralization assays for vaccine design. This review explores how this might be achieved.