Soluble Fc gamma receptors: interaction with ligands and biological consequences

Cathepsin S Is More Abundant in Serum of Mycobacterium avium subsp. paratuberculosis-Infected Dairy Cows

Mycobacterium avium subsp. paratuberculosis (MAP) is the causative agent of bovine paratuberculosis, a chronic granulomatous enteritis leading to economic losses and posing a risk to human health due to its zoonotic potential. The pathogen cannot reliably be detected by standard methods, and immunological procedures during the infection are not well understood. Therefore, the aim of our study was to explore host-pathogen interactions in MAP-infected dairy cows and to improve diagnostic tests. Serum proteomics analysis using quantitative label-free LC-MS/MS revealed 60 differentially abundant proteins in MAP-infected dairy cows compared to healthy controls from the same infected herd and 90 differentially abundant proteins in comparison to another control group from an uninfected herd. Pathway enrichment analysis provided new insights into the immune response to MAP and susceptibility to the infection. Furthermore, we found a higher abundance of Cathepsin S (CTSS) in the serum of MAP-infected dairy cows, which is involved in multiple enriched pathways associated with the immune system. Confirmed with Western blotting, we identified CTSS as a potential biomarker for bovine paratuberculosis. This study enabled a better understanding of procedures in the host-pathogen response to MAP and improved detection of paratuberculosis-diseased cattle.

Association between FCGR3B copy number variations and susceptibility to autoimmune diseases: a meta-analysis

OBJECTIVE

This study determined whether FCGR3B copy number variations (CNVs) were associated with susceptibility to autoimmune diseases.

METHODS

A meta-analysis was conducted to determine the association between FCGR3B CNVs and susceptibility to autoimmune diseases by comparing low FCGR3B CN (<2 to ≥2) and high FCGR3B CN (>2 to ≤2).

RESULTS

In all, 28 comparative studies from 15 reports involving 12,160 patients and 11,103 controls were included in this meta-analysis. The meta-analysis showed a significant association between low FCGR3B CN and autoimmune diseases (OR=1.496, 95% CI=1.301-1.716, p=1.0×10(-9)). Subgroup analysis according to ethnicity indicated an association between low FCGR3B CN and autoimmune diseases in Caucasians (OR=1.482, 95% CI=1.219-1.801, p=7.7×10(-6)) and Asians (OR=1.498, 95% CI=1.306-1.717, p=1.0×10(-9)). Meta-analysis according to the type of autoimmune disease indicated a significant association of low FCGR3B CN with systemic lupus erythematosus (SLE; OR=1.797, 95% CI=1.562-2.068, p<1.0×10(-9)), primary Sjogren's syndrome (pSS; OR=2.263, 95% CI=1.316-3.892, p=0.003), and Wegener's granulomatosis (WG; OR=1.973, 95% CI=1.178-3.302, p=0.010), but not with rheumatoid arthritis (RA; OR=1.333, 95% CI=0.947-1.877, p=0.099). However, the meta-analysis showed no association between high FCGR3B CN and SLE, RA, pSS, and WG.

CONCLUSIONS

Thus, the results of this meta-analysis indicated that low FCGR3B CN increased susceptibility to autoimmune diseases, especially SLE, pSS, and WG.

Targeting the Fc receptor in autoimmune disease

INTRODUCTION

The Fc receptors (FcRs) and their interactions with immunoglobulin and innate immune opsonins, such as C-reactive protein, are key players in humoral and cellular immune responses. As the effector mechanism for some therapeutic monoclonal antibodies, and often a contributor to the pathogenesis and progression of autoimmunity, FcRs are promising targets for treating autoimmune diseases.

AREAS COVERED

This review discusses the nature of different FcRs and the various mechanisms of their involvement in initiating and modulating immunocyte functions and their biological consequences. It describes a range of current strategies in targeting FcRs and manipulating their interaction with specific ligands, while presenting the pros and cons of these approaches. This review also discusses potential new strategies including regulation of FcR expression and receptor crosstalk.

EXPERT OPINION

FcRs are appealing targets in the treatment of inflammatory autoimmune diseases. However, there are still knowledge limitations and technical challenges, the most important being a better understanding of the individual roles of each of the FcRs and enhancement of the specificity in targeting particular cell types and specific FcRs.

FcgammaR polymorphisms: Implications for function, disease susceptibility and immunotherapy

Leukocyte Fcgamma receptors (FcgammaR) confer potent cellular effector functions to the specificity of IgG. FcgammaR-induced leukocyte functions, including antibody-dependent cellular cytotoxicity, phagocytosis, superoxide generation, degranulation, cytokine production and regulation of antibody production, are essential for host defense and immune regulation. The efficacy of IgG-induced FcgammaR function displays inter-individual heterogeneity due to genetic polymorphisms of three FcgammaR subclasses, FcgammaRIIa (CD32a), FcgammaRIIIa (CD16a), and FcgammaRIIIb (CD16b). FcgammaR polymorphisms have been associated with infectious and autoimmune disease, or with disease severity. FcgammaR polymorphisms may furthermore serve as markers for therapeutic efficacy and side-effects of treatment with monoclonal antibodies. In this review, FcgammaR function and the relevance of FcgammaR polymorphisms as prognostic markers for inflammatory disease and antibody-based immunotherapy are discussed.

Fc alpha RI/CD89 circulates in human serum covalently linked to IgA in a polymeric state

The FcR for IgA CD89/FcalphaRI, is a type I receptor glycoprotein, expressed on myeloid cells, with important immune effector functions. In vitro CD89 can be released from CD89-expressing cells upon activation. Little information is available on the existence of this soluble molecule in vivo. Using specific and sensitive ELISA techniques (detection limit 50 pg/ml), we were not able to detect circulating CD89 in human sera. However, using Western blotting, a 30-kDa soluble CD89 molecule was demonstrated in both serum and plasma. Moreover, using a specific semiquantitative dot-blot system, we found CD89 in all human sera tested (mean concentration 1900 ng/ml). Size fractionation of human serum using gel filtration chromatography showed that the CD89 molecule was predominantly present in larger molecular mass fractions. Direct complexes between IgA and CD89 were demonstrated by anti-IgA affinity purification, and when analyzed under nonreducing conditions appeared to be covalently linked. Size fractionation of affinity-purified IgA showed the presence of soluble CD89 only in the high molecular mass fractions of IgA, but not in monomeric IgA. High molecular mass complexes of CD89-IgA could be distinguished from J chain containing dimeric IgA. These data show that CD89 circulates in complex with IgA, and suggest that CD89 might contribute to the formation of polymeric serum IgA.

Soluble Fcgamma receptor IIIb alters the function of polymorphonuclear neutrophils but extends their survival

We have established that polymorphonuclear neutrophil (PMN)-binding anti-Fcgamma receptor IIIb (FcgammaRIIIb) autoantibodies (autoAb) inhibit the function of these cells but extend their survival. Here, we show that recombinant FcgammaRIIIb (rFcgammaRIIIb), as well as purified FcgammaRIIIb (pFcgammaRIIIb), deteriorated the PMN adherence and respiratory burst in a dose-dependent manner. Furthermore, rFcgammaRIIIb and pFcgammaRIIIb reduced the level of annexin V-binding PMN from 23.6 +/- 1.6 % to 6.3 +/- 1.0 and 11.0 +/- 1.0 %, respectively, while human serum albumin exerted no effects. Incubation of rFcgammaRIIIb with those autoAb binding to soluble FcgammaRIIIb resulted in the attachment of such immune complexes (IC) to the cells, thereby also delaying apoptosis (44.9 +/- 5.9 versus 18.0 +/- 2.0 % annexin V-binding PMN after 16 hours). Soluble FcgammaRIIIb, in concert with FcgammaRIIIb / anti-FcgammaRIIIb IC, produced similar effects in that the percentage of annexin V-binding PMN declined to 16.0 +/-1.9 %. It was thus suggested that FcgammaRIIIb / anti-FcgammaRIIIb IC inserted the Fc region of their IgG into the membrane FcgammaRIIIb. Such an interpretation is consistent with our finding that, whereas aggregated IgG and anti-FcgammaRIIIb monoclonal Ab prevented membrane FcgammaRIIIb / IC interaction, neither soluble FcgammaRIIIb, nor anti-cgammaRII did so. We conclude that the function and the life span of PMN are influenced synergistically by soluble FcgammaRIIIb and anti-FcgammaRIIIb autoAb.

[Structural basis of the interaction between immunoglobulins and Fc receptors provided by NMR spectroscopy]

Fc gamma Receptors (Fc gamma R) are membrane glycoproteins that bind the Fc portion of immunoglobulin G (IgG). The cross linking of Fc gamma R-bound IgG by multivalent antigens allows clustering of the Fc gamma R and initiates a variety of effector mechanisms which play a key role in immune defenses against pathogens. The Fc region is composed of two identical polypeptide chains, which are related to each other by a two-fold axis. Recent elucidation of the crystal structure of human Fc gamma RII provided two distinct views of modes of IgG-Fc gamma R interactions, which is controversial against each other. Nuclear magnetic resonance (NMR) spectroscopy provides a unique and irreplaceable tool to solve these issues. We recently studied the interaction between the Fc fragment of mouse IgG2b and the extracellular domain of mouse Fc gamma RII by this method. We showed that Fc gamma RII binds to a negatively charged area of the CH2 domain, corresponding to the lower hinge region, and that the binding of Fc gamma RII onto one of the two symmetrically related sites on the Fc induces a conformational change in the other site. This conformational change may account for the 1:1 stoichiometry that we and others observed between Fc gamma R and Fc. We therefore propose a model that explains why the interaction between IgG molecules and Fc gamma R does not trigger cellular responses in the absence of cross linking by multivalent antigens and does not lead to spontaneous inflammatory responses that would be deleterious for the organism.

Human receptors for immunoglobulin G: key elements in the pathogenesis of rheumatic disease

The structural diversity of Fc gammaR provides a mechanism by which IgG can elicit a broad range of cell responses. Fc gammaR vary in their affinity for IgG, their preference for IgG subclasses, the cell types in which they are expressed, and the intracellular signals which they elicit--stimulatory or inhibitory. Expansion in our knowledge of structure-function relationships among Fc gammaR has identified them as heritable risk factors for disease susceptibility and valuable targets for therapeutic modulation of the immune system.

The 3.2-A crystal structure of the human IgG1 Fc fragment-Fc gammaRIII complex

The immune response depends on the binding of opsonized antigens to cellular Fc receptors and the subsequent initiation of various cellular effector functions of the immune system. Here we describe the crystal structures of a soluble Fc gamma receptor (sFc gammaRIII, CD16), an Fc fragment from human IgG1 (hFc1) and their complex. In the 1:1 complex the receptor binds to the two halves of the Fc fragment in contact with residues of the C gamma2 domains and the hinge region. Upon complex formation the angle between the two sFc gammaRIII domains increases significantly and the Fc fragment opens asymmetrically. The high degree of amino acid conservation between sFc gammaRIII and other Fc receptors, and similarly between hFc1 and related immunoglobulins, suggest similar structures and modes of association. Thus the described structure is a model for immune complex recognition and helps to explain the vastly differing affinities of other Fc gammaR-IgG complexes and the Fc epsilonRI alpha-IgE complex.

Fcalpha receptor (CD89) mediates the development of immunoglobulin A (IgA) nephropathy (Berger's disease). Evidence for pathogenic soluble receptor-Iga complexes in patients and CD89 transgenic mice

The pathogenesis of immunoglobulin A (IgA) nephropathy (IgAN), the most prevalent form of glomerulonephritis worldwide, involves circulating macromolecular IgA1 complexes. However, the molecular mechanism(s) of the disease remain poorly understood. We report here the presence of circulating soluble FcalphaR (CD89)-IgA complexes in patients with IgAN. Soluble CD89 was identified as a glycoprotein with a 24-kD backbone that corresponds to the expected size of CD89 extracellular domains. To demonstrate their pathogenic role, we generated transgenic (Tg) mice expressing human CD89 on macrophage/monocytes, as no CD89 homologue is found in mice. These mice spontaneously developed massive mesangial IgA deposition, glomerular and interstitial macrophage infiltration, mesangial matrix expansion, hematuria, and mild proteinuria. The molecular mechanism was shown to involve soluble CD89 released after interaction with IgA. This release was independent of CD89 association with the FcRgamma chain. The disease was induced in recombination activating gene (RAG)2(-/-) mice by injection of serum from Tg mice, and in severe combined immunodeficiency (SCID)-Tg mice by injection of patients' IgA. Depletion of soluble CD89 from serum abolished this effect. These results reveal the key role of soluble CD89 in the pathogenesis of IgAN and provide an in vivo model that will be useful for developing new treatments.

Regulation of production of soluble Fc gamma receptors type III in normal and pathological conditions

CD16 (Fc gamma R type III), a low affinity IgG Fc receptor, is found in two forms, a transmembrane Fc gamma RIIIa expressed by NK cells and monocytes and a phosphatidylinositol-linked Fc gamma RIIIb present on neutrophils. Exposure of neutrophils to inflammatory signals induces a rapid loss of CD16 expression and release of a soluble form of CD16 (sCD16). Soluble CD16 circulates in plasma, levels being reduced in sera from patients with multiple myeloma. In the present manuscript the authors summarize work that aimed to better understand: (i) the role of proteinases in sCD16 production and CD16 membrane shedding; and (ii) the regulation of sCD16 levels in multiple myeloma patients and the possible biological consequences of its decrease in this disease. Soluble CD16 was purified from human serum. Its N-terminal sequencing demonstrated that it originates from neutrophil CD16 and its C-terminal sequencing showed that the cleavage site was between Val 196 and Ser 197, close to the membrane anchor. Analysis of the effect of protease inhibitors revealed that the cleavage leading to sCD16 production by PMA-activated neutrophils was metalloproteinase-dependent. In addition, membrane and sCD16 were sensitive to serine proteinases released by azurophil granules or added under purified form. The reduction of sCD16 levels that occurs in patients with multiple myeloma was associated with a slight decrease in circulating neutrophils, but not with a significant defect in sCD16 production by neutrophils, as detected in vitro. Moreover, addition of a recombinant sCD16 to plasmocytoma lines did not significantly modify their proliferation and Ig secretion.

Soluble CD16 in plasma cell dyscrasias

Soluble forms of Fc gammaR type III (sFc gammaRIII or sCD16) are present in many biological fluids. Their main ligand is IgG in the form of complexes. In plasma, sCD16 essentially derive from cleavage of membrane CD16 (or Fc gammaRIII) present on neutrophils and, to a lesser extent, on NK cells. Determination of sCD16 serum level during monoclonal gammopathies has demonstrated markedly reduced levels in multiple myeloma and in monoclonal gammopathy of undetermined significance (MGUS) rapidly evolving to multiple myeloma, compared to stable MGUS or controls, indicating a prognostic value for this biological parameter. The biology and functions of sCD16 are described, together with the biological significance of modifications of the sCD16 serum level in monoclonal gammopathies.

N-glycan structures of a recombinant mouse soluble Fcgamma receptor II

N-glycans of a recombinant mouse soluble Fcgamma receptor II (sFcgammaRII) expressed in baby hamster kidney cells were released from glycopeptides by digestion with glycoamidase A (from sweet almond), and the reducing ends of the oligosaccharides were reductively aminated with 2-aminopyridine. The derivatized N-glycans were separated and structurally identified by a three-dimensional high-performance liquid chromatography (HPLC) mapping technique on three kinds of HPLC columns [Takahashi, et al. (1995) Anal. Biochem. 226:139-46]. Eighteen different major N-glycan structures were identified, of which six were neutral (45%), five mono-sialyl (49%), one di-sialyl (4.6%), five tri-sialyl (1.1%), and one tetra-sialyl (0.3%). All N-glycan structures determined were complex type with fucosylation at the N-acetylglucosamine residue of the reducing end, and N-acetylneuraminic acid, when present, was alpha-(2,3)-linked. The existence of a unique structure containing both N-acetylgalactosamine and alpha-(2,3)-N-acetylneuraminic acid residues at the reducing ends, as below, was confirmed by MALDI-TOF mass spectrometry. Carbohydrate sequence [see text]

Identification of the cleavage site involved in production of plasma soluble Fc gamma receptor type III (CD16)

CD16 (FcgammaR type III) is a low-affinity IgG Fc receptor (R) that exists in two isoforms, a transmembrane FcgammaRIIIa expressed by NK cells and monocytes, and a phosphatidylinositol-linked FcgammaRIIIb expressed by neutrophils. A soluble form of CD16 (sCD16) circulates in plasma. The cleavage site and the nature of the enzyme(s) involved in production of sCD16 were investigated. Soluble CD16 was purified to apparent homogeneity from human serum by eight steps, including anion exchange and immunoaffinity chromatography. Serum sCD16 was sequenced at both ends, as well as a recombinant form of sCD16 used as control. N-terminal sequencing demonstrated that serum sCD16 originates from neutrophil FcgammaRIIIb and C-terminal sequencing suggested that the cleavage site is between Val 196 and Ser 197, close to the membrane anchor. Addition of a hydroxamate-based inhibitor of Zn2+ metalloproteinases (RU36156) led to a dramatic decrease of sCD16 production by phorbol 12-myristate 13-acetate-activated neutrophils, whereas inhibitors of serine proteinases had no significant effect, showing the metalloproteinase dependence of this cleavage process.