A dot-immunobinding assay for the demonstration of soluble Fc gamma receptors

Channel catfish soluble FcmuR binds conserved linear epitopes present on Cmu3 and Cmu4

A linear epitope on catfish IgM has been identified as the docking site for the catfish soluble FcmuR (IpFcRI). Western blot analyses and latex bead binding assays identified the consensus octapeptide motif FxCxVxHE located at the second cysteine that forms the intrachain disulfide bond of the catfish Cmu3 and Cmu4 immunolglobulin (Ig) domains as the IpFcRI binding sites. Furthermore, molecular modeling of catfish Cmu3 and Cmu4 confirmed that the octapeptide in both of these domains is accessible for IpFcRI interactions. In addition, since this octapeptide motif is also found in other vertebrate Ig domains, IpFcRI binding to Ig heavy (H) and light (L) chains from rainbow trout, chicken, mouse, rabbit, and goat were examined by Western blot analyses and latex bead binding assays. IpFcRI readily bound reduced rainbow trout (Igmu), chicken (Ignu), mouse (Igmu, Iggamma1, Iggamma2a, Iggamma2b, and Igalpha), rabbit (Igmu and Iggamma) and goat (Iggamma) IgH chains, and mouse Igkappa and Iglambda, and chicken Iglambda IgL chains. IpFcRI also bound mouse IgM, IgA and IgG subclasses when examined under native conditions.

Fc-binding molecules specific for human IgG1 and IgG3 are present in Echinococcus granulosus protoscoleces

In this work we describe the presence of Fc-binding activity on the suckers and tegument of E. granulosus protoscoleces. A fraction (PSA-Fc+) from protoscolex somatic antigens was isolated by affinity chromatography on human Fc-gamma1-Sepharose. Analysis by SDS-PAGE of PSA-Fc+ showed that it contained two major components. Using mouse F(ab')2-human Fc chimaeric monoclonal antibodies we verified that PSA-Fc+ bound mainly to human Fc-gamma1 and Fc-gamma3 isotypes. In addition, one of the components of PSA-Fc+ showed proteolytic activity. Both, Fc-binding and proteolytic activities localized on the protoscolex surface, may play a relevant role in the host-parasite interaction.

Human placental Fc gamma-binding proteins in the maternofetal transfer of IgG

Annexin II, a member of the annexin family of Ca2+ and phospholipid binding proteins, is present in human placenta. Placental annexin II has low affinity FcR activity, and is present as a heterotetramere on syncytiotrophoblast apical cell membrane extracellular surface. In addition to annexin II, transmembraneous leukocyte FcRIII is present on syncytiotrophoblast apical membrane. Either one, or both molecules may mediate the binding of IgG and thereby facilitate its transport through the syncytiotrophoblast layer. However, the presence of other maternal plasma proteins in syncytiotrophoblasts that are not transported to the human fetus is suggestive of nonspecific fluid phase endocytosis. The MHC class I like FcR, similar to the receptor found in neonatal rodent intestine, FcRn, is present intracellularly in human syncytiotrophoblasts, as is its light chain beta 2-microglobulin. The hFcRn is not detected on the apical plasma membrane. The placental hFcRn co-localizes with IgG in syncytiotrophoblast granules. It is likely that hFcRn binds and transcytoses IgG through the syncytiotrophoblast. Protected transfer of IgG may occur within syncytiotrophoblast endocytotic vesicles prior to release in the villous stroma and subsequent translocation into the lumen of fetal stem vessels by uptake and transport in endothelial caveolae.

Co-localization of the neonatal Fc gamma receptor and IgG in human placental term syncytiotrophoblasts

Transfer of maternal IgG through the human placenta furnishes the newborn with passive immunity to a number of infectious agents. The exact mechanism of this transfer is still unknown, but it is agreed that it involves active receptor-mediated transport. The neonatal Fc receptor is a major histocompatibility complex class I-like receptor originally identified in the intestines of newborn rodents. A similar receptor has recently been detected in human placental syncytiotrophoblasts. Using multilabeling fluorescence immunohistochemistry and confocal laser scanning microscopy, we found that the neonatal Fc receptor co-localizes with IgG and beta 2-microglobulin in granules of human placental syncytiotrophoblast. The Fc receptor is not detected on syncytiotrophoblast apical plasma membrane. Localization to the outermost cellular barrier between the fetal and maternal blood further strengthens the role of the Fc receptor in transplacental transport of IgG.

Identification of a soluble Fc gamma-binding molecule (annexin II) in human serum using a competitive ELISA

We have previously produced a monoclonal antibody (mAb), B1D6, reactive with a 37 kD placental IgG Fc-binding molecule (FcR), recently identified as annexin II. Annexin II is an intracellular molecule found in several cell types, including endothelium and monocytes. Since soluble Fc-binding molecules are of importance in the regulation of the immune response, we have now used B1D6 in a competitive ELISA to study levels of soluble annexin II in human sera. Soluble annexin II was detected in all sera studied. The highest levels were observed in patients with infectious mononucleosis. Gel filtration of sera revealed annexin II in fractions corresponding to a molecular weight of 40-60 kD. In Western blot analysis a molecule of approximately 37 kD was found. The pI of soluble annexin II was about 7.5-8 as demonstrated by chromatofocusing. Annexin II belongs to a family of phospholipid-binding molecules involved in anti-inflammatory responses, and elevated levels of annexin II in serum may be important for the suppression of an immune response.

Fc gamma-receptor activity of placental annexin II

We have previously produced a MoAb, B1D6, against a placental FcR. The antigen isolated using F(ab')2-fragments of B1D6 exhibits Fc-binding properties with low affinity for IgG. The antigen is a single-chained glycoprotein with a molecular weight of approximately 37 kDa and a pI of about 7.0-8.5. Amino acid sequences from enzymatic digests of the antigen indicated that it is annexin II. Immunoreactivity using anti-annexin antisera and purified placental annexin II have further established the specificity of B1D6 to annexin II. The B1D6 epitope appears to be intramembraneous and intracellular on placental syncytiotrophoblasts, monocytes and other cells investigated.