Structure Of Fc Research Articles

Human neutrophil Fc gamma receptor distribution and structure.

Fc receptors (FcR) for IgG on human cells were analyzed with two monoclonal antibodies, 3G8 and 4F7. Fab fragments of both hybridoma IgGs inhibited binding to neutrophils of soluble rabbit IgG complexes and sheep erythrocytes (E) coated with rabbit IgG. The number of sites for 3G8 Fab was 135,000 per neutrophil, roughly equivalent to the number of sites for rabbit IgG in immune complexes (112,000 per cell). We did not observe human IgG1 binding sites on neutrophils, although binding of IgG1 to FcR-bearing lines U937 and HL-60 was readily demonstrated. Other cell types bearing 3G8 binding sites were mature chronic myelogenous leukemia cells, eosinophils, 6% of E-rosetting lymphocytes, and 15% of Ig-bearing peripheral lymphocytes. No binding of 3G8 Fab was observed on the FcR-bearing cell lines U937, HL-60 Raji, Daudi, and K562 or on blood monocytes. However, 15% of monocytes cultured for 7 days and 60% of lung macrophages expressed 3G8 antigen. When analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, the neutrophil FcR immunoprecipitated with 3G8 or 4F7 Fab-Sepharose displayed a broad band extending from Mr 73,000 to 51,000; in many experiments this band was resolved into two poorly separated components, centered at Mr 66,000 and 53,000. These results show that human neutrophil FcR for IgG is different from that on monocytes, with respect to both antigenic composition and binding of monomeric IgG1.

Mouse spleen cells and sensitized sheep red blood cells: An Fc-rosette-forming system allowing the detection of “activated” Fc structures

From a variety of Fc receptor-bearing cell/sensitized red blood cell combinations, mouse spleen cells, and sensitized SRBC were selected as an Fc-specific EA rosette assay system because only this mixture combined a high percentage (about 50%) of rosette-forming cells with complete absence of spontaneous rosettes and showed no influence of complement on the rosette formation. From studies on the minimal structural requirement of IgG both for mediation and inhibition of EA rosettes using IgG and several well-defined fragments, it appeared that both the C H2 and the C H3 domain of Fe are needed for optimal interaction with the lymphocyte Fc receptor. Finally, it was demonstrated that the assay system is able to detect “activated” Fc structures (here: heat-aggregated IgG) and to differentiate between varying amounts of such structures.

Immunochemical aspects of Fc-mediated binding of human IgG subclasses to group A, C and G streptococci

Nineteen human IgG myeloma proteins were tested in a sensitive assay for binding to 60 strains of β-hemolytic streptococci. Human group A, C and G streptococci demonstrated strong reactivity with positive binding of all four human IgG subclasses. Group C and G streptococci were capable of binding twice as much IgG as group A strains. β-Hemolytic bovine group G streptococci showed a low degree of reactivity with IgG1 and IgG4. Kinetic experiments revealed that the IgG binding was a time-dependent, displaceable process. Studies performed with Fab and Fc fragments showed that Fc structures are involved in the binding of intact IgG molecules to the bacterial cell surface. In addition, radiolabelied Fab fragments from two monoclonal components exhibited a definite reactivity with group G streptococci. Scatchard plots of the IgG-streptococcal interaction showed a linear section at low saturation levels. The equilibrium constants derived from such plots were 8–9 × 10 7 1./mole. The equilibrium constant for the interaction of IgG with staphylococcal protein A as measured with the Cowan I reference strain was 5 × 10 7 1./mole.

Electronic Structure in Amorphous Alloy Fe76P24

Electronic structure of amorphous metallic alloy FC 76 P 24 is calculated by using an atomic structural model and the atomic sphere approximation to the KKR method. Potential parameters are determined by superposing the charge density of neutral atoms. The strong hybridization with Fe–3d and P–3p band is observed and the electron energy stability is attributed to the charge transfer and the hybridization.

Isotypes of IgG: Comparison of the primary structures of three pairs of isotypes which differ in their ability to activate complement

The comparison of three pairs of isotypic Cγ‡ primary structures is made with reference to the known crystallographic structure of Fab and Fc from human IgGl. Contiguous residue positions are assigned to sub-regions based on their occurrence in secondary structures, extended chains or the loops which connect them, in the crystallographic models. Simple inspection reveals that residue positions having identical amino acids in all six Cγ are found mainly in extended chain sub-regions forming parts of β-sheets. However, average structural dissimilarity (ASD) values in loops which mediate V H to Cγl and Cγ2-Cγ3 interactions are as low as those obtained for extended chain subregions, implying that these structures are conserved among IgGs. In contrast, loop structures which are candidates for Cγl-Cγ2 interactions show high values of ASD, indicating little structural homology is conserved. Longitudinal Fab-Fc interactions, if they occur at all, probably occur via remarkably different residue contacts in different IgGs. Our analysis yields no clear-cut evidence for a pathway by which isotypic IgGs diverged in biologic effector functions. Great variation in primary structure is seen in the ‘hinge’ region, and in some loop and extended chain subregions exposed to solvent in the Cγ2 Cγ2 domain. These variations may account for the ability or inability of the six IgGs compared to interact with the first component of complement (C1) since some structural features in those regions are shared among those Fcs which bind C1. We speculate that the C1. binding site is determined by the extended chain formed by residues Lys-290 to Glu-295 of human IgGl heavy chain.

Structure and Function of Immunoglobulin Domains

Abstract The ability of various fragments of human myeloma IgG1 to inhibit rosette formation between human anti-D-coated human red blood cells and human polymorphonuclear leukocytes has been investigated. Although IgG and Fc showed a dose-dependent inhibition of rosettes at equimolar concentrations, neither of the fragments corresponding to the Cγ2 and Cγ3 homology regions obtained by acid-tryptic cleavage of Fc was able to inhibit rosette formation. The pepsin fragment of Fc, pFc′, which represents the complete Cγ3 domain, was also unable to prevent rosette formation. Reduction and alkylation of IgG or Fc markedly diminished cytophilic activity as measured by this system. These data indicate that the site in human IgG1 bound by granulocytes is dependent on the full quaternary structure of Fc, a requirement in marked contrast to that noted for binding by macrophages and monocytes.

Selective Proteolysis of the J Chain Component in Human Polymeric Immunoglobulin

To clarify the losses that have been observed in the J chain portion of human IgM and IgA, were carried out studies on the enzymatic susceptibility of the J polypeptide. When Waldenström macroglobulins and myeloma IgA polymers were subjected to limited proteolysis with various endopeptidases, only subtilisin was found to attack the J chain component. The pattern of cleavage was a function of the polymer species. The J chain in IgM was highly susceptible to digestion, quantitative cleavage being achieved at very low enzyme to IgM ratios and without significant changes in the remaining pentamer structure. Analyses of the digestion products showed that the initial cleavage occurred at an exposed region midway in the J sequence and was followed by extensive degradation of the carboxy-terminal segment. These findings indicated that the observed loss of the IgM J component can be explained by the inadvertent introduction of subtilisin in vitro or by the attack of in vivo enzymes with a specificity similar to subtilisin. In contrast, the IgA J chain was found to be much more resistant to subtilisin proteolysis; its cleavage required higher enzyme concentrations and was accompanied by significant degradation of the alpha-chains. Thus, it appears unlikely that the IgA J polypeptide is degraded by either in vitro or in vivo enzymes unless its accessibility is first enhanced by changes in the IgA Fc structure.

Physicochemical and immunological properties of a μ chain disease protein

The physicochemical and immunological properties of a μ heavy chain disease protein (BO) have been investigated. The native protein has a mol. wt of 520,000 daltons and it is constituted of 10 polypeptide chains interlinked by 30 interchain disulfide bonds. It was shown that each polypeptide chain contains the Cμ4, Cμ3 and Cμ2 domains and the 160–170 missing residues are probably located at the level of the V H−Cμ1 regions. Protein BO was shown to contain one residue of galactosamine per polypeptide chain. Primary structure data indicate that significant amino acids changes occur in the Fcμ region of this protein as compared with the Fc structure of a Waldenström IgM molecule.

Temperature and pressure dependence of self-diffusion, viscosity and nuclear relaxation times in liquid water

A scaled particle-quansilattice model for liquid water, which is based on the ice VII and ice fc structures, is shown to be capable of explaining several anomalous properties of water. The non-Arrhenius temperature dependence of coefficients of self-diffusion and viscosity and nuclear relaxation times are quantitatively interpreted. Negative volumes of activation, which are observed for viscosity and nuclear relaxation, are also computed to be in good agreement with experiment.

Immunological memory in mice. II. Cell interactions in the secondary immune response studies by means of immunoglobulin allotype markers.

Congenic mice, differing genetically only at the loci coding for immunoglobulin H chain (or Fc) structures, have been used to study cell interactions in the 7S (gammaG(2a)) antibody response to sheep erythrocytes (SRBC), as detected by the Jerne plaque-forming cell (PFC) method. The interaction between thymus and bone marrow cells was studied in adult thymectomized irradiated recipients, protected with syngeneic bone marrow and injected with thymus cells from the partner congenic strain. All of the gammaG(2a) PFC detected in the spleens of these mice were of bone marrow allotype. Adoptive secondary immune responses were then studied to determine whether a similar interaction between memory cells and bone marrow derived cells could be detected. Primed spleen cells from the partner congenic strain, or a subpopulation of these cells obtained by BSA density gradient fractionation, were injected into irradiated recipients alone, or together with syngeneic nonimmune spleen or bone marrow cells. All gammaG(2a) PFC detected in these experiments were of primed cell allotype. There was no evidence that antibody forming cell precursors in normal spleen or bone marrow participate in the adoptive secondary immune response detected 7 days after transfer of primed spleen cells. This was true regardless of whether the bone marrow cells were injected at the time of transfer, or were injected 1-2 wk earlier and allowed to become established in the spleens of recipient mice. Although no specific cell interaction was seen, bone marrow (and, to a lesser degree, normal spleen) cells were found to have a nonspecific enhancing effect on the adoptive secondary response when they were injected together with the primed spleen cells. This enhancement was not evident if the bone marrow cells were injected 1 or 2 wk prior to primed cell transfer.

Phagocytosis in subacute bacterial endocarditis. Localization of the primary opsonic site to Fc fragment.

The opsonic properties of immune gammaG-globulins isolated from patients with chronic septicemic conditions, principally subacute bacterial endocarditis were studied. Opsonic capacity as well as complement-fixing properties of gamma-globulins appeared to be closely associated with integrity of Fc structures. Progressive pepsin digestion of immune gammaG-globulins, as monitored by successive loss of Gm(a) and Gm(b) antigens, abolished opsonic activity. Colostral gammaA, containing agglutinating antibacterial antibodies but no demonstrable complement-fixing activity, was devoid of opsonic capacity. Reduction of gamma-globulin opsonins with 0.01 or 0.1 M mercaptoethanol progressively abolished opsonic activity in parallel with loss of ability of treated gamma-globulins to fix complement with bacteria. Treatment of gamma-globulin opsonins with 0.01 M sodium metaperiodate also produced complete loss of opsonic capacity in parallel with loss of Gm(b) Fc antigens. These findings, together with antiopsonic effects demonstrable with anti-gamma-globulin factors showing primary reactivity with Fc structures, indicate that the opsonic property of immune gamma-globulins requires the participation of structures integral to the Fc region of gamma-globulin.