A hapten-induced conformational change accompanies the cryoprecipitation of an immunoglobulin

Decreased IgG-Fc gamma RII dissociation kinetics in the presence of a protein antigen

Total internal reflection fluorescence microscopy has been used to examine the interaction of a mouse monoclonal IgG2b, in the absence and presence of its protein antigen, with mouse Fc gamma RII in substrate-supported planar membranes. Equilibrium association and kinetic dissociation constants were measured for the antibody S6-34.11, which is specific for bovine prothrombin fragment 1 (BF1). These measurements showed that BF1 induces a statistically significant decrease (30-40%) in the IgG-Fc gamma RII dissociation kinetics. A corresponding increase in the equilibrium association constant was not observed, perhaps because the statistical accuracy of the equilibrium measurements is lower than that for the kinetic measurements. The consequences of these results for understanding the mechanism by which macrophages recognize and ingest opsonized targets are discussed.

Cryoprecipitation properties of a high-affinity monoclonal IgM anti-fluorescyl antibody

A high affinity (Ka = 2-3 x 10(10) M-1) murine monoclonal anti-fluorescein IgM antibody (18-2-3) exhibiting low temp. insolubility in the absence of bound ligand has served as a model to study cryoprecipitation. Insolubility of 18-2-3 at low temp. (4 degrees C) had been shown to be reversible at higher temp. and in the presence of fluorescyl ligand, indicating antigen binding site involvement. The primary objectives were to isolate and identify structural component(s) responsible for insolubility at low temp. Procedures developed for production and isolation of the monomeric subunit (IgMs) involved thiol reduction and gel filtration separation in the presence of the zwitterionic detergent, CHAPS. Fab and (Fc)5 fragments generated by papain digestion were purified sequentially by gel filtration (Sephadex G-200) and affinity chromatography (fluorescein-Sepharose). A protocol for covalent labeling of Fab fragments with the fluorescent chromophore 2,5 DNS-Cl was developed in order to measure steady-state fluorescence polarization. In kinetic and equilibrium cryoprecipitation assays, nonliganded 18-2-3 IgM and IgMs demonstrated insolubility. Bound fluorescyl ligand, increased ionic strength (0.5 M NaCl) or basic pH (greater than 8.0) abrogated cryoprecipitation. Isolated Fab or (Fc)5 fragments did not exhibit low temp. insolubility. Decreased cryoprecipitation occurred when (Fc)5 fragments were added to nonliganded 18-2-3 IgM. Fluorescence polarization of 2,5 DNS Fab 18-2-3 fragments indicated lack of Fab-Fab aggregation. Results suggested that electrostatic interactions involving 18-2-3 antibody combining sites with interactive sites in the Fc region of homologous IgM were responsible for the phenomenon of cryoprecipitation.

Dual conformations of an immunoglobulin light-chain dimer: heterogeneity of antigen specificity and idiotope profile may result from multiple variable-domain interaction mechanisms

The structure of an immunoglobulin antigen-binding fragment (Fab) has been thought to be invariantly defined by well-conserved amino acid residues in the variable domains of the heavy and light chains. These conserved residues enable folding of the polypeptide segments into the characteristic immunoglobulin fold domains and are the major controllers of interactions between domains. However, crystallographic studies of some immunoglobulin light-chain dimers have suggested and the crystallographic structure of the Fab in an Fab-neuraminidase complex may have proven that antibodies are not restricted to a single, invariant relative positioning of the two variable domains. We propose that in some cases the detailed quaternary structural relationships between the variable domains of heavy and light chains are not restricted to those of the canonical Fab. It is unclear whether alterations of these relationships occur only after complex formation with antigen or, if in ligand-free solution, Fab conformers might coexist in relative concentrations determined by isomerization rates. In the latter case, antibody-presenting lymphocytes may be polyspecific, and the specificity of lymphocytes might be modulated by anti-idiotopic antibodies complexed to cell surface receptors. In either case, the idiotopic repertoire displayed by an antibody or lymphocyte surface receptor might be changed by the presence or absence of antigen.