High-affinity rat anti-fluorescein monoclonal antibody with unique fine specificity properties including differential recognition of dynamic ligand analogues

Resolution of rabbit polyclonal anti-fluorescein Fab (IgG) fragments into subpopulations differing in affinity and spectral properties of bound ligand

Fab fragments derived from ten different IgG populations of hyperimmune rabbit polyclonal anti-fluorescein antibodies were further resolved into subfractions based on differences in time-dependent dissociation from an FITC-adsorbent in the presence of 0.1 M fluorescein at 4 degrees C. Fab fragments separated into subpopulations based on specific dissociation times of 0.1 day, 1.0 day, 10 days and 100 days from the adsorbent. Finally, after the 100 days elution step incubation with 6.0 M guanidine-HCl was included to determine total protein concentration of specific anti-fluorescein Fab fragments. Yields of specifically eluted Fab fragments ranged from 12.7 to 84.1% of the total Fab population originally incubated with the adsorbent. All Fab polyclonal populations and subpopulations analyzed quenched the fluorescence of the bound ligand by 90% or greater. None of the plots of protein concentration versus percent yield of the total specific antibody obtained for each of the five resolved fractions constituting a specific polyclonal population conformed to Gaussian distributions. All resolved Fab subpopulations retained bound fluorescein ligand that exhibited significant bathochromic shifts in absorbancy. Based on the extent of the red-shift the antibodies segregated into one of two general spectral families showing either a peak shift to 505-507 nm or to 518-520 nm. The red-shift to 518-520 nm appeared unique to rabbit anti-fluorescein antibodies, since corresponding large shifts have not been observed with antibodies derived from other species (e.g. mouse, rat, chicken, etc.). K(d) values determined for the resolved fractions confirmed a continuous progression in affinity from the 0.1day through the 100 days elution. Preliminary isoelectric focusing analyses revealed progressive selection for relatively more homogeneous fractions, especially in the 100 days resolved fraction.

Biochemical purification and partial characterization of a murine macrophage surface receptor possessing specificity for small aromatic moieties including fluorescein

Binding properties and requirements for internalization of hapten-protein antigens, such as fluorescein-polyderivatized bovine serum albumin (FITC-BSA) and poly-D-lysine (FITC-PDL), by murine macrophage was consistent with surface receptor recognition of fluorescyl moieties (Cherukuri et al., Mol. Immunol. 34, 21-32, 1997; Cherukuri et al., Cytometry 31, 110-124, 1998). Ligand binding properties of the putative macrophage receptor pointed toward specificity for various aromatic moieties including phenylalanine, phenyloxazolone and fluorescein (Cherukuri and Voss, Mol. Immunol. 35, 115-125, 1998). Purification of the hapten-recognizing receptor from J774 macrophage cells involved subcellular fractionation of plasma membrane fractions, and affinity chromatography of solubilized membranes employing a phenyl-Sepharose adsorbent with subsequent specific elution of receptor using fluorescein ligand. The final product was a protein with a molecular mass of approximately 180 kDa. Characterization of the purified receptor involved absorption and fluorescence spectroscopy, circular dichroism, fluorescence quenching analyses, various ligand binding assays and an immunological analysis. Spectroscopic analyses revealed that the receptor possessed aromatic amino acids while circular dichroism suggested significant alpha-helical secondary structure. Binding specificity of the purified receptor was confirmed in a spectrofluorometric assay where the fluorescence of fluorescein ligand was quenched approximately 97%. Finally, specific binding activity of the receptor with FITC-BSA was demonstrated in Western blot analysis under native conditions. Receptor purity was confirmed in amino acid sequencing analysis when the amino-terminal residue was found to be totally blocked. Results are discussed in terms of the possible identity of the isolated macrophage receptor and its biological-immunological role.

Backbone dynamics of homologous fibronectin type III cell adhesion domains from fibronectin and tenascin

BACKGROUND

Fibronectin type III domains are found as autonomously-folded domains in a large variety of multidomain proteins, including extracellular matrix proteins. A subset of these domains employ an Arg-Gly-Asp (RGD) tripeptide motif to mediate contact with cell-surface receptors (integrins). This motif mediates protein-protein interactions in a diverse range of biological processes, such as in tissue development, would healing and metastasis. The molecular basis for affinity and specificity of cell adhesion via type III domains has not been clearly established. The tenth type III domain from fibronectin (FNfn10) and the third type III domain from tenascin-C (TNfn3) have 27% sequence identity and share the same overall protein fold, but present the RGD motifs in different structural contexts. The dynamical properties of the RGD motifs may affect the specificity and affinity of the FNfn10 and TNfn3 domains. Structure-dynamics correlations for these structurally homologous proteins may reveal common molecular features which are important to the dynamical properties of proteins.

RESULTS

The intramolecular dynamics of the protein backbones of FNfn10 and TNfn3 have been studied by 15N nuclear spin relaxation. The FG loop in FNfn10, which contains the RGD motif, exhibits extensive flexibility on picosecond to nanosecond timescales, but motions on microsecond to millisecond timescales are not observed. The equivalent region in TNfn3 is as rigid as regular elements of secondary structure. The CC' loop also is more flexible on picosecond-nanosecond timescales in FNfn10 than in TNfn3. Conformational exchange, reflecting flexibility on microsecond-millisecond timescales, is observed in beta strands A and B of both FNfn10 and TNfn3.

CONCLUSIONS

Comparison of the structures of the FNfn10 and TNfn3 reveals several features related to their different dynamical properties. The larger amplitude motions of loops in FNfn10 are consistent with the hypothesis that flexibility of these regions facilitates induced-fit recognition of fibronectin by multiple receptors. Similarly, the more rigid loops of TNfn3 may reflect greater specificity for particular integrins. The correlations observed between structural features and dynamical properties of the homologous type III domains indicate the influence of hydrogen bonding and hydrophobic packing on dynamical fluctuations in proteins.

Transition-state theory and secondary forces in antigen--antibody complexes

Secondary forces, defined as those interactions between the antigen (epitope including the surrounding environment) and areas immediately adjacent to the antibody active site, were investigated using monofluorescein-derivatized synthetic peptides of varying electrostatic properties. Secondary forces were quantitated by measuring the unimolecular rate constants at two different temperatures using the high-affinity anti-fluorescein monoclonal antibody 4-4-20 complexed with fluorescein-derivatized synthetic peptides. Unimolecular rate constants were correlated with transition-state theory to explain secondary effects. An acidic peptide produced a large temperature-dependent effect upon binding including a significant enthalpic factor (+33.28 kcal/mol) relative to the binding of fluorescein ligand (+23.96 kcal/mol). Binding of a basic peptide produced both a relatively smaller temperature effect and enthalpy factor than fluorescein ligand. The antibody-ligand binding results were interpreted invoking the concepts of thermally averaged metatypic (liganded) states of the antibody as well as potential biochemical interactions between the antigen and accessible surface regions of the antibody's complementarity determining regions.

Anti-metatype antibody stabilization of Fv 4-4-20 variable domain dynamics

Anti-metatype (anti-Met) antibodies are immunoglobulins that specifically recognize and stabilize antibodies in their liganded or metatypic state, but lack specificity for either the hapten or the unliganded antibody. Autologous anti-Met antibodies were previously observed in vivo, suggesting that a metatypic autoantibody response could play a physiological role in the immune network, e.g. controlling the clearance of immune complexes from circulation. The first elicited anti-Met antibodies were against the fluorescein-liganded high affinity murine anti-fluorescein monoclonal antibody 4-4-20. The fluorescein-hapten system has proved to be an invaluable tool for both the recognition and characterization of the metatypic response by utilization of its spectral properties. In this investigation, hydrostatic pressure measurements, in conjunction with fluorescence spectroscopy, were performed on the recombinant Fv derivative (Fv 4-4-20) of the high affinity anti-fluorescein monoclonal antibody 4-4-20 complexed to anti-Met antibodies to study the influence of anti-Met antibodies of Fv 4-4-20 intervariable domain interactions. Anti-Met antibodies bound to liganded Fv 4-4-20 were observed to cause a change in the fluorescence properties of fluorescein that was not observed when anti-Met antibodies were bound to the liganded parent immunoglobulin. The variation of these spectral properties upon addition of anti-Met antibodies was shown to be correlated with dissociation of the variable domains in Fv 4-4-20 in response to its interaction with the anti-Met antibody. The ability to cause variable domain dissociation was dependent on whether monoclonal or polyclonal anti-Met antibodies were bound to the metatype. A model was proposed that elucidated the interaction of anti-Met antibodies, polyclonal and monoclonal, with variable domains of the primary anti-antigen antibody.

Antibody networks and imaging: elicitation of anti-fluorescein antibodies in response to the metatypic state of fluorescein-specific monoclonal antibodies

Studies are described regarding generation of anti-hapten antibodies starting with a monoclonal Ig immunogen in the ligand-induced conformation or metatypic state. Liganded monoclonal Ab1 antibodies represent the unique feature of the study since previous reports investigating internal imaging in the original Idiotype Network Hypothesis [Jerne, 1974 (Ann. Immun. 125C, 373-389)] were based on the non-liganded or idiotypic state [as reviewed in: Rodkey, 1980 (Microbiol. Rev. 44, 631-659); Kohler et al., 1979 (In: Methods in Enzymology: Antibodies, Antigens and Molecular Mimicry, pp. 3-35); Greenspan and Bona, 1993 (FASEB J. 7,437-444)]. Affinity-labeled liganded murine monoclonal anti-fluorescein antibodies served as immunogens administered both in the syngenic and xenogenic modes to determine if the metatypic state elicited anti-hapten antibodies through imaging-like mechanisms. Polyclonal and monoclonal anti-Ab1 reagents in various hosts were assayed for anti-fluorescein and/or anti-metatype specificity. Significant anti-fluorescein responses were measured indicating that the metatypic state directly or indirectly stimulates an anti-hapten antibody population.