The increased flexibility of CDR loops generated in antibodies by Congo red complexation favors antigen binding

Interaction of Supramolecular Congo Red and Congo Red-Doxorubicin Complexes with Proteins for Drug Carrier Design

Targeted immunotherapy has expanded to simultaneous delivery of drugs, including chemotherapeutics. The aim of the presented research is to design a new drug carrier system. Systems based on the use of proteins as natural components of the body offer the chance to boost safety and efficacy of targeted drug delivery and excess drug removal. Congo red (CR) type supramolecular, self-assembled ribbon-like structures (SRLS) were previously shown to interact with some proteins, including albumin and antibodies complexed with antigen. CR can intercalate some chemotherapeutics including doxorubicin (Dox). The goal of this work was to describe the CR-Dox complexes, to analyze their interaction with some proteins, and to explain the mechanism of this interaction. In the present experiments, a model system composed of heated immunoglobulin light chain Lλ capable of CR binding was used. Heat aggregated immunoglobulins (HAI) and albumin were chosen as another model system. The results of experiments employing methods such as gel filtration chromatography and dynamic light scattering confirmed the formation of the CR-Dox complex of large size and properties different from the free CR structures. Electrophoresis and chromatography experiments have shown the binding of free CR to heated Lλ while CR-Dox mixed structures were not capable of forming such complexes. HAI was able to bind both free CR and CR-Dox complexes. Albumin also bound both CR and its complex with Dox. Additionally, we observed that albumin-bound CR-Dox complexes were transferred from albumin to HAI upon addition of HAI. DLS analyses showed that interaction of CR with Dox distinctly increased the hydrodynamic diameter of CR-Dox compared with a free CR supramolecular structure. To our knowledge, individual small proteins such as Lλ may bind upon heating a few molecules of Congo red tape penetrating protein body due to the relatively low cohesion of the dye micelle. If, however, the compactness is high (in the case of, e.g., CR-Dox) large ribbon-like, micellar structures appear. They do not divide easily into smaller portions and cannot attach to proteins where there is no room for binding large ligands. Such binding is, however, possible by albumin which is biologically adapted to form complexes with different large ligands and by tightly packed immune complexes and heat aggregated immunoglobulin-specific protein complex structures of even higher affinity for Congo red than albumin. The CR clouds formed around them also bind the CR-Dox complexes. The presented research is essential in the search for optimum solutions for SRLS application in immuno-targeting therapeutic strategies, especially with the use of chemotherapeutics.

Structure and Location of Protein Sites Binding Self-Associated Congo Red Molecules with Intercalated Drugs as Compact Ligands-Theoretical Studies

In the search for new carriers capable of transporting toxic drugs to a target, particular attention has been devoted to supramolecular systems with a ribbon-like micellar structure of which Congo red is an example. A special promise of the possible use of such systems for directing drugs to a target emerges from their particular affinity to immune complexes and as an independent property, binding many organic compounds including drugs by intercalation. Serum albumin also appeared able to bind micellar particles of such systems. It may protect them against dilution in transport. The mathematical tool, which relies on analysis of the distribution of polarity and hydrophobicity in protein molecules (fuzzy oil drop model), has been used to find the location of binding area in albumin as well as anchorage site for Congo red in heated IgG light chain used as a model presenting immunoglobulin-like structures. Results confirm the suggested formerly binding site of Congo red in V domain of IgG light chain and indicated the cleft between pseudo-symmetric domains of albumin as the area of attachment for the dye.

Intramolecular immunological signal hypothesis revived--structural background of signalling revealed by using Congo Red as a specific tool

Micellar structures formed by self-assembling Congo red molecules bind to proteins penetrating into functionrelated unstable packing areas. Here, we have used Congo red - a supramolecular protein ligand to investigate how the intramolecular structural changes that take place in antibodies following antigen binding lead to complement activation. According to our findings, Congo red binding significantly enhances the formation of antigen-antibody complexes. As a result, even low-affinity transiently binding antibodies can participate in immune complexes in the presence of Congo red, although immune complexes formed by these antibodies fail to trigger the complement cascade. This indicates that binding of antibodies to the antigen may not, by itself, fulfill the necessary conditions to generate the signal which triggers effector activity. These findings, together with the results of molecular dynamics simulation studies, enable us to conclude that, apart from the necessary assembling of antibodies, intramolecular structural changes generated by strains which associate high- affinity bivalent antibody fitting to antigen determinants are also required to cross the complement activation threshold.

The use of Titan yellow dye as a metal ion binding marker for studies on the formation of specific complexes by supramolecular Congo red

Congo red (CR) and other self-assembling compounds creating supramolecular structures of rod- or ribbon-like architecture form specific complexes with cellulose and also with many proteins, including antibodies bound to the antigen and amyloids in particular. The mechanism of complexation and structure of these complexes are still poorly recognized despite the importance of the problem for medicine. This work proposes the progress in electron microscopy studies of amyloid-dye complexes by labeling supramolecular ligand CR with silver ions as a marker. Silver ions are introduced to CR carried by the strongly binding silver dye Titan yellow, which in addition form comicellar structures with CR. Silver carried by self-assembled dye molecules forms in the resulting metal nanoparticles, making the specific amyloid ligand CR perceptible in EM studies.

Prevalence and gene characteristics of antibodies with cofactor-induced HIV-1 specificity

The healthy immune repertoire contains a fraction of antibodies that bind to various biologically relevant cofactors, including heme. Interaction of heme with some antibodies results in induction of new antigen binding specificities and acquisition of binding polyreactivity. In vivo, extracellular heme is released as a result of hemolysis or tissue damage; hence the post-translational acquisition of novel antigen specificities might play an important role in the diversification of the immunoglobulin repertoire and host defense. Here, we demonstrate that seronegative immune repertoires contain antibodies that gain reactivity to HIV-1 gp120 upon exposure to heme. Furthermore, a panel of human recombinant antibodies was cloned from different B cell subpopulations, and the prevalence of antibodies with cofactor-induced specificity for gp120 was determined. Our data reveal that upon exposure to heme, ∼24% of antibodies acquired binding specificity for divergent strains of HIV-1 gp120. Sequence analyses reveal that heme-sensitive antibodies do not differ in their repertoire of variable region genes and in most of the molecular features of their antigen-binding sites from antibodies that do not change their antigen binding specificity. However, antibodies with cofactor-induced gp120 specificity possess significantly lower numbers of somatic mutations in their variable region genes. This study contributes to the understanding of the significance of cofactor-binding antibodies in immunoglobulin repertoires and of the influence that the tissue microenvironment might have in shaping adaptive immune responses.

The use of supramolecular structures as protein ligands

Congo red dye as well as other eagerly self-assembling organic molecules which form rod-like or ribbon-like supramolecular structures in water solutions, appears to represent a new class of protein ligands with possible wide-ranging medical applications. Such molecules associate with proteins as integral clusters and preferentially penetrate into areas of low molecular stability. Abnormal, partly unfolded proteins are the main binding target for such ligands, while well packed molecules are generally inaccessible. Of particular interest is the observation that local susceptibility for binding supramolecular ligands may be promoted in some proteins as a consequence of function-derived structural changes, and that such complexation may alter the activity profile of target proteins. Examples are presented in this paper.

The indirect generation of long-distance structural changes in antibodies upon their binding to antigen

An allosteric mechanism for the generation of long-distance structural alterations in Fab fragments of antibodies in immune complexes has been postulated and tested in theoretical and experimental analysis. The flexing and/or torsion-derived forces exerted on the elbow region in Fab arms of bivalent antibodies upon binding to antigen were assumed to drive the disruption of hydrogen bonds which stabilize N- and C-terminal chain fragments in V-domains. This allows an extra movement in the elbow followed by a relaxation in the Fab arm and may generate long-distance effects if, in particular, the structural changes are generated asymmetrically involving one chain of the Fab arm only. This mechanism was studied by simulation of molecular dynamics. The local instability in the area involving the site of packing of the N-terminal chain fragment allows penetration and binding of the supramolecular dye Congo red that hence becomes an indicator of the initiated relaxation process and is also the prospective ligand in studies of designing drugs. The susceptibility to dye binding was observed in complexation of bivalent antibodies only, supplying the evidence that constraints associating the interaction with randomly distributed antigenic determinants drive the local structural changes in the V-domain followed by long-distance effects.

Albumin binds self-assembling dyes as specific polymolecular ligands

Self-assembling dyes with a structure related to Congo red (e.g. Evans blue) form polymolecular complexes with albumin. The dyes, which are lacking a self-assembling property (Trypan blue, ANS) bind as single molecules. The supramolecular character of dye ligands bound to albumin was demonstrated by indicating the complexation of dye molecules outnumbering the binding sites in albumin and by measuring the hydrodynamic radius of albumin which is growing upon complexation of self-assembling dye in contrast to dyes lacking this property. The self-assembled character of Congo red was also proved using it as a carrier introducing to albumin the intercalated nonbonding foreign compounds. Supramolecular, ordered character of the dye in the complex with albumin was also revealed by finding that self-assembling dyes become chiral upon complexation. Congo red complexation makes albumin less resistant to low pH as concluded from the facilitated N-F transition, observed in studies based on the measurement of hydrodynamic radius. This particular interference with protein stability and the specific changes in digestion resulted from binding of Congo red suggest that the self-assembled dye penetrates the central crevice of albumin.