Affinity purification of polyclonal antibodies using immobilized multimeric peptides

Reversible cyclic peptide libraries for the discovery of affinity ligands

A novel strategy is presented for the identification of cyclic peptide ligands from combinatorial libraries of reversible cyclic depsipeptides. A method for the solid-phase synthesis of individual cyclic depsipeptides and combinatorial libraries of these compounds is proposed, which employs lactic acid (Lact) and the dipeptide ester (Nα-Ac)-Ser(Ala)- as linkers for dilactonization. Upon alkaline treatment of the beads selected by screening a model library, the cyclic depsipeptides are linearized and released from the solid support to the liquid phase, to be sequenced via single-step tandem mass spectrometry (MS/MS). The protocol presented for library synthesis provides for wide structural diversity. Two model sequences, VVWVVK and AAWAAR, were chosen to present different structural examples for depsipeptide libraries and demonstrate the process of sequence determination by mass spectrometry. Further, a case study using the IgG binding cyclic depsipeptide cyclo[(Nα-Ac)-S(A)-RWHYFK-Lact-E] is presented to demonstrate the process of library screening and sequence determination on the selected beads. Finally, a method is shown for synthesis of the irreversible cyclic peptide corresponding to the proposed depsipeptide structure, to make the ligand stable to the aqueous acid and alkaline conditions encountered in affinity chromatographic applications. The cyclic peptide ligand was synthesized on a poly(methacrylate) resin and used for chromatographic binding of the target IgG.

Understanding ligand-protein interactions in affinity membrane chromatography for antibody purification

Affinity chromatography with Protein A beads has become the conventional unit operation for the primary capture of monoclonal antibodies. However, Protein A activated supports are expensive and ligand leakage is an issue to be considered. In addition, the limited production capabilities of the chromatographic process drive the research towards feasible alternatives. The use of synthetic ligands as Protein A substitutes has been considered in this work. Synthetic ligands, that mimic the interaction between Protein A and the constant fragment (Fc) of immunoglobulins, have been immobilized on cellulosic membrane supports. The resulting affinity membranes have been experimentally characterized with pure immunoglobulin G (IgG). The effects of the membrane support and of the spacer arm on the ligand-ligate interaction have been studied in detail. Experimental data have been compared with molecular dynamic simulations with the aim of better understanding the interaction mechanisms. Molecular dynamic simulations were performed in explicit water, modelling the membrane as a matrix of overlapped glucopyranose units. Electrostatic charges of the ligand and spacer were calculated through ab initio methods to complete the force field used to model the membrane. The simulations enabled to elucidate how the interactions of surface, spacer and ligand with IgG, contribute to the formation of the bond between protein and affinity membrane.

A new approach for producing polyclonal antibodies using impure antigens

A new convenient approach has been designed to produce polyclonal antibodies (PcAb). The approach is based on the principle of the immunoglobulin (Ig) class switch in the immune response. We produced six different antibodies (Ab) against calcineurin A subunit (CNA). CNA, His-tagged calcineurin A subunit (His-CNA), single chain calcineurin (CNB-CNA) and single chain calcineurin-calmodulin complex (CaM-CNB-CNA) were expressed in Escherichia coli (E. coli) BL21 strain, and they were used to immunize male BALB/c mice. These Ab were examined by enzyme-linked immunosorbent assay (ELISA) and western blot analysis. The results clearly demonstrated that the specificity of the Ab produced by different protein samples was much higher than that of the Ab produced by a single sample. We used CNA, CaM-CNB-CNA and His-CNA to immunize mice in turn and obtained monospecific PcAb against CNA fortunately by our new approach. Remarkably, our approach not only offered a simple and general alternative to other methods for producing PcAb described previously, but also disclosed a novel process of immunization that could be used to produce monoclonal antibodies (mAb).

Membrane adsorbers as purification tools for monoclonal antibody purification

Downstream purification processes for monoclonal antibody production typically involve multiple steps; some of them are conventionally performed by bead-based column chromatography. Affinity chromatography with Protein A is the most selective method for protein purification and is conventionally used for the initial capturing step to facilitate rapid volume reduction as well as separation of the antibody. However, conventional affinity chromatography has some limitations that are inherent with the method, it exhibits slow intraparticle diffusion and high pressure drop within the column. Membrane-based separation processes can be used in order to overcome these mass transfer limitations. The ligand is immobilized in the membrane pores and the convective flow brings the solute molecules very close to the ligand and hence minimizes the diffusional limitations associated with the beads. Nonetheless, the adoption of this technology has been slow because membrane chromatography has been limited by a lower binding capacity than that of conventional columns, even though the high flux advantages provided by membrane adsorbers would lead to higher productivity. This review considers the use of membrane adsorbers as an alternative technology for capture and polishing steps for the purification of monoclonal antibodies. Promising industrial applications as well as new trends in research will be addressed.

Future of antibody purification

Antibody purification seems to be safely ensconced in a platform, now well-established by way of multiple commercialized antibody processes. However, natural evolution compels us to peer into the future. This is driven not only by a large, projected increase in the number of antibody therapies, but also by dramatic improvements in upstream productivity, and process economics. Although disruptive technologies have yet escaped downstream processes, evolution of the so-called platform is already evident in antibody processes in late-stage development. Here we perform a wide survey of technologies that are competing to be part of that platform, and provide our [inherently dangerous] assessment of those that have the most promise.

Isolation of antigens and antibodies by affinity chromatography

Antibody-antigen binding constants are commonly strong enough for an effective affinity purification of antibodies (by immobilized antigens) or antigens (by immobilized antibodies) to work out a straightforward purification method. A drawback is that antibodies are large protein molecules and subject to denaturation under conditions required for the elution from the complex. Structures of antigens can vary but usually antigens are also equally subject to similar problems. The lability of the components can sometimes make the procedure sophisticated, but usually in all cases it is possible to find a satisfactory approach. In certain cases, specific interactions of the Fc part of antibodies are more facile to exploit for their purification.

Sequence-simplification and chimeric assembly: new models of peptide antigen modification

Sequence-simplified variants of a 15-mer peptide antigen, identified by amino acid side chains in alternating positions were synthesized introducing glycine residues alternatively in the parent peptide sequence and used to induce antibodies in rabbit. They reacted to a significant extent with anti-parent peptide antibodies, and in addition, affinity purified antibodies against these halved forms recognized with similar affinity and specificity, the starting peptide in affinity chromatography, optical biosensor and enzyme linked immunosorbent assay (ELISA) experiments, while no cross-reactivity was detected between reduced antigens. These findings suggest that a peptide antigen can display two molecular surfaces of recognition, identified by side chains of residues in alternating positions. Each surface can even take part in antigen/antibody interaction independently, thus indicating the possibility to select and assembly sequence-simplified forms belonging to different epitopes, also deriving from different molecules, to generate new structures incorporating a two-fold antigen/antibody specificity. Two "chimeric" forms were then synthesized starting from the P15 and P13 complementary peptides, both able to bind interleukin 2. These structures, showing simultaneously trans-surfaces of recognition belonging to both parent forms, have been found to retain antigenic properties against antibodies of simplified P15 derivatives showing the same molecular surface of recognition. In addition, anti-chimeric antibodies recognized both P15 and P13 starting peptides, while no cross-antibody recognition was observed between chimeric antigens.

Topological mimicry of cross-reacting enantiomeric peptide antigens

Rabbit polyclonal antibodies against multimeric peptide antigens were found to cross-react to a significant extent with topologically related variants of the parent antigen, where the chirality of each amino acid residue (inverso derivatives), or the peptide sequence orientation (retro derivatives), was inverted or where both modifications were simultaneously introduced (retro-inverso derivatives). All peptide variants displayed similar recognition properties for antibodies and similar dose-dependent inhibitory effects on the interaction between immobilized parent antigen and corresponding antibodies. Importance of peptide side chain topology on antigenicity was evaluated analyzing the recognition properties of two sequence-simplified parent peptide variants, one lacking of the side chains in the sequence odd position and the other in even position. These two variants, prepared introducing glycine residues alternatively in the parent peptide sequence, were found to cross-react to a significant extent with the original antibody raised against the parent peptide. Analysis of molecular models of peptide enantiomeric variants in the elongated all-trans configuration suggested that the topological equivalence of alternating side chains could lead to the formation of similar recognition surfaces, thus mimicking the parent peptide antigenic structure.

Antigenicity of topochemically related peptides

Antibodies raised in rabbits against multimeric all-L peptides (MAP's) were first made monospecific by affinity chromatography on immobilized antigen columns and then tested for their ability to cross-react with topologically related variants of the parent antigen, where the chirality of each amino-acid residue (inverso derivatives), or the peptide sequence orientation (retro derivatives), was inverted, or where both modifications were simultaneously introduced (retro-inverso derivatives). Retro, inverso, and retro-inverso forms of the parent peptide were prepared, both in the linear as well as in the BSA-conjugated form, and found to cross-react to a significant extent with affinity purified polyclonal antibodies raised against the parent peptide. Peptide variants displayed similar dose-dependent inhibitory effects on the interaction between immobilized parent antigen and affinity purified antibodies. Analysis of molecular models of the peptide variants in the trans-configuration suggested that the topological equivalence of alternating side chains in the series of related peptides may be responsible for the observed cross-recognition, leading to the formation of similar recognition surfaces which could mimic the parent peptide antigenic structure.