Demonstration of an in vivo generated sub-picomolar affinity fully human monoclonal antibody to interleukin-8

The high specificity and affinity of monoclonal antibodies make them attractive as therapeutic agents. In general, the affinities of antibodies reported to be high affinity are in the high picomolar to low nanomolar range and have been affinity matured in vitro. It has been proposed that there is an in vivo affinity ceiling at 100 pM and that B cells producing antibodies with affinities for antigen above the estimated ceiling would have no selective advantage in antigen-induced affinity maturation during normal immune responses. Using a transgenic mouse producing fully human antibodies, we have routinely generated antibodies with sub-nanomolar affinities, have frequently rescued antibodies with less than 10 pM affinity, and now describe the existence of an in vivo generated anti-hIL-8 antibody with a sub-picomolar equilibrium dissociation constant. This confirms the prediction that antibodies with affinities beyond the proposed affinity ceiling can be generated in vivo. We also describe the technical challenges of determining such high affinities. To further understand the importance of affinity for therapy, we have constructed a mathematical model to predict the relationship between the affinity of an antibody and its in vivo potency using IL-8 as a model antigen.     

High-affinity binding measurements of antibodies to cell-surface-expressed antigens

A simple method that allows affinity measurements of antibodies to integral membrane proteins is described. Kinetic Exclusion Assay was used to determine the concentration of free antibody that remains in solution after equilibrium has been established between the antibody and the cell-surface-expressed antigen, from which the equilibrium dissociation constant (Kd) was determined. It eliminates the requirement for soluble antigen and modifications such as radio-labeling or fluorescent labeling of the antibody. For one of the cell-surface-expressed antigens, it was determined that the affinity of the antibody to the cell-surface-expressed antigen was similar to that of the purified, soluble form of the antigen. In addition to the simplicity of the approach, the method provides a true measure of the affinity/avidity of the antibody to the native form of cell-surface-expressed targets, including antigens that cannot be produced in soluble forms, and to unknown cell surface antigens.     

Probing the binding mechanism and affinity of tanezumab, a recombinant humanized anti-NGF monoclonal antibody, using a repertoire of biosensors

We describe the use of four complementary biosensors (Biacore 3000, Octet QK, ProteOn XPR36, and KinExA 3000) in characterizing the kinetics of human nerve growth factor (NGF) binding to a humanized NGF-neutralizing monoclonal antibody (tanezumab, formerly known as RN624). Tanezumab is a clinical candidate as a therapy for chronic pain. Our measurements were consistent with the NGF/tanezumab binding affinity being tighter than 10 pM due to the formation of an extremely stable complex that had an estimated half-life exceeding 100 h, which was beyond the resolution of any of our methods. The system was particularly challenging to study because NGF is an obligate homodimer, and we describe various assay orientations and immobilization methods that were used to minimize avidity in our experiments while keeping NGF in as native a state as possible. We also explored the interactions of NGF with its natural receptors, TrkA and P75, and how tanezumab blocks them. The Biacore blocking assay that we designed was used to quantify the potency of tanezumab and is more precise and reproducible than the currently available cell-based functional assays.     

PURE ribosome display and its application in antibody technology

Ribosome display utilizes formation of the mRNA–ribosome–polypeptide ternary complex in a cell-free protein synthesis system to link genotype (mRNA) to phenotype (polypeptide). However, the presence of intrinsic components, such as nucleases in the cell-extract-based cell-free protein synthesis system, reduces the stability of the ternary complex, which would prevent attainment of reliable results. We have developed an efficient and highly controllable ribosome display system using the PURE (Protein synthesis Using Recombinant Elements) system. The mRNA–ribosome–polypeptide ternary complex is highly stable in the PURE system, and the selected mRNA can be easily recovered because activities of nucleases and other inhibitory factors are very low in the PURE system. We have applied the PURE ribosome display to antibody engineering approaches, such as epitope mapping and affinity maturation of antibodies, and obtained results showing that the PURE ribosome display is more efficient than the conventional method. We believe that the PURE ribosome display can contribute to the development of useful antibodies. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.     

Kinetic analysis of unpurified native antigens available in very low quantities and concentrations

Affinity measurements of antigen–antibody interactions are generally performed using known concentrations of purified or recombinant materials. In addition, many technologies that measure affinity require the interacting components to be present in at least microgram quantities. Specifically, if the antigen is either available only in low quantities or unable to be purified, or if the quantity is unknown, then the measurement of affinity can be very difficult. Using the Kinetic Exclusion Assay (KinExA) technology, here we describe a method that overcomes the requirement for large amounts of purified and known quantities of antigen. We used this method to precisely measure the affinity of fully human anti-human interleukin 13 (IL13) monoclonal antibodies to IL13 produced in native form from primary T cells derived from a variety of species, including human. These antigens were available only in the limited quantities present in the conditioned cell culture medium, and the affinity was measured directly without further purification.     

Kinetic exclusion assay of monoclonal antibody affinity to the membrane protein Roundabout 1 displayed on baculovirus

The reliable assessment of monoclonal antibody (mAb) affinity against membrane proteins in vivo is a major issue in the development of cancer therapeutics. We describe here a simple and highly sensitive method for the evaluation of mAbs against membrane proteins by means of a kinetic exclusion assay (KinExA) in combination with our previously developed membrane protein display system using budded baculovirus (BV). In our BV display system, the membrane proteins are displayed on the viral surface in their native form. The BVs on which the liver cancer antigen Roundabout 1 (Robo1) was displayed were adsorbed onto magnetic beads without fixative (BV beads). The dissociation constant (K_d, ∼10⁻¹¹ M) that was measured on the Robo1 expressed BV beads correlated well with the value from a whole cell assay (the coefficient of determination, R² = 0.998) but not with the value for the soluble extracellular domains of Robo1 (R² = 0.834). These results suggest that the BV–KinExA method described here provides a suitably accurate K_d evaluation of mAbs against proteins on the cell surface.     

Directed evolution of an anti-prion protein scFv fragment to an affinity of 1 pM and its structural interpretation

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative prion disease affecting cattle that is transmissible to humans, manifesting as a variant of Creutzfeldt-Jakob disease (vCJD) likely following the consumption of meat contaminated with BSE prions. High-affinity antibodies are a prerequisite for the development of simple, highly sensitive and non-invasive diagnostic tests that are able to detect even small amounts of the disease-associated PrP conformer (PrP(Sc)). We describe here the affinity maturation of a single-chain Fv antibody fragment with a binding affinity of 1 pM to a peptide derived from the unstructured region of bovine PrP (BoPrP (90-105)). This is the tightest peptide-binding antibody reported to date and may find useful application in diagnostics, especially when PrP(Sc) is pretreated by denaturation and/or proteolysis for peptide-like presentation. Several rounds of directed evolution and off-rate selection with ribosome display were performed using an antibody library generated from a single PrP binder with error-prone PCR and DNA-shuffling. As the correct determinations of affinities in this range are not straightforward, competition biosensor techniques and KinExA methods were both applied and compared. Structural interpretation of the affinity improvement was performed based on the crystal structure of the original prion binder in complex with the BoPrP (95-104) peptide by modeling the corresponding mutations.     

Characterizing high-affinity antigen/antibody complexes by kinetic- and equilibrium-based methods

Two biophysical methods, Biacore and KinExA, were used to kinetically and thermodynamically characterize high-affinity antigen/antibody complexes. Three to five independent experiments were performed on each platform with three different antigen/antibody complexes possessing nanomolar to picomolar equilibrium dissociation constants. By monitoring the dissociation phase on Biacore for 4 h, we were able to measure dissociation rate constants (kd) on the order of 1 x 10(-5)s(-1). To characterize high-affinity interactions by KinExA, samples needed to be equilibrated for up to 35 h to reach equilibrium. In the end, we show that similar kinetic rate constants and affinities were determined with both solution-phase and solid-phase methodologies. These results help further validate both interaction technologies and illustrate their suitability for characterizing extremely high-affinity interactions.