Multi epitope-targeting immunoprecipitation using F(ab') fragments with high affinity and specificity for the enhanced detection of a peptide with matrix-assisted laser desorption ionization-time-of-flight mass spectrometry.

Human plasma has been frequently studied using mass spectrometry for new biomarker discovery, although detection of low-abundance biological molecules can be challenging due to sample complexity and dynamic protein concentration ranges of plasma proteins. While immunoprecipitation coupled with mass spectrometric analysis is an essential method for overcoming this difficulty, its sensitivity can be insufficient to detect clinically relevant circulating biomarkers because of limited antibody affinity or specificity. To increase antibody affinity, we developed a strategy using a F(ab') fragment coupled to polyethylene glycol. We produced hetero-F(ab')-(PEG)24 beads composed of two monoclonal antiamyloid ß antibodies (6E10 and 4G8) that are specific for different epitopes of amyloid ß and assessed the detection limit of amyloid ß(1-28)-spiked human plasma. In human plasma, the detection limit of amyloid ß(1-28) was 6.14 pM, which was 25- to 50-fold more sensitive than single IgG-protein G beads. In addition, an introduction of polyethylene glycol as a linker reduced nonspecific binding, leading to highly specific MS detection. Finally, the present IP method enabled the detection of endogenous amyloid ß(1-40) in 250 µL of human plasma with matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS). This technique provides a powerful approach for enhancing the sensitivity and specificity of immunoprecipitation (IP)-MS for detection of low-abundance peptides in plasma and has the potential to accelerate MS-based clinical applications.

Flexible antibodies with nonprotein hinges.

There is a significant need for antibodies that can bind targets with greater affinity. Here we describe a novel strategy employing chemical semisynthesis to produce symmetroadhesins: antibody-like molecules having nonprotein hinge regions that are more flexible and extendible and are capable of two-handed binding. Native chemical ligation was carried out under mild, non-denaturing conditions to join a ligand binding domain (Aß peptide) to an IgG1 Fc dimer via discrete oxyethylene oligomers of various lengths. Two-handed Aß-Fc fusion proteins were obtained in quantitative yield and shown by surface plasmon resonance to bind an anti-Aß antibody with a K(D) at least two orders of magnitude greater than the cognate Aß peptide. MALDI-TOF MS analysis confirmed the protein/nonprotein/protein structure of the two-handed molecules, demonstrating its power to characterize complex protein-nonprotein hybrids by virtue of desorption/ionization mediated by peptide sequences contained therein. We anticipate many applications for symmetroadhesins that combine the target specificity of antibodies with the novel physical, chemical and biological properties of nonprotein hinges.