Feasibility of measuring antigen-antibody interaction forces using a scanning force microscope.

Area of Science:

• Biophysics
• Surface Science
• Immunology

Background:

• Measuring antigen-antibody binding forces is crucial for understanding immune responses and developing diagnostics.
• Scanning Force Microscopy (SFM) offers high-resolution force measurements, but adapting it for specific molecular interactions requires careful probe design.

Purpose of the Study:

• To evaluate the efficacy of molecular affinity scanning force microscopy (MASFM) using a spherical bead probe for quantifying antigen-antibody binding forces.
• To analyze force-separation distance profiles to differentiate specific antigen-antibody interactions from nonspecific forces.

Main Methods:

• Developed a MASFM configuration with a glass bead probe functionalized with fluorescein antigen and a substrate with immobilized antifluorescyl IgG antibody.
• Measured adhesion forces and force-separation profiles between specific antigen-antibody pairs and control surfaces.
• Analyzed force discontinuities, mean adhesion forces, and force histograms, and attempted to fit data to a Poisson discrete-force model.

Main Results:

• Specific antigen-antibody interactions exhibited more force discontinuities and higher mean adhesion forces compared to nonspecific interactions.
• Nonspecific forces, likely from spacer layer deformation, significantly contributed to the measured forces.
• Mean forces did not correlate with protein activation enthalpies, and force histograms for specific and nonspecific interactions were indistinguishable at lower force values.

Conclusions:

• The use of a large spherical probe in MASFM is counterproductive for measuring specific antigen-antibody binding due to excessive sampling of nonspecific forces.
• Nonspecific interactions, potentially mediated by spacer layer deformation, complicate the interpretation of specific binding events.
• Alternative probe designs or surface chemistries may be necessary to improve the specificity and resolution of MASFM for molecular binding studies.