Force spectroscopy of the leukocyte function-associated antigen-1/intercellular adhesion molecule-1 interaction

Area of Science:

• Biophysics
• Cellular Adhesion
• Immunology

Background:

• Leukocyte adhesion is critical for immune responses, involving interactions like leukocyte function-associated antigen-1 (LFA-1) with intercellular adhesion molecule-1 (ICAM-1).
• Understanding these interactions under physiological conditions, including blood flow and external forces, is essential.
• The mechanical properties of cell adhesion molecules significantly impact their function.

Purpose of the Study:

• To investigate the binding properties of the LFA-1/ICAM-1 interaction under steady-state and external pulling forces.
• To elucidate the energy landscape and energetic determinants of the LFA-1/ICAM-1 complex.
• To define how cofactors and mechanical forces modulate LFA-1/ICAM-1 binding.

Main Methods:

• Atomic force microscopy (AFM) was employed to measure single-molecule unbinding events between LFA-1 and ICAM-1.
• An experimental setup involved an LFA-1-expressing T cell hybridoma attached to an AFM cantilever and an ICAM-1 expressing surface.
• Force loading rates were varied over three orders of magnitude to analyze the force spectrum.

Main Results:

• AFM measurements revealed distinct inner (steep) and outer (wide) activation barriers governing LFA-1/ICAM-1 dissociation.
• Magnesium ions (Mg2+), a cofactor, increased the unbinding force in the slow loading regime, stabilizing the complex.
• EDTA, a chelator, suppressed the inner activation barrier, reducing the complex's resistance to force.

Conclusions:

• The equilibrium dissociation constant of LFA-1/ICAM-1 is primarily regulated by the outer activation barrier's energetics.
• The complex's ability to withstand external pulling forces is determined by the divalent cation-dependent inner activation barrier.
• These findings provide insights into the mechanical regulation of leukocyte adhesion.