Cellular activation of leukocyte function-associated antigen-1 and its affinity are regulated at the I domain...

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• The integrin LFA-1 (leukocyte function-associated antigen 1) plays a crucial role in immune cell adhesion.
• Its ligand-binding interface, located in the I domain, includes the metal ion-dependent adhesion site (MIDAS).
• ICAM-1 (intercellular adhesion molecule 1) is a key ligand for LFA-1.

Purpose of the Study:

• To investigate the role of the I domain allosteric site (IDAS) in regulating LFA-1/ICAM-1 binding.
• To determine how intracellular signaling affects LFA-1 activation at the IDAS.
• To characterize the impact of IDAS mutations on LFA-1 binding kinetics and conformational changes.

Main Methods:

• Site-directed mutagenesis to create inhibitory and activating mutations at the IDAS.
• Stimulation with phorbol 12-myristate 13-acetate (PMA) to induce LFA-1 activation.
• Binding assays using soluble ICAM-1 and a novel CD18 activation epitope mAb.
• Surface plasmon resonance (SPR) to measure binding kinetics (Kd and Ka).

Main Results:

• Inhibitory IDAS mutations are dominant over activating cytoplasmic signals.
• Activating IDAS mutations enhance LFA-1 binding to ICAM-1 in response to PMA without causing constitutive binding.
• IDAS mutations significantly alter the dissociation rate (Kd) of ICAM-1 binding, demonstrating a 6-fold change.
• SPR data revealed differences in Kd but identical association rates (Ka) between wild-type and mutant LFA-1.

Conclusions:

• The IDAS is a critical regulator of LFA-1 binding to ICAM-1, modulated by intracellular signals.
• IDAS mutations directly influence LFA-1 affinity by altering the dissociation kinetics.
• Regulation at the IDAS controls LFA-1 affinity states, involving conformational changes in the I domain and CD18.