Ligand intercellular adhesion molecule 1 has a necessary role in activation of integrin lymphocyte...

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• Leukocyte integrin lymphocyte function-associated molecule (LFA-1) mediates T-cell adhesion via its ligand, intercellular adhesion molecule 1 (ICAM-1).
• Inside-out signaling, transduced indirectly through T-cell receptors, is the known mechanism for LFA-1 activation.
• The high-affinity state of LFA-1 is associated with a specific epitope recognized by monoclonal antibody 24.

Purpose of the Study:

• To investigate the precise signaling events and conformational changes required for LFA-1 high-affinity binding.
• To determine if ligand binding itself is a prerequisite for full LFA-1 activation.
• To characterize the role of the monoclonal antibody 24 epitope in LFA-1 function.

Main Methods:

• Utilized monoclonal antibody 24 to detect the high-affinity LFA-1 epitope.
• Investigated epitope expression in response to agonist signaling versus ICAM-1 binding.
• Employed a fixation protocol to assess receptor behavior after initial ligand interaction.
• Observed LFA-1 localization at T-cell and ICAM-1 transfectant contact sites.

Main Results:

• The monoclonal antibody 24 epitope is expressed upon LFA-1 binding to ICAM-1, not solely by initial agonist signals.
• This epitope is localized to LFA-1 at the cell-cell contact interface.
• T cells require prior ICAM-1 exposure for secondary firm binding, even after receptor fixation.
• Ligand binding induces a conformational change in LFA-1, contributing to its activation.

Conclusions:

• Full activation of LFA-1 necessitates an initial interaction with its ligand, ICAM-1, in addition to inside-out signaling.
• LFA-1 activation involves an 'induced fit' mechanism, where ligand binding imposes an external conformational change.
• This finding reveals a previously unrecognized, ligand-dependent step in leukocyte integrin activation.