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I domain of beta 2 integrin lymphocyte function-associated antigen-1 contains a binding site for ligand intercellular
1Leukocyte Adhesion Laboratory, Imperial Cancer Research Fund, London, United Kingdom.
Insights
The Lymphocyte function-associated antigen-1 (LFA-1) I domain binds intercellular adhesion molecule-1 (ICAM-1), identifying the first ligand binding site in beta 2 integrins. This finding is crucial for understanding leukocyte adhesion and immune responses.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Lymphocyte function-associated antigen-1 (LFA-1) is a critical beta 2 integrin mediating leukocyte adhesion and function.
- LFA-1 interacts with its ligand, intercellular adhesion molecule-1 (ICAM-1), but the precise binding site on LFA-1 remained unidentified.
- The homologous domains in von Willebrand factor are known to bind ligands, suggesting a similar role for the LFA-1 I domain.
Purpose of the Study:
- To identify the ICAM-1 binding site on LFA-1.
- To characterize the function of the isolated LFA-1 I domain.
- To establish the I domain as a key region for LFA-1-ICAM-1 interactions.
Main Methods:
- Expression of the LFA-1 I domain as an isolated functional unit.
- Binding assays using purified recombinant ICAM-1.
- Monoclonal antibody epitope mapping to assess antibody interference with LFA-1-ICAM-1 binding.
- Inhibition assays of LFA-1-dependent T cell adhesion to ICAM-1.
Main Results:
- The isolated LFA-1 I domain was shown to bind directly to purified recombinant ICAM-1.
- The I domain contains epitopes for numerous anti-LFA-1 antibodies, many of which block LFA-1-ICAM-1 interaction.
- The I domain effectively inhibited LFA-1-dependent T cell adhesion to ICAM-1.
Conclusions:
- The LFA-1 I domain is definitively identified as the ICAM-1 binding region.
- This study represents the first identification of a ligand binding site within a beta 2 integrin.
- The findings provide critical insights into the molecular mechanisms of leukocyte adhesion and immune cell trafficking.
Abstract:
Lymphocyte function-associated antigen-1 (LFA-1) is a beta 2 integrin that participates in a broad range of leukocyte functions through binding to its ligand intercellular adhesion molecule-1 (ICAM-1). The location of the ICAM-1 binding site on LFA-1 is not known. A approximately 200-amino acid "inserted" or "I" domain, which is part of the beta 2 integrin alpha subunit, is homologous to the "A" domains found in the adhesive protein von Willebrand factor and in a number of other proteins. In von Willebrand factor, the A domains are involved in ligand binding, but their function in the other proteins is still unclear. In this report, we show that the LFA-1 I domain contains a binding site for ICAM-1, which can be expressed as an isolated functional unit. The I domain contains the epitopes for 18 out of 20 anti-LFA-1 monoclonal antibodies, many of which interfere with the interaction between LFA-1 and ICAM-1. The I domain binds directly to purified recombinant ICAM-1 and also inhibits LFA-1-dependent T cell adhesion to ICAM-1. This report establishes the I domain as an ICAM-1 binding region in LFA-1 and the first ligand binding site to be identified in a beta 2 integrin.