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Intercellular adhesion molecule-1 dimerization and its consequences for adhesion mediated by lymphocyte function
1Center for Immunology, Washington University School of Medicine, St. Louis, Missouri, USA.
Insights
Intercellular adhesion molecule-1 (ICAM-1) forms dimers, enhancing its binding to LFA-1. This homodimerization is crucial for ICAM-1
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Intercellular adhesion molecule-1 (ICAM-1, CD54) is a key immune cell adhesion molecule.
- ICAM-1 functions as a ligand for integrins like LFA-1 and Mac-1, mediating immune and inflammatory responses.
Purpose of the Study:
- To investigate the homodimerization of ICAM-1.
- To determine the functional significance of ICAM-1 homodimerization in ligand binding and cell adhesion.
Main Methods:
- Expression and modification of recombinant soluble ICAM-1.
- Binding assays using purified LFA-1.
- Monoclonal antibody binding studies on cell surface ICAM-1 mutants.
- Chemical cross-linking of cell surface ICAM-1.
Main Results:
- Modified soluble ICAM-1 forms dimers, indicating homophilic interaction sites in its ectodomain.
- Soluble ICAM-1 dimers exhibit high-avidity binding to LFA-1 (Kd = 8 nM), unlike monomers.
- Cell surface ICAM-1 exists as dimers, confirmed by antibody binding and cross-linking experiments.
- Dimeric cell surface ICAM-1 demonstrates enhanced potency as an LFA-1 ligand compared to monomers.
Conclusions:
- ICAM-1 undergoes functionally significant homodimerization.
- ICAM-1 dimers serve as more potent ligands for LFA-1-mediated adhesion.
- This study provides the first evidence of specific, functionally important homodimerization for a cell surface integrin ligand.
Abstract:
Intercellular adhesion molecule-1 (ICAM-1, CD54) is a ligand for the integrins lymphocyte function associated-1 (LFA-1, CD11a/CD18) and complement receptor-3 (Mac-1, CD11b/CD18) making it an important participant in many immune and inflammatory processes. Modified recombinant soluble ICAM-1 formed dimers. This result indicated that the ectodomain of ICAM-1 contains homophilic interaction sites. Soluble ICAM-1 dimers bind to solid-phase purified LFA-1 with high avidity (dissociation constant [Kd] = 8 nM) in contrast to soluble ICAM-1 monomers whose binding was not measurable. Cell surface ICAM-1 was found to be dimeric based on two distinct criteria. First, a monoclonal antibody specific for monomeric soluble ICAM-1, CA7, binds normal ICAM-1 poorly at the cell surface; this antibody, however, binds strongly to two mutant forms of ICAM-1 when expressed at the cell surface, thus identifying elements required for dimer formation. Second, chemical cross-linking of cell surface ICAM-1 on transfected cells and tumor necrosis factor-activated endothelial cells results in conversion of a portion of ICAM-1 to a covalent dimer. Cell surface ICAM-1 dimers are more potent ligands for LFA-1-dependent adhesion than ICAM-1 monomers. While many extracellular matrix-associated ligands of integrins are multimeric, this is the first evidence of specific, functionally important homodimerization of a cell surface integrin ligand.