Integrins are large heterodimeric cell surface receptors of glycoproteins and have many critical functions in many aspects of cell physiology and pathology.
b2 integrins is specific for leukocytes mediating critical steps in cell adhesion and diapedesis during immune responses. LFA-1 is one of the most abound
b2 integrins. ICAM-1 is the major ligand for LFA-1 and expresses both on leukocytes and endothelial cells. The leukocytes expressing LFA-1 do not adhere spontaneously to cells or surfaces bearing ICAM-1, so it is generally accepted that ICAM-1 doesn’t bind with the resting LFA-1. Objectives: To investigate whether pure ICAM-1 binds LFA-1 and induces the LFA-1 conformational change from resting state to activated state. Methods: We used different concentration of ICAM-1 with or without Protein Kinase C activator to treat Thp-1 cells. Immuno-fluorescence staining with m24 monoclonal antibody, a conformational marker, and flowcytometry was used to detected the LFA-1 conformational change. Result: Without the presence of any other stimulator, the ICAM-1 bound to LFA-1 on Thp-1 cells and induced LFA-1 into an activated state. The conformational change of LFA-1 induced by ICAM-1 is concentration dependent. PMA alone induced LFA-1 conformational change to a detectable degree but much lower than the ICAM-1 induction. Conclusion: Contradicting to the previous believe, our data illustrated that the presence of ICAM-1 alone on the endothelial cell is sufficient to induce the conformational change of LFA-1. The fact that leukocyte bearing LFA-1 can’t stably adhere to ICAM-1 coated surface must due to different reason. We believe that the LFA-1 conformational change alone is not enough to mediate the stable interact between LFA-1 and ICAM-1, may be due to the weak binding force. Rather, LFA-1 clustering may be necessary and critical for mediating the firm cell adhesion through LFA-1 and ICAM-1 interaction.
Supported by NIH: GM54715 and AHA: 0240017N