Lymphocyte function-associated (LFA) molecules and intercellular adhesion molecules (ICAMs) are important both in antigen-specific interactions in immune responses and in interactions with endothelial cells during leukocyte localization in inflammation. LFA-1 is an alphabeta heterodimer and a member of the integrin family. It binds to 3 ICAMS, (ICAM-1, ICAM-2, and ICAM-3) that are members of the Ig superfamily (IgSF) and are closely related (35-55%) to one another. ICAM-4 and ICAM-5 are further homologues that bind more weakly to LFA-I. The physiologic importance of these molecules is demonstrated by effect of deficiency of LFA-1 in an inherited disease, LAD; effect of LFA-1 and ICAM-1 deficiency in mice; and effect of mAb to these molecules in inhibiting antigen-specific T lymphocyte responses, graft rejection, and animal models of disease. Studies of the function of these molecules in health and disease, the regulation of adhesiveness through them, and the structure of their interaction sites is of great importance, and is leading to the development of novel anti-inflammatory agents. LFA-1 transduces signals bidirectionally across the plasma membrane that regulates binding to extracellular ligands and intracellular signaling. The conformational changes in the extracellular and cytoplasmic domains and the mechanisms that couple them are of great interest. We will determine the mechanism of transduction of conformational change from one domain to another in the extracellular portion of LFA-1 using mutations that stabilize particular conformational states or affect inter-domain linkage, small molecule antagonists with newly defined allosteric mechanisms of action, antibodies to activation epitopes, and measurement of ligand binding. Distinct overall conformational states of LFA-1 will be defined by electron microscopy, and ligand binding headpiece conformations by crystal structures. The overall structure of ICAM-1, the basis for LFA-1 recognition of different ICAMs, and the basis for formation of one-dimensional lattices of ICAM-1 and LFA-1 will be defined in structural and mutational studies. The contributions to cellular adhesiveness of clustering in the membrane and intermediate and high affinity states of LFA-1 will be examined. The dynamics of cytoplasmic domain conformational change will be examined by FRET. The importance of dimerization and the cytoplasmic domain interactions of ICAM-1 will be examined in the formation of ICAM-l-enriched microvillous processes that are stimulated by engagement of LFA-1 and that form a cup-like structure surrounding 9 transmigratory leukocytes.
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