Small Molecule Antagonists of Complement Receptor Type 3 Block Adhesion and Adhesion-Dependent Oxidative Burst in Human Polymorphonuclear Leukocytes

The leukocyte integrin complement receptor type 3 (CR3, Mac-1, CD11b/CD18) is the predominant β2 integrin receptor of polymorphonuclear leukocytes (PMNs). This cell surface receptor plays a central role in innate immunity against pathogens as well as being a major cellular effector of inflammation and tissue injury. Two small molecules, compounds 1 and 2, have been identified, that interact with CR3 and prevent CR3 from binding to its natural ligand, C3bi. Compounds 1 and 2 have IC50 values of 0.14 and 0.33 μM, respectively, for the inhibition of binding of monomeric C3bi-alkaline phosphatase to immobilized CR3. Both compounds also inhibit binding of CR3 to biotinylated sheep red blood cells opsonized with C3bi, with IC50 values in the micromolar range. Inhibition of ligand binding by the compounds is not easily reversed and requires light, suggesting the formation of a covalent adduct through photoactivation. Compounds 1 and 2 also inhibit adhesion of human PMNs to fibrinogen in response to tumor necrosis factor (TNF) or PMA, with IC50values of 2.5 to >10 μM. They block the adhesion-dependent production of H2O2 stimulated by TNF or phorbol 12-myristate 13-acetate (PMA) with IC50 values of 0.2 to 0.8 μM and 1 to 3 μM, respectively. Limited structure-activity relationship studies based on compound 2 indicate the importance of the two benzothiazole rings, an ethyl side chain, and the length of the carbon chain linking the rings. Further modification of these groups may help in making compounds appropriate for in vivo studies.