Dual Regulation of Ligand Binding by CD11b I Domain

Abstract

The role of coagulation factor X as a ligand for CD11b/CD18 (Mac-1, α_Mβ₂) in leukocyte adhesion was investigated. A factor X peptide, (G)L²³⁸YQAKRFKV²⁴⁶(G), blocked ligand binding to CD11b/CD18 and prevented monocyte procoagulant activity. This peptide also inhibited monocytic THP-1 cell adhesion to tumor necrosis factor α-stimulated endothelium and blocked neutrophil migration through tumor necrosis factor α-activated endothelial cell monolayers. In contrast, other factor X-derived peptides were ineffective. Radiolabeled peptide (G)LYQAKRFKV(G) bound specifically and saturably to isolated recombinant CD11b I domain. Functionally, the factor X sequence (G)LYQAKRFKV(G) dose-dependently inhibited THP-1 cell attachment to intercellular adhesion molecule 1 (ICAM-1) transfectants (IC₅₀ = ∼50 μg/ml), indistinguishably from anti-CD18 monoclonal antibodies 60.3 and IB4. In contrast, peptide (G)LYQAKRFKV(G) failed to reduce binding of¹²⁵I-fibrinogen to immobilized CD11b I domain, which was abolished by the fibrinogen-derived peptide KYG¹⁹⁰WTVFQKRLDGSV²⁰². By Lineweaver-Burke analysis, peptide (G)LYQAKRFKV(G) inhibited factor X binding to CD11b/CD18 in a noncompetitive fashion, and intact factor X did not reduce monocyte-endothelial cell interaction. These data suggest that the factor X sequence (G)LYQAKRFKV(G) defines an ICAM-1-binding site on CD11b I domain physically distinct from and nonoverlapping with the fibrinogen interacting region(s). Engagement of this site induces a conformational change in the holoreceptor, which disrupts a distant factor X-binding site required for monocyte procoagulant activity. These observations demonstrate a dual regulatory role of CD11b I domain in ligand binding and provide a molecular basis for the recently reported anti-inflammatory properties of factor X homologous sequencesin vivo.