Monoclonal antibodies that block the activity of leukocyte function-associated antigen 1 recognize three discrete epitopes in the inserted domain of CD11a

Abstract

The epitopes recognized by eight independently isolated monoclonal antibodies to the α chain of human and murine leukocyte function-associated antigen 1 (LFA-1), all able to inhibit receptor function, were identified. Initial localization of epitopes was accomplished using chimeric proteins constructed by splicing fragments of cD-NAs encoding the α subunit of LFA-1 (CD11a) and the α subunit of the closely related leukocyte integrin, Mac-1 (CD11b). Antibody binding to CD11a/CD11b chimeras, expressed in the 293 human kidney cell line, demonstrated that the epitopes recognized by six monoclonal antibodies to human CD11a were located in a ∼200-amino acid sequence found in all β2-integrin α subunits, termed the inserted (I) domain. Three distinct epitopes within the I domain (IdeA, IdeB, and IdeC) were identified using a series of mutants in which sequences from murine CD11a were substituted into human CD11a. A series of mutants incorporating single amino acid substitutions was used to identify individual amino acids essential for antibody binding. The location of these residues accounts for the binding specificity of LFA-1-blocking antibodies and identifies particular conserved sequences (residues 126-150) in the I domain of CD11a and homologous sequences in other β2-integrin α subunits that may be important for ligand binding.