• Overview
• Leukocyte Transmigration
• JAMs
• Selectins
• Integrins
• VLA-4/VCAM-1
• LFA-1/ICAM-1

Adhesion Molecules of Leukocyte Trafficking :
LFA-1/ICAM-1 Interaction

LFA-1

In the early 1980’s Springer identified lymphocyte function-associated antigen-1 (LFA-1, CD11a/CD18) to be the predominant integrinin lymphocytes (Sanchez-Madrid et al., 1983). LFA-1 consists of an a L subunit (also designated as CD11a) and a b 2 subunit (CD18). LFA-1 contains three functionally important domains: 1) the EF hand-like motif in the a subunit (Tuckwell et al., 1992), 2) the I domain also in the a subunit (Huang and Springer, 1995) and 3) the b I domain, a conserved sequence in the b 2 subunit homologous to the a subunit’s I domain (D’Souza et al., 1994). The EF hand-like motif is responsible for Ca 2+ binding and mediates LFA-1 clustering, whereas, the a L I domain contains the ICAM-1 adhesion site. Within the a L I domain is a metal ion dependent adhesion site (MIDAS) that chelated Mg 2+ or Mn 2+, and is directly involved in the binding of LFA-1 to ICAM-1. Away from the ICAM-1 binding site, the a L I domain contains an I domain allosteric site (IDAS) (Huth et al., 2000) that regulates the conformation of LFA-1 and consequently its affinity for ICAM-1.

ICAM-1

The binding partner of LFA-1 is ICAM-1, a member of the immunoglobulin superfamily. ICAM-1 is expressed as both a monomer and a noncovalently linked dimer on a wide variety of cells including macrophages, leukocytes, keratinocytes, and vascular endothelial cells (Reilly et al., 1995; Miller et al., 1995). In addition, its expression can be upregulated on the vascular endothelium by lymphokines released during an inflammatory response. The ICAM-1 monomer is a transmembrane glycoprotein containing five extracellular immunoglobulin-like domains, a transmembrane domain and a short cytoplasmic domain. The LFA-1 binding site centers on Glu-34, located near the middle of domain 1 (D1) in b strand C. The second domain has been found to play a more accessory role in LFA-1 by serving to maintain the structural integrity of the binding site in the first domain (Stanley and Hogg, 1998).

The structural basis of the LFA-1/ICAM-1 interaction

During LFA-1/ICAM-1 complex formation, the I domain of LFA-1 interacts with domain 1 (D1) of ICAM-1 (Bella et al., 1998). Coordinating this interaction is a divalent cation (Mg 2+ or Mn 2+) that chelates to the I domain of LFA-1 (Qu and Leahy, 1995). Based on mutagenesis and epitope mapping studies, the MIDAS region of the LFA-1 I domain has been identified as the binding site for ICAM-1 (McDowall et al., 1998). Structural studies have revealed the MIDAS region to be a relatively flat surface of approximately 10 x10 Å. Interacting with the MIDAS binding site is a similarly flat surface on domain 1 of ICAM-1. Amino acid residues in the MIDAS region that have been shown to be essential for ICAM-1 binding include Leu-205, Glu-241, and Thr-243 (Edwards et al., 1995; Edwards et al., 1998). The affinity of the complex was also reduced when other amino acid residues in the I domain were mutated to an alanine. Included among these residues are Glu-146, Thr-175, and His-264. The complementary binding surface on ICAM-1 includes Glu-34, Met-64, Tyr-66, Asn-68, and Glu-73. The interaction between the binding surfaces of LFA-1 and ICAM-1 appears to be regulated by the I domain allosteric site (IDAS), which is located in I domain, but distal to the ICAM-1 binding surface (Huth et al., 2000).

Affinity modulation of the LFA-1/ICAM-1 interaction

Like most integrins, LFA-1 is expressed on the cell surface in one of two affinity states: low and high (Diamond and Springer, 1994; Hughes and Pfaff, 1998). Resting leukocytes express a form of LFA-1 that binds to ICAM-1 with low affinity. Depending on the cell type, LFA-1 is activated by different external signals. Changes in the affinity state of LFA-1 can be experimentally induced by extracellular divalent cations Mg 2+ or Mn 2+. The activation of LFA-1 by Mg 2+ appears to be a complex process that is initiated by the binding of the divalent cations to the MIDAS of the b I domain of the b subunit. This is then followed by the transduction of signal across the interchain junction to the b -propeller domain, which subsequently induces the a I domain to expose its high affinity binding site for ICAM-1 (Lu et al., 2001). Based on structural homology analysis of the known Mac-1 ( a M b 2 ) structures (Emsley et al., 2000; Lee et al., 1995), it has been proposed that the high affinity state of LFA-1 stemmed from a displacement of the a 7 helix of the LFA-1 I domain. Such predicted conformational changes of I domain are supported by antibody binding studies (Ma et al., 2002).