A novel leukocyte adhesion deficiency caused by expressed but nonfunctional β2 integrins Mac-1 and LFA-1

Abstract

The adhesive response of circulating leukocytes to inflammatory stimuli is now well documented (1, 2). After such signals, leukocytes adhere to the blood vasculature using selectin-mediated interactions, and this stage leads to activation of their integrins. The β2 or leukocyte integrins lymphocyte function-associated molecule (LFA)-1 (CD11a/CD18) has a major role in the firm adhesion of leukocytes to endothelium and in their migration across this barrier. In addition, LFA-1 cooperates with the T-cell receptor in antigen-stimulated T-cell priming (3) and, in general, participates in the formation of leukocyte-leukocyte contacts. Mac-1 (CD11b/CD18) is a major phagocytic receptor operating in association with the third β2 integrin, p150,95 (CD11c/CD18), and both recognize as ligands fibrinogen and the complement fragment iC3b (4, 5).Expression of integrins on the cell membrane is not a guarantee of their ability to function as adhesion receptors. Integrins must undergo conversion from inactive to active ligand-binding status, which occurs through a process of clustering and/or altered conformation (6, 7). The stimulus for this activation is initiated by the triggering of other membrane receptors, a route of signal transduction that has been termed "inside out" signaling. The integrins bind divalent cations such as Mg 2+ or Mn 2+ in order to function, and an alternative means of directly altering integrin activity is through extracellular exposure to these cations. It is thought that these latter procedures mimic the conformational changes brought about by integrin-activating signals generated intracellularly. Special anti-integrin monoclonal antibodies (MABs) can serve as reporters of this activation. For example, MAB 24 recognizes an epitope on high-affinity β2 integrin (7) and detects a conformational change in the form of interdomain movement involving the ligand binding I domain on the integrin α subunit (8).Valuable information about the functioning of the β2 integrins has come from study of the leukocyte adhesion deficiency (LAD)-1 syndrome. LAD-1 is an autosomal recessive disorder caused by mutation in the CD18 gene on chromosome 21 that leads to absent or aberrant biosynthesis of the β2 subunit of leukocyte integrins (9-11). This lesion is reflected in the absence or markedly diminished expression on the leukocyte cell surface of the β2 integrins, CD11a/CD18 (LFA-1), CD11b/CD18 (Mac-1), and CD11c/CD18 (p150,95). The patient phenotype is variable but reflects the level of β2 integrins expressed. Patients with <1% expression suffer from life-threatening infections and require bone marrow transplantation for long-term survival (12). In patients with 1%-10% expression, defects in leukocyte mobility, adherence, and endocytosis lead to periodic periodontitis, skin infections, and retarded wound healing with dysplastic scarring. The heterozygotic relatives of the patients have ∼40%-60% normal levels of β2 inte- In the leukocyte adhesion deficiency (LAD)-1 syndrome, there is diminished expression of β2 (CD18) inte...

Distinct regulatory mechanisms control integrin adhesive processes during tissue morphogenesis

“…b1 and b2, the second I-like domain mutation, S196F (S123 in b3, S134 in b1, S138 in b2), was designed to bind to a subset of ECM molecules but prevent the resulting conformational changes essential to transmit outside-in signals (Bajt and Loftus, 1994;Hogg et al, 1999;Chen et al, 2006b). This allele was identified previously in Drosophila and found to behave effectively as a missense ''null'' (Jannuzi et al, 2002(Jannuzi et al, , 2004, but detailed phenotypic analyses were not performed.…”

“…Comparing the rescue capacities of the S196F and D192A/S194A mutants allowed us to uncouple ECM binding from ligandinduced outside-in conformational changes to determine their relative contributions to morphogenesis: while the D192A/S194A mutant should not bind ECM, the S196F mutant should retain minimal ECM binding capacity but fail to relay outside-in signals due to conformational constraints (Bajt and Loftus, 1994;Hogg et al, 1999;Chen et al, 2006b). While GBR, DC, and MTJs were differentially sensitive to perturbation of some aspects of integrin function, we found that all failed in S196F rescue embryos.…”

On the potential involvement of CD11d in co-stimulating the production of interferon- by natural killer cells upon interaction with neutrophils via intercellular adhesion molecule-3

“…Highly purified neutrophils, NK cells and slanDC were isolated and cultured for 18 h as previously described, 5 with or without 100 ng/mL LPS plus the IL-15/IL-18 combination (both at 10 ng/mL), in the presence of 10 mg/mL αCD11a/CD18 efalizumab, a humanized IgG1 antibody derived from the murine MHM24 clone 10 which, in preliminary experiments, confirmed its ability to prevent the binding of CD11a to either ICAM-1 11 or ICAM-3 12 in a specific T-cell adhesion assay 13 , 10 mg/mL αCD18 or αCD18 F(ab')2 fragments [clone IB4 (mouse IgG2a)], 10 mg/mL αDC-SIGN [clone 120507 (mouse IgG2b) 14 or clone MR-1 (mouse IgG1) 15 ], as well as corresponding isotype control antibodies. Monocytederived dendritic cells (moDC) were generated as previously described.…”

Integrin‐dependent pathologies

“…In both mild and severe LAD-1 clinical complications are essentially caused by bacterial and not viral or fungal infections, suggesting essentially neutrophil and monocyte defects with relatively normal levels of lymphocytes relying on α 4 β 1 -integrindependent adhesion (VLA-4) for transmigration. More recently, variant forms of LAD-1 have been reported in which β 2 -integrins are normally expressed on the surface of leucocytes but fail to support their adhesion to the endothelial cell [33][34][35][36]. In one of these cases, LAD-1 was caused by a point mutation in the β 2 -integrin, rendering it non-responsive to insideout activation of leucocytes [33].…”

“…More recently, variant forms of LAD-1 have been reported in which β 2 -integrins are normally expressed on the surface of leucocytes but fail to support their adhesion to the endothelial cell [33][34][35][36]. In one of these cases, LAD-1 was caused by a point mutation in the β 2 -integrin, rendering it non-responsive to insideout activation of leucocytes [33]. In the other cases, β 2 -integrin-dependent leucocyte adhesion deficiencies were extended to a failure of β 1 -and/or β 3 -integrin function.…”