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...

SUMMARYLeucocyte adhesion deficiency (LAD) is a hereditary disorder caused by mutations in the CD18 (b2 integrin) gene. Four missense mutations have been identified in three patients. CD18(A270V) supports, at a diminished level, CD11b/CD18 (Mac-1, aMb2 integrin) and CD11c/CD18 (p150,95, aXb2 integrin) expression and function but not CD11a/CD18 (LFA-1, aLb2 integrin) expression. Conversely, CD18(A341P) supports a limited level of expression and function of CD11a/CD18, but not of the other two CD11/CD18 antigens. CD18(C590R) and CD18(R593C) show a decreasing capacity to associate with the CD11a, CD11c and CD11b subunits. Transfectants expressing the CD11a/CD18 with the C590R and R593C mutations are more adhesive than transfectants expressing wild-type LFA-1, and express the reporter epitope of the monoclonal antibody 24 constitutively. Thus, the four mutations affect CD18 differently in its capacities to support CD11/CD18 expression and adhesion. These results not only provide a biochemical account for the clinical diversity of patients with leucocyte adhesion deficiency, but also offer novel insights into the structural basis of interaction between the a and b subunits, which is an integral component in our understanding of integrin-mediated adhesion and its regulation.

The cysteine-rich region (CRR) of the L L2 integrin subunit was replaced by that of L L1 to give the chimera L L2NV1. L L2NV1 can combine with K KL to form a variant leukocytefunction-associated antigen (LFA)-1 on COS cell surface, suggesting that the specificity of the L L2 interaction with K KL does not lie in the CRR. Unlike those expressing wild-type LFA-1, COS cells expressing K KLL L2NV1 are constitutively active in intercellular adhesion molecule (ICAM)-1 adhesion. These results suggest that activation of LFA-1 involves the release of an intramolecular constraint, which is maintained, in part, by the authentic L L2 CRR. z 1998 Federation of European Biochemical Societies.

Of the 56 cysteines in the extracellular domain of the CD18 antigen (L L2 integrin subunit), corresponding ones are not found in 12 positions in the L L4, L L7, or L L8 integrin subunits. These 12 cysteines were mutated to alanines, either singly or in pairs, in CD18. All these mutants can support the expression of all three CD11/CD18 integrins. Transfectants expressing these variant integrins are generally more adhesive than the wild-type, suggesting that the cysteine residues, perhaps by engaging in disulphide bonds, may contribute to the maintenance of the CD11/CD18 integrins in a resting state. ß