Molecular characterization of leukocyte adhesion deficiency in six patients

Clinical and Genetic Spectrum of a Large Cohort of Patients With Leukocyte Adhesion Deficiency Type 1 and 3: A Multicentric Study From India

Leukocyte adhesion deficiency (LAD) syndrome is a group of inborn errors of immunity characterized by a defect in the cascade of the activation and adhesion leading to the failure of leukocyte to migrate to the site of tissue injury. Three different types of LAD have been described. The most common subtype is LAD type 1 (LAD1) caused due to defects in the ITGβ2 gene. LAD type 2 (LAD2) is caused by mutations in the SLC35C1 gene leading to a generalized loss of expression of fucosylated glycans on the cell surface and LAD type 3 (LAD3) is caused by mutations in the FERMT3 gene resulting in platelet function defects along with immunodeficiency. There is a paucity of data available from India on LAD syndromes. The present study is a retrospective analysis of patients with LAD collated from 28 different centers across India. For LAD1, the diagnosis was based on clinical features and flow cytometric expression of CD18 on peripheral blood leukocytes and molecular confirmation by Sanger sequencing. For patients with LAD3 diagnosis was largely based on clinical manifestations and identification of the pathogenic mutation in the FERMT3 gene by next-generation Sequencing. Of the total 132 cases diagnosed with LAD, 127 were LAD1 and 5 were LAD3. The majority of our patients (83%) had CD18 expression less than 2% on neutrophils (LAD1°) and presented within the first three months of life with omphalitis, skin and soft tissue infections, delayed umbilical cord detachment, otitis media, and sepsis. The patients with CD18 expression of more than 30% (LAD1⁺) presented later in life with skin ulcers being the commonest manifestation. Bleeding manifestations were common in patients with LAD3. Persistent neutrophilic leukocytosis was the characteristic finding in all patients. 35 novel mutations were detected in the ITGβ2 gene, and 4 novel mutations were detected in the FERMT3 gene. The study thus presents one of the largest cohorts of patients from India with LAD, focusing on clinical features, immunological characteristics, and molecular spectrum.

Molecular characterization of leukocyte adhesion deficiency-I in Indian patients: identification of 9 novel mutations

PURPOSE

Leukocyte adhesion deficiency type-I (LAD-I) is caused by mutations in the ITGB2 gene, encoding the β2-subunit of β2-integrin (CD18) which leads to markedly reduced expression of CD18 on leukocytes resulting into recurrent life threatening infections. Here we aim to identify the molecular defects underlying LAD-I in Indian patients and correlate with the clinical presentation.

METHODS

Blood was collected from 30 patients and their parents for absolute neutrophil count, expression of CD18 and CD11 by flow cytometry and DNA extraction. PCR and DNA sequencing of the ITGB2 gene was done for mutation characterization.

RESULTS

Phenotypically, 22 patients were LAD-I(0), 1 was LAD-I(-) and 7 were LAD-I(+) showing no expression and reduced expression of CD18 respectively. Nine novel mutations in 15 patients and 11 known mutations in 16 patients were detected. Prenatal diagnosis was performed for 5 families.

CONCLUSION

In this study 30 patients were phenotypically and genotypically evaluated for a less known disease LAD-I. Unavailability of curative options to majority of the patients and high cost of supportive care emphasize the need to increase awareness about a suspicious case so that timely management can be given to the patient and prenatal diagnosis can be offered to their families.

Characterization of single amino acid substitutions in the β2 integrin subunit of patients with leukocyte adhesion deficiency (LAD)-1

Leukocyte adhesion deficiency 1 (LAD-1) is caused by defects in the β2 integrin subunit. We studied 18 missense mutations, 14 of which fail to support the surface expression of the β2 integrins. Integrins with the β2-G150D mutation fail to bind ligands, possibly due to the failure of the α1 segment of the βI domain to assume an α-helical structure. Integrins with the β2-G716A mutation are not maintained in their resting states, and the patient has the severe phenotype of LAD-1. The β2-S453N and β2-P648L mutants support the expression of integrins and adhesion functions. They should be re-classified as polymorphic variants.

Hematologically important mutations: leukocyte adhesion deficiency (first update)

Leukocyte adhesion deficiency (LAD) is an immunodeficiency caused by defects in the adhesion of leukocytes (especially neutrophils) to the blood vessel wall. As a result, patients with LAD suffer from severe bacterial infections and impaired wound healing, accompanied by neutrophilia. In LAD-I, mutations are found in ITGB2, the gene that encodes the β subunit of the β(2) integrins. This syndrome is characterized directly after birth by delayed separation of the umbilical cord. In the rare LAD-II disease, the fucosylation of selectin ligands is disturbed, caused by mutations in SLC35C1, the gene that encodes a GDP-fucose transporter of the Golgi system. LAD-II patients lack the H and Lewis Le(a) and Le(b) blood group antigens. Finally, in LAD-III (also called LAD-I/variant) the conformational activation of the hematopoietically expressed β integrins is disturbed, leading to leukocyte and platelet dysfunction. This last syndrome is caused by mutations in FERMT3, encoding the kindlin-3 protein in all blood cells that is involved in the regulation of β integrin conformation.

A novel 3' splice-site mutation and a novel gross deletion in leukocyte adhesion deficiency (LAD)-1

A patient was diagnosed with leukocyte adhesion deficiency-1. She was born in 1996 and her parents are not known to be related. Her leukocytes expressed less than 2% of the CD18 antigens relative to normal individuals. Molecular analysis revealed that she is a compound heterozygote. She inherited a 27,703bp deletion from her father (g.43201_PTTG1IP:10890del27703), spanning from intron 11 of the gene for the β2 integrin (ITGB2, CD18, NG_007270.2) to intron 2 of the gene for the Pituitary Tumor-Transforming Gene 1 Interacting Protein (PTTG1IP, NC_000021.8). The maternal allele has a g.23457C>A mutation at position -10 in intron 2 of the ITGB2 gene, resulting in the activation of a cryptic 3' splice site in intron 2 to include 43 intronic nucleotides (r.[59-43_59-1ins;59-10C>A]).

Characterization of 11 new cases of leukocyte adhesion deficiency type 1 with seven novel mutations in the ITGB2 gene

BACKGROUND

Leukocyte adhesion deficiency type 1 (LAD I) is an autosomal recessive disorder caused by mutations in the ITGB2 gene, encoding the beta2 integrin family. Severe recurrent infections, impaired wound healing, and periodontal diseases are the main features of disease.

METHODS

In order to investigate clinical and molecular manifestations of new LAD I cases, 11 patients diagnosed in one center during 7 years were studied. Patients were screened for the ITGB2 gene mutations, using polymerase chain reaction, followed by single-strand conformation polymorphism and sequencing.

RESULTS

The most common first presenting feature of the patients was omphalitis. The mean age of cord separation was 19.9 +/- 1 days. The most common clinical manifestations of the patients during the follow-up period included omphalitis, skin ulcers with poor healing, sepsis, and otitis media. During the follow-up, eight patients died. Eight homozygous changes, including seven novel mutations, were detected: two splicing (IVS4-6C>A, IVS7+1G>A), three missense (Asp128Tyr, Ala239Thr, and Gly716Ala), and three frameshift deletions (Asn282fsX41, Tyr382fsX9, and Lys636fsX22).

CONCLUSION

Our results indicate that different mutations underlie the development of LAD I. Definitive molecular diagnosis is valuable for genetic counseling and prenatal diagnosis. Regarding clinical presentations, it seems that omphalitis is the most consistent finding seen in LAD I infants.

Clinical significance of complement deficiencies

The complement system is composed of more than 30 serum and membrane-bound proteins, all of which are needed for normal function of complement in innate and adaptive immunity. Historically, deficiencies within the complement system have been suspected when young children have had recurrent and difficult-to-control infections. As our understanding of the complement system has increased, many other diseases have been attributed to deficiencies within the complement system. Generally, complement deficiencies within the classical pathway lead to increased susceptibility to encapsulated bacterial infections as well as a syndrome resembling systemic lupus erythematosus. Complement deficiencies within the mannose-binding lectin pathway generally lead to increased bacterial infections, and deficiencies within the alternative pathway usually lead to an increased frequency of Neisseria infections. However, factor H deficiency can lead to membranoproliferative glomerulonephritis and hemolytic uremic syndrome. Finally, deficiencies within the terminal complement pathway lead to an increased incidence of Neisseria infections. Two other notable complement-associated deficiencies are complement receptor 3 and 4 deficiency, which result from a deficiency of CD18, a disease known as leukocyte adhesion deficiency type 1, and CD59 deficiency, which causes paroxysmal nocturnal hemoglobinuria. Most inherited deficiencies of the complement system are autosomal recessive, but properidin deficiency is X-linked recessive, deficiency of C1 inhibitor is autosomal dominant, and mannose-binding lectin and factor I deficiencies are autosomal co-dominant. The diversity of clinical manifestations of complement deficiencies reflects the complexity of the complement system.

The extracellular domain of CD11d regulates its cell surface expression

A mAb targeting the CD11d subunit of the leukocyte integrin CD11d/CD18 decreases intraspinal inflammation and oxidative damage leading to improved neurological outcomes in rodent models of SCI. CD11d/CD18 is the fourth member of the beta2-integrin family. Current evidence indicates that CD11d/CD18 is regulated differently than other beta2-integrins, suggesting that CD11d(+) leukocytes play a distinct role in inflammation. Although the transcriptional control of CD11d expression has been evaluated, control of the intracellular distribution of CD11d has not been addressed. For this reason and as a result of the potential of CD11d as a therapeutic target for SCI and possibly other CNS injuries, we investigated the intracellular localization and surface expression of CD11d in cultured cells. CD11d and CD18 were fused at their C-termini with YFP and mRFP, respectively. Flow cytometry and confocal microscopy demonstrated that rCD11d-YFP is expressed on the cell surface of leukocyte cell lines expressing CD18. In contrast, in heterologous cell lines, CD11d-YFP is retained intracellularly in the TGN. Coexpression of CD11d-YFP and CD18-mRFP relieves this intracellular restriction and allows the CD11d/CD18 heterodimer to be surface-expressed. Based on domain-swapping experiments with CD25, the extracellular domain of CD11d is required and sufficient for the observed intracellular retention in heterologous cells. Furthermore, the transmembrane and C-terminus are also required for proper heterodimerization with CD18 and localization to the plasma membrane. These findings suggest that multiple CD11d domains play a role in controlling intracellular location and association with CD18.

Leukocyte adhesion deficiency type 1: an important consideration in the clinical differential diagnosis of prepubertal periodontitis. A case report and review of the literature

Leukocyte adhesion deficiency type 1 (LAD-1) is a rare, inherited immunodeficiency that affects 1 in 1 million people yearly and usually presents with recurrent, indolent bacterial infections of the skin, mouth, and respiratory tract and impaired pus formation and wound healing. Features of this disease result from mutations in the region of the CD18 gene, which is encoded on chromosome 21q22.3. This gene codes for the common subunit of the leukocyte integrins LFA-1, Mac 1, and p150,95. Failure to produce a functional subunit results in the defective expression of all 3 leukocyte integrins, and the leukocytes of LAD have subnormal adhesion properties. We present a case of the moderate-to-severe form of LAD in a 3-year-old girl who initially presented with generalized swelling and erythema of the gingiva, with slight tooth mobility and a nonhealing labial ulceration. Her medical history was significant for recurrent urinary tract infections. Periodontal pathogens, including Capnocytophaga, Eikenella corrodens, and Candida albicans, were cultured. The patient had a significantly elevated white blood cell count and absolute neutrophil count. The diagnosis of LAD was confirmed with flow cytometry, which revealed significantly decreased subunits. Twenty-four months after the diagnosis was made and after a series of granulocyte transfusions and bone marrow transplantations, she expired as the result of respiratory failure.

Reversion mutations in patients with leukocyte adhesion deficiency type-1 (LAD-1)

Leukocyte adhesion deficiency type-1 (LAD-1) is an autosomal recessive immunodeficiency caused by mutations in the beta2 integrin, CD18, that impair CD11/CD18 heterodimer surface expression and/or function. Absence of functional CD11/CD18 integrins on leukocytes, particularly neutrophils, leads to their incapacity to adhere to the endothelium and migrate to sites of infection. We studied 3 LAD-1 patients with markedly diminished neutrophil CD18 expression, each of whom had a small population of lymphocytes with normal CD18 expression (CD18(+)). These CD18(+) lymphocytes were predominantly cytotoxic T cells, with a memory/effector phenotype. Microsatellite analyses proved patient origin of these cells. Sequencing of T-cell subsets showed that in each patient one CD18 allele had undergone further mutation. Interestingly, all 3 patients were young adults with inflammatory bowel disease. Somatic reversions of inherited mutations in primary T-cell immunodeficiencies are typically associated with milder clinical phenotypes. We hypothesize that these somatic revertant CD18(+) cytotoxic T lymphocytes (CTLs) may have altered immune regulation. The discovery of 3 cases of reversion mutations in LAD-1 at one center suggests that this may be a relatively common event in this rare disease.

Epitope mapping of monoclonal antibody to integrin alphaL beta2 hybrid domain suggests different requirements of affinity states for intercellular adhesion molecules (ICAM)-1 and ICAM-3 binding

Integrin undergoes different activation states by changing its quaternary conformation. The integrin beta hybrid domain acts as a lever for the transmission of activation signal. The displacement of the hybrid domain can serve to report different integrin activation states. The monoclonal antibody (mAb) MEM148 is a reporter antibody that recognizes Mg/EGTA-activated but not resting integrin alpha(L) beta2. Herein, we mapped its epitope to the critical residue Pro374 located on the inner face of the beta2 hybrid domain. Integrin alpha(L) beta2 binds to its ligands ICAM-1 and ICAM-3 with different affinities. Integrin is proposed to have at least three affinity states, and the position of the hybrid domain differs in each. We made use of the property of mAb MEM148 to analyze and correlate these affinity states in regard to alpha(L) beta2/intercellular adhesion molecule (ICAM) binding. Our study showed that Mg/EGTA-activated alpha(L)beta2 can adopt a different conformation from that activated by activating mAbs KIM185 or MEM48. Unlike ICAM-1 binding, which required only one activating agent, alpha(L) beta2/ICAM-3 binding required both Mg/EGTA and an activating mAb. This suggests that alpha(L)beta2 with intermediate affinity is sufficient to bind ICAM-1 but not ICAM-3, which requires a high affinity state. Furthermore, we showed that the conformation adopted by alpha(L)beta2 in the presence of Mg/EGTA, depicting an intermediate activation state, could be reverted to its resting conformation.

The integrin alpha L beta 2 hybrid domain serves as a link for the propagation of activation signal from its stalk regions to the I-like domain

Integrin activation involves global conformational changes as demonstrated by various functional and structural analyses. The integrin beta hybrid domain is proposed to be involved in the propagation of this activation signal. Our previous study showed that the integrin beta(2)-specific monoclonal antibody 7E4 abrogates monoclonal antibody KIM185-activated but not Mg(2+)/EGTA-activated leukocyte function-associated antigen-1 (LFA-1; alpha(L)beta(2))-mediated adhesion to ICAM-1. Here we investigated the allosteric inhibitory property of 7E4. By using human/mouse chimeras and substitution mutations, the epitope of 7E4 was mapped to Val(407), located in the mid-region of the beta(2) hybrid domain. Two sets of constitutively active LFA-1 variants were used to examine the effect of 7E4 on LFA-1/ICAM-1 binding. 7E4 attenuated the binding of variants that have modifications to regions membrane proximal with respect to the beta(2) hybrid domain. In contrast, the inhibitory effect was minimal on variants with alterations in the alpha(L) I- and beta(2) I-like domains preceding the hybrid domain. Furthermore, 7E4 abrogated LFA-1/ICAM-1 adhesion of phorbol 12-myristate 13-acetate-treated MOLT-4 cells. Our data demonstrate that interaction between the hybrid and I-like domain is critical for the regulation of LFA-1-mediated adhesion.

Chemotaxis of non-compressed blood polymorphonuclear leukocytes from an adolescent with severe leukocyte adhesion deficiency

We have defined the defect in a child with severe leukocyte adhesion deficiency-1 (LAD) as resulting from a single amino acid shift in CD18 (from a C to T mutation at position 533) that prevents heterodimerization with the CD11 antigens to produce beta(2) integrins-the first reported patient homozygous for this defect. Although beset by frequent infections, the patient has survived to adolescence despite the lack of these important adhesion molecules. Consistent with his clinical course is the ability of his PMN to respond chemotactically in slide preparations, albeit with difficulty because of their poor purchase on substrate. The operant adhesins are unknown; his polymorphonuclear leukocytes (PMN) remain chemotactically responsive in the presence of antibodies to alphavbeta(3) and beta(1) integrins and to integrin-associated protein (IAP). These findings indicate that not all patients with severe LAD are candidates for early bone marrow transplantation.

Genetic analysis of patients with leukocyte adhesion deficiency: genomic sequencing reveals otherwise undetectable mutations

OBJECTIVE

The aim of this study was to analyze mutations in DNA from patients with leukocyte adhesion deficiency (LAD), an immunodeficiency caused by absence of the beta(2) subunit (CD18) of the leukocyte integrins LFA-1 (CD11a/CD18), Mac-1 (CD11b/CD18), p150,95 (CD11c/CD18), and CR4 (CD11d/CD18).

METHODS

We developed genomic DNA PCR sequencing to detect mutations not only in exons but also in introns.

RESULTS

Eight LAD patients were analyzed, of which five had homozygous mutations, i.e., a 0.8-kb deletion, a branchpoint mutation in intron 5 causing mRNA missplicing, a nonsense mutation, and two missense mutations. Four of these mutations are novel. We cotransfected the two mutant CD18 proteins with normal CD11a, b, or c in COS cells. This resulted in absence of all three beta(2) integrins on the surface of cells transfected with CD18(252Arg). However, CD18(593Cys) supported some LFA-1 and p150,95 formation in COS cells. The other three patients were compound heterozygotes in which only one allele had previously been characterized, because the other alleles were undetectable at the cDNA level. We identified the unknown mutations as a novel two-nucleotide deletion, a nonsense mutation, and a single nucleotide deletion.

CONCLUSION

Our method allows identification of mutations in CD18 from genomic DNA. This opens the possibility of early prenatal diagnosis of LAD and reliable carrier detection.

Characterization of four CD18 mutants in leucocyte adhesion deficient (LAD) patients with differential capacities to support expression and function of the CD11/CD18 integrins LFA-1, Mac-1 and p150,95

Leucocyte adhesion deficiency (LAD) is a hereditary disorder caused by mutations in the CD18 (beta2 integrin) gene. Four missense mutations have been identified in three patients. CD18(A270V) supports, at a diminished level, CD11b/CD18 (Mac-1, alphaMbeta2 integrin) and CD11c/CD18 (p150,95, alphaXbeta2 integrin) expression and function but not CD11a/CD18 (LFA-1, alphaLbeta2 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 alpha and beta subunits, which is an integral component in our understanding of integrin-mediated adhesion and its regulation.

Hematologically important mutations: leukocyte adhesion deficiency

Leukocyte adhesion deficiency (LAD) is an immunodeficiency caused by defects in the adhesion of leukocytes (especially neutrophils) to the blood vessel wall. As a result, patients with LAD suffer from severe bacterial infections and impaired wound healing. In LAD-I, mutations are found in INTG2, the gene that encodes the beta subunit of the beta(2) integrins. In the rare LAD-II disease, the fucosylation of selectin ligands is disturbed, caused by mutations in the gene for a GDP-fucose transporter of the Golgi. This article summarizes all known patient mutations and polymorphisms in these genes.

The N-terminal region and the mid-region complex of the integrin beta 2 subunit

In the primary sequence of the integrin beta subunit, the N-terminal region (NTR) and mid-region are separated by the I-like domain. To determine the spatial relationship and functional properties of the integrin beta(2) NTR and mid-region, we constructed beta(2)/beta(7) chimeras in which the NTR, I-like domain, and the mid-region of the beta(2) subunit were replaced by those of beta(7). Changing either the beta(2) NTR or mid-region, but not the I-like domain to that of beta(7) did not affect LFA-1 (alpha(L)beta(2)) formation and surface expression. Thus, the specificity of alpha(L)beta(2) pairing is conferred by the I-like domain but not the NTR or mid-region. Using these chimeras, the epitopes of six anti-beta(2) mAbs (H52, 7E4, AZN-L18, AZN-L27, KIM202, and MEM-148) were mapped. All except H52 require both the NTR and mid-region for epitope expression. Since these mAbs have distinct properties in terms of epitope expression and effect on LFA-1 binding to ICAM-1, we conclude that the beta(2) NTR and mid-region interact extensively. Although the I-like domain is located between the NTR and mid-region, its removal does not affect the folding of the beta(2) NTR/mid-region complex because this complex alone can be expressed as a soluble protein and precipitated by the appropriate mAbs. Finally, the mAbs H52 and 7E4, abrogated KIM185- but not Mg/EGTAinduced LFA-1/ICAM-1 binding and the epitope of MEM-148 is expressed on Mg/EGTA-activated but not resting LFA-1. These results suggest that the NTR/mid-region complex is involved in the regulation of LFA-1 function.

Primary immunodeficiency mutation databases

Primary immunodeficiencies are intrinsic defects of immune systems. Mutations in a large number of cellular functions can lead to impaired immune responses. More than 80 primary immunodeficiencies are known to date. During the last years genes for several of these disorders have been identified. Here, mutation information for 23 genes affected in 14 immunodefects is presented. The proteins produced are employed in widely diverse functions, such as signal transduction, cell surface receptors, nucleotide metabolism, gene diversification, transcription factors, and phagocytosis. Altogether, the genetic defect of 2,140 families has been determined. Diseases with X-chromosomal origin constitute about 70% of all the cases, presumably due to full penetrance and because the single affected allele causes the phenotype. All types of mutations have been identified; missense mutations are the most common mutation type, and truncation is the most common effect on the protein level. Mutational hotspots in many disorders appear in CPG dinucleotides. The mutation data for the majority of diseases are distributed on the Internet with a special database management system, MUTbase. Despite large numbers of mutations, it has not been possible to make genotype-phenotype correlations for many of the diseases.

Demystified...adhesion molecule deficiencies

The basic physiology of leucocyte emigration from the intravascular space into the tissues is now known to be dependent on a class of cell surface molecules that have come to be known as adhesion molecules. Many cell-cell interactions are dependent on adhesion and signal transduction via the various adhesion molecules, particularly the integrins. The study of the functions of these molecules has been enhanced by the development of blocking and activating monoclonal antibodies, knockout mice, and by the rare "experiments of nature" in the human population, in whom there is absence or dysfunction of one of the adhesion molecules. This review describes these leucocyte adhesion defects and discusses how they have provided important insights into the function of these molecules.

Two Novel Frame Shift, Recurrent and De Novo Mutations in the ITGB2 (CD18) Gene Causing Leukocyte Adhesion Deficiency in a Highly Inbred North African Population

We have identified four different mutations causing leukocyte adhesion Deficiency (LAD) in the ITGB2 gene of patients from a highly inbred population. Two were novel single-bp deletions (1497delG and 1920delG) causing frame shift and the two others were the missense mutations G284S and R593C. In our study, the G284S was a recurrent mutation while the R593C occurred de novo. We have also characterized a novel Xba1 polymorphic site located at the 5' end of the ITGB2 locus. Family studies showed that the 1497delG mutation segregated with this marker and the intragenic AvaII polymorphic marker, suggesting the presence of a founder effect. The observation of a heterogeneous spectrum including de novo and recurrent mutations causing LAD in a highly inbred population is rather unexpected. In view of the literature published on the molecular genetics of LAD and considering the ethnic origin of the patients studied, our findings confirm the heterogeneity of the mutations causing LAD and point out potential mutational hot spots in the ITGB2 gene.

Effect of integrin beta 2 subunit truncations on LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) assembly, surface expression, and function

LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) are members of the beta2 integrins involved in leukocyte function during immune and inflammatory responses. We aimed to determine a minimized beta2 subunit that forms functional LFA-1 and Mac-1. Using a series of truncated beta2 variants, we showed that the subregion Q23-D300 of the beta2 subunit is sufficient to combine with the alphaL and alphaM subunits intracellularly. However, only the beta2 variants terminating after Q444 promote cell surface expression of LFA-1 and Mac-1. Thus, the major cysteine-rich region and the three highly conserved cysteine residues at positions 445, 447, and 449 of the beta2 subunit are not required for LFA-1 and Mac-1 surface expression. The surface-expressed LFA-1 variants are constitutively active with respect to ICAM-1 adhesion and these variants express the activation reporter epitope of the mAb 24. In contrast, surface-expressed Mac-1, both the wild type and variants, require 0. 5 mM MnCl2 for adhesion to denatured BSA. These results suggest that the role of the beta2 subunit in LFA-1- and Mac-1-mediated adhesion may be different.

Structural and functional studies with antibodies to the integrin beta 2 subunit. A model for the I-like domain

To establish a structure and function map of the beta2 integrin subunit, we mapped the epitopes of a panel of beta2 monoclonal antibodies including function-blocking, nonblocking, and activating antibodies using human/mouse beta2 subunit chimeras. Activating antibodies recognize the C-terminal half of the cysteine-rich region, residues 522-612. Antibodies that do not affect ligand binding map to residues 1-98 and residues 344-521. Monoclonal antibodies to epitopes within a predicted I-like domain (residues 104-341) strongly inhibit LFA-1-dependent adhesion. These function-blocking monoclonal antibodies were mapped to specific residues with human --> mouse knock-out or mouse --> human knock-in mutations. Combinatorial epitopes involving residues distant in the sequence provide support for a specific alignment between the beta-subunit and I domains that was used to construct a three-dimensional model. Antigenic residues 133, 332, and 339 are on the first and last predicted alpha-helices of the I-like domain, which are adjacent on its "front." Other antigenic residues in beta2 and in other integrin beta subunits are present on the front. No antigenic residues are present on the "back" of the domain, which is predicted to be in an interface with other domains, such as the alpha subunit beta-propeller domain. Most mutations in the beta2 subunit in leukocyte adhesion deficiency are predicted to be buried in the beta2 subunit I-like domain. Two long insertions are present relative to alpha-subunit I-domains. One is tied down to the back of the I-like domain by a disulfide bond. The other corresponds to the "specificity-determining loop" defined in beta1 and beta3 integrins and contains the antigenic residue Glu(175) in a disulfide-bonded loop located near the "top" of the domain.

A novel point mutation in CD18 causing the expression of dysfunctional CD11/CD18 leucocyte integrins in a patient with leucocyte adhesion deficiency (LAD)

Leucocyte adhesion deficiency type 1 (LAD-1) is characterized by the incapacity of leucocytes to carry out their adhesion functions via their CD11/CD18 antigens, which are also referred to as the leucocyte integrins. The patients generally suffer from poor wound healing and recurrent bacterial and fungal infections. In severe cases, the infections are often systemic and life-threatening. A LAD patient (AW) of moderate phenotype has been identified but, unlike most other cases, the level of CD11/CD18 antigens on her leucocytes are uncharacteristically high for a LAD patient. Molecular analysis revealed that she is a compound heterozygote for CD18 mutations. She has inherited a D231H mutation from her father and a G284S mutation from her mother. By transfection studies, it was established that the G284S mutation does not support CD11/CD18 antigen expression on the cell surface. In contrast, the D231H mutation does not affect CD18 forming integrin heterodimers with the CD11 antigens on the cell surface. However, the expressed integrins with the D231H mutation are not adhesive to ligands.

Genetic analysis of integrin function in man: LAD-1 and other syndromes

The integrins are cell membrane receptors composed of alpha and beta subunits which orchestrate adhesive events in all tissues of the body (Hynes, R.O., 1992. Integrins: versatility, modulation, and signalling in cell adhesion. Cell 69, 11-25; and Hynes, R.O., 1999. Cell adhesion: old and new questions. Trends Cell Biol. 9, M33-37). At present 18 alpha subunits and 8 beta subunits have been identified which are loosely organised into families. There are three inherited autosomal recessive diseases in man which involve germline mutations in genes coding for integrins. Leukocyte adhesion deficiency-1 (LAD-1) is the result of mutations in the beta2 subunit of the CD11/CD18 integrins, LFA-1, Mac-1, p150,95 and alphadbeta2. The bleeding disorder Glanzmann thrombasthenia is caused by mutations in either the alpha or beta subunit of the platelet integrin, alphaIIbbeta3. Thirdly, it is now recognised than one of the variants of the usually lethal skin blistering disorder, epidermolysis bullosa (JEB-PA), is caused by mutation in either the alpha or beta subunit of the epithelial hemidesmosome integrin, alpha6beta4. Many of the mutations cause defective alphabeta heterodimer formation. The majority of the beta subunit mutations are in the conserved N-terminal region known as the betaI domain. It is suggested that this region participates in alphabeta heterodimer formation.

A novel CD18 genomic deletion in a patient with severe leucocyte adhesion deficiency: a possible CD2/lymphocyte function-associated antigen-1 functional association in humans

Leucocyte adhesion deficiency (LAD) is an autosomal-recessive genetic disease that is characterized clinically by severe bacterial infections and caused by mutations in the CD18 gene that codes for the beta2 integrin subunit. A patient with a severe LAD phenotype was studied and the molecular basis of the disease was identified as a single homozygous defect in a Herpes virus saimiri (HVS)-transformed T-cell line. The defect identified involves a deletion of 171 bp in the cDNA that encodes part of the proteic extracellular domain. This genetic abnormality was further studied at the genomic DNA level and found to consist of a deletion of 169 bp (from -37 of intron 4 to +132 of exon 5), which abolishes the normal splicing and results in the total skipping of exon 5. The 171-bp shortened 'in-frame' mRNA not only resulted in the absence of CD18 expression on the cell surface but also in its absence in the cytoplasm of HVS T-cell lines. Functionally, the LAD-derived HVS T-cell lines showed a severe, selective T-cell activation impairment in the CD2 (but not in the CD3) pathway. This defect was not reversible when exogenous interleukin-2 (IL-2) was added, suggesting that there is also a functional interaction of the lymphocyte function-associated antigen-1 (LFA-1) protein in the CD2 signal transduction pathway in human T cells, as has been previously reported in mice and in the human Papillon-Lefèvre syndrome. Thus, HVS transformation is not only a suitable model for T-cell immunodeficiency studies and characterization, but is also a good system for investigating the immune system in pathological conditions. It may also be used in the future in cellular models for in vitro gene-therapy trials.

A novel leukocyte adhesion deficiency caused by expressed but nonfunctional beta2 integrins Mac-1 and LFA-1

In the leukocyte adhesion deficiency (LAD)-1 syndrome, there is diminished expression of beta2(CD18) integrins. This is caused by lesions in the beta2-subunit gene and gives rise to recurrent bacterial infections, impaired pus formation, and poor wound healing. We describe a patient with clinical features compatible with a moderately severe phenotype of LAD-1 but who expresses the beta2 integrins lymphocyte function- associated molecule (LFA)-1 and Mac-1 at 40%-60% of normal levels. This level of expression should be adequate for normal integrin function, but both the patient's Mac-1 on neutrophils and LFA-1 on T cells failed to bind ligands such as fibrinogen and intercellular adhesion molecule (ICAM)-1, respectively, or to display a beta2-integrin activation epitope after adhesion-inducing stimuli. Unexpectedly, divalent cation treatment induced the patient's T cells to bind to ICAM-2 and ICAM-3. Sequencing of the patient's two CD18 alleles revealed the mutations S138P and G273R. Both mutations are in the beta2-subunit conserved domain, with S138P a putative divalent cation coordinating residue in the metal ion-dependent adhesion site (MIDAS) motif. After K562 cell transfection with alpha subunits, the mutated S138P beta subunit was coexpressed but did not support function, whereas the G273R mutant was not expressed. In summary, the patient described here exhibits failure of the beta2 integrins to function despite adequate levels of cell-surface expression.

The role of the cysteine-rich region of the beta2 integrin subunit in the leukocyte function-associated antigen-1 (LFA-1, alphaLbeta2, CD11a/CD18) heterodimer formation and ligand binding

The cysteine-rich region (CRR) of the beta2 integrin subunit was replaced by that of beta1 to give the chimera beta2NV1. Beta2NV1 can combine with alphaL to form a variant leukocyte-function-associated antigen (LFA)-1 on COS cell surface, suggesting that the specificity of the beta2 interaction with alphaL does not lie in the CRR. Unlike those expressing wild-type LFA-1, COS cells expressing alphaL beta2NV1 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 beta2 CRR.

Expression of the H52 epitope on the beta2 subunit is dependent on its interaction with the alpha subunits of the leukocyte integrins LFA-1, Mac-1 and p150,95 and the presence of Ca2+

Integrin-mediated adhesion is a divalent cation-dependent process. Whether divalent cations directly participate in ligand binding or exert their effects indirectly by affecting the overall structure of the integrin heterodimers is not known. In this study we describe the epitope of the mAb H52 which has been mapped to a predicted disulfide-bonded loop (C386 and C400) in the beta2 integrin subunit. In the presence of Ca2+ and Mg2+, the H52 epitope is expressed on the monomeric beta2 subunit, the LFA-1 and Mac-1 heterodimers but not on p150,95, thus implying that this epitope is masked in p150,95. However, expression of the H52 epitope on Mac-1, but not on LFA-1, or the monomeric beta2 subunit, is dependent on the presence of Ca2+, thus suggesting that the chelation of Ca2+ causes a conformational change in Mac-1 which results in the loss of the epitope. These results suggest that expression of the H52 epitope on the beta2 subunit is dependent on its interaction with the different alpha subunits. Since the epitope itself is not required for heterodimer formation nor for ligand binding, occupancy of a Ca2+ binding site(s) must therefore affect the alphabeta subunit interactions, and thus the overall conformation of Mac-1.

A novel form of hereditary myeloperoxidase deficiency linked to endoplasmic reticulum/proteasome degradation

Myeloperoxidase (MPO) deficiency is a common inherited disorder linked to increased susceptibility to infection and malignancy. We identified a novel missense mutation in the MPO gene at codon 173 whereby tyrosine is replaced with cysteine (Y173C) that is associated with MPO deficiency and assessed its impact on MPO processing and targeting in transfectants expressing normal or mutant proteins. Although the precursor synthesized by cells expressing the Y173C mutation (MPOY173C) was glycosylated, associated with the molecular chaperones calreticulin and calnexin, and acquired heme, it was neither proteolytically processed to mature MPO subunits nor secreted. After prolonged association with calreticulin and calnexin in the endoplasmic reticulum, MPOY173C was degraded. Furthermore, the 20S proteasome inhibitor N-acetyl-L-leucinyl-L-leucinyl-L-norleucinyl inhibited its degradation, suggesting that the proteasome mediates proteolysis of MPOY173C and, thus, participates in quality control in this novel form of hereditary MPO deficiency.