Heterogeneity in the molecular defect leading to the leukocyte adhesion deficiency

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.

Pyoderma gangrenosum in a child with congenital partial deficiency of leucocyte adherence glycoproteins

Congenital deficiency of beta 2 integrin leucocyte adhesion molecules is a rare immunodeficiency and is often fatal. Neutrophils are unable to bind to ligands on the endothelium, and so cannot leave the circulation during inflammation or infection. When leucocyte adhesion deficiency (LAD) is caused by abnormally low expression of beta 2 integrins, it is termed LAD type 1. We describe a 5-year-old girl with a history of recurrent bacterial infections since early childhood who developed necrotic skin ulcers resembling pyoderma gangrenosum and a persistent circulating neutrophilia. Histologically, the lesions showed deep ulceration with a diffuse lymphohistiocytic infiltrate, but with a relative sparsity of neutrophils. Subsequent investigation revealed a complete absence of CD11a/CD18 beta 2 integrins on the surface of the patient's neutrophils, confirming the diagnosis of LAD type 1. The ulcers responded to treatment with oral prednisolone and colchicine.

Characterization of two new CD18 alleles causing severe leukocyte adhesion deficiency

Leukocyte adhesion deficiency (LAD) is an autosomal recessive disease caused by heterogeneous mutations within the gene encoding the common beta subunit (CD18) of the three leukocyte integrins LFA-1 (CD11a/CD18), Mac-1 (CD11b/CD18), and p150,95 (CD11c/CD18). Based on the level of expression of CD18 on patient leukocytes, two phenotypes of LAD have been defined (severe and moderate) which correlate with the severity of the disease. We have investigated the molecular basis of the disease in two unrelated severe patients (HS and ZJO). Both patients share a complete absence of CD18 protein precursor and cell surface expression, but they differ in the level of CD18 mRNA, which is normal in HS and undetectable by Northern blot in ZJO. Determination of the primary structure of the patient HS CD18 mRNA revealed a 10-base pair deletion between nucleotides 190-200 (CD18 exon 3), which eliminates residues 41-43 and causes a frameshift into a premature termination codon 17 base pairs downstream from the deleted region. The 10-base pair frameshift deletion maps to a region of the CD18 gene where aberrant mRNA processing has been detected in HS and two other unrelated LAD patients. In the ZJO patient, amplification of lymphoblast CD18 mRNA demonstrated the presence of a non-sense mutation in the third nucleotide of the triplet encoding Cys534 (TGC-->TGA), within exon 12. Both genetic abnormalities were also detected at the genomic level, and affect the restriction pattern of their corresponding genes, thus enabling the detection of the mutant alleles among healthy heterozygous alleles in family studies. The identification of two new LAD CD18 alleles, either carrying a non-sense mutation (ZJO) or a partial gene deletion (HS), further illustrates the heterogeneity of the genetic alterations in LAD.

Familial genetic defect in a case of leukocyte adhesion deficiency

Leukocyte adhesion deficiency (LAD) is an inherited immunodeficiency disorder caused by CD18 subunit abnormality dependent defective expression of beta 2 integrins on the surface of leukocytes. On analysis of the CD18 molecular defect in a female Japanese patient with a severe deficiency LAD phenotype, neither CD11a nor CD18 molecules could be detected on the patient's EBV-transformed B lymphoblastoid cell line. The mRNA of the patient's B cells was normal in size, but was diminished in quantity, to approximately half normal levels. Sequencing of the CD18 cDNA of the patient revealed a C605 to T transition, resulting in a Pro178-->Leu substitution. This was heterozygous in the genomic DNA, and shown to be of maternal origin by family study. Only a few transcripts from the other allele without the Pro178-->Leu mutation were detectable. Northern blot analysis revealed reduced CD18 mRNA levels, not only in the patient, but also in the father and brother. These results indicate that our case is a compound heterozygote with two different mutant alleles: one causing a single amino acid substitution and the other causing defective expression of mRNA.

Characterization of the lymphocyte activation gene 3-encoded protein. A new ligand for human leukocyte antigen class II antigens

The lymphocyte activation gene 3 (LAG-3), expressed in human activated T and natural killer (NK) cells, is closely related to CD4 at the gene and protein levels. We report here the initial characterization of the LAG-3-encoded protein. We have generated two monoclonal antibodies after immunization of mice with a 30-amino acid peptide that corresponds to an exposed extra loop region present in the LAG-3 immunoglobulin-like first domain. The reactivity of these reagents is directed against LAG-3 since they recognize both membrane-expressed and soluble recombinant LAG-3 molecules produced in a baculovirus expression system. The two antibodies are likely to react with the same or closely related epitope (termed LAG-3.1) exposed on the LAG-3 first domain extra loop, as assessed in competition experiments on LAG-3-expressing activated lymphocytes. Cellular distribution analysis indicated that the LAG-3.1 epitope is expressed on activated T (both CD4+ and CD8+ subsets) and NK cells, and not on activated B cells or monocytes. In immunoprecipitation experiments performed on activated T and NK cell lysates, a 70-kD protein was detected after SDS-PAGE analysis. 45-kD protein species were also immunoprecipitated. Both the 70- and 45-kD proteins were shown to be N-glycosylated. In Western blot analysis, only the former molecule was recognized by the anti-LAG-3 antibodies, demonstrating that it is LAG-3 encoded. These anti-LAG-3 antibodies were used to investigate whether the LAG-3 protein interacts with the CD4 ligands. By using a high-level expression cellular system based on COS-7 cell transfection with recombinant CDM8 vectors and a quantitative cellular adhesion assay, we demonstrate that rosette formation between LAG-3-transfected COS-7 cells and human leukocyte antigen (HLA) class II-bearing B lymphocytes is specifically dependent on LAG-3/HLA class II interaction. In contrast to CD4, LAG-3 does not bind the human immunodeficiency virus gp120. This initial characterization will guide further studies on the functions of this molecule, which may play an important role in immune responses mediated by T and NK lymphocytes.

Both LFA-1-positive and -deficient T cell clones require the CD2/LFA-3 interaction for specific cytolytic activation

We investigated the capacity of T lymphocytes from a leukocyte adhesion-deficient (LAD) patient to respond to alloantigen. Leukocytes of this patient completely lacked LFA-1 surface expression due to the absence of mRNA coding for the LFA-1 beta chain. Despite the absence of LFA-1, T lymphocytes obtained from this patient, cultured with allogeneic stimulator cells (lymphoblastoid B cells JY), were capable of lysing JY cells. Furthermore, two T cell clones (one CD4+ and one CD8+), generated from this lymphocyte culture, specifically lysed the allogeneic lymphoblastoid JY cells. The cytolytic capacity of LFA-1-negative T lymphocytes and T cell clones was comparable to that of control LFA-1-positive T cells with allospecificity against JY. Detailed analysis of the CD4 positive and LFA-1-negative T cell clone demonstrated that it specifically recognized HLA-DQ. Antibody inhibition studies showed that the CTL/target cell interaction was mediated through the CD2/LFA-3 adhesion pathway. LFA-1 expressed by the target cells did not participate in the CTL/target cell conjugate formation and contributed only minimally to the cytotoxic activity. Moreover, when allogeneic LFA-1-deficient B cells, bearing the appropriate HLA-DQ alloantigen, were used as target cells, significant levels of specific cytotoxicity were measured, further excluding a role for LFA-1 in this interaction. The adhesion molecules, VLA-4, CD44 and L-selectin (LECAM1) were not involved. These results demonstrate that LFA-1-negative T lymphocytes can exert allospecific cytotoxicity and that CTL/target cell contact is mediated through the CD2/LFA-3 route. This observation may explain in part why in LAD patients viral infections, cleared largely by T cells, are less frequently observed than bacterial infections, in which phagocytic cells play a major role.

Anti-(human LFA-1) monoclonal antibodies bind P815 murine tumour cells

Using anti-CD11a and anti-CD18 monoclonal antibodies (mAbs) directed respectively against the alpha and the beta chains of LFA-1, we obtained an important and specific staining of P815 murine tumour cells. Both ascitic and cultured cells displayed a positive staining. Other murine tumours of haematopoietic origin, as well as lymphocytes or lymphoblasts from DBA/2 mice, were not labelled by the same monoclonal antibodies. These results were surprising since, to our knowledge, no case of cross-reaction between species has been reported with LFA-1. Moreover, competition assays showed that epitopes recognized by the two anti-CD11a antibodies were different from those identified by H35.89.9, a mAb raised against the murine LFA-1 alpha chain. Using allogeneic cytotoxic T lymphocytes, we also showed that anti-(human LFA-1) mAbs were unable to block the lysis of P815 by these effector cells. Thus, the putative functional properties of these structures, as well as their importance from an antigeneic point of view, remain to be assessed.

Primary membrane T cell immunodeficiencies

Primary membrane T cell immunodeficiencies (ID) have recently been characterized. In this paper we describe the main findings about the leukocyte adhesion deficiencies (LAD), the ID with low expression of the T cell receptor/CD3 complex, and the Omenn's syndrome. LAD is a consequence of mutations in the beta-chain-encoding gene of the leukocyte adhesion proteins. Functional consequences mainly affect phagocytic cells which are incapable of transendothelial migration. Effector T lymphocyte functions are, however, also impaired, i.e., helper T cell activity and cytotoxicity. The latter defect may account for the inability of LAD patients to reject HLA nonidentical bone marrow. Low expression of the T cell receptor CD3 complex is a rare entity characterized by a profoundly diminished expression of the whole complex on all T cells. The basic defect has not yet been unravelled. Interestingly, such T cells differentiate normally and can be activated by some antigens while anti-CD3 and anti-CD2 antibodies are not efficient. In five patients with Omenn's syndrome (combined immunodeficiency with eosinophilia), oligoclonal T cells were detected in blood, skin, and gut. These T cells are also in vivo activated. Since in one family, one sibling presented with typical SCID, i.e., alymphocytosis, and another with the Omenn's syndrome, it is proposed that the latter syndrome may correspond to a form of leakiness of SCID as found in the mice SCID model.

Distinct mutations in two patients with leukocyte adhesion deficiency and their functional correlates

Two patients with leukocyte adhesion deficiency (LAD), one with a moderate phenotype (patient 14) and one with a severe phenotype (patient 2) who had been shown to have a normal sized beta subunit protein precursor, were analyzed in an attempt to determine the molecular basis for their disease. RNase mapping located possible mutations to two distinct but adjacent regions of the beta subunit cDNA. Sequencing of patient-derived cDNA clones in this region revealed a C for T difference at amino acid 149 in patient 14 which resulted in the substitution of a leucine for a proline, and an A for G substitution at amino acid 169 in patient 2 which mutated a glycine to an arginine. The mutated amino acids are in a region of the cDNA that is highly conserved between the beta subunits of the integrin family and are identical in all known integrin beta subunits. Co-transfection of the beta subunit cDNA containing the patient 2 mutation with the wild-type alpha subunit of LFA-1 in a mammalian expression system resulted in no expression of LFA-1. In the case of the mutation in patient 14 there was markedly diminished expression of LFA-1 with loss of function and loss of the epitope for a number of anti-beta mAbs. Normal half-life of the mutant beta subunits, and previous demonstration of a lack of alpha/beta complex formation during biosynthesis in patient cells, suggest a defect in association with the alpha subunit. Association with beta is required for expression of the alpha subunit of LFA-1. Loss of functional expression with both of these beta subunit mutations suggests that they lie in a site critical for association with the alpha subunit.

Transfection of cells from patients with leukocyte adhesion deficiency with an integrin beta subunit (CD18) restores lymphocyte function-associated antigen-1 expression and function

Leukocyte adhesion deficiency (LAD) is an inherited immunodeficiency disease that is characterized by the deficient expression of the leukocyte adhesion glycoproteins lymphocyte function-associated antigen-1 (LFA-1), Mac-1, and p150,95. This loss of expression is attributed to heterogeneous defects in the common beta subunit shared by these glycoproteins. Here we demonstrate that expression of the LFA-1 alpha beta heterodimer in EBV-transformed B lymphoblastoid cells from LAD patients can be recovered after transfection with the beta subunit cDNA contained in an EBV-based vector. Four patients with differing severities of LAD comprising three distinct classes of mutations were studied. Flow cytometry analysis of stably transfected patient cells revealed near normal levels of expression of both the alpha and beta chains of LFA-1, and immunoprecipitation studies confirmed that fully processed alpha and beta chains were being expressed at the cell surface. In addition, Northern analysis of mRNA expression also demonstrated that the transfected LAD patient cells were expressing high quantities of exogenous beta subunit mRNA. Functional studies such as homotypic adhesion and adhesion to a purified counterreceptor for LFA-1, intracellular adhesion molecule-1, demonstrated that LFA-1 function had been restored in the stably transfected LAD patient cell lines. These studies unequivocally show that the defect in cells from patients with LAD is in the leukocyte integrin beta subunit.

The leukocyte cell surface receptor(s) for the iC3b product of complement

CR3 is probably the major adhesion molecule on monocytes and neutrophils. Its function as a phagocytic receptor for iC3b-coated particles has been well characterized. CR3 also has binding affinity for other ligands, including those that compete with iC3b such as fibrinogen, factor X, and beta-glucan, and those that do not such as bacterial LPS. CR3 binding to endothelial cells probably plays an important role in the extravascular migration of monocytes and neutrophils, but the ligand that it recognizes on endothelial cells has not been identified. Structurally CR3 belongs to the integrin family, and it shares a common subunit with p150,95 and LFA-1. The expression of these three membrane antigens appear to be limited to leukocytes, and they are sometimes referred to collectively as the leukocyte integrins. All three antigens have a common binding affinity for bacterial LPS. p150,95 also has affinity for iC3b, but p150,95/iC3b-dependent cellular responses has not been demonstrated. Its status as a complement receptor therefore awaits further experimental support.

Effects of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on the expression of LFA-1 in the moderate phenotype of leukocyte adhesion deficiency (LAD)

Leukocyte adhesion deficiency (LAD) is a recessive autosomal disease characterized by life-threatening recurrent bacterial infections, associated with defective functions of leukocytes due to deficient membrane expression of leukocyte adhesion glycoproteins. These proteins, LFA-1, Mac-1 (CR3), and p150,95 are alpha 1 beta 1 heterodimers composed of different alpha chains noncovalently associated with a common beta chain. Patients with the severe phenotype of the disease completely lack the three glycoproteins on leukocyte surfaces, while patients with the moderate phenotype can express low levels of leukocyte adhesion proteins (1-10%). We have studied a patient with the moderate phenotype of LAD. Polymorphonuclear functions such as chemotaxis and adherence were altered, natural killer activity was low, and cytotoxic T-lymphocyte activity was abolished. Previous biochemical studies showed a conserved synthesis of both the LFA-1 alpha-chain precursor and the beta-chain precursor with, occasionally, some amount of alpha-beta complexes in the cytosol. beta chain-specific mRNA transcripts of normal size were detected at normal levels in patients' cells. Attempts to increase the transcription of the beta gene by in vitro treatment with TNF-alpha or IFN-gamma were successful but did not result in increased membrane expression of the alpha-beta complexes.