The primary structure of the beta-subunit of the cell surface adhesion glycoproteins LFA-1, CR3 and p150,95 and its relationship to the fibronectin receptor

CD11d is a novel antigen on chicken leukocytes

In life sciences, antibodies are among the most commonly used tools for identifying, tracking, quantifying and isolating molecules, mainly proteins. However, it has recently become clear that antibodies often fall short with respect to specificity and selectivity and in many cases target proteins are not even known. When commercial availability of antibodies is scarce, e.g. for targeting proteins from farm animals, researchers face additional challenges: they often have to rely on cross-reactive antibodies, which are poorly characterized for their exact target, their actual cross-reactivity and the desired application. In this study, we aimed at identifying the true target of mouse monoclonal antibody 8F2, which was generated against chicken PBMC and used for decades in research, while it's actual target molecule remained unknown. We used 8F2 antibody for immunoprecipitation in chicken PBMC and subsequently identified its true target as CD11d, which was never described in chicken lymphocytes before, by quantitative LC-MSMS. The most abundant interactor of CD11d was identified as integrin beta 2. The existence of this alpha integrin was therefore clearly proven on protein level and provides a first basis to further assess the role of CD11d in chickens in future studies. Data are available via ProteomeXchange with identifier PXD017248. SIGNIFICANCE: Our studies determined CD11d as the true target of a previously uncharacterized mouse monoclonal antibody 8F2, generated against chicken peripheral blood derived mononuclear cells (PBMC). This is therefore now first member of alpha integrins in chickens, that existence was now clearly identified on protein level. The additional identification of CD11d interactors provides information on integrin-dependent regulation of signaling networks, allowing further functional studies.

Aggregatibacter actinomycetemcomitans leukotoxin causes activation of lymphocyte function-associated antigen 1

Repeats-in-toxin leukotoxin (LtxA) produced by the oral bacterium Aggregatibacter actinomycetemcomitans kills human leukocytes in a lymphocyte function-associated antigen 1 (LFA-1, integrin α_L /β₂ )-dependent manner, although the mechanism for this interaction has not been identified. The LtxA internalisation by LFA-1-expressing cells was explored with florescence resonance energy transfer (FRET) microscopy using a cell line that expresses LFA-1 with a cyan fluorescent protein-tagged cytosolic α_L domain and a yellow fluorescent protein-tagged β₂ domain. Phorbol 12-myristate 13-acetate activation of LFA-1 caused transient cytosolic domain separation. However, addition of LtxA resulted in an increase in FRET, indicating that LtxA brings the cytosolic domains closer together, compared with the inactive state. Unlike activation, this effect was not transient, lasting more than 30 min. Equilibrium constants of LtxA binding to the cytoplasmic domains of both α_L and β₂ were determined using surface plasmon resonance. LtxA has a strong affinity for the cytosolic domains of both the α_L and β₂ subunits (K_d  = 15 and 4.2 nM, respectively) and a significantly lower affinity for the cytoplasmic domains of other integrin α_M , α_X , and β₃ subunits (K_d  = 400, 180, and 230 nM, respectively), used as controls. Peptide fragments of α_L and β₂ show that LtxA binds membrane-proximal domain of α_L and intermediate domain of β₂ .

Antibody against integrin lymphocyte function-associated antigen 1 inhibits HIV type 1 infection in primary cells through caspase-8-mediated apoptosis

HIV-1 infection induces formation of a virological synapse wherein CD4, chemokine receptors, and cell-adhesion molecules such as lymphocyte function-associated antigen 1 (LFA-1) form localized domains on the cell surface. Studies show that LFA-1 on the surface of HIV-1 particles retains its adhesion function and enhances virus attachment to susceptible cells by binding its counterreceptor intercellular adhesion molecule 1 (ICAM-1). This virus-cell interaction augments virus infectivity by facilitating binding and entry events. In this study, we demonstrate that inhibition of the LFA-1/ICAM-1 interaction by a monoclonal antibody leads to decreased virus production and spread in association with increased apoptosis of HIV-infected primary T cells. The data indicate that the LFA-1/ICAM-1 interaction may limit apoptosis in HIV-1-infected T cells. This phenomenon appears similar to anoikis wherein epithelial cells are protected from apoptosis conferred by ligand-bound integrins. These results have implications for further understanding HIV pathogenesis and replication in peripheral compartments and lymphoid organs.

Complement system activation in cardiac and skeletal muscle pathology: friend or foe?

A major goal in current cardiology practice is to determine optimal strategies for minimizing myocardial necrosis and optimizing cardiac repair following an acute myocardial infarction. Temporally regulated activation and suppression of innate immunity may be critical for achieving this goal. Extensive experimental data in various animal models have indicated that inhibiting complement activation offers protection to cardiac tissue after ischemia/reperfusion. However, the results of clinical studies using complement inhibitors (mainly at the C5 level) in patients with acute myocardial infarction have largely been disappointing. In cases in which complement activation participates in the initial events of muscle cell destruction, as in autoimmune myocarditis or autoimmune muscle disorders, inhibition of complement activation is expected to prove a successful treatment. In other pathologic conditions in which complement is recruited by degenerating or dying muscle cells, as in ischemia, the ideal approach is probably to modulate rather than abruptly blunt complement activation. Beneficial effects of complement action with regard to waste disposal, recruitment of stem cells, regeneration, angiogenesis, and better utilization of energy sources under hypoxic conditions may also prove important for successful disease treatment. Patient outcome after myocardial infarction almost certainly depend upon the combined activation of several distinct but potentially interrelated signaling pathways, suggesting that a combination of treatments targeted to different pathways should be the therapy of choice, and modulation of complement could be one of them.

Proteomic-based identification of CD4-interacting proteins in human primary macrophages

Background

Human macrophages (Mφ) express low levels of CD4 glycoprotein, which is constitutively recycled, and 40–50% of its localization is intracellular at steady-state. Although CD4-interacting proteins in lymphoid cells are well characterised, little is known about the CD4 protein interaction-network in human Mφ, which notably lack LCK, a Src family protein tyrosine kinase believed to stabilise CD4 at the surface of T cells. As CD4 is the main cellular receptor used by HIV-1, knowledge of its molecular interactions is important for the understanding of viral infection strategies.

Methodology/Principal Findings

We performed large-scale anti-CD4 immunoprecipitations in human primary Mφ followed by high-resolution mass spectrometry analysis to elucidate the protein interaction-network involved in induced CD4 internalization and degradation. Proteomic analysis of CD4 co-immunoisolates in resting Mφ showed CD4 association with a range of proteins found in the cellular cortex, membrane rafts and components of clathrin-adaptor proteins, whereas in induced internalization and degradation CD4 is associated with components of specific signal transduction, transport and the proteasome.

Conclusions/Significance

This is the first time that the anti-CD4 co-immunoprecipitation sub-proteome has been analysed in human primary Mφ. Our data have identified important Mφ cell surface CD4-interacting proteins, as well as regulatory proteins involved in internalization and degradation. The data give valuable insights into the molecular pathways involved in the regulation of CD4 expression in Mφ and provide candidates/targets for further biochemical studies.

Proteomic analysis of plasma membrane lipid rafts of HL-60 cells

Neutrophils acquire phagocytic activity as they differentiate. Recently, plasma membrane lipid rafts have been shown to play important roles in the process of phagocytosis in neutrophils. To characterize the proteins involved in phagocytosis and to elucidate the process by which they acquire phagocytic activity, we investigated by nano-LC-MS/MS analysis the changes in protein composition of plasma membrane lipid rafts during DMSO-induced differentiation of the human leukemia cell line HL-60 cells into neutrophilic lineage. Based on the spectrum counts of 147 proteins identified, 25 proteins were upregulated and 49 were downregulated by DMSO treatment. CD11b/CD18 subunits of beta2-integrin Mac-1, CD35, and GPI-80, which are known to be upregulated during differentiation, were dominantly detected in the lipid rafts of DMSO-treated cells. Many known membrane proteins, G proteins, and cytoskeletal proteins were also detected and they showed characteristic distributions. Absolute quantification of nine proteins in the lipid rafts using internal standard peptides labeled with stable isotopes showed that the amount of protein almost corresponded to the results obtained by spectrum count. Identified proteins, expression of which was altered by DMSO treatment, are expected to be candidate proteins involved in differentiation and functions of neutrophils.

Aggregatibacter actinomycetemcomitans leukotoxin requires beta-sheets 1 and 2 of the human CD11a beta-propeller for cytotoxicity

Aggregatibacter actinomycetemcomitans leukotoxin (Ltx) is a repeats-in-toxin (RTX) cytolysin that kills human leukocyte function-associated antigen-1 (LFA-1; alpha(L)/beta(2))-bearing cells. In order to determine whether the alpha(L) portion of the heterodimer is involved in Ltx recognition, we transfected human, mouse and bovine alpha(L) cDNAs into J-beta(2).7, an alpha(L)-deficient cell line, and looked for restoration of Ltx susceptibility. Cells expressing either bovine or human alpha(L) in conjunction with human beta(2) were efficiently killed by Ltx, an indication that bovine alpha(L) could substitute for its human counterpart in critical regions used by Ltx for attachment to LFA-1. On the other hand, cells expressing murine alpha(L) and human beta(2) were not susceptible to the lethal effects of Ltx indicating that the toxin recognition sites are not present in the corresponding mouse sequence. To further identify the region(s) of alpha(L) recognized by Ltx, we constructed and evaluated a panel of chimeric human/murine alpha(L) genes in J-beta(2).7 cells. Analysis of the alpha(L) mutant panel showed that the presence of human N-terminal 128 amino acids on a mouse CD11a background, a region that includes beta-sheets 1 and 2 of the beta-propeller of the human alpha(L) chain, was sufficient for Ltx cytolysis.

The Leukocyte Integrin Gene CD11d Is Repressed by Gut-enriched Kruppel-like Factor 4 in Myeloid Cells

The myeloid-specific leukocyte integrin CD11d encodes the alphaD subunit for the alphaDbeta2 receptor. A yeast one-hybrid screen showed that a longer isoform of gut-enriched Kruppel-like factor 4 (GKLF) we term GKLFa interacts with the CD11d promoter. Purified GST-GKLFa protein was shown to bind within the -61 to -44 region that overlaps a binding site for the CD11d transcriptional activators Sp1 and transforming growth factor beta-inducible early gene-1 (TIEG1). Transfection of GKLF/GKLFa in myeloid cells reduced CD11d promoter activity, whereas, down-regulation of GKLF/GKLFa with small interfering RNAs led to up-regulation of CD11d expression. Differentiation of myeloid cells with phorbol ester led to activation of the CD11d promoter and reduced occupancy of the promoter by GKLF/GKLFa but an increased occupancy by TIEG1 in vivo. Binding of GKLF/GKLFa, Sp1, and TIEG1 to the CD11d promoter in vivo is dependent on their zinc finger DNA binding domains. GKLFa physically associates with the histone deacetylases (HDAC) 1 and 2, and both HDACs are bound to the CD11d promoter in vivo but released after exposure of myeloid cells to phorbol ester suggesting that GKLF/GKLFa recruits HDACs to effect repression.

The Zinc Finger Transcription Factor Transforming Growth Factor β-Inducible Early Gene-1 Confers Myeloid-specific Activation of the Leukocyte Integrin CD11d Promoter

CD11d encodes the alpha(D) subunit for a leukocyte integrin that is expressed on myeloid cells. In this study we show that the -100 to -20 region of the CD11d promoter confers myeloid-specific activation of the CD11d promoter. Transforming growth factor beta-inducible early gene-1 (TIEG1) was isolated in a yeast one-hybrid screen using the -100 to -20 region of the CD11d promoter as bait. Purified GST.TIEG1 protein was able to bind within the -61 to -45 region that overlaps a shorter binding site for Sp1. Transient overexpression of TIEG1 activated the CD11d promoter specifically in myeloid cells, whereas, down-regulation of TIEG1 with small interfering TIEG1 RNA also down-regulated expression of CD11d. In vivo, TIEG1 does not physically interact with Sp1. Cotransfection and electrophoretic mobility shift analyses of TIEG1, Sp1, and Sp3 revealed that TIEG1 competes with these Sp proteins for binding to overlapping sites in the CD11d promoter. Although TIEG1 and Sp1 are ubiquitously expressed in myeloid and non-myeloid cells, chromatin immunoprecipitation assays revealed differential occupancy of the CD11d promoter by these factors. In undifferentiated myeloid and non-myeloid cells, occupancy of the CD11d promoter by TIEG1 is similar. Upon differentiation of myeloid cells and subsequent up-regulation of CD11d expression, TIEG1 occupancy increases. In contrast, occupancy by TIEG1 remains low in non-myeloid cells exposed to phorbol ester. We propose that up-regulation of CD11d expression following differentiation of myeloid cells is mediated through increased binding of TIEG1 binding to the CD11d promoter.

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.

Role of the β1-integrin cytoplasmic tail in mediating invasin-promoted internalization of Yersinia

Invasin of Yersinia pseudotuberculosis binds to β1-integrins on host cells and triggers internalization of the bacterium. To elucidate the mechanism behind the β1-integrin-mediated internalization of Yersinia, a β1-integrin-deficient cell line, GD25, transfected with wild-type β1A, β1B or different mutants of the β1A subunit was used. Both β1A and β1B bound to invasin-expressing bacteria, but only β1A was able to mediate internalization of the bacteria. The cytoplasmic region of β1A, differing from β1B, contains two NPXY motifs surrounding a double threonine site. Exchanging the tyrosines of the two NPXYs to phenylalanines did not inhibit the uptake, whereas a marked reduction was seen when the first tyrosine (Y783) was exchanged to alanine. A similar reduction was seen when the two nearby threonines (TT788-9) were exchanged with alanines. It was also noted that cells affected in bacterial internalization exhibited reduced spreading capability when seeded onto invasin, suggesting a correlation between the internalization of invasin-expressing bacteria and invasin-induced spreading. Likewise, integrins defective in forming peripheral focal complex structures was unable to mediate uptake of invasin-expressing bacteria.

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.

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.

Defining the repeating elements in the cysteine-rich region (CRR) of the CD18 integrin beta 2 subunit

The cysteine-rich region (CRR) of the integrin beta subunits is organised into four repeating elements. By expression of a panel of truncated beta 2 subunits, and CRR segments fused to the C-terminal end of a CD4 soluble fragment, the segment required for the expression of two monoclonal antibody conformational epitopes was determined. This segment, E482-Q574, contains 16 cysteines representing two repeating units. We have thus defined the CRR unit motif of 'xC---C---C---CxCxxCxC---Cx', where 'x' represents a single residue, and '---' represents a stretch of four to 14 residues.

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.

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.

Structural and functional characterization of the leukocyte integrin gene CD11d. Essential role of Sp1 and Sp3

CD11d encodes the latest alpha-subunit of the leukocyte integrin family to be discovered, and it is expressed predominantly in myelomonocytic cells. We have isolated a genomic clone that contains CD11d and showed this gene to be 11,461 bp downstream and oriented in the same direction as the related CD11c gene. CD11d transcription begins 69-79 nucleotides upstream of the ATG codon. Transfection analysis of CD11d-luc reporter constructs revealed that the -173 to +74 region is sufficient to confer leukocyte-specific expression of luciferase in myelomonocytic cells (THP1 and HL60), B-cells (IM9), and T-cells (Jurkat). Transfection analysis showed that down-regulation of CD11d expression by phorbol ester was myelomonocyte-specific and is mediated by one or more cis-elements within the -173 to +74 region. In vitro DNase I footprint analysis and electrophoretic mobility shift analysis showed that Sp1 and Sp3 bind at -63 to -40. Deletion of the Sp-binding site significantly reduced CD11d promoter activity. Overexpression of either Sp1 or Sp3 in THP1 cells led to activation of the CD11d promoter even in the presence of phorbol ester, whereas down-regulation of either factor by antisense oligonucleotides decreased CD11d promoter activity. In contrast, overexpression of Sp3 in IM9 and Jurkat cells down-regulated CD11d promoter expression. In vivo genomic footprinting revealed that the -63 to -40 region is bound by a Sp protein in unstimulated HL60 cells but not in phorbol ester-stimulated HL60 cells. In contrast, this site is bound in both unstimulated and phorbol ester-stimulated IM9 and Jurkat cells. Together, these results show that myelomonocyte-specific phorbol ester down-regulation of CD11d is mediated through both Sp1 and Sp3.

The role of dendritic cells in the innate immune system

Dendritic cells (DCs) are bone-marrow-derived leucocytes that are specialised antigen-presenting cells capable of stimulating a primary T-lymphocyte response to specific antigen. In this chapter we discuss the role DCs play in the innate response acting as a critical link with the adaptive response and the influence of the innate response on dendritic cells.

Identification of a beta 2 (CD18) molecule in a teleost species, Ictalurus punctatus Rafinesque

Beta 2, in combination with the alpha subunit, is responsible for tight adhesion of leukocytes, especially neutrophils and macrophages, in areas of inflammation. Although identified in mammalian and avian species; the beta 2 or CD18 molecule has yet to be identified in fish. The present investigation has identified a full-length channel catfish, Ictalurus punctatus, cDNA beta 2 molecule composed of 2.8 kb and a deduced amino acid sequence of 772 amino acids. The catfish molecule has an amino acid homology ranging from 54 to 63% with mouse, bovine, rabbit, human and chicken. The channel catfish molecule retains several characteristics of mammalian beta 2 molecules, such as cysteine-rich repeat regions, N-linked glycosylation sites, and several proposed signal sequences. Expression of the beta 2 molecule on the catfish neutrophil cytoplasmic membranes is increased upon phorbol dibutyrate stimulation of the cells. Based on Western blotting and the immunoprecipitation test, the channel catfish beta 2 molecule has a molecular mass of approximately 95 kD, essentially the same as that for mammalian species. However, two additional molecules, perhaps alpha chains, of unexpected molecular mass appear to co-precipitate in the SPIT with the 95 kD CD18 molecule. These results confirm the existence and expression of a beta 2 gene in channel catfish, a species phylogenetically distant from mammalian species.

Expression of the Third Component of Complement, C3, in Regenerating Limb Blastema Cells of Urodeles

In this study we have shown that complement component C3 is expressed in the regenerating tissue during urodele limb regeneration. C3 was expressed in the dedifferentiated regeneration blastema and in the redifferentiated limb tissues in the axolotl, Amblystoma mexicanum, and in Notophthalmus viridescens. This expression was verified by immunofluorescent staining using an Ab against axolotl C3 and by in situ hybridization with an axolotl C3 cDNA probe. In the early stages of regeneration C3 appeared to be equally present in all mesenchymal cells and in the wound epithelium, whereas in the later stages it was mainly expressed in the differentiating muscle cells. Since no expression was seen in the developing limb, it appears that the C3 expression was specific to the regeneration process. We then demonstrated by hybridization experiments that a blastema cell line of myogenic origin expresses C3. All these findings implicate C3 in the dedifferentiation process and may indicate a new role for this molecule in muscle differentiation.

alphadbeta2 integrin is expressed on human eosinophils and functions as an alternative ligand for vascular cell adhesion molecule 1 (VCAM-1)

The beta2 family of integrins, CD11a, CD11b, CD11c, and alphad, are expressed on most leukocytes. We show that the newest member of this family, alphad, is expressed on human eosinophils in peripheral blood, and surface expression can be upregulated within minutes by phorbol ester or calcium ionophore A23187. Culture of eosinophils with interleukin 5 (IL-5) leads to a two- to fourfold increase in alphad levels by 3-7 d without a change in alpha4 integrin expression. Eosinophils isolated from late phase bronchoalveolar lavage fluids express alphad at levels similar to that seen after 3 d of IL-5 culture. Regarding alphadbeta2 ligands, in both freshly isolated and IL-5-cultured eosinophils, as well as alphadbeta2-transfected Chinese hamster ovary cells, alphadbeta2 can function as a ligand for vascular cell adhesion molecule 1 (VCAM-1). This conclusion is based on the ability of monoclonal antibodies to alphad, beta2, or VCAM-1 to block cell attachment in static adhesion assays. In experiments with eosinophils, the relative contribution of alphadbeta2 integrin- mediated adhesion is enhanced after IL-5 culture. These experiments demonstrate that alphadbeta2 is an alternative ligand for VCAM-1, and this integrin may play a role in eosinophil adhesion to VCAM-1 in states of chronic inflammation.

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.

CCAAT/enhancer binding protein alpha is a regulatory switch sufficient for induction of granulocytic development from bipotential myeloid progenitors

The transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha) regulates a number of myeloid cell-specific genes. To delineate the role of C/EBPalpha in human granulopoiesis, we studied its expression and function in human primary cells and bipotential (granulocytic/monocytic) myeloid cell lines. We show that the expression of C/EBPalpha initiates with the commitment of multipotential precursors to the myeloid lineage, is specifically upregulated during granulocytic differentiation, and is rapidly downregulated during the alternative monocytic pathway. Conditional expression of C/EBPalpha alone in stably transfected bipotential cells triggers neutrophilic differentiation, concomitant with upregulation of the granulocyte-specific granulocyte colony-stimulating factor receptor and secondary granule protein genes. Moreover, induced expression of C/EBPalpha in bipotential precursors blocks their monocytic differentiation program. These results indicate that C/EBPalpha serves as a myeloid differentiation switch acting on bipotential precursors and directing them to mature to granulocytes.

RTX toxins recognize a beta2 integrin on the surface of human target cells

Actinobacillus actinomycetemcomitans leukotoxin and Escherichia coli alpha-hemolysin are RTX toxins that kill human immune cells. We have obtained a monoclonal antibody (295) to a cell surface molecule present on toxin-sensitive HL60 cells that can inhibit cytolysis by both RTX toxins. Utilization of this monoclonal antibody for immunoaffinity purification of detergent-solubilized target cell membranes yielded two polypeptide chains of approximate molecular masses of 100 and 170 kDa. Microsequencing of tryptic peptides from the two proteins showed complete homology with CD11a and CD18, the two subunits of the beta2 integrin, lymphocyte function-associated antigen 1 (LFA-1). Anti-CD11a and CD18 monoclonal antibodies also inhibited RTX toxin-mediated cytolysis. Direct binding experiments demonstrated the ability of an immobilized RTX to bind LFA-1 heterodimers present in a detergent lysate of human HL60 target cells. Transfection of CD11a and CD18 integrin genes into a cell line (K562) that is not sensitive to either RTX toxin resulted in LFA-1 expressing cells, KL/4, that were sensitive to both toxins. These experiments identify LFA-1 as a cell surface receptor that mediates toxicity of members of this family of pore-forming toxins.

Leukocyte adhesion--structure and function of human leukocyte beta2-integrins and their cellular ligands

Leukocyte adhesion is of pivotal functional importance and this has resulted in extensive research and rapid development in the field. Leukocyte adhesion involves members of three molecular families: integrins, members of the immunoglobulin superfamily and carbohydrate binding selectins and sialoadhesins. Recently, considerable structural information on leukocyte integrins and members of the immunoglobulin superfamily of adhesion molecules has been obtained. This fact, combined with the identification of several novel adhesion molecules, has increased our understanding of how they function at the molecular level. Furthermore, the important issue of how integrins are activated to become adhesive is rapidly advancing. It is clearly evident that the knowledge accumulated from basic research will increasingly be applied in clinical medicine. In this review we focus on two important families of adhesion molecules, the leukocyte-specific beta2-integrins and their ligands, the intercellular adhesion molecules. Emphasis is put on their structural/functional relationships, their mode of regulation and on novel adhesion molecules recently discovered.

Folding of the conserved domain but not of flanking regions in the integrin beta2 subunit requires association with the alpha subunit

We have used immunoprecipitation with mAbs to probe folding during biosynthesis of the beta2 integrin subunit of lymphocyte function-associated antigen 1 (LFA-1; CD11a/CD18) before and after association with the alphaL subunit. An evolutionarily conserved region is present in the beta2 subunit between amino acid residues 102 and 344. mAbs to one subregion before the conserved region, and two subregions after the conserved domain, immunoprecipitated both the unassociated beta'2 precursor and mature alphaL/beta2 complex, suggesting portions of these subregions are folded before association with alphaL. An activating mAb to the C-terminal cysteine-rich region, KIM127, preferentially bound to the unassociated beta subunit, suggesting that it may bind to an epitope that is in an alphabeta interface in unactivated LFA-1. By contrast, mAbs to five different epitopes in the conserved region did not react with unassociated beta'2 precursor, suggesting that this region folds after alphaL association and is intimately associated with the alphaL subunit in the alphaL/beta2 complex. mAbs to two different epitopes that involve the border between the conserved region and the C-terminal segment, were fully or partially reactive with the beta'2 precursor, suggesting that this region is partially folded before association with alphaL. The findings suggest that the conserved region is a distinct folding and hence structural unit, and is intimately associated with the alpha subunit.

Interaction of monocytoid cells with the mucosal addressin MAdCAM-1 via the integrins VLA-4 and LPAM-1

The differentiation of myeloid cells into macrophages and granulocytes is accompanied by marked changes in adhesive phenotype. Here we seek to understand the regulation of expression and functionality of the VLA-4 (alpha 4 beta 1), LPAM-1 (alpha 4 beta 7) and HML-1 (alpha E beta 7) integrins on monocytes/macrophages and granulocytes, given that these integrins including LFA-1 (alpha L beta 2) mediate the entry, retention and signalling events of pathogenic leucocytes within chronically inflamed tissues. Phorbol ester-induced monocytic differentiation of the promyelocyte cell line HL60 led to increases in the steady-state levels of beta 2 and beta 7 mRNA transcripts, requiring a period of 10 and 24 h, respectively, of de novo protein synthesis. There was a parallel de novo expression of LPAM-1 on the cell surface, despite the fact that alpha 4 mRNA transcripts were rapidly down-regulated. At 72 h, HML-1 was not coexpressed with LPAM-1 on HL60 cells, although it was weakly expressed on peripheral blood monocytes/macrophages after a prolonged period of in vitro culture. Retinoic acid-induced granulocytic differentiation of HL60 cells led to the appearance of low levels of LPAM-1 at the cell surface. LPAM-1 was not found expressed on peripheral blood neutrophils, raising the possibility that it is transiently expressed during granulocyte differentiation. In accord with the above findings, differentiated monocytes and HL60 cells bound to recombinant MAdCAM-1 in an alpha 4- and beta 7-integrin-dependent fashion, whereas a population of undifferentiated HL60 cells and Mn(+2)-activated monocytes bound in an alpha 4-integrin-dependent beta 7-integrin-independent manner via VLA-4 expressed abundantly at all stages of differentiation. Four h after attachment, some of these VLA-4+ LPAM-1- HL60 cells could be seen to start spreading. These finding suggest that MAdCAM-1 can bind to VLA-4 when LPAM-1 is absent, and thus has the potential to recruit both VLA-4-bearing monocytes and VLA-4+ LPAM-1+ macrophages into chronically inflamed tissues.

KIM127, an antibody that promotes adhesion, maps to a region of CD18 that includes cysteine-rich repeats

A series of fusion proteins have been generated between human and mouse CD18. These proteins have been used to carry out preliminary mapping studies on a number of anti-CD18 antibodies including KIM127 an antibody that promotes CD18-dependent adhesion. This antibody maps to a region of the CD18 molecule between amino acids 406 and 570 in a region containing cysteine-rich repeats.

Differential expression of VLA beta 1 (CD29) on monocytes from patients with endometriosis

PROBLEM

Previous studies have established that in vitro proliferation of endometrial cells is enhanced by peripheral blood monocytes (PBM) and suppressed by peritoneal macrophages (PM) from patients with endometriosis but only suppressed by PBM and PM obtained from normal subjects. The functional activity of PBM and PM is influenced by the engagement of numerous cell surface receptors with their respective physiological ligands.

METHOD

In this study, PBM and PM from fertile women (Group 1), women with unexplained infertility (Group 2), and women with limited (Group 3) or severe (Group 4) endometriosis were isolated in order to analyze these cells for the expression of CD54, CD58 and HLA-DR (immunoglobulin supergene antigens) CD18 and CD29 (integrins) and CD44 (an addresin). These cell surface antigens are involved in monocyte/macrophage trafficking, activation, signal transduction and/or adhesion.

RESULTS

No differences were detected in the percentage of PBM expressing CD18, CD44, CD54, CD58, or HLA-DR among the four groups of subjects. Furthermore, the density of these antigens expressed on PBM was identical in patients and control subjects. In contrast, the percentage of PBM expressing CD29 (also known as VLA beta 1) and the density of CD29 expressed per cell were significantly reduced (P < 0.01) in patients with limited endometriosis compared to controls and patients with severe disease. Interestingly, although the percentage of CD29+ PBM from women with severe endometriosis was not statistically different from the percentage of CD29+ PBM from controls, the density of CD29 expressed per cell was significantly elevated among patients with severe disease. Analysis of PM from the four subject groups revealed no differences in CD29 expression or density. However, the percentage of PM expressing CD18 was significantly decreased in patients with limited (but not severe) endometriosis.

CONCLUSION

Since both CD18 and CD29 play a role in cell trafficking and/or adhesion, alterations in their expression among patients with endometriosis suggest that these integrin beta chains may play a role in the pathogenesis of the disease.

Molecular characterization of leukocyte adhesion deficiency in six patients

Leukocyte adhesion deficiency (LAD) is caused by defects in the CD18 gene, which codes for the common beta 2 subunit of the leukocyte integrins LFA-1, Mac-1 and p150,95. Failure to produce a functional beta 2 subunit results in the defective expression of all three leukocyte integrins, and the leukocytes of LAD patients have subnormal adhesion properties. Six patients with LAD were studied. Patient B was homozygous and carried a G284S mutation. A two-bp (GA) deletion at position 1256 (1256 delta GA) was found in the cDNA of patient C, who also had an abnormally large mRNA of 4.3 kb. Patients E and K were siblings and were heterozygous at the genomic level. One defective allele contained a mutation in intron 6/7 which created a preemptive 3' splice site. The resulting mRNA has 12 extra bases at the junction of exons 6 and 7, coding for four extra residues PSSQ in the protein. The same allele also carried a R586W mutation. The other allele was transcribed at a low level and was not characterized. Patient G carried a L149P mutation in one allele; again, the other allele was not characterized due to low transcription levels. Patient R carried two mutant alleles with G284S and R593C mutations respectively. The G284S mutation and the 1256 delta GA deletion have not been reported previously. CD18 cDNA carrying the abnormalities were cotransfected with normal CD11a or CD11b cDNA into COS cells. Expression of the LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) antigens on COS cells was not detected, suggesting that these two mutations are sufficient to account for LAD.

Crystal structure of the A domain from the alpha subunit of integrin CR3 (CD11b/CD18)

We have determined the high resolution crystal structure of the A domain from the alpha chain of integrin CR3. The domain adopts a classic alpha/beta "Rossmann" fold and contains an unusual Mg2+ coordination site at its surface. One of the coordinating ligands is the glutamate side chain from another A domain molecule. We suggest that this site represents a general metal ion-dependent adhesion site (MIDAS) for binding protein ligands. We further propose that the beta subunits of integrins contain a MIDAS motif within a modified A domain. Our crystal structure will allow reliable models to be built for other members of the A domain superfamily and should facilitate development of novel adhesion modulatory drugs.

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.

Adhesion molecules and their role in cancer metastasis

This article describes various adhesion molecules and reviews evidence to support a mechanistic role for adhesion molecules in the process of cancer metastasis. A variety of evidence supports the involvement of specific adhesion molecules in metastasis. 1. For example, some cancer cells metastasize to specific organs, irrespective of the first organ encountered by the circulating cancer cells. This ability to colonize a specific organ has been correlated with the preferential adhesion of the cancer cells to endothelial cells derived from the target organ. This suggests that cancer cell/endothelial cell adhesion is involved in cancer cell metastasis and that adhesion molecules are expressed on the endothelium in an organ-specific manner. 2. Further, inclusion of peptides that inhibit cell adhesion, such as the YIGSR- or RGD-containing peptides, is capable of inhibiting experimental metastasis. 3. Metastasis can be enhanced by acute or chronic inflammation of target vessels, or by treatment of animals with inflammatory cytokines, such as interleukin-1. In vitro, cancer cell/endothelial cell adhesion can be enhanced by pretreating the endothelial cell monolayer with cytokines, such as interleukin-1 or tumor necrosis factor-alpha. This suggests that, in addition to organ-specific adhesion molecules, a population of inducible endothelial adhesion molecules is involved and is relevant to metastasis. 4. Further support for this model is found in the comparison to leukocyte/endothelial adhesion during leukocyte trafficking. Convincing evidence exists, both in vivo and in vitro, to demonstrate an absolute requirement for leukocyte/endothelial adhesion before leukocyte extravasation can occur. The relevance of this comparison to metastasis is reinforced by the observation that some of the adhesion molecules involved in leukocyte/endothelial adhesion are also implicated in cancer cell/endothelial adhesion. The involvement of adhesion molecules suggests a potential therapy for metastasis based on interrupting adhesive interactions that would augment other treatments for primary tumors.

A novel, tissue-specific integrin subunit, beta nu, expressed in the midgut of Drosophila melanogaster

The integrins are a family of cell surface receptors for extracellular matrix proteins and counter-receptors on other cells. We have used the polymerase chain reaction to identify a novel integrin receptor beta subunit in Drosophila melanogaster. The deduced amino acid sequence of this subunit, which we have termed beta v (beta-neu), indicates that it has several unusual properties. The beta v subunit is roughly 33% identical with each of the previously sequenced vertebrate and Drosophila beta subunits and is lacking four of the 56 cysteine residues characteristic of most members of this protein family. The expression of the beta v gene is strikingly restricted. It is temporally regulated, with maximal expression occurring at 12–15 hours of embryonic development. In situ hybridization analyses and antibody localization on whole-mount embryos reveal that beta v expression is tissue-specific and confined to the developing midgut endoderm and its precursors during embryogenesis. Tissue specificity of expression is maintained through later stages of development as beta v transcripts are found exclusively in the larval midgut. Within this structure, beta v transcripts are especially concentrated in the cells of the midgut imaginal islands which give rise to the adult midgut.

De novo expression of intercellular adhesion molecule 1 (ICAM-1, CD54) in pancreas cancer

We examined the expression of intercellular--adhesion molecule-I (ICAM-I, CD54) in 6 surgically removed pancreatic tumors and 8 pancreatic tumor cell lines. Immunohistochemistry revealed a varying percentage of ICAM-I-positive pancreas tumor cells, while normal pancreatic tissue (except for slight reactivity of endothelial cells) was not stained. The presence of the ICAM-I molecule on the cell surface and the expression of ICAM-I mRNA were investigated for 8 different pancreatic tumor cell lines. Three of these (Capan-I, Capan-2, QGP-I) expressed ICAM-I constitutively. In 4 of the ICAM-I-negative pancreas cancer cell lines, it was possible to induce a remarkable expression of ICAM-I by incubating the cells in the presence of inflammatory cytokines, whereas one cell line, 818-4, remained ICAM-I-negative. The responsiveness to either IFN-gamma, TNF-alpha, or IL-I beta treatment was shown to vary from cell line to cell line, indicating complex mechanisms that regulate the expression of ICAM-I at both, the transcriptional and the post-transcriptional level. Interestingly, ICAM-I is shed by pancreatic tumor cells, since soluble sICAM-I was detected in the cell-culture supernatants. In comparison with normal sera, the mean level of sICAM-I in sera of patients with pancreas carcinoma is elevated 2-fold.

Identification of a 14-kDa laminin binding protein (HLBP14) in human melanoma cells that is identical to the 14-kDa galactoside binding lectin

The carbohydrate moieties present on laminin play a crucial role in the multiple biological activities of this basement membrane glycoprotein. We report the identification of a human laminin binding protein with an apparent molecular mass of 14 kDa on sodium dodecyl sulfate-polyacrylamide gels that was found, after purification and amino acid microsequencing, to be identical to the previously described 14-kDa galactoside binding soluble L-14 lectin. We have designated this human laminin binding protein as HLBP14. HLBP14 was purified from human melanoma cells in culture by laminin affinity chromatography and gel electroelution. We demonstrate that HLBP14 binds specifically to the poly-N-acetyllactosamine residues of murine laminin and does not bind to other glycoproteins that do not contain such structures, such as fibronectin. HLBP14 was eluted from a murine laminin column by lactose, N-acetyllactosamine, and galactose but not by other control saccharides, including glucose, fucose, mannose, and melibiose. It did not bind to laminin treated with endo-beta-galactosidase. Lactose also eluted HLBP14 off a human laminin affinity column, implying that human laminin also contains poly-N-acetyllactosamine residues. On immunoblots, polyclonal antibodies raised against HLBP14 recognized HLBP14 as well as 31- and 67-kDa molecules that are also laminin binding proteins, indicating that these proteins share common epitopes. L-14, a dimeric lactose binding lectin, is expressed in a wide variety of tissues. Although the expression of this molecule has been linked to a variety of biological events, the elucidation of its specific functions has been elusive. The observation that HLBP14, a human cancer cell laminin binding protein, is identical to L-14 strongly suggests that the functions attributed to this lectin could be mediated, at least in part, through its ability to interact with the poly-N-acetyllactosamine residues of laminin. HLBP14 could potentially play a role during tumor invasion and metastasis by modulating the interactions between cancer cells and laminin.

Molecular basis for a severe case of leukocyte adhesion deficiency

The leukocyte integrins LFA-1, Mac-1 and p150,95 (CD11a/CD18, CD11b/CD18, CD11c/CD18) mediate crucial leukocyte adhesive functions in immune and inflammatory reactions. Leukocyte adhesion deficiency (LAD) disease is caused by the defective expression of these adhesion molecules on leukocytes, and is characterized by recurrent infections and impaired pus formation due to the blockade of leukocyte migration into inflamed tissues. LAD is originated by heterogeneous mutations affecting the CD18 gene and, based on the severity of the deficiency, two phenotypes (severe and moderate) have been defined. Biochemical and genetic studies have allowed the classification of five different types of LAD. We have identified a type V LAD patient (severe phenotype, and normal size and levels of both CD18 precursor and CD18 mRNA), and determined its molecular basis. Reverse transcription-polymerase chain reaction and cloning and sequencing of CD18 cDNA derived from this patient revealed three silent mutations and a missense mutation that leads to the substitution of glycine at position 169 for an arginine. Analysis of patient-derived cDNA clones revealed the concomitant presence of aberrant splicing within the 5' region of the CD18 gene. The description of an identical mutation at residue 169 in an unrelated severe LAD patient raises the possibility that severe LAD type V is caused by a unique genetic defect.