Urinary B-cell-activating factor of the tumour necrosis factor family (BAFF) in systemic lupus erythematosus

INTRODUCTION

We examined the clinical relevance of urinary concentrations of B-cell-activating factor of the tumour necrosis factor family (BAFF) and a proliferation-inducing ligand (APRIL) in systemic lupus erythematosus (SLE).

METHODS

We quantified urinary BAFF (uBAFF) by enzyme-linked immunosorbent assay in 85 SLE, 28 primary Sjögren syndrome (pSS), 40 immunoglobulin A nephropathy (IgAN) patients and 36 healthy controls (HCs). Urinary APRIL (uAPRIL) and monocyte chemoattractant protein 1 (uMCP-1) were also quantified. Overall and renal SLE disease activity were assessed using the Systemic Lupus Erythematosus Disease Activity Index 2000.

RESULTS

uBAFF was detected in 12% (10/85) of SLE patients, but was undetectable in HCs, IgAN and pSS patients. uBAFF was detectable in 28% (5/18) of SLE patients with active nephritis vs 5/67 (7%) of those without ( p = 0.03), and uBAFF was significantly higher in active renal patients ( p = 0.02) and more likely to be detected in patients with persistently active renal disease. In comparison, uAPRIL and uMCP-1 were detected in 32% (25/77) and 46% (22/48) of SLE patients, respectively. While no difference in proportion of samples with detectable uAPRIL was observed between SLE, HCs and IgAN patients, both uAPRIL and uMCP-1 were significantly detectable in higher proportions of patients with active renal disease.

CONCLUSIONS

uBAFF was detectable in a small but a significant proportion of SLE patients but not in other groups tested, and was higher in SLE patients with active renal disease.

CD11b immunophenotyping identifies inflammatory profiles in the mouse and human lungs

The development of easily accessible tools for human immunophenotyping to classify patients into discrete disease endotypes is advancing personalized therapy. However, no systematic approach has been developed for the study of inflammatory lung diseases with often complex and highly heterogeneous disease etiologies. We have devised an internally standardized flow cytometry approach that can identify parallel inflammatory alveolar macrophage phenotypes in both the mouse and human lungs. In mice, lung innate immune cell alterations during endotoxin challenge, influenza virus infection, and in two genetic models of chronic obstructive lung disease could be segregated based on the presence or absence of CD11b alveolar macrophage upregulation and lung eosinophilia. Additionally, heightened alveolar macrophage CD11b expression was a novel feature of acute lung exacerbations in the SHIP-1(-/-) model of chronic obstructive lung disease, and anti-CD11b antibody administration selectively blocked inflammatory CD11b(pos) but not homeostatic CD11b(neg) alveolar macrophages in vivo. The identification of analogous profiles in respiratory disease patients highlights this approach as a translational avenue for lung disease endotyping and suggests that heterogeneous innate immune cell phenotypes are an underappreciated component of the human lung disease microenvironment.

SHIP-1 deficiency in the myeloid compartment is insufficient to induce myeloid expansion or chronic inflammation

SHIP-1 has an important role in controlling immune cell function through its ability to downmodulate PI3K signaling pathways that regulate cell survival and responses to stimulation. Mice deficient in SHIP-1 display several chronic inflammatory phenotypes including antibody-mediated autoimmune disease, Crohn's disease-like ileitis and a lung disease reminiscent of chronic obstructive pulmonary disease. The ileum and lungs of SHIP-1-deficient mice are infiltrated at an early age with abundant myeloid cells and the mice have a limited lifespan primarily thought to be due to the consolidation of lungs with spontaneously activated macrophages. To determine whether the myeloid compartment is the key initiator of inflammatory disease in SHIP-1-deficient mice, we examined two independent strains of mice harboring myeloid-restricted deletion of SHIP-1. Contrary to expectations, conditional deletion of SHIP-1 in myeloid cells did not result in consolidating pneumonia or segmental ileitis typical of germline SHIP-1 deficiency. In addition, other myeloid cell abnormalities characteristic of germline loss of SHIP-1, including flagrant splenomegaly and enhanced myelopoiesis, were absent in mice lacking SHIP-1 in myeloid cells. This study indicates that the spontaneous inflammatory disease characteristic of germline SHIP-1 deficiency is not initiated solely by LysM-positive myeloid cells but requires the simultaneous loss of SHIP-1 in other hematolymphoid lineages.

Gain- and loss-of-function Lyn mutant mice define a critical inhibitory role for Lyn in the myeloid lineage

To investigate the role of the Lyn kinase in establishing signaling thresholds in hematopoietic cells, a gain-of-function mutation analogous to the Src Y527F-activating mutation was introduced into the Lyn gene. Intriguingly, although Lyn is widely expressed within the hematopoietic system, these mice displayed no propensity toward hematological malignancy. By contrast, analysis of aging cohorts of both loss- and gain-of-function Lyn mutant mice revealed that Lyn(-/-) mice develop splenomegaly, increased numbers of myeloid progenitors, and monocyte/macrophage (M phi) tumors. Biochemical analysis of cells from these mutants revealed that Lyn is essential in establishing ITIM-dependent inhibitory signaling and for activation of specific protein tyrosine phosphatases within myeloid cells. Loss of such inhibitory signaling may predispose mice lacking this putative protooncogene to tumorigenesis.

Fyn and Lyn phosphorylate the Fc receptor gamma chain downstream of glycoprotein VI in murine platelets, and Lyn regulates a novel feedback pathway

Activation of platelets by collagen is mediated by the complex glycoprotein VI (GPVI)/Fc receptor gamma (FcR gamma chain). In the current study, the role of 2 Src family kinases, Fyn and Lyn, in GPVI signaling has been examined using murine platelets deficient in one or both kinases. In the fyn(-/-) platelets, tyrosine phosphorylation of FcR gamma chain, phopholipase C (PLC) activity, aggregation, and secretion are reduced, though the time of onset of response is unchanged. In the lyn(-/-) platelets, there is a delay of up to 30 seconds in the onset of tyrosine phosphorylation and functional responses, followed by recovery of phosphorylation and potentiation of aggregation and alpha-granule secretion. Tyrosine phosphorylation and aggregation in response to stimulation by collagen-related peptide is further attenuated and delayed in fyn(-/-)lyn(-/-) double-mutant platelets, and potentiation is not seen. This study provides the first genetic evidence that Fyn and Lyn mediate FcR immune receptor tyrosine-based activation motif phosphorylation and PLC gamma 2 activation after the ligation of GPVI. Lyn plays an additional role in inhibiting platelet activation through an uncharacterized inhibitory pathway. (Blood. 2000;96:4246-4253)

Ryk-deficient mice exhibit craniofacial defects associated with perturbed Eph receptor crosstalk

Secondary palate formation is a complex process that is frequently disturbed in mammals, resulting in the birth defect cleft palate. Gene targeting has identified components of cytokine/growth factor signalling systems such as Tgf-alpha/Egfr, Eph receptors B2 and B3 (Ephb2 and Ephb3, respectively), Tgf-beta2, Tgf-beta3 and activin-betaA (ref. 3) as regulators of secondary palate development. Here we demonstrate that the mouse orphan receptor 'related to tyrosine kinases' (Ryk) is essential for normal development and morphogenesis of craniofacial structures including the secondary palate. Ryk belongs to a subclass of catalytically inactive, but otherwise distantly related, receptor protein tyrosine kinases (RTKs). Mice homozygous for a null allele of Ryk have a distinctive craniofacial appearance, shortened limbs and postnatal mortality due to feeding and respiratory complications associated with a complete cleft of the secondary palate. Consistent with cleft palate phenocopy in Ephb2/Ephb3-deficient mice and the role of a Drosophila melanogaster Ryk orthologue, Derailed, in the transduction of repulsive axon pathfinding cues, our biochemical data implicate Ryk in signalling mediated by Eph receptors and the cell-junction-associated Af-6 (also known as Afadin). Our findings highlight the importance of signal crosstalk between members of different RTK subfamilies.

Genomic structure and expression of the mouse growth factor receptor related to tyrosine kinases (Ryk)

We report the genomic organization of the mouse orphan receptor related to tyrosine kinases (Ryk), a structurally unclassified member of the growth factor receptor family. The mouse RYK protein is encoded by 15 exons distributed over a minimum of 81 kilobases. Genomic DNA sequences encoding a variant protein tyrosine kinase ATP-binding motif characteristic of RYK are unexpectedly found in two separate exons. A feature of the gene is an unmethylated CpG island spanning exon 1 and flanking sequences, including a TATA box-containing putative promoter and single transcription start site. Immunohistochemical examination of RYK protein distribution revealed widespread but developmentally regulated expression, which was spatially restricted within particular adult organs. Quantitative reduction of Southern blotting stringency for the detection of Ryk-related sequences provided evidence for a retroprocessed mouse pseudogene and a more distantly related gene paralogue. Extensive cross-species reactivity of a mouse Ryk kinase subdomain probe and the cloning of a Ryk orthologue from Caenorhabditis elegans demonstrate that Ryk and its relatives encode widely conserved members of a novel receptor tyrosine kinase subfamily.

Activation of the Ras/mitogen-activated protein kinase pathway by kinase-defective epidermal growth factor receptors results in cell survival but not proliferation

Signalling by the epidermal growth factor (EGF) receptor (EGFR) has been studied intensively, but for most cell types the analysis is complicated by the fact that EGFR not only homodimerizes but can also form heterodimers with other EGFR family members. Heterodimerization is a particular problem in the study of EGFR mutants, where the true phenotype of the mutants is confounded by the contribution of the heterodimer partner to signal transduction. We have made use of the murine hemopoietic cell line BaF/3, which does not express EGFR family members, to express wild-type (WT) EGFR, three kinase-defective EGFR mutants (V741G, Y740F, and K721R), or a C-terminally truncated EGFR (CT957) and have measured their responses to EGF. We found that under the appropriate conditions EGF can stimulate cell proliferation of BaF/3 cells expressing WT or CT957 EGFRs but not that of cells expressing the kinase-defective mutants. However, EGF promotes the survival of BaF/3 cells expressing either of the kinase-defective receptors (V741G and Y740F), indicating that these receptors can still transmit a survival signal. Analysis of the early signalling events by the WT, V741G, and Y740F mutant EGF receptors indicated that EGF stimulates comparable levels of Shc phosphorylation, Shc-GRB-2 association, and activation of Ras, B-Raf, and Erk-1. Blocking the mitogen-activated protein kinase (MAPK) signalling pathway with the specific inhibitor PD98059 abrogates completely the EGF-dependent survival of cells expressing the kinase-defective EGFR mutants but has no effect on the EGF-dependent proliferation mediated by WT and CT957 EGFRs. Similarly, the Src family kinase inhibitor PP1 abrogates EGF-dependent survival without affecting proliferation. However blocking phosphatidylinositol-3-kinase or JAK-2 kinase with specific inhibitors does arrest growth factor-dependent cell proliferation. Thus, EGFR-mediated mitogenic signalling in BaF/3 cells requires an intact EGFR tyrosine kinase activity and appears to depend on the activation of both the JAK-2 and PI-3 kinase pathways. Activation of the Src family of kinases or of the Ras/MAPK pathway can, however, be initiated by a kinase-impaired EGFR and is linked to survival.

Common in vitro substrate specificity and differential Src homology 2 domain accessibility displayed by two members of the Src family of protein-tyrosine kinases, c-Src and Hck

Hck and Src are members of the Src family of protein- tyrosine kinases that carry out distinct and overlapping functions in vivo (Lowell, C. A., Niwa, M., Soriano, P., and Varmus, H. E. (1996) Blood 87, 1780-1792). In an attempt to understand how Hck and Src can function both independently and in concert, we have compared 1) their in vitro substrate specificity and 2) the accessibility of their Src homology 2 (SH2) domain. Using several synthetic peptides, we have demonstrated that Hck and Src recognize similar structural features in the substrate peptides, suggesting that both kinases have the intrinsic ability to carry out overlapping cellular functions by phosphorylating similar cellular proteins in vivo. Using a phosphotyrosine-containing peptide that has previously been shown to bind the SH2 domain of Src family kinases with high affinity, we found that although Src could bind to the phosphopeptide, Hck showed no interaction. The inability of Hck to bind the phosphopeptide was not a result of a stable intramolecular interaction between its SH2 domain and C-terminal regulatory phosphotyrosine residue (Tyr-520), as most Hck molecules in the purified Hck preparation were not tyrosine-phosphorylated. In contrast to intact Hck, a recombinant truncation analog of Hck was able to bind the phosphopeptide with an affinity similar to that of the Src SH2 domain, suggesting that conformational constraints are imposed on intact Hck that limit accessibility of its SH2 domain to the phosphopeptide. Furthermore, the difference in SH2 domain accessibility is a potential mechanism that enables Src and Hck to perform their respective unique functions by 1) targeting them to different subcellular compartments, whereupon they phosphorylate different cellular proteins, and/or 2) facilitating direct binding to their cellular substrates.

Polygenic autoimmune traits: Lyn, CD22, and SHP-1 are limiting elements of a biochemical pathway regulating BCR signaling and selection

A B lymphocyte hyperactivity syndrome resembling systemic lupus erythematosus characterizes mice lacking the src-family kinase Lyn. Lyn is not required to initiate B cell antigen receptor (BCR) signaling but is an essential inhibitory component. lyn-/- B cells have a delayed but increased calcium flux and exaggerated negative selection responses in the presence of antigen and spontaneous hyperactivity in the absence of antigen. As in invertebrates, genetic effects of loci with only one functional allele can be used to analyze signaling networks in mice, demonstrating that negative regulation of the BCR is a complex quantitative trait in which Lyn, the coreceptor CD22, and the tyrosine phosphatase SHP-1 are each limiting elements. The biochemical basis of this complex trait involves a pathway requiring Lyn to phosphorylate CD22 and recruit SHP-1 to the CD22/BCR complex.

Inhibition of the B cell by CD22: a requirement for Lyn

Mice in which the Lyn, Cd22, or Shp-1 gene has been disrupted have hyperactive B cells and autoantibodies. We find that in the absence of Lyn, the ability of CD22 to become tyrosine phosphorylated after ligation of mIg, to recruit SHP-1, and to suppress mIg-induced elevation of intracellular [Ca2+] is lost. Therefore, Lyn is required for the SHP-1-mediated B cell suppressive function of CD22, accounting for similarities in the phenotypes of these mice.

Biochemical characterization of mutant EGF receptors expressed in the hemopoietic cell line BaF/3

The Epidermal Growth Factor (EGF) receptor appears to require a fully active tyrosine kinase domain to transmit mitogenic signals. However, waved-2 mice carrying a mutation in the alpha-helix C of their EGF-R, which abolishes tyrosine kinase activity, only display a mild phenotype and are fully viable. This suggests that the mutant EGF-R signals through heterodimerization with endogenous, kinase active members of the EGF-R family such as ErbB-2 or ErbB-4. We have examined the biochemistry of EGF-Rs carrying mutations in the alpha-helix C of the human EGF-R (V741G and Y740F), in the ATP binding site (K721R) and at the C-terminus (CT957), by expression in BaF/3 cells which are devoid of EGF-R family members. The in vitro kinase activity of the alpha-helix C EGF-R mutants was severely impaired as a result of reduced phosphotransfer activity without appreciable changes in the affinity for either ATP or peptide substrate. Surprisingly, EGF stimulation of cells carrying the different mutant or wild type EGF-Rs resulted in tyrosine phosphorylation of EGF-R proteins; this phosphorylation was abolished in crude plasma membrane preparations, and appears to be due to activation of a membrane-associated or a cytosolic kinase. Receptor-mediated internalization of EGF was profoundly suppressed in the V741G, K721R and CT957 receptor mutant, and high affinity EGF binding was undetectable in the V741G and K721R receptors. We conclude that specific residues in the C-helix of the EGF-R kinase are essential for full kinase activity; mutations in this region do not affect ATP binding, but impair the receptors' phosphotransfer ability. High affinity binding of EGF is not dependent on tyrosine kinase activity or sequences in the C-terminus.

Lyn tyrosine kinase is essential for erythropoietin-induced differentiation of J2E erythroid cells

Erythropoietin stimulates the immature erythroid J2E cell line to terminally differentiate and maintains the viability of the cells in the absence of serum. In contrast, a mutant J2E clone (J2E-NR) fails to mature in response to erythropoietin; however, it remains viable in the presence of the hormone. We have shown previously that intracellular signalling is disrupted in the J2E-NR cell line and that tyrosine phosphorylation is dramatically reduced after erythropoietin stimulation. In this study we investigated the defect in J2E-NR cells that is responsible for their inability to differentiate. Screening of numerous signalling molecules revealed that the lyn tyrosine kinase appeared to be absent from J2E-NR cells. On closer examination, both lyn mRNA and protein content were reduced >500-fold. Consistent with a defect in lyn, amphotropic retroviral infection of J2E-NR cells with lyn restored the ability of the cells to synthesize haemoglobin and enabled the cells to mature morphologically. Conversely, the ability of J2E cells to differentiate in response to epo was severely curtailed when antisense lyn oligonucleotides or a dominant negative lyn were introduced into the cells. However, erythropoietin-supported viability was unaffected by reducing lyn activity. The ability of two other erythropoietin-responsive cell lines (R11 and R24) to differentiate in response to the hormone was also reduced by dominant negative lyn. Finally, co-immunoprecipitation and yeast two-hybrid analyses indicated that lyn directly associated with the erythropoietin receptor complex. These data indicate for the first time an essential role for lyn in erythropoietin-initiated differentiation of J2E cells but not in the maintenance of cell viability.

Lyn, a src-like tyrosine kinase

Lyn is a member of the src family of non-receptor protein tyrosine kinases that is predominantly expressed in haematopoietic tissues. Like all members of the src family, lyn is thought to participate in signal transduction from cell surface receptors that lack intrinsic tyrosine kinase activity. It is associated with a number of cell surface receptors including the B cell antigen receptor and Fc epsilon RI. Lyn deficient mice develop autoimmune disease characterised by autoantibodies in serum and the deposition of immune complexes in the kidney, a pathology reminiscent of systemic lupus erythematosus. Lyn deficient mice also have impaired signalling involving Fc epsilon RI in mast cells, resulting in defective allergic responses.

Autophosphorylation induces autoactivation and a decrease in the Src homology 2 domain accessibility of the Lyn protein kinase

Lyn is a member of the Src family of protein-tyrosine kinases that can readily undergo autophosphorylation in vitro. The site of autophosphorylation is Tyr397 which corresponds to the consensus autophosphorylation site of other Src family tyrosine kinases. The rate of autophosphorylation is concentration-dependent, indicating that the reaction follows an intermolecular mechanism. Autophosphorylation results in a 17-fold increase in protein-tyrosine kinase activity. Kinetic analysis demonstrates that phosphorylation of a substrate peptide by Lyn following autophosphorylation occurs with a 63-fold decrease in Km but no significant change in Vmax, suggesting that autophosphorylation relieves the conformational constraint that prevents binding of the substrate peptide to the active site of the kinase. Using a phosphotyrosine-containing peptide (pYEEI) that has previously been shown to bind to the Src homology 2 (SH2) domain of Src family tyrosine kinases with high affinity, we found that autophosphorylation results in a significant decrease in accessibility of the Lyn SH2 domain, indicating that conformational changes in the protein kinase domain induced by autophosphorylation can be propagated to the SH2 domain. Our study suggests that autophosphorylation plays an important role in regulating Lyn by modulating both its kinase activity and its interaction with other phosphotyrosine-containing molecules.

Multiple defects in the immune system of Lyn-deficient mice, culminating in autoimmune disease

Mice homozygous for a disruption at the Lyn locus display abnormalities associated with the B lymphocyte lineage and in mast cell function. Despite reduced numbers of recirculating B lymphocytes, Lyn-/- mice are immunoglobulin M (IgM) hyperglobulinemic. Immune responses to T-independent and T-dependent antigens are affected. Lyn-/- mice fail to mediate an allergic response to IgE cross-linking, indicating that activation of LYN plays an indispensable role in Fc epsilon RI signaling. Lyn-/- mice have circulating autoreactive antibodies, and many show severe glomerulonephritis caused by the deposition of IgG immune complexes in the kidney, a pathology reminiscent of systemic lupus erythematosus. Collectively, these results implicate LYN as having an indispensable role in immunoglobulin-mediated signaling, particularly in establishing B cell tolerance.

Identification of a duplication of the mouse Lyn gene

The Lyn gene encodes a PTK that is believed to participate in the transduction of signals from a variety of cell membrane receptors. Here we report the genomic organisation of the mouse Lyn gene and show that, while the promoter and exons 11-13 are present in single copy, sequences corresponding to the first coding exon are duplicated and this duplication extends into intron 10. Two sets of genomic clones representing the duplicated regions have been isolated and characterised. Nucleotide sequence analysis of these clones has revealed minimal sequence divergence between the two, suggesting that the duplication is a recent event. This is supported by Southern blot analysis of DNA from other mammalian species showing that the duplication is confined to the mouse. Aside from the duplicated sequences, the overall structure of the mouse Lyn gene is similar to that of other Src family members. These data suggest that the process of duplication which generated the Src family of PTK is an ongoing process and provide an insight into the molecular evolution of this group of genes.

A mutation in the epidermal growth factor receptor in waved-2 mice has a profound effect on receptor biochemistry that results in impaired lactation

The mutant mouse waved-2 (wa-2) is strikingly similar to transforming growth factor alpha-deficient mice generated by gene targeting in embryonic stem cells. We confirm that wa-2 is a point mutation (T-->G resulting in a valine-->glycine substitution at residue 743) in the gene encoding the epidermal growth factor (EGF) receptor. wa-2 fibroblastic cells lack high-affinity binding sites for EGF, and the rate of internalization of EGF is retarded. Although the tyrosine kinase activity of wa-2 EGF receptors is significantly impaired, NIH 3T3 cells lacking endogenous EGF receptors but overexpressing recombinant wa-2 EGF receptor cDNA are mitogenically responsive to EGF. While young and adult wa-2 mice are healthy and fertile, 35% of wa-2 mice born of homozygous wa-2 mothers die of malnutrition because of impaired maternal lactation.

Membrane-proximal Ig-like domain of Fc gamma RIII (CD16) contains residues critical for ligand binding

Ag-Ab complexes are cleared from the circulation through a complex system of receptors for the Fc portion of Ig (FcRs). Fc gamma RIII (CD16) is a low affinity FcR for IgG that is composed of two highly homologous Ig-like extracellular domains. Using secondary structure predictions, we located a strongly hydrophilic region in the second Ig-like domain of Fc gamma RIII that is predicted to lie between beta-strands C and C'. Substitutions of seven out of eight amino acids in this region abolished binding to IgG. Substitution of a conformationally adjacent amino acid in a bend just before beta-strand F and an amino acid in the B-C loop also affected ligand binding. However, amino acid substitutions in two different predicted loops in the second Ig-like domain as well as substitutions to three predicted loops in the first Ig-like domain had no effect on function. A chimeric Fc gamma RIII molecule lacking the second Ig-like domain was unable to bind IgG further, suggesting the presence of the binding site in the second domain. Neutralizing mAbs that inhibit Fc gamma RIII interaction with IgG were mapped to the E-F loop in the membrane proximal domain of Fc gamma RIII, providing further evidence of the importance of this region of the molecule in ligand interaction.

Membrane-proximal Ig-like domain of Fc gamma RIII (CD16) contains residues critical for ligand binding

Ag-Ab complexes are cleared from the circulation through a complex system of receptors for the Fc portion of Ig (FcRs). Fc gamma RIII (CD16) is a low affinity FcR for IgG that is composed of two highly homologous Ig-like extracellular domains. Using secondary structure predictions, we located a strongly hydrophilic region in the second Ig-like domain of Fc gamma RIII that is predicted to lie between beta-strands C and C'. Substitutions of seven out of eight amino acids in this region abolished binding to IgG. Substitution of a conformationally adjacent amino acid in a bend just before beta-strand F and an amino acid in the B-C loop also affected ligand binding. However, amino acid substitutions in two different predicted loops in the second Ig-like domain as well as substitutions to three predicted loops in the first Ig-like domain had no effect on function. A chimeric Fc gamma RIII molecule lacking the second Ig-like domain was unable to bind IgG further, suggesting the presence of the binding site in the second domain. Neutralizing mAbs that inhibit Fc gamma RIII interaction with IgG were mapped to the E-F loop in the membrane proximal domain of Fc gamma RIII, providing further evidence of the importance of this region of the molecule in ligand interaction.

The cytoplasmic domain of the integrin lymphocyte function-associated antigen 1 beta subunit: sites required for binding to intercellular adhesion molecule 1 and the phorbol ester-stimulated phosphorylation site

We have defined the regions of the cytoplasmic domain of the leukocyte integrin lymphocyte function-associated antigen 1 (LFA-1) that are required for active binding of its extracellular domain to intercellular adhesion molecule 1 (ICAM-1). The NH2-terminal 28 amino acids in the cytoplasmic domain are dispensable, but a segment of 5 amino acids including three contiguous threonines (758-760) and Phe 766 in the COOH-terminal third of the cytoplasmic domain are required for binding to ICAM-1. Mutation and phosphoamino acid analysis show that Ser 756 is the major residue phosphorylated in response to phorbol ester. Furthermore, multiple mutations demonstrate that serine phosphorylation can be dissociated from phorbol ester-stimulated binding of LFA-1 to ICAM-1. The sites we have defined are previously unremarked, are well conserved in the beta 1, beta 3, and beta 7 integrin subunits, and may be of broad importance in regulating adhesiveness of integrins.

Regulation of adhesion of ICAM-1 by the cytoplasmic domain of LFA-1 integrin beta subunit

Interactions between cytotoxic lymphocytes and their targets require the T cell antigen receptor (TCR) and the integrin lymphocyte function-associated molecule-1 (LFA-1, CD11a/CD18). LFA-1 is not constitutively avid for its counter-receptors, intercellular adhesion molecules (ICAMs)-1 and -2. Cross-linking of the TCR transiently converts LFA-1 to a high avidity state and thus provides a mechanism for regulating cellular adhesion and de-adhesion in an antigen-specific manner. Truncation of the cytoplasmic domain of the beta, but not the alpha, subunit of LFA-1 eliminated binding to ICAM-1 and sensitivity to phorbol esters. Thus, LFA-1 binding to ICAM-1 was found to be regulated by the cytoplasmic domain of the beta subunit of LFA-1.

ICAM-1 (CD54): a counter-receptor for Mac-1 (CD11b/CD18)

While the leukocyte integrin lymphocyte function-associated antigen (LFA)-1 has been demonstrated to bind intercellular adhesion molecule (ICAM)-1, results with the related Mac-1 molecule have been controversial. We have used multiple cell binding assays, purified Mac-1 and ICAM-1, and cell lines transfected with Mac-1 and ICAM-1 cDNAs to examine the interaction of ICAM-1 with Mac-1. Stimulated human umbilical vein endothelial cells (HUVECs), which express a high surface density of ICAM-1, bind to immunoaffinity-purified Mac-1 adsorbed to artificial substrates in a manner that is inhibited by mAbs to Mac-1 and ICAM-1. Transfected murine L cells or monkey COS cells expressing human ICAM-1 bind to purified Mac-1 in a specific and dose-dependent manner; the attachment to Mac-1 is more temperature sensitive, lower in avidity, and blocked by a different series of ICAM-1 mAbs when compared to LFA-1. In a reciprocal assay, COS cells cotransfected with the alpha and beta chain cDNAs of Mac-1 or LFA-1 attach to immunoaffinity-purified ICAM-1 substrates; this adhesion is blocked by mAbs to ICAM-1 and Mac-1 or LFA-1. Two color fluorescence cell conjugate experiments show that neutrophils stimulated with fMLP bind to HUVEC stimulated with lipopolysaccharide for 24 h in an ICAM-1-, Mac-1-, and LFA-1-dependent fashion. Because cellular and purified Mac-1 interact with cellular and purified ICAM-1, we conclude that ICAM-1 is a counter receptor for Mac-1 and that this receptor pair is responsible, in part, for the adhesion between stimulated neutrophils and stimulated endothelial cells.

On the species specificity of the interaction of LFA-1 with intercellular adhesion molecules

Species restrictions in immune cell interactions have been demonstrated both in Ag-specific responses of T lymphocytes and the phenomenon of natural attachment. To determine the possible contribution of adhesion receptors to these restrictions, we have studied binding between the murine and human homologues of LFA-1 (CD11a/CD18) and ICAM employing purified human LFA-1 and ICAM-1 (CD54) bound to solid substrates. Murine cell lines bind to purified human LFA-1 through ICAM-1 and at least one other counter-receptor. This provides evidence for multiple counter-receptors for LFA-1 in the mouse as well as in the human. In contrast to binding of murine ICAM-1 to human LFA-1, murine LFA-1 does not bind to human ICAM-1. The species specificity maps to the LFA-1 alpha subunit, because mouse x human hybrid cells expressing the human alpha subunit associated with a mouse beta subunit bind to human ICAM-1, whereas those with a human beta subunit associated with a murine alpha subunit do not. Increased adhesiveness for ICAM-1 stimulated by phorbol esters could be demonstrated for hybrid LFA-1 molecules with human alpha and murine beta subunits.

On the species specificity of the interaction of LFA-1 with intercellular adhesion molecules

Species restrictions in immune cell interactions have been demonstrated both in Ag-specific responses of T lymphocytes and the phenomenon of natural attachment. To determine the possible contribution of adhesion receptors to these restrictions, we have studied binding between the murine and human homologues of LFA-1 (CD11a/CD18) and ICAM employing purified human LFA-1 and ICAM-1 (CD54) bound to solid substrates. Murine cell lines bind to purified human LFA-1 through ICAM-1 and at least one other counter-receptor. This provides evidence for multiple counter-receptors for LFA-1 in the mouse as well as in the human. In contrast to binding of murine ICAM-1 to human LFA-1, murine LFA-1 does not bind to human ICAM-1. The species specificity maps to the LFA-1 alpha subunit, because mouse x human hybrid cells expressing the human alpha subunit associated with a mouse beta subunit bind to human ICAM-1, whereas those with a human beta subunit associated with a murine alpha subunit do not. Increased adhesiveness for ICAM-1 stimulated by phorbol esters could be demonstrated for hybrid LFA-1 molecules with human alpha and murine beta subunits.

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 integrin LFA-1 reconstituted by cDNA transfection in a nonhematopoietic cell line is functionally active and not transiently regulated

The functional activity of lymphocyte function-associated antigen 1 (LFA-1) on leukocytes can be regulated by T-cell receptor (TCR) stimulation and pharmacologic agents. It was of interest to determine if functionally active LFA-1 could be reconstituted on a nonhematopoietic, LFA-1-negative cell line. We report the expression of LFA-1 and diethylaminoethyl (DEAE) Mac-1 alpha beta heterodimers on the cell surface of a fibroblastoid cell line, COS, by DEAE dextran cotransfection of the alpha and beta subunit cDNAs. Immunoprecipitation studies demonstrated that the alpha and beta subunit was expressed in heterodimers. The alpha or beta subunit was expressed at lower levels after transfection with the alpha or beta subunit cDNA alone. Cotransfection of the alpha and beta subunit cDNAs, but not transfection of alpha or beta alone, was sufficient to reconstitute intercellular adhesion molecule-1 (ICAM-1) binding activity. Consistent with this observation, LFA-1 on the fibroblastoid cells possesses the activation epitope defined by the L16 monoclonal antibody (mAb). This epitope marks the conversion of LFA-1 from the low to high avidity state on peripheral blood T lymphocytes (PBLs) and is constitutively present on activated cell lines. In contrast to LFA-1 on leukocytes, the functional activity of LFA-1 on fibroblastoid cells was not influenced by phorbol ester treatment. Furthermore, the use of agents that interfere with intracellular signaling, a protein kinase C inhibitor, cAMP analogue, or the combination of a phosphodiesterase inhibitor and adenyl cyclase activator, did not affect the binding of COS cells expressing LFA-1 to purified ICAM-1.

Mechanisms for regulating expression of membrane isoforms of Fc gamma RIII (CD16)

Granulocyte and natural killer (NK) cell Fc receptors for immunoglobulin G (CD16) differ in only a few amino acids, yet have phosphatidylinositol glycan (PIG) or polypeptide membrane anchors, respectively. Mutagenesis shows that anchoring is regulated by a serine residue near the PIG anchor attachment site in the extracellular domain. The NK cell isoform was not expressed on the surface of COS cells unless cotransfected with a subunit that was expressed in NK cells and that was identical to the gamma subunit of the high affinity IgE Fc receptor (Fc epsilon RI). However, the CD16 sequence and not expression of the gamma subunit is dominant in regulating PIG reanchoring.

Natural killer cell and granulocyte Fc gamma receptor III (CD16) differ in membrane anchor and signal transduction

CD16 is a low affinity Fc gamma R III expressed on granulocytes, macrophages and large granular lymphocytes, the mediators of antibody-dependent cellular cytotoxicity and NK. The occupancy of CD16 by aggregated IgG on large granular lymphocytes induces expression of activation markers, release of inflammatory mediators and triggering of effector functions such as antibody-dependent cellular cytotoxicity. Recently we and others described that CD16 is anchored to the membrane of granulocytes via a phosphatidylinositol glycan moiety. Here we show that the CD16 molecule expressed on NK cells, cultured monocytes, and lung macrophages is not phosphatidylinositol glycan moiety anchored. It is not released with phosphatidylinositol-specific phospholipase C, and after removal of N-linked carbohydrate is 5 to 7 kDa larger than the granulocyte CD16 molecule, strongly suggesting the presence of transmembrane and cytoplasmic protein domains. Redirected killing of hybridoma targets expressing anti-CD16 surface Ig shows that NK cell CD16 is unable to do so. These findings demonstrate that NK cell and granulocyte CD16 have different membrane anchors and indicate that the type of membrane anchor is an important biologic mechanism for regulating the functional capacity of surface receptors.

Natural killer cell and granulocyte Fc gamma receptor III (CD16) differ in membrane anchor and signal transduction

CD16 is a low affinity Fc gamma R III expressed on granulocytes, macrophages and large granular lymphocytes, the mediators of antibody-dependent cellular cytotoxicity and NK. The occupancy of CD16 by aggregated IgG on large granular lymphocytes induces expression of activation markers, release of inflammatory mediators and triggering of effector functions such as antibody-dependent cellular cytotoxicity. Recently we and others described that CD16 is anchored to the membrane of granulocytes via a phosphatidylinositol glycan moiety. Here we show that the CD16 molecule expressed on NK cells, cultured monocytes, and lung macrophages is not phosphatidylinositol glycan moiety anchored. It is not released with phosphatidylinositol-specific phospholipase C, and after removal of N-linked carbohydrate is 5 to 7 kDa larger than the granulocyte CD16 molecule, strongly suggesting the presence of transmembrane and cytoplasmic protein domains. Redirected killing of hybridoma targets expressing anti-CD16 surface Ig shows that NK cell CD16 is unable to do so. These findings demonstrate that NK cell and granulocyte CD16 have different membrane anchors and indicate that the type of membrane anchor is an important biologic mechanism for regulating the functional capacity of surface receptors.

The structure of the murine Fc receptor for IgG. Assignment of intrachain disulfide bonds, identification of N-linked glycosylation sites, and evidence for a fourth form of Fc receptor

The Fc receptor (Fc gamma R) of the murine macrophage cell line, J774, was purified by immunoaffinity chromatography then subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and amino-terminal sequencing. FcR material judged to be pure by these criteria was digested with a number of enzymes to identify the cysteine residues engaged in disulfide bonds within the native structure. The results clearly establish that the mouse macrophage Fc gamma R contains two intrachain disulfide bonds, each of which connects adjacent cysteine residues within the two putative extracellular domains of the molecule. In addition, each disulfide-bonded domain was shown to contain two authentic sites of N-linked glycosylation. Extensive peptide sequencing resulted in the unexpected identification of peptide fragments from a fourth Fc gamma R whose sequences were highly homologous to sequences surrounding the two Cys residues in the amino-terminal domain of both alpha and beta 1 Fc gamma R. The fourth Fc gamma R contains a disulfide-bonded amino-terminal domain similar to beta 1 Fc gamma R.

The structure of the murine Fc receptor for IgG. Assignment of intrachain disulfide bonds, identification of N-linked glycosylation sites, and evidence for a fourth form of Fc receptor

The Fc receptor (Fc gamma R) of the murine macrophage cell line, J774, was purified by immunoaffinity chromatography then subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and amino-terminal sequencing. FcR material judged to be pure by these criteria was digested with a number of enzymes to identify the cysteine residues engaged in disulfide bonds within the native structure. The results clearly establish that the mouse macrophage Fc gamma R contains two intrachain disulfide bonds, each of which connects adjacent cysteine residues within the two putative extracellular domains of the molecule. In addition, each disulfide-bonded domain was shown to contain two authentic sites of N-linked glycosylation. Extensive peptide sequencing resulted in the unexpected identification of peptide fragments from a fourth Fc gamma R whose sequences were highly homologous to sequences surrounding the two Cys residues in the amino-terminal domain of both alpha and beta 1 Fc gamma R. The fourth Fc gamma R contains a disulfide-bonded amino-terminal domain similar to beta 1 Fc gamma R.

The mouse Fc receptor for IgG (Ly-17): molecular cloning and specificity

A cDNA clone encoding the mouse Ly-17+ Fc receptor for IgG, isolated from a myelomonocytic cell line, was sequenced and expression of mRNA and the functional Fc gamma R investigated. The receptor is a 301 amino acid transmembrane glycoprotein with two homologous extracellular domains that are also homologous to members of the Ig superfamily. The receptor has four sites of N-linked glycosylation and a long 94 amino acid cytoplasmic tail. Northern analysis, immune complex binding, and serological studies demonstrate that the receptor encoded by the cDNA clone binds mouse IgG gamma 1/2b and rabbit IgG complexes.

The murine Fc receptor for immunoglobulin: purification, partial amino acid sequence, and isolation of cDNA clones

The murine Fc receptor for IgG (Fc gamma R) was purified to homogeneity by immunoaffinity chromatography from detergent lysates of the macrophage cell line J774. Microsequencing of intact protein yielded a single amino-terminal sequence, which was confirmed and extended to 20 residues by the isolation of an overlapping peptide. The isolation of additional proteolytic fragments obtained by using Staphylococcus aureus V8 protease, cyanogen bromide, and lysine C proteinase, facilitated sequence analysis of a total of 119 amino acid residues. Codon usage charts were used to construct oligonucleotide probes based on the amino acid sequences of three nonoverlapping peptides. These probes were used to screen a cDNA library derived from the WEHI-3B myelomonocytic cell line, and a single cDNA clone (pFc24) to which all three probes hybridized was isolated. This clone, containing a 1.02-kilobase cDNA insert, has been characterized by restriction mapping and partial DNA sequencing, and it has been shown to encode the Fc gamma R. The sequence at the 5' end of the clone contained the coding information for the amino-terminal sequence of the Fc gamma R as well as a putative 13-amino acid signal sequence. The 3' end of the clone encoded a peptide identified in purified receptor preparations. Thus, the presence of coding information at the 5' and 3' ends of this clone suggests that full-length Fc receptor cDNA spans greater than 1 kilobase.

The mouse Ly-17 locus identifies a polymorphism of the Fc receptor

The mouse Ly-17.2 alloantigen has recently been defined with both conventional and monoclonal antibodies; it identifies a locus, sited on chromosome 1, the products of which were considered to be specific for B cells. Using another Ly-17.2-specific monoclonal antibody (described herein), the tissue distribution of the Ly-17.2 antigen was shown to extend to a subpopulation of T lymphocytes and to neutrophils. This distribution is remarkably similar to that of the Fc receptor for immunoglobulin. Indeed, we now demonstrate that the Ly-17 locus codes for a polymorphism of the Fc receptor, a conclusion based upon (a) an identical tissue distribution of Ly-17.2 and FcR on both normal and tumor tissue; (b) specific inhibition of EA rosette formation by F(ab')2 fragments of anti-Ly-17.2; (c) inhibition of the binding of the 2.4G2 monoclonal rat antimouse Fc receptor antibody by Ly-17.2 antibody; (d) precipitation of an identical series of molecules by our Ly-17.2-specific antibody and by the recognized Fc receptor-specific antibody (2.4G2); and (e) the demonstration by coprecipitation that the Ly-17.2 specificity is present on Fc receptor molecules. The studies suggest that the xenogeneic monoclonal antibody (2.4G2) which recognizes an invariant site on the FcR molecule and the polymorphic site are closely associated. In addition, the studies firmly map a gene coding for or regulating the expression of the FcR to chromosome 1.

The cell surface phenotype of mouse neutrophils

Using monoclonal antibodies, mouse peritoneal neutrophils were typed for the presence of 23 cell surface alloantigens, the expression of which was quantitated by flow cytofluorometry and compared with that of lymphocytes. The H-2K and H-2D alloantigens and beta 2-microglobulin were present on all neutrophils, but Ia and Qa antigens were not detected. It was found that Ly-5.1, Ly-15.2, Ly-21.2, Ly-24.2 (Pgp-1) and Ly-25.1 were present on greater than 90% of neutropils; Ly-6.2 and Ly-27.2 were absent, but Ly-28.2 (encoded by an Ly-6 linked gene), was present on greater than 90% of neutrophils. As expected, the lymphocyte-specific antigens Ly-1.1, Ly-2.2, Ly-3.1, Ly-7.2, Ly-12.1 and Thy-1.2 were absent from the neutrophils. When compared with lymphocytes, marked differences in alloantigen expression on neutrophils were seen for Ly-5.1, Ly-24.2 and Ly-28.2. These studies should be of value in the study of neutrophil structure and function.

Gm-3.2, a new granulocyte/macrophage alloantigen

A monoclonal antibody defining a unique mouse neutrophil cell-surface antigen, Gm-3.2, is described. Gm-3.2 is found on all neutrophils in peritoneal exudates and in bone marrow, and is also present on macrophages activated by thioglycolate but is absent from lymphoid, kidney, liver, heart, and red cells. Gm-3.2 is a differentiation antigen of myeloid cells, as granulocyte/macrophage colony-forming cells are Gm-3.2- while mature neutrophils are Gm-3.2+. Strain distribution pattern analysis shows linkage of the Gm-3 locus to the Ly-4, B2m, H-3 complex on chromosome 2.

Monoclonal antibody to murine neutrophils: identification of the Gm-2.2 specificity

A neutrophil-specific alloantigen (Gm-2.2) was defined by a monoclonal antibody, 5119-4/7. The Gm-2.2 antigen is found only on bone marrow neutrophils or calcium caseinate-induced neutrophils and is absent from all lymphoid cells examined as well as adherent thioglycollate-induced peritoneal exudate cells and the nonhemopoietic tissues, kidney, liver, heart, brain, and red blood cells. Furthermore, unlike mature neutrophils, granulocyte/macrophage progenitor cells are Gm-2.2-, suggesting that Gm-2.2 is a differentiation antigen for the neutrophil series. The Gm-2 locus is linked to, but distinct from, the Ly-6 locus.

Monoclonal antibody to murine neutrophils: identification of the Gm-2.2 specificity

A neutrophil-specific alloantigen (Gm-2.2) was defined by a monoclonal antibody, 5119-4/7. The Gm-2.2 antigen is found only on bone marrow neutrophils or calcium caseinate-induced neutrophils and is absent from all lymphoid cells examined as well as adherent thioglycollate-induced peritoneal exudate cells and the nonhemopoietic tissues, kidney, liver, heart, brain, and red blood cells. Furthermore, unlike mature neutrophils, granulocyte/macrophage progenitor cells are Gm-2.2-, suggesting that Gm-2.2 is a differentiation antigen for the neutrophil series. The Gm-2 locus is linked to, but distinct from, the Ly-6 locus.