A molecular model of RGD ligands. Antibody D gene segments that direct specificity for the integrin alpha IIb beta 3

Cell Adhesion by Integrins

Integrins are heterodimeric cell surface receptors ensuring the mechanical connection between cells and the extracellular matrix. In addition to the anchorage of cells to the extracellular matrix, these receptors have critical functions in intracellular signaling, but are also taking center stage in many physiological and pathological conditions. In this review, we provide some historical, structural, and physiological notes so that the diverse functions of these receptors can be appreciated and put into the context of the emerging field of mechanobiology. We propose that the exciting journey of the exploration of these receptors will continue for at least another new generation of researchers.

Structural and functional aspects of decorsin and its analog as recognized by integrin αIIbβ3

Decorsin is an antagonist of platelet glycoprotein integrin αIIbβ3 on platelets; the protein is 39 amino acids long with three disulfide bridges in its tertiary structure. To demonstrate decorsin's mechanism of action, we applied the computational virtual technique and platelet aggregation inhibition assay, which showed that the flanking amino-acid residues of the Arg-Gly-Asp (RGD) motif play an important role in platelet aggregation. The computational simulations revealed that the RGD motif mainly contributes to the stability of the complex when decorsion interacts with integrin αIIbβ3. However, the C-terminal residues, such as 34A→W and 35D→R, was also found to possibly play a key role in their binding structures. Moreover, we produced a decorsin analog (A34W plus D35R decorsin), in which the 34A (alanine) and 35D (aspartic acid) residues were respectively substituted by W (tryptophan) and R (arginine). This isoform was then recombinantly expressed in Escherichia coli. Intriguingly, this mutant type showed higher anti-platelet aggregation activity than the wildtype. Our study may further contribute to finding decorsin mutants with higher anti-platelet aggregation activity.

Tailored placement of a turn-forming PA tag into the structured domain of a protein to probe its conformational state

Placement of a tag sequence is usually limited to either terminal of the target protein, reducing the potential of epitope tags for various labeling applications. The PA tag is a dodecapeptide (GVAMPGAEDDVV) that is recognized by a high-affinity antibody NZ-1. We determined the crystal structure of the PA tag/NZ-1 complex and found that NZ-1 recognized a central segment of the PA tag peptide in a tight β-turn configuration, suggesting its compatibility with the insertion into a loop. This possibility was tested and confirmed using multiple integrin subunits and semaphorin. More specifically, the PA tag can be inserted at multiple locations within the αIIb subunit of the fibrinogen receptor αIIbβ3 integrin without affecting the structural and functional integrity, while maintaining its high affinity toward NZ-1. The large choice of the sites for "epitope grafting" enabled the placement of the PA tag at a location whose accessibility is modulated during the biological action of the receptor. Thus, we succeeded in converting a general anti-tag antibody into a special reporter/activator anti-β1 integrin antibody that can be classified as a ligand-induced binding site antibody.

Surface modification and endothelialization of biomaterials as potential scaffolds for vascular tissue engineering applications

This review highlights the recent developments of surface modification and endothelialization of biomaterials in vascular tissue engineering applications.

Structural requirements for activation in alphaIIb beta3 integrin

Integrins are postulated to undergo structural rearrangement from a low affinity bent conformer to a high affinity extended conformer upon activation. However, some reports have shown that a bent conformer is capable of binding a ligand, whereas another report has shown that integrin extension does not absolutely lead to activation. To clarify whether integrin affinity is indeed regulated by the so-called switchblade-like movement, we have engineered a series of mutant αIIbβ3 integrins that are constrained specifically in either a bent or an extended conformation. These mutant αIIbβ3 integrins were expressed in mammalian cells, and fibrinogen binding to these cells was examined. The bent integrins were created through the introduction of artificial disulfide bridges in the β-head/β-tail interface. Cells expressing bent integrins all failed to bind fibrinogen unless pretreated with DTT to disrupt the disulfide bridges. The extended integrins were created by introducing N-glycosylation sites in amino acid residues located close to the α-genu, where the integrin legs fold backward. Among these mutants, activation was maximized in one integrin with an N-glycosylation site located behind the α-genu. This extension-induced activation was completely blocked when the swing-out of the hybrid domain was prevented. These results suggest that the bent and extended conformers represent low affinity and high affinity conformers, respectively, and that extension-induced activation depends on the swing-out of the hybrid domain. Taken together, these results are consistent with the current hypothesis that integrin affinity is regulated by the switchblade-like movement of the integrin legs.

Tenectin is a novel alphaPS2betaPS integrin ligand required for wing morphogenesis and male genital looping in Drosophila

Morphogenesis of the adult structures of holometabolous insects is regulated by ecdysteroids and juvenile hormones and involves cell-cell interactions mediated in part by the cell surface integrin receptors and their extracellular matrix (ECM) ligands. These adhesion molecules and their regulation by hormones are not well characterized. We describe the gene structure of a newly described ECM molecule, tenectin, and demonstrate that it is a hormonally regulated ECM protein required for proper morphogenesis of the adult wing and male genitalia. Tenectin's function as a new ligand of the PS2 integrins is demonstrated by both genetic interactions in the fly and by cell spreading and cell adhesion assays in cultured cells. Its interaction with the PS2 integrins is dependent on RGD and RGD-like motifs. Tenectin's function in looping morphogenesis in the development of the male genitalia led to experiments that demonstrate a role for PS integrins in the execution of left-right asymmetry.

Anticoagulant and antithrombotic activity of a new peptide pENW (pGlu-Asn-Trp)

Objectives

The aim was to test a newly discovered oligopeptide, pENW (pGlu-Asn-Trp), for its anticoagulant and antithrombotic activity in vivo, and try to investigate its underlying mechanisms.

Methods

We measured coagulation time by the glass slide method and bleeding time by cutting of mice tails. The thrombosis models employed here included an arterio-venous shunt model and inferior vena ligation model. An ELISA (enzyme-linked immunosorbent assay) was used to analyse t-PA/PAI (tissue-type plasminogen activator/plasminogen activator inhibitor) in the blood drawn from the rats with thrombosis. The ultrastructural changes of the endothelium in the vessels developing thrombosis were observed under a transmission electron microscope.

Key findings

We found that pENW-treated mice exhibited a prolonged coagulation time in a dose-dependent manner, but not an extended haemorrhage time. On the other hand, pENW significantly inhibited thrombus formation in both arterio-venous shunt models and inferior vena ligation models. Plasma t-PA/PAI was significantly higher as measured by ELISA. Transmission electron microscope photos of pENW-treated groups also displayed a better condition than model controls, with less erythrocytes in the vascular lumens. In addition, pENW concentration-dependently inhibited aggregation of platelets induced by ADP (adenosine 5′-diphosphate sodium salt) in rabbit platelet-rich plasma.

Conclusions

These findings support the suggestion that pENW possesses antithrombotic activity and could be a promising drug in the prevention and treatment of unwanted clot formation.

Effect of molecular mobility of polymeric implants on soft tissue reactions: an in vivo study in rats

Although numerous different polymers are used as implants or otherwise studied for many other biotechnical applications, there is a lack of basic models that correlate polymer characteristics with foreign body reactions. This study aims at developing one such model by systematically studying surface molecular mobility of polymeric implants in soft tissues in vivo. Changing the length of the alkyl side chain of poly(alkyl methacrylates) (PAMAs), provides an interesting opportunity to study the surface molecular mobility with minimal changes of the hydrophobicity of the surface. Thus, in this study three different PAMAs, with increasingly surface mobility; poly (isobutyl methacrylate) (PIBMA), poly(butyl methacrylate) (PBMA), and poly(lauryl methacralate) (PLMA) along with pure titanium (Ti) substrates were implanted in the dorsum of Sprague-Dawley rats. Inflammatory cell recruitment, cell adhesion, and cytokine release were studied after 1, 3, and 28 days of implantation. Total number of inflammatory cells in the exudate was measured but no correlation between surface mobility and cell recruitment where found. However, the number of surface associated cells where significantly lower on the surfaces with high molecular mobility (PLMA and PBMA). The histological evaluation performed after 28 days revealed thicker fibrous capsule and a higher number of blood vessels on the low molecular mobility surface (PIBMA). After 28 days the cell activity was higher on the high molecular mobility surfaces (PLMA and PBMA) compared with PIBMA, based on the cytokine release. None of the surfaces induced any significant cell-death. On the basis of the results of this study we conclude that there is a significant difference in biological response to surfaces with different in molecular mobility. This might affect the wound healing process and the biocompatibility of biomaterials.

Novel tools for functional analysis of CD11c: activation-specific, activation-independent, and activating antibodies

Functions and binding properties of four CD11c-specific mAbs are described here. The mAb 496B stimulated, while 496K inhibited ligand binding of CD11c. The stimulatory mAb, 496B, as well as the inhibitory mAbs BU15 and 496 K appear to act allosterically, as they do not bind the CD11c I domain. The mAb 3.9 bound preferentially to activated forms of CD11c and the binding was divalent cation dependent. CD11c binding to 3.9 recapitulates many of the integrin-ligand interactions. Our data suggest that 3.9 is a competitive antagonist, BU15 and 496K are allosteric antagonists, and 496B is an allosteric agonist of CD11c. These mAbs provide a set of tools to study the functions of the dendritic cell marker, CD11c.

Tyr178 of beta3 is critical for alphaIIb maturation and macromolecular ligand binding to alphaIIbbeta3

To explore the structural basis of ligand binding to alphaIIbbeta3, we conducted a site-directed mutagenesis of Y178, which is located in the ligand-specificity region (C177-C184) of the beta3 subunit. Two mutant beta3 constructs, Y178A and Y178I, were transfected into CHO cells and co-expressed with human alphaIIb subunit on the cell surface. Our results showed that the Y178A mutation affected processing and cell surface exposure of recombinant alphaIIbbeta3 receptor, abrogated the binding of PAC-1, a ligand-mimetic antibody, to alphaIIbbeta3 pre-treated with the known activator DTT. The Y178A mutation also resulted in reduced adhesion of alphaIIbbeta3 on immobilized fibrinogen. In contrast, the interaction of alphaIIbbeta3 with the small molecular ligand RGDS was unaffected by Y178A mutation, as evidenced by the elevated LIBS-1 epitope expression following RGDS addition. Interestingly however, Y178I mutation did not affect the receptor synthesis and function at all. As for post-receptor occupancy, neither Y178A nor Y178I prevented alphaIIbbeta3 translocation to focal adhesion contacts. These results suggest that Y178 is involved in alphaIIb maturation and alphaIIbbeta3 complex expression. This residue is also critical for alphaIIbbeta3 interaction with its macromolecular ligand or ligand-mimetic mAb, possibly due to its involvement in other ligand-binding sites distinct from the RGD-binding pocket. We also propose that a residue with appropriate side-chain size and hydrophobicity at position 178 is spatially required for formation of the correct tertiary structure of the site.

A Polysaccharide Carrier to Effectively Deliver Native Phosphodiester CpG DNA to Antigen-Presenting Cells

Oligodeoxynucleotides containing unmethylated CpG sequences (CpG DNAs) activate the vertebrate innate immune system via toll-like receptor 9 (TLR-9). Although CpG DNA is a promising immunotherapeutic agent, its short circulation time in biological fluids due to nuclease is the major drawback. This paper proposes that a natural polysaccharide called schizophyllan (SPG) can be used as an effective CpG DNA carrier because SPG can complex with CpG DNA and the resultant complex shows the nuclease resistance of the bound DNA. In order to increase cellular uptake in vitro, we chemically attached spermine, cholesterol, arginine octamer, or RGD peptide to SPG. The complexes made of the chemically modified SPG and CpG DNA having a phosphorothioate (PS) or phosphodiester (PO) backbone led to increased secretion of cytokines of about 4- to 15-fold, compared with the uncomplexed dose. Furthermore, when PO CpG DNA was complexed with unmodified SPG, the IL-12 level increased by almost 3- to 11-fold compared with the naked dose. The PO CpG DNA/unmodified SPG complex data suggested that unmodified SPG might effectively deliver PO in vivo due to the electrically neutral nature of unmodified SPG. When the complexed CpG DNAs were injected intraperitoneally, a large amount of IL-12 production was observed compared with the uncomplexed material. Both in vivo and vitro assays indicated that the SPG complex may be of use for CpG DNA therapy.

Mechanistic Insights from a Refined Three-dimensional Model of Integrin αIIbβ3

The integrin alpha(IIb)beta(3) plays an important role in platelet function, and abnormalities of this protein result in a serious bleeding disorder, known as Glanzmann thrombasthenia. Although crystallographic data exist for the related integrin alpha(V)beta(3), to date, there are no high resolution structures of integrin alpha(IIb)beta(3) available in the literature. Therefore, it is still unclear how specific elements of the alpha(IIb) subunit contribute to integrin alpha(IIb)beta(3) function. Here we describe a refined model of the alpha(IIb) N-terminal portion of integrin alpha(IIb)beta(3) obtained by using the alpha(V)beta(3) template combined with a new method for predicting the conformations of the unique alpha(IIb) loop regions comprising residues 71-85, 114-125, and 148-164. The refined model was probed based on a structural prediction that differentiates it from standard homology models: specifically, that Lys-118 of alpha(IIb) contacts Glu-171 of beta(3). To test this hypothesis experimentally, the mutant integrin chains alpha(IIb) K118C and beta(3) E171C were cotransfected into HEK 293 cells. We show that the cells expressed the mutants alpha(IIb)beta(3) on their surface as a disulfide-linked dimer, supporting the close proximity between alpha(IIb) Lys-118 and beta(3) Glu-171 predicted from the refined model. This validated model provides a specific structural context for the analysis and interpretation of structure-function relations of integrin alpha(IIb)beta(3). In addition, it suggests mechanistic hypotheses pertaining to both naturally occurring mutations responsible for Glanzmann thrombasthenia and to point mutations that affect ligand binding.

Function-blocking integrin alphavbeta6 monoclonal antibodies: distinct ligand-mimetic and nonligand-mimetic classes

We have generated a panel of potent, selective monoclonal antibodies that bind human and mouse alpha(v)beta(6) integrin with high affinity (up to 15 pm). A subset of these antibodies blocked the binding of alpha(v)beta(6) to the transforming growth factor-beta1 latency-associated peptide with IC(50) values as low as 18 pm, and prevented the subsequent alpha(v)beta(6)-mediated activation of transforming growth factor-beta1. The antibodies also inhibited alpha(v)beta(6) binding to fibronectin. The blocking antibodies form two biochemical classes. One class, exemplified by the ligand-mimetic antibody 6.8G6, bound to the integrin in a divalent cation-dependent manner, contained an RGD motif or a related sequence in CDR3 of the heavy chain, was blocked by RGD-containing peptides, and was internalized by alpha(v)beta(6)-expressing cells. Despite containing an RGD sequence, 6.8G6 was specific for alpha(v)beta(6) and showed no cross-reactivity with the RGD-binding integrins alpha(v)beta(3), alpha(v)beta(8),or alpha(IIb)beta(3). The nonligand-mimetic blocking antibodies, exemplified by 6.3G9, were cation-independent, were not blocked by RGD-containing peptides, were not internalized, and did not contain RGD or related sequences. These two classes of antibody were unable to bind simultaneously to alpha(v)beta(6), suggesting that they may bind overlapping epitopes. The "ligand-mimetic" antibodies are the first to be described for alpha(v)beta(6) and resemble those described for alpha(IIb)beta(3). We also report for the first time the relative abilities of divalent cations to promote alpha(v)beta(6) binding to latency-associated peptide and to the ligand-mimetic antibodies. These antibodies should provide valuable tools to study the ligand-receptor interactions of alpha(v)beta(6) as well as the role of alpha(v)beta(6) in vivo.

Integrin αIIbβ3 specific synthetic human monoclonal antibodies and HCDR3 peptides that potently inhibit platelet aggregation

The interaction of fibrinogen with integrin alphaIIbbeta3 (GPIIb/IIIa), in part mediated by an RGD tripeptide motif, is an essential step in platelet aggregation. Based on their inhibition of platelet aggregation, three integrin alphaIIbbeta3 inhibitors are clinically approved. The clinically most widely used integrin alphaIIbbeta3 inhibitor abciximab is a chimeric mouse/human antibody that induces thrombocytopenia, often severe, in 1-2% of patients due to a human anti-mouse antibody (HAMA) response. In addition, unlike other ligands mimicking small molecular drugs, abciximab cross-reacts with integrin alphavbeta3 and alphaMbeta2. Here we used phage display to select monoclonal antibodies specific to integrin alphaIIbbeta3 from a synthetic human antibody library based on the randomized HCDR3 sequence VGXXXRADXXXYAMDV. The selected antibodies revealed a strong consensus in HCDR3 (V(V/W)CRAD(K/R)RC) and high specificity toward integrin alphaIIbbeta3 but not to other RGD binding integrins such as alphavbeta3, alphavbeta5, and alpha5beta1. The selected antibodies as well as three synthetic peptides (VWCRADRRC, VWCRADKRC, and VVCRADRRC) whose sequences were derived from the HCDR3 sequences of the selected antibodies strongly inhibited the interaction between integrin alphaIIbbeta3 and fibrinogen and platelet aggregation ex vivo. To our knowledge, these are the first fully human monoclonal antibodies that are specific to integrin alphaIIbbeta3 and can potently inhibit platelet aggregation.

Human IgG monoclonal anti-alpha(IIb)beta(3)-binding fragments derived from immunized donors using phage display

Previous studies of the immune response in polytransfused Glanzmann thrombasthenia (GT) patients and in autoimmune thrombocytopenic purpura (AITP) have relied on serum analysis and have shown the frequent development of Abs directed against the alpha(IIb)beta(3) integrin. However, little is known about the molecular diversity of the humoral immune response to alpha(IIb)beta(3) due to the paucity of mAbs issuing from these pathologies. We have isolated human IgG anti-alpha(IIb)beta(3) binding fragments using combinatorial libraries of single-chain IgG created from the B cells of a GT and an AITP patient, both with serum Abs. Ab screening was performed using activated platelets or activated alpha(IIb)beta(3)-expressing Chinese hamster ovary cells. Sequencing of selected phage Abs showed that a broad selection of genes from virtually all V gene families had been used, indicating the diversity of the immune response. About one-half of the V(H) and V(L) segments of our IgG anti-alpha(IIb)beta(3) fragments displayed extensive hypermutations in the complementarity-determining region, supporting the idea that an Ag-driven immune response was occurring in both patients. The H chain complementarity-determining region 3 analysis of phage Abs revealed motifs other than the well-known RGD and KQAGDV integrin-binding sequences. To our knowledge, our study is the first to illustrate multiple human IgG anti-alpha(IIb)beta(3) reactivities and structural variations linked to the anti-platelet human immune response. Human alpha(IIb)beta(3) Abs preferentially directed against the activated form of the integrin were further characterized because platelet alpha(IIb)beta(3) inhibitors are potential therapeutic reagents for treating acute coronary syndromes. Currently available alpha(IIb)beta(3) antagonists do not specifically recognize the activated form of the integrin.

Amino acid residues in the alpha IIb subunit that are critical for ligand binding to integrin alpha IIbbeta 3 are clustered in the beta-propeller model

Several distinct regions of the integrin alpha(IIb) subunit have been implicated in ligand binding. To localize the ligand binding sites in alpha(IIb), we swapped all 27 predicted loops with the corresponding sequences of alpha(4) or alpha(5). 19 of the 27 swapping mutations had no effect on binding to both fibrinogen and ligand-mimetic antibodies (e.g. LJ-CP3), suggesting that these regions do not contain major ligand binding sites. In contrast, swapping the remaining 8 predicted loops completely blocked ligand binding. Ala scanning mutagenesis of these critical predicted loops identified more than 30 discontinuous residues in repeats 2-4 and at the boundary between repeats 4 and 5 as critical for ligand binding. Interestingly, these residues are clustered in the predicted beta-propeller model, consistent with this model. Most of the critical residues are located at the edge of the upper face of the propeller, and several critical residues are located on the side of the propeller domain. None of the predicted loops in repeats 1, 6, and 7, and none of the four putative Ca(2+)-binding predicted loops on the lower surface of the beta-propeller were important for ligand binding. The results map an important ligand binding interface at the edge of the top and on the side of the beta-propeller toroid, centering on repeat 3.

Platelet-associated anti-GPIIb-IIIa autoantibodies in chronic immune thrombocytopenic purpura recognizing epitopes close to the ligand-binding site of glycoprotein (GP) IIb

Localization of epitopes for platelet-associated (PA) anti-GPIIb-IIIa (alpha(IIb)beta(3)) autoantibodies in chronic immune thrombocytopenic purpura remains elusive. Previous studies suggest that PA antibodies recognize the tertiary structure of intact glycoprotein (GP) IIb-IIIa. To localize their epitopes using antigen-capture enzyme-linked immunosorbent assay (ELISA), the reactivity of 34 PA anti-GPIIb-IIIa antibodies was examined with recombinant GPIIb-IIIa having a defect in ligand-binding sites in either GPIIb or GPIIIa, and no major conformational change was induced: KO variant GPIIb-IIIa was attributed to a 2-amino acid insertion between residues 160 and 161 in the W3 4-1 loop in GPIIb, and CAM variant GPIIb-IIIa was attributed to D119Y in GPIIIa. In one third (11 of 34) of the patients, PA antibodies showed a marked decrease (less than 50%) in reactivity with KO compared with wild-type GPIIb-IIIa. Their reactivity was also impaired against GPIIbD163A-IIIa. In sharp contrast, they reacted normally with CAM GPIIb-IIIa. OP-G2, a ligand-mimetic monoclonal antibody, markedly inhibited their binding to GPIIb-IIIa in patients with impaired binding to KO GPIIb-IIIa, but small GPIIb-IIIa antagonists did not. In addition, a newly developed sensitive ELISA indicated that autoantibodies showing impaired binding to KO are more potent inhibitors for fibrinogen binding. The present data suggest that certain PA anti-GPIIb-IIIa autoantibodies recognize epitopes close to the ligand-binding site in GPIIb, but not in GPIIIa.

Multiple discontinuous ligand-mimetic antibody binding sites define a ligand binding pocket in integrin alpha(IIb)beta(3)

Integrin alpha(IIb)beta(3), a platelet fibrinogen receptor, is critically involved in thrombosis and hemostasis. However, how ligands interact with alpha(IIb)beta(3) has been controversial. Ligand-mimetic anti-alpha(IIb)beta(3) antibodies (PAC-1, LJ-CP3, and OP-G2) contain the RGD-like RYD sequence in their CDR3 in the heavy chain and have structural and functional similarities to native ligands. We have located binding sites for ligand-mimetic antibodies in alpha(IIb) and beta(3) using human-to-mouse chimeras, which we expect to maintain functional integrity of alpha(IIb)beta(3). Here we report that these antibodies recognize several discontinuous binding sites in both the alpha(IIb) and beta(3) subunits; these binding sites are located in residues 156-162 and 229-230 of alpha(IIb) and residues 179-183 of beta(3). In contrast, several nonligand-mimetic antibodies (e.g. 7E3) recognize single epitopes in either subunit. Thus, binding to several discontinuous sites in both subunits is unique to ligand-mimetic antibodies. Interestingly, these binding sites overlap with several (but not all) of the sequences that have been reported to be critical for fibrinogen binding (e.g. N-terminal repeats 2-3 but not repeats 4-7, of alpha(IIb)). These results suggest that ligand-mimetic antibodies and probably native ligands may make direct contact with these discontinuous binding sites in both subunits, which may constitute a ligand-binding pocket.

Antiidiotypic antibody recognizes an amiloride binding domain within the alpha subunit of the epithelial Na+ channel

We previously raised an antibody (RA6.3) by an antiidiotypic approach which was designed to be directed against an amiloride binding domain on the epithelial Na+ channel (ENaC). This antibody mimicked amiloride in that it inhibited transepithelial Na+ transport across A6 cell monolayers. RA6.3 recognized a 72-kDa polypeptide in A6 epithelia treated with tunicamycin, consistent with the size of nonglycosylated Xenopus laevis alphaENaC. RA6.3 specifically recognized an amiloride binding domain within the alpha-subunit of mouse and bovine ENaC. The deduced amino acid sequence of RA6.3 was used to generate a three-dimensional model structure of the antibody. The combining site of RA6.3 was epitope mapped using a novel computer-based strategy. Organic residues that potentially interact with the RA6.3 combining site were identified by data base screening using the program LUDI. Selected residues docked to the antibody in a manner corresponding to the ordered linear array of amino acid residues within an amiloride binding domain on the alpha-subunit of ENaC. A synthetic peptide spanning this domain inhibited the binding of RA6.3 to alphaENaC. This analysis provided a novel approach to develop models of antibody-antigen interaction as well as a molecular perspective of RA6.3 binding to an amiloride binding domain within alphaENaC.

Combinatorial chemistry reveals a new motif that binds the platelet fibrinogen receptor, gpIIbIIIa

Among cell adhesion molecules, the classic Arg-Gly-Asp (RGD) motif is the best studied. We used combinatorial chemical and affinity immunochemical methods to find a novel motif of unnatural peptide ligands for the fibrinogen receptor of platelets, gpIIbIIIa (alphaIIbbeta3). The new d-amino acid motif, p(f/y)l, is unique among the ligands that bind the RGD pocket: It lacks the carboxylic acid group that is believed to coordinate with calcium in the MIDAS motif of the receptor. With an IC50 of 14 microM for the most potent compound, these linear p(f/y)l peptides had affinities similar to those of linear peptides containing RGD, and reversed sequences failed to compete with binding up to 1 mM. As the new motif was so different, molecular modeling was employed to suggest a model for molecular recognition. A reversed binding mechanism common for d-amino acid mimics of natural l-amino acid peptides offers an attractive hypothesis that suggests three points of contact similar to those made by the RGD-mimicking monoclonal antibody, OPG2. Interestingly, the model proposes that pi-electrons in the new motif may substitute for the carboxylate group present in all other RGD-types of ligands. Although modeling linear peptides is subjective, the pi-bonding model provides intriguing possibilities for medicinal chemistry after appropriate confirmatory studies.

Structural motifs in rheumatoid T-cell receptors

The linkage of rheumatoid arthritis (RA) to HLA-DR haplotypes, high levels of HLA-DR expression, and T-cell infiltration in the joints, indicate a central role for the interaction of T-cell receptors (TCR) with antigen (Ag) + major histocompatibility complex (MHC) complexes in pathogenesis. Receptor analysis in RA has uncovered a restricted heterogeneity of TCR transcripts, suggesting an antigen-driven response. We analyzed the sequence and structural features of RA-associated TCRs in light of the recently published TCR crystal structures. The surface-exposed residues of the third complementarity-determining region (CDR3s) showed preferential use of certain amino acid residues when sequences derived from synovial fluid or tissue were compared with those derived from peripheral blood, particularly for alpha chains. Sequence alignment of oligoclonal synovial TCR CDR3s revealed groupings with similar CDR3 lengths and amino acid compositions, which suggests shared antigen recognition. Given the limitations of analyzing TCR sequences without knowing their structures, we developed several in vivo-activated synovial-tissue Vbeta17 + RA T-cell clones. Two Vbeta17/V alpha7 clones with different CDR3 sequences were analyzed by molecular modeling. Although distinct topologic features were seen, a central patch of residues with similar chemical and geometric characteristics was present in both. Electrostatic maps revealed similar binding surfaces of both alpha domains and central patches, with differences in the beta domains. This suggests that an alpha-domain-focused binding trajectory would allow shared antigen recognition by these TCRs. These studies support recognition of a limited diversity of Ag + MHC complexes by synovial RA TCRs.

Two novel probes reveal tubular and vascular Arg-Gly-Asp (RGD) binding sites in the ischemic rat kidney

We have previously demonstrated that RGD peptides prevent tubular obstruction in ischemic acute renal failure (ARF) suggested that exposed unoccupied integrin receptors represent the target for such therapy. The present study investigated the topography of RGD binding sites and integrin receptors in ischemic rat kidneys. Two RGD peptides were synthesized: a cyclic biotinylated (Bt) RGD peptide and a linear RGD peptide (GRGDSP) labeled with rhodamine green (RhoG). Rats were subjected to 45 minutes of renal artery occlusion kidneys were harvested at different times post-ischemia, and stained with RGD peptides and a panel of antibodies to integrins. In control, Bt-RGD staining was undetectable in alkaline phosphatase histochemistry, whereas immunofluorescence detection with Rho-streptavidin conjugate as well as RhoG-GRGDSP staining faintly decorated the basolateral aspect of the proximal tubular cells in a punctate fashion. In contrast, ischemic kidneys showed binding to the basolateral and apical aspects of proximal tubules, peritubular capillaries, and desquamated cells within tubular lumen. The most conspicuous staining of ischemic kidneys was obtained with antibodies to the beta 1 (labeling of the apical aspect of proximal and distal tubules, as well as desquamated cells obstructing tubular lumen) and the alpha V (glomeruli, tubular epithelia, intima of blood vessels stained faintly, while the obstructing cellular conglomerates showed intense staining) subunits. Double staining with Bt-RGD and antibodies against the beta 1 and alpha V beta 3 integrins showed co-localization of staining within the tubules and vasculature, respectively. In vitro attachment of HL-60 leukocytes to the endothelial cells was inhibited by the cyclic RGD peptide. In conclusion, expression of RGD binding sites and beta 1 integrin subunits along the apical aspect of tubular epithelia and on the surface of desquamated cells is in concert with the hypothesis on the pathogenetic role of RGD-recognizing integrins in tubular obstruction. The expression of RGD binding sites along the intimal surface of blood vessels in ischemic kidneys suggests an additional target for RGD peptides in vascular endothelial cells.

Molecular determinants of arg-gly-asp ligand specificity for beta3 integrins

The Arg-Tyr-Asp (RYD) and Arg-Gly-Asp (RGD) sequences within the third complementarity-determining region of the heavy chain (H3) of murine recombinant Fab molecules OPG2 and AP7, respectively, are responsible for their specific binding to the platelet integrin alphaIIbbeta3. In this study, we evaluated the influence of divalent cation composition and single amino acid substitutions at key positions within H3 on the selectivity of these Fab molecules for integrin alphaIIbbeta3 versus the vitronectin receptor alphaVbeta3. The parent Fab molecule OPG2 (H3 sequence, HPFYRYDGGN) binds selectively to alphaIIbbeta3 and not at all to any other RGD-cognitive integrin, particularly alphaVbeta3, under any divalent cation conditions. The binding of the AP7 Fab molecule (HPFYRGDGGN) to alphaIIbbeta3 is not affected by the relative composition of calcium, magnesium or manganese. However, AP7 binding to alphaVbeta3, either expressed by M21 cells or as the purified integrin, is supported by manganese and inhibited by calcium. If the flanking asparagine 108 residue within the AP7 H3 loop is replaced by alanine (HPFYRGDGGA), the resulting Fab molecule AP7.4 binds selectively to alphaVbeta3 in a cation-dependent manner, but does not bind at all to alphaIIbbeta3 under any conditions. AP7.4 binding to alphaVbeta3 is supported by manganese, completely inhibited by calcium, and largely unaffected by magnesium. This behavior mimics that of the adhesive protein, osteopontin, another ligand that binds preferentially to alphaVbeta3. Despite these differences in specificity for alphaIIbbeta3 and alphaVbeta3, AP7 and AP7.4 remain selective for the beta3 integrins and do not bind to cell lines that express the RGD-cognitive integrins alphaVbeta5 or alpha5beta1. These results confirm that subtle changes in the amino acid composition immediately flanking the RGD or RYD motifs can have a profound effect on beta3 integrin specificity, most likely because they influence the juxtaposition of the arginine and aspartate side chains within the extended RGD loop sequence.

Crystal structure of NMC-4 fab anti-von Willebrand factor A1 domain

We have solved the crystal structure of the Fab fragment of NMC-4, a mouse monoclonal antibody that binds to the A1 domain of von Willebrand factor (vWF). Two Asp and three Tyr residues in the complementarity determining regions 1 and 3 of the heavy chain exhibited a spatial orientation suggestive of a dominant role in establishing contact with the antigen. A cluster of Asp and Tyr residues occurs also in a region of the platelet glycoprotein (GP) Ib alpha amino terminal domain known to be critically involved in vWF binding. Thus, the structural information obtained with NMC-4 may prove relevant to understand the stereochemical bases of the GP Ib alpha-vWF interaction essential for thrombus formation at sites of vascular lesion.

Molecular requirements for assembly and function of a minimized human integrin alphaIIbbeta3

Integrin subunit compatibility within and between species plays a major role in heterodimer assembly and ligand specificity. As an example, human alphaIIb pairs only with human beta3 and does not assemble a heterodimer with beta3 from other species. We use interspecies subunit chimeras to identify molecular requirements for subunit compatibility and show that species-restricted heterodimer assembly depends on a unique hexapeptide VGSDNH in an extended loop of the hypothetical human beta3 MIDAS domain. This allows us to express alphaIIb(1-233) and beta3(111-318) as a soluble, mini-integrin that retains RGD-dependent ligand recognition. Thus, in the case of one integrin, alphaIIbbeta3, the molecular requirements for integrin subunit compatibility and ligand recognition are intimately related.

Critical residues of integrin alphaIIb subunit for binding of alphaIIbbeta3 (glycoprotein IIb-IIIa) to fibrinogen and ligand-mimetic antibodies (PAC-1, OP-G2, and LJ-CP3)

Integrin alphaIIbbeta3 plays a critical role in platelet aggregation through its interaction with fibrinogen. Elucidation of the mechanisms of alphaIIbbeta3-fibrinogen interaction is critical to understanding hemostasis and thrombosis. Here we report that mutations of Gly-184, Tyr-189, Tyr-190, Phe-191, and Gly-193 within the predicted turn structure of the third amino-terminal repeat of alphaIIb significantly block binding of alphaIIbbeta3 to soluble fibrinogen. These mutations also block binding of alphaIIbbeta3 to ligand-mimetic monoclonal antibodies PAC-1, OP-G2, LJ-CP3, which have an RGD-related RYD sequence in their antigen-binding sites. These mutations do not significantly affect the expression of alphaIIbbeta3, in contrast to most of the natural alphaIIb mutations occurring in Glanzmann's thrombasthenic patients. The data suggest that these residues are critically involved in alphaIIbbeta3-ligand interactions.

Regulation of conformation and ligand binding function of integrin alpha5beta1 by the beta1 cytoplasmic domain

We have studied the role of the cytoplasmic domain in the conformation and affinity modulation of the integrin beta1. Expression of a conformation-dependent anti-beta1 antibody 15/7 correlates with activation in wild-type beta1. Truncation of 16 carboxyl-terminal residues in the cytoplasmic domain (the 762t beta1 mutant) induces constitutive expression of the 15/7 epitope at a high level (which probably reflects a major conformational change of the extracellular domain) but does not activate ligand binding. The dissociation of epitope expression and affinity suggests that the epitope expression reflects the conformation of nonligand binding sites of the extracellular domain of beta1 but does not necessarily reflect that of the ligand binding sites. Indeed we discovered that the 15/7 epitope is in fact located in the nonligand binding region of beta1 (within residues 354-425). The 762t mutant has apparently normal alpha/beta association, suggesting that the overexpression of the 15/7 epitope is not due to alpha/beta dissociation. The data suggest that the carboxyl-terminal 16 residues of the beta1 cytoplasmic domain are critical for properly modulating conformation and affinity of beta1 integrins.

The amino-terminal one-third of alpha IIb defines the ligand recognition specificity of integrin alpha IIb beta 3

The integrin alpha subunits play a major role in the regulation of ligand binding specificity. To gain further insight into the regions of the alpha subunits that regulate ligand specificity, we have utilized alpha v / alpha IIb chimeras to identify regions of alpha IIb that when substituted for the homologous regions of alpha v switched the ligand binding phenotype of alpha v beta 3 to that of alpha IIb beta 3. We report that the ligand recognition specificity of beta 3 integrins is regulated by the amino-terminal one-third of the alpha subunit. Substitution of the amino-terminal portion of alpha v with the corresponding 334 residues of alpha IIb reconstituted reactivity with both alpha IIb beta 3-specific activation-dependent (PAC1) and -independent (OPG2) ligand mimetic antibodies in addition to small highly specific activation-independent ligands. In contrast, substitution of the amino-terminal portion alone or the divalent cation repeats alone were not sufficient to change ligand binding specificity. These data in combination with previous studies demonstrate that integrin ligand recognition requires cooperation between elements in both the alpha and beta subunits and indicate that the ligand binding pocket is a structure assembled from elements of both the alpha and beta subunits.

Biochemical and functional characterization of a new murine monoclonal antibody against human platelet glycoprotein IIIa

A new murine monoclonal antibody, MDP-1, specific for human platelet glycoprotein IIIa has been produced and characterized. Following SDS-polyacrylamide gel electrophoresis, MDP-1 reacted with a 94kDa protein immobilized on a nitrocellulose membrane. Upon reduction, MDP-1 no longer bound to the 94kDa protein indicating an epitope requiring at least one disulfide bond. On crossed immunoelectrophoresis MDP-1 reacted to the same peak as the GP IIb-IIIa complex-specific antibody AP-2. After dissociation of the GP IIb-IIIa complex with EDTA, AP-2 showed no reactivity while MDP-1 bound to a new peak that was broader and anodal to the original GP IIb-IIIa peak, consistent with GP IIIa. MDP-1 inhibited ADP and thrombin induced aggregation. In addition, MDP-1 inhibited ADP induced release of ATP, but did not inhibit thrombin stimulated ATP release. Following chymotrypsin digestion, MDP-1 bound to a cleaved GP IIIa protein (nonreduced M, = 122 kDa) consistent with opening of the major disulfide loop. A second cleavage resulted in a 63 kDa species that reacted with MDP-1. Scatchard analysis revealed 22 000 molecules of MDP-1 bound per platelet, and indicated a type of binding consistent with positive cooperativity. The antibody bound equally well to stimulated and unstimulated platelets. MDP-1 binding was inhibited by a polyclonal anti-PI(A1) antibody, but bound to platelets from a PI(A1) negative individual indicating a binding site close to but not identical to the PI(A1) epitope. In addition, MDP-1 binding was not inhibited by Arg-Gly-Asp-Ser (RGDS) suggesting that it is not directed to the RGD binding site on GP IIIa.

Defining topological similarities among ion transport proteins with anti-amiloride antibodies

The structural features of amiloride binding sites on amiloride-sensitive transport proteins have received limited characterization. An antibody that recognizes limited regions of amiloride and can mimic, in binding specificity, certain amiloride-sensitive transport proteins was used as a model to elucidate potential amino acid residue relationships that might define putative amiloride contact sites. Analysis of the structure of this antibody has allowed us to identify sequence relationships among several Na+ selective transport proteins. A structure-based relational database was employed to re-examine sequence homologies among these ion transport proteins. A search of the protein sequence databank identified representative amino acid tracts among amiloride sensitive proteins involving planar residues that might be involved in interacting with amiloride. Computer models of sites within transmembrane domains of NHE1 and NHE2 isoforms of the Na+/H+ exchanger reflective of these planar tracts indicate that amiloride probably spans two helices for interaction with the Na+/H+ exchanger. Structural analysis of this monoclonal anti-amiloride antibody appears to mimic some of the salient features of amiloride binding sites on these proteins.

Staphylococcus aureus induces platelet aggregation via a fibrinogen-dependent mechanism which is independent of principal platelet glycoprotein IIb/IIIa fibrinogen-binding domains

Platelet aggregation by bacteria is felt to play an important role in the pathogenesis of infective endocarditis. However, the mechanisms involved in bacterium-induced platelet aggregation are not well-defined. In the present study, we examined the mechanisms by which Staphylococcus aureus causes rabbit platelet aggregation in vitro. In normal plasma, the kinetics of S. aureus-induced platelet aggregation were rapid and biphasic. The onset and magnitude of aggregation phase 1 varied with the bacterium-platelet ratio, with maximal aggregation observed at a ratio of 5:1. The onset of aggregation phase 2 was delayed in the presence of apyrase (an ADP hydrolase), suggesting that this later aggregation phase may be triggered by secreted ADP. The onset of aggregation phase 2 was delayed in the presence of prostaglandin I2-treated platelets, and this phase was absent when paraformaldehyde-fixed platelets were used, implicating platelet activation in this process. Platelet aggregation phase 2 was dependent on S. aureus viability and an intact bacterial cell wall, and it was mitigated by antibody directed against staphylococcal clumping factor (a fibrinogen-binding protein) and by the cyclooxygenase inhibitor indomethacin. Similarly, aggregation phase 2 was either delayed or absent in three distinct transposon-induced S. aureus mutants with reduced capacities to bind fibrinogen in vitro. In addition, a synthetic pentadecapeptide, corresponding to the staphylococcal binding domain in the C terminus of the fibrinogen delta-chain, blocked aggregation phase 2. However, phase 2 of aggregation was not inhibited by two synthetic peptides (alone or in combination) analogous to the two principal fibrinogen-binding domains on the platelet glycoprotein (GP) IIb/IIIa integrin receptor: (i) a recognition site on the IIIa molecule for the Arg-Gly-Asp (RGD) sequence of the fibrinogen alpha-chain and (ii) a recognition site on the IIb molecule for a dodecapeptide sequence of the fibrinogen delta-chain. This differs from ADP-induced platelet aggregation, which relies on an intact platelet GP IIb/IIIa receptor with an accessible RGD sequence and dodecapeptide recognition site for fibrinogen. Furthermore, a monoclonal antibody directed against the RGD recognition site on rabbit platelet GP IIb/IIIa receptors failed to inhibit rabbit platelet aggregation by S. aureus. Collectively, these data suggest that S. aureus-induced platelet aggregation requires bacterial binding to fibrinogen but is not principally dependent upon the two major fibrinogen-binding domains on the platelet GP IIb/IIIa integrin receptor, the RGD and dodecapeptide recognition sites.

The exchange of Arg-Gly-Asp (RGD) and Arg-Tyr-Asp (RYD) binding sequences in a recombinant murine Fab fragment specific for the integrin alpha IIb beta 3 does not alter integrin recognition

The murine monoclonal antibody OPG2 is an excellent paradigm of natural RGD ligands and binds specifically to alpha IIb beta 3 integrin. A reactive Arg103-Tyr104-Asp105 (RYD) tripeptide is located in an extended loop, the third complementarity-determining region of the heavy chain (H3). When compared to other RGD ligands, the RYD tripeptide of OPG2 is unique, in that the side chains are fixed in a stable orientation that we have defined by x-ray crystallography. In this study, we express OPG2 H chain segments (Fd) and kappa chains as components of active, Fab heterodimers by coinfection of Spodoptera frugiperda cell lines with recombinant baculoviruses containing cDNA specific for each protein. Recombinant AP7 Fd segments are generated from the parent OPG2 Fd segments by replacement of Tyr104 with Gly, while recombinant AP7E Fd segments are produced from AP7 Fd segments, by exchange of Asp105 with Glu. Neither the free Fd segments nor the free kappa chains of OPG2 or AP7 can bind to alpha IIb beta 3. The AP7 Fab fragment, like the parent OPG2 Fab, binds strongly to purified alpha IIb beta 3 but weakly, if at all, to purified alpha V beta 3. The affinity of OPG2 and AP7 Fab fragments for gel-filtered platelets, whether nonstimulated or activated by 0.2 microM phorbol 12-myristate 13-acetate, is identical. As with other natural RGD ligands, the binding of recombinant OPG2 Fab or AP7 Fab fragments to purified alpha IIb beta 3 or to gel-filtered platelets is completely inhibited by the peptide RGDW or by addition of EDTA, AP7E Fab fragments do not bind at all to either purified alpha IIb beta 3 or platelets. Our results demonstrate, for the first time within a natural protein ligand, that the tripeptides RGD and RYD exhibit equivalent binding capacity and specificity for the integrin alpha IIb beta 3.

Repertoire cloning of a human IgG inhibitor of alpha IIB beta 3 function. The OG idiotype

A patient (OG) with Glanzmann thrombasthenia became refractory to platelet transfusion following the production of IgG antibodies (Ab1) specific for the integrin subunit beta 3. We generated recombinant VH and VL cDNA libraries using IgG-specific mRNA isolated from OG peripheral blood B-lymphocytes that had been selected for binding to antigen (alpha IIb beta 3 adsorbed to plastic dishes). These antigen-specific B-lymphocytes contain rearranged VH DNA segments that belong exclusively to the VH4 gene family. Recombinant Fab were expressed on the surface of filamentous phage coinfected with VH and VL segments cloned into the phagemid pHEN1 or the phage fd-tet-DOG1. To facilitate selection of the desired recombinant Ab1 Fab, we developed a rabbit polyclonal antibody specific for affinity-purified OG anti-beta 3 Fab (Ab2). Ab2 reacts specifically with Ab1, and this interaction is inhibited by purified alpha IIb beta 3. Following three rounds of phage selection on Ab2 adsorbed to plastic dishes and random reassociation of heavy and light chains, we isolated Ab1 Fab and tested their binding to alpha IIb beta 3. Five Id-positive Fab were selected for further characterization. These Fab use one of two VH genes (H21 or H23) complexed with one of three V lambda genes. Subsequent sequence data demonstrated that all three lambda genes are the same clone L22 which uses a germline V lambda gene segment. Fab using H23 bind to alpha IIb beta 3, while those using H21 do not. Based on sequence homology, both H21 and H23 use VH gene segments belonging to the VH4 gene family. Thus, the idiotype OG is restricted to the VH4 gene family and is the first sequenced prototype of human antibodies that bind close to or at a functional epitope(s) of alpha IIb beta 3.

Topography of ligand-induced binding sites, including a novel cation-sensitive epitope (AP5) at the amino terminus, of the human integrin beta 3 subunit

Changes in ligand binding ability of the integrin alpha IIb beta 3 can be monitored by the concomitant expression of ligand-inducible binding sites (LIBS). A new LIBS, the hexapeptide sequence GPNICT (residues 1-6) at the amino terminus of beta 3 recognized by the murine monoclonal antibody (mAb) AP5, is sensitive both to the binding of ligand and to micromolar differences in divalent cation levels. Calcium or magnesium can completely inhibit the binding of AP5 to alpha IIb beta 3 on platelets, with ID50 values of 80 and 1500 microM, respectively. The inhibitory effect of calcium plus magnesium is cumulative. In the presence of 1 mM calcium plus 1 mM magnesium, the peptide RGDW overcomes this inhibition and induces maximal binding of AP5. Maximal AP5 binding is also induced by a molar excess of EDTA. The unique location of the AP5 LIBS was determined by comparing the binding of LIBS-specific mAb to recombinant human-Xenopus beta 3 chimeras produced in a baculovirus expression system. AP5 defines one region at the amino terminus beta 3 1-6. A second region, defined by mAb D3GP3, is probably located within beta 3 422-490, confirming the finding of Kouns et al. (Kouns, W. C., Newman, P.J., Puckett, K. J., Miller, A. A., Wall, C. D., Fox, C. F., Seyer, J. M., and Jennings, L. K. (1991) Blood 78, 3215-3223). The third region, encompassing at most residues 490-690, and perhaps more precisely located within 602-690 (Du X., Gu, M., Weise, J. W., Nagaswami, C., Bennett, J. S., Bowditch, R., and Ginsberg, M. H. (1993) J. Biol. Chem. 268, 23087-23092), is recognized by the four mAb, anti-LIBS2, anti-LIBS3, anti-LIBS6, and P41. Since its exposure is uniquely regulated by both divalent cations and ligand, the amino terminus of beta 3 may be involved in control of ligand binding by divalent cation mobilization.

Crystal structure of the OPG2 Fab. An antireceptor antibody that mimics an RGD cell adhesion site

Cell surface receptors called integrins mediate diverse cell adhesion phenomena through recognition of the sequence arginine-glycine-aspartic acid (RGD) present in proteins such as fibronectin and fibrinogen. Platelet aggregation in hemostasis is mediated by the binding of fibrinogen to the gpIIb/IIIa integrin. The OPG2 antibody binds the gpIIb/IIIa receptor and acts as a ligand mimic due to the presence of an arginine-tyrosine-aspartic acid (RYD) sequence in the CDR3 loop of the heavy chain. The RYD loop and side chains are ordered in the 2.0-A resolution crystal structure of the Fab fragment from this antireceptor antibody. Moreover, the RYD loop assumes two clearly defined conformations that may correspond to the orientations of the loop in the free state or bound to integrin. This molecule will serve as a tool for understanding protein-integrin recognition in platelet aggregation and other RGD-mediated cell adhesion interactions.

A synthetic peptide with anti-platelet activity derived from a CDR of an anti-GPIIb-IIIa antibody

A monoclonal antibody, AC7, directed against the RGD (Arg-Gly-Asp) binding site on the GpIIIa subunit of the platelet fibrinogen receptor, interacts only with activated platelet. In order to identify the regions of AC7 that interact with the receptor, cDNA sequences of AC7 immunoglobulin heavy and light chain variable regions were determined. Among the six complementarity-determining regions (CDRs) of AC7, the CDR3 heavy chain (H3) contains homology to the RGDF sequence within fibrinogen. A synthetic peptide encompassing the H3 region (H3, RQMIRGYFDV) inhibited platelet aggregation and fibrinogen binding to platelet (IC50 = 700 microM). The inhibitory potencies of modified H3 peptides suggest that the RGYF sequence within the H3 peptide mimic the receptor recognition sequence in fibrinogen.

Isolated VH4 heavy chain variable regions bind DNA characterization of a recombinant antibody heavy chain library derived from patient(s) with active SLE

In many autoimmune diseases autoantibodies are intimately involved in disease manifestations. Molecular characterization of these autoantibodies should provide insights into the pathogenesis of these diseases, as well as suggest novel avenues for development of therapeutics. While some prior studies suggest that DNA binding may be a characteristic of individual heavy chain variable regions, the ability of these V regions to bind DNA in isolation has not been investigated. We have utilized a bacterial vector for cloning and expressing isolated antibody heavy chain variable regions. RNA was extracted from peripheral blood mononuclear cells of patients with active SLE, cDNA synthesized and heavy chain V regions amplified with VH specific oligonucleotide primers. The VH fragments were cloned into a bacterial expression plasmid including the pelB leader peptide to direct appropriate expression. Recombinant antibodies were screened for binding to 32P-labeled double-stranded plasmid DNA and later also characterized for binding to single-stranded DNA. Binding was confirmed by standard ELISA methodology. Sequence analysis of seven DNA binding VH fragments revealed that they utilized the VH gene family previously described to be associated with autoimmune responses, with a JH6 segment. On VH sequence analysis only one residue substitution in the consensus sequence is needed to form a VH4 germline gene. Potential contact residues with DNA were delineated by three-dimensional structure analysis. We concluded that the DNA binding characteristics of VH regions can be examined in the absence of light chain. DNA binding specificity appears to be a property of the germline VH4 gene. Analysis of such V regions can aid in the identification of hypervariable region contact residues important for DNA binding.

An echistatin-like Arg-Gly-Asp (RGD)-containing sequence in the heavy chain CDR3 of a murine monoclonal antibody that inhibits human platelet glycoprotein IIb/IIIa function

We describe the production and biochemical characterization of the first GPIIb/IIIa-inhibiting monoclonal antibody that contains an RGD sequence in the CDR3 region of the heavy chain. Monoclonal antibodies obtained by immunizing mice with human platelets were screened using consecutive ELISAs based on human platelets and immuno-affinity-purified glycoprotein (GP) IIb/IIIa coated on microtitre plates. Out of 30 monoclonal antibodies reacting with GPIIb/IIIa, one, MA-16N7C2, potently inhibited platelet aggregation induced by ADP, thrombin, arachidonic acid, collagen, U46619, adrenaline and platelet-activating factor, whereas ristocetin-induced aggregation was unaffected. MA-16N7C2 (IgG2a) bound approximately 4 times faster to activated than to resting platelets, with a Kdcalc of 6.6nM and of 17.5nM, respectively. Equilibrium binding studies to non-activated platelets showed a Kd of 18.2nM with 41 x 10(3) binding sites per platelet. The antibody recognized GPIIb/IIIa only as a Ca(2+)-dependent complex. MA-16N7C2 blocked fibrinogen and von Willebrand factor binding to GPIIb/IIIa in a competitive manner with a Ki of 8.5nM and 13.2nM, respectively. Sequence analysis revealed a RGD-containing sequence with homology to disintegrins, in the CDR3 region of the heavy chain. That this RGD-containing sequence could be involved in the interaction of the antibody to GPIIb/IIIa was finally indicated by showing that the binding is completely and competitively inhibited by echistatin.

Molecular and structural analysis of nuclear localizing anti-DNA lupus antibodies

To determine the structure of three nuclear localizing lupus anti-DNA immunoglobulins (Igs) and to search for clues to mechanisms of cellular and/or nuclear access, their H- and L-chain variable region sequences were determined and subjected to three-dimensional modeling. Although the results indicate heterogeneity in their primary structures, the H chains are encoded by 3 members of the J558 VH gene family with a common tertiary conformation that is not shared by a J558-encoded nonnuclear localizing anti-DNA control Ig. Furthermore, at least two of the Igs share a conformational motif in the H-chain CDR3, and all three Igs contain multiple positively charged amino acids in their CDRs, resembling nuclear localization signals that direct protein nuclear import. Notably, each VH and VK gene is also found recurrently among previously described autoantibodies. Molecular analysis further indicates that both germline-encoded and significantly mutated V genes can generate nuclear localizing anti-DNA Ig.

Structural properties of a subset of nephritogenic anti-DNA antibodies

Structural analysis of lupus autoantibodies is beginning to provide clues to the molecular basis for antigenic specificity and pathogenicity. The present analysis indicates that multiple light and heavy chains contain residues which can facilitate DNA binding, reaffirming the notion that there are multiple ways that different amino acids combine to form an antigen-binding pocket with affinity for dsDNA and ssDNA. Furthermore, this analysis suggests that these conformations and contact residues are intrinsic to germline sequences, although amino acid changes at critical locations (somatically introduced) modulate antigen binding, and appear to influence the capacity of individual immunoglobulin to form immune deposits. Analysis of additional individual immunoglobulins with closely related V-region sequences and differing pathogenic properties will be required to resolve the contribution of specific motifs to pathogenecity.