Selectin blockade prevents antigen-induced late bronchial responses and airway hyperresponsiveness in allergic sheep

Antigen challenge can elicit an allergic inflammatory response in the airways that involves eosinophils, basophils, and neutrophils and that is expressed physiologically as a late airway response (LAR) and airway hyperresponsiveness (AHR). Although previous studies have suggested that E-selectin participates in these allergic airway responses, there is little information concerning the role of L-selectin. To address this question, we examined the effects of administering an L-selectin-specific monoclonal antibody, DU1-29, as well as three small molecule selectin binding inhibitors, on the development of early airway responses (EAR), LAR and AHR in allergic sheep undergoing airway challenge with Ascaris suum antigen. Sheep treated with aerosol DU1-29 before antigen challenge had a significantly reduced LAR and did not develop postchallenge AHR. No protective effect was seen when sheep were treated with a nonspecific control monoclonal antibody. Treatment with DU1-29 also reduced the severity of the EAR to antigen. Similar results were obtained with each of the three small molecule selectin inhibitors at doses that depended on their L-, but not necessarily E-selectin inhibitory capacity. The inhibition of the EAR with one of the inhibitors, TBC-1269, was associated with a reduction in histamine release. Likewise, treatment with TBC-1269 reduced the number of neutrophils recovered in bronchoalveolar lavage (BAL) during the time of LAR and AHR. TBC-1269, given 90 min after antigen challenge also blocked the LAR and the AHR, but this protection was lost if the treatment was withheld until 4 h after challenge, a result consistent with the proposed time course of L-selectin involvement in leukocyte trafficking. These are the first data indicating that L-selectin may have a unique cellular function that modulates allergen-induced pulmonary responses.

A tissue plasminogen activator/P-selectin fusion protein is an effective thrombolytic agent

BACKGROUND

P-selectin is expressed on the surface of activated endothelial cells and platelets. We hypothesized that a tissue plasminogen activator (TPA)/P-selectin fusion protein would have not only thrombolytic activity but also might target TPA to the thrombi. In addition, it seemed possible that this chimeric protein would competitively inhibit the binding of native P-selectin on endothelial cells and platelets to leukocytes and thus further promote thrombolysis.

METHODS AND RESULTS

The full-length, plasminogen activator inhibitor-1-resistant form of TPA (TPAIR) together with two TPAIR/P-selectin fusion constructs (P280IR and P121IR) were expressed with the use of baculovirus vectors. After infection of Sf21 cells with the recombinant baculovirus, recombinant TPAIR and P-selectin/TPAIR fusion proteins were purified with the use of metal ion chromatography. The intact protease activity of TPAIR and the ligand binding capability of P-selectin were confirmed through indirect chromogenic and cell binding assays, respectively. These molecules were assessed both in vitro and in vivo for thrombolytic activity. In vitro clot lysis assays indicated equal efficacy of TPAIR, P280IR, and P121IR (P > .5). The in vivo efficacy was tested in a cyclic flow variation model with the use of the rat mesenteric artery. Compared with saline control treatment, reduction in cyclic flow variations was significant (P < .05) and similar (P > .5) among TPAIR, P280IR, and P121IR. No significant bleeding was noted among treated animals.

CONCLUSIONS

Chimeric proteins P280IR and P121IR have clot lysis activities that are similar to TPAIR both in vitro and in vivo. These chimeric proteins also bind to P-selectin ligand in vitro. Thus, these proteins may provide an efficient method of targeting TPA to the thrombotic region. Further experimental analysis with the use of larger animal coronary occlusion models should help determine the future value of these proteins as clinical therapeutic agents.

Single amino acid residues in the E- and P-selectin epidermal growth factor domains can determine carbohydrate binding specificity

E-selectin and P-selectin are two closely related vascular cell adhesion proteins. Each selectin has an amino-terminal C-type lectin domain that is thought to possess the carbohydrate binding site that binds the sialylated Lewisx antigen (sLex or CD15s) (Neu5Acalpha2-3Galbeta1-4(Fucalpha1-3)GlcNAc). In addition to the sLex carbohydrate, P-selectin binds sulfated proteoglycan, 3-sulfated galactosyl ceramide (sulfatide), and heparin. Both E- and P-selectin have an EGF-like (EGF) domain that is immediately adjacent to and COOH-terminal to the lectin domain. We report that mutagenic substitution of single amino acid residues in either the P- or E-selectin EGF domain can dramatically alter selectin binding to sLex, heparin, or sulfatide. Substitution of E- and P-selectin EGF domain residue Ser128 with an arginine results in E- and P-selectin proteins that have lost the requirement for alpha1-3-linked fucose and are thus able to bind to sialyllactosamine. A similar phenotype is reported for an E-selectin mutation within the lectin domain. Additionally, we have determined that conservative substitution of EGF domain residues 124 and 128 can alter E-selectin binding such that it is able to adhere to heparin or sulfatide and can reduce P-selectin adherence to these ligands. The distance between the substituted EGF domain amino acid residues and the primary carbohydrate binding site within the lectin domain and their relative positioning as determined by the three-dimensional crystal structure of the E-selectin lectin and EGF domains (Graves, B. J., Crowther, R. L., Chandran, C., Rumberger, J. B., Li, S., Huang, D.-S., Presky, D. H., Familletti, P. C., Wolitzky, B. A., and Burns, D. K. (1994) Nature 367, 532-538) suggest that there is little direct contact between the two domains. However, we report mutant binding characteristics which indicate that selectin oligosaccharide binding may be modulated by both domains and that wild-type E- and P-selectin/sLex binding interactions may be significantly different from those previously hypothesized.

Structure-function analysis of P-selectin-sialyl LewisX binding interactions. Mutagenic alteration of ligand binding specificity

P-selectin is a vascular cell adhesion molecule that is expressed on the surface of platelets and endothelial cells in response to inflammatory stimuli. It is believed to aid in the binding and recruitment of leukocytes to inflamed tissue. P-selectin adhesion to leukocytes is mediated by the amino-terminal lectin domain that binds the sialyl LewisX (sLeX) carbohydrate (Neu5Acalpha2-3Galbeta1-4(Fucalpha1-3)GlcNAc). Neither the three-dimensional structure of P-selectin nor the protein-carbohydrate interactions that mediate the binding of P-selectin to the sLeX carbohydrate have been determined. The most closely related protein for which a ligand-bound three-dimensional structure has been resolved is the rat mannose-binding protein (Weis, W. I., Drickamer, K., and Hendrickson, W. A. (1992) Nature 360, 127-134). Using the known binding interactions that occur between the rat mannose-binding protein and its ligand (oligomannose) as a template, we have used site-directed mutagenesis to substitute Ala-77 with lysine. This substitution changed P-selectin-carbohydrate binding specificity from sLeX to oligomannose. Further substitution altered the binding preference from mannose to galactose in a predictable manner. These results indicate that P-selectin binds sLeX in a shallow cleft that is similar to the mannose-binding protein saccharide-binding cleft. Additionally, we present an extensive mutagenic analysis of P-selectin Lys-113, a residue that has previously been implicated in P-selectin binding to both sLeX and 3-sulfated galactosylceramide (sulfatide). Our analysis demonstrates that Lys-113 is probably not involved in P-selectin binding to either sulfatide or sLeX. Functionally, it appears that P-selectin has retained a conserved carbohydrate and calcium coordination site that enables it to bind carbohydrate in a manner that is quite similar to that which has been determined for the rat mannose-binding protein.

Rational design and synthesis of small molecule, non-oligosaccharide selectin inhibitors: (alpha-D-mannopyranosyloxy)biphenyl-substituted carboxylic acids

The calcium dependent E-selectin/sialyl Lewisx (sLex) interaction plays a key role in inflammation where it mediates the rolling of leukocytes prior to firm adhesion and extravasation from the vasculature. A model of E-selectin/sLex binding, along with previously reported structure-activity relationships of sLex-related oligosaccharide, was used in the rational design of non-oligosaccharide inhibitors of this pivotal interaction. A palladium-mediated biaryl-coupling (Suzuki) reaction was used as the key step to prepare a number of substituted biphenyls which were assayed for their ability to inhibit the binding of E-, P-, and L-selectin-IgG fusion proteins to sLex expressed on the surface of HL60 cells. Some of the compounds developed had greater in vitro potency than the parent sLex tetrasaccharide and are currently being evaluated in in vivo models of inflammation to select a candidate for clinical development.

A single amino acid residue can determine the ligand specificity of E-selectin

E-selectin (ELAM-1) is a member of the selectin family of cellular adhesion molecules. This family of proteins possesses an amino-terminal Ca(2+)-dependent lectin or carbohydrate recognition domain that is essential for ligand binding. A known E-selectin ligand is the carbohydrate antigen, sialyl Lewis(x) (sLe(x) (Neu5Ac alpha 2-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc). We have developed a model of E-selectin binding to the sLe(x) tetrasaccharide, (Neu5Ac alpha 2-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc), using the NMR-determined, E-selectin-bound solution conformation of sLe(x) (Cooke, R.M., Hale, R.S., Lister, S. G., Shah, G., and Weir, M. P. (1994) Biochemistry 33, 10591-10596) together with the x-ray crystallographic structures of E-selectin (Graves, B. J., Crowther, R. L., Chandran, C., Rumberger, J. B., Li, s., Huang, K.-S., Presky, D. H., Familletti, P. C., Wolitzky, B. A., and Burns, D. K. (1994) Nature 367, 532-538) (ligand unbound) and a related C-type animal lectin, the mannose-binding protein (Weis, W. I., Drickamer, K., and Hendrickson, A. (1992) Nature 360, 127-134) (ligand bound). Analysis of this model indicated that the alteration of one E-selectin amino acid, alanine 77, to a lysine residue might shift binding specificity from sLe(x) to mannose. The results presented here show that an E-selectin mutant protein possessing this change displays preferential binding to mannose containing oligosaccharides and that further mutagenesis of this mannose-binding selectin confers galactose recognition in a predictable manner. These mutagenesis data support the presented model of the detailed interactions between E-selectin and the sLe(x) oligosaccharide.