Proteolysis-Targeting Chimeras Enhance T Cell Bispecific Antibody-Driven T Cell Activation and Effector Function through Increased MHC Class I Antigen Presentation in Cancer Cells

The availability of Ags on the surface of tumor cells is crucial for the efficacy of cancer immunotherapeutic approaches using large molecules, such as T cell bispecific Abs (TCBs). Tumor Ags are processed through intracellular proteasomal protein degradation and are displayed as peptides on MHC class I (MHC I). Ag recognition through TCRs on the surface of CD8⁺ T cells can elicit a tumor-selective immune response. In this article, we show that proteolysis-targeting chimeras (PROTACs) that target bromo- and extraterminal domain proteins increase the abundance of the corresponding target-derived peptide Ags on MHC I in both liquid and solid tumor-derived human cell lines. This increase depends on the engagement of the E3 ligase to bromo- and extraterminal domain protein. Similarly, targeting of a doxycycline-inducible Wilms tumor 1 (WT1)-FKBP12^F36V fusion protein, by a mutant-selective FKBP12^F36V degrader, increases the presentation of WT1 Ags in human breast cancer cells. T cell-mediated response directed against cancer cells was tested on treatment with a TCR-like TCB, which was able to bridge human T cells to a WT1 peptide displayed on MHC I. FKBP12^F36V degrader treatment increased the expression of early and late activation markers (CD69, CD25) in T cells; the secretion of granzyme β, IFN-γ, and TNF-α; and cancer cell killing in a tumor-T cell coculture model. This study supports harnessing targeted protein degradation in tumor cells, for modulation of T cell effector function, by investigating for the first time, to our knowledge, the potential of combining a degrader and a TCB in a cancer immunotherapy setting.

The INNs and outs of antibody nonproprietary names

An important step in drug development is the assignment of an International Nonproprietary Name (INN) by the World Health Organization (WHO) that provides healthcare professionals with a unique and universally available designated name to identify each pharmaceutical substance. Monoclonal antibody INNs comprise a –mab suffix preceded by a substem indicating the antibody type, e.g., chimeric (-xi-), humanized (-zu-), or human (-u-). The WHO publishes INN definitions that specify how new monoclonal antibody therapeutics are categorized and adapts the definitions to new technologies. However, rapid progress in antibody technologies has blurred the boundaries between existing antibody categories and created a burgeoning array of new antibody formats. Thus, revising the INN system for antibodies is akin to aiming for a rapidly moving target. The WHO recently revised INN definitions for antibodies now to be based on amino acid sequence identity. These new definitions, however, are critically flawed as they are ambiguous and go against decades of scientific literature. A key concern is the imposition of an arbitrary threshold for identity against human germline antibody variable region sequences. This leads to inconsistent classification of somatically mutated human antibodies, humanized antibodies as well as antibodies derived from semi-synthetic/synthetic libraries and transgenic animals. Such sequence-based classification implies clear functional distinction between categories (e.g., immunogenicity). However, there is no scientific evidence to support this. Dialog between the WHO INN Expert Group and key stakeholders is needed to develop a new INN system for antibodies and to avoid confusion and miscommunication between researchers and clinicians prescribing antibodies.

BI 6727, a Polo-like kinase inhibitor with improved pharmacokinetic profile and broad antitumor activity

PURPOSE

Antimitotic chemotherapy remains a cornerstone of multimodality treatment for locally advanced and metastatic cancers. To identify novel mitosis-specific agents with higher selectivity than approved tubulin-binding agents (taxanes, Vinca alkaloids), we have generated inhibitors of Polo-like kinase 1, a target that functions predominantly in mitosis.

EXPERIMENTAL DESIGN

The first compound in this series, suitable for i.v. administration, has entered clinical development. To fully explore the potential of Polo-like kinase 1 inhibition in oncology, we have profiled additional compounds and now describe a novel clinical candidate.

RESULTS

BI 6727 is a highly potent (enzyme IC(50) = 0.87 nmol/L, EC(50) = 11-37 nmol/L on a panel of cancer cell lines) and selective dihydropteridinone with distinct properties. First, BI 6727 has a pharmacokinetic profile favoring sustained exposure of tumor tissues with a high volume of distribution and a long terminal half-life in mice (V(ss) = 7.6 L/kg, t(1/2) = 46 h) and rats (V(ss) = 22 L/kg, t(1/2) = 54 h). Second, BI 6727 has physicochemical and pharmacokinetic properties that allow in vivo testing of i.v. as well as oral formulations, adding flexibility to dosing schedules. Finally, BI 6727 shows marked antitumor activity in multiple cancer models, including a model of taxane-resistant colorectal cancer. With oral and i.v. routes of administration, the total weekly dose of BI 6727 is most relevant for efficacy, supporting the use of a variety of well-tolerated dosing schedules.

CONCLUSION

These findings warrant further investigation of BI 6727 as a tailored antimitotic agent; clinical studies have been initiated.

The small-molecule inhibitor BI 2536 reveals novel insights into mitotic roles of polo-like kinase 1

BACKGROUND

The mitotic kinases, Cdk1, Aurora A/B, and Polo-like kinase 1 (Plk1) have been characterized extensively to further understanding of mitotic mechanisms and as potential targets for cancer therapy. Cdk1 and Aurora kinase studies have been facilitated by small-molecule inhibitors, but few if any potent Plk1 inhibitors have been identified.

RESULTS

We describe the cellular effects of a novel compound, BI 2536, a potent and selective inhibitor of Plk1. The fact that BI 2536 blocks Plk1 activity fully and instantaneously enabled us to study controversial and unknown functions of Plk1. Cells treated with BI 2536 are delayed in prophase but eventually import Cdk1-cyclin B into the nucleus, enter prometaphase, and degrade cyclin A, although BI 2536 prevents degradation of the APC/C inhibitor Emi1. BI 2536-treated cells lack prophase microtubule asters and thus polymerize mitotic microtubules only after nuclear-envelope breakdown and form monopolar spindles that do not stably attach to kinetochores. Mad2 accumulates at kinetochores, and cells arrest with an activated spindle-assembly checkpoint. BI 2536 prevents Plk1's enrichment at kinetochores and centrosomes, and when added to metaphase cells, it induces detachment of microtubules from kinetochores and leads to spindle collapse.

CONCLUSIONS

Our results suggest that Plk1's accumulation at centrosomes and kinetochores depends on its own activity and that this activity is required for maintaining centrosome and kinetochore function. Our data also show that Plk1 is not required for prophase entry, but delays transition to prometaphase, and that Emi1 destruction in prometaphase is not essential for APC/C-mediated cyclin A degradation.

BI 2536, a potent and selective inhibitor of polo-like kinase 1, inhibits tumor growth in vivo

Fine-mapping of the cell-division cycle, notably the identification of mitotic kinase signaling pathways, provides novel opportunities for cancer-drug discovery. As a key regulator of multiple steps during mitotic progression across eukaryotic species, the serine/threonine-specific Polo-like kinase 1 (Plk1) is highly expressed in malignant cells and serves as a negative prognostic marker in specific human cancer types . Here, we report the discovery of a potent small-molecule inhibitor of mammalian Plk1, BI 2536, which inhibits Plk1 enzyme activity at low nanomolar concentrations. The compound potently causes a mitotic arrest and induces apoptosis in human cancer cell lines of diverse tissue origin and oncogenome signature. BI 2536 inhibits growth of human tumor xenografts in nude mice and induces regression of large tumors with well-tolerated intravenous dose regimens. In treated tumors, cells arrest in prometaphase, accumulate phosphohistone H3, and contain aberrant mitotic spindles. This mitotic arrest is followed by a surge in apoptosis, detectable by immunohistochemistry and noninvasive optical and magnetic resonance imaging. For addressing the therapeutic potential of Plk1 inhibition, BI 2536 has progressed into clinical studies in patients with locally advanced or metastatic cancers.

BI-D1870 is a specific inhibitor of the p90 RSK (ribosomal S6 kinase) isoforms in vitro and in vivo

Hormones and growth factors induce the activation of a number of protein kinases that belong to the AGC subfamily, including isoforms of PKA, protein kinase B (also known as Akt), PKC, S6K p70 (ribosomal S6 kinase), RSK (p90 ribosomal S6 kinase) and MSK (mitogen- and stress-activated protein kinase), which then mediate many of the physiological processes that are regulated by these extracellular agonists. It can be difficult to assess the individual functions of each AGC kinase because their substrate specificities are similar. Here we describe the small molecule BI-D1870, which inhibits RSK1, RSK2, RSK3 and RSK4 in vitro with an IC(50) of 10-30 nM, but does not signi-ficantly inhibit ten other AGC kinase members and over 40 other protein kinases tested at 100-fold higher concentrations. BI-D1870 is cell permeant and prevents the RSK-mediated phorbol ester- and EGF (epidermal growth factor)-induced phosphoryl-ation of glycogen synthase kinase-3beta and LKB1 in human embry-onic kidney 293 cells and Rat-2 cells. In contrast, BI-D1870 does not affect the agonist-triggered phosphorylation of substrates for six other AGC kinases. Moreover, BI-D1870 does not suppress the phorbol ester- or EGF-induced phosphorylation of CREB (cAMP-response-element-binding protein), consistent with the genetic evidence indicating that MSK, and not RSK, isoforms mediate the mitogen-induced phosphorylation of this transcription factor.

A phase I repeated dose escalation study of the Polo-like kinase 1 inhibitor BI 2536 in patients with advanced solid tumours

2038

Background: BI 2536 is a novel highly potent and selective inhibitor of the serine-threonine kinase polo-like kinase 1 (Plk1), which is a key regulator of cell cycle progression. Objectives of this trial were the assessment of the maximum tolerated dose (MTD), overall safety, pharmacokinetics and efficacy of BI 2536 given intravenously. Methods: Sequential cohorts of 3 to 6 patients (pts) with pretreated advanced or metastatic solid tumours received intravenous infusions of BI 2536 on days 1 and 8 of a 3-week treatment course following a toxicity guided dose escalation design. Further treatment courses were administered to pts in the absence of disease progression and if toxicity after a 3-week treatment course had resolved. Results: A total of 42 pts was treated at doses of 25 mg (n=3), 50 mg (n=3), 100 mg (n=22), 125 mg (n=5), 150 mg (n=6) and 200 mg (n=3). Reversible CTCAE grade ≥ 3 neutropenia in 14/42 pts represented the main drug related toxicity with an incidence of 3/5 in the 125 mg cohort, 4/6 in the 150 mg and 2/2 in the 200 mg dose cohorts. Dose limiting toxicity (DLT) was defined as drug related toxicity prohibiting administration of the day 8 dose of BI 2536 (hematologic: CTCAE ≥ 3 grade, non-hematologic toxicity: CTCAE ≥ 2). No DLT other than d8 neutropenia was observed. The MTD was defined at 100 mg for the given day 1 and 8 schedule. Further related adverse events (AE’s) were of mild to moderate intensity (CTCAE grade ≤ 2). There were no related AEs resulting in study discontinuation. Preliminary PK analysis showed dose proportionality of Cmax and AUC0-∞ with a high clearance (∼ 1500 mL/min) and a high volume of distribution (∼ 2000 L). No accumulation from d1 to d8 occurred. Patients were treated for up to 8 courses without evidence of accumulating toxicity. No objective responses were observed according to RECIST criteria in this heavily pretreated patient population. Conclusions: In summary BI 2536 is a Plk1 inhibitor with a favorable PK and safety profile at the tested dose and schedule. Neutropenia as a mechanism-related toxicity indicates target inhibition in vivo.

[Table: see text]

A phase I study of two administration schedules of the Polo-like kinase 1 inhibitor BI 2536 in patients with advanced solid tumors

3069

Background: BI 2536 is a novel highly potent and selective inhibitor of the serine-threonine Polo-like kinase 1 (Plk1), a key regulator of cell cycle progression. Objectives of this trial were the assessment of the maximum tolerated dose (MTD), overall safety, pharmacokinetics and preliminary efficacy of BI 2536 given intravenously. Methods: Sequential cohorts of 3–6 patients (pat) with advanced or metastatic solid tumors received infusions of BI 2536 following a toxicity guided dose escalation design. In the first part of the trial, a single administration was given every 21 days (d1). After completion of accrual BI 2536 was administered on three consecutive days (d1–3) in a dose intensified schedule in additional patient cohorts. Further treatment courses were given in the absence of disease progression and after recovery of toxicity after a 3-week observation period. Results: Of the 52 entered patients a total of 40 pts were treated at doses of 25 (n=3), 50 (n=3), 100 (n=3), 200 (n=25) and 250 mg (n=6) in the d1 schedule. The MTD for the d1 schedule was defined at 200 mg. Dose limiting toxicity (DLT) consisted of neutropenic infection and occurred in 2/6 patients at 250 mg. Reversible CTCAE grade ≥ 3 neutropenia represented the main drug related toxicity with an incidence of 13/25 in the 200 mg and 5/6 in the 250 mg dose cohorts. Alopecia was reported in 20% of pat. Further common drug related adverse events (AE’s) consisted of nausea (38%), anorexia (25%) fatigue (18%), vomiting (18%) and mucositis (12%) and were mostly of mild to moderate intensity (CTCAE grade ≤ 2). PK analysis showed dose proportionality of Cmax and AUC0-∞ with a high clearance (∼ 1400 mL/min) and a high volume of distribution (∼ 1200 L). A correlation between exposure and neutropenia was observed. Patients were treated for up to 10 courses without evidence of accumulating toxicity. One partial response was observed in a patient with metastatic squamous cell head and neck cancer treated at 250 mg. The DLT was determined at 3x70 mg in the dose intensified schedule with the MTD still pending. Conclusion: In summary BI 2536 is a Plk1 inhibitor with a favorable safety and PK profile. Neutropenia as a mechanism-related toxicity indicates target inhibition in vivo. First signs of antitumor activity were observed.

[Table: see text]

Syntaxin 17 is abundant in steroidogenic cells and implicated in smooth endoplasmic reticulum membrane dynamics

The endoplasmic reticulum (ER) consists of subcompartments that have distinct protein constituents, morphological appearances, and functions. To understand the mechanisms that regulate the intricate and dynamic organization of the endoplasmic reticulum, it is important to identify and characterize the molecular machinery involved in the assembly and maintenance of the different subcompartments. Here we report that syntaxin 17 is abundantly expressed in steroidogenic cell types and specifically localizes to smooth membranes of the ER. By immunoprecipitation analyses, syntaxin 17 exists in complexes with a syntaxin regulatory protein, rsly1, and/or two intermediate compartment SNARE proteins, rsec22b and rbet1. Furthermore, we found that syntaxin 17 is anchored to the smooth endoplasmic reticulum through an unusual mechanism, requiring two adjacent hydrophobic domains near its carboxyl terminus. Converging lines of evidence indicate that syntaxin 17 functions in a vesicle-trafficking step to the smooth-surfaced tubular ER membranes that are abundant in steroidogenic cells.

SNARE protein trafficking in polarized MDCK cells

A key feature of polarized epithelial cells is the ability to maintain the specific biochemical composition of the apical and basolateral plasma membrane domains. This polarity is generated and maintained by the continuous sorting of apical and basolateral components in the secretory and endocytic pathways. Soluble N-ethyl maleimide-sensitive factor attachment protein receptors (SNARE) proteins of vesicle-associated membrane protein (VAMP) and syntaxin families have been suggested to play a role in the biosynthetic transport to the apical and basolateral plasma membranes of polarized cells, where they likely mediate membrane fusion. To investigate the involvement of SNARE proteins in membrane trafficking to the apical and basolateral plasma membrane in the endocytic pathway we have monitored the recycling of various VAMP and syntaxin molecules between intracellular compartments and the two plasma membrane domains in Madin-Darby canine kidney (MDCK) cells. Here we show that VAMP8/endobrevin cycles through the apical but not through the basolateral plasma membrane. Furthermore, we found that VAMP8 localizes to apical endosomal membranes in nephric tubule epithelium and in MDCK cells. This asymmetry in localization and cycling behavior suggests that VAMP8/endobrevin may play a role in apical endosomal trafficking in polarized epithelium cells.

nSec1 binds a closed conformation of syntaxin1A

The Sec1 family of proteins is proposed to function in vesicle trafficking by forming complexes with target membrane SNAREs (soluble N-ethylmaleimide-sensitive factor [NSF] attachment protein [SNAP] receptors) of the syntaxin family. Here, we demonstrate, by using in vitro binding assays, nondenaturing gel electrophoresis, and specific neurotoxin treatment, that the interaction of syntaxin1A with the core SNARE components, SNAP-25 (synaptosome-associated protein of 25 kD) and VAMP2 (vesicle-associated membrane protein 2), precludes the interaction with nSec1 (also called Munc18 and rbSec1). Inversely, association of nSec1 and syntaxin1A prevents assembly of the ternary SNARE complex. Furthermore, using chemical cross-linking of rat brain membranes, we identified nSec1 complexes containing syntaxin1A, but not SNAP-25 or VAMP2. These results support the hypothesis that Sec1 proteins function as syntaxin chaperons during vesicle docking, priming, and membrane fusion.

Vesicle-associated membrane protein 4 is implicated in trans-Golgi network vesicle trafficking

The trans-Golgi network (TGN) plays a pivotal role in directing proteins in the secretory pathway to the appropriate cellular destination. VAMP4, a recently discovered member of the vesicle-associated membrane protein (VAMP) family of trafficking proteins, has been suggested to play a role in mediating TGN trafficking. To better understand the function of VAMP4, we examined its precise subcellular distribution. Indirect immunofluorescence and electron microscopy revealed that the majority of VAMP4 localized to tubular and vesicular membranes of the TGN, which were in part coated with clathrin. In these compartments, VAMP4 was found to colocalize with the putative TGN-trafficking protein syntaxin 6. Additional labeling was also present on clathrin-coated and noncoated vesicles, on endosomes and the medial and trans side of the Golgi complex, as well as on immature secretory granules in PC12 cells. Immunoprecipitation of VAMP4 from rat brain detergent extracts revealed that VAMP4 exists in a complex containing syntaxin 6. Converging lines of evidence implicate a role for VAMP4 in TGN-to-endosome transport.

SNARE interactions are not selective. Implications for membrane fusion specificity

The SNARE hypothesis proposes that membrane trafficking specificity is mediated by preferential high affinity interactions between particular v (vesicle membrane)- and t (target membrane)-SNARE combinations. The specificity of interactions among a diverse set of SNAREs, however, is unknown. We have tested the SNARE hypothesis by analyzing potential SNARE complexes between five proteins of the vesicle-associated membrane protein (VAMP) family, three members of the synaptosome-associated protein-25 (SNAP-25) family and three members of the syntaxin family. All of the 21 combinations of SNAREs tested formed stable complexes. Sixteen were resistant to SDS denaturation, and most complexes thermally denatured between 70 and 90 degreesC. These results suggest that the specificity of membrane fusion is not encoded by the interactions between SNAREs.

Three novel proteins of the syntaxin/SNAP-25 family

Intracellular membrane traffic is thought to be regulated in part by soluble N-ethylmaleimide-sensitive factor-attachment protein receptors (SNAREs) through the formation of complexes between these proteins present on vesicle and target membranes. All known SNARE-mediated fusion events involve members of the syntaxin and vesicle-associated membrane protein families. The diversity of mammalian membrane compartments predicts the existence of a large number of different syntaxin and vesicle-associated membrane protein genes. To further investigate the spectrum of SNAREs and their roles in membrane trafficking we characterized three novel members of the syntaxin and SNAP-25 (synaptosome-associated protein of 25 kDa) subfamilies. The proteins are broadly expressed, suggesting a general role in vesicle trafficking, and localize to distinct membrane compartments. Syntaxin 8 co-localizes with markers of the endoplasmic reticulum. Syntaxin 17, a divergent member of the syntaxin family, partially overlaps with endoplasmic reticulum markers, and SNAP-29 is broadly localized on multiple membranes. SNAP-29 does not contain a predicted membrane anchor characteristic of other SNAREs. In vitro studies established that SNAP-29 is capable of binding to a broad range of syntaxins.

Localization, dynamics, and protein interactions reveal distinct roles for ER and Golgi SNAREs

ER-to-Golgi transport, and perhaps intraGolgi transport involves a set of interacting soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins including syntaxin 5, GOS-28, membrin, rsec22b, and rbet1. By immunoelectron microscopy we find that rsec22b and rbet1 are enriched in COPII-coated vesicles that bud from the ER and presumably fuse with nearby vesicular tubular clusters (VTCs). However, all of the SNAREs were found on both COPII- and COPI-coated membranes, indicating that similar SNARE machinery directs both vesicle pathways. rsec22b and rbet1 do not appear beyond the first Golgi cisterna, whereas syntaxin 5 and membrin penetrate deeply into the Golgi stacks. Temperature shifts reveal that membrin, rsec22b, rbet1, and syntaxin 5 are present together on membranes that rapidly recycle between peripheral and Golgi-centric locations. GOS-28, on the other hand, maintains a fixed localization in the Golgi. By immunoprecipitation analysis, syntaxin 5 exists in at least two major subcomplexes: one containing syntaxin 5 (34-kD isoform) and GOS-28, and another containing syntaxin 5 (41- and 34-kD isoforms), membrin, rsec22b, and rbet1. Both subcomplexes appear to involve direct interactions of each SNARE with syntaxin 5. Our results indicate a central role for complexes among rbet1, rsec22b, membrin, and syntaxin 5 (34 and 41 kD) at two membrane fusion interfaces: the fusion of ER-derived vesicles with VTCs, and the assembly of VTCs to form cis-Golgi elements. The 34-kD syntaxin 5 isoform, membrin, and GOS-28 may function in intraGolgi transport.

The binding of T cell-expressed P-selectin glycoprotein ligand-1 to E- and P-selectin is differentially regulated

The HECA452 carbohydrate epitope, also termed cutaneous lymphocyte antigen, is known to bind to E-selectin and defines a human T cell subset preferentially found in inflamed skin. Activated T cells can express a functional form of the P-selectin glycoprotein ligand-1 (PSGL-1), the major ligand known for P-selectin. Here we show that PSGL-1 can exist in two forms, of which only one carries the HECA452 epitope and binds to E-selectin, while the other only binds to P-selectin. We have analyzed the glycoprotein ligands for E- and P-selectin on the mouse CD8+ T cell clone 4G3 at 4, 8, and 12 days after antigen-specific activation. Only at day 4 did the cells bind to E-selectin, whereas cells at all three activation stages bound to P-selectin. Expression of the HECA452 epitope correlated with E-selectin binding. In affinity isolation experiments, PSGL-1 was isolated as the major ligand by E-selectin-IgG and by P-selectin-IgG; however, PSGL-1 only bound to E-selectin at day 4, whereas it bound to P-selectin at all three activation stages. Immunoprecipitated PSGL-1 from cells at day 4, but not from cells at days 8 and 12, was recognized in immunoblots by monoclonal antibody HECA452. In immunoblots of total extracts of cells at day 4, HECA452 recognized a 240/140-kDa pair of protein bands as the major antigen. These bands could be completely removed by depletion of cell extracts with anti-PSGL-1 antibodies. Our data suggest that the carbohydrate requirements for binding of PSGL-1 to P-selectin differ from those necessary for binding to E-selectin. Furthermore, we conclude that the major glycoprotein carrier for the HECA452 epitope on activated 4G3 cells is PSGL-1.

The P-selectin glycoprotein ligand-1 is important for recruitment of neutrophils into inflamed mouse peritoneum

The P-selectin glycoprotein ligand-1 (PSGL-1) is a high-affinity ligand of P-selectin on myeloid cells and certain subsets of lymphoid cells. We generated the rat monoclonal antibody (MoAb) 2PH1 that recognizes an epitope within the first 19 amino acids at the N-terminus of the processed form of mouse PSGL-1. This antibody blocks attachment of mouse myeloid cells to P-selectin under both static and flow conditions. Intravenous administration of saturating amounts of 2PH1 reduced the number of rolling leukocytes in venules of the acutely exposed mouse cremaster muscle by 79% (+/-5.7%), whereas an anti-P-selectin MoAb reduced it completely. Examining the effect of the MoAb 2PH1 on the recruitment of neutrophils into chemically inflamed mouse peritoneum showed that blocking PSGL-1 inhibited neutrophil accumulation in the peritoneum by 82% (+/-7%) at 2 hours and by 59% (+/-7.9%) at 4 hours after stimulation. A similar effect was seen with the MoAb against P-selectin. Simultaneous administration of both antibodies at the 4-hour time point blocked neutrophil accumulation by 86% (+/-4.2%), arguing for an additional partner molecule for PSGL-1 besides P-selectin. This is the first demonstration of the importance of PSGL-1 in the recruitment of mouse neutrophils into inflamed tissue.

P-selectin glycoprotein ligand-1 mediates rolling of mouse bone marrow-derived mast cells on P-selectin but not efficiently on E-selectin

It has been shown recently that mast cells play an essential role as a source of tumor necrosis factor-alpha production during neutrophil recruitment to sites of bacterial infection. Increased numbers of mast cells are indeed noted at sites of wound healing and inflammation. These cells are either recruited from the bone marrow or proliferate locally under cytokine stimulation. Little is known about how mast cell progenitors extravasate into tissue. Using antibody-like fusion proteins of mouse E-selectin and P-selectin, we have analyzed the ability of immature mouse bone marrow-derived mast cells (BMMC) to interact with the endothelial selectins. The P-selectin glycoprotein ligand-1 (PSGL-1) was affinity-isolated from detergent extracts of surface biotinylated BMMC with both selectin-IgG fusion proteins. However, only P-selectin-IgG, but not E-selectin-IgG showed significant interaction with intact BMMC as tested by flow cytometry and cell attachment assays with the immobilized fusion proteins under flow and non-flow conditions at physiological shear stress. Thus, in spite of carrying the necessary carbohydrate modifications which enable solubilized PSGL-1 to bind avidly to E-selectin, PSGL-1 on the surface of BMMC is presented in a way that prevents it from interacting efficiently with E-selectin. Affinity-purified rabbit antibodies against mouse PSGL-1 almost completely blocked the interaction of BMMC with P-selectin-IgG in flow cytometry as well as in cell adhesion assays under static and under flow conditions. Our data reveal that PSGL-1 is the major binding site for P-selectin on mouse BMMC progenitors, but does not support efficient interactions with E-selectin.

The E-selectin-ligand ESL-1 is located in the Golgi as well as on microvilli on the cell surface

Neutrophils and subsets of lymphocytes bind to E-selectin, a cytokine inducible adhesion molecule on endothelial cells. The E-selectin-ligand-1 (ESL-1) is a high affinity glycoprotein ligand which participates in the binding of mouse myeloid cells to E-selectin. The sequence of mouse ESL-1 is highly homologous to the cysteine rich FGF receptor (CFR) in chicken and the rat Golgi protein MG160. We have analysed the subcellular distribution of ESL-1 by indirect immunofluorescence, flow cytometry, various biochemical techniques and by immunogold scanning electron microscopy. We could localize ESL-1 in the Golgi as well as on the cell surface of 32Dc13 cells and neutrophils. Cell surface staining was confirmed by cell surface biotinylation and by cell surface immunoprecipitations in which antibodies only had access to surface proteins on intact cells. In addition, ESL-1(high) and ESL-1(low) expressing cells, sorted by flow cytometry, gave rise to high and low immunoprecipitation signals for ESL-1, respectively. Based on immunogold labeling of intact cells, we localized ESL-1 on microvilli of 32Dc13 cells and of the lymphoma cell line K46. Quantitative evaluation determined 80% of the total labeling for ESL-1 on microvilli of K46 cells while 69% of the labeling for the control antigen B220 was found on the planar cell surface. These data indicate that ESL-1 occurs at sites on the leukocyte cell surface which are destined for the initiation of cell contacts to the endothelium.

L-selectin from human, but not from mouse neutrophils binds directly to E-selectin

L-Selectin on neutrophils as well as inducible E- and P-selectin on endothelium are involved in the recruitment of neutrophils into inflamed tissue. Based on cell attachment assays, L-selectin was suggested to function as a carbohydrate presenting ligand for E- and P-selectin. However, previous affinity isolation experiments with an E-selectin-Ig fusion protein had failed to detect L-selectin among the isolated E-selectin ligands from mouse neutrophils. We show here that L-selectin from human neutrophils, in contrast to mouse neutrophils, can be affinity-isolated as a major ligand from total cell extracts using E-selectin-Ig as affinity probe. Binding of human L-selectin to E-selectin was direct, since purified L-selectin could be reprecipitated with E-selectin-Ig. Recognition of L-selectin was abolished by sialidase-treatment, required Ca2+, and was resistant to treatment with endoglycosidase F. Binding of L-selectin to a P-selectin-Ig fusion protein was not observed. In agreement with the biochemical data, the anti-L-selectin mAb DREG56 inhibited rolling of human neutrophils on immobilized E-selectin-Ig but not on P-selectin-Ig. No such inhibitory effect was seen with the anti-mouse L-selectin mAb MEL14 on mouse neutrophils. Rolling of E-selectin transfectants on purified and immobilized human L-selectin was inhibited by mAb DREG56. We conclude that L-selectin on human neutrophils is a major glycoprotein ligand among very few glycoproteins that can be isolated by an E-selectin affinity matrix. The clear difference between human and mouse L-selectin suggests that E-selectin-binding carbohydrate moieties are attached to different protein scaffolds in different species.

P-selectin glycoprotein ligand-1 (PSGL-1) on T helper 1 but not on T helper 2 cells binds to P-selectin and supports migration into inflamed skin

We have shown recently that mouse Th1 cells but not Th2 cells are selectively recruited into inflamed sites of a delayed-type hypersensitivity (DTH) reaction of the skin. This migration was blocked by monoclonal antibodies (mAb) against P- and E-selectin. Here we show that Th1 cells bind to P-selectin via the P-selectin glycoprotein ligand-1 (PSGL-1). This is the only glycoprotein ligand that was detectable by affinity isolation with a P-selectin-Ig fusion protein. Binding of Th1 cells to P-selectin, as analyzed by flow cytometry and in cell adhesion assays, was completely blocked by antibodies against PSGL-1. The same antibodies blocked partially the migration of Th1 cells into cutaneous DTH reactions. This blocking activity, in combination with that of a mAb against E-selectin, was additive. PSGL-1 on Th2 cells, although expressed at similar levels as on Th1 cells, did not support binding to P-selectin. Thus, the P-selectin-binding form of PSGL-1 distinguishes Th1 cells from Th2 cells. Furthermore, PSGL-1 is relevant for the entry of Th1 cells into inflamed areas of the skin. This is the first demonstration for the importance of PSGL-1 for mouse leukocyte recruitment in vivo.

The E-selectin-ligand ESL-1 is a variant of a receptor for fibroblast growth factor

E-SELECTIN is an inducible cell-adhesion molecule on endothelial cells, which mediates the binding of neutrophils and functions as a Ca(2+)-dependent lectin. We have recently identified a 150K glycoprotein as the major ligand for E-selectin on myeloid cells, using a recombinant antibody-like form of mouse E-selectin as an affinity probe. Here we report the isolation of a mouse complementary DNA for this E-selectin ligand (ESL-1). The predicted amino-acid sequence of ESL-1 is 94% identical (over 1,078 amino acids) to the recently identified chicken cysteine-rich fibroblast growth-factor receptor, except for a unique 70-amino-acid aminoterminal domain of mature ESL-1. Fucosylation of ESL-1 is imperative for affinity isolation with E-selectin-IgG. A fucosylated, recombinant antibody-like form of ESL-1, but not of L-selectin, supports adhesion of E-selectin-transfected Chinese hamster ovary cells. Antibodies against ESL-1 block the binding of mouse myeloid cells to E-selectin. ESL-1, with a structure essentially identical to that of a receptor, thus functions as a cell adhesion ligand of E-selectin.