Tools for anti-inflammatory drug design:In vitro models of leukocyte migration

Assaying Microglia Functions In Vitro

Microglia, the main immune modulators of the central nervous system, have key roles in both the developing and adult brain. These functions include shaping healthy neuronal networks, carrying out immune surveillance, mediating inflammatory responses, and disposing of unwanted material. A wide variety of pathological conditions present with microglia dysregulation, highlighting the importance of these cells in both normal brain function and disease. Studies into microglial function in the context of both health and disease thus have the potential to provide tremendous insight across a broad range of research areas. In vitro culture of microglia, using primary cells, cell lines, or induced pluripotent stem cell derived microglia, allows researchers to generate reproducible, robust, and quantifiable data regarding microglia function. A broad range of assays have been successfully developed and optimised for characterizing microglial morphology, mediation of inflammation, endocytosis, phagocytosis, chemotaxis and random motility, and mediation of immunometabolism. This review describes the main functions of microglia, compares existing protocols for measuring these functions in vitro, and highlights common pitfalls and future areas for development. We aim to provide a comprehensive methodological guide for researchers planning to characterise microglial functions within a range of contexts and in vitro models.

How Have Leukocyte In Vitro Chemotaxis Assays Shaped Our Ideas about Macrophage Migration?

Simple Summary

The migration of immune cells is vital during inflammatory responses. Macrophages, which are a subset of immune cells, are unique in the ways they migrate because they can switch between different mechanism of migration. This crucial feature of macrophage migration has been underappreciated in the literature because technologies used to study macrophage migration were not able to efficiently detect those subtle differences between macrophages and other immune cells. This review article describes popular technologies used to study macrophage migration and critically assesses their advantages and disadvantages in macrophage migration studies.

Abstract

Macrophage chemotaxis is crucial during both onset and resolution of inflammation and unique among all leukocytes. Macrophages are able to switch between amoeboid and mesenchymal migration to optimise their migration through 3D environments. This subtle migration phenotype has been underappreciated in the literature, with macrophages often being grouped and discussed together with other leukocytes, possibly due to the limitations of current chemotaxis assays. Transwell assays were originally designed in the 1960s but despite their long-known limitations, they are still one of the most popular methods of studying macrophage migration. This review aims to critically evaluate transwell assays, and other popular chemotaxis assays, comparing their advantages and limitations in macrophage migration studies.

High expression of CCL2 in tumor cells and abundant infiltration with CD14 positive macrophages predict early relapse in breast cancer

Macrophages are important for the function of the innate immune system, and in solid tumors, they represent a significant proportion of the tumor mass. Tumor-associated macrophages (TAM) have a M2 phenotype and show a multitude of pro-tumoral functions, promoting tumor cell survival, proliferation, and dissemination. CCL2, synthesized by tumor and stromal cells, initiates a chemokine cascade inducing these processes. We studied by immunohistochemistry (IHC) the frequency of TAMs and CCL2 expressing cells in three groups of primary tumor (PT)-recurrence (R) pairs, where relapse was recorded within 2 years (group 1), between 5 and 10 years (group 2), and after 10 years (group 3). In our study all established breast cancers were heavily infiltrated by CD68 positive cells. Both in PTs and in R lesions the infiltration was more abundant in the peritumoral than in the intratumoral stroma. The mean frequency of M2 marker and CD14 positive cells in the intratumoral stroma and CCL2 expressing tumor cells was higher in the Rs as compared to the corresponding PTs. In PTs, a high frequency of CD14 positive cells and a high expression of CCL2 by tumor cells was associated with an early recurrence. The findings support the current understanding of immune cell orchestrated development, progression and metastatic spread of breast cancer. Our study showed that a high frequency of CCL2 positive tumor cells and CD14 positive TAMs are significant risk factors for rapid tumor recurrence. Potential targets for intervention are discussed.

Rapid assay of stem cell functionality and potency using electric cell-substrate impedance sensing

Regenerative medicine studies using autologous bone marrow mononuclear cells (BM-MNCs) have shown improved clinical outcomes that correlate to in vitro BM-MNC invasive capacity. The current Boyden-chamber assay for testing invasive capacity is labor-intensive, provides only a single time point, and takes 36 hours to collect data and results, which is not practical from a clinical cell delivery perspective. To develop a rapid, sensitive and reproducible invasion assay, we employed Electric Cell-substrate Impedance Sensing (ECIS) technology. Chemokine-directed BM-MNC cell invasion across a Matrigel-coated Transwell filter was measurable within minutes using the ECIS system we developed. This ECIS-Transwell chamber system provides a rapid and sensitive test of stem and progenitor cell invasive capacity for evaluation of stem cell functionality to provide timely clinical data for selection of patients likely to realize clinical benefit in regenerative medicine treatments. This device could also supply robust unambiguous, reproducible and cost effective data as a potency assay for cell product release and regulatory strategies.

Effect of adhesion and chemokine presentation on T-lymphocyte haptokinesis

Motility is critical for the function of T-lymphocytes. Motility in T-lymphocytes is driven by the occupancy of chemokine receptors by chemokines, and modulated by adhesive interactions. However, it is not well understood how the combination of adhesion and chemokine binding affects T-lymphocyte migration. We used microcontact printing on polymeric substrates to measure how lymphocyte migration is quantitatively controlled by adhesion and chemokine ligation. Focusing only on random motion, we found that T-lymphocytes exhibit biphasic motility in response to the substrate concentration of either ICAM-1 or VCAM-1, and generally display more active motion on ICAM-1 surfaces. Furthermore, we examined how the combination of the homeostatic chemokines CCL19 and CCL21 contribute to motility. By themselves, CCL19 and CCL21, ligands for CCR7, elicit biphasic motility, but their combination synergistically increases CCR7 mediated chemokinesis on ICAM-1. By presenting CCL21 with ICAM-1 on the surface with soluble CCL19, we observed random motion that is greater than what is observed with soluble chemokines alone. These data suggest that ICAM-1 has a greater contribution to motility than VCAM-1 and that both adhesive interactions and chemokine ligation work in concert to control T-lymphocyte motility.

In situ generation of tunable porosity gradients in hydrogel-based scaffolds for microfluidic cell culture

Compared with preformed anisotropic matrices, an anisotropic matrix that allows users to alter its properties and structure in situ after synthesis offers the important advantage of being able to mimic dynamic in vivo microenvironments, such as in tissues undergoing morphogenesis or in wounds undergoing tissue repair. In this study, porous gradients are generated in situ in a hydrogel comprising enzymatically crosslinked gelatin hydroxyphenylpropionic acid (GTN-HPA) conjugate and carboxylmethyl cellulose tyramine (CMC-TYR) conjugate. The GTN-HPA component acts as the backbone of the hydrogel, while CMC-TYR acts as a biocompatible sacrificial polymer. The hydrogel is then used to immobilize HT1080 human fibrosarcoma cells in a microfluidic chamber. After diffusion of a biocompatible cellulase enzyme through the hydrogel in a spatially controlled manner, selective digestion of the CMC component of the hydrogel by the cellulase gives rise to a porosity gradient in situ instead of requiring its formation during hydrogel synthesis as with other methods. The influence of this in situ tunable porosity gradient on the chemotactic response of cancer cells is subsequently studied both in the absence and presence of chemoattractant. This platform illustrates the potential of hydrogel-based microfluidics to mimic the 3D in vivo microenvironment for tissue engineering and diagnostic applications.

Sensitive and accurate quantification of human leukocyte migration using high-content Discovery-1 imaging system and ATPlite assay

Migration is a fundamental aspect of leukocyte behavior and represents a significant therapeutic target clinically. However, most migration assays used in research are relatively low throughput and not easily compatible with rapid analysis or high-throughput screening (HTS) protocols required for drug screening assays. We therefore investigated the quantification of the migration of human leukocytes using the Molecular Devices high-content Discovery-1 platform or PerkinElmer ATPlite assay compared to manual counting. We have conducted extensive assay validation, investigating the detection limits, sensitivity, and precision of each method to count human leukocytes. Leukocyte migration assays were conducted using 96-well HTS-Transwell plates and the potent chemokine stromal cell-derived factor-1 (SDF-1). We reveal that the Discovery-1 and ATPlite methods developed here provide useful approaches to quantify leukocyte migration in an HTS manner with high levels of detection, sensitivity, and precision.

Anaesthetic impairment of immune function is mediated via GABA(A) receptors

Background

GABA_A receptors are members of the Cys-loop family of neurotransmitter receptors, proteins which are responsible for fast synaptic transmission, and are the site of action of wide range of drugs [1]. Recent work has shown that Cys-loop receptors are present on immune cells, but their physiological roles and the effects of drugs that modify their function in the innate immune system are currently unclear [2]. We are interested in how and why anaesthetics increase infections in intensive care patients; a serious problem as more than 50% of patients with severe sepsis will die [3]–[6]. As many anaesthetics act via GABA_A receptors [7], the aim of this study was to determine if these receptors are present on immune cells, and could play a role in immunocompromising patients.

Principal Findings

We demonstrate, using RT-PCR, that monocytes express GABA_A receptors constructed of α1, α4, β2, γ1 and/or δ subunits. Whole cell patch clamp electrophysiological studies show that GABA can activate these receptors, resulting in the opening of a chloride-selective channel; activation is inhibited by the GABA_A receptor antagonists bicuculline and picrotoxin, but not enhanced by the positive modulator diazepam. The anaesthetic drugs propofol and thiopental, which can act via GABA_A receptors, impaired monocyte function in classic immunological chemotaxis and phagocytosis assays, an effect reversed by bicuculline and picrotoxin.

Significance

Our results show that functional GABA_A receptors are present on monocytes with properties similar to CNS GABA_A receptors. The functional data provide a possible explanation as to why chronic propofol and thiopental administration can increase the risk of infection in critically ill patients: their action on GABA_A receptors inhibits normal monocyte behaviour. The data also suggest a potential solution: monocyte GABA_A receptors are insensitive to diazepam, thus the use of benzodiazepines as an alternative anesthetising agent may be advantageous where infection is a life threatening problem.

Anaesthetic impairment of immune function is mediated via GABA(A) receptors

BACKGROUND

GABA(A) receptors are members of the Cys-loop family of neurotransmitter receptors, proteins which are responsible for fast synaptic transmission, and are the site of action of wide range of drugs. Recent work has shown that Cys-loop receptors are present on immune cells, but their physiological roles and the effects of drugs that modify their function in the innate immune system are currently unclear. We are interested in how and why anaesthetics increase infections in intensive care patients; a serious problem as more than 50% of patients with severe sepsis will die. As many anaesthetics act via GABA(A) receptors, the aim of this study was to determine if these receptors are present on immune cells, and could play a role in immunocompromising patients.

PRINCIPAL FINDINGS

We demonstrate, using RT-PCR, that monocytes express GABA(A) receptors constructed of α1, α4, β2, γ1 and/or δ subunits. Whole cell patch clamp electrophysiological studies show that GABA can activate these receptors, resulting in the opening of a chloride-selective channel; activation is inhibited by the GABA(A) receptor antagonists bicuculline and picrotoxin, but not enhanced by the positive modulator diazepam. The anaesthetic drugs propofol and thiopental, which can act via GABA(A) receptors, impaired monocyte function in classic immunological chemotaxis and phagocytosis assays, an effect reversed by bicuculline and picrotoxin.

SIGNIFICANCE

Our results show that functional GABA(A) receptors are present on monocytes with properties similar to CNS GABA(A) receptors. The functional data provide a possible explanation as to why chronic propofol and thiopental administration can increase the risk of infection in critically ill patients: their action on GABA(A) receptors inhibits normal monocyte behaviour. The data also suggest a potential solution: monocyte GABA(A) receptors are insensitive to diazepam, thus the use of benzodiazepines as an alternative anesthetising agent may be advantageous where infection is a life threatening problem.

The serpinopathies studying serpin polymerization in vivo

The serpinopathies result from point mutations in members of the serine protease inhibitor or serpin superfamily. They are characterized by the formation of ordered polymers that are retained within the cell of synthesis. This causes disease by a "toxic gain of function" from the accumulated protein and a "loss of function" as a result of the deficiency of inhibitors that control important proteolytic cascades. The serpinopathies are exemplified by the Z (Glu342Lys) mutant of α₁-antitrypsin that results in the retention of ordered polymers within the endoplasmic reticulum of hepatocytes. These polymers form the intracellular inclusions that are associated with neonatal hepatitis, cirrhosis, and hepatocellular carcinoma. A second example results from mutations in the neurone-specific serpin-neuroserpin to form ordered polymers that are retained as inclusions within subcortical neurones as Collins' bodies. These inclusions underlie the autosomal dominant dementia familial encephalopathy with neuroserpin inclusion bodies or FENIB. There are different pathways to polymer formation in vitro but not all form polymers that are relevant in vivo. It is therefore essential that protein-based structural studies are interpreted in the context of human samples and cell and animal models of disease. We describe here the biochemical techniques, monoclonal antibodies, cell biology, animal models, and stem cell technology that are useful to characterize the serpin polymers that form in vivo.

Large-scale investigation of the olfactory receptor space using a microfluidic microwell array

The mammalian olfactory system is able to discriminate among tens of thousands of odorant molecules. In mice, each odorant is sensed by a small subset of the approximately 1000 odorant receptor (OR) types, with one OR gene expressed by each olfactory sensory neuron (OSN). However, the sum of the large repertoire of OR-OSN types and difficulties with heterologous expression have made it almost impossible to analyze odorant-responsiveness across all OR-OSN types. We have developed a microfluidic approach that allowed us to screen over 20,000 single cells at once in microwells. By using calcium imaging, we were able to detect and analyze odorant responses of about 2900 OSNs simultaneously. Importantly, this technique allows for both the detection of rare responding OSNs as well as the identification of OSN populations broadly responsive to odorants of unrelated structures. This technique is generally applicable for screening large numbers of single cells and should help to characterize rare cell behaviors in fields such as toxicology, pharmacology, and cancer research.

Distinct locomotive patterns of granulocytes, monocytes and lymphocytes in a stable concentration gradient of chemokines

The pattern of leukocyte locomotion can be changed in many pathological situations, but its accurate analysis is difficult because of technological limitation. In the present study, by using a newly developed time-lapse videomicroscopic technique, we have analyzed the locomotive patterns of leukocytes in a stable concentration gradient of chemokines. Granulocytes, monocytes, and lymphocytes were purified from adult human peripheral blood. Locomotive behavior of the leukocytes was analyzed by an optical assay using a microchannel producing a stable concentration gradient of chemokines. Videomicroscopic analysis showed distinct locomotive patterns of granulocytes, monocytes, and lymphocytes. Granulocytes were intrinsically motile, vigorously moving in random direction without any chemokine stimulation. Upon stimulation with CXCL8/IL-8, the speed of migration was increased from 13.3 +/- 2.8 to 19.4 +/- 2.5 microm/min (P = 0.002, n = 100) and they moved toward the chemokine, although many cells still wandered very much. Stimulation with CCL5/RANTES or CXCL12/SDF-1alpha induced similar changes in locomotive pattern. On the other hand, most lymphocytes did not polarize or move spontaneously without chemokine stimulation. Stimulation with CXCL12 induced directional migration in 37% of the lymphocytes at a speed of 9.6 +/- 1.6 microm/min (n = 100). The movement pattern of monocytes was similar to that of granulocytes in that they tend to become polarized and move spontaneously, but they moved at a very slow speed ranging from 3.9 to 4.2 microm/min even with chemokine stimulation. The new optical assay may be useful for many diagnostic as well as basic studies.

Immobilized chemoattractant peptides mediate adhesion and distinct calcium-dependent cell signaling in human neutrophils

Chemotaxis is the stimulated directional migration of cells in response to chemotactic factors, manifested for instance during leukocyte interaction with chemoattractants in inflammation. The N-formyl-Met-Leu-Phe (fMLF) bacterial peptide family is particularly potent in attracting and activating neutrophilic granulocytes. To accomplish defined circumstances for recruitment and activation of cells, we fabricated semitransparent gold-coated glass coverslips functionalized with chemoattractant fMLF receptor peptide agonist analogues. Peptides based on a common leading four-amino-acid sequence Gly-Gly-Gly-Cys were thus coupled to two potent fMLF receptor agonists, N-formyl-Tyr-Nle-Phe-Leu-Nle-Gly-Gly-Gly-Cys and N-formyl-Met-Leu-Phe-Gly-Gly-Gly-Cys, and a formylated control peptide, N-formyl-Gly-Gly-Gly-Cys. They were anchored via the SH group of Cys either directly to the gold surface or a mixed self-assembled monolayer composed of maleimide- and hydroxyl-terminated oligo(ethylene glycol) alkyldisulfides. The overall peptide immobilization procedure was characterized with ellipsometry, contact angle measurement, and infrared spectroscopy. When exposed to granulocytes, the agonist surface rapidly recruited neutrophils and the cells responded with extensive spreading and intracellular calcium transients within minutes. The reference peptide generated no such activation, and the cells maintained a more spherical morphology, suggesting that we have been able to immobilize chemoattractant receptor agonist peptides with retained bioactivity. This is a crucial step in designing surfaces with specific effects on cellular behavior.

Study of synthetic peptides derived from the PKI55 protein, a protein kinase C modulator, in human neutrophils stimulated by the methyl ester derivative of the hydrophobic N-formyl tripeptide for-Met-Leu-Phe-OH

Elucidation of the involvement of protein kinase C subtypes in several diseases is an important challenge for the future development of new drug targets. We previously identified the PKI55 protein, which acts as a protein kinase C modulator, establishing a feedback loop of inhibition. The PKI55 protein is able to penetrate the cell membrane of activated human T-lymphocytes and to inhibit the activity of alpha, beta(1) and beta(2) protein kinase C isoforms. The present study aimed to identify the minimal amino acid sequence of PKI55 that is able to inhibit the enzyme activity of protein kinase C. Peptides derived from both C- and N-terminal sequences were synthesized and initially assayed in rat brain protein kinase C to identify which part of the entire protein maintained the in vitro effects described for PKI55, and then the active peptides were tested on the isoforms alpha, beta(1), beta(2), gamma, delta, epsilon and zeta to identify their specific inhibition properties. Specific protein kinase C isoforms have been associated with the activation of specific signal transduction pathways involved in inflammatory responses. Thus, the potential therapeutic role of the selected peptides has been studied in polymorphonuclear leukocytes activated by the methyl ester derivative of the hydrophobic N-formyl tripeptide for-Met-Leu-Phe-OH to evaluate their ability to modulate chemotaxis, superoxide anion production and lysozyme release. These studies have shown that only chemotactic function is significantly inhibited by these peptides, whereas superoxide anion production and lysozyme release remain unaffected. Western blotting experiments also demonstrated a selective reduction in the levels of the protein kinase C beta(1) isoform, which was previously demonstrated to be associated with the polymorphonuclear leukocyte chemotactic response.

Can anti-migratory drugs be screened in vitro? A review of 2D and 3D assays for the quantitative analysis of cell migration

The aim of the present review is to detail and analyze the pros and cons of in vitro tests available to quantify the anti-migratory effects of anti-cancer drugs for their eventual use in combating the dispersal of tumor cells, a clinical need which currently remains unsatisfied. We therefore briefly sum up why anti-migratory drugs constitute a promising approach in oncology while at the same time emphasizing that migrating cancer cells are resistant to apoptosis. To analyze the pros and cons of the various in vitro tests under review we also briefly sum up the molecular and cellular stages of cancer cell migration, an approach that enables us to argue both that no single in vitro test is sufficient to characterize the anti-migratory potential of a drug and that standardization is needed for the efficient quantitative analysis of cell locomotion in a 3D environment. Before concluding our review we devote the final two parts (i) to the description of new prototypes which, in the near future, could enter the screening process with a view to identifying novel anti-migratory compounds, and (ii) to the anti-migratory compounds currently developed against cancer, with particular emphasis on how these compounds were selected before entering the clinical trial phase.

Homeostatic Lymphoid Chemokines Synergize with Adhesion Ligands to Trigger T and B Lymphocyte Chemokinesis

Homeostatic chemokines such as CCL19, CCL21, and CXCL13 are known to elicit chemotaxis from naive T and B cells and play a critical role in lymphocyte homing to appropriate zones within secondary lymphoid organs (SLO). Here we tested whether CCL21 and CXCL13 modulate murine lymphocyte motility in the absence of concentration gradients, using videomicroscopy to directly observe the migration of single cells. CCL21 treatment of T cells induced rapid polarization and sustained random migration with average speeds of 5.16 +/- 2.08 microm/min; B cell migration (average velocity 4.10 +/- 1.58 microm/min) was similarly induced by CXCL13. Migration required the presence of both chemokine and adhesion ligands and was sustained for >24 h. Furthermore, in in vitro assays modeling the relative infrequency of Ag-specific T cell-dendritic cell (DC) encounters during primary immune responses, we found that CCL21 addition to T-DC cocultures accelerated the kinetics of CD69 up-regulation and enhanced by 2-fold the proliferation of Ag-specific T cells in a manner dependent on G-protein-coupled receptor signaling in T cells. These results suggest that homeostatic chemokines could substantially impact the dynamics and priming of lymphocytes within SLO even in the absence of significant concentration gradients.

A microsystem for sensing and patterning oxidative microgradients during cell culture

We present the design, modeling, fabrication and testing of a microsystem for the electrolytic patterning and sensing of oxidative microgradients within 1 x 1 mm2 area during cell culture. The system employs an array of microfabricated electrodes (3-40 microm in width) embedded in gas-permeable microchannels to generate precise doses of dissolved oxygen (ranging from 10 fmol O2 mm(-2) s(-1) to 100 nmol O2 mm(-2) s(-1)) via electrolysis. The microgradients generated by different microelectrodes in the array can be superimposed to pattern multi-dimensional oxygen profiles not possible with other methods. We demonstrate the patterning, sensing and quantification of dissolved oxygen microgradients in the 0 to 40% dO2 range using this microsystem. Reactive oxygen species generation and dosing is also quantified. Lastly, we demonstrate how the microtechnology enables new types of experiments in three different cell culture models: localized hyperoxia-induced apoptosis in C2C12 myoblasts, dynamic aerotaxis assays of Bacillus subtilis, and studies of calcium release in an ischemia/re-oxygenation myoblast model.

Microfluidic system for measuring neutrophil migratory responses to fast switches of chemical gradients

Experimental systems that provide temporal and spatial control of chemical gradients are required for probing into the complex mechanisms of eukaryotic cell chemotaxis. However, no current technique can simultaneously generate stable chemical gradients and allow fast gradient changes. We developed a microfluidic system with microstructured membranes for exposing neutrophils to fast and precise changes between stable, linear gradients of the known chemoattractant Interleukin-8 (IL-8). We observed that rapidly lowering the average concentration of IL-8 within a gradient, while preserving the direction of the gradient, resulted in temporary neutrophil depolarization. Fast reversal of the gradient direction while increasing or decreasing the average concentration also resulted in temporary depolarization. Neutrophils adapted and maintained their directional motility, only when the average gradient concentration was increased and the direction of the gradient preserved. Based on these observations we propose a two-component temporal sensing mechanism that uses variations of chemokine concentration averaged over the entire cell surface and localized at the leading edge, respectively, and directs neutrophil responses to changes in their chemical microenvironment.

Blockade of chemokine-induced signalling inhibits CCR5-dependent HIV infection in vitro without blocking gp120/CCR5 interaction

BACKGROUND

Cellular infection with human immunodeficiency virus (HIV) both in vitro and in vivo requires a member of the chemokine receptor family to act as a co-receptor for viral entry. However, it is presently unclear to what extent the interaction of HIV proteins with chemokine receptors generates intracellular signals that are important for productive infection.

RESULTS

In this study we have used a recently described family of chemokine inhibitors, termed BSCIs, which specifically block chemokine-induced chemotaxis without affecting chemokine ligands binding to their receptors. The BSCI termed Peptide 3 strongly inhibited CCR5 mediated HIV infection of THP-1 cells (83 +/- 7% inhibition assayed by immunofluoresence staining), but had no effect on gp120 binding to CCR5. Peptide 3 did not affect CXCR4-dependent infection of Jurkat T cells.

CONCLUSION

These observations suggest that, in some cases, intracellular signals generated by the chemokine coreceptor may be required for a productive HIV infection.

A high-throughput chemotaxis assay for pharmacological characterization of chemokine receptors: Utilization of U937 monocytic cells

INTRODUCTION

Higher-throughput chemotaxis assays have had limited use in chemokine receptor pharmacology studies mainly because of the unavailability of optimal assay formats in addition to an incompatibility of chemotactic cell backgrounds with other pharmacological assays. Here, we developed a high-throughput 96-well chemotaxis assay for leukocytic cell lines and identified the human U937 monocytic line as an excellent cell background for both chemotaxis and the high-throughput calcium mobilization Fluorescent Imaging Plate Reader (FLIPR) assay.

METHODS

Optimal chemotactic conditions were developed using the Neuroprobe MBA96 nondisposable and the Millipore MultiScreen-MIC disposable apparatuses with responses to CXC chemokine receptor (CXCR)-4 endogenously expressed on the human H9 T lymphoma line, and confirmed with Jurkat T cell and U937 monocytic cell lines.

RESULTS

The U937 cell line was chosen for site-directed mutagenesis studies with CC chemokine receptor (CCR)-7 because this cell line did not endogenously express this receptor, it demonstrated a good chemotaxis index, and it showed an exceptional ability to mobilize calcium measured via FLIPR. Using the Millipore MultiScreen-MIC and FLIPR assays, alanine substitutions at K130 and Q227 caused threefold shifts in potency for the CCR7 ligand, CCL19, whereas that at K137 had no effect.

DISCUSSION

Because these CCR7 mutations have previously been shown not to affect ligand binding, our results here show that these residues are specifically involved in receptor activation signals critical to chemotaxis and underscore the importance of using the U937 cell background to confirm results of chemotaxis with those of the FLIPR assay.

Identification of CC chemokine receptor 7 residues important for receptor activation

The binding pocket of family A GPCRs that bind small biogenic amines is well characterized. In this study we identify residues on CC chemokine receptor 7 (CCR-7) that are involved in agonist-mediated receptor activation but not in high affinity ligand binding. The mutations also affect the ability of the ligands to induce chemotaxis. Two of the residues, Lys3.33(137) and Gln5.42(227), are consistent with the binding pocket described for biogenic amines, while Lys3.26(130) and Asn7.32(305), are found at, or close to, the cell surface. Our observations are in agreement with findings from other peptide and chemokine receptors, which indicate that receptors that bind larger ligands contain contact sites closer to the cell surface in addition to the conventional transmembrane binding pocket. These findings also support the theory that chemokine receptors require different sets of interactions for high affinity ligand binding and receptor activation.