Requirements for hyaluronic acid binding by CD44: a role for the cytoplasmic domain and activation by antibody

CD44 Intracellular Domain: A Long Tale of a Short Tail

CD44 is a single-chain transmembrane receptor that exists in multiple forms due to alternative mRNA splicing and post-translational modifications. CD44 is the main cell surface receptor of hyaluronan as well as other extracellular matrix molecules, cytokines, and growth factors that play important roles in physiological processes (such as hematopoiesis and lymphocyte homing) and the progression of various diseases, the predominant one being cancer. Currently, CD44 is an established cancer stem cell marker in several tumors, implying a central functional role in tumor biology. The present review aims to highlight the contribution of the CD44 short cytoplasmic tail, which is devoid of any enzymatic activity, in the extraordinary functional diversity of the receptor. The interactions of CD44 with cytoskeletal proteins through specific structural motifs within its intracellular domain drives cytoskeleton rearrangements and affects the distribution of organelles and transport of molecules. Moreover, the CD44 intracellular domain specifically interacts with various cytoplasmic effectors regulating cell-trafficking machinery, signal transduction pathways, the transcriptome, and vital cell metabolic pathways. Understanding the cell type- and context-specificity of these interactions may unravel the high complexity of CD44 functions and lead to novel improved therapeutic interventions.

Induction of ferroptosis using functionalized iron-based nanoparticles for anti-cancer therapy

Ferroptosis, a cell death pathway that is induced in response to iron, has recently attracted remarkable attention given its emerging therapeutic potential in cancer cells. The need for a promising modality to improve chemotherapy's efficacy through this pathway has been urgent in recent years, and this non-apoptotic cell death pathway accumulates reactive oxygen species (ROS) and is subsequently involved in lipid peroxidation. Here, we report cancer-targeting nanoparticles that possess highly efficient cancer-targeting ability and minimal systemic toxicity, thereby leading to ferroptosis. To overcome the limit of actual clinical application, which is the ultimate goal due to safety issues, we designed safe nanoparticles that can be applied clinically. Nanoparticles containing ferroptosis-dependent iron and FDA-approved hyaluronic acid (FHA NPs) are fabricated by controlling physicochemical properties, and the FHA NPs specifically induce ROS production and lipid peroxidation in cancer cells without affecting normal cells. The excellent in vivo anti-tumor therapeutic effect of FHA NPs was confirmed in the A549 tumor-bearing mice model, indicating that the induction of FHA NP-mediated cell death via the ferroptosis pathway could serve as a powerful platform in anticancer therapy. We believe that this newly proposed FHA NP-induced ferroptosis strategy is a promising system that offers the potential for efficient cancer treatment and provides insight into the safe design of nanomedicines for clinical applications.

The state of CD44 activation in cancer progression and therapeutic targeting

CD44, a non-kinase transmembrane glycoprotein, is ubiquitously expressed on various types of cells, especially cancer stem cells (CSCs), and has been implicated in cancer onset and aggressiveness. The major ligand for the CD44, hyaluronan (HA), binds to and interacts with CD44, which in turn triggers downstream signaling cascades, thereby promoting cellular behaviors such as proliferation, motility, invasiveness and chemoresistance. The CD44-HA interaction is cell-specific and strongly affected by the state of CD44 activation. Therefore, the binding of HA to CD44 is essential for the activation of CD44 during which the detailed regulatory mechanism needs to be clarified. Different CD44 activation states distribute in human carcinoma and normal tissue; however, whether CD44 activation is a critical requirement for tumor initiation, progression and notorious CSC properties remains to be clarified. A deeper understanding of the regulation of CD44 activation may facilitate the development of novel targeted drugs in the future. Here, we review the current findings concerning the states of CD44 activation on the cell surface, the underlying regulatory mechanisms of CD44 activation, the known role for CD44 activation in tumor progression and CSC hallmarks, as well as the potential of HA-coated nanoparticle for targeting activated CD44 for cancer therapy.

Epitope Mapping of the Anti-CD44 Monoclonal Antibody (C44Mab-46) Using the REMAP Method

CD44 functions as a major hyaluronan receptor on most cell types, with roles in cell adhesion, migration, proliferation, differentiation, and survival. The CD44 gene comprises 20 exons, with alternative splicing producing many different isoforms. CD44 variant isoforms exhibit tissue-specific expression patterns and have been studied as therapeutic targets for several cancers; therefore, anti-CD44 monoclonal antibodies (mAbs) are useful for investigating CD44 expression in various cancers. Previously, we established an anti-CD44 mAb, C₄₄Mab-46 (IgG₁, κ), by immunizing mice with the CD44v3-10 ectodomain. Although C₄₄Mab-46 recognized all CD44 isoforms, the binding epitope of C₄₄Mab-46 has not been determined. In this study, we first checked the reactivity of C₄₄Mab-46 to several CD44v3-10 deletion mutants such as dN79, dN124, dN147, and dN224. We found the N-terminus of the C₄₄Mab-46-binding epitope between residues 147 and 224 of CD44v3-10. We next investigated this epitope using a novel mapping system: RIEDL insertion for epitope mapping (REMAP) method. We constructed 31 CD44 standard (CD44s) mutants where the RIEDL tag was inserted into the expected epitope region in CD44s. We observed that the C₄₄Mab-46 epitope constituted five amino acids: ₁₇₄-TDDDV-₁₇₈ of CD44s. Thus, the REMAP method could be used to determine mAb binding epitopes for membrane proteins.

Dendritic cell entry to lymphatic capillaries is orchestrated by CD44 and the hyaluronan glycocalyx

CD44 anchors the hyaluronan glycocalyx on migrating dendritic cells to permit docking to the endothelial receptor LYVE-1, thus orchestrating lymphatic trafficking through modulating glycocalyx density.

DCs play a vital role in immunity by conveying antigens from peripheral tissues to draining lymph nodes, through afferent lymphatic vessels. Critical to the process is initial docking to the lymphatic endothelial receptor LYVE-1 via its ligand hyaluronan on the DC surface. How this relatively weak binding polymer is configured for specific adhesion to LYVE-1, however, is unknown. Here, we show that hyaluronan is anchored and spatially organized into a 400–500 nm dense glycocalyx by the leukocyte receptor CD44. Using gene knockout and by modulating CD44-hyaluronan interactions with monoclonal antibodies in vitro and in a mouse model of oxazolone-induced skin inflammation, we demonstrate that CD44 is required for DC adhesion and transmigration across lymphatic endothelium. In addition, we present evidence that CD44 can dynamically control the density of the hyaluronan glycocalyx, regulating the efficiency of DC trafficking to lymph nodes. Our findings define a previously unrecognized role for CD44 in lymphatic trafficking and highlight the importance of the CD44:HA:LYVE-1 axis in its regulation.

Mechanical Cues Affect Migration and Invasion of Cells From Three Different Directions

Cell migration and invasion is a key driving factor for providing essential cellular functions under physiological conditions or the malignant progression of tumors following downward the metastatic cascade. Although there has been plentiful of molecules identified to support the migration and invasion of cells, the mechanical aspects have not yet been explored in a combined and systematic manner. In addition, the cellular environment has been classically and frequently assumed to be homogeneous for reasons of simplicity. However, motility assays have led to various models for migration covering only some aspects and supporting factors that in some cases also include mechanical factors. Instead of specific models, in this review, a more or less holistic model for cell motility in 3D is envisioned covering all these different aspects with a special emphasis on the mechanical cues from a biophysical perspective. After introducing the mechanical aspects of cell migration and invasion and presenting the heterogeneity of extracellular matrices, the three distinct directions of cell motility focusing on the mechanical aspects are presented. These three different directions are as follows: firstly, the commonly used invasion tests using structural and structure-based mechanical environmental signals; secondly, the mechano-invasion assay, in which cells are studied by mechanical forces to migrate and invade; and thirdly, cell mechanics, including cytoskeletal and nuclear mechanics, to influence cell migration and invasion. Since the interaction between the cell and the microenvironment is bi-directional in these assays, these should be accounted in migration and invasion approaches focusing on the mechanical aspects. Beyond this, there is also the interaction between the cytoskeleton of the cell and its other compartments, such as the cell nucleus. In specific, a three-element approach is presented for addressing the effect of mechanics on cell migration and invasion by including the effect of the mechano-phenotype of the cytoskeleton, nucleus and the cell's microenvironment into the analysis. In precise terms, the combination of these three research approaches including experimental techniques seems to be promising for revealing bi-directional impacts of mechanical alterations of the cellular microenvironment on cells and internal mechanical fluctuations or changes of cells on the surroundings. Finally, different approaches are discussed and thereby a model for the broad impact of mechanics on cell migration and invasion is evolved.

A Review on the Scope of Photothermal Therapy-Based Nanomedicines in Preclinical Models of Colorectal Cancer

Oncologic thermal ablation involves the use of hyperthermic temperatures to damage and treat solid cancers. Thermal ablation is being investigated as a method of treatment in colorectal cancers and has the potential to complement conventional anticancer treatments in managing local recurrence and metastatic disease. Photothermal therapy utilizes photosensitive agents to generate local heat and induce thermal ablation. There is growing interest in developing nanotechnology platforms to deliver such photosensitive agents. An advantage of nanomedicines is their multifunctionality, with the capability to deliver combinations of chemotherapeutics and cancer-imaging agents. To date, there have been no clinical studies evaluating photothermal therapy-based nanomedicines in colorectal cancers. This review presents the current scope of preclinical studies, investigating nanomedicines that have been developed for delivering multimodal photothermal therapy to colorectal cancers, with an emphasis on potential clinical applications.

An osteopontin/CD44 immune checkpoint controls CD8+ T cell activation and tumor immune evasion

Despite breakthroughs in immune checkpoint inhibitor (ICI) immunotherapy, not all human cancers respond to ICI immunotherapy and a large fraction of patients with the responsive types of cancers do not respond to current ICI immunotherapy. This clinical conundrum suggests that additional immune checkpoints exist. We report here that interferon regulatory factor 8 (IRF8) deficiency led to impairment of cytotoxic T lymphocyte (CTL) activation and allograft tumor tolerance. However, analysis of chimera mice with competitive reconstitution of WT and IRF8-KO bone marrow cells as well as mice with IRF8 deficiency only in T cells indicated that IRF8 plays no intrinsic role in CTL activation. Instead, IRF8 functioned as a repressor of osteopontin (OPN), the physiological ligand for CD44 on T cells, in CD11b+Ly6CloLy6G+ myeloid cells and OPN acted as a potent T cell suppressor. IRF8 bound to the Spp1 promoter to repress OPN expression in colon epithelial cells, and colon carcinoma exhibited decreased IRF8 and increased OPN expression. The elevated expression of OPN in human colon carcinoma was correlated with decreased patient survival. Our data indicate that myeloid and tumor cell-expressed OPN acts as an immune checkpoint to suppress T cell activation and confer host tumor immune tolerance.

Polysaccharide-Based Controlled Release Systems for Therapeutics Delivery and Tissue Engineering: From Bench to Bedside

Polysaccharides or polymeric carbohydrate molecules are long chains of monosaccharides that are linked by glycosidic bonds. The naturally based structural materials are widely applied in biomedical applications. This article covers four different types of polysaccharides (i.e., alginate, chitosan, hyaluronic acid, and dextran) and emphasizes their chemical modification, preparation approaches, preclinical studies, and clinical translations. Different cargo fabrication techniques are also presented in the third section. Recent progresses in preclinical applications are then discussed, including tissue engineering and treatment of diseases in both therapeutic and monitoring aspects. Finally, clinical translational studies with ongoing clinical trials are summarized and reviewed. The promise of new development in nanotechnology and polysaccharide chemistry helps clinical translation of polysaccharide-based drug delivery systems.

CRISPR-mediated deletion of the PECAM-1 cytoplasmic domain increases receptor lateral mobility and strengthens endothelial cell junctional integrity

AIMS

PECAM-1 is an abundant endothelial cell surface receptor that becomes highly enriched at endothelial cell-cell junctions, where it functions to mediate leukocyte transendothelial migration, sense changes in shear and flow, and maintain the vascular permeability barrier. Homophilic interactions mediated by the PECAM-1 extracellular domain are known to be required for PECAM-1 to perform these functions; however, much less is understood about the role of its cytoplasmic domain in these processes.

MAIN METHODS

CRISPR/Cas9 gene editing technology was employed to generate human endothelial cell lines that either lack PECAM-1 entirely, or express mutated PECAM-1 missing the majority of its cytoplasmic domain (∆CD-PECAM-1). The endothelial barrier function was evaluated by Electric Cell-substrate Impedance Sensing, and molecular mobility was assessed by fluorescence recovery after photobleaching.

KEY FINDINGS

We found that ∆CD-PECAM-1 concentrates normally at endothelial cell junctions, but has the unexpected property of conferring increased baseline barrier resistance, as well as a more rapid rate of recovery of vascular integrity following thrombin-induced disruption of the endothelial barrier. Fluorescence recovery after photobleaching analysis revealed that ∆CD-PECAM-1 exhibits increased mobility within the plane of the plasma membrane, thus allowing it to redistribute more rapidly back to endothelial cell-cell borders to reform the vascular permeability barrier.

SIGNIFICANCE

The PECAM-1 cytoplasmic domain plays a novel role in regulating the rate and extent of vascular permeability following thrombotic or inflammatory challenge.

Hyaluronic acid enhances cell migration and invasion via the YAP1/TAZ-RHAMM axis in malignant pleural mesothelioma

Most malignant mesotheliomas (MPMs) frequently show activated forms of Yes-associated protein 1 (YAP1) and transcriptional co-activator with PDZ-binding motif (TAZ), which transcriptionally regulates the receptor for hyaluronic acid-mediated motility (RHAMM). As RHAMM is involved in cell migration and invasion in various tumors, we speculated that hyaluronic acid (HA) in pleural fluid might affect the progression of mesothelioma by stimulating cell migration and invasion through RHAMM. The level of RHAMM expression was decreased by YAP1/TAZ knockdown, and conversely increased by forced expression of the active form of YAP1, suggesting that RHAMM was regulated by YAP1/TAZ in MPM cells. Cell migration and invasion were also decreased by YAP1/TAZ or RHAMM knockdown. Notably, HA treatment increased cell motility and invasion, and this was abolished by RHAMM knockdown, suggesting that HA may augment local progression of MPM cells via RHAMM. Furthermore, treatment with fluvastatin, which regulates RHAMM transcription by modulating YAP1/TAZ activity, decreased the motility and invasion of MPM cells. Collectively, these data suggest that HA is an “unfavorable” factor because it promotes malignancy in mesothelioma and that the YAP1/TAZ-RHAMM axis may have potential value as a therapeutic target for inhibition of disease progression in MPM.

Selective killing of breast cancer cells expressing activated CD44 using CD44 ligand-coated nanoparticles in vitro and in vivo

The cell surface glycoprotein CD44 is expressed in cancer cells and has been used as a therapeutic target in preclinical studies. However, the ubiquitous expression of CD44 in numerous cell types, including hematopoietic cells, has hindered its application in targeted therapy. Here, we demonstrated that CD44 was activated on breast cancer cells but was inactive on normal cells in vitro and in vivo. We analyzed 34 clinical primary tumor and normal breast tissues and demonstrated that CD44 was in an active state on breast cancer cells but in an inactive state on normal cells. Furthermore, based on the binding property of CD44 with its ligand hyaluronan (HA), we self-assembled HA-coated nanoparticles and studied their selective targeting efficacy. Our results indicate that HA-coated nanoparticles bearing the CD44 ligand selectively targeted cancer cells both in vitro and in vivo, killing breast cancer cells while sparing normal cells. Our study suggested that the active state of CD44 plays a crucial role in the selective targeting of breast cancer cells by avoiding nonspecific toxicity to CD44-quiescent normal cells. These findings may provide a new idea for the selective targeting of cancer cells in other human cancers.

Binding of Hyaluronan to the Native Lymphatic Vessel Endothelial Receptor LYVE-1 Is Critically Dependent on Receptor Clustering and Hyaluronan Organization

The lymphatic endothelial receptor LYVE-1 has been implicated in both uptake of hyaluronan (HA) from tissue matrix and in facilitating transit of leukocytes and tumor cells through lymphatic vessels based largely on in vitro studies with recombinant receptor in transfected fibroblasts. Curiously, however, LYVE-1 in lymphatic endothelium displays little if any binding to HA in vitro, and this has led to the conclusion that the native receptor is functionally silenced, a feature that is difficult to reconcile with its proposed in vivo functions. Nonetheless, as we reported recently, LYVE-1 can function as a receptor for HA-encapsulated Group A streptococci and mediate lymphatic dissemination in mice. Here we resolve these paradoxical findings and show that the capacity of LYVE-1 to bind HA is strictly dependent on avidity, demanding appropriate receptor self-association and/or HA multimerization. In particular, we demonstrate the prerequisite of a critical LYVE-1 threshold density and show that HA binding may be elicited in lymphatic endothelium by surface clustering with divalent LYVE-1 mAbs. In addition, we show that cross-linking of biotinylated HA in streptavidin multimers or supramolecular complexes with the inflammation-induced protein TSG-6 enables binding even in the absence of LYVE-1 cross-linking. Finally, we show that endogenous HA on the surface of macrophages can engage LYVE-1, facilitating their adhesion and transit across lymphatic endothelium. These results reveal LYVE-1 as a low affinity receptor tuned to discriminate between different HA configurations through avidity and establish a new mechanistic basis for the functions ascribed to LYVE-1 in matrix HA binding and leukocyte trafficking in vivo.

CD44 is a multidomain signaling platform that integrates extracellular matrix cues with growth factor and cytokine signals

The reception and integration of the plethora of signals a cell receives from its microenvironment is decisive in determining cell behavior. Perturbation of extracellular cues, or an inappropriate response to or integration of these signals lies at the root of many diseases such as cancer. The transmembrane protein CD44 contributes to the reception of a broad variety of microenvironmental components, including extracellular matrix constituents such as hyaluronic acid, as well as growth factors and cytokines. In this chapter, we review the range of extracellular cues that are recognized by CD44, and show how CD44 serves to integrate this information at several levels through the mechanisms by which it contributes to transduction of these various microenvironmental signals.

Advances and advantages of nanomedicine in the pharmacological targeting of hyaluronan-CD44 interactions and signaling in cancer

Extensive experimental evidence in cell and animal tumor models show that hyaluronan-CD44 interactions are crucial in both malignancy and resistance to cancer therapy. Because of the intimate relationship between the hyaluronan-CD44 system and tumor cell survival and growth, it is an increasingly investigated area for applications to anticancer chemotherapeutics. Interference with the hyaluronan-CD44 interaction by targeting drugs to CD44, targeting drugs to the hyaluronan matrix, or interfering with hyaluronan matrix/tumor cell-associated CD44 interactions is a viable strategy for cancer treatment. Many of these methods can decrease tumor burden in animal models but have yet to show significant clinical utility. Recent advances in nanomedicine have offered new valuable tools for cancer detection, prevention, and treatment. The enhanced permeability and retention effect has served as key rationale for using nanoparticles to treat solid tumors. However, the targeted and uniform delivery of these particles to all regions of tumors in sufficient quantities requires optimization. An ideal nanocarrier should be equipped with selective ligands that are highly or exclusively expressed on target cells and thus endow the carriers with specific targeting capabilities. In this review, we describe how the hyaluronan-CD44 system may provide such an alternative in tumors expressing specific CD44 variants.

RHAMM deficiency disrupts folliculogenesis resulting in female hypofertility

The postnatal mammalian ovary contains the primary follicles, each comprising an immature oocyte surrounded by a layer of somatic granulosa cells. Oocytes reach meiotic and developmental competence via folliculogenesis. During this process, the granulosa cells proliferate massively around the oocyte, form an extensive extracellular matrix (ECM) and differentiate into cumulus cells. As the ECM component hyaluronic acid (HA) is thought to form the backbone of the oocyte-granulosa cell complex, we deleted the relevant domain of the Receptor for HA Mediated Motility (RHAMM) gene in the mouse. This resulted in folliculogenesis defects and female hypofertility, although HA-induced signalling was not affected. We report that wild-type RHAMM localises at the mitotic spindle of granulosa cells, surrounding the oocyte. Deletion of the RHAMM C-terminus in vivo abolishes its spindle association, resulting in impaired spindle orientation in the dividing granulosa cells, folliculogenesis defects and subsequent female hypofertility. These data reveal the first identified physiological function for RHAMM, during oogenesis, and the importance of this spindle-associated function for female fertility.

Terminal sialic acids on CD44 N-glycans can block hyaluronan binding by forming competing intramolecular contacts with arginine sidechains

Specific sugar residues and their linkages form the basis of molecular recognition for interactions of glycoproteins with other biomolecules. Seemingly small changes, like the addition of a single monosaccharide in the covalently attached glycan component of glycoproteins, can greatly affect these interactions. For instance, the sialic acid capping of glycans affects protein-ligand binding involved in cell-cell and cell-matrix interactions. CD44 is a single-pass transmembrane glycoprotein whose binding with its carbohydrate ligand hyaluronan (HA), an extracellular matrix component, mediates processes such as leukocyte homing, cell adhesion, and tumor metastasis. This binding is highly regulated by glycosylation of the N-terminal extracellular hyaluronan-binding domain (HABD); specifically, sialic acid capped N-glycans of HABD inhibit ligand binding. However, the molecular mechanism behind this sialic acid mediated regulation has remained unknown. Two of the five N-glycosyation sites of HABD have been previously identified as having the greatest inhibitory effect on HA binding, but only if the glycans contain terminal sialic acid residues. These two sites, Asn25 and Asn120, were chosen for in silico glycosylation in this study. Here, from extensive standard molecular dynamics simulations and biased simulations, we propose a molecular mechanism for this behavior based on spontaneously-formed charge-paired hydrogen bonding interactions between the negatively-charged sialic acid residues and positively-charged Arg sidechains known to be critically important for binding to HA, which itself is negatively charged. Such intramolecular hydrogen bonds would preclude associations critical to hyaluronan binding. This observation suggests how CD44 and related glycoprotein binding is regulated by sialylation as cellular environments fluctuate.

Neutrophils at work

In this Review we discuss data demonstrating recently recognized aspects of neutrophil homeostasis in the steady state, granulopoiesis in 'emergency' conditions and interactions of neutrophils with the adaptive immune system. We explore in vivo observations of the recruitment of neutrophils from blood to tissues in models of blood-borne infections versus bacterial invasion through epithelial linings. We examine data on novel aspects of the activation of NADPH oxidase and the heterogeneity of phagosomes and, finally, consider the importance of two neutrophil-derived biological agents: neutrophil extracellular traps and ectosomes.

Novel di-tertiary-butyl phenylhydrazones as dual cyclooxygenase-2/5-lipoxygenase inhibitors: synthesis, COX/LOX inhibition, molecular modeling, and insights into their cytotoxicities

Although dual inhibition of Cyclooxygenase-2 (COX-2) and 5-Lipoxygenase (5-LOX) enzymes is highly effective than targeting COX or LOX alone, there are only a few reports of examining such compounds in case of colorectal cancers (CRC). In the present work we report that the novel di-tert-butyl phenol-based dual inhibitors DTPSAL, DTPBHZ, DTPINH, and DTPNHZ exhibit significant cytotoxicity against human CRC cell lines. Molecular docking studies revealed a good fit of these compounds in the COX-2 and 5-LOX protein cavities. The inhibitors show significant inhibition of COX-2 and 5-LOX activities and are effective against a panel of human colon cancer cell lines including HCA-7, HT-29, SW480 and intestinal Apc10.1 cells as well as the hyaluronan synthase-2 (Has2) enzyme over-expressing colon cancer cells, through inhibition of the Hyaluronan/CD44v6 cell survival pathway. Western blot analysis and qRT-PCR analyses indicated that the di-tert-butyl phenol-based dual inhibitors reduce the expression of COX-2, 5-LOX, and CD44v6 in human colon cancer HCA-7 cells, while the combination of CD44v6shRNA and DTPSAL has an additional inhibitory effect on CD44v6 mRNA expression. The synergistic inhibitory effect of Celecoxib and Licofelone on CD44v6 mRNA expression suggests that the present dual inhibitors down-regulate cyclooxygenase and lipoxygenase enzymes through CD44v6. The compounds also exhibited enhanced antiproliferative potency compared to standard dual COX/LOX inhibitor, viz. Licofelone. Importantly, the HA/CD44v6 antagonist CD44v6shRNA in combination with synthetic compounds had a sensitizing effect on the cancer cells which enhanced their antiproliferative potency, a finding which is crucial for the anti-proliferative potency of the novel synthetic di-tert-butyl phenol based dual COX-LOX inhibitors in colon cancer cells.

Inter-α-inhibitor impairs TSG-6-induced hyaluronan cross-linking

Under inflammatory conditions and in the matrix of the cumulus-oocyte complex, the polysaccharide hyaluronan (HA) becomes decorated covalently with heavy chains (HCs) of the serum glycoprotein inter-α-inhibitor (IαI). This alters the functional properties of the HA as well as its structural role within extracellular matrices. The covalent transfer of HCs from IαI to HA is catalyzed by TSG-6 (tumor necrosis factor-stimulated gene-6), but TSG-6 is also known as a HA cross-linker that induces condensation of the HA matrix. Here, we investigate the interplay of these two distinct functions of TSG-6 by studying the ternary interactions of IαI and TSG-6 with well defined films of end-grafted HA chains. We demonstrate that TSG-6-mediated cross-linking of HA films is impaired in the presence of IαI and that this effect suppresses the TSG-6-mediated enhancement of HA binding to CD44-positive cells. Furthermore, we find that the interaction of TSG-6 and IαI in the presence of HA gives rise to two types of complexes that independently promote the covalent transfer of heavy chains to HA. One type of complex interacts very weakly with HA and is likely to correspond to the previously reported covalent HC·TSG-6 complexes. The other type of complex is novel and binds stably but noncovalently to HA. Prolonged incubation with TSG-6 and IαI leads to HA films that contain, in addition to covalently HA-bound HCs, several tightly but noncovalently bound molecular species. These findings have important implications for understanding how the biological activities of TSG-6 are regulated, such that the presence or absence of IαI will dictate its function.

Intracellular domain fragment of CD44 alters CD44 function in chondrocytes

The hyaluronan receptor CD44 undergoes sequential proteolytic cleavage at the cell surface. The initial cleavage of the CD44 extracellular domain is followed by a second intramembranous cleavage of the residual CD44 fragment, liberating the C-terminal cytoplasmic tail of CD44. In this study conditions that promote CD44 cleavage resulted in a diminished capacity to assemble and retain pericellular matrices even though sufficient non-degraded full-length CD44 remained. Using stable and transient overexpression of the cytoplasmic domain of CD44, we determined that the intracellular domain interfered with anchoring of the full-length CD44 to the cytoskeleton and disrupted the ability of the cells to bind hyaluronan and assemble a pericellular matrix. Co-immunoprecipitation assays were used to determine whether the mechanism of this interference was due to competition with actin adaptor proteins. CD44 of control chondrocytes was found to interact and co-immunoprecipitate with both the 65- and 130-kDa isoforms of ankyrin-3. Moreover, this interaction with ankyrin-3 proteins was diminished in cells overexpressing the CD44 intracellular domain. Mutating the putative ankyrin binding site of the transiently transfected CD44 intracellular domain diminished the inhibitory effects of this protein on matrix retention. Although CD44 in other cells types has been shown to interact with members of the ezrin/radixin/moesin (ERM) family of adaptor proteins, only modest interactions between CD44 and moesin could be demonstrated in chondrocytes. The data suggest that release of the CD44 intracellular domain into the cytoplasm of cells such as chondrocytes exerts a competitive or dominant-negative effect on the function of full-length CD44.

Development of drug loaded nanoparticles for tumor targeting. Part 1: Synthesis, characterization, and biological evaluation in 2D cell cultures

Nanoparticles (NPs) are being extensively studied as carriers for drug delivery, but they often have limited penetration inside tumors. We envision that by targeting an endocytic receptor on the cell surface, the uptake of NPs can be significantly enhanced through receptor mediated endocytosis. In addition, if the receptor is recycled to the cell surface, the NP cargo can be transported out of the cells, which is then taken up by neighboring cells thus enhancing solid tumor penetration. To validate our hypothesis, in the first of two articles, we report the synthesis of doxorubicin (DOX)-loaded, hyaluronan (HA) coated silica nanoparticles (SNPs) containing a highly fluorescent core to target CD44, a receptor expressed on the cancer cell surface. HA was conjugated onto amine-functionalized SNPs prepared through an oil-water microemulsion method. The immobilization of the cytotoxic drug DOX was achieved through an acid sensitive hydrazone linkage. The NPs were fully characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta potential measurements, thermogravimetric analysis (TGA), UV-vis absorbance, and nuclear magnetic resonance (NMR). Initial biological evaluation experiments demonstrated that compared to ligand-free SNPs, the uptake of HA-SNPs by the CD44-expressing SKOV-3 ovarian cancer cells was significantly enhanced when evaluated in the 2D monolayer cell culture. Mechanistic studies suggested that cellular uptake of HA-SNPs was mainly through CD44 mediated endocytosis. HA-SNPs with immobilized DOX were endocytosed efficiently by the SKOV-3 cells as well. The enhanced tumor penetration and drug delivery properties of HA-SNPs will be evaluated in 3D tumor models in the subsequent paper.

Lipid-Based Nanovectors for Targeting of CD44-Overexpressing Tumor Cells

Hyaluronic acid (HA) is a naturally occurring glycosaminoglycan that exists in living systems, and it is a major component of the extracellular matrix. The hyaluronic acid receptor CD44 is found at low levels on the surface of epithelial, haematopoietic, and neuronal cells and is overexpressed in many cancer cells particularly in tumour initiating cells. HA has been therefore used as ligand attached to HA-lipid-based nanovectors for the active targeting of small or large active molecules for the treatment of cancer. This paper describes the different approaches employed for the preparation, characterization, and evaluation of these potent delivery systems.

Hyaluronan-CD44 interactions as potential targets for cancer therapy

It is becoming increasingly clear that signals generated in tumor microenvironments are crucial to tumor cell behavior, such as survival, progression and metastasis. The establishment of these malignant behaviors requires that tumor cells acquire novel adhesion and migration properties to detach from their original sites and to localize to distant organs. CD44, an adhesion/homing molecule, is a major receptor for the glycosaminoglycan hyaluronan, which is one of the major components of the tumor extracellular matrix. CD44, a multistructural and multifunctional molecule, detects changes in extracellular matrix components, and thus is well positioned to provide appropriate responses to changes in the microenvironment, i.e. engagement in cell-cell and cell-extracellular matrix interactions, cell trafficking, lymph node homing and the presentation of growth factors/cytokines/chemokines to co-ordinate signaling events that enable the cell responses that change in the tissue environment. The potential involvement of CD44 variants (CD44v), especially CD44v4-v7 and CD44v6-v9, in tumor progression has been confirmed for many tumor types in numerous clinical studies. The downregulation of the standard CD44 isoform (CD44s) in colon cancer is postulated to result in increased tumorigenicity. CD44v-specific functions could be caused by their higher binding affinity than CD44s for hyaluronan. Alternatively, CD44v-specific functions could be caused by differences in associating molecules, which may bind selectively to the CD44v exon. This minireview summarizes how the interaction between hyaluronan and CD44v can serve as a potential target for cancer therapy, in particular how silencing CD44v can target multiple metastatic tumors.

Spiral and square microstructured surfaces: the effect of the decreasing size of photo-immobilized hyaluronan domains on cell growth

Spiral and squared micropatterned surfaces of decreasing dimensions were realized by photo-immobilizing a photoreactive hyaluronan (Hyal) derivative on silanized glass substrates. The microstructured surfaces were observed by atomic force microscope and scanning electron microscope. Scanning electron microscope analysis revealed the presence of a spiral (ranging from 100 microm down to 1 microm in the central part) and a square pattern consisting of a central square of 100 microm x 100 microm and squares of different dimensions decreasing from the centre to the edges of the micropatterned area (2 microm x 1 microm). Three cell types were tested on all the microstructured surfaces: human coronary artery endothelial cells (HCAEC), human dermal fibroblasts (C54), and NIH 3T3 fibroblasts. Cell adhesion analysis demonstrated that HCAEC and C54 did not adhere to the immobilized Hyal on silanized glass but adapted their shape to the different sizes of the square and spiral patterns. Also, in both geometric patterns, the reduction of the adhesive glass width induced C54 to create bonds amongst themselves. NIH 3T3 cells adhered inside the squares and the spiral but reducing the adhesive glass width induced NIH 3T3 to adhere to immobilized Hyal. This fact is explained by the interactions between the cells and the immobilized Hyal as a consequence of the CD44/Hyal binding.

Delivery of CD44 shRNA/nanoparticles within cancer cells: perturbation of hyaluronan/CD44v6 interactions and reduction in adenoma growth in Apc Min/+ MICE

Our studies have shown that constitutive interactions between hyaluronan and CD44 on tumor cells induces various anti-apoptotic cell survival pathways through the formation of a multimeric signaling complex that contains activated receptor tyrosine kinases. Inhibition of the hyaluronan-CD44 interactions on tumor cells by hyaluronan-CD44 interaction antagonists suppresses these activities by disassembling the complex. Although the anti-tumor activity of hyaluronan-oligosaccharides, a hyaluronan-CD44 interaction antagonist, is effective in sensitizing tumor cells to chemotherapeutic agents and reducing tumor growth in xenografts, hyaluronan-oligosaccharide alone was not effective in reducing tumor progression in Apc Min/+ mice. We now show in vitro and in vivo that targeted inhibition of the expression of CD44v6 depletes the ability of the colon tumor cells to signal through hyaluronan-CD44v6 interactions. First, we cloned oligonucleotides coding CD44v6 shRNA into a conditionally silenced pSico vector. Second, using pSico-CD44v6 shRNA and a colon-specific Fabpl promoter-driven Cre recombinase expression vector packaged into transferrin-coated nanoparticles, we successfully delivered the CD44v6 shRNA within pre-neoplastic and neoplastic colon malignant cells. Third, using the Apc Min/+ mice model, we demonstrated that inhibition of the CD44v6 expression reduces the signaling through a hyaluronan/CD44v6-pErbB2-Cox-2 interaction pathway and reduced adenoma number and growth. Together, these data provide insight into the novel therapeutic strategies of short hairpin RNA/nanoparticle technology and its potential for silencing genes associated with colon tumor cells.

CD44 mediates successful interstitial navigation by killer T cells and enables efficient antitumor immunity

Although T lymphocytes are constitutively nonadherent cells, they undergo facultative polarity during migration and upon interaction with cells presenting cognate antigen, suggesting that cell polarity might be critical for target cell destruction. Using two-photon imaging of tumor-infiltrating T lymphocytes, we found that CD44, a receptor for extracellular matrix proteins and glycosaminoglycans, was crucial for interstitial T cell navigation and, consequently, efficient tumor cell screening. CD44 functioned as a critical regulator of intratumoral movement by stabilizing cell polarity in migrating T cells, but not during target cell interactions. Stable anterior-posterior asymmetry was maintained by CD44 independently of its extracellular domain. Instead, migratory polarity depended on the recruitment of ezrin, radixin, moesin (ERM) proteins by the intracellular domain of CD44 to the posterior cellular protrusion. Our results formally demonstrate that CD44-dependent T lymphocyte locomotion within target sites represents an essential immunologic checkpoint that determines the potency of T cell effector functions.

Shifts in bone marrow cell phenotypes caused by spaceflight

Bone marrow cells were isolated from the humeri of C57BL/6 mice after a 13-day flight on the space shuttle Space Transportation System (STS)-118 to determine how spaceflight affects differentiation of cells in the granulocytic lineage. We used flow cytometry to assess the expression of molecules that define the maturation/activation state of cells in the granulocytic lineage on three bone marrow cell subpopulations. These molecules included Ly6C, CD11b, CD31 (platelet endothelial cell adhesion molecule-1), Ly6G (Gr-1), F4/80, CD44, and c-Fos. The three subpopulations were small agranular cells [region (R)1], larger granular cells (R2), which were mostly neutrophils, and very large, very granular cells (R3), which had properties of macrophages. Although there were no composite phenotypic differences between total bone marrow cells isolated from spaceflight and ground-control mice, there were subpopulation differences in Ly6C (R1 and R3), CD11b (R2), CD31 (R1, R2, and R3), Ly6G (R3), F4/80 (R3), CD44(high) (R3), and c-Fos (R1, R2, and R3). In particular, the elevation of CD11b in the R2 subpopulation suggests neutrophil activation in response to landing. In addition, decreases in Ly6C, c-Fos, CD44(high), and Ly6G and an increase in F4/80 suggest that the cells in the bone marrow R3 subpopulation of spaceflight mice were more differentiated compared with ground-control mice. The presence of more differentiated cells may not pose an immediate risk to immune resistance. However, the reduction in less differentiated cells may forebode future consequences for macrophage production and host defenses. This is of particular importance to considerations of future long-term spaceflights.

Structural basis for CD44 recognition by ERM proteins

CD44 is an important adhesion molecule that functions as the major hyaluronan receptor which mediates cell adhesion and migration in a variety of physiological and pathological processes. Although full activity of CD44 requires binding to ERM (ezrin/radixin/moesin) proteins, the CD44 cytoplasmic region, consisting of 72 amino acid residues, lacks the Motif-1 consensus sequence for ERM binding found in intercellular adhesion molecule (ICAM)-2 and other adhesion molecules of the immunoglobulin superfamily. Ultracentrifugation sedimentation studies and circular dichroism measurements revealed an extended monomeric form of the cytoplasmic peptide in solution. The crystal structure of the radixin FERM domain complexed with a CD44 cytoplasmic peptide reveals that the KKKLVIN sequence of the peptide forms a beta strand followed by a short loop structure that binds subdomain C of the FERM domain. Like Motif-1 binding, the CD44 beta strand binds the shallow groove between strand beta5C and helix alpha1C and augments the beta sheet beta5C-beta7C from subdomain C. Two hydrophobic CD44 residues, Leu and Ile, are docked into a hydrophobic pocket with the formation of hydrogen bonds between Asn of the CD44 short loop and loop beta4C-beta5C from subdomain C. This binding mode resembles that of NEP (neutral endopeptidase 24.11) rather than ICAM-2. Our results reveal a characteristic versatility of peptide recognition by the FERM domains from ERM proteins, suggest a possible mechanism by which the CD44 tail is released from the cytoskeleton for nuclear translocation by regulated intramembrane proteolysis, and provide a structural basis for Smad1 interactions with activated CD44 bound to ERM protein.

CD44 suppresses TLR-mediated inflammation

The cell adhesion molecule CD44, which is the major hyaluronan receptor, has been implicated in the binding, endocytosis, and metabolism of hyaluronan. Previous studies have revealed that CD44 plays crucial roles in a variety of inflammatory diseases. In recent years, TLRs, which are ancient microbial pattern recognition receptors, have been shown to initiate an innate immune response and have been linked to a variety of inflammatory diseases. The present study shows that CD44 negatively regulates in vivo inflammation mediated by TLRs via NF-kappaB activation, which leads to proinflammatory cytokine production. Furthermore, our results show that CD44 directly associates with TLR2 when stimulated by the TLR2 ligand zymosan and that the cytoplasmic domain of CD44 is crucial for its regulatory effect on TLR signaling. This study indicates that CD44 plays a protective role in TLR-mediated inflammation and is the first to demonstrate a direct association between CD44 and a TLR.

Structural basis of the cytoplasmic tail of adhesion molecule CD43 and its binding to ERM proteins

CD43/leukosialin/sialophorin is the major adhesion molecule in most hematopoietic cells and belongs to the sialomucin superfamily. In leukocyte emigration and activation, the exclusion of CD43 from the immunological synapse is an essential step. While the exclusion requires binding of the cytoplasmic region to ERM (ezrin/radixin/moesin) proteins, the detailed specific nature of the interaction between CD43 and ERM proteins is obscure. We have characterized the conformational properties of the CD43 cytoplasmic region, consisting of 124 amino acid residues, by hydrodynamic and spectroscopic measurements. Sedimentation equilibrium and velocity studies of ultracentrifugation revealed that the CD43 cytoplasmic peptide exists in a monomeric and extended form in solution. The crystal structure of the complex between the radixin FERM (4.1 and ERM) domain and the CD43 juxtamembrane region peptide reveals that the nonpolar region of the peptide binds subdomain C of the FERM domain. CD43 lacks the Motif-1 sequence for FERM binding found in the FERM-intercellular adhesion molecule-2 complex but possesses two conserved leucine residues that dock into the hydrophobic pocket of subdomain C without forming a 3(10)-helix. The FERM-binding site on CD43 is overlapped with the functional nuclear localization signal sequence. Our structure suggests that regulation of ERM binding may be coupled with regulated intramembrane proteolysis of CD43 followed by the nuclear transfer of the cytoplasmic peptide.

Interaction of CD44 and hyaluronan is the dominant mechanism for neutrophil sequestration in inflamed liver sinusoids

Adhesion molecules known to be important for neutrophil recruitment in many other organs are not involved in recruitment of neutrophils into the sinusoids of the liver. The prevailing view is that neutrophils become physically trapped in inflamed liver sinusoids. In this study, we used a biopanning approach to identify hyaluronan (HA) as disproportionately expressed in the liver versus other organs under both basal and inflammatory conditions. Spinning disk intravital microscopy revealed that constitutive HA expression was restricted to liver sinusoids. Blocking CD44-HA interactions reduced neutrophil adhesion in the sinusoids of endotoxemic mice, with no effect on rolling or adhesion in postsinusoidal venules. Neutrophil but not endothelial CD44 was required for adhesion in sinusoids, yet neutrophil CD44 avidity for HA did not increase significantly in endotoxemia. Instead, activation of CD44-HA engagement via qualitative modification of HA was demonstrated by a dramatic induction of serum-derived HA-associated protein in sinusoids in response to lipopolysaccharide (LPS). LPS-induced hepatic injury was significantly reduced by blocking CD44-HA interactions. Administration of anti-CD44 antibody 4 hours after LPS rapidly detached adherent neutrophils in sinusoids and improved sinusoidal perfusion in endotoxemic mice, revealing CD44 as a potential therapeutic target in systemic inflammatory responses involving the liver.

Hyaluronan in tissue injury and repair

A hallmark of tissue injury and repair is the turnover of extracellular matrix components. This review focuses on the role of the glycosaminoglycan hyaluronan in tissue injury and repair. Both the synthesis and degradation of extracellular matrix are critical contributors to tissue repair and remodeling. Fragmented hyaluronan accumulates during tissue injury and functions in ways distinct from the native polymer. There is accumulating evidence that hyaluronan degradation products can stimulate the expression of inflammatory genes by a variety of immune cells at the injury site. CD44 is the major cell-surface hyaluronan receptor and is required to clear hyaluronan degradation products produced during lung injury; impaired clearance of hyaluronan results in persistent inflammation. However, hyaluronan fragment stimulation of inflammatory gene expression is not dependent on CD44 in inflammatory macrophages. Instead, hyaluronan fragments utilize both Toll-like receptor (TLR) 4 and TLR2 to stimulate inflammatory genes in macrophages. Hyaluronan also is present on the cell surface of lung alveolar epithelial cells and provides protection against tissue damage by interacting with TLR2 and TLR4 on these parenchymal cells. The simple repeating structure of hyaluronan appears to be involved in a number of important aspects of noninfectious tissue injury and repair that are dependent on the size and location of the polymer as well as the interacting cells. Thus, the interactions between the endogenous matrix component hyaluronan and its signaling receptors initiate inflammatory responses, maintain structural cell integrity, and promote recovery from tissue injury.

PKC-induced stiffening of hyaluronan/CD44 linkage; local force measurements on glioma cells

Interaction of cells with hyaluronan (HA) rich extracellular matrix involves the membrane receptor CD44. HA-CD44 interactions are particularly important in the development of glioma pathogenesis for its implication in tumor cells spreading. Highly motile states rely on the spaciotemporal regulation of HA-CD44 interactions occurring in specific cytoskeletal-supported membrane organization such as microvilli or the leading edge observed in migrating cell. We used AFM-based force measurement to probe the HA-CD44 interaction at localized regions at the surface of living glioma cells expressing high level of the CD44 standard isoform. We show that unstimulated cells interact with HA over their entire surfaces and are highly deformable when force is exerted on individual HA molecules bound to membrane CD44 receptors. Conversely, in PKC-activated cells the probed interactions are concentrated at the leading edge of the cells with reduced membrane deformability. Taken together, our results show that PKC-enhanced motility in glioma cells is associated with a redistribution of CD44 receptors at the leading edges concomitant with a stiffer anchoring of CD44 to the cell surface involving the actin cytoskeleton.

Galectin-9 inhibits CD44-hyaluronan interaction and suppresses a murine model of allergic asthma

RATIONALE

Galectin-9 (Gal-9) belongs to the galectin family, which exhibits affinity for beta-galactosides. Gal-9 has a variety of biological activities; however, its role in allergic inflammation is unknown.

OBJECTIVES

We evaluated the effect of a stable form of the human protein on allergic airway inflammation in a mite allergen-induced asthma model.

METHODS

Human stable Gal-9 was given by intravenous injection to mice during antigen challenge. The effect of Gal-9 on airway inflammation and airway hyperresponsiveness (AHR) was then evaluated.

MEASUREMENTS AND MAIN RESULTS

Gal-9 reduced AHR as well as Th2-associated airway inflammation. Furthermore, administration of Gal-9 as well as anti-CD44 monoclonal antibody inhibited the infiltration of peripheral blood Th2 cells into the airway. Interestingly, Gal-9 directly bound the CD44 adhesion molecule and inhibited interactions with hyaluronan (HA). Consistent with the concept that CD44-HA interactions mediate the migration of T cells into the lung, Gal-9 blocked CD44-dependent adhesion of BW5147 mouse T cells to HA.

CONCLUSIONS

We conclude that Gal-9 inhibits allergic inflammation of the airway and AHR by modulating CD44-dependent leukocyte recognition of the extracellular matrix.

Chronic GvHD decreases antiviral immune responses in allogeneic BMT

Chronic graft-versus-host disease (cGvHD) is associated with functional immunodeficiency and an increased risk of opportunistic infections in allogeneic bone marrow transplantation (BMT). We used a parent to F1 model of allogeneic BMT to test the hypothesis that cGvHD leads to impaired antigen-specific antiviral immunity and compared BM transplant recipients with cGvHD to control groups of allogeneic BM transplant recipients without GvHD. Mice with and without cGvHD received a nonlethal dose of murine cytomegalovirus (MCMV) +100 days after transplantation. Recipients with cGvHD had more weight loss and higher viral loads in the spleen and liver. MCMV infection led to greater than 25-fold expansion of donor spleen-derived MCMV peptide-specific tetramer-positive CD8(+) T cells in blood of transplant recipients with and without cGvHD, but mice with cGvHD had far fewer antigen-specific T cells in peripheral tissues and secondary lymphoid organs. The immunosuppression associated with cGvHD was confirmed by vaccinating transplant recipients with and without cGvHD using a recombinant Listeria expressing MCMV early protein (Lm-MCMV). Secondary adoptive transfer of lymphocytes from donor mice with or without cGvHD into lymphopenic congenic recipients showed that cGvHD impaired tissue-specific homing of antigen-specific T cells. These results indicate that cGvHD causes an intrinsic immunosuppression and explain, in part, the functional immunodeficiency in allogeneic transplant recipients.

In vivo CD44-CD49d complex formation in autoimmune disease has consequences on T cell activation and apoptosis resistance

CD44 is involved in leukocyte migration and activation and has recently been reported to contribute to leukocyte extravasation by associating with CD49d. We explored whether similar changes in CD44 activity are seen in vivo using murine alopecia areata (AA) as a chronic, organ-related autoimmune disease model system. Expression of the activated, hyaluronan-binding form of CD44, and of CD49d, was elevated in draining lymph node cells (LNC) of AA-affected mice as compared to control mice. LNC of AA mice displayed increased motility, proliferative activity and apoptosis resistance, which were equally well inhibited by anti-CD44 and anti-CD49d. The latter is the sequelae of the association between CD44 and CD49d that is seen in activated lymphocytes. Significantly, due to CD44-CD49d complex formation, CD44 gains access to focal adhesion kinase and CD49d gains access to CD44-associated lck and ezrin, such that downstream kinases become activated via CD44 or CD49d engagement. Thus, by their association, CD44 and CD49d mutually avail themselves of the partner's signaling pathways and the ligand binding of each one triggers signaling pathways of both. This strongly influences the lymphocytes' activation state and function.

Urothelial CD44 FacilitatesEscherichia coliInfection of the Murine Urinary Tract

Escherichia coli is the most common pathogen found in urinary tract infections (UTIs), mainly affecting children and women. We report that CD44, a hyaluronic acid (HA) binding protein that mediates cell-cell and cell-matrix interactions, facilitates the interaction of E. coli with urothelial cells and thus the infection of the host. We found that CD44 is constitutively expressed on urothelial cells and that HA accumulates in E. coli-induced UTI. In CD44-deficient mice, the bacterial outgrowth was dramatically less compared with wild-type mice despite similar granulocyte influx in the bladder and in the kidney as well as comparable cytokines/chemokines levels in both genotypes. E. coli was able to bind HA, which adhered to CD44-positive tubular epithelial cells. Most importantly, the interaction of CD44 on tubular epithelial cells with HA facilitated the migration of E. coli through the epithelial monolayer. The results provide evidence that CD44 on urothelial cells facilitates E. coli UTI. Disruption of the interaction between CD44 and HA in the bladder may provide a new approach to prevent and to treat UTI.

CD44 differentially activates mouse NK T cells and conventional T cells

NK T (NKT) cells are an important component of the innate immune system and recognize the MHC class I-like CD1d molecule. NKT cells possess significant immunoregulatory activity due to their rapid secretion of large quantities of pro- and anti-inflammatory cytokines following CD1d-dependent stimulation. Because the innate immune system is programmed to respond to a multitude of diverse stimuli and must be able to quickly differentiate between pathogenic and endogenous signals, we hypothesized that, apart from stimulation via the TCR (e.g., CD1d-dependent activation), there must be multiple activation pathways that can be triggered through other cell surface receptors on NKT cells. Therefore, we analyzed the ability of CD44, a structurally diverse cell surface receptor expressed on most cells, to stimulate murine NKT cells, compared with conventional T cells. Stimulation of CD44 through Ab cross-linking or binding to its natural ligands hyaluronan and osteopontin induced NKT cells to secrete cytokines, up-regulate activation markers, undergo morphological changes, and resist activation-induced cell death, whereas conventional T cells only exhibited changes in morphology and protection from activation-induced cell death. This CD44-specific stimulation of NKT cells correlated with their ability to bind hyaluronan. Thus, fundamental differences in CD44 function between these lymphocyte subsets suggest an important biological role for CD44 in the innate immune response.

Use of CD44 by CD4+ Th1 and Th2 lymphocytes to roll and adhere

Localization of circulating lymphocytes to a site of inflammation is paramount for the development and maintenance of an immune response. In vitro studies using cell lines have previously demonstrated that rolling and adhesion of lymphocytes on endothelium requires CD44 interactions with hyaluronan (HA). To date, whether CD44 has a role in mediating CD4(+)-polarized T-helper 1 (Th1) and Th2 lymphocyte interactions with the endothelium in vivo is yet to be determined. In this study we used intravital microscopy to demonstrate that both Th1 and Th2 lymphocytes use CD44 to roll and adhere to tumor necrosis factor-alpha (TNFalpha)-activated microvasculature. Furthermore, chimeric studies imply that CD44 expression by both the endothelium and lymphocytes is essential for these interactions to occur. HA was also necessary for T cell-endothelial cell interactions in vivo and Th1 and Th2 cells rolled on immobilized HA in vitro via CD44. In vitro, both Th1 and Th2 lymphocytes have increased expression of CD44 and greater binding of fluorescent HA than naive cells. The interactions of Th1 and Th2 cells were entirely dependent upon both P-selectin and CD44 in vivo, but did not appear to be counter ligands in vitro. Taken together, these results suggest that CD44 and HA are key to both Th1 and Th2 lymphocyte interactions with the TNFalpha-activated endothelium and raises the possibility of cooperativity between the P-selectin/PSGL-1 and HA/CD44 pathways for Th1 and Th2 rolling in vivo.

Mouse B cell activation is inhibited by CD44 cross-linking

Lymphocyte activation and trafficking are indispensable to the immune system. CD44 is an adhesion molecule with known importance in T cell activation, lymphocyte trafficking, and tumor metastasis. Although CD44 has been shown to participate in the activation, rolling and adhesion, and homing of T cells, the role of CD44 on B cells is relatively unknown. The effects of CD44 cross-linking on murine B cell activation via CD40L was explored using the anti-CD44 mAbs RK3G9 and IM7. When immobilized on a plate, both RK3G9 and IM7 were found to strongly inhibit B cell proliferation and Ig production, especially at lower cell input concentrations. IgE inhibition was especially prominent. In contrast, soluble RK3G9 added to the B cell cultures had no effect. The inhibitory effect of anti-CD44 on B cell activation was not influenced by the addition of the anti-FcgammaRII, indicating that Fc cross-linking did not play a role in this inhibition. As Ig production requires several days for both B cell proliferation and differentiation to occur, the effects of delayed addition of immobilized anti-CD44 mAbs were studied, and the results indicated no inhibition after 96 hrs of culture. Finally, B cells were activated by either LPS or anti-IgM F(ab')2. While LPS-induced B cell activation was inhibited by immobilized anti-CD44 mAbs, anti-IgM activation was refractory. Interestingly, addition of both anti-IgM and CD40L or LPS resulted in some modulation of the inhibitory activity. These results suggest that CD44 cross-linking could control polyclonal B cell activation by CD40L, but allow sIgM/CD40L activation to continue.

Effect of CD44 deficiency on in vitro and in vivo osteoclast formation

In vitro studies have shown that CD44 is involved in the fusion process of osteoclast precursor cells. Yet, in vivo studies do not support this, since an osteopetrotic phenotype has not been described for CD44 knock-out (CD44 k.o.) mice. This discrepancy may suggest that the role of CD44 in fusion may depend on the microenvironment of osteoclast formation. We investigated osteoclast formation of CD44 k.o. and wild-type mice under three conditions: in vitro, both on plastic and on bone and in vivo by analyzing osteoclast number, and size in long bones from wild-type and CD44 k.o. mice. Bone marrow cells from wild-type and CD44 k.o. mice were analyzed for their capacity to form osteoclasts on plastic and on bone in the presence of macrophage colony stimulating factor (M-CSF) and receptor activator of NF-kB ligand (RANKL). On plastic, the number of multinucleated tartrate resistant acid phosphatase (TRAP) positive cells in CD44 k.o. cultures was twofold higher than in wild-type cultures. On bone, however, equal numbers of osteoclasts were formed. Interestingly, the total number of osteoclasts formed on bone proved to be higher than on plastic for both genotypes, strongly suggesting that osteoclastogenesis was stimulated by the bone surface, and that CD44 is not required for osteoclast formation on bone. Functional analyses showed that bone resorption was similar for both genotypes. We further studied the osteoclastogenic potential of wild-type bone marrow cells in the presence of CD44 blocking antibodies. Osteoclastogenesis was not affected by these antibodies, a further indication that CD44 is not required for the formation of multinucleated cells. Finally, we analyzed the in vivo formation of osteoclasts by analyzing long bones from wild-type and CD44 k.o. mice. Morphometric analysis revealed no difference in osteoclast number, nor in number of nuclei per osteoclasts or in osteoclast size. Our in vitro experiments on plastic showed an enhanced formation of osteoclasts in the absence of CD44, thus suggesting that CD44 has an inhibitory effect on osteoclastogenesis. However, when osteoclasts were generated on bone, no differences in number of multinucleated cells nor in bone resorption were seen. These observations are in agreement with in vivo osteoclast characteristics, where no differences between wild-type and CD44 k.o. bones were encountered. Therefore, the modulating role of CD44 in osteoclast formation appears to depend on the microenvironment.

Suppressor activity and potency among regulatory T cells is discriminated by functionally active CD44

CD4CD25(+) regulatory T cells are fundamental to the maintenance of peripheral tolerance and have great therapeutic potential. However, efforts in this regard have been hampered by limiting cell numbers in vivo, an anergic phenotype in vitro, and a rudimentary understanding of the molecular basis for the functional state of CD4CD25(+) regulatory T cells (Treg cells). Here we show heterogeneity of suppressor activity among activated CD4CD25(+) Treg cells and that, within this population, the functionally active, hyaluronan-binding form of CD44 (CD44(act)) is strikingly correlated with superior suppressor activity. Within 16 hours after in vitro activation, CD44(act) can discriminate enhanced suppressive function in in vitro proliferation assays and in an in vivo bone marrow engraftment model. The expression of other surface markers and that of Foxp3 are similar irrespective of hyaluronan binding and associated degree of suppressor potency. Furthermore, CD44(act) is induced on resting CD4CD25(+) cells in vivo by allogeneic stimulation, with similar functional consequences. These results reveal a cell-surface marker that delineates functional activity within a population of activated CD4CD25(+) regulatory T cells, thereby providing a potential tool for identifying regulatory activity and enriching for maximal suppressor potency.

Regulation of hyaluronan binding by F-actin and colocalization of CD44 and phosphorylated ezrin/radixin/moesin (ERM) proteins in myeloid cells

Proinflammatory cytokines such as TNF-alpha up-regulate the expression of the cell adhesion molecule, CD44, and induce hyaluronan (HA) binding in peripheral blood monocytes (PBM). Here we show that in PBM, TNF-alpha induced cytoskeletal rearrangement, increased threonine phosphorylation of ERM proteins, and induced the redistribution and colocalization of phospho-ERM proteins (P-ERM) with CD44. In the myeloid progenitor cell line, KG1a, hyaluronan binding occurred in the pseudopod where CD44, P-ERM, and F-actin were highly localized. Hyaluronan binding correlated with high expression of both CD44 and P-ERM clustered in a single pseudopod. Disruption of polymerized actin reduced hyaluronan binding in both PBM and KG1a cells and abolished CD44 clustering and the pseudopod in KG1a cells. The pseudopod was not required for the clustering of CD44, the colocalization with P-ERM, or hyaluronan binding. However, treatment with a kinase inhibitor abolished ERM phosphorylation and reduced hyaluronan binding. Furthermore, expression of CD44 lacking the putative ERM binding site resulted in reduced hyaluronan binding. Taken together, these data suggest that CD44-mediated hyaluronan binding in human myeloid cells is regulated by P-ERM and the actin cytoskeleton.

Expression levels and functional aspects of the hyaluronan receptor CD44. Effects of insulin, glucose, IGF-I, or growth hormone on human arterial smooth muscle cells

An increased amount of hyaluronan (HA) in the arterial wall is a feature of the diabetic macroangiopathy. The functional consequences of accumulated HA are mediated through binding to CD44. The regulation of this receptor by diabetic metabolic and hormonal factors is, however unknown. The objective of this study was to examine the influence of glucose, insulin, insulin-like growth factor I (IGF-I), and human growth hormone (hGH) on the formation and function of the HA receptor CD44 in cultures of human aortic smooth muscle cells (SMCs). Migration of nonproliferating SMCs were determined by estimating the area covered by cells 6 days after removal of a barrier. Cellular content of standard CD44 and its isoforms, CD44v3 and CD44v6, and HA-binding capacity were measured using a modified enzyme-linked immunosorbent assay procedure. The analysis is made either with antibodies against CD44 or with HA as a ligand. The migration assay showed that glucose, insulin, and IGF-I were able to stimulate SMC migration (2 P < .01). Anti-CD44 antibody inhibited the stimulated migration at most concentrations. Insulin increased HA binding at 100 to 1000 micro U/mL insulin (2 P < .03). CD44 expression was only elevated at 1000 micro U/mL insulin (2 P < .03), whereas CD44 content decreased at 2 ng/mL hGH and increased at 16 ng/mL hGH (2 P < .01). Glucose and IGF-I reduced the amount of the variant isoform CD44v3 (2 P < .01) but did not change the amount of total CD44. CD44v6 was not present on human arterial SMCs. In conclusion, the present data obtained with human arterial SMCs in vitro support a role of CD44 and its isoform, CD44v3, in the SMC response to the metabolic and hormonal disorders of diabetes.

The regulation and activation of CD44 by natural killer (NK) cells and its role in the production of IFN-gamma

Natural killer (NK) cells can express high levels of CD44, and signaling through CD44 has been shown to enhance NK cell cytotoxic activity. However, little is known about the factors that regulate CD44-mediated activation of NK cells. The studies reported here reveal that resting NK cells constitutively express CD44 that is in an inactive form that does not bind to hyaluronan (HA), the principal known ligand for CD44. After infection of mice with the intracellular parasite Toxoplasma gondii, however, a population of NK cells that expressed activated CD44 emerged. To determine how expression and activation of CD44 by resting NK cells were regulated, the role of cytokines in these events was assessed. These studies revealed that whereas stimulation of resting NK cells with interleukin-12 (IL-12) or IL-18 caused increased expression of CD44, only IL-2 or IL-15 led to the upregulation and activation of CD44. The cytokine-induced upregulation and activation of CD44 was independent of NK cell proliferation. To determine the functional consequences of CD44 activation, the effects of low molecular weight HA (LMWHA) on the production of interferon-gamma (IFN-gamma) by IL-2-activated NK cells were assessed. These studies showed that HA alone had little effect on the production of IFN-gamma, but when used in combination with IL-2, IL-12, or IL-18, LMWHA was a potent enhancer of IFN-gamma production. Together, these studies indicate an important role for proinflammatory cytokines in the activation of CD44 on NK cells and identify a novel pathway to enhance the ability of activated NK cells to produce IFN-gamma.