Monoclonal antibodies to CD44 and their influence on hyaluronan recognition

CD44 correlates with longevity and enhances basal ATF6 activity and ER stress resistance

The naked mole rat (NMR) is the longest-lived rodent, resistant to multiple age-related diseases including neurodegeneration. However, the mechanisms underlying the NMR's resistance to neurodegenerative diseases remain elusive. Here, we isolated oligodendrocyte progenitor cells (OPCs) from NMRs and compared their transcriptome with that of other mammals. Extracellular matrix (ECM) genes best distinguish OPCs of long- and short-lived species. Notably, expression levels of CD44, an ECM-binding protein that has been suggested to contribute to NMR longevity by mediating the effect of hyaluronan (HA), are not only high in OPCs of long-lived species but also positively correlate with longevity in multiple cell types/tissues. We found that CD44 localizes to the endoplasmic reticulum (ER) and enhances basal ATF6 activity. CD44 modifies proteome and membrane properties of the ER and enhances ER stress resistance in a manner dependent on unfolded protein response regulators without the requirement of HA. HA-independent role of CD44 in proteostasis regulation may contribute to mammalian longevity.

Critical Involvement of CD44 in T Helper Type 2 Cell-Mediated Eosinophilic Airway Inflammation in a Mouse Model of Acute Asthma

Interactions between CD44 and hyaluronan (HA) are crucial for recruiting leukocytes to inflamed tissues. This review summarizes findings from our studies of the roles of CD44-HA interactions in leukocyte trafficking, with a particular focus on airway T helper type 2 (Th2) cells in mouse models of acute asthma. In a mite allergen-induced model of acute asthma, intraperitoneal injection of anti-CD44 monoclonal antibodies blocked lymphocytes and eosinophils from accumulating in the lung, and suppressed both the antigen-induced increase in Th2 cytokines in the bronchoalveolar lavage fluid (BALF) and airway hyperresponsiveness (AHR). CD44 deficiency was associated with decreased mite allergen-induced Th2 cell-mediated airway inflammation and AHR in sensitized mice. Asthmatic responses to antigen-sensitized splenic CD4+ T cells transferred from CD44-deficient mice were weaker than in wild-type mice. Administration of anti-CD44 monoclonal antibodies preferentially suppressed the airway accumulation of antigen-specific Th2 cells induced by antigen challenge, without affecting Th1 and Th17 cells. Increased HA-binding ability of CD44 and expression of Neu1 sialidase were observed on antigen-specific Th2 cells compared with antigen-specific Th1 and Th17 cells. Finally, in a mouse model of acute asthma, neuraminidase 1-deficient SM/J mice exhibited a lower Th2 cytokine concentration and a lower absolute Th2 cell number in the BALF, as well as an attenuated AHR. Our findings indicate that CD44 critically contributes to the antigen challenge-induced airway accumulation of antigen-specific Th2 cells, without affecting Th1 and Th17 cells, in mice. Furthermore, neuraminidase 1 activity is necessary for the interaction between HA and CD44, and Th2 cell-mediated airway inflammation.

The role of CD44 in cancer chemoresistance: A concise review

CD44 is a cell surface adhesion molecule, which is overexpressed on cancer stem cells. The interaction of CD44 with hyaluronan is responsible for tumor development, metastasis, and expression of the chemoresistant phenotype. The overexpression of CD44 impedes the cytotoxic effect of chemotherapy medications in various cancers. Therefore, the high expression of CD44 is associated with a poor prognosis in affected patients. This high expression of CD44 in various cancers has provided an ample opportunity for the treatment of patients with chemoresistant malignancy. This review aims to demonstrate the various cross-talk between CD44 and intracellular and extracellular factors and highlight its role in developing chemoresistant tumors in some troublesome cancers.

Distinct effects of chondroitin sulfate on hematopoietic cells and the stromal microenvironment in bone marrow hematopoiesis

The bone marrow (BM) microenvironment, known as the BM niche, regulates hematopoiesis but is also affected by interactions with hematopoietic cells. Recent evidence indicates that extracellular matrix components are involved in these interactions. Chondroitin sulfate (CS), a glycosaminoglycan, is a major component of the extracellular matrix; however, it is not known whether CS has a physiological role in hematopoiesis. Here, we analyzed the functions of CS in hematopoietic and niche cells. CSGalNAcT1, which encodes CS N-acetylgalactosaminyltransferase-1 (T1), a key enzyme in CS biosynthesis, was highly expressed in hematopoietic stem and progenitor cells (HSPCs) and endothelial cells (ECs), but not in mesenchymal stromal cells (MSCs) in BM. In T1 knockout (T1KO) mice, a greater number of HSPCs existed compared with the wild-type (WT), but HSPCs from T1KO mice showed significantly impaired repopulation in WT recipient mice on serial transplantation. RNA sequence analysis revealed the activation of IFN-α/β signaling and endoplasmic reticulum stress in T1KO HSPCs. In contrast, the number of WT HSPCs repopulated in T1KO recipient mice was larger than that in WT recipient mice after serial transplantation, indicating that the T1KO niche supports repopulation of HSPCs better than the WT niche. There was no obvious difference in the distribution of vasculature and MSCs between WT and T1KO BM, suggesting that CS loss alters vascular niche functions without affecting its structure. Our results revealed distinct roles of CS in hematopoietic cells and BM niche, indicating that crosstalk between these components is important to maintain homeostasis in BM.

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.

CD44 Receptor Mediates Urate Crystal Phagocytosis by Macrophages and Regulates Inflammation in A Murine Peritoneal Model of Acute Gout

Gout is a chronic arthritis caused by the deposition of poorly soluble monosodium urate monohydrate (MSU) crystals in peripheral joints. Resident macrophages initiate inflammation in response to MSU mediated by NF-κB nuclear translocation and NLRP3 inflammasome activation. We investigated the role of CD44, a transmembrane receptor, in mediating MSU phagocytosis by macrophages. We used an antibody that sheds the extracellular domain (ECD) of CD44 to study the role of the receptor and its associated protein phosphatase 2A (PP2A) in macrophage activation. We also studied the significance of CD44 in mediating MSU inflammation in-vivo. Cd44^−/− BMDMs showed reduced MSU phagocytosis, LDH release, IL-1β expression and production compared to Cd44^+/+ BMDMs. Elevated CD44 staining was detected intracellularly and CD44 colocalized with α-tubulin as a result of MSU exposure and ECD-shedding reduced MSU phagocytosis in murine and human macrophages. Anti-CD44 antibody treatment reduced NF-κB p65 subunit nuclear levels, IL-1β expression, pro-IL-1β and IL-8 production in MSU stimulated THP-1 macrophages (p < 0.01). The effect of the antibody was mediated by an enhancement in PP2A activity. CD44 ECD-shedding reduced the conversion of procaspase-1 to active caspase-1, caspase-1 activity and resultant generation of mature IL-1β in macrophages. Neutrophil and monocyte influx and upregulated production of IL-1β was evident in wildtype mice. MSU failed to trigger neutrophil and monocyte recruitment in Cd44^−/− mice and lower IL-1β levels were detected in peritoneal lavages from Cd44^−/− mice (p < 0.01). Anti-CD44 antibody treatment reduced neutrophil and monocyte recruitment and resulted in reduced lavage IL-1β levels in the same model. CD44 plays a biologically significant role in mediating phagocytosis of MSU and downstream inflammation and is a novel target in gout treatment.

Respiratory Syncytial Virus Infection of Human Lung Fibroblasts Induces a Hyaluronan-Enriched Extracellular Matrix That Binds Mast Cells and Enhances Expression of Mast Cell Proteases

Human lung fibroblasts (HLFs) treated with the viral mimetic polyinosine-polycytidylic acid (poly I:C) form an extracellular matrix (ECM) enriched in hyaluronan (HA) that avidly binds monocytes and lymphocytes. Mast cells are important innate immune cells in both asthma and acute respiratory infections including respiratory syncytial virus (RSV); however, the effect of RSV on HA dependent mast cell adhesion and/or function is unknown. To determine if RSV infection of HLFs leads to the formation of a HA-enriched ECM that binds and enhances mast cell activity primary HLFs were infected with RSV for 48 h prior to leukocyte binding studies using a fluorescently labeled human mast cell line (LUVA). Parallel HLFs were harvested for characterization of HA production by ELISA and size exclusion chromatography. In separate experiments, HLFs were infected as above for 48 h prior to adding LUVA cells to HLF wells. Co-cultures were incubated for 48 h at which point media and cell pellets were collected for analysis. The role of the hyaladherin tumor necrosis factor-stimulated gene 6 (TSG-6) was also assessed using siRNA knockdown. RSV infection of primary HLFs for 48 h enhanced HA-dependent LUVA binding assessed by quantitative fluorescent microscopy. This coincided with increased HLF HA synthase (HAS) 2 and HAS3 expression and decreased hyaluronidase (HYAL) 2 expression leading to increased HA accumulation in the HLF cell layer and the presence of larger HA fragments. Separately, LUVAs co-cultured with RSV-infected HLFs for 48 h displayed enhanced production of the mast cell proteases, chymase, and tryptase. Pre-treatment with the HA inhibitor 4-methylumbelliferone (4-MU) and neutralizing antibodies to CD44 (HA receptor) decreased mast cell protease expression in co-cultured LUVAs implicating a direct role for HA. TSG-6 expression was increased over the 48-h infection. Inhibition of HLF TSG-6 expression by siRNA knockdown led to decreased LUVA binding suggesting an important role for this hyaladherin for LUVA adhesion in the setting of RSV infection. In summary, RSV infection of HLFs contributes to inflammation via HA-dependent mechanisms that enhance mast cell binding as well as mast cell protease expression via direct interactions with the ECM.

Hyaluronan in immune dysregulation and autoimmune diseases

The tissue microenvironment contributes to local immunity and to the pathogenesis of autoimmune diseases - a diverse set of conditions characterized by sterile inflammation, immunity against self-antigens, and destruction of tissues. However, the specific factors within the tissue microenvironment that contribute to local immune dysregulation in autoimmunity are poorly understood. One particular tissue component implicated in multiple autoimmune diseases is hyaluronan (HA), an extracellular matrix (ECM) polymer. HA is abundant in settings of chronic inflammation and contributes to lymphocyte activation, polarization, and migration. Here, we first describe what is known about the size, amount, and distribution of HA at sites of autoimmunity and in associated lymphoid structures in type 1 diabetes, multiple sclerosis, and rheumatoid arthritis. Next, we examine the recent literature on HA and its impact on adaptive immunity, particularly in regards to the biology of lymphocytes and Foxp3+ regulatory T-cells (Treg), a T-cell subset that maintains immune tolerance in healthy individuals. We propose that HA accumulation at sites of chronic inflammation creates a permissive environment for autoimmunity, characterized by CD44-mediated inhibition of Treg expansion. Finally, we address potential tools and strategies for targeting HA and its receptor CD44 in chronic inflammation and autoimmunity.

Transforming growth factor-β-mediated CD44/STAT3 signaling contributes to the development of atrial fibrosis and fibrillation

Atrial fibrillation (AF) is associated with atrial fibrosis. Inhibition of atrial fibrosis might be a plausible approach for AF prevention and therapy. This study is designed to evaluate the potential role of CD44, a membrane receptor known to regulate fibrosis, and its related signaling in the pathogenesis of atrial fibrosis and AF. Treatment of cultured rat atrial fibroblasts with transforming growth factor-β (TGF-β, a key mediator of atrial fibrosis) led to a higher expression of hyaluronan (HA), CD44, STAT3, and collagen (a principal marker of fibrosis) than that of ventricular fibroblasts. In vivo, TGF-β transgenic mice and AF patients exhibited a greater expression of HA, CD44, STAT3, and collagen in their atria than wild-type mice and sinus rhythm subjects, respectively. Treating TGF-β transgenic mice with an anti-CD44 blocking antibody resulted in a lower expression of STAT3 and collagen in their atria than those with control IgG antibody. Programmed stimulation triggered less AF episodes in TGF-β transgenic mice treated with anti-CD44 blocking antibody than in those with control IgG. Blocking CD44 signaling with anti-CD44 antibody and mutated CD44 plasmids attenuated TGF-β-induced STAT3 activation and collagen expression in cultured atrial fibroblasts. Deletion and mutational analysis of the collagen promoter along with chromatin immunoprecipitation demonstrated that STAT3 served as a vital transcription factor in collagen expression. TGF-β-mediated HA/CD44/STAT3 pathway plays a crucial role in the development of atrial fibrosis and AF. Blocking CD44-dependent signaling may be a feasible way for AF management.

Hyaluronan Nanoparticles Selectively Target Plaque-Associated Macrophages and Improve Plaque Stability in Atherosclerosis

Hyaluronan is a biologically active polymer, which can be formulated into nanoparticles. In our study, we aimed to probe atherosclerosis-associated inflammation by using hyaluronan nanoparticles and to determine whether they can ameliorate atherosclerosis. Hyaluronan nanoparticles (HA-NPs) were prepared by reacting amine-functionalized oligomeric hyaluronan (HA) with cholanic ester and labeled with a fluorescent or radioactive label. HA-NPs were characterized in vitro by several advanced microscopy methods. The targeting properties and biodistribution of HA-NPs were studied in apoe^-/- mice, which received either fluorescent or radiolabeled HA-NPs and were examined ex vivo by flow cytometry or nuclear techniques. Furthermore, three atherosclerotic rabbits received ⁸⁹Zr-HA-NPs and were imaged by PET/MRI. The therapeutic effects of HA-NPs were studied in apoe^-/- mice, which received weekly doses of 50 mg/kg HA-NPs during a 12-week high-fat diet feeding period. Hydrated HA-NPs were ca. 90 nm in diameter and displayed very stable morphology under hydrolysis conditions. Flow cytometry revealed a 6- to 40-fold higher uptake of Cy7-HA-NPs by aortic macrophages compared to normal tissue macrophages. Interestingly, both local and systemic HA-NP-immune cell interactions significantly decreased over the disease progression. ⁸⁹Zr-HA-NPs-induced radioactivity in atherosclerotic aortas was 30% higher than in wild-type controls. PET imaging of rabbits revealed 6-fold higher standardized uptake values compared to the muscle. The plaques of HA-NP-treated mice contained 30% fewer macrophages compared to control and free HA-treated group. In conclusion, we show favorable targeting properties of HA-NPs, which can be exploited for PET imaging of atherosclerosis-associated inflammation. Furthermore, we demonstrate the anti-inflammatory effects of HA-NPs in atherosclerosis.

Lubricin/Proteoglycan 4 Binding to CD44 Receptor: A Mechanism of the Suppression of Proinflammatory Cytokine-Induced Synoviocyte Proliferation by Lubricin

OBJECTIVE

To evaluate the binding of recombinant human proteoglycan 4 (rhPRG4) to CD44 receptor and its consequences on cytokine-induced synoviocyte proliferation.

METHODS

The binding of rhPRG4 to CD44 and competition with high molecular weight (HMW) hyaluronic acid (HA) was evaluated using a direct enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance. Sialidase A and O-glycosidase digestion of rhPRG4 was performed, and CD44 binding was evaluated using ELISA. Rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS) were stimulated with interleukin-1β (IL-1β) or tumor necrosis factor α (TNFα) for 48 hours in the presence or absence of rhPRG4 or HMW HA at 20, 40, and 80 μg/ml, and cell proliferation was measured. The contribution of CD44 was assessed by coincubation with a CD44 antibody (IM7). The antiproliferative effect of rhPRG4 was investigated following treatment of PRG4(-/-) mouse synoviocytes with IL-1β or TNFα in the presence or absence of IM7.

RESULTS

Recombinant human PRG4 bound CD44 and interfered with the binding of HMW HA to CD44. Removal of sialic acid and O-glycosylations significantly increased CD44 binding by rhPRG4 (P < 0.001). Both rhPRG4 and HMW HA at 40 and 80 μg/ml significantly suppressed IL-1β-induced proliferation of RA FLS (P < 0.05). Recombinant human PRG4 at 20, 40, and 80 μg/ml significantly suppressed TNFα-induced RA FLS proliferation (P < 0.05). CD44 neutralization reversed the effect of rhPRG4 on IL-1β- and TNFα-stimulated RA FLS and the effect of HMW HA on IL-1β-stimulated RA FLS. Recombinant human PRG4 inhibited cytokine-induced proliferation of PRG4(-/-) synoviocytes, which could be prevented by blocking CD44.

CONCLUSION

PRG4 (lubricin) is a novel putative ligand for CD44 and may control synoviocyte overgrowth in inflammatory arthropathies via a CD44-mediated mechanism.

Immunosurveillance of the liver by intravascular effector CD8(+) T cells

Effector CD8(+) T cells (CD8 TE) play a key role during hepatotropic viral infections. Here, we used advanced imaging in mouse models of hepatitis B virus (HBV) pathogenesis to understand the mechanisms whereby these cells home to the liver, recognize antigens, and deploy effector functions. We show that circulating CD8 TE arrest within liver sinusoids by docking onto platelets previously adhered to sinusoidal hyaluronan via CD44. After the initial arrest, CD8 TE actively crawl along liver sinusoids and probe sub-sinusoidal hepatocytes for the presence of antigens by extending cytoplasmic protrusions through endothelial fenestrae. Hepatocellular antigen recognition triggers effector functions in a diapedesis-independent manner and is inhibited by the processes of sinusoidal defenestration and capillarization that characterize liver fibrosis. These findings reveal the dynamic behavior whereby CD8 TE control hepatotropic pathogens and suggest how liver fibrosis might reduce CD8 TE immune surveillance toward infected or transformed hepatocytes.

The where, when, how, and why of hyaluronan binding by immune cells

Hyaluronan is made and extruded from cells to form a pericellular or extracellular matrix (ECM) and is present in virtually all tissues in the body. The size and form of hyaluronan present in tissues are indicative of a healthy or inflamed tissue, and the interactions of hyaluronan with immune cells can influence their response. Thus, in order to understand how inflammation is regulated, it is necessary to understand these interactions and their consequences. Although there is a large turnover of hyaluronan in our bodies, the large molecular mass form of hyaluronan predominates in healthy tissues. Upon tissue damage and/or infection, the ECM and hyaluronan are broken down and an inflammatory response ensues. As inflammation is resolved, the ECM is restored, and high molecular mass hyaluronan predominates again. Immune cells encounter hyaluronan in the tissues and lymphoid organs and respond differently to high and low molecular mass forms. Immune cells differ in their ability to bind hyaluronan and this can vary with the cell type and their activation state. For example, peritoneal macrophages do not bind soluble hyaluronan but can be induced to bind after exposure to inflammatory stimuli. Likewise, naïve T cells, which typically express low levels of the hyaluronan receptor, CD44, do not bind hyaluronan until they undergo antigen-stimulated T cell proliferation and upregulate CD44. Despite substantial knowledge of where and when immune cells bind hyaluronan, why immune cells bind hyaluronan remains a major outstanding question. Here, we review what is currently known about the interactions of hyaluronan with immune cells in both healthy and inflamed tissues and discuss how hyaluronan binding by immune cells influences the inflammatory response.

Clodronate treatment significantly depletes macrophages in chickens

Macrophages function as phagocytes and antigen-presenting cells in the body. As has been demonstrated in mammals, administration of clodronate [dichloromethylene bisphosphonate (Cl2MBP)] encapsulated liposomes results in depletion of macrophages. Although this compound has been used in chickens, its effectiveness in depleting macrophages has yet to be fully determined. Here, we show that a single administration of clodronate liposomes to chickens results in a significant depletion of macrophages within the spleen and lungs of chickens up to 4 d post-treatment. This finding suggests that, in order to obtain depletion of macrophages in chickens for greater than 5 d, it is necessary to administer clodronate liposomes 4 d apart. The study also showed that 2 treatments of clodronate liposomes at 4-day intervals resulted in the depletion of macrophages for up to 10 d. The findings of the present study will encourage more precise studies to be done on the potential roles of macrophages in immune responses and in the pathogenesis of microbial infections in chickens.

Osteopontin facilitates ultraviolet B-induced squamous cell carcinoma development

BACKGROUND

Osteopontin (OPN) is a matricellular glycoprotein that is markedly expressed in cutaneous squamous cell carcinomas (cSCCs) and in actinic keratoses implicating its role in photocarcinogenesis.

OBJECTIVE

To determine whether OPN facilitates the development of cSCC and its function.

METHODS

cSCCs development was compared between wild-type (WT) and OPN-null mice subjected to UVB irradiation for 43 weeks. UVB-induced OPN expression was determined by Western blot, immunoprecipitation, ELISA, and semi-quantitative RT-PCR. Epidermal layer and TUNEL analyses assessed if OPN mediates UVB-induced epidermal hyperplasia or suppresses UVB-induced apoptosis of basal keratinocytes, respectively. In vitro experiments determined whether OPN enhances cell survival of UVB-induced apoptosis and its potential mechanisms. Immunohistochemical analyses of epidermis assessed the expression of CD44 and focal adhesion kinase (FAK), molecules that mediate OPN survival function.

RESULTS

Compared to female WT mice, OPN-null mice did not develop cSCCs. UVB irradiation stimulated OPN protein expression in the dorsal skin by 11h and remains high at 24-48h. OPN did not mediate UVB-induced epidermal hyperplasia; instead, it protected basal keratinocytes from undergoing apoptosis upon UVB exposure. Likewise, the addition of OPN suppressed UVB-induced OPN-null cSCC cell apoptosis, the activation of caspase-9 activity, and increased phosphorylation of FAK at Y397. Furthermore, the expression of CD44 and FAK in WT mice epidermis was greater than that of OPN-null mice prior to and during early acute UVB exposure.

CONCLUSION

These data support the hypothesis that chronic UVB-induced OPN expression protects the survival of initiated basal keratinocytes and, consequently, facilitates cSCC develop.

Opposing effects of high- and low-molecular weight hyaluronan on CXCL12-induced CXCR4 signaling depend on CD44

The tumor microenvironment makes a decisive contribution to the development and dissemination of cancer, for example, through extracellular matrix components such as hyaluronan (HA), and through chemokines that regulate tumor cell behavior and angiogenesis. Here we report a molecular link between HA, its receptor CD44 and the chemokine CXCL12 in the regulation of cell motility and angiogenesis. High-molecular-weight HA (hHA) was found to augment CXCL12-induced CXCR4 signaling in both HepG2iso cells and primary human umbilical vein endothelial cells, as evidenced by enhanced ERK phosphorylation and increased cell motility. The augmentation of CXCR4 signaling translated into increased vessel sprouting and angiogenesis in a variety of assays. Small HA oligosaccharides (sHA) efficiently inhibited these effects. Both siRNA-mediated reduction of CD44 expression and antibodies that block the interaction of CD44 with HA provided evidence that CXCL12-induced CXCR4 signaling depends on the binding of hHA to CD44. Consistently, CD44 and CXCR4 were found to physically interact in the presence of CXCL12, an interaction that could be inhibited by sHA. These findings provide novel insights into how microenvironmental components interact with cell surface receptors in multi-component complexes to regulate key aspects of tumor growth and progression.

CD44 antibodies and immune thrombocytopenia in the amelioration of murine inflammatory arthritis

Antibodies to CD44 have been used to successfully ameliorate murine models of autoimmune disease. The most often studied disease model has been murine inflammatory arthritis, where a clear mechanism for the efficacy of CD44 antibodies has not been established. We have recently shown in a murine passive-model of the autoimmune disease immune thrombocytopenia (ITP) that some CD44 antibodies themselves can induce thrombocytopenia in mice, and the CD44 antibody causing the most severe thrombocytopenia (IM7), also is known to be highly effective in ameliorating murine models of arthritis. Recent work in the K/BxN serum-induced model of arthritis demonstrated that antibody-induced thrombocytopenia reduced arthritis, causing us to question whether CD44 antibodies might primarily ameliorate arthritis through their thrombocytopenic effect. We evaluated IM7, IRAWB14.4, 5035-41.1D, KM201, KM114, and KM81, and found that while all could induce thrombocytopenia, the degree of protection against serum-induced arthritis was not closely related to the length or severity of the thrombocytopenia. CD44 antibody treatment was also able to reverse established inflammation, while thrombocytopenia induced by an anti-platelet antibody targeting the GPIIbIIIa platelet antigen, could not mediate this effect. While CD44 antibody-induced thrombocytopenia may contribute to some of its therapeutic effect against the initiation of arthritis, for established disease there are likely other mechanisms contributing to its efficacy. Humans are not known to express CD44 on platelets, and are therefore unlikely to develop thrombocytopenia after CD44 antibody treatment. An understanding of the relationship between arthritis, thrombocytopenia, and CD44 antibody treatment remains critical for continued development of CD44 antibody therapeutics.

CD44 regulates the apoptotic response and promotes disease development in chronic lymphocytic leukemia

The cell-surface glycoprotein CD44 is expressed in chronic lymphocytic leukemia (CLL), but its functional role in this disease is poorly characterized. We therefore investigated the contribution of CD44 to CLL in a murine disease model, the Eµ-TCL1 transgenic mouse, and in CLL patients. Surface CD44 increased during murine CLL development. CD44 expression in human CLL was induced by stimulation with interleukin 4/soluble CD40 ligand and by stroma cell contact. Engagement of CD44 by its natural ligands, hyaluronic acid or chondroitin sulfate, protected CLL cells from apoptosis, while anti-CD44 small interfering RNAs impaired tumor cell viability. Deletion of CD44 during TCL1-driven murine leukemogenesis reduced the tumor burden in peripheral blood and spleen and led to a prolonged overall survival. The leukemic cells from these CD44 knockout animals revealed lower levels of antiapoptotic MCL1, a higher propensity to apoptosis, and a diminished B-cell receptor kinase response. The inhibitory anti-CD44 antibodies IM7 and A3D8 impaired the viability of CLL cells in suspension cultures, in stroma contact models, and in vivo via MCL1 reduction and by effector caspase activation. Taken together, CD44 expression in CLL is mediated by the tumor microenvironment. As a coreceptor, CD44 promotes leukemogenesis by regulating stimuli of MCL1 expression. Moreover, CD44 can be addressed therapeutically in CLL by specific antibodies.

Regulation of Antigen-Experienced T Cells: Lessons from the Quintessential Memory Marker CD44

Despite the widespread use of the cell-surface receptor CD44 as a marker for antigen (Ag)-experienced, effector and memory T cells, surprisingly little is known regarding its function on these cells. The best-established function of CD44 is the regulation of cell adhesion and migration. As such, the interactions of CD44, primarily with its major ligand, the extracellular matrix (ECM) component hyaluronic acid (HA), can be crucial for the recruitment and function of effector and memory T cells into/within inflamed tissues. However, little is known about the signaling events following engagement of CD44 on T cells and how cooperative interactions of CD44 with other surface receptors affect T cell responses. Recent evidence suggests that the CD44 signaling pathway(s) may be shared with those of other adhesion receptors, and that these provide contextual signals at different anatomical sites to ensure the correct T cell effector responses. Furthermore, CD44 ligation may augment T cell activation after Ag encounter and promote T cell survival, as well as contribute to regulation of the contraction phase of an immune response and the maintenance of tolerance. Once the memory phase is established, CD44 may have a role in ensuring the functional fitness of memory T cells. Thus, the summation of potential signals after CD44 ligation on T cells highlights that migration and adhesion to the ECM can critically impact the development and homeostasis of memory T cells, and may differentially affect subsets of T cells. These aspects of CD44 biology on T cells and how they might be modulated for translational purposes are discussed.

CD44 is critical for airway accumulation of antigen-specific Th2, but not Th1, cells induced by antigen challenge in mice

CD44 is a cell adhesion molecule involved in lymphocyte infiltration of inflamed tissues. We previously demonstrated that CD44 plays an important role in the development of airway inflammation in a murine model of allergic asthma. In this study, we investigated the role of CD44 expressed on CD4(+) T cells in the accumulation of T-helper type 2 (Th2) cells in the airway using CD44-deficient mice and anti-CD44 monoclonal antibodies. Antigen-induced Th2-mediated airway inflammation and airway hyperresponsiveness (AHR) in sensitized mice were reduced by CD44-deficiency. These asthmatic responses induced by the transfer of antigen-sensitized splenic CD4(+) T cells from CD44-deficient mice were weaker than those from WT mice. Lack of CD44 failed to induce AHR by antigen challenge. Expression level and hyaluronic acid receptor activity of CD44, as well as Neu1 sialidase expression on antigen-specific Th2 cells, were higher than those on antigen-specific Th1 cells. Anti-CD44 antibody preferentially suppressed the accumulation of those Th2 cells in the airway induced by antigen challenge. Our findings indicate that CD44 expressed on CD4(+) T cells plays a critical role in the accumulation of antigen-specific Th2 cells, but not Th1 cells, in the airway and in the development of AHR induced by antigen challenge.

Physiological function of hyaluronan in mammalian oocyte maturation

Despite its structural simplicity, hyaluronan exhibits a broad spectrum of biological activities. Cumulus expansion observed during oocyte maturation in mammals is also induced by hyaluronan accumulation in cumulus-oocyte complexes. It has been demonstrated that this volumetric change in cumulus-oocyte complexes correlates with the progression of oocyte maturation. We have investigated the molecular mechanism of oocyte maturation in mammals, focusing on hyaluronan accumulation in cumulus-oocyte complexes during cumulus expansion. In this review, we describe the physiological function of hyaluronan, emphasizing the progression of oocyte maturation in mammals based on our previous findings.

Anti-inflammatory protein TSG-6 secreted by activated MSCs attenuates zymosan-induced mouse peritonitis by decreasing TLR2/NF-κB signaling in resident macrophages

Human mesenchymal stem/progenitor cells (hMSCs) repair tissues and modulate immune systems but the mechanisms are not fully understood. We demonstrated that hMSCs are activated by inflammatory signals to secrete the anti-inflammatory protein, TNF-α-stimulated gene 6 protein (TSG-6) and thereby create a negative feedback loop that reduces inflammation in zymosan-induced peritonitis. The results demonstrate for the first time that TSG-6 interacts through the CD44 receptor on resident macrophages to decrease zymosan/TLR2-mediated nuclear translocation of the NF-κB. The negative feedback loop created by MSCs through TSG-6 attenuates the inflammatory cascade that is initiated by resident macrophages and then amplified by mesothelial cells and probably other cells of the peritoneum. Because inflammation underlies many pathologic processes, including immune responses, the results may explain the beneficial effects of MSCs and TSG-6 in several disease models.

Modulation of hyaluronan production by CD44 positive glioma cells

This study examines the functional relationship between glioma cell production of hyaluronan (HA), known to play a role in glioma invasion, expression of its CD44 receptor, and glioma cell viability. Production of HA by CD44 positive mouse G26 and human U373 glioma cell lines was evaluated and compared to that of a CD44 positive mouse fibroblast-like L929 cell line. We found that both G26 and U373 MG glioma cells, but not L929 fibroblast-like cells, synthesized HA. The synthesis of HA by glioma cells was found during the proliferative phase as well as post-confluency, as detected by fluorophore-assisted carbohydrate electrophoresis. Eighty to ninety percent of the HA synthesized was secreted into the medium and 10-20% remained associated with the cells. To examine a possible mechanistic link between the CD44-HA interaction and endogenous HA production, glioma cells were treated with either anti-CD44 antibodies (clones KM201 or IM7) or HA oligosaccharides (hexamer oligoHA-6 or decamer oligoHA-10). We found that oligoHA-10, which was previously shown to compete effectively with the CD44-HA interaction, enhanced glioma HA synthesis by approximately 1.5-fold, without affecting cell viability. IM7 treatment of human U373 glioma cells resulted in over 50% decrease of HA production, which was associated with changes in cell size and apoptosis. Taken together, these data show that CD44 specific ligands, such as the IM7 antibody or oligoHA-10 could down-regulate or up-regulate glioma HA production, respectively. Our results suggest that interference with CD44/HA may lead to the discovery and development of new treatment modalities for glioma.

Involvement of the MEKK1 signaling pathway in the regulation of epicardial cell behavior by hyaluronan

During embryonic development, cells comprising the outermost layer of the heart or epicardium play a critical role in the formation of the coronary vasculature. Thus, uncovering the molecular mechanisms that govern epicardial cell behavior is imperative to better understand the etiology of cardiovascular diseases. In this study, we investigated the function of hyaluronan (HA), a major component of the extracellular matrix, in the modulation of epicardial signaling. We show that stimulation of epicardial cells with high molecular weight HA (HMW-HA) promotes the association of MEKK1 with the HA receptor CD44 and induces MEKK1 phosphorylation. This leads to the activation of two distinct pathways, one ERK-dependent and another NFkappaB-dependent. Furthermore, HMW-HA stimulates epicardial cells to differentiate and invade, as suggested by increased vimentin expression and enhanced invasion through a collagen matrix. Blockade of CD44, transfection with a kinase-inactive MEKK1 construct or the use of ERK1/2 and NFkappaB inhibitors significantly abrogates the invasive response to HMW-HA. Together, these findings suggest an important role for HA in the regulation of epicardial cell fate via activation of MEKK1 signaling cascades.

Targeting CD44 in mast cell regulation

IMPORTANCE OF THE FIELD

Accumulating evidence suggests that mast cells are involved in a wide variety of immune responses including chronic inflammation, immune tolerance and tumor immunity. Mast cells originate from hematopoietic stem cells and undergo terminal differentiation in the tissues, in which they are ultimately resident. Heterogeneity of tissue mast cells is, therefore, one of the key concepts for a better understanding of various immune responses.

AREAS COVERED IN THIS REVIEW

This review describes the candidate genes involved in regulation of cutaneous mast cell differentiation, with a particular attention to CD44, which is the primary receptor for hyaluronan.

WHAT THE READER WILL GAIN

CD44 is involved in various aspects of cutaneous inflammation. Regarding mast cells, CD44 is upregulated upon differentiation and maturation of mast cells, and plays a critical role in regulation of cutaneous mast cell number. Since both degradation and decrease of hyaluronan are often observed upon chronic inflammation, CD44 might be involved in modulation of local immune responses through regulation of cutaneous mast cell functions.

TAKE HOME MESSAGE

Understanding of cutaneous immune responses should require clarification of local mast cell functions, a part of which is regulated by extracellular matrix components and their membrane receptors.

CD44 regulates survival and memory development in Th1 cells

Optimal immunity to microorganisms depends upon the regulated death of clonally expanded effector cells and the survival of a cohort of cells that become memory cells. After activation of naive T cells, CD44, a widely expressed receptor for extracellular matrix components, is upregulated. High expression of CD44 remains on memory cells and despite its wide usage as a "memory marker," its function is unknown. Here we report that CD44 was essential for the generation of memory T helper 1 (Th1) cells by promoting effector cell survival. This dependency was not found in Th2, Th17, or CD8(+) T cells despite similar expression of CD44 and the absence of splice variants in all subsets. CD44 limited Fas-mediated death in Th1 cells and its ligation engaged the phosphoinositide 3-kinase-Akt kinase signaling pathway that regulates cell survival. The difference in CD44-regulated apoptosis resistance in T cell subpopulations has important implications in a broad spectrum of diseases.

Size-dependent regulation of Snail2 by hyaluronan: its role in cellular invasion

Hyaluronan (HA) induces changes in cellular behavior that are crucial during both embryonic development and cancer progression. However, the biological effects of varying sizes of HA and the signal transduction mechanisms that these polymers may activate remain unclear. In this study, we demonstrate that pulse stimulation of mouse embryonic fibroblasts with high-molecular-weight (HMW) HA, but not HA of lower molecular sizes, leads to increases in Snail2 protein which are dependent on NFkappaB activity. Involvement of CD44, the main HA receptor, in these responses was determined by use of a CD44 blocking antibody and CD44 siRNA. Both the blockade and silencing of CD44 significantly abrogate the increases in nuclear factor kappaB (NFkappaB) activity and Snail2 protein following HMW-HA stimulation. Furthermore, we show that HMW-HA induces cellular invasion and that inhibition of CD44, Snail2, or NFkappaB significantly decreases this response. These studies elucidate a novel HA/Snail2 functional connection through CD44 and NFkappaB that is important for the induction of cellular invasion and is dependent on HA size.

Involvement of CD44 in mast cell proliferation during terminal differentiation

By using the recently established culture system that reproduces the terminal differentiation process of connective tissue-type mast cells, we found significant transcriptional induction of CD44. As CD44 is a primary receptor for hyaluronan (HA), which is one of the major extracellular matrix components, we investigated the role of CD44 in cutaneous mast cells. When co-cultured with fibroblasts, mouse bone marrow-derived cultured mast cells (BMMCs) were found to form clusters in an HA-dependent manner. As compared with BMMCs derived from the wild-type mice, those from the CD44(-/-) mice exhibited impaired growth during the co-cultured period. Furthermore, in the peritoneal cavities and ear tissues, mature mast cells were fewer in number in the CD44(-/-) mice than in the wild-type mice. We investigated roles of CD44 in mast cell proliferation by reconstituting BMMCs into the tissues of mast cell-deficient, Kit(W)/Kit(W-v) mice, and found that the number of metachromatic cells upon acidic toluidine blue staining in the tissues transplanted with CD44(-/-) BMMCs was not significantly changed for 10 weeks, whereas that in the tissues transplanted with the CD44(+/+) BMMCs was significantly increased. These results suggest that CD44 plays a crucial role in the regulation of the cutaneous mast cell number.

Role of CD44s and CD44v6 on human breast cancer cell adhesion, migration, and invasion

The interaction between the transmembrane receptor CD44 on epithelial tumor cells and its ligand hyaluronan in the surrounding extracellular matrix is important in tumor progression and metastasis. CD44 is encoded by a single 20-exon gene and expressed in standard form (CD44s), as well as a myriad of CD44 variants (CD44v) generated by alternative splicing of the CD44 mRNA. Previously, we demonstrated that hyaluronan (HA) production is increased at tumor-stroma interface in invasive and metastatic human breast cancers when compared with benign or premalignant lesions. We hypothesize that CD44 expression on breast cancer cells is a major contributing factor to cell adhesion, migration and invasion. To evaluate this hypothesis we examined the effects of 3 distinct anti-CD44s and 2 anti-CD44v6 monoclonal antibodies on breast cancer cell lines that expressed high and low CD44s and CD44v6. Using these antibodies we assessed the role of CD44 in cell adhesion, cell motility, and cell invasion using immobilized HA-coated wells, wound healing assays, and modified Boyden chamber respectively. Our results showed that anti-CD44s could inhibit breast cancer cell adhesion, motility and invasion, while anti-CD44v6 inhibits cell motility. In conclusion, our data suggests that CD44s is involved in breast cancer cell adhesion, motility and invasion through interaction with HA but CD44v6 is involved only in cell motility. Furthermore we concluded that antibodies against different epitopes on CD44 mediate distinct functional effects on breast cancer cells.

CD44-specific antibody treatment and CD44 deficiency exert distinct effects on leukocyte recruitment in experimental arthritis

CD44, the leukocyte adhesion receptor for hyaluronan, has been considered a therapeutic target on the basis of the robust anti-inflammatory effect of CD44-specific antibodies in animal models of immune-mediated diseases. However, CD44 deficiency does not provide substantial protection against inflammation. Using intravital video microscopy in a murine model of rheumatoid arthritis, we show that CD44 deficiency and anti-CD44 antibody treatment exert disparate effects on leukocyte recruitment in inflamed joints. Leukocyte rolling, which is increased in CD44-deficient mice, is promptly abrogated in anti-CD44-treated wild-type mice. CD44-specific antibodies also trigger platelet deposition on granulocytes and subsequent depletion of this leukocyte subset in the circulation. These in vivo effects require CD44 cross-linking and are reproducible with an antibody against Gr-1, a molecule that, like CD44, is highly expressed on granulocytes. Anticoagulant pretreatment, which prevents platelet deposition, mitigates both granulocyte depletion and the suppressive effect of CD44-specific antibody on joint swelling. Our observations suggest that cross-linking of prominent cell surface molecules, such as CD44 or Gr-1, can initiate a rapid self-elimination program in granulocytes through engagement of the coagulation system. We conclude that the robust anti-inflammatory effect of CD44-specific antibodies in arthritis is primarily the result of their ability to trigger granulocyte depletion.

Galectin-9 suppresses tumor metastasis by blocking adhesion to endothelium and extracellular matrices

We previously described an inverse correlation between galectin-9 (Gal-9) expression and metastasis in patients with malignant melanoma and breast cancer. This study verified the ability of Gal-9 to inhibit lung metastasis in experimental mouse models using highly metastatic B16F10 melanoma and Colon26 colon cancer cells. B16F10 cells transfected with a secreted form of Gal-9 lost their metastatic potential. Intravenous Gal-9 administration reduced the number of metastases of both B16F10 and Colon26 cells in the lung, indicating that secreted Gal-9 suppresses metastasis. Analysis of adhesive molecule expression revealed that B16F10 cells highly express CD44, integrin alpha1, alpha 4, alpha V, and beta1, and that Colon26 cells express CD44, integrin alpha2, alpha 5, alpha V, and beta1, suggesting that Gal-9 may inhibit the adhesion of tumor cells to vascular endothelium and the extracellular matrix (ECM) by binding to such adhesive molecules. Indeed, Gal-9 suppressed the binding of hyaluronic acid to CD44 on both B16F10 and Colon26 cells, and also suppressed the binding of vascular cell adhesion molecule-1 to very late antigen-4 on B16F10 cells. Furthermore, Gal-9 inhibited the binding of tumor cells to ECM components, resulting in the suppression of tumor cell migration. The present results suggest that Gal-9 suppresses both attachment and invasion of tumor cells by inhibiting the binding of adhesive molecules on tumor cells to ligands on vascular endothelium and ECM.

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.

Role of the Hyaluronan Receptor CD44 During Porcine Oocyte Maturation

Previous our studies have shown that CD44, the principal receptor for hyaluronan, is present on cumulus cells during oocyte maturation. Although hyaluronan-CD44 interaction has been implicated in cumulus expansion and/or oocyte maturation, the full significance of CD44 remains unknown. The objective of the present study was to further investigate the role of CD44 in cumulus expansion and oocyte maturation in pigs. We demonstrate here in that CD44 has a key role in oocyte maturation but not in cumulus expansion. Previous studies have reported the physiological significance of cumulus expansion in oocyte maturation. However, our results suggest that cumulus expansion is a necessary condition for oocyte maturation, but that it is not sufficient on its own. Furthermore, western blot analysis demonstrated that the CD44 of the in vitro-matured cumulus-oocyte complexes (COCs) had a larger molecular weight and more terminal sialic acid, which has been proven to inhibit the hyaluronan-binding ability of the receptor, than the CD44 of the in vivo-matured COCs, indicating that the hyaluronan-CD44 interactions during in vitro maturation might be insufficient compared with those in vivo. The insufficient interactions of hyaluronan-CD44 during in vitro maturation may cause the inferior capacity of fertilization and development of oocytes matured in vitro.

Potential roles for hyaluronan and CD44 in kainic acid-induced mossy fiber sprouting in organotypic hippocampal slice cultures

The most well-documented synaptic rearrangement associated with temporal lobe epilepsy is mossy fiber sprouting (MFS). MFS is a pronounced expansion of granule cell mossy fiber axons into the inner dentate molecular layer. The recurrent excitatory network formed by MFS is hypothesized to play a critical role in epileptogenesis, which is the transformation of the normal brain into one that is prone to recurrent spontaneous seizures. While many studies have focused on the functional consequences of MFS, relatively few have investigated the molecular mechanisms underlying the increased propensity of mossy fibers to invade the inner molecular layer. We hypothesized that changes in two components of the extracellular matrix, hyaluronan and its primary receptor, CD44, contribute to MFS. Hyaluronan contributes to laminar-specificity in the hippocampus and increases in hyaluronan and CD44 are associated with temporal lobe epilepsy. We tested our hypothesis in an in vitro model of MFS using a combination of histological and biochemical approaches. Application of kainic acid (KA) to organotypic hippocampal slice cultures induced robust MFS into the inner dentate molecular layer compared with vehicle-treated controls. Degradation of hyaluronan with hyaluronidase significantly reduced but did not eliminate KA-induced MFS, suggesting that hyaluronan played a permissive role in MFS, but that loss of hyaluronan signaling alone was not sufficient to block mossy fiber reorganization. Comparison of CD44 expression with MFS revealed that when CD44 expression in the molecular layers was high, MFS was minimal and when CD44 expression/function was reduced following KA treatment or with function blocking antibodies, MFS was increased. The time course of KA-induced reductions in CD44 expression was identical to the temporal progression of KA-induced MFS reported previously in hippocampal slice cultures, suggesting that reduced CD44 expression may help promote MFS. Understanding the molecular mechanisms underlying MFS may lead to therapeutic interventions that limit epileptogenesis.

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.

Enhanced cell surface CD44 variant (v6, v9) expression by osteopontin in breast cancer epithelial cells facilitates tumor cell migration: novel post-transcriptional, post-translational regulation

Osteopontin (OPN) is a glycosylated, secreted phosphoprotein that functions both as a cell attachment and chemotactic factor. Elevated expression of OPN confers enhanced metastatic ability on transformed cells, suggesting that OPN may contribute to the malignant progression of tumors. Migration of mammary carcinoma cells is stimulated by OPN via interactions with integrins and CD44 cell surface receptors. We hypothesized that OPN modulates specific CD44 isoform expression to facilitate breast cancer cell migration. The 21NT tumorigenic human breast cancer cell line was examined for regulation of CD44 expression at both the mRNA and protein levels in response to an engineered increase in OPN expression under CMV promoter control. Significant up-regulation of CD44s isoform mRNA expression was observed, but no change in CD44v6, v8, v9 or v10 mRNA levels. While there were elevated levels of CD44s, v6 and v9 protein at the cell surface, at the level of total cellular protein only CD44s and v6 were markedly increased. This suggests that OPN can regulate CD44 expression at both transcriptional and post-transcriptional (both amount and localization of protein) levels. To validate the functional consequence of OPN regulation of CD44 expression, we demonstrate that OPN-mediated cell migration was reduced by exposure to a anti-pan CD44 antibody, and to anti-CD44v6 and anti-CD44v9 function-blocking antibodies. Our data provide evidence that in 21NT cells OPN enhances CD44s mRNA expression, increases cell surface expression of CD44 variant forms without a change in mRNA levels, and stimulates cell migration.

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.

Chiasmatic neurons in the ventral diencephalon of mouse embryos--changes in arrangement and heterogeneity in surface antigen expression

We have investigated the changes in arrangement of the SSEA-1 immunoreactive chiasmatic neurons in the mouse ventral diencephalon from embryonic day (E) 9 to the end of gestation. A regionally specific staining of SSEA-1 was first detected in the ventricular layer of the caudal diencephalon at E10 and later at E11 on the cells in the subventricular layer. At E12, these cells formed the characteristic V-shaped configuration caudal to the optic axons in the chiasm. At E13-E15, this neuronal array changes gradually to a configuration that facilitates contact with the optic axons only at the midline and the initial segment of the optic tract. Colocalization studies showed that CD44 was localized strongly on the neurons in the central but not lateral domains of the array, suggesting existence of heterogeneity in these neurons in terms of surface antigen presentation. This difference between the central and lateral domains raises the possibility that the chiasmatic neurons may regulate the patterning of axon orders at the midline and the optic tract through presentation of distinct combination of guidance cues at these strategic positions in the optic pathway. Furthermore, exogenous Lewis-x/SSEA-1 inhibited neurite outgrowth from the E14 retinal explants; this inhibition was observed in neurites from both ventral temporal and dorsal nasal retina. These findings suggest an action of this surface carbohydrate on the control of axon growth and guidance in the mouse optic pathway.

Hyaluronan fragments induce endothelial cell differentiation in a CD44- and CXCL1/GRO1-dependent manner

Hyaluronan is a glycosaminoglycan of the extracellular matrix. In tumors and during chronic inflammatory diseases, hyaluronan is degraded to smaller fragments, which are known to stimulate endothelial cell differentiation. In this study, we have compared the molecular mechanisms through which hyaluronan dodecasaccharides (HA12), and the known angiogenic factor, fibroblast growth factor 2 (FGF-2), induce capillary endothelial cell sprouting in a three-dimensional collagen gel. The gene expression profiles of unstimulated and HA12- or FGF-2-stimulated endothelial cells were compared using a microarray analysis approach. The data revealed that both FGF-2 and HA12 promoted endothelial cell morphogenesis in a process depending on the expression of ornithine decarboxylase (Odc) and ornithine decarboxylase antizyme inhibitor (Oazi) genes. Among the genes selectively up-regulated in response to HA12 was the chemokine CXCL1/GRO1 gene. The notion that the induction of CXCL1/GRO1 is of importance for HA12-induced endothelial cell sprouting was supported by the fact that morphogenesis was inhibited by antibodies specifically neutralizing the CXCL1/GRO1 protein product. HA12-stimulated endothelial cell differentiation was exerted via binding to CD44 since it was inhibited by antibodies blocking CD44 function. Our data show that hyaluronan fragments and FGF-2 affect endothelial cell morphogenesis by the induction of overlapping but also by distinct sets of genes.

TSG-6 modulates the interaction between hyaluronan and cell surface CD44

Interactions between CD44 and hyaluronan are implicated in the primary adhesion of lymphocytes to endothelium at inflammatory locations. Here we show that preincubation of hyaluronan with full-length recombinant TSG-6 or its Link module domain (Link_TSG6) enhances or induces the binding of hyaluronan to cell surface CD44 on constitutive and inducible cell backgrounds, respectively. These effects are blocked by CD44-specific antibodies and are absent in CD44-negative cells. Enhancement of CD44-mediated interactions of lymphoid cells with hyaluronan by TSG-6 proteins was seen under conditions of flow at shear forces that occur in post-capillary venules. Increases in the number of rolling cells were observed on substrates comprising TSG-6-hyaluronan complexes as compared with a substrate containing hyaluronan alone. In ligand competition experiments, cell surface-bound TSG-6-hyaluronan complexes were more potent than hyaluronan alone in inhibiting cell adhesion to immobilized hyaluronan. Link_TSG6 mutants with impaired hyaluronan binding function had a reduced ability to modulate ligand binding by cell surface CD44. However, some mutants that exhibited close to wild-type hyaluronan binding were found to have either reduced or increased activity, suggesting that some amino acid residues outside of the hyaluronan binding site might be involved in protein self-association, potentially leading to the formation of cross-linked hyaluronan fibers. In turn, cross-linked hyaluronan could increase the binding avidity of CD44 by inducing receptor clustering. The ability of TSG-6 to modulate the interaction of hyaluronan with CD44 has important implications for CD44-mediated cell activity at sites of inflammation, where TSG-6 is expressed.

Structure of the Regulatory Hyaluronan Binding Domain in the Inflammatory Leukocyte Homing Receptor CD44

Adhesive interactions involving CD44, the cell surface receptor for hyaluronan, underlie fundamental processes such as inflammatory leukocyte homing and tumor metastasis. Regulation of such events is critical and appears to be effected by changes in CD44 N-glycosylation that switch the receptor "on" or "off" under appropriate circumstances. How altered glycosylation influences binding of hyaluronan to the lectin-like Link module in CD44 is unclear, although evidence suggests additional flanking sequences peculiar to CD44 may be involved. Here we show using X-ray crystallography and NMR spectroscopy that these sequences form a lobular extension to the Link module, creating an enlarged HA binding domain and a formerly unidentified protein fold. Moreover, the disposition of key N-glycosylation sites reveals how specific sugar chains could alter both the affinity and avidity of CD44 HA binding. Our results provide the necessary structural framework for understanding the diverse functions of CD44 and developing novel therapeutic strategies.

CD44-regulated intracellular proliferation of Listeria monocytogenes

CD44 has been implicated in immune and inflammatory processes. We have analyzed the role of CD44 in the outcome of Listeria monocytogenes infection in murine bone marrow-derived macrophages (BMM). Surprisingly, a dramatically decreased intracellular survival of L. monocytogenes was observed in CD44(-/-) BMM. CD44(-/-) heart or lung fibroblast cultures also showed reduced bacterial levels. Moreover, livers from CD44(-/-)-infected mice showed diminished levels of L. monocytogenes. In contrast, intracellular growth of Salmonella enterica serovar Typhimurium was the same in CD44(-/-) and control BMM. The CD44-mediated increased bacterial proliferation was not linked to altered BMM differentiation or to secretion of soluble factors. CD44 did not mediate listerial uptake, and it played no role in bacterial escape from the primary phagosome or formation of actin tails. Furthermore, CD44-enhanced listerial proliferation occurred in the absence of intracellular bacterial spreading. Interestingly, coincubation of BMM with hyaluronidase or anti-CD44 antibodies that selectively inhibit hyaluronan binding increased intracellular listerial proliferation. Treatment of cells with hyaluronan, in contrast, diminished listerial growth and induced proinflammatory transcript levels. We suggest that L. monocytogenes takes advantage of the CD44-mediated signaling to proliferate intracellularly, although binding of CD44 to certain ligands will inhibit such response.

Perturbation of CD44 function affects chiasmatic routing of retinal axons in brain slice preparations of the mouse retinofugal pathway

Neurons generated early in development of the ventral diencephalon have been shown to play a key role in defining the midline and the caudal boundary of the optic chiasm in the mouse retinofugal pathway. These functions have been attributed to a surface bound adhesion molecule, CD44 that is expressed in these chiasmatic neurons. In this study, we investigated the effects of perturbing normal CD44 functions on axon routing in brain slice preparations of the mouse retinofugal pathway. Two CD44 antibodies (Hermes-1 and IM7) were used that bind to distinct epitopes on the extracellular domain of the molecule. We found that both antibodies produced dramatic defects in routing of the retinal axons that arrive early in the chiasm. In preparations of embryonic day 13 (E13) and E14 pathways, the crossed component in the chiasm was significantly reduced after antibody treatment. However, such reduction in axon crossing was not observed in E15 chiasm, indicating that the lately generated crossed axons lost their responses to CD44. Furthermore, the anti-CD44 treatment produced a reduction in the uncrossed component in the E15 but not in younger pathways, suggesting a selective response of the lately generated axons, mostly from ventral temporal retina, but not those generated earlier, to the CD44 at the chiasmatic midline in order to make their turn for the uncrossed pathway. These findings provide evidence that a normal function of CD44 molecules in the chiasmatic neurons is essential for axon crossing and axon divergence at the mouse optic chiasm.