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Trafficking of human CD34 ؉ stem/progenitor cells (HSCs/HPCs) is regulated by chemokines, cytokines, proteolytic enzymes, and adhesion molecules. We report that the adhesion receptor CD44 and its major ligand, hyaluronic acid (HA), are essential for homing into the bone marrow (BM) and spleen of nonobese diabetic/ severe combined immunodeficient (NOD/ SCID) mice and engraftment by human HSCs. Homing was blocked by anti-CD44 monoclonal antibodies (mAbs) or by soluble HA, and it was significantly impaired after intravenous injection of hyaluronidase. Furthermore, stromal cellderived factor-1 (SDF-1) was found to be a rapid and potent stimulator of progenitor adhesion to immobilized HA, leading to formation of actin-containing protrusions with CD44 located at their tips. HPCs migrating on HA toward a gradient of SDF-1 acquired spread and polarized morphology with CD44 concentrating at the pseudopodia at the leading edge. These morphologic alterations were not observed when the progenitors were first exposed to anti-CD44 mAbs, demonstrating a crosstalk between CD44 and CXCR4 signaling. Unexpectedly, we found that HA is expressed on human BM sinusoidal endothelium and endosteum, the regions where SDF-1 is also abundant. Taken IntroductionThe outcome of hematopoietic stem cell transplantation is influenced by the ability of the cells to home and repopulate their specialized bone marrow (BM) niches. The crosstalk between the hematopoietic stem/progenitor cells (HSCs/HPCs) and the microenvironment, which regulates homing to the BM, is not fully elucidated. Data indicate that transplanted HSCs/HPCs lodge into their BM niches by a sequence of highly regulated events that mimic the migration of leukocytes to inflammatory sites. This process includes tethering and rolling on E-and P-selectins, firm adhesion to the vessel wall, transendothelial extravasation, and migration through the extracellular matrix (ECM). [1][2][3] This multistep process is mediated by an interplay between chemokines, growth factors, proteolytic enzymes, and adhesion molecules. 4,5 The chemokine stromal cell-derived factor-1 (SDF-1), also named CXCL-12, and its receptor, CXCR4, play key roles in human HSC trafficking and repopulation. 6 This chemokine, expressed by both human and murine BM endothelium and stroma, 7,8 is the most powerful chemoattractant of HSCs/HPCs 9,10 that also regulates their survival. 11,12 It induces the integrin-mediated firm arrest of human HPCs under physiologic shear flow, facilitates their transendothelial migration, 3,8 and regulates homing 13 and BM engraftment. 14 Furthermore, SDF-1 is also required for the retention of murine stem and progenitor cells within the BM. 15,16 HSCs/HPCs express several types of adhesion molecules that are responsible for cell-cell and cell-ECM interactions 17 ; among them CD44 is of particular interest.The importance of CD44 in cell migration is reported for a variety of normal and malignant cells. 18 CD44 is a multifunctional and multistructural receptor that has a large array of isoforms....

CD44 is a multistructural and multifunctional cell surface molecule involved in cell proliferation, cell differentiation, cell migration, angiogenesis, presentation of cytokines, chemokines, and growth factors to the corresponding receptors, and docking of proteases at the cell membrane, as well as in signaling for cell survival. All these biological properties are essential to the physiological activities of normal cells, but they are also associated with the pathologic activities of cancer cells. Experiments in animals have shown that targeting of CD44 by antibodies, antisense,and CD44-soluble proteins markedly reduces the malignant activities of various neoplasms, stressing the therapeutic potential of anti-CD44 agents. Furthermore, because alternative splicing and posttranslational modifications generate many different CD44 sequences, including, perhaps, tumor-specific sequences, the production of anti-CD44 tumor-specific agents may be a realistic therapeutic approach. However, in many cancers (renal cancer and non-Hodgkin's lymphomas are exceptions), a high level of CD44 expression is not always associated with an unfavorable outcome. On the contrary, in some neoplams CD44 upregulation is associated with a favorable outcome. Even worse, in many cases different research grows analyzing the same neoplastic disease reached contradictory conclusions regarding the correlation between CD44 expression and disease prognosis, possibly due to differences in methodology. These problems must be resolved before applying anti-CD44 therapy to human cancers.

We report here that joint inflammation in collagen-induced arthritis is more aggravated in CD44-knockout mice than in WT mice, and we provide evidence for molecular redundancy as a causal factor. Furthermore, we show that under the inflammatory cascade, RHAMM (receptor for hyaluronan-mediated motility), a hyaluronan receptor distinct from CD44, compensates for the loss of CD44 in binding hyaluronic acid, supporting cell migration, up-regulating genes involved with inflammation (as assessed by microarrays containing 13,000 cDNA clones), and exacerbating collagen-induced arthritis. Interestingly, we further found that the compensation for loss of the CD44 gene does not occur because of enhanced expression of the redundant gene (RHAMM), but rather because the loss of CD44 allows increased accumulation of the hyaluronic acid substrate, with which both CD44 and RHAMM engage, thus enabling augmented signaling through RHAMM. This model enlightens several aspects of molecular redundancy, which is widely discussed in many scientific circles, but the processes are still ill defined

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Inflammatory destruction of insulin-producing ␤ cells in the pancreatic islets is the hallmark of insulin-dependent diabetes mellitus, a spontaneous autoimmune disease of non-obese diabetic mice resembling human juvenile (type I) diabetes. Histochemical analysis of diabetic pancreata revealed that mononuclear cells infiltrating the islets and causing autoimmune insulitis, as well as local islet cells, express the CD44 receptor; hyaluronic acid, the principal ligand of CD44, is detected in the islet periphery and islet endothelium. Injection of anti-CD44 mAb 1 hr before cell transfer of diabetogenic splenocytes and subsequently on alternate days for 4 weeks induced considerable resistance to diabetes in recipient mice, reflected by reduced insulitis. Contact sensitivity to oxazolone was not influenced by this treatment. A similar antidiabetic effect was observed even when the anti-CD44 mAb administration was initiated at the time of disease onset: i.e., 4 -7 weeks after cell transfer. Administration of the enzyme hyaluronidase also induced appreciable resistance to insulin-dependent diabetes mellitus, suggesting that the CD44 -hyaluronic acid interaction is involved in the development of the disease. These findings demonstrate that CD44-positive inflammatory cells may be a potential therapeutic target in insulin-dependent diabetes.T he inflammatory cascade in affected organs of autoimmune diseases is a complex process that involves triggering of the immunological response, release of chemokines, cytokines, and toxic agents (e.g., reactive oxygen) by the activated cells, stimulation of endothelial cells, up-regulation of cell surface adhesion molecules, transendothelial cell migration, and a shift in the Th1͞Th2 balance in favor of the Th1 cells (1). Hence, the destructive autoimmune inflammatory process depends substantially (but not exclusively) on cell migration and cell interaction with matrix components of target organs. The destruction of pancreatic islet ␤-cells in insulin-dependent diabetes mellitus (IDDM) by invading leukocytes and the consequent deterioration of the insulin-dependent glucose homeostasis is an excellent example of such an autoimmune process (2, 3), although neither the nature of the triggering self-antigen nor the molecules associated with its recognition and presentation have been unequivocally identified.Whereas the function of selectins and integrins in supporting inflammatory cell migration and lodgment has been well established (4), the role of cell surface CD44 has only recently attracted attention (5). Alternative splicing and͞or posttranslation modifications generate many CD44 isoforms. The large array of CD44 isoforms is mainly attributable to the insertion of amino acid sequences, encoded by different combinations of 10 variant exons, into a membrane proximal position of the extracellular domain. Transcripts in which these variant exons are spliced out encode the most common and widely expressed 85-kDa isoform, known as standard CD44. The expression of CD44 isoforms containing s...

The receptors for insulin and insulin-like growth factor-I (IGF-I) belong to the family of receptor protein tyrosine kinases [1]. Although a vast body of data supports the concept that insulin stimulates cell growth in vitro and in vivo, the question of whether insulin is physiologically a growth factor remains controversial (for review see [2]). Even more controversial is the question of whether insulin is capable of inducing mitogenic effects through its own receptor, or whether the growth-promoting effects of insulin result from its weak interaction with the IGF-I receptor or occur within insulin/IGF-I receptor hybrids [3,4], or via interphosphorylation of the IGF-I receptor by the insulin receptor tyrosine kinase [5]. The response possibly depends on the cell type and its given supply of insulin and IGF-I receptors as well as the subsets of intracellular signalling molecules that are activated by either receptor. (We use the term IGF-I receptor for simplicity to designate the type 1 IGF receptor which binds both IGF-I and II and probably mediates the mitogenic effects of both growth factors [6].) Diabetologia (1997) Summary Insulin has traditionally been considered as a hormone essential for metabolic regulation, while the insulin-like growth factors (IGF-I and IGF-II) are postulated to be more specifically involved in growth regulation. The conventional wisdom is that they share each other's effects only at high concentrations, due to their weak affinity for the heterologous receptor. We discuss here the evidence that in the proper cellular context, insulin can be mitogenic at physiologic concentrations through its own receptor. We studied the insulin and IGF-I binding characteristics of a new model suitable for analysing insulin receptor mediated mitogenesis; that is, a T-cell lymphoma line that depends on insulin for growth, but is unresponsive to IGFs. The cells showed no specific binding of 125 I-IGF-I and furthermore, no IGF-I receptor mRNA was detected by RNAse protection assay in the LB cells, in contrast with mouse brain and thymus. The cells bound at saturation about 3000 insulin molecules to receptors that had normal characteristics in terms of affinity, kinetics, pH dependence and negative co-operativity. A series of insulin analogues competed for 125 I-insulin binding with relative potencies comparable to those observed in other insulin target cells. The full sequence of the insulin receptor cDNA was determined and found to be identical to the published sequence of the murine insulin receptor cDNA. The LB cell line is therefore an ideal model with which to investigate insulin mitogenic signalling without interference from the IGF-I receptor. Using this model, we have started approaching the molecular basis of insulin-induced mitogenesis, in particular the role of signalling kinetics in choosing between mitogenic and metabolic pathways. [Diabetologia (1997) 40: S 25-S 31]

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