MUC18, a member of the immunoglobulin superfamily, is expressed on bone marrow fibroblasts and a subset of hematological malignancies

Differential CD146 expression on circulating versus tissue endothelial cells in rectal cancer patients: implications for circulating endothelial and progenitor cells as biomarkers for antiangiogenic therapy

PURPOSE

Circulating endothelial cells (CECs) and progenitor cells are currently evaluated as potential biomarkers of antiangiogenic therapy. CD146 is considered a panendothelial-specific marker, but its utility as a CEC marker in cancer patients remains unclear.

PATIENTS AND METHODS

We analyzed the expression of CD146 on mononuclear blood cells, primary tissue endothelial cells, and malignant and normal tissues by flow cytometric and immunohistochemical analyses. Furthermore, we measured the circulation kinetics of CD146+ cells before, and then 3 and 12 days after vascular endothelial growth factor (VEGF) antibody blockade by bevacizumab infusion in rectal cancer patients enrolled in a phase I trial.

RESULTS

In the peripheral blood of these cancer patients, over 90% of the CD146+ cells were CD45+ hematopoietic cells. CD146 expression was primarily detected on a subset of CD3+CD4+ lymphocytes, and was undetectable on CD34+CD133+CD45(dim) progenitor cells or CD31(bright)CD45- viable CECs. In contradistinction, CD146 was detectable in tissues on both cellular components of tumor vessel wall: CD31(bright)CD45- endothelial cells and alpha-SMA+ pericytes. Unlike viable CECs and progenitor cells, CD146+ cell concentration in the peripheral blood of cancer patients did not decrease during VEGF blockade.

CONCLUSION

CD146 is fairly homogeneously expressed on vascular endothelium but not on viable CECs or progenitor cells. The vast majority of CD146+ blood cells are lymphocytes, and VEGF blockade by bevacizumab did not significantly change their number in rectal cancer patients. These results underscore the need for further characterization and identification of new markers for CEC subpopulations for their development as biomarkers of antiangiogenic therapy.

The Lutheran glycoprotein: a multifunctional adhesion receptor

The Lutheran blood group system, which comprises one of the largest families of human red blood cell (RBC) antigens, resides on two immunoglobulin superfamily (IgSF) proteins: Lutheran and basal cell adhesion molecule (B-CAM). These two glycoproteins arise via alternative splicing of mRNA from a single gene and differ in structure only in the lengths of their cytoplasmic tails. Both are expressed on RBCs as well as a variety of other cell types, and they are overexpressed on sickle RBCs (SS RBC). B-CAM/Lu is the critical receptor for SS RBC adhesion to the extracellular matrix protein laminin, an interaction thought to contribute to the pathogenesis of sickle cell-related vasoocclusive events. Recent work has also shown that B-CAM/Lu on RBCs can undergo activation as a result of adrenergic signaling pathways. The high affinity of B-CAM/Lu for laminin is also thought to contribute to various developmental processes, including organogenesis, vascular development, erythropoiesis, and smooth muscle development and organization. Interestingly, the B-CAM spliceoform seems to be overexpressed by a variety of different malignant tumors and may be involved, along with other adhesion receptor proteins, in malignant transformation and tumor metastasis. Studies of B-CAM/Lu have thus expanded from defining antigen-specific polymorphisms to investigations of processes involved in sickle cell disease, human development, and cancer biology.

HETEROGENEITY AMONGST SPLENIC STROMAL CELL LINES WHICH SUPPORT DENDRITIC CELL HEMATOPOIESIS

Long-term cultures (LTC) producing dendritic cells (DC) have been previously established from spleen. LTC support the development of nonadherent cells comprising small DC progenitors and immature DC. Similarly, the splenic stroma STX3, derived from a LTC which ceased DC production, can support DC development from precursors in overlaid bone marrow. The STX3 stroma is an immortalised mixed population of endothelial cells and elongated spindle-shaped cells, thought to be fibroblasts. The stromal cell components of STX3 have been studied here. A panel of 102 cell lines was established by single-cell sorting. A range of clone morphology, including cobblestone cells and elongated spindle-shaped cells, was reflective of heterogeneity in STX3. However, similar expression levels for the endothelial genes ACVRL1/ ALK1, COL18A1, and MCAM in 13 splenic stromal cell lines suggested that both cell types had endothelial origin. The hematopoietic support function of stromal clones was tested in coculture assays with a bone marrow cell overlay. Splenic stromal cell lines with different morphology were both supporters and nonsupporters of hematopoiesis, in terms of foci formation or release of suspension cells. Cloning of STX3 led to the isolation of a panel of splenic endothelial cell lines heterogeneous in terms of morphology and hematopoietic support function.

Isolation of C15: A novel antibody generated by phage display against mesenchymal stem cell-enriched fractions of adult human marrow

Adult bone marrow stroma contains a source of mesenchymal stem cells (MSC) that have the capacity to self-renew and differentiate into multiple stromal lineages. These rare cells can be visualised indirectly by the formation of heterogeneous colonies, containing stem cells and their differentiated progeny in long-term culture. If MSC and their associated progenitor and precursor populations are to reach their full therapeutic potential, markers will be required to identify and characterize specific bone marrow stromal subsets. We sought to use phage display to generate antibodies against bone marrow mononuclear cells (BMMNC) enriched for colony forming cells. Initially, we identified our target cell population by comparing the colony forming efficiency (CFE) of CD49a-positive, STRO-1-positive and CD45-negative BMMNC subpopulations with unseparated BMMNC. Selection with anti-CD49a gave the greatest enrichment (19-fold) of colony forming cells and in light of these findings, we generated phage antibodies against CD49a-positive BMMNC by simultaneous positive/negative selection. A dominant clone (C15), generated after 3 rounds of selection, has been isolated and sequenced, then characterized for cell and tissue specificity. Sequence analysis showed that the V(H) and V(L) gene segments of C15 aligned most closely to the VH26/DP-47 and IGLV3S1/DPL16 germline V segments found in the synthetic repertoire. C15 bound to 4% of freshly isolated BMMNC and localized to osteoblastic cells and proximal marrow cells in areas of active bone formation in sections of osteophyte. C15 binding was upregulated in cultured bone marrow stromal cells (BMSC) and was also detected on bone-derived cell lines. This report demonstrates that phage display is a powerful tool for the isolation of antibodies against rare cell populations, and provides a platform for the future application of this technology in the search for antigens on MSC and other rare cell populations.

Gene delivery in bone tissue engineering: progress and prospects using viral and nonviral strategies

Bone tissue loss as a consequence of the natural aging process or as a result of trauma and degenerative disease has led to the need for procedures to generate cartilage and bone for a variety of orthopedic applications. The ability to transfer genes into multipotential mesenchymal stem cells, while still in its infancy, offers considerable therapeutic hope in a variety of musculoskeletal disorders. However, the choice of gene delivery method is key. This review examines the various techniques and methods currently available to enable gene transfer into a target population from viral methods (transduction) to nonviral (transfection) methods and the limitations associated with each method. The potential applications and current understanding of each method are presented. Given the demographic challenge of an aging population, the ultimate goal remains the development of simple, safe, and reproducible strategies for gene delivery that will address the pressing orthopedic clinical imperatives of many.

Mesenchymal stem cells

It has become clear that adult mammalian bone marrow contains not one but two ostensibly discrete populations of adult stem cells. The first and by far the most fully characterized are the hematopoietic stem cells responsible for maintaining lifelong production of blood cells. The biological characteristics and properties of the second marrow resident population of stem cells, variously termed bone marrow stromal cells or mesenchymal stem cells, are in contrast much less well understood. In vitro, cultures established from single-cell suspensions of bone marrow from a wide range of mammalian species generate colonies of adherent marrow stromal cells, each derived from a single precursor cell termed a colony-forming unit-fibroblast (CFU-F). Culture conditions have been developed to expand marrow stromal cells in vitro while maintaining the capacity of these cells to differentiate into bone, fat, and cartilage. A significant portion of our current knowledge of this population of cells is based on analysis of the properties of these culture expanded cells, not on the primary colony-initiating cells. In this article, we will focus on methodologies to prospectively isolate stromal progenitors from mouse and human bone marrow and will review current data that suggest stromal progenitors in the bone marrow in situ are associated with the outer surfaces of blood vessels and may share identity with vascular pericytes.

Multipotent stromal cells derived from the infrapatellar fat pad of the knee

Tissue engineering approaches for promoting the repair of skeletal tissues have focused on cell-based therapies involving multipotent stromal cells. Recent studies have identified such cells in several tissues in the adult human, including skin, muscle, bone marrow, and subcutaneous fat. This study examined the hypothesis that the infrapatellar fat pad of the adult knee contains progenitor cells that have the ability to differentiate into chondrocytes, osteoblasts, or adipocytes under appropriate culture conditions. Cells isolated from the fat pad stroma had a profile of cell-surface molecules similar but not identical to that of bone marrow-derived mesenchymal stem cells. Using defined culture conditions, fat pad-derived stromal cells were induced to differentiate cells with phenotypic characteristics of: (1) chondrocytes, synthesizing cartilage matrix molecules; (2) adipocytes, producing lipid vacuoles and leptin; or (3) osteoblasts, forming mineralized tissue. The culture conditions also modulated the expression of characteristic gene markers for each lineage. This study supports the hypothesis that multipotent stromal cells are present in many connective tissues in the adult human. Given its location and accessibility, the fat pad may prove to be a potential source of progenitor cells for musculoskeletal tissue engineering.

Perivascular niche of postnatal mesenchymal stem cells in human bone marrow and dental pulp

Mesenchymal stem cell populations have previously been identified in adult bone marrow and dental pulp that are capable of regenerating the bone marrow and dental pulp microenvironments, respectively. Here we show that these stem cell populations reside in the microvasculature of their tissue of origin. Human bone marrow stromal stem cells (BMSSCs) and dental pulp stem cells (DPSCs) were isolated by immunoselection using the antibody, STRO-1, which recognizes an antigen on perivascular cells in bone marrow and dental pulp tissue. Freshly isolated STRO-1 positive BMSSCs and DPSCs were tested for expression of vascular antigens known to be expressed by endothelial cells (von Willebrand factor, CD146), smooth muscle cells, and pericytes (alpha-smooth muscle actin, CD146), and a pericyte-associated antigen (3G5), by immunohistochemistry, fluorescence-activated cell sorting (FACS), and/or immunomagnetic bead selection. Both BMSSCs and DPSCs lacked expression of von Willebrand factor but were found to be positive for alpha-smooth muscle actin and CD146. Furthermore, the majority of DPSCs expressed the pericyte marker, 3G5, while only a minor population of BMSSCs were found to be positive for 3G5. The finding that BMSSCs and DPSCs both display phenotypes consistent with different perivascular cell populations, regardless of their diverse ontogeny and developmental potentials, may have further implications in understanding the factors that regulate the formation of mineralized matrices and other associated connective tissues.

Origin and differentiation of human and murine stroma

Stromal cells generated in long-term cultures appear to follow a vascular smooth muscle differentiation pathway. Such a pathway, comprising several steps hallmarked by the expression of cytoskeletal and extracellular matrix markers, is found not only for bone marrow stromal cells, but also for stromal cells generated from the different developmental sites of hematopoiesis (yolk sac, aorta-gonad-mesonephros region, fetal liver, and spleen). Factors responsible for this differentiation pathway and its functional significance are discussed. The mesenchymal founder cell might be, at least for bone marrow, a mesenchymal stem cell (MSC), giving rise to stromal cells, endothelial cells, adipocytes, osteoblasts, and chondrocytes. A feature that distinguishes the MSC lineage from that of the hematopoietic stem cell lineage is that differentiation pathways are not strictly delineated, since even apparently fully differentiated cells from a given lineage have the potential to convert into another lineage (phenotype "plasticity") and intermediate cell phenotypes are observed. A stochastic Repression/Induction model that would account for this plasticity is proposed.

Surface protein characterization of human adipose tissue-derived stromal cells

Human bone marrow stromal cells are a multipotent population of cells capable of differentiating into a number of mesodermal lineages as well as supporting hematopoeisis. Their distinct protein and gene expression phenotype is well characterized in the literature. Human adipose tissue presents an alternative source of multipotent stromal cells. In this study, we have defined the phenotype of the human adipose tissue-derived stromal cells in both the differentiated and undifferentiated states. Flow cytometry and immunohistochemistry show that human adipose tissue-derived stromal cells have a protein expression phenotype that is similar to that of human bone marrow stromal cells. Expressed proteins include CD9, CD10, CD13, CD29, CD34, CD44, CD 49(d), CD 49(e), CD54, CD55, CD59, CD105, CD106, CD146, and CD166. Expression of some of these proteins was further confirmed by PCR and immunoblot detection. Unlike human bone marrow-derived stromal cells, we did not detect the STRO-1 antigen on human adipose tissue-derived stromal cells. Cells cultured under adipogenic conditions uniquely expressed C/EBPalpha and PPARdelta, two transcriptional regulators of adipogenesis. Cells cultured under osteogenic conditions were more likely to be in the proliferative phases of the cell cycle based on flow cytometric analysis of PCNA and Ki67. The similarities between the phenotypes of human adipose tissue-derived and human bone marrow-derived stromal cells could have broad implications for human tissue engineering.

Expression of a human cell adhesion molecule, MUC18, in prostate cancer cell lines and tissues

BACKGROUND

Over expression of huMUC18, a cell adhesion molecule in the immunoglobulin gene superfamily, causes a non-metastatic human melanoma cell line to become metastatic in a nude mouse system. To determine if MUC18 expression correlates with the malignant progression of prostate cancer, we investigated differential expression of human MUC18 (huMUC18) in normal prostate epithelial cells, prostate cancer cell lines, and prostatic normal and cancer tissues.

METHODS

RT-PCR and Western blot analyses were used to analyze the expression of MUC18 mRNA and protein in four human prostate cancer cell lines, cultured primary normal prostate epithelial cells, normal prostate and malignant prostate tissues. Immunohistochemistry was used to determine the expression of MUC18 antigen in prostatic tissues at different stages of malignancy.

RESULTS

Human MUC18 mRNA and protein was expressed in three different prostate cancer cell lines (TSU-PR1, DU145, and PC-3), but not in one prostate cancer cell line (LNCaP.FGC). HuMUC18 protein was also expressed at high levels in extracts prepared from tissue sample sections containing high grade prostatic intraepithelial neoplasia (PIN), but weakly expressed in extracts prepared from either cultured primary normal prostatic epithelial cells or the normal prostate gland. Immunohistochemical analysis showed that huMUC18 was expressed at higher levels in the epithelial cells of high-grade PIN and prostatic carcinomas and in cells of a lymph node metastasis compared to that in normal or benign hyperplastic epithelium (BPH).

CONCLUSIONS

We therefore conclude that MUC18 is expressed at higher levels in pre-malignant and malignant prostatic epithelium, including metastasis. We suggest that over-expression of MUC18 may be a new marker of human prostate cancer and also implicates its possible role in development and progression of prostate cancer.

Bone marrow stromal stem cells: nature, biology, and potential applications

Bone marrow stromal cells are progenitors of skeletal tissue components such as bone, cartilage, the hematopoiesis-supporting stroma, and adipocytes. In addition, they may be experimentally induced to undergo unorthodox differentiation, possibly forming neural and myogenic cells. As such, they represent an important paradigm of post-natal nonhematopoietic stem cells, and an easy source for potential therapeutic use. Along with an overview of the basics of their biology, we discuss here their potential nature as components of the vascular wall, and the prospects for their use in local and systemic transplantation and gene therapy.

Circulating skeletal stem cells

We report the isolation of adherent, clonogenic, fibroblast-like cells with osteogenic and adipogenic potential from the blood of four mammalian species. These cells phenotypically resemble but are distinguishable from skeletal stem cells found in bone marrow (stromal stem cells, "mesenchymal stem cells"). The osteogenic potential of the blood-borne cells was proven by an in vivo transplantation assay in which either polyclonal or single colony-derived strains were transplanted into the subcutis of immunocompromised mice, and the donor origin of the fully differentiated bone cells was proven using species-specific probes. This is the first definitive proof of the existence of circulating skeletal stem cells in mammals.

Erythroid cell adhesion molecules Lutheran and LW in health and disease

The Lutheran and LW glycoproteins are blood group-active proteins found at the surface of human red cells. The Lutheran glycoprotein (Lu gp) is a member of the immunoglobulin superfamily (IgSF) that binds the extracellular matrix protein laminin, in particular, laminin isoforms containing the alpha 5 subunit. The LW glycoprotein (LW gp), also an IgSF member, has substantial sequence homology with the family of intercellular adhesion molecules (ICAMs). LW gp binds the integrin very late antigen-4 (VLA-4, alpha 4 beta 1) and alpha V-containing integrins. Studies on the expression of LW and Lu gps during erythropoiesis utilizing in vitro cultures of haemopoietic progenitor cells have shown that LW gp expression precedes that of Lu gp. These observations have led to the suggestion that LW gp on erythroblasts may interact with VLA-4 on macrophages to stabilize erythroblastic islands in normal bone marrow and that Lu gp may facilitate trafficking of more mature erythroid cells to the sinusoidal endothelium where alpha 5-containing laminins are known to be expressed. Levels of Lu gp and LW gp expression on sickle red cells are greater than on normal red cells and sickle red cells adhere to alpha 5-containing laminins. These data suggest that the Lu and LW molecules may contribute to the vaso-occlusive events associated with episodes of acute pain in sickle cell disease.

The role of CD146 (Mel-CAM) in biology and pathology

CD146, also known as Mel-CAM, MUC18, A32 antigen, and S-Endo-1, is a membrane glycoprotein which functions as a Ca(2+)-independent cell adhesion molecule involved in heterophilic cell-cell interactions. Based on homology of the nucleotide sequence, CD146 is classified as a member of the immunoglobulin gene superfamily, since it contains the characteristic V-V-C2-C2-C2 immunoglobulin-like domain structure. Using immunohistochemistry with CD146-specific antibodies, CD146 expression has been demonstrated in a relatively limited spectrum of normal human tissues and malignant neoplasms. The lineage-specific expression pattern of CD146 can be useful in the differential diagnosis of certain lesions including melanomas and various types of gestational trophoblastic lesions. Although the biological role of CD146 in normal tissue and malignant tumours remains unclear, CD146 has been suggested to play an important role in tumour progression, implantation and placentation. CD146 expression can promote tumour progression in human melanoma, possibly through enhanced interaction between melanoma cells and endothelial cells. In contrast, CD146 may act as a tumour suppressor in breast carcinoma. CD146 expression is frequently lost in breast carcinomas and overexpression of CD146 in breast carcinoma cells results in a more cohesive cell growth and the formation of smaller tumours in nude mice. During implantation and placentation, CD146 expressed by the intermediate trophoblast in the placental site binds to its putative receptor in uterine smooth muscle cells and limits trophoblastic invasion in the myometrium. In conclusion, CD146 is a recently identified novel cell adhesion molecule and its biological functions and role as a diagnostic marker in pathology are now being recognized. Identification of the receptor for CD146 and the development of experimental models that can account for the complex interactions between CD146-expressing cells and their microenvironment are needed to investigate further the functions of this molecule in biology and in pathological states.

Further characterization of cells expressing STRO-1 in cultures of adult human bone marrow stromal cells

Primitive cells of the osteoblast lineage are not well characterized but are known to be present within the STRO-1+ fraction of adult human bone and marrow. A survey of human osteosarcoma cell lines revealed that STRO-1 is expressed by MG-63 but not SaOS-2. Among murine cell lines tested, expression of STRO-1 was detected in the bipotential (adipocyte/osteoblast) line BMS-2 but not the committed osteoblast precursor MC3T3-E1. A proportion of cultured adult human bone marrow stromal cells (BMSCs) consistently expressed the STRO-1 antigen. The expression of a range of cell surface antigens was studied in relation to STRO-1 by flow cytometry and several, including the bone/liver/kidney isoform of alkaline phosphatase (ALP), were found to subtype the STRO-1+ population of BMSCs. Further, BMSCs dual-labeled with antibodies recognizing STRO-1 and ALP could be assigned to one of four fractions: STRO-1-/ALP-, STRO-1+/ALP-, STRO-1+/ALP+, and STRO-1-/ALP+. Cells from each fraction could be isolated in high purity and, when recultured, remained viable and exhibited a limited degree of phenotypic stability. Using reverse transcriptase-polymerase chain reaction, cells in the four fractions were found to express different levels of transcripts for the parathyroid hormone receptor (PTHr) and bone sialoprotein (BSP). The expression of transcripts for the nuclear transcription factor core-binding factor alpha 1/osteoblast-specific factor-2 (CBFA1/OSF2) was restricted to those fractions expressing STRO-1 and/or ALP. Treatment with 10 nM dexamethasone consistently increased the proportion of cells present in those fractions which expressed the highest levels of transcripts for PTHr and BSP (STRO-1+/ALP+ and STRO-1-/ALP+) while simultaneously decreasing the proportion present in the STRO-1+/ALP- fraction. In conclusion, the expression of STRO-1 in vitro remains a characteristic of less well differentiated cells of the osteoblast lineage; in cultures of BMSCs and in established human osteosarcoma cell lines, there is an inverse association between the expression of STRO-1 and ALP; dual labeling of BMSCs with monoclonal antibodies recognizing STRO-1 and ALP permits the identification and isolation of cells of the osteoblast lineage at different stages of differentiation.

Influence of MUC18/MCAM/CD146 expression on human melanoma growth and metastasis in SCID mice

The cell surface glycoprotein MUC18MCAM/CD146 was originally defined as a marker of melanoma progression and has been suspected to be directly linked to the metastatic process of this malignancy. In order to address this question, 2 MCAM negative human melanoma cell lines, SK-2 and XP44RO(Mel), were transfected with MCAM-encoding cDNA. Surface MCAM expression on SK-2 and XP44RO(Mel) transfectants was similar to that observed in naturally occurring MCAM positive human melanoma cells and transfectants demonstrated MCAM-dependent increase in homotypic adhesion in vitro. The growth behavior of 7 MCAM transfectants and their respective vector controls was evaluated in SCID mice. Tumor size at 4-5 weeks after s.c. implantation was highly variable, but did not correlate with MCAM expression. Despite massive primary tumor formation at the injection site, no spontaneous metastasis was observed with any of the investigated MCAM transfectants. The influence of MCAM expression on lung metastases formation in an experimental metastasis assay was system dependent, converting only XP44RO(Mel) transfectants into metastatic cells, although increased homotypic adhesion, leading to formation of tumor cell clusters, was observed with transfectants of both cell lines in vitro. Our findings indicate that MCAM expression of human melanoma cells has an influence on later stages of the metastatic process only, namely, extravasation and establishment of new foci of growth, but is per se not sufficient for this process.