Human mesenchymal stem cells modulate allogeneic immune cell responses

Mesenchymal stem cells (MSCs) are multipotent cells found in several adult tissues. Transplanted allogeneic MSCs can be detected in recipients at extended time points, indicating a lack of immune recognition and clearance. As well, a role for bone marrow-derived MSCs in reducing the incidence and severity of graft-versus-host disease (GVHD) during allogeneic transplantation has recently been reported; however, the mechanisms remain to be investigated. We examined the immunomodulatory functions of human MSCs (hMSCs) by coculturing them with purified subpopulations of immune cells and report here that hMSCs altered the cytokine secretion profile of dendritic cells (DCs), naive and effector T cells (T helper 1 [T(H)1] and T(H)2), and natural killer (NK) cells to induce a more anti-inflammatory or tolerant phenotype. Specifically, the hMSCs caused mature DCs type 1 (DC1) to decrease tumor necrosis factor alpha (TNF-alpha) secretion and mature DC2 to increase interleukin-10 (IL-10) secretion; hMSCs caused T(H)1 cells to decrease interferon gamma (IFN-gamma) and caused the T(H)2 cells to increase secretion of IL-4; hMSCs caused an increase in the proportion of regulatory T cells (T(Regs)) present; and hMSCs decreased secretion of IFN-gamma from the NK cells. Mechanistically, the hMSCs produced elevated prostaglandin E2 (PGE(2)) in co-cultures, and inhibitors of PGE(2) production mitigated hMSC-mediated immune modulation. These data offer insight into the interactions between allogeneic MSCs and immune cells and provide mechanisms likely involved with the in vivo MSC-mediated induction of tolerance that could be therapeutic for reduction of GVHD, rejection, and modulation of inflammation.

Mesenchymal stem cell-natural killer cell interactions: evidence that activated NK cells are capable of killing MSCs, whereas MSCs can inhibit IL-2-induced NK-cell proliferation

In recent years, mesenchymal stem cells (MSCs) have been shown to inhibit T-lymphocyte proliferation induced by alloantigens or mitogens. However, no substantial information is available regarding their effect on natural killer (NK) cells. Here we show that MSCs sharply inhibit IL-2-induced proliferation of resting NK cells, whereas they only partially affect the proliferation of activated NK cells. In addition, we show that IL-2-activated NK cells (but not freshly isolated NK cells) efficiently lyse autologous and allogeneic MSCs. The activating NK receptors NKp30, NKG2D, and DNAM-1 represented the major receptors responsible for the induction of NK-mediated cytotoxicity against MSCs. Accordingly, MSCs expressed the known ligands for these activating NK receptors-ULBPs, PVR, and Nectin-2. Moreover, NK-mediated lysis was inhibited when IFN-gamma-exposed MSCs were used as target cells as a consequence of the up-regulation of HLA class I molecules at the MSC surface. The interaction between NK cells and MSCs resulted not only in the lysis of MSCs but also in cytokine production by NK cells. These results should be taken into account when evaluating the possible use of MSCs in novel therapeutic strategies designed to improve engraftment or to suppress graft-versus-host disease (GVHD) in bone marrow transplantation.

Isolation and direct characterization of resident microglial cells from the normal and inflamed central nervous system

In addition to the major population of infiltrating leukocytes recovered from inflamed rat central nervous system (CNS), all of which expressed high levels of leukocyte common antigen CD45, many cells were coisolated that were MRC OX42+ (complement receptor 3/CD11b) but expressed low-to-moderate levels of CD45 and major histocompatibility complex (MHC) class I molecules. Most cells from normal CNS, in contrast, lay within this latter, CD45low population. From previous in situ immunohistochemical studies, the fortuitously isolated CD45low cells were probably resident (ramified) microglia. Using irradiation chimeras, we show that resident microglia respond to inflammation by upregulating CD45, CD4, and MHC class I molecules with a minority of these cells increasing their expression of MHC class II molecules. A 3- to 4-fold increase in the number of microglia isolated from inflamed CNS provided indirect evidence that the cells had proliferated. In normal CNS, a very small population of blood-derived CD45high-expressing cells are present; most MHC class II expression is associated with these few cells and not with the resident microglia.

Cell surface antigens on human marrow-derived mesenchymal cells are detected by monoclonal antibodies

Human bone marrow has been shown to contain mesenchymal cells, which fabricate the connective tissue network of the marrow called the stroma. A subset of these marrow-derived mesenchymal cells can be isolated, expanded in culture, and then induced to differentiate into bone-producing osteoblasts and ultimately osteocytes when placed in the proper environment. At present, there are no methods for definitively identifying these cells in human marrow tissue or following their differentiation into osteogenic phenotypes. Therefore, we culture-expanded, marrow-derived mesenchymal cells from human donors and used these cells to immunize cells from human donors and used these cells to immunize mice whose spleens were used to generate hybridoma cell lines, which secrete antibodies to antigens on the cell surface of these culture-expanded mesenchymal cells. Hybridoma culture supernatants were successively screened against highly enriched samples of culture-expanded, marrow-derived mesenchymal cells in cryosections and live cell cultures to identify unique cell surface antigens. Positive clones were then screened against cell suspensions of whole and fractionated marrow to identify hybridomas whose supernatants were nonreactive with marrow hemopoietic cells. Three hybridoma cell lines, SH2, SH3, and SH4, were identified; these hybridomas secrete antibodies that recognize antigens on the cell surface of marrow-derived mesenchymal cells, but fail to react with marrow-derived hemopoietic cells. Additional tissue screening reveals unique tissue distributions for each of the recognized antigens, which suggests different antigen recognition for each antibody. However, all three antibodies fail to react with the cell surface of osteoblasts or osteocytes, suggesting that the antigens recognized by these antibodies are developmentally regulated and specific for primitive or early-stage cells of the osteogenic lineage.

Management of malignant gastrointestinal stromal tumours

Gastrointestinal stromal tumours (GISTs) are the most common form of mesenchymal tumour of the gastrointestinal tract. Clinically, they range from small indolent tumours curable with surgery alone to aggressive cancers. Making a distinction between an indolent and a malignant GIST is unreliable with conventional histopathological techniques. The presence of metastases at the time of diagnosis confirms malignancy, but all GISTs should be regarded as having malignant potential. GISTs characteristically express the KIT protein, a transmembrane tyrosine kinase receptor for stem-cell factor. Most GISTs have a mutation in the KIT proto-oncogene that translates into a gain-of-function constitutive activation of the KIT kinase. KIT activation seems to be an early tumour-promoting event in pathogenesis. Commonly, malignant GISTs show high-level primary resistance to conventional chemotherapy. Imatinib mesylate is an orally administered selective inhibitor of certain tyrosine kinases including KIT. Most patients with advanced malignant GISTs achieve clinical benefit and significant antitumour responses with imatinib mesylate. Responses have been durable, and most patients tolerate the drug well at clinically effective doses. Imatinib mesylate is the first effective systemic therapy for advanced GIST.

Epithelial-mesenchymal interactions in the pathogenesis of asthma

During lung development, repair, and inflammation, local production of cytokines (eg, transforming growth factor-beta) and growth factors (eg, epidermal growth factor) by epithelial and mesenchymal cells mediate bidirectional growth control effectively creating an epithelial-mesenchymal trophic unit. In asthma the bronchial epithelium is highly abnormal, with structural changes involving separation of columnar cells from their basal attachments and functional changes including increased expression and release of proinflammatory cytokines, growth factors, and mediator-generating enzymes. Beneath this damaged structure there is an increase in the number of subepithelial myofibroblasts that deposit interstitial collagens causing thickening and increased density of the subepithelial basement membrane. Our recent studies suggest that the extent of epithelial damage in asthma may be the result of impaired epidermal growth factor receptor-mediated repair. In view of the close spatial relationship between the damaged epithelium and the underlying myofibroblasts, we propose that impaired epithelial repair cooperates with the T(H)2 environment to shift the set point for communication within the trophic unit. This leads to myofibroblast activation, excessive matrix deposition, and production of mediators that propagate and amplify the remodeling responses throughout the airway wall.

Lymphostromal interactions in thymic development and function

The generation of a peripheral T-cell pool is essential for normal immune system function. CD4+ and CD8+ T cells are produced most efficiently in the thymus, which provides a complexity of discrete cellular microenvironments. Specialized stromal cells, that make up such microenvironments, influence each stage in the maturation programme of immature T-cell precursors. Progress has recently been made in elucidating events that regulate the development of intrathymic microenvironments, as well as mechanisms of thymocyte differentiation. It is becoming increasingly clear that the generation and maintenance of thymic environments that are capable of supporting efficient T-cell development, requires complex interplay between lymphoid and stromal compartments of the thymus.

Immune-induced epithelial to mesenchymal transition in vivo generates breast cancer stem cells

The breast cancer stem cell (BCSC) hypotheses suggest that breast cancer is derived from a single tumor-initiating cell with stem-like properties, but the source of these cells is unclear. We previously observed that induction of an immune response against an epithelial breast cancer led in vivo to the T-cell-dependent outgrowth of a tumor, the cells of which had undergone epithelial to mesenchymal transition (EMT). The resulting mesenchymal tumor cells had a CD24(-/lo)CD44(+) phenotype, consistent with BCSCs. In the present study, we found that EMT was induced by CD8 T cells and the resulting tumors had characteristics of BCSCs, including potent tumorigenicity, ability to reestablish an epithelial tumor, and enhanced resistance to drugs and radiation. In contrast to the hierarchal cancer stem cell hypothesis, which suggests that breast cancer arises from the transformation of a resident tissue stem cell, our results show that EMT can produce the BCSC phenotype. These findings have several important implications related to disease progression and relapse.

Veto-Like Activity of Mesenchymal Stem Cells: Functional Discrimination Between Cellular Responses to Alloantigens and Recall Antigens

Trans-differentiation of stem cells shows promise for use in tissue repair medicine. Although poorly defined, mesenchymal stem cells (MSC) appear useful for applications in repair medicine. Despite the low frequency of MSC, they are relatively easy to expand. The expression of MHC class II on MSC, however, could deter their use in repair medicine, since these molecules could stimulate an allogeneic host response. This study sought to compare the immune stimulatory and suppressive effects of MSC. Primary human MSC were cultured from bone marrow aspirates and then passaged at least three times before use in assays. Morphologically, MSC were symmetrical; were SH2(+), MHC class II(+), CD45(-), CD44(+), CD31(-), CD14(-), proly-4-hydroxylase(-); and showed normal karyotype patterns and elevated telomerase activities. MSC elicited significant stimulatory responses when cocultured with allogeneic PBMC. Despite the production of different types of growth factors, allogeneic effects of MSC could not be explained by the production of these growth factors. One-way MLR reactions were significantly blunted by third-party MSC. Similar suppression was not observed for responses to three different recall Ags. Based on these functional differences by MSC in responses to allo- and recall Ags, we examined whether MSC could exert veto-like functions. We showed that MSC could blunt the cytotoxic effects of allogeneic-induced effectors to mitogen-activated targets. The results showed that although MSC elicited allogeneic responses in a model that mimics a graft-vs-host reaction, they also exerted veto-like activity, but caused no effect on responses to recall Ags.

Mesenchymal stem cells are capable of homing to the bone marrow of non-human primates following systemic infusion

OBJECTIVE

The human bone marrow contains mesenchymal stem cells capable of differentiating along multiple mesenchymal cell lineages. Using a non-human primate model, we sought to determine whether the systemic infusion of baboon-derived mesenchymal stem cells was associated with toxicity and whether these cells were capable of homing to and persisting within the bone marrow.

MATERIALS AND METHODS

Five baboons (Papio anubis) were administered lethal irradiation followed by intravenous autologous hematopoietic progenitor cells combined with either autologous (n = 3) or allogeneic (n = 2) mesenchymal stem cells that had been expanded in culture. In four of these baboons, the mesenchymal stem cells were genetically modified with a retroviral vector encoding either the enhanced green fluorescent protein gene (n = 3) or the human placental alkaline phosphatase gene (n = 1) for tracking purposes. A sixth animal received only intravenous gene marked autologous mesenchymal stem cells but no hematopoietic stem cells or conditioning irradiation.

RESULTS

Following culture, baboon mesenchymal stem cells appeared morphologically as a homogeneous population of spindle-shaped cells that were identified by the monoclonal antibodies SH-3 and SH-4. These cells did not express the hematopoietic markers CD34 or CD45. Baboon mesenchymal stem cells isolated from primary culture were capable of differentiating along both adipogenic and osteogenic lineages. There was no acute or chronic toxicity associated with the intravenous infusion of mesenchymal stem cells. In all five recipients of gene marked mesenchymal stem cells, transgene was detected in post-transplant bone marrow biopsies. In two animals receiving autologous mesenchymal stem cells, including the one non-conditioned recipient, transgene could be detected over 1 year following infusion. In one recipient of allogeneic gene marked mesenchymal stem cells, transgene was detected in the bone marrow at 76 days following infusion.

CONCLUSION

These data demonstrate that baboon mesenchymal stem cells: 1) are not associated with significant toxicity when administered intravenously, 2) are capable of homing to the bone marrow following intravenous infusion, and 3) have the capacity to establish residence within the bone marrow for an extended duration following systemic administration.

Mesenchymal cells of the intestinal lamina propria

The mesenchymal elements of the intestinal lamina propria reviewed here are the myofibroblasts, fibroblasts, mural cells (pericytes) of the vasculature, bone marrow-derived stromal stem cells, smooth muscle of the muscularis mucosae, and smooth muscle surrounding the lymphatic lacteals. These cells share similar marker molecules, origins, and coordinated biological functions previously ascribed solely to subepithelial myofibroblasts. We review the functional anatomy of intestinal mesenchymal cells and describe what is known about their origin in the embryo and their replacement in adults. As part of their putative role in intestinal mucosal morphogenesis, we consider the intestinal stem cell niche. Lastly, we review emerging information about myofibroblasts as nonprofessional immune cells that may be important as an alarm system for the gut and as a participant in peripheral immune tolerance.

MHC class II-positive epithelium and mesenchyme cells are both required for T-cell development in the thymus

T lymphocytes are produced in the thymus from precursors originating in the haemopoietic tissues. On entering the thymus, they undergo a programme of proliferation, T-cell receptor (TCR) gene rearrangement, differentiation and repertoire selection. Although the thymus provides a unique environment for these events, the role of the thymic stroma in regulating specific developmental stages is not well understood. We therefore devised an in vitro system to study the role of individual thymic stromal components in T-cell development. We report here that the development of TCR-CD4-CD8-T-cell precursors into TCR+ cells expressing CD4 and/or CD8 requires the presence of both major histocompatibility complex class II+ epithelial cells and fetal mesenchyme. The requirement for mesenchymal support can be mapped to the initial stages of intrathymic development because the later stages of maturation, from double-positive CD4+CD8+ thymocytes into single-positive CD4+ or CD8+ cells, can be supported by epithelial cells alone. We also show that the requirement for mesenchymal cells can be met by cells of the fibroblast line 3T3 (but not by supernatants from these cells). To our knowledge, these findings provide the first direct evidence that mesenchymal as well as epithelial cells are involved in T-cell development, and suggest that their involvement is stage-specific and likely to be dependent on short-range or contact-mediated interactions.

The airway epithelium is central to the pathogenesis of asthma

Asthma is an inflammatory disorder principally involving the conducting airways and characterised by infiltration of the airway wall with a range of inflammatory cells driven in large part by activation of Th2-type lymphocytes, mast cells and eosinophils. However a key component of asthma is the structural change that involves all of the elements of the airway wall. Here evidence is presented to suggest that the airway epithelium in asthma is fundamentally abnormal with increased susceptibility to environmental injury and impaired repair associated with activation of the epithelial-mesenchymal trophic unit (EMTU). In addition to adopting an activated phenotype, the barrier function of the epithelium is impaired through defective tight junction formation thereby facilitating penetration of potentially toxic or damaging environmental insults. Activated and repairing epithelial cells generate a range of growth factors that are involved in the early life origins of this disease as well as its progression in the form of mucous metaplasia and airway wall remodeling. By placing the epithelium at the forefront of asthma pathogenesis, different approaches to treatment can be devised focused more on protecting vulnerable airways against environmental injury rather than focusing on suppressing airway inflammation or manipulating the immune response.

Human circulating CD14+ monocytes as a source of progenitors that exhibit mesenchymal cell differentiation

Circulating CD14+ monocytes are precursors of phagocytes, such as macrophages and dendritic cells. Here we report primitive cells with a fibroblast-like morphology derived from human peripheral blood CD14+ monocytes that can differentiate into several distinct mesenchymal cell lineages. We named this cell population monocyte-derived mesenchymal progenitor (MOMP). MOMPs were obtained in vitro from human peripheral blood mononuclear cells cultured on fibronectin in the presence of fetal bovine serum alone as a source of growth factors. MOMPs had a unique molecular phenotype-CD14+CD45+CD34+type I collagen+-and showed mixed morphologic and molecular features of monocytes and endothelial and mesenchymal cells. MOMPs were found to be derived from a subset of circulating CD14+ monocytes, and their differentiation required that they bind fibronectin and be exposed to one or more soluble factors derived from peripheral blood CD14- cells. MOMPs could be expanded in culture without losing their original phenotype for up to five passages. The induction of MOMPs to differentiate along multiple limb-bud mesodermal lineages resulted in the expression of genes and proteins specific for osteoblasts, skeletal myoblasts, chondrocytes, and adipocytes. Our findings represent the first evidence that human circulating CD14+ monocytes are a source of progenitors that exhibit mesenchymal cell differentiation.

Organizer-like reticular stromal cell layer common to adult secondary lymphoid organs

Mesenchymal stromal cells are crucial components of secondary lymphoid organs (SLOs). Organogenesis of SLOs involves specialized stromal cells, designated lymphoid tissue organizer (LTo) in the embryonic anlagen; in the adult, several distinct stromal lineages construct elaborate tissue architecture and regulate lymphocyte compartmentalization. The relationship between the LTo and adult stromal cells, however, remains unclear, as does the precise number of stromal cell types that constitute mature SLOs are unclear. From mouse lymph nodes, we established a VCAM-1(+)ICAM-1(+)MAdCAM-1(+) reticular cell line that can produce CXCL13 upon LTbetaR stimulation and support primary B cell adhesion and migration in vitro. A similar stromal population sharing many characteristics with the LTo, designated marginal reticular cells (MRCs), was found in the outer follicular region immediately underneath the subcapsular sinus of lymph nodes. Moreover, MRCs were commonly observed at particular sites in various SLOs even in Rag2(-/-) mice, but were not found in ectopic lymphoid tissues, suggesting that MRCs are a developmentally determined element. These findings lead to a comprehensive view of the stromal composition and architecture of SLOs.

Immunologic properties of human fetal mesenchymal stem cells

OBJECTIVE

Mesenchymal stem cells (MSCs) can be isolated from adult bone marrow and fetal liver. We investigated the immunologic properties of undifferentiated and differentiated human fetal MSCs.

STUDY DESIGN

Expression of HLA class I and II was investigated by flow cytometry and Western blot on undifferentiated fetal MSC and after in vitro differentiation to adipocytes and osteocytes. Alloreactivity was studied after adding fetal MSCs to allogeneic lymphocytes in mixed lymphocyte cultures.

RESULTS

Fetal MSCs expressed HLA class I but not HLA class II. The presence of interferon gamma (IFN-gamma) in the growth medium for 2 days initiated the intracellular synthesis of HLA class II, but 7 days of exposure was required for cell surface expression. Neither undifferentiated nor differentiated fetal MSCs induced proliferation of allogenic lymphocytes. Fetal MSCs treated with IFN-gamma suppressed alloreactive lymphocytes.

CONCLUSION

Undifferentiated and differentiated fetal MSCs do not elicit alloreactive lymphocyte proliferation. The results suggest that fetal MSCs have potentials for allogenic transplantation.

MHC class II-positive microglia in human brain: association with Alzheimer lesions

Cells of the mononuclear phagocytic system (MPS) present foreign antigen on their cell surfaces bound to major histocompatibility complex (MHC) class II molecules. Previous studies of normal human brain samples reported MHC class II expression primarily by perivascular MPS cells and white matter microglial cells. Marked increases in MHC class II-expressing microglia have been shown in many neuropathologic disorders, including Alzheimer's disease (AD). A close morphologic association between these cells and Alzheimer senile plaque beta-amyloid has been demonstrated. The present study used a mixed aldehyde fixative to enhance the localization of MHC class II-expressing MPS cells in non-AD and AD brain. Two antibodies against MHC class II (HLA-DR; LN3), as well as the lectin Ricinus communis agglutinin (which recognizes both ramified and activated microglia) were used for light and electron microscopic analyses. We now report that MHC class II-expressing ramified microglia are distributed in a uniform reticular array throughout the grey, as well as the white matter in non-AD cases. In AD cases, immunolabelled cells had the morphology of activated microglia, with darkly stained plump somata and short, thick processes. Microglia clustered around senile plaque amyloid and neurofibrillary tangles (NFT), rather than forming the uniform array characteristic of control tissue. Finally, we report that perivascular MPS cells are found in a morphologic relationship with vascular amyloid identical to that seen between microglial cells and senile plaque beta-amyloid. These data suggest that MHC class II-expressing cells may be involved in the degradation of NFT-laden neurons and the posttranslational modification of extracellular-NFT epitopes. In addition, both parenchymal and perivascular MPS cells are ideally situated to uptake and process the beta-amyloid protein precursor and deposit beta-amyloid on senile plaques, NFT, and the cerebrovasculature.

Microglia: phagocyte and glia cell

Microglia are the resident immune cells of the brain, and are located within the brain parenchyme behind the blood-brain barrier. They originate from mesodermal hemapoietic precursors and are slowly turned over and replenished by proliferation in the adult central nervous system. In the healthy brain resting, ramified microglia function as supportive glia cells, and their activation status is regulated by neurons through soluble mediators and cell-cell contact. However, in response to brain pathology microglia become activated: acquisition of innate immune cell functions render microglia competent to react towards brain injury through tissue repair or induction of immune responses. In certain pathological conditions, however, microglia activation may sustain a chronic inflammation of the brain, leading to neuronal dysfunction and cell death. This might be mediated by the microglial release of extracellular toxic reactive oxygen and nitrogen species. Nevertheless, in the future microglia may potentially be harnessed for therapeutical purposes.

Monoclonal antibodies reactive with human osteogenic cell surface antigens

Monoclonal antibodies (McAbs) against the surface of osteoblastic cells have been used to characterize the osteogenic lineage. In view of the paucity of probes against the surface of normal human osteogenic cells, we sought to generate McAbs which could be used for both in vivo and in vitro studies. We raised a series of McAbs against early osteoblastic cell surface antigens by immunizing mice with human mesenchymal stem cells (MSCs) that had been directed into the osteogenic lineage in vitro. After screening against the surface of osteogenic cells at various stages of differentiation in vitro, as well as evaluating in situ reactivity with human fetal limbs, we isolated three hybridoma cell lines referred to as SB-10, SB-20, and SB-21. Immunocytochemical analyses during osteogenic differentiation demonstrate that SB-10 reacts with MSCs and osteoprogenitors, but no longer reacts with cells once alkaline phosphatase (APase) is expressed. Flow cytometry documents that SB-10 is expressed on the surface of all purified, culture-expanded human MSCs, thus providing further evidence that these cells are a homogeneous population. By contrast, SB-20 and SB-21 do not react with the progenitor cells in situ, but bind to a subset of the APase-positive osteoblasts. None of these antibodies stain terminally differentiated osteocytes in sections of developing bone. Furthermore, these McAbs were not observed to react in samples from chick, rat, rabbit, canine, or bovine bone, although selected extraskeletal human tissues were immunostained. In all cell and tissue specimens examined, SB-20 immunostaining is identical to that observed with SB-21. We have used these McAbs to refine our understanding of the discrete cellular transitions that constitute the osteogenic cell lineage. We suggest a refined model for understanding osteoblast differentiation that is based on the proposition that the sequential acquisition and loss of specific cell surface molecules can be used to define positions of individual cells within the osteogenic cell lineage.

Fibroblast-Like Synoviocytes of Mesenchymal Origin Express Functional B Cell-Activating Factor of the TNF Family in Response to Proinflammatory Cytokines

Immunohistochemical analysis revealed that the intimal lining cells of synovial tissue of inflamed joints of patients with rheumatoid arthritis differed from that of normal joints or of diseased joints in osteoarthritis in that they stained with mAb specific for the B cell-activating factor of the TNF family (BAFF; also called BLyS). We generated fibroblast-like synoviocytes (FLS) cell lines that were bereft of myelomonocytic cells to examine whether mesenchymal-derived FLS could express this critical B cell survival factor. We found that FLS expressed low amounts of BAFF mRNA relative to that of myelomonocytic cells. However, when various cytokines/factors were added to such FLS cell lines, we found that IFN-gamma or TNF-alpha were unique in that they could induce significant increases in BAFF mRNA and protein. Even minute amounts of IFN-gamma primed FLS for TNF-alpha, allowing the latter to stimulate significantly higher levels of BAFF mRNA and protein than could TNF-alpha alone. Consistent with this, B cells cocultured with IFN-gamma and/or TNF-alpha-treated FLS had a significantly greater viability than B cells cocultured with nontreated FLS. The enhanced protection of B cells afforded by IFN-gamma/TNF-alpha-treated FLS was inhibited by the addition of BAFF-R:Fc fusion protein. We conclude that the proinflammatory cytokines IFN-gamma and TNF-alpha can induce mesenchymal-derived FLS to express functional BAFF in vitro. The induced expression of BAFF on FLS by proinflammatory cytokines may enhance the capacity of such cells to protect B cells from apoptosis in inflammatory microenvironments in vivo.

Immunogenicity of adult mesenchymal stem cells: lessons from the fetal allograft

Herein we review recent data that support host tolerance of allogeneic adult mesenchymal stem cells (MSC). Evidence is emerging that donor MSC deploy a very powerful array of mechanisms that allow escape from host allogeneic responses. These mechanisms include limited expression of alloantigen by the stem cell and cell contact-dependent and -independent mechanisms. MSC modulate host dendritic cell and T cell function, promoting induction of suppressor or regulatory T cells. These effects are complemented by the induction of divisional arrest anergy in T cells and by stem cell production of soluble immunomodulatory factors, including interleukin-10, transforming growth factor-beta, prostaglandin E2, and hepatocyte growth factor. In addition, MSC express the enzyme indoleamine 2,3-dioxygenase, which creates a tryptophan-depleted milieu that promotes immunosuppression. We propose that these observations show striking similarity to emerging data on the maternal acceptance of the fetal allograft. This comparison suggests new approaches to determine the contribution of different mechanisms to the successful use of MSC in regenerative medicine.

Immunological and biochemical studies of collagen type transition during in vitro chrondrogenesis of chick limb mesodermal cells

This work describes an approach to monitor chondrogenesis of stage-24 chick limb mesodermal cells in vitro by analyzing the onset of type II collagen synthesis with carboxymethyl-cellulose chromatography, immunofluorescence, and radioimmunoassay. This procedure allowed specific and quantitative determination of chondrocytes in the presence of fibroblasts and myoblasts, both of which synthesize type I collagen. Chondrogenesis was studied in high-density cell preparations on tissue culture plastic dishes and on agar base. It was found that stage-24 limb mesenchymal cells initially synthesized only type I collagen. With the onset of chondrogenesis, a gradual transition to type II collagen synthesis was observed. In cell aggregates formed over agar, type II collagen synthesis started after 1 day in culture and reached levels of 80-90 percent of the total collagen synthesis at 6-8 days. At that time, the cells in the center of the aggregates had acquired the typical chondrocyte phenotype and stained only with type II collagen antibodies, whereas the peripheral cells had developed into a "perichondrium" and stained with type I and type II collagen antibodies. On plastic dishes plated with 5 X 10(6) cells per 35mm dish, cartilage nodules developed after 4-6 days, but the type II collagen synthesis only reached levels of 10-20 percent of the total collagen. The majority of the cells differentiated into fibroblasts and myoblasts and synthesized type I collagen. These studies demonstrate that analysis of cell specific types of collagen provides a useful method for detailing the specific events in the differentiation of mesenchymal cells in vitro.

IL-21 inhibits IFN-gamma production in developing Th1 cells through the repression of Eomesodermin expression

Exposure of naive Th cell precursors (Thp) to IL-21 inhibits IFN-gamma production from developing Th1 cells. The inhibition of IFN-gamma seen in IL-21-treated Thp cells is specific as the expression of other Th1 cytokines is unaffected. Recently, it has been reported that Eomesodermin (Eomes), a member of the T-box gene family, is expressed in developing CD8+ T cells and plays an important role in regulating IFN-gamma production and cytolytic effector function. In this study, we show that Eomes mRNA and protein are also expressed in developing Th1 cells, and exposure of naive Thp cells to IL-21 results in a decrease in Eomes expression. Moreover, the repression of Eomes expression by IL-21 is not due to an indirect effect of IL-21 on the expression of IFN-gamma or STAT4 and is independent of STAT1 and T-bet expression. Finally, we show that ectopic expression of Eomes prevents the inhibition of IFN-gamma production from IL-21-treated Thp cells. Taken together, these results demonstrate that Eomes plays a role in regulating IFN-gamma production in CD4+ T cells and IL-21 inhibits IFN-gamma production in developing Th1 cells through the repression of Eomes expression.

Phenotypic characterization and ontogeny of mesodermal-derived and endocrine epithelial components of the human thymic microenvironment

Using murine monoclonal antibodies TE-4 and TE-7 raised against human thymic stroma, we identified two distinct and mutually exclusive thymic microenvironment components: the thymic endocrine epithelium (TE-4+) and mesodermal-derived fibrous stroma (TE-7+). TE-4-reactive epithelium did not react with antibody TE-7, contained thymosin alpha 1 and keratin, and expressed other known markers of thymic endocrine epithelium (A2B5 and p19). Moreover, TE-4+ thymic epithelial cells strongly expressed class I (HLA-A, -B and -C) and class II (Ia-like) major histocompatibility complex (MHC) antigens. In contrast, TE-7+ thymic fibrous stroma did not react with antibody TE-4, did not contain thymosin alpha 1 nor keratin, and did not express the thymic endocrine epithelium markers A2B5 and p19. TE-7+ thymic stromal cells weakly expressed class I and did not express class II MHC antigens. Both TE-4+ and TE-7+ thymic microenvironment compartments were identifiable in thymus from 7 wk gestation through adult life. At 7 wk fetal gestation, TE-7+ stroma surrounded a cylindrical TE-4+, A2B5+ thymic epithelial rudiment. Between 10 and 15 wk fetal gestation, TE-7+ thymic stroma surrounded early thymic lobules. By 15 wk fetal gestation, antibody TE-4 defined subcapsular cortical and medullary zones of endocrine thymic epithelium, while antibody TE-7 bound to interlobular fibrous septae, vessels, and thymic fibrous capsule. While otherwise specific for endocrine thymic epithelium, antibody TE-4 reacted with the basal layer of squamous epithelium in skin, tonsil, conjunctiva, and upper esophagus.

Differential requirement for mesenchyme in the proliferation and maturation of thymic epithelial progenitors

Formation of a mature thymic epithelial microenvironment is an essential prerequisite for the generation of a functionally competent T cell pool. It is likely that recently identified thymic epithelial precursors undergo phases of proliferation and differentiation to generate mature cortical and medullary thymic microenvironments. The mechanisms regulating development of immature thymic epithelial cells are unknown. Here we provide evidence that expansion of embryonic thymic epithelium is regulated by the continued presence of mesenchyme. In particular, mesenchymal cells are shown to mediate thymic epithelial cell proliferation through their provision of fibroblast growth factors 7 and 10. In contrast, differentiation of immature thymic epithelial cells, including acquisition of markers of mature cortical and medullary epithelium, occurs in the absence of ongoing mesenchymal support. Collectively, our data define a role for mesenchymal cells in thymus development, and indicate distinct mechanisms regulate proliferation and differentiation of immature thymic epithelial cells. In addition, our findings may aid in studies aimed at developing strategies to enhance thymus reconstitution and functioning in clinical certain contexts where thymic epithelial cell function is perturbed.