Intestinal epithelial cell-derived semaphorin 7A negatively regulates development of colitis via αvβ1 integrin

Immunological functions of the neuropilins and plexins as receptors for semaphorins

Semaphorins were originally identified as axon-guidance molecules that function during neuronal development. However, cumulative evidence indicates that semaphorins also participate in immune responses, both physiological and pathological, and they are now considered to be potential diagnostic and/or therapeutic targets for a range of diseases. The primary receptors for semaphorins are neuropilins and plexins, which have cell type-specific patterns of expression and are involved in multiple signalling responses. In this Review, we focus on the roles of neuropilin 1 (NRP1) and plexins in the regulation of the immune system, and we summarize recent advances in our understanding of their pathological implications.

Differential marker expression by cultures rich in mesenchymal stem cells

Background

Mesenchymal stem cells have properties that make them amenable to therapeutic use. However, the acceptance of mesenchymal stem cells in clinical practice requires standardized techniques for their specific isolation. To date, there are no conclusive marker (s) for the exclusive isolation of mesenchymal stem cells. Our aim was to identify markers differentially expressed between mesenchymal stem cell and non-stem cell mesenchymal cell cultures. We compared and contrasted the phenotype of tissue cultures in which mesenchymal stem cells are rich and rare. By initially assessing mesenchymal stem cell differentiation, we established that bone marrow and breast adipose cultures are rich in mesenchymal stem cells while, in our hands, foreskin fibroblast and olfactory tissue cultures contain rare mesenchymal stem cells. In particular, olfactory tissue cells represent non-stem cell mesenchymal cells. Subsequently, the phenotype of the tissue cultures were thoroughly assessed using immuno-fluorescence, flow-cytometry, proteomics, antibody arrays and qPCR.

Results

Our analysis revealed that all tissue cultures, regardless of differentiation potential, demonstrated remarkably similar phenotypes. Importantly, it was also observed that common mesenchymal stem cell markers, and fibroblast-associated markers, do not discriminate between mesenchymal stem cell and non-stem cell mesenchymal cell cultures. Examination and comparison of the phenotypes of mesenchymal stem cell and non-stem cell mesenchymal cell cultures revealed three differentially expressed markers – CD24, CD108 and CD40.

Conclusion

We indicate the importance of establishing differential marker expression between mesenchymal stem cells and non-stem cell mesenchymal cells in order to determine stem cell specific markers.

An epithelial armamentarium to sense the microbiota

Intestinal epithelial cells were once thought to be inert, non-responsive cells that simply acted as a physical barrier that prevents the contents of the intestinal lumen from accessing the underlying tissue. However, it is now clear that these cells express a full repertoire of Toll- and Nod-like receptors, and that their activation by components of the microbiota is vital for the development of a functional epithelium, maintenance of barrier integrity, and defense against pathogenic organisms. Additionally, mounting evidence suggests that epithelial sensing of bacteria plays a significant role in the management of the numbers and types of microbes present in the gut microbiota via the production of antimicrobial peptides and other microbe-modulatory products. This is a critical process, as it is now becoming apparent that alterations in the composition of the microbiota can predispose an individual to a wide variety of chronic diseases. In this review, we will discuss the bacterial pattern recognition receptors that are known to be expressed by the intestinal epithelium, and how each of them individually contributes to these vital protective functions. Moreover, we will review what is known about the communication between epithelial cells and various classes of underlying leukocytes, and discuss how they interact with the microbiota to form a three-part relationship that maintains homeostasis in the gut.

Intestinal macrophages: well educated exceptions from the rule

Macrophages are the most abundant mononuclear phagocytes in the healthy intestinal lamina propria and have emerged as crucial sentinels for the maintenance of tissue homeostasis. Matching the dynamic mucosal landscape, CX3C chemokine receptor (CX3CR)1-expressing macrophages are relatively short lived, and as opposed to most other tissue macrophages, are continuously replaced from blood monocytes that acquire in the healthy tissue context a robust noninflammatory gene expression signature. By contrast, during gut inflammation, monocytes differentiate in the gut into proinflammatory effector cells, as well as migratory antigen-presenting cells. Manipulation of monocyte fates in the intestine might hold promise for the disease management of inflammatory bowel disorders.

Contributions of dendritic cells and macrophages to intestinal homeostasis and immune defense

Intestinal mononuclear phagocytes have collectively emerged as key players in the maintenance of gut homeostasis, the development of gut inflammation and its resolution. Moreover, recent intense research efforts of many laboratories have revealed evidence for critical labor division between lamina propria-resident CD103(+) dendritic cells and CX3CR1(+) macrophages. In depth understanding of the respective activities of these cells in the mucosal landscape might pave the way for novel treatments of inflammatory bowel disorders (IBD).

Semaphorin7A and its receptors: pleiotropic regulators of immune cell function, bone homeostasis, and neural development

Semaphorins form a large, evolutionary conserved family of cellular guidance signals. The semaphorin family contains several secreted and transmembrane proteins, but only one GPI-anchored member, Semaphorin7A (Sema7A). Although originally identified in immune cells, as CDw108, Sema7A displays widespread expression outside the immune system. It is therefore not surprising that accumulating evidence supports roles for this protein in a wide variety of biological processes in different organ systems and in disease. Well-characterized biological effects of Sema7A include those during bone and immune cell regulation, neuron migration and neurite growth. These effects are mediated by two receptors, plexinC1 and integrins. However, most of what is known today about Sema7A signaling concerns Sema7A-integrin interactions. Here, we review our current knowledge of Sema7A function and signaling in different organ systems, highlighting commonalities between the cellular effects and signaling pathways activated by Sema7A in different cell types. Furthermore, we discuss a potential role for Sema7A in disease and provide directions for further research.

Intestinal epithelial cells as mediators of the commensal-host immune crosstalk

Commensal bacteria regulate the homeostasis of host effector immune cell subsets. The mechanisms involved in this commensal-host crosstalk are not well understood. Intestinal epithelial cells (IECs) not only create a physical barrier between the commensals and immune cells in host tissues, but also facilitate interactions between them. Perturbations of epithelial homeostasis or function lead to the development of intestinal disorders such as inflammatory bowel diseases (IBD) and intestinal cancer. IECs receive signals from commensals and produce effector immune molecules. IECs also affect the function of immune cells in the lamina propria. Here we discuss some of these properties of IECs that define them as innate immune cells. We focus on how IECs may integrate and transmit signals from individual commensal bacteria to mucosal innate and adaptive immune cells for the establishment of the unique mucosal immunological equilibrium.

Semaphorin 7a+ regulatory T cells are associated with progressive idiopathic pulmonary fibrosis and are implicated in transforming growth factor-β1-induced pulmonary fibrosis

RATIONALE

Lymphocytes are increasingly associated with idiopathic pulmonary fibrosis (IPF). Semaphorin 7a (Sema 7a) participates in lymphocyte activation.

OBJECTIVES

To define the relationship between Sema 7a and lymphocytes in IPF.

METHODS

We characterized the significance of Sema 7a+ lymphocytes in humans with IPF and in a mouse model of lung fibrosis caused by lung-targeted, transgenic overexpression of TGF-β1. We determined the site of Sema 7a expression in human and murine lungs and circulation and used adoptive transfer approaches to define the relevance of lymphocytes coexpressing Sema7a and the markers CD19, CD4, or CD4+CD25+FoxP3+ in TGF-β1-induced murine lung fibrosis.

MEASUREMENTS AND MAIN RESULTS

Subjects with IPF show expression of Sema 7a on lung CD4+ cells and circulating CD4+ or CD19+ cells. Sema 7a expression is increased on CD4+ cells and CD4+CD25+FoxP3+ regulatory T cells, but not CD19+ cells, in subjects with progressive IPF. Sema 7a is expressed on lymphocytes expressing CD4 but not CD19 in the lungs and spleen of TGF-β1-transgenic mice. Sema 7a expressing bone marrow-derived cells induce lung fibrosis and alter the production of T-cell mediators, including IFN-γ, IL-4, IL-17A, and IL-10. These effects require CD4 but not CD19. In comparison to Sema 7a-CD4+CD25+FoxP3+ cells, Sema7a+CD4+CD25+FoxP3+ cells exhibit reduced expression of regulatory genes such as IL-10, and adoptive transfer of these cells induces fibrosis and remodeling in the TGF-β1-exposed murine lung.

CONCLUSIONS

Sema 7a+CD4+CD25+FoxP3+ regulatory T cells are associated with disease progression in subjects with IPF and induce fibrosis in the TGF-β1-exposed murine lung.

Systemic macrophage depletion inhibits Helicobacter bilis-induced proinflammatory cytokine-mediated typhlocolitis and impairs bacterial colonization dynamics in a BALB/c Rag2-/- mouse model of inflammatory bowel disease

Helicobacter bilis, an enterohepatic helicobacter, is associated with chronic hepatitis in aged immunocompetent inbred mice and inflammatory bowel disease (IBD) in immunodeficient mice. To evaluate the role of macrophages in H. bilis-induced IBD, Rag2(-/-) BALB/c or wild-type (WT) BALB/c mice were either sham dosed or infected with H. bilis Missouri strain under specific-pathogen-free conditions, followed by an intravenous injection of a 0.2-ml suspension of liposomes coated with either phosphate-buffered saline (control) or clodronate (a macrophage depleting drug) at 15 weeks postinfection (wpi). At 16 wpi, the ceca of H. bilis-infected Rag2(-/-) mice treated with control liposomes had significantly higher histopathological lesional scores (for cumulative typhlitis index, inflammation, edema, epithelial defects, and hyperplasia) and higher counts of F4/80(+) macrophages and MPO(+) neutrophils compared to H. bilis-infected Rag2(-/-) mice treated with clodronate liposomes. In addition, cecal quantitative PCR analyses revealed a significant suppression in the expression of macrophage-related cytokine genes, namely, Tnfa, Il-1β, Il-10, Cxcl1, and iNos, in the clodronate-treated H. bilis-infected Rag2(-/-) mice compared to the H. bilis-infected Rag2(-/-) control mice. Finally, cecal quantitative PCR analyses also revealed a significant reduction in bacterial colonization in the clodronate-treated Rag2(-/-) mice. Taken together, our results suggest that macrophages are critical inflammatory cellular mediators for promoting H. bilis-induced typhlocolitis in mice.

Semaphorins in bone development, homeostasis, and disease

Semaphorins were originally identified as axon guidance cues in the development of the nervous system. In recent years, numerous studies have determined that they are also involved in organogenesis, vascularization/angiogenesis, oncogenesis, and immune responses. In addition, the mechanisms underlying the diverse functions of semaphorins and their receptors have been identified. Recently, significant advances have been made in our understanding of the roles of semaphorins in bone remodeling, particularly the regulation of osteoclast and osteoblast differentiation and migration. Moreover, dysregulated semaphorin expression causes severe bone diseases, including osteoporosis and osteopetrosis. This review focuses on advanced findings on the role of semaphorins/receptors and their intracellular signaling in the regulation of bone homeostasis.

Semaphorin 7A inhibits platelet production from CD34+ progenitor cells

BACKGROUND

The multifunctional protein semaphorin 7A (Sema7A) may have regulatory effects on blood cell differentiation via its receptors β1-integrin and plexin C1. As thrombocytopenia can be treated with transfusion of ex vivo CD34(+) cell-derived megakaryocytes, we investigated the effect of Sema7A on differentiation of CD34(+) progenitor cells into megakaryocytes and platelets.

METHODS

Megakaryocytes and platelets were differentiated with a specific cytokine cocktail (CC) from CD34(+) progenitor cells in the presence or absence of Sema7A. Expression of cell markers CD41, CD42a and CD61 or detection of the activation of the signal mediator focal adhesion kinase (FAK) was performed by flow cytometry, cytokine secretion by Luminex technology, and megakaryocyte cell density and morphology by microscopic studies. Sema7A levels in vivo were assessed by real-time PCR and ELISA in hematological patients undergoing chemotherapy.

RESULTS

CD34(+) progenitor cells expressed the receptors for Sema7A. Expression of CD41, CD42a and CD61 was markedly reduced in the presence of Sema7A, after CC-dependent platelet production from CD34(+) progenitor cells. As revealed by microscopic analysis, megakaryocyte cell density was significantly lower in the presence of Sema7A as compared with controls. Blocking of CD29 abrogated the Sema7A-mediated inhibition. Sema7A activated FAK in CD34(+) progenitor cells and significantly increased secretion of the proinflammatory cytokines IL-6, IL-8 and GM-CSF. Finally, Sema7A levels were up-regulated in 50% of patients after chemotherapy.

CONCLUSIONS

Sema7A markedly reduces the production rates of megakaryocytes and platelets from CD34(+) progenitor cells. Hence, up-regulation of Sema7A may be a major risk factor for a reduced platelet repopulation after hematopoietic stem cell transplantation.