Roles of the semaphorin family in immune regulation

New Promising Routes in Peptic Ulcers: Toll-like Receptors and Semaphorins

Peptic ulcers (PU) are one of the commonest yet problematic diseases found to be existing in the majority of the population. Today, drugs from a wide range of therapeutic classes are available for the management of the disease. Still, the complications of the condition are difficult to tackle and the side effect profile is quite a concern. The literature indicates that Toll-like receptors (TLRs) and Semaphorins (SEMAs) have been under study for their various pharmacological actions over the past few decades. Both these signalling pathways are found to regulate immunological and inflammatory responses. Moreover, receptors and signalling molecules from the family of TLRs and SEMAs are found to have bacterial recognition and antibacterial properties which are essential in eradicating Helicobacter pylori (H. pylori), one of the major causative agents of PU. Our understanding of SEMAs, a class of proteins involved in cell signalling, is relatively less developed compared to TLRs, another class of proteins involved in the immune response. SEMAs and TLRs play different roles in biological processes, with SEMAs primarily involved in guiding cell migration and axon guidance during development, while TLRs are responsible for recognizing pathogens and initiating an immune response. Here, in this review, we will discuss in detail the signalling cascade of TLRs and SEMAs and thereby understand its association with PU for future therapeutic targeting. The review also aims at providing an overview of the study that has been into exploring the role of these signalling pathways in the management of PU.

Soluble Sema4D cleaved from osteoclast precursors by TACE suppresses osteoblastogenesis

Bone remodelling is mediated by orchestrated communication between osteoclasts and osteoblasts which, in part, is regulated by coupling and anti-coupling factors. Amongst formally known anti-coupling factors, Semaphorin 4D (Sema4D), produced by osteoclasts, plays a key role in downmodulating osteoblastogenesis. Sema4D is produced in both membrane-bound and soluble forms; however, the mechanism responsible for producing sSema4D from osteoclasts is unknown. Sema4D, TACE and MT1-MMP are all expressed on the surface of RANKL-primed osteoclast precursors. However, only Sema4D and TACE were colocalized, not Sema4D and MT1-MMP. When TACE and MT1-MMP were either chemically inhibited or suppressed by siRNA, TACE was found to be more engaged in shedding Sema4D. Anti-TACE-mAb inhibited sSema4D release from osteoclast precursors by ~90%. Supernatant collected from osteoclast precursors (OC-sup) suppressed osteoblastogenesis from MC3T3-E1 cells, as measured by alkaline phosphatase activity, but OC-sup harvested from the osteoclast precursors treated with anti-TACE-mAb restored osteoblastogenesis activity in a manner that compensates for diminished sSema4D. Finally, systemic administration of anti-TACE-mAb downregulated the generation of sSema4D in the mouse model of critical-sized bone defect, whereas local injection of recombinant sSema4D to anti-TACE-mAb-treated defect upregulated local osteoblastogenesis. Therefore, a novel pathway is proposed whereby TACE-mediated shedding of Sema4D expressed on the osteoclast precursors generates functionally active sSema4D to suppress osteoblastogenesis.

Locally Secreted Semaphorin 4D Is Engaged in Both Pathogenic Bone Resorption and Retarded Bone Regeneration in a Ligature-Induced Mouse Model of Periodontitis

It is well known that Semaphorin 4D (Sema4D) inhibits IGF-1-mediated osteogenesis by binding with PlexinB1 expressed on osteoblasts. However, its elevated level in the gingival crevice fluid of periodontitis patients and the broader scope of its activities in the context of potential upregulation of osteoclast-mediated periodontal bone-resorption suggest the need for further investigation of this multifaceted molecule. In short, the pathophysiological role of Sema4D in periodontitis requires further study. Accordingly, attachment of the ligature to the maxillary molar of mice for 7 days induced alveolar bone-resorption accompanied by locally elevated, soluble Sema4D (sSema4D), TNF-α and RANKL. Removal of the ligature induced spontaneous bone regeneration during the following 14 days, which was significantly promoted by anti-Sema4D-mAb administration. Anti-Sema4D-mAb was also suppressed in vitro osteoclastogenesis and pit formation by RANKL-stimulated BMMCs. While anti-Sema4D-mAb downmodulated the bone-resorption induced in mouse periodontitis, it neither affected local production of TNF-α and RANKL nor systemic skeletal bone remodeling. RANKL-induced osteoclastogenesis and resorptive activity were also suppressed by blocking of CD72, but not Plexin B2, suggesting that sSema4D released by osteoclasts promotes osteoclastogenesis via ligation to CD72 receptor. Overall, our data indicated that ssSema4D released by osteoclasts may play a dual function by decreasing bone formation, while upregulating bone-resorption.

The Role of Semaphorins and Their Receptors in Innate Immune Responses and Clinical Diseases of Acute Inflammation

Semaphorins are a group of proteins that have been studied extensively for their critical function in neuronal development. They have been shown to regulate airway development, tumorigenesis, autoimmune diseases, and the adaptive immune response. Notably, emerging literature describes the role of immunoregulatory semaphorins and their receptors, plexins and neuropilins, as modulators of innate immunity and diseases defined by acute injury to the kidneys, abdomen, heart and lungs. In this review we discuss the pathogenic functions of semaphorins in clinical conditions of acute inflammation, including sepsis and acute lung injury, with a focus on regulation of the innate immune response as well as potential future therapeutic targeting.

The intervention of valproic acid on the tumorigenesis induced by an environmental carcinogen of PAHs

This study investigated whether valproic acid (VPA, a histone deacetylase inhibitor) can interfere with the carcinogenicity of polycyclic aromatic hydrocarbons (PAHs). A typical representative compound of PAHs, 7,12-Dimethylbenz[a]anthracene (DMBA), was used to induce rat breast cancer. The results showed that therapeutic concentration of VPA (50 and 100 mg/kg) delayed the occurrence of tumors, reduced tumor formation rate and attenuated tumors growth, and have a protective effect on normal tissues. The macrophage-mediated inflammatory response was found to be associated with the observed effect of VPA. In addition, we screened and validated a possible gene, Sema3c, which was involved in DMBA-induced breast cancer development and can be inhibited by VPA.

Semaphorin 4D from CD15+ Granulocytes via ADAM10-Induced Cleavage Contributes to Antibody Production in Bullous Pemphigoid

Autoreactive B-cell activation and antibody production are critical events for the development of bullous pemphigoid (BP). However, the mechanism that is involved in the modulation of B-cell activation and autoantibody generation has not been fully understood. Semaphorin 4D (Sema4D, or CD100) plays important roles in immune regulation related to B cells, but its implications in BP remain obscure. The aim of our study was to characterize Sema4D and the underlying mechanism contributing to the autoimmune features of BP. We found that soluble Sema4D (sSema4D) levels were elevated and correlated with disease severity and activity in serum and blister fluids from patients with BP. Additionally, Sema4D-expressing cells accumulated in subepidermal blisters of BP lesions. In patient-derived peripheral blood mononuclear cells, by promoting the differentiation of B cells into plasmablasts, sSema4D boosted anti-BP180/anti-BP230 antibody production in a time- and dose-dependent manner, which may be attributed to CD72-mediated activation of Akt/NF-κB phosphorylated (p-)65/ERK cascades in B cells. We determined that a disintegrin and metalloproteinase 10 is a proteolytic enzyme for the cleavage of sSema4D from CD15+ granulocytes instead of T cells, which is probably responsible for the high concentration of sSema4D in BP blister fluid and serum. These findings suggest that Sema4D is a crucial participant in BP pathogenesis.

The oviductal transcriptome is influenced by a local ovarian effect in the sow

BACKGROUND

Oviducts participate in fertilization and early embryo development, and they are influenced by systemic and local circulation. Local functional interplay between ovary, oviduct and uterus is important, as deduced from the previously observed differences in hormone concentrations, presence of sperm, or patterns of motility in the oviduct after unilateral ovariectomy (UO). However, the consequences of unilateral ovariectomy on the oviductal transcriptome remain unexplored. In this study, we have investigated the consequences of UO in a higher animal model as the pig.

METHODS

The influence of UO was analyzed on the number of ovulations on the contra ovary, which was increased, and on the ipsilateral oviductal transcriptome. Microarray analysis was performed and the results were validated by PCR. Differentially expressed genes (DEGs) with a fold change ≥ 2 and a false discovery rate of 10 % were analyzed by Ingenuity Pathway Analysis (IPA) to identify the main biofunctions affected by UO.

RESULTS

Data revealed two principal effects in the ipsilateral oviduct after UO: i) down-regulation of genes involved in the survival of sperm in the oviduct and early embryonic development, and ii) up-regulation of genes involved in others functions as protection against external agents and tumors.

CONCLUSIONS

Results showed that unilateral ovariectomy results in an increased number of ovulation points on the contra ovary and changes in the transcriptome of the ipsilateral oviduct with consequences on key biological process that could affect fertility output.

Asian Citrus Psyllid Expression Profiles Suggest Candidatus Liberibacter Asiaticus-Mediated Alteration of Adult Nutrition and Metabolism, and of Nymphal Development and Immunity

The Asian citrus psyllid (ACP) Diaphorina citri Kuwayama (Hemiptera: Psyllidae) is the insect vector of the fastidious bacterium Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus greening disease, or Huanglongbing (HLB). The widespread invasiveness of the psyllid vector and HLB in citrus trees worldwide has underscored the need for non-traditional approaches to manage the disease. One tenable solution is through the deployment of RNA interference technology to silence protein-protein interactions essential for ACP-mediated CLas invasion and transmission. To identify psyllid interactor-bacterial effector combinations associated with psyllid-CLas interactions, cDNA libraries were constructed from CLas-infected and CLas-free ACP adults and nymphs, and analyzed for differential expression. Library assemblies comprised 24,039,255 reads and yielded 45,976 consensus contigs. They were annotated (UniProt), classified using Gene Ontology, and subjected to in silico expression analyses using the Transcriptome Computational Workbench (TCW) (http://www.sohomoptera.org/ACPPoP/). Functional-biological pathway interpretations were carried out using the Kyoto Encyclopedia of Genes and Genomes databases. Differentially expressed contigs in adults and/or nymphs represented genes and/or metabolic/pathogenesis pathways involved in adhesion, biofilm formation, development-related, immunity, nutrition, stress, and virulence. Notably, contigs involved in gene silencing and transposon-related responses were documented in a psyllid for the first time. This is the first comparative transcriptomic analysis of ACP adults and nymphs infected and uninfected with CLas. The results provide key initial insights into host-parasite interactions involving CLas effectors that contribute to invasion-virulence, and to host nutritional exploitation and immune-related responses that appear to be essential for successful ACP-mediated circulative, propagative CLas transmission.

Singapore grouper iridovirus-encoded semaphorin homologue (SGIV-sema) contributes to viral replication, cytoskeleton reorganization and inhibition of cellular immune responses

Semaphorins are a large, phylogenetically conserved family of proteins that are involved in a wide range of biological processes including axonal steering, organogenesis, neoplastic transformation, as well as immune responses. In this study, a novel semaphorin homologue gene belonging to the Singapore grouper iridovirus (SGIV), ORF155R (termed SGIV-sema), was cloned and characterized. The coding region of SGIV-sema is 1728 bp in length, encoding a predicted protein with 575 aa. SGIV-sema contains a ~370 aa N-terminal Sema domain, a conserved plexin-semaphorin-integrin (PSI) domain, and an immunoglobulin (Ig)-like domain near the C terminus. SGIV-sema is an early gene product during viral infection and predominantly distributed in the cytoplasm with a speckled and clubbed pattern of appearance. Functionally, SGIV-sema could promote viral replication during SGIV infection in vitro, with no effect on the proliferation of host cells. Intriguingly, ectopically expressed SGIV-sema could alter the cytoskeletal structure of fish cells, characterized by a circumferential ring of microtubules near the nucleus and a disrupted microfilament organization. Furthermore, SGIV-sema was able to attenuate the cellular immune response, as demonstrated by decreased expression of inflammation/immune-related genes such as IL-8, IL-15, TNF-α and mediator of IRF3 activation (MITA), in SGIV-sema-expressing cells before and after SGIV infection. Ultimately, our study identified a novel, functional SGIV gene that could regulate cytoskeletal structure, immune responses and facilitate viral replication.

Semaphorin 4A is dynamically regulated during thymocyte development in mice

Semaphorins are important regulators of peripheral T and B-cell mediated immune responses in mice and humans. Modulatory roles of semaphorins in T cell development are also being characterized. We carefully analyzed the gene expression and protein levels of semaphorins 4A, 4D, and 7A at various developmental stages of T cell maturation in the thymus of C57BL/6 mice. Sema7a was expressed at very low levels, while Sema4d was abundant at all developmental stages of mouse thymocytes. We found the most interesting pattern of gene regulation and protein localization for semaphorin 4A. Both semaphorin 4A mRNA and protein were clearly detected on the earliest progenitors and were downregulated through thymic development. SEMA4A protein also showed a distinct cortico-medullary pattern of localization. Our findings contribute to an understanding of the complex roles played by semaphorins in the network of spatially and temporally regulated cues underpinning T cell development in the thymus.

Effect of cancer-associated mutations in the PlexinB1 gene

Background

Semaphorins act as chemotactic cues for cell movement via their transmembrane receptors, plexins. Somatic missense mutations in the plexinB1 gene coupled with overexpression of the protein frequently occur in prostate tumours, indicating a role for plexinB1 in the pathogenesis of prostate cancer.

Results

Two specific mutations found in prostate cancer enhance RhoD binding and one other mutation results in loss of inhibition of Rac-dependent Pak1 phosphorylation and lamellipodia formation and in impairment of trafficking of plexinB1 to the membrane. None of the three characterised mutations affect PDZRhoGEF binding, RhoA activity, the interaction of plexinB1with the oncogenes ErbB2 or c-Met or ErbB2 phosphorylation. The mutations have the net effect of increasing cell motility by blocking plexinB1-mediated inhibition of Rac while enhancing the interaction with RhoD, an anti-migratory factor.

Conclusions

PlexinB1 mutations block plexinB1-mediated signalling pathways that inhibit cell motility.

Semaphorins in angiogenesis and tumor progression

The semaphorins were initially described as axon guidance factors, but have recently been implicated in a variety of physiological and developmental functions, including regulation of immune response, angiogenesis, and migration of neural crest cells. The semaphorin family contains more than 30 genes divided into seven subfamilies, all of which are characterized by the presence of a sema domain. The semaphorins transduce their signals by binding to one of the nine receptors belonging to the plexin family, or, in the case of the class 3 semaphorins, by binding to one of the two neuropilin receptors. Additional receptors, which form complexes with these primary semaphorin receptors, are also frequently involved in semaphorin signaling. Recent evidence suggests that some semaphorins can act as antiangiogenic and/or antitumorigenic agents whereas other semaphorins promote tumor progression and/or angiogenesis. Furthermore, loss of endogenous inhibitory semaphorin expression or function on one hand, and overexpression of protumorigenic semaphorins on the other hand, is associated with the progression of some tumor types.

Hormonal regulation and distinct functions of semaphorin-3B and semaphorin-3F in ovarian cancer

Semaphorins comprise a family of molecules that influence neuronal growth and guidance. Class-3 semaphorins, semaphorin-3B (SEMA3B) and semaphorin-3F (SEMA3F), illustrate their effects by forming a complex with neuropilins (NP-1 or NP-2) and plexins. We examined the status and regulation of semaphorins and their receptors in human ovarian cancer cells. A significantly reduced expression of SEMA3B (83 kDa), SEMA3F (90 kDa), and plexin-A3 was observed in ovarian cancer cell lines when compared with normal human ovarian surface epithelial cells. The expression of NP-1, NP-2, and plexin-A1 was not altered in human ovarian surface epithelial and ovarian cancer cells. The decreased expression of SEMA3B, SEMA3F, and plexin-A3 was confirmed in stage 3 ovarian tumors. The treatment of ovarian cancer cells with luteinizing hormone, follicle-stimulating hormone, and estrogen induced a significant upregulation of SEMA3B, whereas SEMA3F was upregulated only by estrogen. Cotreatment of cell lines with a hormone and its specific antagonist blocked the effect of the hormone. Ectopic expression of SEMA3B or SEMA3F reduced soft-agar colony formation, adhesion, and cell invasion of ovarian cancer cell cultures. Forced expression of SEMA3B, but not SEMA3F, inhibited viability of ovarian cancer cells. Overexpression of SEMA3B and SEMA3F reduced focal adhesion kinase phosphorylation and matrix metalloproteinase-2 and matrix metalloproteinase-9 expression in ovarian cancer cells. Forced expression of SEMA3F, but not SEMA3B in ovarian cancer cells, significantly inhibited endothelial cell tube formation. Collectively, our results suggest that the loss of SEMA3 expression could be a hallmark of cancer progression. Furthermore, gonadotropin- and/or estrogen-mediated maintenance of SEMA3 expression could control ovarian cancer angiogenesis and metastasis.

The role of chemokines in B cell chronic lymphocytic leukaemia: pathophysiological aspects and clinical impact

Chemokines are centrally involved in leukocyte migration, homing and haematopoiesis. Besides these physiological aspects, their role in pathological processes especially with respect to solid tumour and haematological neoplasias is well established. In this context, the focus was set here on disclosing their contribution in B cell chronic lymphocytic leukaemia (B-CLL), which is regarded as the most characteristic low-grade lymphoma. Up to now, it has been demonstrated that several chemokines are involved in migration of B-CLL cells to lymph nodes, secondary lymphoid organs and bone marrow. Moreover, some chemokines are known to have an anti-apoptotic effect and thus contribute to the survival of B-CLL cells. By interfering with both of these aspects, new therapeutic targets for this yet incurable disease may be developed. Furthermore, a correlation can be drawn between the concentration of some chemokines in patients' serum, the expression of their respective receptors on B-CLL cells and well-established predictive clinical parameters. Consequently, further systematic investigation of the chemokine network may lead to the identification of new diagnostic and prognostic markers. This review focuses on the impact of chemokines and their receptors on B-CLL pathophysiology and points out potential implications for both treatment and diagnosis.

Targeting human glioma cells using HSV-1 amplicon peptide display vector

Targeting cell infection using herpes simplex virus type 1 (HSV-1) vectors is a complicated issue as the process involves multiple interactions of viral envelope glycoproteins and cellular host surface proteins. In this study, we have inserted a human glioma-specific peptide sequence (denoted as MG11) into a peptide display HSV-1 amplicon vector replacing the heparan sulfate-binding domain of glycoprotein C (gC). The modified MG11:gC envelope recombinant vectors were subsequently packaged into virions in the presence of helper virus deleted for gC. Our results showed that the tropism of these HSV-1 recombinant virions was increased for human glioma cells in culture as compared with wild-type virions. The binding of these recombinant virions could also be blocked effectively by pre-incubating the cells with the glioma-specific peptide, indicating that MG11 peptide and the recombinant virions competed for the same or similar receptor-binding sites on the cell surface of human glioma cells. Furthermore, preferential homing of these virions was shown in xenograft glioma mouse model following intravascular delivery. Taken together, these results validated the hypothesis that HSV-1 binding to cells can be redirected to human gliomas through the incorporation of MG11 peptide sequence to the virions.

Semaphoring Vascular Morphogenesis

In vertebrate embryos, development of an architecturally optimized blood vessel network allows the efficient transport of oxygen and nutrients to all other tissues. The final shape of the vascular system results from vasculogenesis and angiogenesis, during which motile endothelial cells (ECs) modify their integrin-mediated interactions with the extracellular matrix (ECM) in response to pro- and anti-angiogenic factors. There is mounting evidence that different members of the semaphorin (SEMA) family of neural guidance cues participate in developmental and postnatal vessel formation and patterning as well. It turns out that paracrine secretion of class 3 SEMA (SEMA3) by nonendothelial tissues cooperates with vascular endothelial growth factor in regulating EC precursor migration and assembly during vasculogenesis and funnels navigating blood vessel through tissue boundaries during sprouting angiogenesis. Autocrine loops of endothelial SEMA3 instead appears to regulate vascular remodeling, which occurs through blood vessel intussusception and fusion. SEMA3 activity both on the vascular and nervous systems relies upon their ability to hamper the affinity of integrin receptors towards ECM ligands. Indeed, signaling from SEMA-activated plexin receptors negatively regulates cell-ECM adhesive interactions by inhibiting two key integrin activators, such as the small GTPase R-Ras and the focal adhesion protein talin.

CD72, a coreceptor with both positive and negative effects on B lymphocyte development and function

INTRODUCTION

B lymphocytes remain in a resting state until activated by antigenic stimuli through interaction with the B cell receptor (BCR). Coreceptors on B cells can modulate the thresholds for signaling through the BCR for growth and differentiation. CD72 is a B cell coreceptor that has been shown to interact with CD100, a semaphorin, and to enhance BCR signaling.

DISCUSSION

CD72 ligation induces a variety of early signaling events such as activation of the Src kinases Blk and Lyn and the non-src kinase Btk leading to activation of the mitogen-activated protein (MAP) kinases, events usually associated with positive signaling. CD72 signals can enable Btk-deficient B cells to overcome their unresponsiveness to BCR signaling. On the other hand, BCR-mediated signals are enhanced in CD72-deficient cells but are reduced in CD100 null cells. The dual effects of CD72 on B cells can be explained by its association with positive and negative signaling molecules. Thus, CD72 interacts with SHP-1, an SH2-domain containing protein tyrosine phosphatase, a negative regulator of signaling, and Grb2, an adaptor protein associated with the Ras/MAPK pathway. Ligation of CD72 also triggered its association with CD19, a positive modulator of B cell receptor signaling. We propose a dual signaling hypothesis to explain the growth and differentiation promoting properties of CD72. Deficiency in either CD72 or CD100 leads to autoimmunity in mouse models. CD72 expression and polymorphisms exhibit some association with autoimmune diseases such as lupus, Sjogren's syndrome, and type 1 diabetes.

Semaphorin 3B is a 1,25-Dihydroxyvitamin D3-induced gene in osteoblasts that promotes osteoclastogenesis and induces osteopenia in mice

The vitamin D endocrine system is important for skeletal homeostasis. 1,25-Dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] impacts bone indirectly by promoting intestinal absorption of calcium and phosphate and directly by acting on osteoblasts and osteoclasts. Despite the direct actions of 1,25(OH)(2)D(3) in bone, relatively little is known of the mechanisms or target genes that are regulated by 1,25(OH)(2)D(3) in skeletal cells. Here, we identify semaphorin 3B (SEMA3B) as a 1,25(OH)(2)D(3)-stimulated gene in osteoblastic cells. Northern analysis revealed strong induction of SEMA3B mRNA by 1,25(OH)(2)D(3) in MG-63, ST-2, MC3T3, and primary osteoblastic cells. Moreover, differentiation of these osteogenic cells enhanced SEMA3B gene expression. Biological effects of SEMA3B in the skeletal system have not been reported. Here, we show that osteoblast-derived SEMA3B alters global skeletal homeostasis in intact animals and osteoblast function in cell culture. Osteoblast-targeted expression of SEMA3B in mice resulted in reduced bone mineral density and aberrant trabecular structure compared with nontransgenic littermates. Histomorphometry studies indicated that this was likely due to increased osteoclast numbers and activity. Indeed, primary osteoblasts obtained from SEMA3B transgenic mice stimulated osteoclastogenesis to a greater extent than nontransgenic osteoblasts. This study establishes that SEMA3B is a 1,25(OH)(2)D(3)-induced gene in osteoblasts and that osteoblast-derived SEMA3B impacts skeletal biology in vitro and in vivo. Collectively, these studies support a putative role for SEMA3B as an osteoblast protein that regulates bone mass and skeletal homeostasis.

Semaphorin regulation of cellular morphology

Semaphorin proteins, although initially characterized as repulsive neuronal guidance cues, are now appreciated as major contributors to morphogenesis and homeostasis for a wide range of tissue types. Semaphorin-mediated long- and short-range repulsive, and attractive, guidance has profound influences on cellular morphology. The diversity of semaphorin receptor complexes utilized by various semaphorin ligands, the ability of semaphorins themselves to serve as receptors, and the myriad of intracellular signaling components that comprise semaphorin signaling cascades all contribute to cell-type-specific responses to semaphorins. Analysis of the molecular and cellular mechanisms underlying semaphorin function in neural and vascular systems provides insight into principles governing how this large protein family contributes to organogenesis, function, and disease.

A novel role for the semaphorin Sema4D in the induction of allo-responses

Sema4D (CD100), a member of the neuro-semaphorin family of proteins, has recently been shown to play a role in modulating certain immune responses. We tested the requirement of Sema4D expression on T cells in the induction of T cell allo-immune responses. Sema4D-/- T cells showed reduced expansion in vitro upon stimulation with allo-geneic antigen presenting cells (APCs) when compared to wild-type (wt) T cells. Similar in vitro results were observed using anti-Sema4D mAbs. Further studies demonstrated that the reduced proliferation was not due to intrinsic T cell defects, and that the cytotoxic functions were preserved. After allo-geneic bone marrow transplant (BMT), recipients of Sema4D-/- T cells showed reduced mortality and graft-versus-host disease (GVHD) target organ damage. Allo-geneic dendritic cells (DCs) cocultured with Sema4D-/- responder T cells secreted less TNF-alpha and IL-12p70 compared to wt T cells. Similar reduction of DC function was observed with anti-Sema4D mAbs. Given the preservation of CTL function we evaluated graft-versus-leukemia (GVL) responses. When BALB/c recipient mice were challenged with the P815 murine mastocytoma cell line (H2(d)) the recipients of allo-geneic Sema4D-/- B6 T cells showed a significant improvement in tumor free survival when compared to syngeneic recipients, thus demonstrating preservation of GVL, albeit of a lesser magnitude than allo-geneic wt T cells. In summary, Sema4D plays a significant role in mediating in vitro and in vivo allo-geneic responses by modulating T cell-APC interactions.

Regulated surface expression and shedding support a dual role for semaphorin 4D in platelet responses to vascular injury

Semaphorin 4D (sema4D; CD100) is an integral membrane protein and the ligand for two receptors, CD72 and plexin-B1. Soluble sema4D has been shown to evoke angiogenic responses from endothelial cells and impair monocyte migration, but the origin of soluble sema4D, particularly at sites of vascular injury, has been unclear. Here we show that platelets express sema4D and both of its receptors and provide evidence that these molecules promote thrombus formation. We also show that the surface expression of sema4D and CD72 increases during platelet activation, followed by the gradual shedding of the sema4D extracellular domain. Shedding is blocked by metalloprotease inhibitors and abolished in mouse platelets that lack the metalloprotease ADAM17 (TACE). Mice that lack sema4D exhibit delayed arterial occlusion after vascular injury in vivo, and their platelets show impaired collagen responses in vitro. In resting platelets, as in B lymphocytes, CD72 is associated with the protein tyrosine phosphatase SHP-1. Platelet activation causes dissociation of the complex, as does the addition of soluble sema4D. These findings suggest a dual role for sema4D in vascular responses to injury. As thrombus formation begins, platelet-associated sema4D can bind to its receptors on nearby platelets, promoting thrombus formation. As thrombus formation continues, sema4D is shed from the platelet surface and becomes available to interact with receptors on endothelial cells and monocytes, as well as continuing to interact with platelets.

Intracellular Kinases in Semaphorin Signaling

Originally identified as collapse-inducing and repellent proteins for neuronal processes, semaphorins are now implicated in a diverse array of cellular responses, contributing not only to embryonic development, but also to the maintenance of tissue integrity in the adult organism. In addition, semaphorins play a role in the pathological context. Some Semaphorins can act at a distance, facilitating the navigation of cells or axonal process, whilst others evoke responses in a contact-dependent fashion. The intracellular signaling mechanisms employed by the semaphorins are beginning to be determined, and much work in recent years implicates a host of intracellular kinases in mediating Semaphorin function. These include the tyrosine kinase Fyn and the serine/threonine kinases Cdk5, GSK3, MAPK, and LIMK, and the lipid kinase PI3K. What follows is a review of this work with respect to their functions in mediating specific semaphorin-induced responses.

Semaphorin 7A is a negative regulator of T cell responses

Semaphorins play an essential role in axonal guidance, and emerging evidence points to diverse functions of several Semaphorin family members in the immune system. Semaphorin 7A (Sema7A) promotes axonal growth in the central nervous system. Here, we show that Sema7A also plays a critical role in negative regulation of T cell activation and function. T cells deficient in Sema7A exhibit enhanced homeostatic and antigen-induced proliferative response. Moreover, autoreactive Sema7A-deficient T cells mediate aggressive autoimmune disease. The deficiency in Sema7A leads to defective TCR downmodulation and T cell hyperresponsiveness. These results demonstrate an important role of Sema7A in limiting autoimmune responses and add to growing evidence of shared signaling pathways used by the immune and nervous systems.

In-tandem insight from basic science combined with clinical research: CD38 as both marker and key component of the pathogenetic network underlying chronic lymphocytic leukemia

The absence of mutations in the IgV genes, together with the presence of ZAP-70 and CD38, are the most reliable negative prognostic markers for chronic lymphocytic leukemia (CLL) patients. Several lines of evidence indicate that CD38 may be not only a diagnostic marker but also a key element in the pathogenetic network in CLL. First, CD38 is a receptor that induces proliferation and increases survival of CLL cells. Second, CD38 signals start upon interaction with the CD31 ligand expressed by stromal and nurse-like cells. Third, CD38/CD31 contacts up-regulate CD100, a semaphorin involved in sustaining CLL growth. Fourth, evidence that nurselike cells express high levels of CD31 and plexin-B1, the high-affinity ligand for CD100, offers indirect confirmation for this model of receptor cross-talk. Elements of variation in the clinical course of CD38(+) CLL patients include (1) potential intersection with ZAP-70, a kinase involved in the CD38 signaling pathway in T and natural killer (NK) cells, and (2) the effects of genetic polymorphisms of the receptors involved, at least of CD38 and CD31. Consequently, CD38 together with ZAP-70 appear to be the key elements of a coreceptor pathway that may sustain the signals mediated by the B-cell receptor and potentially by chemokines and their receptors. This would result in acquisition of increased survival potential, providing clues to the poorer prognosis of CD38(+) patients.

The neuropilins and their role in tumorigenesis and tumor progression

The neuropilins were originally described as receptors for the six axon guidance factors belonging to the class-3 semaphorins. They were subsequently found to function in addition as receptors for specific splice forms of angiogenic factors belonging to the VEGF family. The neuropilins are expressed in many types of cancer cells, in endothelial cells and in additional many types of normal diploid cell types. Recent findings indicate that the neuropilins and their associated plexin and tyrosine-kinase VEGF receptors play a regulatory role in developmental angiogenesis as well as in tumor angiogenesis. The neuropilin ligands belonging to the semaphorin family as well as the various VEGF's function as modulators of angiogenesis and tumor angiogenesis. Furthermore, since many types of cancer cells express neuropilins and neuropilin associated receptors, it is not surprising that various neuropilin ligands can modulate the behavior of cancer cells directly leading to the potentiation or inhibition of tumor progression.

Reactive astrocytes in neural repair and protection

Reactive astrocytosis occurs prominently in response to all forms of CNS injury or disease. The functions of reactive astrocytes are not well understood, and both harmful and beneficial activities have been attributed to these cells. The basic process of reactive astrocytosis is conserved in vertebrate evolution, suggesting fitness-enhancing benefits, but scar-forming reactive astrocytes are often regarded as uniformly detrimental to clinical outcome, in particular, when implicated as inhibitors of axon regeneration. Transgenic mouse models are providing new means to study the activities of reactive astrocytes after CNS insults in vivo. Recent studies point toward roles for reactive astrocytes in restricting inflammation and protecting neurons and oligodendrocytes, thereby helping to limit tissue degeneration and preserve function after CNS injury.

Axotrophin and leukaemia inhibitory factor (LIF) in transplantation tolerance

Immune self-tolerance is controlled by a subset of T lymphocytes that are regulatory (Treg) and epigenetically programmed to suppress auto-reactive immune effector cells in vivo. By extrapolation, donor-specific transplantation tolerance might be controlled by donor-specific Treg that have acquired the appropriate epigenetic program for tolerance. Although such tolerance has yet to be achieved in man, proof of concept comes from mouse models where regulatory transplantation tolerance can be induced within the complex micro-environment of the spleen or draining lymph node. By studying whole spleen cell populations in a murine model of transplantation tolerance we have incorporated a complexity of environmental factors when looking for specific features that characterize tolerance versus aggression. This approach has revealed unexpected patterns of gene activity in tolerance and most notably that a novel stem cell gene, axotrophin, regulates T lymphocyte responsiveness both in terms of proliferation and in release of leukaemia inhibitory factor (LIF). Since LIF is a regulator of stem cells in addition to being a key neuropoietic cytokine, these preliminary results linking both axotrophin and LIF to transplantation tolerance lead us to propose that regulatory pathways encoded during the epigenetic development of Treg cells are related to pathways that regulate fate determination of stem cells.

CD38 and CD100 lead a network of surface receptors relaying positive signals for B-CLL growth and survival

This work addresses the question whether CD38, a negative prognostic marker in B-cell chronic lymphocytic leukemia (B-CLL), plays a role in neoplastic B-cell growth and survival. We show that CD38+ B-CLL cells bind to murine fibroblasts transfected with the CD31 ligand. The interaction triggers an extensive remodeling of the B-CLL membrane, with relocalization of BCR/CD19 to the CD38/CD31 contact areas, and it also increases cell survival and proliferation. A second event is the up-modulation of the survival receptor CD100, restricted to proliferating cells, and a concomitant decrease of CD72 (low-affinity CD100 ligand and negative regulator of immune responses). The most efficient signals are delivered through sequential interactions between CD38/CD31 and CD100/plexin-B1 (high-affinity CD100 ligand), as inferred by coculture experiments using specific transfectants and blocking monoclonal antibodies (mAbs). The finding that nurselike cells from B-CLL patients express CD31 and plexin-B1, which deliver growth and survival signals to CD38+/CD100+ B-CLL cells, further confirms the model proposed. These findings show that a set of normal receptors and ligands ruling physiologic signaling pathways in B lymphocytes becomes detrimental when expressed in the context of B-CLL cells, ultimately leading to the generation of a tumor reservoir.

Genetic background regulates semaphorin gene expression and epileptogenesis in mouse brain after kainic acid status epilepticus

The host response to neural injury, which can include axonal sprouting and synaptic reorganization is likely to be under tight genetic regulatory control at the level of the genome and may be implicated in epileptogenesis. Despite its importance, however, the molecular basis of synaptic reorganization is unclear. We have studied the development of synaptic reorganization, semaphorin gene expression, and epileptogenesis in hippocampus of epileptogenic sensitive (FVB/NJ) and epileptogenic resistant (C57BL/6J) mice (i.e. distinct genetic backgrounds) after kainic acid-induced status epilepticus. Our results support the hypothesis that disruption of transcriptional regulation of axon guidance genes leads to a differential loss of tonic neuropilin-2 dependent activation of semaphorin 3F receptors on hippocampal neurons on distinct genetic backgrounds. This results in rearranged synaptic circuitry and thus promotes epileptogenesis. These findings may define biologic principles underlying the role of semaphorin signaling which may broadly apply to other systems undergoing neural regeneration.

The Regulated Expression of a Diverse Set of Genes during Thymocyte Positive Selection In Vivo

A signal initiated by the newly formed Ag receptor is integrated with microenvironmental cues during T cell development to ensure positive selection of CD4+CD8+ progenitors into functionally mature CD4+ or CD8+ T lymphocytes. During this transition, a survival program is initiated, TCR gene recombination ceases, cells migrate into a new thymic microenvironment, the responsiveness of the Ag receptor is tuned, and the cells commit to a specific T lineage. To determine potential regulators of these processes, we used mRNA microarray analysis to compare gene expression changes in CD4+CD8+ thymocytes from TCR transgenic mice that have received a TCR selection signal with those that had not received a signal. We found 129 genes with expression that changed significantly during positive selection, the majority of which were not previously appreciated. A large number of these changes were confirmed by real-time PCR or flow cytometry. We have combined our findings with gene changes reported in the literature to provide a comprehensive report of the genes regulated during positive selection, and we attempted to assign these genes to positive selection process categories.