CD100/Plexin-B1 interactions sustain proliferation and survival of normal and leukemic CD5+ B lymphocytes

Biology of chronic lymphocytic leukemia in different microenvironments: clinical and therapeutic implications

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of mature monoclonal B cells in peripheral blood, bone marrow, spleen, and lymph nodes. The trafficking, survival, and proliferation of CLL cells is tightly regulated by the surrounding tissue microenvironment and is mediated by antigenic stimulation, close interaction with various accessory cells and exposure to different cytokines, chemokines, and extracellular matrix components. In the last decade there have been major advances in the understanding of the reciprocal interactions between CLL cells and the various microenvironmental compartments. This article discusses the role of the microenvironment in the context of efforts to develop novel therapeutics that target the biology of CLL.

Cross-species array comparative genomic hybridization identifies novel oncogenic events in zebrafish and human embryonal rhabdomyosarcoma

Human cancer genomes are highly complex, making it challenging to identify specific drivers of cancer growth, progression, and tumor maintenance. To bypass this obstacle, we have applied array comparative genomic hybridization (array CGH) to zebrafish embryonal rhabdomyosaroma (ERMS) and utilized cross-species comparison to rapidly identify genomic copy number aberrations and novel candidate oncogenes in human disease. Zebrafish ERMS contain small, focal regions of low-copy amplification. These same regions were commonly amplified in human disease. For example, 16 of 19 chromosomal gains identified in zebrafish ERMS also exhibited focal, low-copy gains in human disease. Genes found in amplified genomic regions were assessed for functional roles in promoting continued tumor growth in human and zebrafish ERMS – identifying critical genes associated with tumor maintenance. Knockdown studies identified important roles for Cyclin D2 (CCND2), Homeobox Protein C6 (HOXC6) and PlexinA1 (PLXNA1) in human ERMS cell proliferation. PLXNA1 knockdown also enhanced differentiation, reduced migration, and altered anchorage-independent growth. By contrast, chemical inhibition of vascular endothelial growth factor (VEGF) signaling reduced angiogenesis and tumor size in ERMS-bearing zebrafish. Importantly, VEGFA expression correlated with poor clinical outcome in patients with ERMS, implicating inhibitors of the VEGF pathway as a promising therapy for improving patient survival. Our results demonstrate the utility of array CGH and cross-species comparisons to identify candidate oncogenes essential for the pathogenesis of human cancer.

Cancer is a complex genetic disease that is often associated with regional gains and losses of genomic DNA segments. These changes result in aberrant gene expression and drive continued tumor growth. Because amplified and deleted DNA segments tend to span large regions of chromosomes, it has been challenging to identify the genes that are required for continued tumor growth and progression. Array comparative genomic hybridization (array CGH) is an effective technology in identifying abnormal copy number variations in cancer genomes. In this study, array CGH was used in a zebrafish model of embryonal rhabdomyosarcoma - a pediatric muscle tumor. Our work shows that the zebrafish cancer genome contains a small number of recurrent DNA copy number changes, which are also commonly amplified in the human disease. Moreover, these chromosomal regions are small, facilitating rapid identification of candidate oncogenes. A subset of genes identified in zebrafish array CGH was prioritized for functional characterization in human ERMS, identifying evolutionarily conserved pathways that regulate proliferation, migration, differentiation, and neovascularization. Our results demonstrate the broad utility of cross-species array CGH comparisons of human and zebrafish cancer and provide a much needed discovery platform for identifying critical cancer-causing genes in a wide range of malignancies.

Neuroimmune semaphorin 4D is necessary for optimal lung allergic inflammation

Neuroimmune semaphorin 4D (Sema4D) was found to be expressed and function in the nervous and immune systems. In the immune system, Sema4D is constitutively expressed on T cells and regulates T cell priming. In addition, it displays a stimulatory function on macrophages, DC, NK cells, and neutrophils. As all these cells are deeply involved in asthma pathology, we hypothesized that Sema4D plays a critical non-redundant regulatory role in allergic airway response. To test our hypothesis, we exposed Sema4D(-/-) and WT mice to OVA injections and challenges in the well-defined mouse model of OVA-induced experimental asthma. We observed a significant decrease in eosinophilic airway infiltration in allergen-treated Sema4D(-/-) mice relative to WT mice. This reduced allergic inflammatory response was associated with decreased BAL IL-5, IL-13, TGFβ1, IL-6, and IL-17A levels. In addition, T cell proliferation in OVA₃₂₃₋₃₃₉-restimulated Sema4D(-/-) cell cultures was downregulated. We also found increased Treg numbers in spleens of Sema4D(-/-) mice. However, airway hyperreactivity (AHR) to methacholine challenges was not affected by Sema4D deficiency in either acute or chronic experimental disease setting. Surprisingly, lung DC number and activation were not affected by Sema4D deficiency. These data provide a new insight into Sema4D biology and define Sema4D as an important regulator of Th2-driven lung pathophysiology and as a potential target for a combinatory disease immunotherapy.

Leukemia surfaceome analysis reveals new disease-associated features

A better description of the leukemia cell surface proteome (surfaceome) is a prerequisite for the development of diagnostic and therapeutic tools. Insights into the complexity of the surfaceome have been limited by the lack of suitable methodologies. We combined a leukemia xenograft model with the discovery-driven chemoproteomic Cell Surface Capture technology to explore the B-cell precursor acute lymphoblastic leukemia (BCP-ALL) surfaceome; 713 cell surface proteins, including 181 CD proteins, were detected through combined analysis of 19 BCP-ALL cases. Diagnostic immunophenotypes were recapitulated in each case, and subtype specific markers were detected. To identify new leukemia-associated markers, we filtered the surfaceome data set against gene expression information from sorted, normal hematopoietic cells. Nine candidate markers (CD18, CD63, CD31, CD97, CD102, CD157, CD217, CD305, and CD317) were validated by flow cytometry in patient samples at diagnosis and during chemotherapy. CD97, CD157, CD63, and CD305 accounted for the most informative differences between normal and malignant cells. The ALL surfaceome constitutes a valuable resource to assist the functional exploration of surface markers in normal and malignant lymphopoiesis. This unbiased approach will also contribute to the development of strategies that rely on complex information for multidimensional flow cytometry data analysis to improve its diagnostic applications.

Semaphorins in cancer: biological mechanisms and therapeutic approaches

The hallmarks of cancer include multiple alterations in the physiological processes occurring in normal tissues, such as cell proliferation, apoptosis, and restricted cell migration. These aberrant behaviors are due to genetic and epigenetic changes that affect signaling pathways controlling cancer cells, as well as the surrounding "normal" cells in the tumor microenvironment. Semaphorins and their receptors (mainly plexins and neuropilins) are aberrantly expressed in human tumors, and multiple family members are emerging as pivotal signals deregulated in cancer. Notably, different semaphorins can promote or inhibit tumor progression, depending on the implicated receptor complexes and responsive cell type. The important role of semaphorin signals in the regulation of tumor angiogenesis, invasion and metastasis has initiated multiple experimental approaches aimed at targeting these pathways to inhibit cancer.

Plexin-B2 regulates the proliferation and migration of neuroblasts in the postnatal and adult subventricular zone

In the postnatal forebrain, the subventricular zone (SVZ) contains a pool of undifferentiated cells, which proliferate and migrate along the rostral migratory stream (RMS) to the olfactory bulb and differentiate into granule cells and periglomerular cells. Plexin-B2 is a semaphorin receptor previously known to act on neuronal proliferation in the embryonic brain and neuronal migration in the cerebellum. We show here that, in the postnatal and adult CNS, Plexin-B2 is expressed in the subventricular zone lining the telencephalic ventricles and in the rostral migratory stream. We analyzed Plxnb2(-/-) mice and found that there is a marked reduction in the proliferation of SVZ cells in the mutant. Plexin-B2 expression is downregulated in the olfactory bulb as interneurons initiate radial migration. BrdU labeling and GFP electroporation into postnatal SVZ, in addition to time-lapse videomicroscopy, revealed that neuroblasts deficient for Plexin-B2 migrate faster than control ones and leave the RMS more rapidly. Overall, these results show that Plexin-B2 plays a role in postnatal neurogenesis and in the migration of SVZ-derived neuroblasts.

Immune plexins and semaphorins: old proteins, new immune functions

Plexins and semaphorins are a large family of proteins that are involved in cell movement and response. The importance of plexins and semaphorins has been emphasized by their discovery in many organ systems including the nervous (Nkyimbeng-Takwi and Chapoval, 2011; McCormick and Leipzig, 2012; Yaron and Sprinzak, 2012), epithelial (Miao et al., 1999; Fujii et al., 2002), and immune systems (Takamatsu and Kumanogoh, 2012) as well as diverse cell processes including angiogenesis (Serini et al., 2009; Sakurai et al., 2012), embryogenesis (Perala et al., 2012), and cancer (Potiron et al., 2009; Micucci et al., 2010). Plexins and semaphorins are transmembrane proteins that share a conserved extracellular semaphorin domain (Hota and Buck, 2012). The plexins and semaphorins are divided into four and eight subfamilies respectively based on their structural homology. Semaphorins are relatively small proteins containing the extracellular semaphorin domain and short intracellular tails. Plexins contain the semaphorin domain and long intracellular tails (Hota and Buck, 2012). The majority of plexin and semaphorin research has focused on the nervous system, particularly the developing nervous system, where these proteins are found to mediate many common neuronal cell processes including cell movement, cytoskeletal rearrangement, and signal transduction (Choi et al., 2008; Takamatsu et al., 2010). Their roles in the immune system are the focus of this review.

T cells from indolent CLL patients prevent apoptosis of leukemic B cells in vitro and have altered gene expression profile

T cells may have a role in sustaining the leukemic clone in chronic lymphocytic leukemia (CLL). In this study, we have examined the ability of T cells from CLL patients to support the survival of the leukemic B cells in vitro. Additionally, we compared global gene expression of T cells from indolent CLL patients with healthy individuals and multiple myeloma (MM) patients. Apoptosis of purified leukemic B cells was inhibited in vitro when co-cultured with increasing numbers of autologous T cells (p < 0.01) but not autologous B and T cells of normal donors. The anti-apoptotic effect exceeded that of the anti-apoptotic cytokine IL-4 (p = 0.002) and was greater with CD8+ cells (p = 0.02) than with CD4+ cells (p = 0.05). The effect was depended mainly on cell-cell contact although a significant effect was also observed in transwell experiments (p = 0.05). About 356 genes involved in different cellular pathways were deregulated in T cells of CLL patients compared to healthy individuals and MM patients. The results of gene expression profiling were verified for 6 genes (CCL4, CCL5 (RANTES), XCL1, XCL2, KLF6, and TRAF1) using qRT-PCR and immunoblotting. Our results demonstrate that CLL-derived T cells can prevent apoptosis of leukemic B cells and have altered expression of genes that may facilitate the survival of the leukemic clone.

Role of microRNAs in Semaphorin function and neural circuit formation

Since the discovery of the first microRNA (miRNA) almost 20 years ago, insight into their functional role has gradually been accumulating. This class of non-coding RNAs has recently been implicated as key molecular regulators in the biology of most eukaryotic cells, contributing to the physiology of various systems including immune, cardiovascular, nervous systems and also to the pathophysiology of cancers. Interestingly, Semaphorins, a class of evolutionarily conserved signalling molecules, are acknowledged to play major roles in these systems also. This, combined with the fact that Semaphorin signalling requires tight spatiotemporal regulation, a hallmark of miRNA expression, suggests that miRNAs could be crucial regulators of Semaphorin function. Here, we review evidence suggesting that Semaphorin signalling is regulated by miRNAs in various systems in health and disease. In particular, we focus on neural circuit formation, including axon guidance, where Semaphorin function was first discovered.

Plexin structures are coming: opportunities for multilevel investigations of semaphorin guidance receptors, their cell signaling mechanisms, and functions

Plexin transmembrane receptors and their semaphorin ligands, as well as their co-receptors (Neuropilin, Integrin, VEGFR2, ErbB2, and Met kinase) are emerging as key regulatory proteins in a wide variety of developmental, regenerative, but also pathological processes. The diverse arenas of plexin function are surveyed, including roles in the nervous, cardiovascular, bone and skeletal, and immune systems. Such different settings require considerable specificity among the plexin and semaphorin family members which in turn are accompanied by a variety of cell signaling networks. Underlying the latter are the mechanistic details of the interactions and catalytic events at the molecular level. Very recently, dramatic progress has been made in solving the structures of plexins and of their complexes with associated proteins. This molecular level information is now suggesting detailed mechanisms for the function of both the extracellular as well as the intracellular plexin regions. Specifically, several groups have solved structures for extracellular domains for plexin-A2, -B1, and -C1, many in complex with semaphorin ligands. On the intracellular side, the role of small Rho GTPases has been of particular interest. These directly associate with plexin and stimulate a GTPase activating (GAP) function in the plexin catalytic domain to downregulate Ras GTPases. Structures for the Rho GTPase binding domains have been presented for several plexins, some with Rnd1 bound. The entire intracellular domain structure of plexin-A1, -A3, and -B1 have also been solved alone and in complex with Rac1. However, key aspects of the interplay between GTPases and plexins remain far from clear. The structural information is helping the plexin field to focus on key questions at the protein structural, cellular, as well as organism level that collaboratoria of investigations are likely to answer.

Targeting the microenvironment in chronic lymphocytic leukemia offers novel therapeutic options

Chronic lymphocytic leukemia (CLL) cells display features consistent with a defect in apoptosis and exhibit prolonged survival in vivo. Survival of these malignant cells is influenced by interactions with non-leukemic cells located in permissive niches in lymphoid organs. Leukemic cells subvert the normal architecture of the lymphoid organs, recruiting stromal cells, dendritic cells and T lymphocytes, all reported as playing active roles in the survival and proliferation of CLL. The same survival-promoting environment also rescues/protects leukemic cells from cytotoxic therapies, giving way to disease relapse. This review summarizes and discusses current knowledge about the intricate network of soluble and cell-bound signals regulating the life and death of CLL cells in different districts. At the same time, it seeks to hone in on which discrete molecular elements are best suited as targets for treating this still incurable disease.

Plexin-B2 and Plexin-D1 in dendritic cells: expression and IL-12/IL-23p40 production

Plexins are a family of genes (A,B,C, and D) that are expressed in many organ systems. Plexins expressed in the immune system have been implicated in cell movement and cell-cell interaction during the course of an immune response. In this study, the expression pattern of Plexin-B2 and Plexin-D1 in dendritic cells (DCs), which are central in immune activation, was investigated. Plexin-B2 and Plexin-D1 are reciprocally expressed in myeloid and plasmacytoid DC populations. Plasmacytoid DCs have high Plexin-B2 but low Plexin-D1, while the opposite is true of myeloid DCs. Expression of Plexin-B2 and Plexin-D1 is modulated upon activation of DCs by TLR ligands, TNFα, and anti-CD40, again in a reciprocal fashion. Semaphorin3E, a ligand for Plexin-D1 and Plexin-B2, is expressed by T cells, and interestingly, is dramatically higher on Th2 cells and on DCs. The expression of Plexins and their ligands on DCs and T cells suggest functional relevance. To explore this, we utilized chimeric mice lacking Plxnb2 or Plxnd1. Absence of Plexin-B2 and Plexin-D1 on DCs did not affect the ability of these cells to upregulate costimulatory molecules or the ability of these cells to activate antigen specific T cells. Additionally, Plexin-B2 and Plexin-D1 were dispensable for chemokine-directed in-vitro migration of DCs towards key DC chemokines, CXCL12 and CCL19. However, the absence of either Plexin-B2 or Plexin-D1 on DCs leads to constitutive expression of IL-12/IL-23p40. This is the first report to show an association between Plexin-B2 and Plexin-D1 with the negative regulation of IL-12/IL-23p40 in DCs. This work also shows the presence of Plexin-B2 and Plexin-D1 on mouse DC subpopulations, and indicates that these two proteins play a role in IL-12/IL-23p40 production that is likely to impact the immune response.

Expansion of CD8+ T cells lacking Sema4D/CD100 during HIV-1 infection identifies a subset of T cells with decreased functional capacity

Sema4D, also known as CD100, is a constitutively expressed immune semaphorin on T cells and NK cells. CD100 has important immune regulatory functions that improve antigen-specific priming by antigen-presenting cells, and can also act as a costimulatory molecule on T cells. We investigated the consequence of HIV-1 infection on CD100 expression by T cells, and whether CD100 expression signifies functionally competent effector cells. CD100 expression on T cells from healthy individuals was compared with HIV-1-infected subjects including elite controllers, noncontrollers, and patients receiving antiretroviral therapy. The frequency and fluorescence intensity of CD100 on CD8(+) and CD4(+) T cells were decreased during HIV-1 infection. Furthermore, the absolute number of CD100-expressing CD8(+) T cells was positively associated with the magnitude of HIV-1-specific T-cell responses. CD8(+) T cells lacking CD100 expression were functionally impaired and present in increased numbers in HIV-1-infected individuals. The number of CD100(-)CD8(+) T cells positively correlated with T-cell immunosenescence, immune activation, and viral load. Loss of CD100 expression appears to result from direct antigen stimulation, as in vitro cytokine exposure and viral replication did not significantly impact CD100 expression. These data suggest that loss of CD100 expression probably plays an important role in dysfunctional immunity in HIV-1 infection.

More than nervous: the emerging roles of plexins

Plexins are the receptors for semaphorins, a large family of axon guidance cues. Accordingly, the role of plexins in the development of the nervous system was the first to be acknowledged. However, the expression of plexins is not restricted to neuronal cells, and recent research has been increasingly focused on the roles of plexin-semaphorin signalling outside of the nervous system. During embryogenesis, plexins regulate the development of many organs, including the cardiovascular system, skeleton and kidney. They have also been shown to be involved in immune system functions and tumour progression. Analyses of the plexin signalling in different tissues and cell types have provided new insight to the versatility of plexin interactions with semaphorins and other cell-surface receptors. In this review we try to summarise the current understanding of the roles of plexins in non-neural development and immunity.

Plexin-B2 negatively regulates macrophage motility, Rac, and Cdc42 activation

Plexins are cell surface receptors widely studied in the nervous system, where they mediate migration and morphogenesis though the Rho family of small GTPases. More recently, plexins have been implicated in immune processes including cell-cell interaction, immune activation, migration, and cytokine production. Plexin-B2 facilitates ligand induced cell guidance and migration in the nervous system, and induces cytoskeletal changes in overexpression assays through RhoGTPase. The function of Plexin-B2 in the immune system is unknown. This report shows that Plexin-B2 is highly expressed on cells of the innate immune system in the mouse, including macrophages, conventional dendritic cells, and plasmacytoid dendritic cells. However, Plexin-B2 does not appear to regulate the production of proinflammatory cytokines, phagocytosis of a variety of targets, or directional migration towards chemoattractants or extracellular matrix in mouse macrophages. Instead, Plxnb2^−/− macrophages have greater cellular motility than wild type in the unstimulated state that is accompanied by more active, GTP-bound Rac and Cdc42. Additionally, Plxnb2^−/− macrophages demonstrate faster in vitro wound closure activity. Studies have shown that a closely related family member, Plexin-B1, binds to active Rac and sequesters it from downstream signaling. The interaction of Plexin-B2 with Rac has only been previously confirmed in yeast and bacterial overexpression assays. The data presented here show that Plexin-B2 functions in mouse macrophages as a negative regulator of the GTPases Rac and Cdc42 and as a negative regulator of basal cell motility and wound healing.

Chronic lymphocytic leukemia: the pathologist's view of lymph node microenvironment

Chronic lymphocytic leukemia (CLL), an indolent B-cell malignancy frequently diagnosed in the elderly, is characterized by the relentless accumulation of CD5+ monoclonal B cells that proliferate in the appropriate tissue microenvironments. Despite many advances achieved by molecular and functional studies, our knowledge of the reciprocal relationship between the CLL cell and its microenvironment at the tissue level is still largely incomplete. In this review we present the relevant current information on the tissue microenvironmental features of CLL, focusing on the events that appear to occur in the lymph node. Special attention is devoted to analyzing the properties of both neoplastic and nonneoplastic bystander cells within proliferation centers, the mysterious structures that likely represent the actual proliferative compartment.

Biology and function of neuroimmune semaphorins 4A and 4D

Semaphorins belong to a family of membrane-bound and secreted molecules that regulate the functional activity of axons in the nervous system. Sema4A and Sema4D were the first semaphorins also found to be expressed in immune cells and were, therefore, termed "immune semaphorins". It is known that Sema4A has three functional receptors, namely Plexin D1, Plexin B1, and Tim-2, whereas Sema4D binds to Plexin B1 and CD72. Recent studies suggest that immune semaphorins play critical roles in many physiological and pathological processes and such. In this review, we summarize the current knowledge on the biology of neuroimmune semaphorins and their corresponding receptors, their distribution in organs and tissues, function in the immune response, and critical regulatory roles in various diseases.

The functional in vitro response to CD40 ligation reflects a different clinical outcome in patients with chronic lymphocytic leukemia

Malignant B lymphocytes from chronic lymphocytic leukemia (CLL) patients maintain the capacity to respond to CD40 ligation, among other microenvironmental stimuli. In this study, we show that (i) leukemic CLL cells stimulated with the soluble form of CD40L in vitro show differential responses in terms of upregulation of surface markers (CD95 and CD80) and induction of chemokines (CCL22 and CCL17) expression/secretion, and that (ii) these changes are mirrored by a distinct activation of intracellular signalling pathways including increase in IKKalpha/beta phosphorylation and upregulation of antiapoptotic proteins (BCL-2 and MCL-1). CLL patients can then be segregated into two distinct functional subsets. We defined the responsive subset of cases CD40L dependent, considering the capacity to respond as a sign of persistent need of this stimulation for the leukemic expansion. Conversely, we named the unresponsive cases CD40L independent, considering them less dependent on this microenvironmental signal, presumably because of a higher autonomous proliferative and survival potential. Importantly, we report that (iii) the two functional subsets show an opposite clinical outcome, with CD40L-independent cases having a shorter time to progression. This indicates that the functional differences observed in vitro may reflect a different leukemic potential in vivo likely responsible for a distinct clinical course.

Plexin-D1 is a novel regulator of germinal centers and humoral immune responses

Long-lived humoral immune responses depend upon the generation of memory B cells and long-lived plasma cells during the germinal center (GC) reaction. These memory compartments, characterized by class-switched IgG and high-affinity Abs, are the basis for successful vaccination. We report that a new member of the plexin family of molecules, plexin-D1, controls the GC reaction and is required for secondary humoral immune responses. Plexin-D1 was not required for B cell maturation, marginal zone precursor development, dark and light zone formation, Igλ(+) and Igκ(+) B cell skewing, B1/B2 development, and the initial extrafollicular response. Plexin-D1 expression was increased following B cell activation, and PlxnD1(-/-) mice exhibited defective GC reactions during T-dependent immune activation. PlxnD1(-/-) B cells showed a defect in migration toward the GC chemokines, CXCL12, CXCL13, and CCL19. Accordingly, PlxnD1(-/-) mice exhibited defective production of IgG1 and IgG2b, but not IgG3 serum Ab, accompanied by reductions in long-lived bone marrow plasmacytes and recall humoral memory responses. These data show a new role for immune plexins in the GC reaction and generation of immunologic memory.

An overview of chronic lymphocytic leukaemia biology

Chronic lymphocytic leukaemia (CLL) is characterised by accumulation of CD5(+) monoclonal B cells in primary and secondary lymphoid tissues. Genetic defects and stimuli originating from the microenvironment concur to the selection and expansion of the malignant clone. Several lines of evidence, including molecular and functional analysis of the monoclonal immunoglobulin, support the hypothesis that stimulation through the B-cell receptor affects life and death of leukaemic cells. The microenvironment also has a critical role in the survival and accumulation of leukaemic cells within lymphoid organs where signals delivered from the surrounding cells are likely crucial in inducing proliferation. Nevertheless, several major biological issues still remain to be solved including regulation of the balance between proliferation and survival of leukaemic cells and the links between emerging gene abnormalities and microenvironment. In this context, mouse models are helpful tools in understanding disease mechanisms and in evaluating the efficacy of novel therapeutic agents.

Inflammatory cytokines and signaling pathways are associated with survival of primary chronic lymphocytic leukemia cells in vitro: a dominant role of CCL2

BACKGROUND

Chronic lymphocytic leukemia cells show prolonged survival in vivo, but rapidly die by spontaneous apoptosis in vitro, unless they are co-cultured with stromal cells or non-malignant leukocytes. The objective of this study was to characterize the survival-inducing cross-talk of chronic lymphocytic leukemia cells with their microenvironment to identify novel therapeutic targets.

DESIGN AND METHODS

We analyzed and compared microarray-based expression profiles of chronic lymphocytic leukemia cells before and after three different survival-inducing culture conditions: (i) stromal cell co-culture, (ii) stromal cell conditioned medium and (iii) high cell density cultures of unsorted peripheral blood mononuclear cells. Cytokine antibody arrays were applied to study the composition of soluble factors present in these cultures.

RESULTS

The different survival-supportive culture conditions induced distinct gene expression changes, the majority of which were common to all three conditions. Pathway analyses identified - in addition to known signaling networks in chronic lymphocytic leukemia - novel pathways, of which Toll-like receptor signaling, nuclear respiratory factor-2 (NRF2)-mediated oxidative stress response, and signaling via triggering receptor expressed on myeloid cells-1 (TREM1) were the most relevant. A high proportion of up-regulated genes were inflammatory cytokines, of which chemokine (C-C motif) ligand 2 (CCL2) was shown to be induced in monocytes by the presence of chronic lymphocytic leukemia cells in vitro. In addition, increased serum levels of this chemokine were detected in patients with chronic lymphocytic leukemia.

CONCLUSIONS

Our data provide several lines of evidence that an inflammatory microenvironment is induced in survival-supportive cultures of chronic lymphocytic leukemia cells which might be directly or indirectly involved in the prolonged survival of the malignant cells.

Plexin-A4-semaphorin 3A signaling is required for Toll-like receptor- and sepsis-induced cytokine storm

Plexins and semaphorins are ligand-receptor pairs that serve as guidance molecules in the nervous system and play some roles in immunity. Plexins are similar to the Toll-like receptors (TLRs) in their evolutionary conservation from flies to mammals. By studying plexin-A4-deficient (Plxna4(-/-)) innate immune cells, in this study we show a novel influence of plexin-A4 on TLR signaling. Plxna4(-/-) cells exhibit defective inflammatory cytokine production upon activation by a spectrum of TLR agonists and bacteria. Plexin-A4 is required for TLR-induced activation of the small guanosine triphosphate hydrolase (GTPase) Rac1 (ras-related C3 botulinum toxin substrate 1). Rac1 activation is accompanied by JNK (c-Jun N-terminal kinase) and NF-κB activation, culminating in TLR-induced binding of NF-κB and AP-1 to the promoters of inflammatory cytokines. Plxna4(-/-) mice are remarkably resistant to TLR agonist-induced inflammation and polymicrobial peritonitis caused by cecal ligation and puncture. Administration of a ligand of plexin-A4, Sema3A (semaphorin 3A), exacerbates the cytokine storm caused by TLR agonists and bacterial sepsis. TLR engagement can induce Sema3A expression, thus completing an autocrine loop. These findings expand the role of plexins to TLR signaling and suggest plexin-A4 and Sema3A as new intervention points for treating sepsis.

Interaction with vascular endothelium enhances survival in primary chronic lymphocytic leukemia cells via NF-kappaB activation and de novo gene transcription

Chronic lymphocytic leukemia (CLL) cells rapidly undergo apoptosis in vitro, suggesting that the in vivo microenvironment provides crucial antiapoptotic signals. Overexpression of the antiapoptotic proteins Bcl-2 and Mcl-1 is a hallmark of CLL, and their expression is further enhanced in the lymphoid tissues. However, the high levels of Mcl-1 found in peripheral blood samples, coupled with its short half-life, led us to hypothesize that it must be actively maintained in the peripheral circulation. Coculture of CLL cells with human vascular endothelial cells significantly enhanced tumor cell survival, an effect that was not observed with normal B cells. This was associated with elevated levels of the antiapoptotic proteins Bcl-2, Mcl-1, and Bcl-X(L) and marked increased expression of CD38 and CD49d, both of which are associated with clinically aggressive disease. Because CD38, CD49d, and some Bcl-2 family genes are transcriptional targets for NF-κB, we assessed NF-κB activation following coculture with endothelial cells. DNA binding of the NF-κB subunit Rel A was significantly increased and strongly correlated with changes in transcription of CD38, CD49d, BCL2, MCL1, and BCLXL, effects that were reversed by a peptide inhibitor of Rel A. These effects were not observed following coculture with nonendothelial cell lines. Therefore, CLL cells receive specific survival signals following interaction with endothelial cells mediated through the activation of NF-κB and the induction of downstream target genes. This type of interaction in the peripheral vasculature may explain the constitutive NF-κB activation and the overexpression of Bcl-2 family proteins commonly seen in this disease.

The role of toll-like receptors in chronic B-cell malignancies

Toll-like receptors (TLR) are key players in host defence from infection. They recognize a specific set of molecular patterns of microbial origin, immediately trigger an innate immune response, and bridge innate and adaptive immunity. TLR have also been shown to play a role in tumor development. In this context, chronic B-cell malignancies are an interesting example as clonal B lymphocytes remain responsive to and dependent on stimuli originating from the microenvironment which then become crucial for maintaining and propagating the disease. Emerging evidences suggest that, among other microenvironmental elements, TLR ligands may play a role in the pathogenesis of chronic B-cell lymphoid malignancies. Conceivably, their manipulation may find a place in specific settings of treatment of these tumors.

Bone marrow mesenchymal stem cells from leukemia patients inhibit growth and apoptosis in serum-deprived K562 cells

BACKGROUND

The regulation of growth and apoptosis in K562 cells by human bone marrow mesenchymal stem cells (MSCs) from leukemia patients was investigated.

METHODS

K562 cells were cocultured with leukemic MSCs under serum deprivation. Cell Counting Kit-8 (CCK-8), PI staining, Annexin V/PI binding and FACS assays were used to investigate cell proliferation, cell cycle status, and apoptosis of K562 cells cultures in the presence or absence of 10% serum. Western blotting was used to determine the levels of Akt, phosphorylated Akt (p-Akt), the BCL-2 family member Bad, and phosphorylated Bad (p-Bad) proteins in K562 cells after coculturing with MSCs. The effects of LY294002 (a specific inhibitor of PI3K) on protein expression were also determined.

RESULTS

K562 cell proliferation was inhibited by coculture with MSCs and the dominant cell cycle was the G0-G1 phase. The proportion of apoptotic K562 cells was decreased and the levels of p-Akt and p-Bad were upregulated after exposing K562 cells to MSCs. However, when LY294002 was used, p-Akt and p-Bad proteins inK562 cells showed a significant reduction, while no distinct variation was seen in the nonphosphorylated Akt and Bad protein levels.

CONCLUSION

Leukemic MSCs can inhibit K562 cell expansion and modulate the cell cycle to a state of relative quiescence. This allows the K562 cells to endure adverse conditions such as serum starvation. The PI3K-Akt-Bad signaling pathway may be involved in this antiapoptotic process via phosphorylation of the Akt and Bad proteins. Blocking MSC-induced transduction of the PI3K-Akt-Bad pathway may be a potential strategy for a targeted therapy to combat leukemia.

Hypoxia-inducible factor-1-mediated regulation of semaphorin 4D affects tumor growth and vascularity

Tumor progression and metastasis depend on the ability of cancer cells to initiate angiogenesis to ensure delivery of oxygen, nutrients, and growth factors to tumor cells and provide access to the systemic circulation. Hypoxia-inducible factor-1 (HIF-1) can activate expression of a broad range of genes that mediate many of the adaptive responses to decreased oxygen concentration, such as enhanced glucose uptake and formation of new blood vessels. Acting through Plexin-B1 on endothelial cells, Semaphorin 4D (Sema4D) has been shown to promote angiogenesis and enhance invasive growth and proliferation in some tumors. Here we show that the gene for Sema4D, the product of which is elevated in head and neck squamous cell carcinoma (HNSCC) cells, contains upstream hypoxia response elements (HRE) and is strongly induced in hypoxia in a HIF-1-dependent manner. Knocking down Sema4D expression with short hairpin (sh) RNA reduces in vitro endothelial cell migration and growth and vascularity of HNSCC xenografts expressing a degradation resistant HIF-1alpha subunit. We also demonstrate a correlation between HIF-1 activity and Sema4D expression in HNSCC specimens. These findings indicate that Sema4D is induced by hypoxia in a HIF-1-dependent manner and influences endothelial cell migration and tumor vascularity. Expression of Sema4D may be a strategy by which carcinomas promote angiogenesis and therefore could represent a therapeutic target for these malignancies.

Semaphorin signaling in cancer cells and in cells of the tumor microenvironment--two sides of a coin

Semaphorins are a large family of secreted and membrane-bound molecules that were initially implicated in the development of the nervous system and in axon guidance. More recently, they have been found to regulate cell adhesion and motility, angiogenesis, immune responses, and tumor progression. Semaphorin receptors, the neuropilins and the plexins, are expressed by a wide variety of cell types, including endothelial cells, bone-marrow-derived cells and cancer cells. Interestingly, a growing body of evidence indicates that semaphorins also have an important role in cancer. It is now known that cancer progression, invasion and metastasis involve not only genetic changes in the tumor cells but also crosstalk between tumor cells and their surrounding non-tumor cells. Through the recruitment of endothelial cells, leukocytes, pericytes and fibroblasts, and the local release of growth factors and cytokines, the tumor microenvironment can mediate tumor-cell survival, tumor proliferation and regulation of the immune response. Moreover, by conferring cancer cells with an enhanced ability to migrate and invade adjacent tissues, extracellular regulatory signals can play a major role in the metastatic process. In this Commentary, we focus on the emerging role of semaphorins in mediating the crosstalk between tumor cells and multiple stromal cell types in the surrounding microenvironment.

Roles of semaphorins in the immune and hematopoietic system

Semaphorins were originally discovered in the nervous system, and have been implicated in repulsive axon guidance during the development of nervous system. Semaphorins are also implicated in tumor progression, by affecting adhesion, migration of malignant cells and angiogenesis, and are involved in normal cardiovascular development. Recently, several semaphorins and their receptors are expressed in a variety of lymphoid and myeloid cells, and affect immune cell functions, including cell proliferation, differentiation, chemotaxis, and cytokine production. This review focuses on recent work on the functions of semaphorins in the immune system and autoimmune diseases.

Microenvironmental influences in chronic lymphocytic leukaemia: the role of antigen stimulation

Several studies suggest that immune-mediated pathways are important in the pathogenesis of chronic lymphocytic leukaemia (CLL). The in vivo accumulation of leukaemic lymphocytes is facilitated by interactions of CLL cells with other cells and soluble factors that probably occur more often within the microenvironment through classical receptor-ligand interactions. These include CD40L-CD40 and chemokine-chemokine receptor interactions as well as B cell receptor (BCR) engagement by (auto)antigens. Indeed, the categorizations of CLL patients based on immunoglobulin heavy variable (IGHV) gene mutations and structure of the clone's BCR suggest that CLL patient outcome could be a reflection of ongoing BCR signalling in the context of other co-signals.

Expression and function of toll like receptors in chronic lymphocytic leukaemia cells

Mature B-cells can recognize microbial antigens via B-cell-receptor (BCR) in a specific way and via Toll-like receptors (TLR) in a costimulatory manner. A wealth of information is gathering on the possible role of antigenic stimulation in the natural history of Chronic Lymphocytic Leukaemia (CLL). However little is known regarding the repertoire and function of TLR in CLL cells. The TLR family includes 10 different transmembrane proteins devoted to recognize specific pathogen-associated molecular patterns and to alarm immunocompetent cells to trigger an immune response. Here, we studied fresh leukaemic cells for the expression pattern of TLR1 to TLR10, NOD1, NOD2 and SIGIRR (also known as TIR8). CLL cells were found to express several pattern recognition receptors including TLR1, TLR2, TLR6, TLR10, NOD1 and NOD2. The specific TLR expressed by CLL cells were functional. Leukaemic cells, upon stimulation with TLR1/2/6 ligands, such as bacterial lipopeptides, activated the nuclear factor-kappaB signalling pathway, expressed CD86 and CD25 activation molecules, and were protected from spontaneous apoptosis. These findings further support the hypothesis that CLL cells resemble antigen-activated B-cells and suggest a potential role of TLR in modulating CLL cell response in the context of specific antigen recognition.

The semaphorins: versatile regulators of tumour progression and tumour angiogenesis

The semaphorins and their receptors, the neuropilins and the plexins, were originally characterized as constituents of the complex regulatory system responsible for the guidance of axons during the development of the central nervous system. However, a growing body of evidence indicates that various semaphorins can either promote or inhibit tumour progression through the promotion or inhibition of processes such as tumour angiogenesis, tumour metastasis and tumour cell survival. This Review focuses on the emerging role of the semaphorins in cancer.

Semaphorins and their receptors in lung cancer

Semaphorins are a large family of secreted, transmembrane and GPI-linked proteins initially characterized in the development of the nervous system and axonal guidance. Semaphorins are expressed in many tissues where they regulate normal development, organ morphogenesis, immunity and angiogenesis. They affect the cytoskeleton, actin filament organization, microtubules and cell adhesion. Semaphorin signaling is transduced by plexins, which in the case of most class-3 semaphorins requires high-affinity neuropilin receptors. The neuropilins also function as receptors for VEGF and other growth factors, and their expression is often abnormal in tumors. In cancer, semaphorins have both tumor suppressor and tumor promoting functions. We review here the current status of semaphorins and their receptors in tumor development with a focus on lung cancer.

Tumor angiogenesis and progression are enhanced by Sema4D produced by tumor-associated macrophages

Increased evidence suggests that cancer-associated inflammation supports tumor growth and progression. We have previously shown that semaphorin 4D (Sema4D), a ligand produced by different cell types, is a proangiogenic molecule that acts by binding to its receptor, plexin B1, expressed on endothelial cells (Conrotto, P., D. Valdembri, S. Corso, G. Serini, L. Tamagnone, P.M. Comoglio, F. Bussolino, and S. Giordano. 2005. Blood. 105:4321–4329). The present work highlights the role of Sema4D produced by the tumor microenvironment on neoplastic angiogenesis. We show that in an environment lacking Sema4D, the ability of cancer cells to generate tumor masses and metastases is severely impaired. This condition can be explained by a defective vascularization inside the tumor. We demonstrate that tumor-associated macrophages (TAMs) are the main cells producing Sema4D within the tumor stroma and that their ability to produce Sema4D is critical for tumor angiogenesis and vessel maturation. This study helps to explain the protumoral role of inflammatory cells of the tumor stroma and leads to the identification of an angiogenic molecule that might be a novel therapeutic target.

CD38 expression in chronic lymphocytic leukemia is regulated by the tumor microenvironment

Chronic lymphocytic leukemia (CLL) is a lymphoproliferative disease with a highly variable outcome. The prognosis of patients with CLL may be predicted using a number of biomarkers, including the level of CD38 expression at the leukemic cell surface. This study investigates the hypothesis that CD38 expression by CLL cells reflects interactions with nonmalignant cells within pseudofollicles in secondary lymphoid tissue where tumor cell proliferation is thought to occur. CD38 expression is higher in tissues that contain pseudofollicles compared with those that do not. In addition, we show that CD38 expression in CLL is dynamic, changes in response to contact with activated CD4(+) T cells, and identifies cells that are primed to proliferate. Finally, we demonstrate close contact between activated CD4(+) T cells and proliferating tumor in primary patient tissue. Proliferating tumor cells in lymph nodes express CD38, which is in turn associated with an increased number of CD31(+) vascular endothelial cells. Although the factors resulting in colocalization of tumor, T cells, and endothelium remain unclear, the existence of these cellular clusters may provide an explanation for the association between CD38 expression and adverse outcome in CLL and suggests novel therapeutic targets.

A Novel Nonobese Diabetic/Severe Combined Immunodeficient Xenograft Model for Chronic Lymphocytic Leukemia Reflects Important Clinical Characteristics of the Disease

We here describe a novel xenograft model of chronic lymphocytic leukemia (CLL) generated by infusion of human primary CLL cells into immunodeficient nonobese/severe combined immunodeficient (NOD/SCID) mice. Combined i.v. and i.p. injection of peripheral blood mononuclear cells (PBMC) from 39 patients with CLL resulted in highly reproducible splenic (37 of 39) and peritoneal (35 of 39) engraftment, which remained stable over a time span of 4 to 8 weeks. By comparison, recovery of leukemic cells from bone marrow (21 of 39) or peripheral blood (8 of 22) was substantially lower. The engraftment pattern of CLL PBMC 4 weeks posttransplant was correlated with clinical disease activity: infusion of PBMC from donors with Binet stage A, lymphocyte doubling time of >12 months, and normal lactate dehydrogenase (LDH) serum levels led to marked engraftment of T cells whereas comparably few tumor cells could be detected. In contrast, NOD/SCID mice receiving PBMC from donors with advanced stage Binet C, lymphocyte doubling time of <12 months, and elevated LDH serum levels exhibited predominant engraftment of tumor cells and comparably low numbers of T cells. These results suggest that this model reflects the heterogeneity and important clinical characteristics of the disease, and thus may serve as a tool for preclinical drug testing and investigation of the pathophysiology of CLL.

Prognostic significance of CD100 expression in soft tissue sarcoma

BACKGROUND

CD100, a class IV semaphorin, promotes angiogenesis, invasive growth, proliferation, and antiapoptosis of cancer cells in vitro. The expression of CD100 in soft tissue sarcoma (STS) and its correlation with clinicopathologic factors and prognostic significance were evaluated.

METHODS

Expression levels of CD100 in patients with localized STS were evaluated immunohistochemically on paraffin-embedded sections from 81 patients, including 47 men and 34 women with a median age of 54 years. Staining intensity was categorized into weaker than (level 1) or equal to that of lymphocytes with a rate of <10% stained tumor cells (level 2) or >10% stained tumor cells (level 3). Ki-67 staining was performed in parallel.

RESULTS

Forty-two tumors (52%) had level 1 CD100 expression, 18 tumors (22%) had level 2 CD100 expression, and 21 tumors (26%) had level 3 CD100 expression. Tumors that had level 2 and 3 CD100 expression were correlated significantly with higher mitotic count, cellularity, ratio of necrosis, and Ki-67 labeling index (LI) compared with tumors that had level 1 CD100 expression. There was no correlation between CD100 expression and other characteristics. Among the 3 levels of CD100 expression, higher expression levels were correlated with poorer overall and disease-free survival. Multivariate analysis revealed that CD100 expression (levels 1 and 2 vs level 3) and tumor size (<or=5 cm vs >5 cm) were independent prognosticators for overall survival (P < .05 for both), and CD100 expression (levels 1 and 2 vs level 3) was an independent prognosticator for disease-free survival (P < .05).

CONCLUSIONS

The results from this study indicated the demonstrated prognostic significance of CD100 expression in STS.

Efficient nucleofection of primary human B cells and B-CLL cells induces apoptosis, which depends on the microenvironment and on the structure of transfected nucleic acids

Accumulation of neoplastic cells in B-cell chronic lymphocytic leukemia (B-CLL) is thought to be due to intrinsic defects in the apoptotic machinery of the leukemic cells or to an altered, survival-stimulating microenvironment in vivo. Despite their long survival in vivo, B-CLL cells undergo rapid spontaneous apoptosis ex vivo. To maintain survival in vitro, we established a coculture system using the human bone marrow-derived stromal cell line HS-5. The microenvironment in these cocultures lead to B-CLL cell survival for at least several months and therefore provided a tool for valid in vitro analysis, mimicking the in vivo situation. Although primary B lymphocytes are notoriously resistant to most gene transfer techniques, we achieved high transfection efficiency and cell viability in this coculture system by using a nucleofection-based strategy. Surprisingly, the introduction of circular plasmid DNA into B cells and B-CLL cells induced rapid apoptosis, which was independent of the type of transgene used, but dependent on the DNA concentration. However, transfection of these cells with mRNA was highly efficient and resulted in sustained cell viability and potent transgene expression. The described procedure represents a new approach to study gene function in primary B cells and B-CLL cells.

Semaphorin signals on the road to cancer invasion and metastasis

Semaphorins are a large family of secreted and membrane-bound molecules initially implicated in the development of the nervous system and in axon guidance. More recently, they have been found to regulate cell adhesion and cell motility, angiogenesis, immune function and tumour progression. Notably, Semaphorins have been implicated with opposite functions in cancer: either as putative tumor suppressors and anti-angiogenic factors, or as mediating tumour angiogenesis, invasion and metastasis. Interestingly, Semaphorins may display divergent activities in different cell types. These multifaceted functions may be explained by the involvement of different kinds of semaphorin receptor complexes, and by the consequent activation of multiple signaling pathways, in different cells or different functional stages. Semaphorin signaling is largely mediated by the Plexins. However, semaphorin receptor complexes may also include Neuropilins and tyrosine kinases implicated in cancer. In this review, we will focus on major open questions concerning the potential role of Semaphorin signals in cancer.

Semaphorin-4D (Sema-4D), the Plexin-B1 ligand, is involved in mouse ovary follicular development

BACKGROUND

Human Plexin-B1 is expressed in two truncated forms. The long form encodes a trans-membranal protein, while the short form, which is bound to the cell surface and partially secreted, possibly serves as a decoy receptor. Plexin receptors are trans-membrane proteins. The sema domain, found in the extracellular region, is common to all plexins, semaphorins, and the scatter factor receptors and is crucial for the biological activity and plexin receptor specificity. Semaphorin-4D/Plexin-B1 binding provides attractive and repulsive cues for the navigation of axonal growth cones, and new studies suggest that this system also plays a role in the regulation of the biological functions of endothelial cells, specifically in the control of angiogenesis. In a previous study, we have demonstrated the expression and possible role of Plexin-B1 in the mouse ovary. The present study was designed to test the hypothesis that Plexin-B1 effects are mediated by Semaphorin-4D.

METHODS

In vivo expression and localization of mouse ovarian Sema-4D were tested by immunohisto-chemistry. The role of Sema-4D in follicular development was examined by in vitro growth of preantral follicles in the presence or absence of Semaphorin-4D, with or without neutralizing antibodies against Plexin-B1. Follicular growth and steroid hormone secretion rates were tested.

RESULTS

Semaphorin-4D is expressed in the mouse ovary in vivo mostly in the granulosa cells and and its expression is modulated by PMSG and hCG. In the presence of Semaphorin-4D, in-vitro constant growth was observed as indicated by follicular diameter during the culture period and elevated steroid hormone secretion rates compared with control. These effects were abolished after addition of neutralizing antibodies against Plexin-B1.

CONCLUSION

In the ovarian follicle, the effect of Plexin-B1 is mediated by sema-4D.

Bone biopsy derived marrow stromal elements rescue chronic lymphocytic leukemia B-cells from spontaneous and drug induced cell death and facilitates an "angiogenic switch"

A novel bone biopsy technique was used to generate a robust stromal cell system to study how stroma modulates CLL B-cell apoptosis and how the leukemic cell-stromal interaction influences secretion of vascular factors. Marrow stromal elements (MSE) rescued CLL B-cells from both spontaneous and drug induced apoptosis, partly due to soluble factors. When CLL B-cells were added to the MSE cultures, a dramatic increase in the secretion of basic fibroblast growth factor and decrease in the secretion of thrombospondin was observed. These results indicate the interaction between CLL B-cells and marrow stromal elements regulates angiogenic switching and may be linked to disease progression.

Epigenetics and the plasticity of differentiation in normal and cancer stem cells

Embryonic stem cells are characterized by their differentiation to all cell types during embryogenesis. In adult life, different tissues also have somatic stem cells, called adult stem cells, which in specific niches can undergo multipotent differentiation. The use of these adult stem cells has considerable therapeutic potential for the regeneration of damaged tissues. In both embryonic and adult stem cells, differentiation is controlled by epigenetic mechanisms, and the plasticity of differentiation in these cells is associated with transcription accessibility for genes expressed in different normal tissues. Abnormalities in genetic and/or epigenetic controls can lead to development of cancer, which is maintained by self-renewing cancer stem cells. Although the genetic abnormalities produce defects in growth and differentiation in cancer stem cells, these cells have not always lost the ability to undergo differentiation through epigenetic changes that by-pass the genomic abnormalities, thus creating the basis for differentiation therapy. Like normal stem cells, cancer stem cells can show plasticity for differentiation. This plasticity of cancer stem cells is also associated with transcription accessibility for genes that are normally expressed in different tissues, including tissues other than those from which the cancers originated. This broad transcription accessibility can also contribute to the behavior of cancer cells by overexpressing genes that promote cell viability, growth and metastasis.

Plexin-A1 and its interaction with DAP12 in immune responses and bone homeostasis

Semaphorins and their receptors have diverse functions in axon guidance, organogenesis, vascularization and/or angiogenesis, oncogenesis and regulation of immune responses. The primary receptors for semaphorins are members of the plexin family. In particular, plexin-A1, together with ligand-binding neuropilins, transduces repulsive axon guidance signals for soluble class III semaphorins, whereas plexin-A1 has multiple functions in chick cardiogenesis as a receptor for the transmembrane semaphorin, Sema6D, independent of neuropilins. Additionally, plexin-A1 has been implicated in dendritic cell function in the immune system. However, the role of plexin-A1 in vivo, and the mechanisms underlying its pleiotropic functions, remain unclear. Here, we generated plexin-A1-deficient (plexin-A1(-/-)) mice and identified its important roles, not only in immune responses, but also in bone homeostasis. Furthermore, we show that plexin-A1 associates with the triggering receptor expressed on myeloid cells-2 (Trem-2), linking semaphorin-signalling to the immuno-receptor tyrosine-based activation motif (ITAM)-bearing adaptor protein, DAP12. These findings reveal an unexpected role for plexin-A1 and present a novel signalling mechanism for exerting the pleiotropic functions of semaphorins.

Semaphorins: a new class of immunoregulatory molecules

The immune and nervous systems play distinct roles in maintaining physiological homeostasis. Recent data indicates that these systems influence one another and share many proteins and pathways that are essential for their normal function and development. Molecules originally shown to be critical for the development of proper immune responses have recently been found to function in the nervous system. Conversely, neuronal guidance cues can modulate immune functions. Although semaphorins were originally identified as axon guidance factors active during neuronal development, several recent studies have identified indispensable functions for these molecules in the immune system. This review provides an overview of the rapidly emerging functions of semaphorins and their receptors in the immune system.

Expression of plexin-B1 in the mouse ovary and its possible role in follicular development

OBJECTIVE

To study the expression of plexin-B1 in the mouse ovary and its possible role in follicular development.

DESIGN

Controlled laboratory study.

SETTING

Research laboratory, Ha'Emek Medical Center.

PATIENT(S)

ICR female mice (n = 36) were studied.

MAIN OUTCOME MEASURE(S)

Plexin-B1 expression by immunohistochemistry, Western blot analysis, and reverse transcriptase-polymerase chain reaction. Microscopic morphometric and morphologic analysis for the evaluation of mouse follicular growth and E2, P, and T secretion from the developed follicles.

RESULT(S)

Plexin-B1 was expressed in the mouse ovary mostly in the granulosa cells. Plexin-B1 gene expression increased significantly by 126.8% +/- 38% in the pregnant mare serum gonadotropin-treated group and increased by 98.7% +/- 15.2 and 262.2% +/- 65.4% in the hCG-treated group 6 h and 24 h after injection, respectively (ANOVA; P < .05). Comparison of the maximum diameters of the control and study groups on day 9 of the culture period revealed a significant difference between the two groups (P < .001). In the control group, the mean maximum diameter was 191 +/- 2.6 microm compared with 136 +/- 5.13 microm in the presence of neutralizing antibodies against plexin-B1. Neutralizing antibody against plexin-B1 decreases E2 and P secretion.

CONCLUSION(S)

This study shows for the first time the expression of plexin-B1 in the mouse ovary. A possible role for plexin-B1 in follicular growth and steroidogenesis, most likely via the granulosa cells, is suggested.

[Involvement of semaphorins in autoimmunity]

Semaphorins were identified originally as guidance cues for developing axons. However, it is becoming clear that several semaphorins are crucially involved in the immune system. For instance, Sema4D (CD100) enhances activation of B cells and dendritic cells, and Sema4A is involved in T cell priming and Th1/Th2 regulation. In addition, recent cumulative evidence reveals their importance in immunological homeostasis. We here focus on our current understanding of the roles of semaphorins in autoimmunity.

The brain within the tumor: new roles for axon guidance molecules in cancers

Slits, semaphorins and netrins are three families of proteins that can attract or repel growing axons and migrating neurons in the developing nervous system of vertebrates and invertebrates. Recent studies have shown that they are widely expressed outside the nervous system and that they may play important roles in cancers. Several of the genes encoding these proteins are localized on chromosomal region associated with frequent loss-of-heterozygosity in tumors and cancer cell lines and there is also significant hypermethylation of their promoter suggesting that they may act as tumor suppressors. In addition, proteins in all these families and their receptors appear to control the vascularization of the tumors. Last, many axon guidance molecules also regulate cell migration and apoptosis in normal and tumorigenic tissues. Overall, this suggests that molecules that could mimick or block the activity of axon guidance molecules may be used as therapeutic agents for the treatment of malignancy.