Nerve growth factor signal transduction in human B lymphocytes is mediated by gp140trk

Nerve growth factor: a neuroimmune crosstalk mediator for all seasons

Neurotrophic factors comprise a broad family of biomolecules - most of which are peptides or small proteins - that support the growth, survival and differentiation of both developing and mature neurons. The prototypical example and best-characterized neurotrophic factor is nerve growth factor (NGF), which is widely recognized as a target-derived factor responsible for the survival and maintenance of the phenotype of specific subsets of peripheral neurons and basal forebrain cholinergic nuclei during development and maturation. In addition to being active in a wide array of non-nervous system cells, NGF is also synthesized by a range of cell types not considered as classical targets for innervation by NGF-dependent neurons; these include cells of the immune-haematopoietic lineage and populations in the brain involved in neuroendocrine functions. NGF concentrations are elevated in numerous inflammatory and autoimmune states such as multiple sclerosis, chronic arthritis, systemic lupus erythematosus and mastocytosis, in conjunction with increased accumulation of mast cells. Intriguingly, NGF seems to be linked also with diabetic pathology and insulin homeostasis. Mast cells and NGF appear involved in neuroimmune interactions and tissue inflammation. As mast cells are capable of producing and responding to NGF, this suggests that alterations in mast cell behaviour could provoke maladaptive neuroimmune tissue responses, including those of an autoimmune nature. Moreover, NGF exerts a modulatory role on sensory nociceptive nerve physiology in the adult, which appears to correlate with hyperalgesic phenomena occurring in tissue inflammation. NGF can therefore be viewed as a multifactorial modulator of neuro-immune-endocrine functions.

Expression of natural killer cell regulatory microRNA by uveal melanoma cancer stem cells

Natural killer (NK) cells are implicated in the control of metastasis in uveal melanoma, a process that has been ascribed to its cancer stem cell subpopulation. NK cell activation is regulated by specific microRNA (miR). The NK cell sensitivity and regulatory miR production of uveal melanoma cancer stem cells was examined. Cancer stem cells enriched from aggressively metastatic MUM2B uveal melanoma cells by selecting CD271+ cells or propagating as non-adherent spheres in stem-cell supportive were more resistant to NK cell cytolysis than cancer stem cells enriched from less aggressively metastatic OCM1 uveal melanoma cells. Both MUM2B and OCM1 cells expressed and secreted NK cell regulatory miRs, including miR 146a, 181a, 20a, and 223. MUM2B cells expressed and secreted miR-155; OCM1 cells did not. Transfecting MUM2B cells with anti-miR-155 increased NK cell sensitivity. CD271+ cells were identified in the blood of patients with metastatic uveal melanoma and were characterized by low expression of melanocyte differentiation determinants and by the ability to form non-adherent spheres in stem-cell supportive media. These cells also expressed NK cell regulatory miRs, including miR-155. These results indicate that uveal melanoma cancer stem cells can vary in their sensitivity to NK cell lysis and their expression of NK cell regulatory miRs. Circulating CD271+ cells from patients with metastatic uveal melanoma manifest cancer stem cell features and express miRs associated with NK cell suppression, including miR-155, that may contribute to metastatic progression.

Attention Deficit Disorder and Allergic Rhinitis: Are They Related?

The association between ADHD and allergy remains controversial. Our previous findings suggest that nerve growth factor may link the nervous and immune systems. The primary objective of this study was to determine if a combination of cetirizine + methylphenidate is effective in children with comorbid ADHD and allergic rhinitis. We also examined the role of nerve growth factor in these comorbidities. Our randomized, double-blind, placebo-controlled, crossover study enrolled 38 children diagnosed with comorbid ADHD and allergy using cetirizine (n = 12), sustained-release methylphenidate (n = 12), or cetirizine + methylphenidate (n = 14). Endpoints compared baseline to posttreatment evaluations for allergic rhinitis and ADHD scores. Serum nerve growth factor levels were measured using ELISA. For allergy endpoints, combination therapy produced results superior to individual therapy. For ADHD, similar scores were achieved for individual therapy; however, combination therapy resulted in improved scores. Nerve growth factor levels were downregulated following this trend. We conclude that ADHD and allergic rhinitis may have common mechanism and represent a comorbid condition that links the nervous system to the immune system. Further studies are needed.

Nerve growth factor downregulates inflammatory response in human monocytes through TrkA

Nerve growth factor (NGF) levels are highly increased in inflamed tissues, but their role is unclear. We show that NGF is part of a regulatory loop in monocytes: inflammatory stimuli, while activating a proinflammatory response through TLRs, upregulate the expression of the NGF receptor TrkA. In turn, NGF, by binding to TrkA, interferes with TLR responses. In TLR-activated monocytes, NGF reduces inflammatory cytokine production (IL-1β, TNF-α, IL-6, and IL-8) while inducing the release of anti-inflammatory mediators (IL-10 and IL-1 receptor antagonist). NGF binding to TrkA affects TLR signaling, favoring pathways that mediate inhibition of inflammatory responses: it increases Akt phosphorylation, inhibits glycogen synthase kinase 3 activity, reduces IκB phosphorylation and p65 NF-κB translocation, and increases nuclear p50 NF-κB binding activity. Use of TrkA inhibitors in TLR-activated monocytes abolishes the effects of NGF on the activation of anti-inflammatory signaling pathways, thus increasing NF-κB pathway activation and inflammatory cytokine production while reducing IL-10 production. PBMC and mononuclear cells obtained from the synovial fluid of patients with juvenile idiopathic arthritis show marked downregulation of TrkA expression. In ex vivo experiments, the addition of NGF to LPS-activated juvenile idiopathic arthritis to both mononuclear cells from synovial fluid and PBMC fails to reduce the production of IL-6 that, in contrast, is observed in healthy donors. This suggests that defective TrkA expression may facilitate proinflammatory mechanisms, contributing to chronic tissue inflammation and damage. In conclusion, this study identifies a novel regulatory mechanism of inflammatory responses through NGF and its receptor TrkA, for which abnormality may have pathogenic implications for chronic inflammatory diseases.

Nerve growth factor: from the early discoveries to the potential clinical use

The physiological role of the neurotrophin nerve growth factor (NGF) has been characterized, since its discovery in the 1950s, first in the sensory and autonomic nervous system, then in central nervous, endocrine and immune systems. NGF plays its trophic role both during development and in adulthood, ensuring the maintenance of phenotypic and functional characteristic of several populations of neurons as well as immune cells. From a translational standpoint, the action of NGF on cholinergic neurons of the basal forebrain and on sensory neurons in dorsal root ganglia first gained researcher's attention, in view of possible clinical use in Alzheimer's disease patients and in peripheral neuropathies respectively. The translational and clinical research on NGF have, since then, enlarged the spectrum of diseases that could benefit from NGF treatment, at the same time highlighting possible limitations in the use of the neurotrophin as a drug. In this review we give a comprehensive account for almost all of the clinical trials attempted until now by using NGF. A perspective on future development for translational research on NGF is also discussed, in view of recent proposals for innovative delivery strategies and/or for additional pathologies to be treated, such as ocular and skin diseases, gliomas, traumatic brain injuries, vascular and immune diseases.

Endogenous neurotrophins and Trk signaling in diffuse large B cell lymphoma cell lines are involved in sensitivity to rituximab-induced apoptosis

Background

Diffuse large B-cell lymphoma (DLBCL) is a common and often fatal malignancy. Immunochemotherapy, a combination of rituximab to standard chemotherapy, has resulted in improved survival. However a substantial proportion of patients still fail to reach sustained remission. We have previously demonstrated that autocrine brain-derived neurotrophic factor (BDNF) production plays a function in human B cell survival, at least partly via sortilin expression. As neurotrophin receptor (Trks) signaling involved activation of survival pathways that are inhibited by rituximab, we speculated that neurotrophins may provide additional support for tumour cell survival and therapeutic resistance in DLBCL.

Methodology/Principal Findings

In the present study, we used two DLBCL cell lines, SUDHL4 and SUDHL6, known to be respectively less and more sensitive to rituximab. We found by RT-PCR, western blotting, cytometry and confocal microscopy that both cell lines expressed, in normal culture conditions, BDNF and to a lesser extent NGF, as well as truncated TrkB and p75^NTR/sortilin death neurotrophin receptors. Furthermore, BDNF secretion was detected in cell supernatants. NGF and BDNF production and Trk receptor expression, including TrkA, are regulated by apoptotic conditions (serum deprivation or rituximab exposure). Indeed, we show for the first time that rituximab exposure of DLBCL cell lines induces NGF secretion and that differences in rituximab sensitivity are associated with differential expression patterns of neurotrophins and their receptors (TrkA). Finally, these cells are sensitive to the Trk-inhibitor, K252a, as shown by the induction of apoptosis. Furthermore, K252a exhibits additive cytotoxic effects with rituximab.

Conclusions/Significance

Collectively, these data strongly suggest that a neurotrophin axis, such NGF/TrkA pathway, may contribute to malignant cell survival and rituximab resistance in DLBCL.

ProNGF mediates death of Natural Killer cells through activation of the p75NTR-sortilin complex

The common neurotrophin receptor P75NTR, its co-receptor sortilin and ligand proNGF, have not previously been investigated in Natural Killer (NK) cell function. We found freshly isolated NK cells express sortilin but not significant amounts of P75NTR unless exposed to interleukin-12 (IL-12), or cultured in serum free conditions, suggesting this receptor is sequestered. A second messenger associated with p75NTR, neurotrophin-receptor-interacting-MAGE-homologue (NRAGE) was identified in NK cells. Cleavage resistant proNGF123 killed NK cells in the presence of IL-12 after 20h and without IL-12 in serum free conditions at 48h. This was reduced by blocking sortilin with neurotensin. We conclude that proNGF induced apoptosis of NK cells may have important implications for limiting the innate immune response.

On Trk--the TrkB signal transduction pathway is an increasingly important target in cancer biology

In the beginning, Trk was an oncogene. Yet Neurotrophin-Trk signaling came to preeminence in the field of neurobiology. Now it is appreciated that Trks regulate important processes in nonneuronal cells and, in addition to their impact on tumors of neural origin, may contribute to the pathogenesis of carcinomas, myelomas, and prostate and lymphoid tumors. Although mutations and rearrangements of Trk are seen only sporadically in human cancers, such as medullary thyroid carcinoma, a number of recent studies indicate that expression of TrkB contributes to tumor pathology. In neuroblastoma, TrkA expression marks good prognosis which TrkB and Brain-derived neurotrophic factor (BDNF) expression marks poor prognosis. Activation of the BDNF/TrkB signal transduction pathway also stimulates tumor cell survival and angiogenesis and contributes to resistance to cytotoxic drugs and anoikis, enabling cells to acquire many of the characteristic features required for tumorigenesis. Small molecule inhibitors, such as Cephalon's CEP-701, are in phase 1 and 2 clinical trials, and a series of AstraZeneca Trk inhibitors are poised to enter the clinic. As monotherapy, inhibitors may be effective only in tumors with activating Trk mutations. Important clinical follow-up will be the assessment of Trk inhibitors in combination with standard chemo- or radiotherapy or other signal transduction pathway inhibitors.

Neurotrophin signaling through tropomyosin receptor kinases contributes to survival and proliferation of non-Hodgkin lymphoma

OBJECTIVE

Neurotrophin receptor signaling has been increasingly recognized as an important factor in the development and progression of a variety of malignancies. In order to analyze the potential contribution of neurotrophin signaling to lymphoma cell survival, we investigated the role of a neurotrophin axis in promoting survival and proliferation of non-Hodgkin lymphoma (NHL) cells.

MATERIALS AND METHODS

The role of neurotrophins in the survival and proliferation of NHL cells was determined by exposing cells to the Trk-specific inhibitor, K252a, and then performing (3)H-thymidine incorporation and Annexin-V/propidium iodide staining. The involvement of nuclear factor-kappaB (NF-kappaB) in this process was studied using Western blot, electrophoretic mobility shift assay, and immunofluorescence assays.

RESULTS

Here we demonstrate that both primary NHL cells and diffuse large B-cell lymphoma cell lines express Trk receptors and their neurotrophin ligands. Furthermore, these cells are sensitive to the Trk-specific inhibitor, K252a, as evidenced by the inhibition of proliferation and/or induction of apoptosis. Analysis of the mechanism into the effects of K252a revealed that, in the OCI-LY3 cell line, K252a induced a subnuclear distribution of NF-kappaB resulting in the sequestration of RelA in the nucleolus, thereby inhibiting NF-kappaB-dependent gene transcription. This results in the loss of interleukin-6 production; a known survival-promoting signal for OCI-LY3, as well as many primary diffuse large B-cell lymphomas.

CONCLUSION

Thus, Trk receptors represent a novel therapeutic target for the treatment of NHL.

Role of endogenous brain-derived neurotrophic factor and sortilin in B cell survival

Brain-derived neurotrophic factor (BDNF), a major neuronal growth factor, is also known to exert an antiapoptotic effect in myeloma cells. Whereas BDNF secretion was described in B lymphocytes, the ability of B cells to produce sortilin, its transport protein, was not previously reported. We studied BDNF production and the expression of its receptors, tyrosine protein kinase receptor B and p75 neurotrophin receptor in the human pre-B, mature, and plasmacytic malignant B cell lines under normal and stress culture conditions (serum deprivation, Fas activation, or their combination). BDNF secretion was enhanced by serum deprivation and exerted an antiapoptotic effect, as demonstrated by neutralization experiments with antagonistic Ab. The precursor form, pro-BDNF, also secreted by B cells, decreases under stress conditions in contrast to BDNF production. Stress conditions induced the membranous expression of p75 neurotrophin receptor and tyrosine protein kinase receptor B, maximal in mature B cells, contrasting with the sequestration of both receptors in normal culture. By blocking Ab and small interfering RNA, we evidenced that BDNF production and its survival function are depending on sortilin, a protein regulating neurotrophin transport in neurons, which was not previously described in B cells. Therefore, in mature B cell lines, an autocrine BDNF production is up-regulated by stress culture conditions and exerts a modulation of apoptosis through the sortilin pathway. This could be of importance to elucidate certain drug resistances of malignant B cells. In addition, primary B lymphocytes contained sortilin and produced BDNF after mitogenic activation, which suggests that sortilin and BDNF might be implicated in the survival and activation of normal B cells also.

Increased nerve growth factor and its receptors in atopic dermatitis: an immunohistochemical study

Evidence suggests that neurotrophins may regulate certain immune functions and inflammation. In the present study, the localization and distribution of nerve growth factor (NGF) and its receptors were explored using immunohistochemical methods, with the aim of detecting the cause of the neurohyperplasia in early lesions of atopic dermatitis (AD). In AD involved skin, strong NGF-immunoreactive (IR) cells were observed in the epidermis. In some cases, a huge number of infiltrating cells with stronger NGF immunoreactivity was seen mainly in the dermal papillae. Some trkA immunoreactivity was observed in the outer membrane of cells in the basal and spinal layers of the epidermis. In the papillary dermis, a larger number of cells demonstrated strong trkA immunoreactivity. The p75 NGFr-IR nerve fibre profiles were increased (900 per mm(2); p<0.001) compared to normal [the involved skin also differed from the uninvolved skin (p<0.05)] in the dermal papillae. These nerve fibres were larger, coarser and branched, some of them terminated at p75 NGFr-IR basal cells, and also revealed a stronger fluorescence staining than the controls or the uninvolved skin. In normal healthy volunteers and AD uninvolved skin, the NGF immunoreactivity was weak in the basal layer of epidermis. Only a few trkA positive cells were seen in the basal layer of the epidermis and upper dermis. The IR epidermal basal cells revealed a striking patchy arrangement with strong p75 NGFr immunostaining in the peripheral part of the cells, and short and thick NGFr-IR nerve fibre profiles appeared as smooth endings scattered in the dermis including the cutaneous accessory organs. Using NGF and p75 NGFr double staining, both immunoreactivities showed a weak staining in the epidermis and dermis in normal and uninvolved skin. In the involved dermis of AD, the intensity of p75 NGFr-IR nerves was stronger in areas where there were also increased numbers of NGF-IR cells. These findings indicate that NGF and its receptors may contribute to the neurohyperplasia of AD.

Differential expression of the neurotrophin receptors p75NTR, TrkA, TrkB and TrkC in human peripheral blood mononuclear cells

Neurotrophins are involved in the pathogenesis of allergic asthma. In addition to their influence on afferent sensory nerves within the lung, it has been shown in the last years that these factors modulate allergic airway inflammation. The knowledge about their immunomodulatory roles on diverse subsets of immune cells is still fragmentary and incomplete. Since neurotrophin receptor surface expression is essential for neurotrophin action, the aim of our study was to systematically investigate the expression pattern of the low affinity pan neurotrophin receptor p75NTR as well as the high-affinity receptors TrkA, TrkB and TrkC in human peripheral blood mononuclear cells. Our results show that each of the receptors has an individual expression pattern in diverse immune cell subtypes. However, there were no differences in neurotrophin receptor expression in healthy controls and patients with allergies.

Nerve growth factor: the central hub in the development of allergic asthma?

Neurotrophins like nerve growth factor (NGF), originally described as nerve growth factors in neuronal development, have been implicated in many physiological processes in the last years. They are now regarded as important factors involved in the resolution of pathological conditions. NGF has profound effects on inflammation, repair and remodeling of tissues. However, in the lung these beneficial effects can transact into disease promoting actions, e.g., in allergic inflammation or respiratory syncytial virus (RSV) infection. Overproduction of NGF then enhances inflammation, and promotes (neuronal) airway hyperreactivity and neurogenic inflammation. We hypothesize that NGF overexpression in certain vulnerable time windows during infancy could be a major risk factor for the development of asthma symptoms.

Endogenous NGF regulates CGRP expression in human monocytes, and affects HLA-DR and CD86 expression and IL-10 production

Our recent results on autocrine nerve growth factor (NGF) synthesis in B lymphocytes, which directly regulates the expression and release of calcitonin gene-related peptide (CGRP), a neuropeptide known to down-regulate immune response, led us to propose an anti-inflammatory action of NGF. In the present work, we investigated whether the endogenous synthesis of NGF can regulate the expression of CGRP in other antigen-presenting cells, such as monocytes, and whether this may have a functional effect. Our data indicate that human monocytes synthesize basal levels of NGF and CGRP and that, following lipopolysaccharide (LPS) stimulation, NGF and CGRP expression are both up-regulated. When endogenous NGF is neutralized, the up-regulation of CGRP expression induced by LPS is inhibited. The expression of membrane molecules involved in T-cell activation such as human leukocyte antigen-DR (HLA-DR) and CD86 is affected by endogenous NGF, and similar effects were obtained using a CGRP(1) receptor antagonist. In addition, NGF deprivation in LPS-treated monocytes significantly decreases interleukin 10 (IL-10) synthesis. Our findings indicate that endogenous NGF synthesis has a functional role and may represent a physiologic mechanism to down-regulate major histocompatibility complex (MHC) class II and CD86 expression and alter the development of immune responses.

Anti-apoptotic effect of nerve growth factor is lost in congenital insensitivity to pain with anhidrosis (CIPA) B lymphocytes

Congenital insensitivity to pain with anhidrosis (CIPA) is identified as a genetic disorder of mutations in the human TrkA known as high affinity receptor of nerve growth factor (NGF). NGF signal through TrkA promotes anti-apoptotic activity in hematopoietic cells including B lymphocytes. Here we studied the effect of NGF on anti-apoptotic activity by using human EBV-immortalized B lymphoblastoid cell lines (EB-LCLs) derived from a patient with CIPA and the associated carriers of CIPA. The TrkA(mt/mt) EB-LCL derived from the CIPA patient and the TrkA(wt/mt) EB-LCL derived from the carrier with the heterozygous TrkA mutation did not show any responses to NGF on anti-apoptotic activity. We concluded that this phenomenon is one of the pathogeneses of CIPA.

Human and murine lymphocyte neurotrophin expression is confined to B cells

Recent reports indicate that autoreactive T cells may produce neurotrophic factors capable of mediating repair and regeneration of damaged neurons. By using semiquantitative RT-PCR, we examined gene expression of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and the trkB BDNF receptor in autoreactive T cells from SWXJ mice immunized with the p104-117 encephalitogen of myelin proteolipid protein (PLP 104-117). We observed antigen-inducible expression of NGF and BDNF, but not NT-3 and trkB, in lymph node cells activated with PLP 104-117. To determine which leukocyte subpopulation expressed neurotrophins, CD4(+), CD8(+), B220(+), CD11b(+), and NK1.1(+) cells were purified from activated primary cultures, and their mRNAs were analyzed. Neurotrophin expression was also measured in CD3(+) T cells purified from mouse CNS during acute onset of experimental autoimmune encephalomyelitis as well as in resting and activated human T cells and B cells purified from peripheral blood of normal subjects. In all cases, we found that neurotrophin expression was confined exclusively to B cells (B220(+)) in both mouse and human. CD3(+), CD4(+), and CD8(+) T cells as well as NK1.1(+) cells and CD11b(+) monocytes and macrophages did not express any detectable BDNF, NGF, NT-3, or trkB under any conditions. Our data indicate that B cells rather than T cells are the predominant if not the only source of leukocyte-derived neurotrophins and as such may provide "protective autoimmunity" in repair and regeneration of the injured nervous system.

The neuroprotective role of inflammation in nervous system Injuries

The contribution of inflammation to the pathogenesis of several nervous system disorders has long been established. Other observations, however, indicate that both inflammatory cells and mediators may also have beneficial functions, assisting in repair and recovery processes. There is compelling evidence to indicate that in the injured nervous system, as in other tissues, macrophages are needed at an early stage after injury in order for healing to take place. Likewise, activated T cells of a particular specificity can reduce the spread of damage. This neuroprotective effect of T cells may be caused, at least in part, by the production of neurotrophic factors such as neurotrophin-3 or brain-derived neurotrophic factor. Interestingly, recent findings indicate that immune cells are able to produce a variety of neurotrophic factors which promote neuronal survival and may also mediate anti-inflammatory effects. Numerous cytokines are induced after nervous system injuries. Some cytokines, such as TNF-alpha, IL-1 and IFN-gamma, are well known for their promotion of inflammatory responses. However, these cytokines also have immunosuppressive functions and their subsequent expression also assists in repair or recovery processes, suggesting a dual role for some pro-inflammatory cytokines. This should be clarified, as it may be crucial in the design of therapeutic strategies to target specific cytokine(s). Finally, there is a growing body of evidence to show that autoreactive IgM antibodies may constitute an endogenous system of tissue repair, and therefore prove of value as a therapeutic strategy. Available evidence would appear to indicate that the inflammatory response observed in several neurological conditions is more complex than previously thought. Therefore, the design of more effective therapies depends on a clear delineation of the beneficial and detrimental effects of inflammation.

[Nerve growth factor (NGF) in inflammation and asthma]

INTRODUCTION

The nerve growth factor (NGF) is known as a factor involved in neuronal growth and survival. From recent studies it may also be considered as a mediator of inflammation, in particular in the airways.

STATE OF ART

Several animal studies have shown that NGF may increase the sensory innervation, and participate in the bronchial hyperresponsiveness and inflammation observed in the airways of asthmatic patients. Different cell types are capable of secreting NGF: inflammatory cells that infiltrate the bronchial mucosa, and structural cells such as epithelial cells, smooth muscle cells and pulmonary fibroblasts. Furthermore, increased NGF levels have been detected in the bronchoalveolar lavage fluid from asthmatic patients.

PERSPECTIVES AND CONCLUSION

Altogether, these results suggest that NGF may play a role in inflammation, bronchial hyperresponsiveness and airway remodelling in asthma, and may lead to a better understanding of the mechanisms occurring in chronic inflammatory diseases, in particular asthma.

Neurotrophins and lung disease

Neurotrophins are growth factors that exert multiple actions on neuronal and nonneuronal cells. Neurotrophin receptors are expressed on central and peripheral neurons, lymphocytes, monocytes, mast cells, and fibroblasts. In accordance with the distribution of their receptors, neurotrophins control the development and function of neurons and regulate inflammatory processes. Production of neurotrophins is altered in asthma, lung cancer, and pulmonary fibrosis. Evidence from animal models has implicated nerve growth factor (NGF) as a mediator of pulmonary inflammation, bronchoconstriction, and airway hyperreactivity, all of which are hallmarks of asthma. NGF regulates the growth of lung tumor cells and cultured lung fibroblasts. Thus neurotrophins, particularly NGF, are candidate molecules for regulating disease processes in asthma, lung cancer, and pulmonary fibrosis.

Expression of nerve growth factor in the airways and its possible role in asthma

Nerve growth factor (NGF), in addition to its essential role in neuronal growth and survival, may also act as an inflammatory mediator. As several animal studies have shown, NGF appears to play a part in the development of airway hyperresponsiveness and in the increased sympathetic and sensory innervation of the lung. It also has a profound effect on airway inflammation and asthma-related symptoms. Sources of NGF in the airways are numerous: inflammatory cells infiltrated into the bronchial mucosa, and structural cells including lung fibroblasts, airway epithelial and smooth muscle cells. These cells, by releasing more NGF in inflammatory conditions, may contribute to the increased NGF levels observed in bronchoalveolar lavage fluid and serum from patients with asthma. Taken together, these results suggest that NGF is an important mediator in both inflammation and asthma.

Interactions between the cells of the immune and nervous system: neurotrophins as neuroprotection mediators in CNS injury

Inflammatory processes in the central nervous system (CNS) are considered neurotoxic, although recent studies suggest that they also can be beneficial and confer neuroprotection (neuroprotective autoimmunity). Cells from the immune system have been detected in CNS injury and found to produce and secrete a variety of neurotrophins such as NGF, BDNF, NT-3 and NT-4/5, and to express (similarly to neuronal cells), members of the tyrosine kinase (Trk) receptor family such as TrkA, TrkB and TrkC. Indeed, autocrine and paracrine interactions are observed at the site of CNS injury, resulting in a variety of homologic-heterologic modulations of immune and neuronal cell function. The end result of the inflammatory process, neurotoxicity and/or neuroprotection, is a function of the fine balance between the two cellular systems, i.e., of the complex signaling relationships between anti-inflammatory neuroprotective factors (neurotrophins and other chemical mediators) and proinflammatory neurotoxic factors (TNF, free radicals, certain cytokines, etc.). Autoimmune neuroprotection is a novel therapeutic approach aimed at shifting the balance between the immune and neuronal cells towards survival pathways in a variety of CNS injuries. This review focuses on data supporting this concept and its future therapeutical implications for optic nerve injury and multiple sclerosis.

Role of nerve growth factor and other trophic factors in brain inflammation

Inflammation in the brain is a double-edged process that may be beneficial in promoting homeostasis and repair, but can also result in tissue injury through the damaging potential of inflammatory mediators. Thus, control mechanisms that minimize the extent of the inflammatory reaction are necessary in order to help preserve brain architecture and restore function. The expression of neurotrophic factors such as nerve growth factor (NGF) is increased after brain injury, in part mediated by effects on astrocytes of pro-inflammatory mediators and cytokines produced by immune cells. Conversely, cells of the immune system express NGF receptors, and NGF signaling modulates immune function. Multiple sclerosis (MS) and the disease model experimental autoimmune encephalomyelitis are neurodegenerative disorders whereby chronic destruction of the brain parenchyma results from an autoaggressive, immune-mediated inflammatory process and insufficient tissue regeneration. Here, we review evidence indicating that the increased production of NGF and other trophic factors in central nervous system (CNS) during these diseases can suppress inflammation by switching the immune response to an anti-inflammatory, suppressive mode in a brain-specific environment. Thus, trophic factors networks in the adult CNS not only protects axons and myelin but appear to also actively contribute to the maintenance of the brain immune privilege. These agents may represent good targets for therapeutic intervention in MS and other chronic CNS inflammatory diseases.

Neurotrophins and the immune system

The neurotrophins are a family of polypeptide growth factors that are essential for the development and maintenance of the vertebrate nervous system. In recent years, data have emerged indicating that neurotrophins could have a broader role than their name might suggest. In particular, the putative role of NGF and its receptor TrkA in immune system homeostasis has become a much studied topic, whereas information on the other neurotrophins is scarce in this regard. This paper reviews what is known about the expression and possible functions of neurotrophins and their receptors in different immune tissues and cells, as well as recent data obtained from studies of transgenic mice in our laboratory. Results from studies to date support the idea that neurotrophins may regulate some immune functions. They also play an important role in the development of the thymus and in the survival of thymocytes.

Brain-immune interactions in sleep

This chapter discusses various levels of interactions between the brain and the immune system in sleep. Sleep-wake behavior and the architecture of sleep are influenced by microbial products and cytokines. On the other hand, sleep processes, and perhaps also specific sleep states, appear to promote the production and/or release of certain cytokines. The effects of immune factors such as endotoxin and cytokines on sleep reveal species specificity and usually strong dependence on parameters such as substance concentration, time relative to administration or infection with microbial products, and phase relation to sleep and/or the light-dark cycle. For instance, endotoxin increased SWS and EEG SWA in humans only at very low concentrations, whereas higher concentrations increased sleep stage 2 only, but not SWS. In animals, increases in NREM sleep and SWA were more consistent over a wide range of endotoxin doses. Also, administration of pro-inflammatory cytokines such as IL-6 and IFN-alpha in humans acutely disturbed sleep while in rats such cytokines enhanced SWS and sleep. Overall, the findings in humans indicate that strong nonspecific immune responses are acutely linked to an arousing effect. Although subjects feel subjectively tired, their sleep flattens. However, some observations indicate a delayed enhancing effect on sleep which could be related to the induction of secondary, perhaps T-cell-related factors. This would also fit with results in animals in which the T-cell-derived cytokine IL-2 enhanced sleep while cytokines with immunosuppressive functions like IL-4 and L-10 suppressed sleep. The most straightforward similarity in the cascade of events inducing sleep in both animals and humans is the enhancing effect of GHRH on SWS, and possibly the involvement of the pro-inflammatory cytokine systems of IL-1 beta and TNF-alpha. The precise mechanisms through which administered cytokines influence the central nervous system sleep processes are still unclear, although extensive research has identified the involvement of various molecular intermediates, neuropeptides, and neurotransmitters (cp. Fig. 5, Section III.B). Cytokines are not only released and found in peripheral blood mononuclear cells, but also in peripheral nerves and the brain (e.g., Hansen and Krueger, 1997; März et al., 1998). Cytokines are thereby able to influence the central nervous system sleep processes through different routes. In addition, neuronal and glial sources have been reported for various cytokines as well as for their soluble receptors (e.g., Kubota et al., 2001a). Links between the immune and endocrine systems represent a further important route through which cytokines influence sleep and, vice versa, sleep-associated processes, including variations in neurotransmitter and neuronal activity may influence cytokine levels. The ability of sleep to enhance the release and/or production of certain cytokines was also discussed. Most consistent results were found for IL-2, which may indicate a sleep-associated increase in activity of the specific immune system. Furthermore, in humans the primary response to antigens following viral challenge is enhanced by sleep. In animals results are less consistent and have focused on the secondary response. The sleep-associated modulation in cytokine levels may be mediated by endocrine parameters. Patterns of endocrine activity during sleep are probably essential for the enhancement of IL-2 and T-cell diurnal functions seen in humans: Whereas prolactin and GH release stimulate Th1-derived cytokines such as IL-2, cortisol which is decreased during the beginning of nocturnal sleep inhibits Th1-derived cytokines. The immunological function of neurotrophins, in particular NGF and BDNF, has received great interest. Effects of sleep and sleep deprivation on this cytokine family are particularly relevant in view of the effects these endogenous neurotrophins can have not only on specific immune functions and the development of immunological memories, but also on synaptic reorganization and neuronal memory formation.

Pan-neurotrophin receptor p75 contributes to neuronal hyperreactivity and airway inflammation in a murine model of experimental asthma

Bronchial asthma represents a severe chronic inflammatory disease with increasing prevalence. The pathogenesis is characterized by complex neuroimmune dysregulation. Although the immunopathogenesis of the disease has been extensively studied, the nature of neuronal dysfunction still remains poorly understood. Recent data indicate that neurotrophins contribute to airway inflammation, broncho-obstruction and airway hyperresponsiveness. Using an established murine model of allergic bronchial asthma, the contribution of the pan-neurotrophin receptor p75(NTR) was defined. This receptor is expressed both in normal and asthmatic lungs and airways. Analysis of p75(NTR-/-) mice, as well as in vivo blocking of p75(NTR), revealed that airway inflammation is to a large extent dependent upon functional receptor expression. Furthermore, neuronal hyperreactivity depends entirely on this receptor. Based on these data, a novel molecular pathway in the neuroimmune pathogenesis of bronchial asthma could be defined.

Upregulation of TrkA neurotrophin receptor expression in the thymic subcapsular, paraseptal, perivascular, and cortical epithelial cells during thymus regeneration

Neuroimmune networks in the thymic microenvironment are thought to be involved in the regulation of T cell development. Here, we report upon an examination of the expression of the TrkA neurotrophin receptor, the high affinity receptor for nerve growth factor, during regeneration following acute involution induced by cyclophosphamide in the rat thymus. Light and electron microscopic immunocytochemistry demonstrated enhanced expression of the TrkA receptor in the subcapsular, paraseptal, perivascular, and cortical epithelial cells during thymus regeneration. In addition, various morphological alterations, suggestive of a hyperfunctional and dynamic state, of the subcapsular, paraseptal, and perivascular epithelial cells were also observed. The presence of TrkA protein in extracts from the control and regenerating rat thymus was confirmed by western blot. Furthermore, RT-PCR analysis supported these results by demonstrating that thymic extracts contain TrkA mRNA at higher levels during thymus regeneration. Thus, our results suggest that the TrkA receptor located on the thymic subcapsular, paraseptal, perivascular, and cortical epithelial cells could play a role in the development of new T cells to replace T cells damaged during thymus regeneration.

Nerve growth factor in serum of children with systemic lupus erythematosus is correlated with disease activity

Nerve growth factor (NGF) is a neurotrophic factor, which is expressed both in the nervous system and in peripheral organs. NGF is also present in mast cells, and in B- and T-lymphocytes, and may play a role in the immune cell development and differentiation. Various cytokines have been shown to affect NGF expression, and NGF is elevated in inflammation and in some autoimmune diseases. Here we have studied NGF concentrations in serum of pediatric patients with systemic lupus erythematosus (SLE) using a two-site enzyme-linked immunosorbent assay (ELISA). We have further correlated the levels of NGF to the inflammatory state of the disease. The mean value of serum NGF in SLE patients was significantly increased compared with controls (3346 vs 627pg/ml). There was a correlation between the activity of SLE and the levels of NGF. The results show that NGF is elevated in childhood SLE and that the levels are correlated with disease activity. The present results suggest that NGF may play a role in the pathogenesis of SLE and may have a prognostic value in evaluating the course of the disease and in outlining the medication.

Neurotrophins reduce degeneration of injured ascending sensory and corticospinal motor axons in adult rat spinal cord

Spinal cord regeneration in adult mammals is limited by neurite outgrowth inhibitors and insufficient availability of outgrowth-promoting agents. Formation of degenerative swellings at the proximal ends of severed axons (terminal clubs), which starts early after injury, also may hinder recovery and their rupture may contribute to secondary spinal cord damage. We investigated whether neurotrophins would reduce these degenerative processes. Adult rats received a transection of the dorsal column sensory and corticospinal motor tracts at T9 and anterograde tracing of the axons from the sciatic nerve and motor cortex, respectively. The highest number of terminal clubs was found at 1 day and approximately half remained present until at least 28 days. A single injection immediately after injury of a mixture of nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 into the lesion site, reduced the number of terminal clubs in the sensory system by approximately half at 1 and 7 days (but not 14) after the lesion. Individual or combinations of two neurotrophins were as effective, suggesting that the neurotrophins protected similar axonal populations. The injected neurotrophins did not affect degeneration of corticospinal motor axons. A 7-day continuous intrathecal infusion of neurotrophin-3 was more effective and also reduced terminal club formation of corticospinal axons by approximately 60%. Spinal tissue loss was not affected by the neurotrophin treatments, suggesting that terminal clubs are not major contributors to the pathogenesis of secondary spinal degeneration during the first two weeks. Thus, neurotrophins can reduce axonal degeneration in the spinal cord after traumatic axonal injury.

NGF modulates CGRP synthesis in human B-lymphocytes: a possible anti-inflammatory action of NGF?

We investigated whether the sensory neuropeptide, calcitonin gene-related peptide (CGRP), could be synthesised by human lymphocytes. Our results indicate that in activated B-cells, there is a strong expression of CGRP gene transcripts, which is almost absent in resting cells. Since B-cells autocrinally produce NGF, the neutralisation of endogenous NGF by anti-NGF antibodies resulted in a marked reduction in CGRP expression in both resting and activated B-cells. Thus, NGF appears to directly affect the synthesis of CGRP in B-cells as in sensory neurons. By regulating CGRP synthesis in lymphocytes and neuronal cells, NGF can influence the intensity and duration of the immune response.

NGF rescues human B lymphocytes from anti-IgM induced apoptosis by activation of PKCzeta

Nerve growth factor (NGF) is a neurotrophic factor acting on both the peripheral and central nervous systems. In addition, it has been shown to modulate B lymphocyte function through receptors consisting of both p75 and TrkA proteins. The low-affinity NGFR, p75, shares structural homology with the B cell antigen, CD40, tumor necrosis factor (TNF) receptor and Fas antigen (APO-1), which play a role in cell apoptosis. We studied the effect of NGF on anti-IgM-induced apoptosis in human B lymphocytes and the role of protein kinase C (PKC) in this effect. Incubation of Ramos cells with anti-IgM (10 microg/ml) induced apoptosis which was observed after 6 h and reached plateau levels after 24 h. Addition of NGF to anti-IgM-treated cells rescued cells from apoptosis. The NGF effect was blocked by anti-NGF antibody and by K252a, a specific inhibitor for the tyrosine kinase activity of TrkA. NGF induced translocation of PKCdelta and PKCalpha from the cytosol to the plasma membrane and translocation of PKCzeta to the nucleus. To examine the role of PKC in the inhibitory effect of NGF on anti-IgM-induced apoptosis, we used inhibitors of PKCalpha and PKCdelta and found that these treatments did not alter the NGF effect. In contrast, treatment of the cells with oligonucleotide antisense directed against the 5' coding sequence of PKCzeta reduced the expression of PKCzeta in the cells and abolished the protective effect of NGF on anti-IgM-induced apoptosis. The translocation of PKCzeta and the protective effect of NGF were inhibited by the phosphatidylinositol 3 (PI3)-kinase inhibitors wortmannin and LY294002. The results of this study indicate that NGF is involved in B cell survival and that this effect is mediated by PI3-kinase-dependent activation of PKCzeta.

Neurophysiologic studies in congenital insensitivity to pain with anhidrosis

Thirteen patients with congenital insensitivity to pain and anhidrosis, carrying a mutation at the TRK-A gene, were studied. Neurologic examination revealed vestigial pain sensitivity, suggesting an incomplete involvement of the affected nerves. All 13 patients manifested normal electrophysiologic studies but striking absence of sympathetic skin responses. We suggest the use of the sympathetic skin response test in the clinical evaluation of patients suspected of having congenital insensitivity to pain and anhidrosis.

Nerve growth factor and neuroimmune interactions in inflammatory diseases

Discovered almost 50 years ago, nerve growth factor (NGF) has been extensively studied in various biological systems. NGF has recently been suggested to play an important role in mediating and/or regulating immune response, in addition to its trophic and tropic effects on nerve growth and regeneration It is clear that in complex interactions between immune cells and nervous system NGF plays a central role. We have only just begun to identify and understand the direct mechanisms by which NGF activates target cells, the precise identity of the target cells, and the particular factors released from target cells. Nerve growth factor together with possibly other neurotrophins such as BDNF (brain-derived nerve growth factor), GDNF (glial-derived nerve growth factor) or NT3 are important modulators of immunity. More detailed studies are needed at the receptor, mediator and cellular levels to better understand the neuroimmunomodulatory properties of neurothrophins and NGF. The nature of the involvement of NGF in inflammation and inflammatory diseases remains a particularly interesting question. By blocking NGF or mediators released upon NGF activation, we are able to control the progress of inflammation, thereby opening many therapeutic opportunities for the future.

Autonomic regulation of neuroimmunological responses: implications for multiple sclerosis

The expression of neural regulatory molecules by immune cells that infiltrate the nervous system upon injury may be a mechanism for cross regulation between the nervous system and the immune system. Several lines of evidence implicate nerve growth factor signaling through its receptors as a potential source of communication between the two systems. The expression of beta-adrenergic receptors and sympathetic innervation of lymphoid organs represents another example of communication between the immune and the nervous system. In this review, we discuss mechanisms of how factors in common between the nervous system and the immune system may result in regulatory circuits which are important in both healthy and diseased states. These studies may have relevance for a number of inflammatory conditions in humans, including multiple sclerosis.

Macrophages express neurotrophins and neurotrophin receptors. Regulation of nitric oxide production by NT-3

In this study we examined the expression of neurotrophins and their receptors in mouse macrophages and the effects of the neurotrophins on nitric oxide secretion. Macrophages expressed TrkB and TrkC but not BDNF, NT-3 or NT-4. LPS induced up-regulation of TrkB and TrkC and of BDNF and NT-3 expression. Treatment of macrophages with NT-3 increased the secretion of nitric oxide in LPS-treated macrophages and this increase was blocked by K252a, a Trk kinase inhibitor. In contrast, BDNF and NT-4 had no significant effects on the induction of nitric oxide. Our results suggest that NT-3 play important roles in the function of macrophages during inflammatory responses and in tissue repair.

Neurotrophic factors in Alzheimer's and Parkinson's disease brain

The biomedical literature on the subject of neurotrophic growth factors has expanded prodigiously. This essay reviews neurotrophic factors (NTF) and their receptors in Alzheimer's disease (AD) and Parkinson's disease (PD) brain and recent updates on receptor signaling. The hypotheses for specific NTF involvement in neurodegenerative diseases in human and as potential therapy are based mainly on experimental animal and in vitro models. There are wide gaps in information on regional synthesis and cell contents of NTFs and their receptors in human brain. Observations on AD brain indicate increases in NGF and decreases in BDNF in surviving neurons of hippocampus and certain neocortical regions and decreases in TrkA in cortex and nucleus basalis. In PD brain, the few data available indicate decreases in neuronal content of GDNF and bFGF in surviving substantia nigra dopaminergic neurons. There are very few data regarding age-dependent effects on NTFs and on their receptors in human brain. Since NTFs in neurons are subject to retrograde and, in at least some cases, to anterograde transport from and to target neurons, their effects may be related to synthesis in local or remote sites or to changes in axoplasmic transport. Also, certain NTFs and their receptors are found to be expressed in activated glia. Thus, comparative in situ data for transcription levels and protein contents for NTFs and their receptors in both sites of neuronal origin and termination in human brain are needed to understand their potential roles in treating human diseases.

Differential regulation of neurotrophin expression by mitogens and neurotransmitters in mouse lymphocytes

In this study, we examined the expression of neurotrophins in mouse lymphocytes and the regulation of their expression by mitogens and neurotransmitters. We found that mixed splenocytes as well as T and B lymphocytes expressed mRNA for all the neurotrophins examined. Differential regulation of the neurotrophins was obtained upon stimulation of the cells. Thus, LPS increased the expression of NGF, BDNF and NT-3 in splenocytes and B cells, whereas Con-A increased the mRNA of NT-3 and NT-4 in T cells and NGF expression in splenocytes. The neurotransmitter substance P and the beta-adrenergic agonist, isoproterenol induced an increase in the expression of NGF. Our results suggest an important role for the different neurotrophins in the function of the immune system and point to a bi-directional interaction between neurotrophins and neurotransmitters in this system.

Nerve growth factor inducer, 4-methyl catechol, potentiates central sensitization associated with acceleration of spinal glutamate release after mustard oil paw injection in rats

1. In rats, injection of mustard oil (MO) into the paw caused a gradual increase in flinching of the injected paw and this algogenic behavior corresponded with an increase in the CSF-Glu level. 2. The nerve growth factor (NGF) inducer, 4-methyl catechol (4MC), enhanced the frequency of flinching and this effect was dose dependent. In addition, spinal CSF-Glu release was significantly above baseline 10 min after MO injection. In contrast, morphine (MOR) pretreatment completely blocked this behavioral and neurohumoral effect. 3. Anti-NGF paw injection attenuated the algogenic behavior and spinal Glu release otherwise observed after 4MC treatment. 4. The results demonstrated that MO-induced hyperalgesia is associated with increased CSF-Glu release and that this effect is potentiated by a NGF inducer. These data also suggest a possible involvement of NGF in the development of central sensitization after acute peripheral nociceptive stimulation.

Nerve growth factor: a neurotrophin with activity on cells of the immune system

Numerous studies published in the last two decades provide evidence that nerve growth factor (NGF), a polypeptide originally discovered because of its neurotrophic activity, acts on a variety of cells of the immune system, including mast cells, eosinophils, and B and T lymphocytes. NGF has been shown to increase during inflammatory responses, autoimmune disorders, parasitic infections, and allergic diseases. Moreover, stress, which is characterized also by activation of a variety of immune cells, causes a significant increase in basal plasma NGF levels. Recently published studies reveal that hematopoietic progenitor cells seem to be able to produce and/or respond to NGF. We report these data and discuss the hypothesis of the possible implication of NGF on the functional activities of immune cells.

Microsphere embolism-induced elevation of nerve growth factor level and appearance of nerve growth factor immunoreactivity in activated T-lymphocytes in the rat brain

Changes in nerve growth factor (NGF) level and type of cells producing NGF were investigated in the rat brain after sustained cerebral embolism. The NGF level was determined by a two-site enzyme immunoassay specific for NGF. The cerebral cortex, striatum, and hippocampus of the embolized hemisphere maximally contained 2.4-, 2.4-, and 1.7-times higher NGF levels than the corresponding regions of the nonembolized hemisphere. A significant increase was transiently observed for 1 week in the cerebral cortex and striatum, whereas the increase was longer lasting, at least of 4 weeks' duration, in the hippocampus. To examine the localization of NGF-like immunoreactivity (NGF-LI), we used a newly developed anti-NGF peptide antiserum that specifically recognized a 30-kDa molecule(s) in the hippocampal extracts or in NGF cDNA-transfected cells, suggesting that the antibody predominantly reacted with the putative NGF precursor protein(s). NGF-LI, which was localized in neurons of the normal or non-embolized hemisphere, was reduced, and on the embolized side new signals emerged in small non-neuronal cells having a round shape. These included cells with common leukocyte antigen CD45 and T-lymphocyte antigen CD3, which did not appear in the normal or non-embolized hemisphere. NGF-LI and CD3 were colocalized in a substantial number of the cells, suggesting that some activated T-lymphocytes produce NGF for neuronal regeneration after sustained cerebral embolism.

Activation of Vav and Ras through the nerve growth factor and B cell receptors by different kinases

Engagement of the B-cell antigen receptor (BCR) or the nerve growth factor receptor (NGFR/TrkA) induces activation of multiple tyrosine kinases, resulting in phosphorylation of numerous intracellular substrates. We show that addition of NGF or anti-IgM antibody leads to the early tyrosine phosphorylation of p95(vav), which is expressed exclusively in hematopoietic cells; NGF, similar to crosslinking the BCR, also results in the rapid activation of Ras. The phosphorylation of Vav and activation of Ras triggered by NGF is mediated through Trk tyrosine kinase, whereas signaling through the BCR uses a different tyrosine kinase. We also show that NGF induces tyrosine phosphorylation of Shc and its association with Grb2. Vav and Ras with the adaptor proteins Shc and Grb2 appear to serve as a link between different receptor-mediated signaling pathways and, in human B cells, may play an important regulatory role in neuroimmune interactions.

Role of nerve growth factor in a mouse model of allergic airway inflammation and asthma

The role of nerve growth factor (NGF), a potent mediator acting in the development and differentiation of both neuronal and immune cells, was examined in a mouse model of allergic asthma. NGF-positive cells were detected in the inflammatory infiltrate of the lung and enhanced levels of NGF were detected in serum and broncho-alveolar lavage fluids. Mononuclear cells in inflamed airway mucosa as well as broncho-alveolar macrophages were identified as one source of NGF production. Splenic mononuclear cells from allergen-sensitized mice produced NGF in response to allergen. They responded to exogenously added NGF with a dose-dependent increase in IL-4 and IL-5 production and augmented IgE and IgG1 synthesis. In contrast, IFN-gamma and IgG2alpha levels remained unaffected. The effects were NGF specific, since they could be blocked by an anti-NGF-antibody. Nasal application of anti-NGF to allergen-sensitized mice significantly reduced IL-4 and prevented development of airway hyperreactivity. These results show that allergic airway inflammation is accompanied by enhanced local NGF production that acts as an amplifier for Th2 effector functions and plays an important role in the development of airway hyperreactivity. Therefore it is suggested that NGF may serve as a link between the immune and nerve system.

The expanding role of nerve growth factor: from neurotrophic activity to immunologic diseases

Numerous studies published in the last 10-15 years have shown that nerve growth factor (NGF), a polypeptide originally discovered in connection with its neurotrophic activity, also acts on cells of the immune system. NGF has been found in various immune organs including the spleen, lymph nodes, and thymus, and cells such as mast cells, eosinophils, and B and T cells. The circulating levels of NGF increase in inflammatory responses, in various autoimmune diseases, in parasitic infections, and in allergic diseases. Stress-related events both in animal models and in man also result in an increase of NGF, suggesting that this molecule is involved in neuroendocrine functions. The rapid release of NGF is part of an alerting signal in response to either psychologically stressful or anxiogenic conditions in response to homeostatic alteration. Thus, the inflammation and stress-induced increase in NGF might alone or in association with other biologic mediators induce the activation of immune cells during immunologic insults. A clearer understanding of the role of NGF in these events may be useful to identify the mechanisms implicated in certain neuroimmune and immune dysfunctions.

Human mast cells express functional TrkA and are a source of nerve growth factor

Mast cells are the principal effector cells in IgE-dependent hypersensitivity reactions. Despite reports that rodent mast cells proliferate in the presence of nerve growth factor (NGF), human mast cells reportedly do not respond to this factor. To determine if human mast cells express the NGF receptors, TrkA tyrosine receptor and the low affinity NGF receptor (LNGFR), we first analyzed the mRNA expression by RT-PCR of TrkA and LNGFR in a human mast cell line (HMC-1) and in human mast cells cultured in the presence of stem cell factor. Both HMC-1 and cultured human mast cells were found to express TrkA but not LNGFR. TrkA protein was demonstrated by Western blot analysis of HMC-1 lysates. Using flow cytometric analysis and mast cell tryptase as a mast cell marker, both HMC-1 cells and cultured human mast cells were shown to coexpress tryptase and TrkA. Treatment of mast cells with NGF resulted in phosphorylation of TrkA on tyrosine residues as detected by immunoblotting with an antiphosphotyrosine antibody. Furthermore, NGF induced the immediate early gene c-fos in HMC-1 cells. HMC-1 cells and cultured human mast cells were also found to express NGF mRNA, and conditioned medium from HMC-1 cells stimulated neurite outgrowth from chicken embryonic sensory ganglia in culture. This effect was blocked by anti-NGF. Thus, mast cells express functional TrkA and synthesize NGF, suggesting a mechanism by which NGF may act as an autocrine factor for human mast cells, and by which mast cells and nerves may interact.