Neurotrophin receptor-like protein immunoreactivity in human lymph nodes

NGFR regulates stromal cell activation in germinal centers

Nerve growth factor receptor (NGFR) is expressed by follicular dendritic cells (FDCs). However, the role of NGFR in the humoral response is not well defined. Here, we study the effect of Ngfr loss on lymph node organization and function, demonstrating that Ngfr depletion leads to spontaneous germinal center (GC) formation and an expansion of the GC B cell compartment. In accordance with this effect, stromal cells are altered in Ngfr-/- mice with a higher frequency of FDCs, characterized by CD21/35, MAdCAM-1, and VCAM-1 overexpression. GCs are located ectopically in Ngfr-/- mice, with lost polarization together with impaired high-affinity antibody production and an increase in circulating autoantibodies. We observe higher levels of autoantibodies in Bcl2 Tg/Ngfr-/- mice, concomitant with a higher incidence of autoimmunity and lower overall survival. Our work shows that NGFR is involved in maintaining GC structure and function, participating in GC activation, antibody production, and immune tolerance.

Neurotrophins: are they involved in immune tolerance in pregnancy?

In this review, an attempt was made to substantiate the possibility for neurotrophins to be involved in the development of immune tolerance based on data accumulated on neurotrophin content and receptor expression in the trophoblast and immune cells, in particular, in natural killer cells. Numerous research results are reviewed to show that the expression and localization of neurotrophins along with their high-affinity tyrosine kinase receptors and low-affinity p75NTR receptor in the mother-placenta-fetus system indicate the important role of neurotrophins as binding molecules in regulating the crosstalk between the nervous, endocrine, and immune systems in pregnancy. An imbalance between these systems can occur with tumor growth and pathological processes observed in pregnancy complications and fetal development anomalies.

NGF and Its Receptors in the Regulation of Inflammatory Response

There is growing interest in the complex relationship between the nervous and immune systems and how its alteration can affect homeostasis and result in the development of inflammatory diseases. A key mediator in cross-talk between the two systems is nerve growth factor (NGF), which can influence both neuronal cell function and immune cell activity. The up-regulation of NGF described in inflamed tissues of many diseases can regulate innervation and neuronal activity of peripheral neurons, inducing the release of immune-active neuropeptides and neurotransmitters, but can also directly influence innate and adaptive immune responses. Expression of the NGF receptors tropomyosin receptor kinase A (TrkA) and p75 neurotrophin receptor (p75NTR) is dynamically regulated in immune cells, suggesting a varying requirement for NGF depending on their state of differentiation and functional activity. NGF has a variety of effects that can be either pro-inflammatory or anti-inflammatory. This apparent contradiction can be explained by considering NGF as part of an endogenous mechanism that, while activating immune responses, also activates pathways necessary to dampen the inflammatory response and limit tissue damage. Decreases in TrkA expression, such as that recently demonstrated in immune cells of arthritis patients, might prevent the activation by NGF of regulatory feed-back mechanisms, thus contributing to the development and maintenance of chronic inflammation.

BDNF and its receptors in human myasthenic thymus: implications for cell fate in thymic pathology

Here we show that in myasthenic thymus several cell types, including thymic epithelial cells (TEC) and immune cells, were the source and the target of the neurotrophic factor brain-derived growth factor (BDNF). Interestingly, many actively proliferating medullary thymocytes expressed the receptor TrkB in vivo in involuted thymus, while this population was lost in hyperplastic or neoplastic thymuses. Furthermore, in hyperplastic thymuses the robust coordinated expression of BDNF in the germinal centers together with the receptor p75NTR on all proliferating B cells strongly suggests that this factor regulates germinal center reaction. Finally, all TEC dying of apoptosis expressed BDNF receptors, indicating that this neurotrophin is involved in TEC turnover. In thymomas both BDNF production and receptor expression in TEC were strongly hindered. This may represent an attempt of tumour escape from cell death.

Distinct cytokine-driven responses of activated blood gammadelta T cells: insights into unconventional T cell pleiotropy

Human Vgamma9/Vdelta2 T cells comprise a small population of peripheral blood T cells that in many infectious diseases respond to the microbial metabolite, (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), expanding to up to 50% of CD3(+) cells. This "transitional response," occurring temporally between the rapid innate and slower adaptive response, is widely viewed as proinflammatory and/or cytolytic. However, increasing evidence that different cytokines drive widely different effector functions in alphabeta T cells provoked us to apply cDNA microarrays to explore the potential pleiotropy of HMB-PP-activated Vgamma9/Vdelta2 T cells. The data and accompanying validations show that the related cytokines, IL-2, IL-4, or IL-21, each drive proliferation and comparable CD69 up-regulation but induce distinct effector responses that differ from prototypic alphabeta T cell responses. For example, the Th1-like response to IL-2 also includes expression of IL-5 and IL-13 that conversely are not induced by IL-4. The data identify specific molecules that may mediate gammadelta T cell effects. Thus, IL-21 induces a lymphoid-homing phenotype and high, unexpected expression of the follicular B cell-attracting chemokine CXCL13/BCA-1, suggesting a novel follicular B-helper-like T cell that may play a hitherto underappreciated role in humoral immunity early in infection. Such broad plasticity emphasizes the capacity of gammadelta T cells to influence the nature of the immune response to different challenges and has implications for the ongoing clinical application of cytokines together with Vgamma9/Vdelta2 TCR agonists.

The Trk tyrosine kinase inhibitor K252a regulates growth of lung adenocarcinomas

The neurotrophin family of growth factors and their receptors support the survival of several neuronal and non-neuronal cell populations during embryonic development and adult life. Neurotrophins are also involved in malignant transformation. To seek the role of neurotrophin signaling in human lung cancer we studied the expression of neurotrophin receptors in human lung adenocarcinomas and investigated the effect of the neurotrophin receptor inhibitor K252a in A549 cell survival and colony formation ability in soft agar. We showed that human lung adenocarcinomas express TrkA and TrkB, but not TrkC; A549 cells, derived from a human lung adenocarcinoma, express mRNA transcripts encoding nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), TrkA, TrkB, and p75, and high protein levels of TrkA and TrkB. Stimulation of cells using NGF or BDNF activates the anti-apoptotic protein Akt. Interestingly, inhibition of neurotrophin receptor signaling using K252a prevents Akt activation in response to NGF or BDNF, induces apoptotic cell death, and diminishes the ability of A549 cells to growth in soft agar. The data suggest that neurotrophin signaling inhibition using k252a may be a valid therapy to treat patients with lung adenocarcinomas.

Immunological role of neuronal receptor vanilloid receptor 1 expressed on dendritic cells

Capsaicin (CP), the pungent component of chili pepper, acts on sensory neurons to convey the sensation of pain. The CP receptor, vanilloid receptor 1 (VR1), has been shown to be highly expressed by nociceptive neurons in dorsal root and trigeminal ganglia. We demonstrate here that the dendritic cell (DC), a key cell type of the vertebrate immune system, expresses VR1. Engagement of VR1 on immature DCs such as by treatment with CP leads to maturation of DCs as measured by up-regulation of antigen-presenting and costimulatory molecules. This effect is present in DCs of VR1+/+ but not VR1-/- mice. In VR1+/+ mice, this effect is inhibited by the VR1 antagonist capsazepine. Further, intradermal administration of CP leads to migration of DCs to the draining lymph nodes in VR1+/+ but not VR1-/- mice. These data demonstrate a powerful influence of a neuroactive ligand on a central aspect of immune function and a commonality of mechanistic pathways between neural and immune functions.

Expression of NGF in hepatocellular carcinoma cells with its receptors in non-tumor cell components

Nerve growth factor (NGF) is suggested to have a role in tumor progression in addition to its role in differentiation and survival of neuronal cells. We investigated expression of NGF and its receptors, TrkA and p75NTR, in hepatocellular carcinomas (HCCs). Although hepatocytes and hepatic stellate cells (HSCs) showed respectively weak and intense NGF immunostaining in the background livers of patients suffering from liver cirrhosis (LC) or chronic hepatitis (CH), intense staining was demonstrated in HCC cells of 33 of 54 (61.1%) tumors. RT-PCR detected NGF mRNA in 7 freshly-isolated HCC samples, and in 2 of 4 cases, in which both background livers and tumors could be analyzed, NGF mRNA was more abundant in the tumors than the background livers. TrkA was detected in the smooth muscle cells of hepatic arteries, but it was negative in tumor cells as well as non-neoplastic hepatocytes. p75NTR and alpha-smooth muscle actin (alphaSMA) was expressed in HSCs in the background liver and fibroblast-like cells in stromal septa, whereas HSCs within the HCC tissues were mostly negative for p75NTR but positive for alphaSMA. This suggests that HSCs in HCC have a different property from those in background livers. Furthermore, the stromal septa contained abundant nerve fibers, which may be related to the increased NGF expression in HCC cells. NGF and its receptors are then thought to have a role in cellular interactions involving HCC cells, HSCs, arterial cells and nerve cells in HCC tissues.

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.

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.

Neurotrophins and neurotrophin receptors in allergic asthma

The neurotrophins nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 (NT-3) and NT-4 play a pivotal role in the development of the nervous system. Despite their well-known effects on neurons, elevated neurotrophin concentrations have been observed under pathological conditions in sera of patients with inflammatory disorders. Patients with asthma feature both airway inflammation and an abnormal airway reactivity to many unspecific stimuli, referred to as airway hyperresponsiveness, which is, at least partly, neuronally controlled. Interestingly, these patients show increased levels of neurotrophins in the blood as well as locally in the lung. It has been demonstrated that neurotrophin release from immune cells is triggered by allergen contact. The presence of neurotrophins and the neurotrophin receptors p75 (p75NTR), tyrosine kinase A (TrkA), TrkB and TrkC have been described in several immune cells. There is strong evidence for an involvement of neurotrophins in regulation of hematopoiesis and, in addition, in modulation of immune cell function in mature cells circulating in blood or resting in lymphatic organs and peripheral tissues. The aim of this review is to demonstrate possible roles of neurotrophins during an allergic reaction in consideration of the temporospatial compartimentalization.

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.

p75NTR in the spleen: age-dependent changes, effect of NGF and 4-methylcatechol treatment, and structural changes in p75NTR-deficient mice

In addition to their well-known actions within the nervous system, neurotrophins and their receptors are involved in immune system functioning, as demonstrated by their wide distribution in lymphoid tissues and their in vitro actions on immunocompetent cells. Nevertheless, the in vivo roles of neurotrophin-receptor systems in lymphoid tissues, as well as the scope of their influence throughout development and adulthood, are yet to be clarified. In the present study, we used combined morphological and immunohistochemical techniques to investigate the presence and cellular localization of p75NTR, the pan-neurotrophin receptor protein, in rat spleen from newborns to aging individuals, and the structural and innervation changes in the spleens of p75NTR-deficient mice. In rats, p75NTR was expressed by splenic nerve fibers and dendritic cells in an age-regulated fashion, with maximal expression detected at 2 weeks. Consistently, the spleens of newborn mice lacking this receptor protein showed no signs of ingrowing sympathetic fibers, along with an absence of defined white pulp areas. The present findings suggest a prolonged role of p75NTR in the physiology of the spleen; at least during the embryonic development period, the receptor may be critical for correct innervation and compartmentalization processes to occur.

Massive lymphocyte apoptosis in the thymus of functionally deficient TrkB mice

The occurrence of TrkB in the murine thymus (15-day and 3-month old) was investigated by Northern blot, Western blot and immunohistochemistry. Furthermore, the thymus of 15-day-old mice carrying a non-functional mutation on trkB was analyzed. Both trkB mRNA and 145 kDa TrkB protein were detected. In addition, isolated lymphocytes and stromal cells also expressed this protein. The thymus of homozygous functionally TrkB-deficient animals showed structural and ultrastructural changes consistent with massive death of cortical lymphocytes, confirmed with TUNEL. Present results suggest a role for TrkB in maintaining the survival or preventing massive death of lymphocytes in the mammalian thymus.

Production of nerve growth factor by mouse hepatocellular carcinoma cells and expression of TrkA in tumor-associated arteries in mice

BACKGROUND & AIMS

Nerve growth factor (NGF) has been suggested to play a role in cancer progression. We found that NGF is specifically elevated in mouse hepatocellular carcinomas (HCCs) by cDNA array analysis. The present study aimed to elucidate expression of NGF and its receptors during hepatocarcinogenesis and under other conditions.

METHODS

Expression of NGF, TrkA, and p75NTR was investigated in HCCs developing and regenerating livers, and primary hepatocyte cultures in B6C3F(1) mice by reverse-transcription polymerase chain reaction, Northern blotting, and/or immunohistochemistry. The biological activity of NGF produced by the HCC cells was studied by using PC12 cells. Nerve fibers in hepatic tumors were immunohistochemically examined.

RESULTS

Although NGF was negative in adult and developing livers, it was markedly elevated in focal hepatocytic lesions from early stages of carcinogenesis. Appreciable levels were also detected in regenerating livers and hepatocytes in culture. The conditioned medium of HCC cells caused PC12 neurite outgrowth, but this was reduced on pretreatment of the conditioned medium with an anti-NGF antibody or NGF antisense expression in HCC cells. Although neither TrkA nor p75NTR was detectable in either HCC or normal hepatic cells, TrkA was shown in the walls of tumor-associated arteries that contain abundant nerve fibers.

CONCLUSIONS

NGF is expressed by hepatocytes during carcinogenesis, regeneration, and primary culture but may have cells other than hepatocytes as the target. TrkA expression and the abundance of nerve fibers in the walls of tumor-associated arteries suggest a possible role for NGF in angiogenesis.

Neurotrophins and neurotrophin receptors in human lung cancer

The expression of neurotrophins (NTs) and related high- and low-affinity receptors was studied in surgical samples of histologically diagnosed human tumors of the lower respiratory tract. The experiment was conducted with 30 non-small cell lung cancer specimens and in eight small cell lung cancer specimens by Western blot analysis and immunohistochemistry to assess expression and distribution of NT and NT receptor proteins in tissues examined. Immunoblots of homogenates from human tumors displayed binding of anti-nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and NT-3 antibodies as well as of anti-tyrosine-specific protein kinase (Trk) A, TrkB, and TrkC receptor antibodies, with similar migration characteristics than those displayed by human beta-NGF and proteins from rat brain. A specific immunoreactivity for NTs and NT receptors was demonstrated in vessel walls, stromal fibroblasts, immune cells, and sometimes within neoplastic cell bodies. Approximately 33% of bronchioloalveolar carcinomas exhibited a strong membrane NGF and TrkA immunoreactivity, whereas 46% adenocarcinomas expressed an intense TrkA immunoreactivity but a weak immunostaining for NGF within tumor cells. Moreover, squamous cell carcinomas developed an intense TrkA immunoreactivity only within stroma surrounding neoplastic cells. A faint BDNF and TrkB immunoreactivity was documented in adenocarcinomas, squamous cell carcinomas, and small cell lung cancers. NT-3 and its corresponding TrkC receptor were found in a small number of squamous cell carcinomas within large-size tumor cells. No expression of low-affinity p75 receptor protein was found in tumor cells. The detection of NTs and NT receptor proteins in tumors of the lower respiratory tract suggests that NTs may be involved in controlling growth and differentiation of human lung cancer and/or influencing tumor behavior.

Neurotrophin receptor-like proteins in the bovine (Bos taurus) lymphoid organs, with special reference to thymus and spleen

Increasing evidence suggests that neurotrophins could regulate immune functions acting directly or indirectly on immunocompetent cells. The indirect pathway involves stromal cells of the primary and secondary lymphoid organs. In the present study the occurrence of Trk proteins (TrkA, TrkB and TrkC), regarded as the high-affinity signal-transducing receptors for neurotrophins, was investigated in cow lymphoid organs using immunohistochemistry. The thymus and spleen of both fetal and adult animals, and the palatine tonsils, lymph nodes and Peyer's patches of adult animals, were analysed. Unidentified cells displaying TrkA-like immunoreactivity were found in the fetal thymus, whereas those expressing this protein in the adult gland were identified as epithelial cells. In the spleen, immunoreactive TrkA was observed in cells of the white pulp. TrkB immunoreactivity in both fetal and adult thymus and spleen was localized in monocyte/macrophage cells. As a rule, TrkC was absent from the thymus and the spleen independent of the animal's age. Different types of stromal cells, but never the lymphocytes themselves, displayed TrkA, TrkB, or TrkC immunoreactivity in the other lymphoid organs analysed. As in other vertebrate species, Trk proteins in the lymphoid organs of the cow were localized in the stromal, non-lymphoid cells, thus suggesting that neurotrophins might regulate the immune function acting indirectly on lymphocytes.

Neurotrophins and asthma

The neurotrophins are a family of peptides that promote survival, growth, and differentiation of neurons. Neurotrophins may also influence the function of nonneuronal cell types, including immune cells. The development and maintenance of asthma is thought to involve the nervous system and the immune system, but the role that neurotrophins play in asthma is unknown. The cellular sources of the neurotrophins include mast cells, lymphocytes, macrophages, epithelial cells, smooth muscle cells, and eosinophils. The activation of neurotrophin receptors in immune cells and neurons involves ligand-induced homodimerization, which leads to activation of intrinsic Trk receptor kinase. The exact consequences of activating these receptors on immune cells is unknown, but rather than having unique actions on immune cells, the neurotrophins appear to act in concert with known immune regulating factors to modulate the maturation, accumulation, proliferation, and activation of immune cells. Neurotrophins can modulate afferent nerve function by stimulating the production of neuropeptides within airway afferent neurons. These neuropeptides may be released from the central terminals of airway afferent neurons, which leads to heightened autonomic reflex activity, and increased reactivity in the airways.

Epithelial rests of Malassez express immunoreactivity of TrkA and its distribution is regulated by sensory nerve innervation

The periodontal ligament is the connective tissue that fills the space between the tooth and its bony socket. It is abundantly innervated by the sensory and sympathetic nerves. We first investigated the immunoreactivity of TrkA, which is a high-affinity receptor of nerve growth factor (NGF), in the periodontal ligament of rats. Immunoreactivity was observed at the epithelial cells in the cervical and furcation regions of the molars. These epithelial cells, which gather together to form clusters or networks, are known as the epithelial rests of Malassez. Immunoreactivity was not observed in other non-neuronal cells, such as osteoblasts, fibroblasts, odontoblasts, cementoblasts, endothelial cells, and/or osteoclasts. On the basis of these findings, we investigated the possible involvement of sensory nerve innervation in the immunoreactivity of the epithelial cells. Denervation of the inferior alveolar nerve resulted in a marked decrease in the distribution area and size of the clusters of immunoreactive cells compared with those of sham-operated rats. These findings suggest that sensory nerve innervation may have a regulatory role in maintenance of the epithelial rests of Malassez expressing TrkA in the periodontal ligament.

Neurotrophin and neurotrophin receptor expression in alveolar macrophages: an immunocytochemical study

Alveolar macrophages play a crucial role in regulating lung immune responses and in maintaining the integrity of the respiratory tract. Neurotrophins (NTs), besides to their neurotrophic activities, exhibit physiological effects in the immune system. In this study, nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), NT-3 and low- (p75) and high affinity (Trks) NT receptors were investigated by immunocytochemistry in cytospin centrifuged preparations of human alveolar macrophages. Approximately 2.5% alveolar macrophages were immunoreactive for NGF, whereas no macrophages displaying immunoreactivity for BDNF or NT-3 were observed. A 3.5% macrophages displayed immunoreactivity for TrkA-receptor protein, 10% for TrkB-receptor protein (full length isoform), and 2% for TrkC-receptor protein. No low-affinity p75 NT and TrkB[-] truncated isoform receptor immunoreactive macrophages were found. These findings support the hypothesis that NTs and the corresponding receptors may play a role in regulating immunological and functional activity of alveolar macrophages via paracrine/autocrine mechanisms.

Expression of the TrkB neurotrophin receptor by thymic macrophages

Increasing evidence suggests that some members of the neurotrophic factor family of neurotrophins could be implicated in the regulation of immune responses. Neurotrophins, as well as their tyrosine kinase signal-transducing receptors (the so-called Trk neurotrophin receptors), have been detected in different lymphoid tissues, although their cellular localization is not well known. In this study we used single and double immunohistochemistry to localize TrkB in situ in the rat thymus (in animals from 0 days to 2 years of age), in cytospin preparations of rat thymic cells, and in two mouse monocyte-macrophage cell lines (RAW 264.7 and J774A.1). We found TrkB protein expression in a subpopulation of cells in the corticomedullary junction, which simultaneously expressed the rat macrophage marker ED1. The density of TrkB-expressing cells increased with age, reaching maximal values at 2 years. Conversely, no evidence of TrkB protein expression could be found in dendritic cells, epithelial cells or thymocytes. Thymic macrophages in cytospin preparations, as well as in the mouse monocyte macrophage cell lines, also expressed TrkB protein. Although the possible function of TrkB in the thymic macrophage remains to be clarified, present findings add further evidence to the proposed role of neurotrophins in the immune system.

Neurotrophins and their receptors in the pigeon caecal tonsil. An immunohistochemical study

Neurotrophins are growth factors which bind to signal-transducing receptors called Trk proteins. The neurotrophins and their receptor proteins are present in the mammalian and avian lymphoid organs, thus suggesting that these factors could act upon cells of the immune system. Nevertheless, little is known about the cellular distribution of neurotrophins and their receptor proteins in avian lymphoid tissues. In this study we use immunohistochemistry to detect the cellular localisation of neurotrophins and their receptor proteins in the pigeon caecal tonsil, used as a model for avian secondary lymphoid organs. Rabbit polyclonal antibodies against neurotrophins (nerve growth factor -NGF-, brain-derived neurotrophic factor -BDNF- and neurotrophin -3 NT-3-) and against specific epitopes of TrkA, TrkB and TrkC proteins were used. Cytokeratins, vimentin, S-100 protein and chromogranin A were studied in parallel to identify cells which seemed to express neurotrophins and Trk proteins. TrkA-like protein was seen in the intestinal epithelium, whereas TrkB-like and TrkC-like proteins was found in cells which we identified as dendritic cells and macrophages. BDNF-like and NT-3-like reactivity was localised in intestinal epithelial cells, especially endocrine cells. Present results add further evidence to the presumptive immune role of neurotrophins and their receptors and the possible functions of these peptides in the caecal tonsil are discussed.