Somatostatin reduces the release of colony-stimulating activity (CSA) from PHA-activated mouse spleen lymphocytes

Peptides as regulators of the immune system: emphasis on somatostatin

Study of the communication between nervous and immune systems culminated in the understanding that cytokines, formerly considered exclusively as immune system-derived peptides, are endogenous to the brain and display central actions. More recently, immune cells have been recognized as a peripheral source of "brain-specific" peptides with immunomodulatory actions. This article reviews studies concerning reciprocal effects of selected cytokines and neuropeptides in the nervous and immune systems, respectively. The functional equivalence of these two categories of communicators is discussed with reference to the example of the actions of neuropeptide somatostatin in the immune system.

Localization of Receptors for Vasoactive Intestinal Peptide, Somatostatin, and Substance P in Distinct Compartments of Human Lymphoid Organs

Regulatory peptides, such as vasoactive intestinal peptide (VIP), somatostatin (SS), or substance P (SP), are considered to play a role in immune regulation. To localize the targets of these peptides in the human immune system, their receptors have been evaluated with in vitro receptor autoradiography in lymph nodes, tonsils, appendix, Peyer's patches, spleen, and thymus. The three peptide receptors were detected in all lymphoid tissues tested, but, unexpectedly, usually in distinct compartments. In lymph nodes, palatine tonsils, vermiform appendix, and Peyer's patches, VIP receptors were found in the CD3 positive zone around lymphoid follicles; SS receptors in the germinal centers of secondary follicles; and SP receptors mainly in interfollicular blood vessels. In the spleen, VIP receptors were detected in periarterial lymphatic sheaths, SS receptors in the red pulp, and SP receptors in the central arteries. In the thymus, VIP receptors were present in cortex and medulla, SS receptors in the medulla, and SP receptors in blood vessels. For comparison, cholecystokinin (CCK)-A and -B receptors were not demonstrated in any of these tissues. These results suggest a strong compartmentalization of the three peptide receptors in human lymphoid tissues and represent the molecular basis for the understanding of a very complex and interactive mode of action of these peptides.

Somatostatin increases mitogen-induced IL-2 secretion and proliferation of human Jurkat T cells via sst3 receptor isotype

The neuropeptide somatostatin (SRIF) modulates normal and leukemia T cell proliferation. However, neither molecular isotypes of receptors nor mechanisms involved in these somatostatin actions have been elucidated as yet. Here we show by using RT-PCR approach that mitogen-activated leukemia T cells (Jurkat) express mRNA for a single somatostatin receptor, sst3. This mRNA is apparently translated into protein since specific somatostatin binding sites (K11 = 78 +/- 3 pM) were detected in semipurified plasma membrane preparations by using 125I-Tyr1-SRIF14 as a radioligand. Moreover, somatostatin inhibits adenylyl cyclase activity with similar efficiency (IC50 = 23 +/- 4 pM) thus strongly suggesting a functional coupling of sst3 receptor to this transduction pathway. The involvement of sst3 receptor in immuno-modulatory actions of somatostatin was assessed by analysis of neuropeptide effects on IL-2 secretion and on proliferation of mitogen-activated Jurkat cells. Our data show that in the concentrations comprised between 10 pM and 10 nM, somatostatin potentiates IL-2 secretion. This effect is correlated with somatostatin-dependent increase of Jurkat cell proliferation since the EC50 concentrations for both actions were almost identical (EC50 = 22 +/- 9 pM and EC50 = 12 +/- 1 pM for IL-2 secretion and proliferation, respectively). Altogether, these data strongly suggest that in mitogen-activated Jurkat cells, somatostatin increases cell proliferation through the increase of IL-2 secretion via a functional sst3 receptor negatively coupled to the adenylyl cyclase pathway.

Expression of somatostatin receptor subtype 2 mRNA in human lymphoid cells

We analyzed the mRNA expression of somatostatin receptor subtypes 1 to 5 (SSTR1-5) in human lymphoid cell lines, human peripheral blood lymphocytes (PBL), and human lymphatic leukemia cells, using the reverse transcription-polymerase chain reaction method. In human lymphoid cell lines, SSTR2 mRNA expression was clearly detectable, and there was no evidence of SSTR1 mRNA expression. SSTR2 mRNA was barely detectable in PBL from healthy individuals but was clearly detectable in EB virus-transformed lymphocytes. Lymphocytes from some of the leukemic patients showed elevated SSTR2 mRNA expression. SSTR2 mRNA expression in PBL was upregulated upon stimulation by PHA. SSTR3 mRNA was also observed in all the cell lines examined, although in one cell line, the expression was weak. Some cell lines showed little or no SSTR4 or 5 mRNA expression. The expression pattern of SSTR2 mRNA suggests that this receptor may have some important roles in lymphocyte activation, development, and/or tumorgenesis.

Measles virus modulates human T-cell somatostatin receptors and their coupling to adenylyl cyclase

The possible role of immunomodulatory peptide somatostatin (SRIF) in measles virus (MV)-induced immunopathology was addressed by analysis of SRIF receptors and their coupling to adenylyl cyclase in mitogen-stimulated Jurkat T cells and human peripheral blood mononuclear cells (PBMC). SRIF-specific receptors were assayed in semipurified membrane preparations by using SRIF14 containing iodinated tyrosine at the first position in the amino acid chain ([125I]Tyr1) as a radioligand. A determination of receptor number by saturation of radioligand binding at equilibrium showed that in Jurkat cells, MV infection led to a dramatic decrease in the total receptor number. The virus-associated disappearance of one (Ki2 = 12 +/- 4 nM [mean +/- standard error of the mean [SEM]]; n = 4) of two somatostatin binding sites identified in control Jurkat cells (Ki1 = 78 +/- 3 pM and Ki2 = 12 +/- 4 nM [mean +/- SEM]; n = 4) was also observed. Almost identical results were obtained for phytohemagglutinin-activated human PBMC. In the absence of MV infection, two somatostatin binding sites were present (Ki1 = 111 +/- 31 pM and Ki2 = 17 +/- 2 nM [mean +/- SEM]; n = 2), whereas in MV-infected cells, only the high-affinity (Ki1 = 48 +/- 15 pM [mean +/- SEM]; n = 2) binding site remained. In addition, MV infection reinforced the inhibitory effects of SRIF on adenylyl cyclase activity, since maximal inhibition at 1 microM peptide was 11% +/- 4% in control cells versus 25% +/- 3% (P < 0.05) in infected Jurkat cells. Moreover, MV infection severely impaired the capacity of adenylyl cyclase to be activated directly (by forskolin) or indirectly (via Gs protein-coupled vasoactive intestinal peptide receptor). An assessment of [methyl-3H]thymidine incorporation showed that SRIF increased proliferative responses to mitogens only in control cells, not in MV-infected cells. Altogether, our data emphasize that MV-associated alteration of SRIF transduction appears to be related to the loss of SRIF-dependent increase of mitogen-induced proliferation.

Functional histology of the neuroendocrine thymus

The primary role of the thymus lies in T-cell differentiation and self-education leading to the establishment of appropriate host immune defenses. However, the view of the thymus as a self-contained organ is no longer valid. It is now clear that intricate interactions of both a stimulatory and inhibitory nature exist between the neuroendocrine and immune system. A broad array of neuroendocrine circuits are networked with the thymus and neuroendocrine-thymic interactions are bidirectional. These interactions are thought to play an important immunomodulatory role during an active immune response, during T-cell ontogeny and in the aging process of the whole organism. The chemical messengers that transmit communicating signals in this network are secreted neuropeptides and their specific receptors. The objective of this review is to provide a comprehensive overview of the morphological substrates of these neuropeptides in the thymus.

Modulation of cytokine production in activated human monocytes by somatostatin

The immunosuppressor effects of the widely distributed neuropeptide somatostatin were examined on purified peripheral blood human monocytes. Somatostatin, at concentrations thought to be physiologic (10(-10)-10(-7) M), regulated monocyte/macrophage responses to (LPS) stimulation, as reflected by interleukin production. In particular, somatostatin had direct inhibitory effects on TNF-alpha, IL-1 beta, and IL-6 secretion by LPS-activated monocytes, while the decrease on IL-8 synthesis was modulated mainly by the action of somatostatin on TNF-alpha and IL-1 beta. In fact, the addition of these two inflammatory cytokines to the monocyte culture medium was able to induce IL-8 expression, as demonstrated by mRNA analysis, also in presence of the neuropeptide. Although somatostatin affected IL-8 production in an indirect way, it suppressed directly the chemotactic response of neutrophils to IL-8. Finally, somatostatin downregulation of monocyte activation was confirmed by the decrease of HLA-DR expression on cell plasma membranes (52% versus 33%). Our results confirm that somatostatin exerts preferential effects on the suppression of immunoreactions by modulating cytokine production and activity.

Expression of the somatostatin gene in human astrocytoma cell lines

Somatostatin (somatotropin release-inhibiting hormone; SRIH) has been demonstrated in neurons of the central nervous system (CNS) as well as in endocrine cells of the pancreas and gastrointestinal tract and can suppress various immune functions including lymphocyte proliferation, immunoglobulin synthesis, and cytokine production. Since astrocytes possess antigen-presenting activity and can secrete a wide array of immunoregulatory and inflammatory cytokines, we studied SRIH gene expression in both astrocyte cell lines and mitogen-stimulated peripheral blood mononuclear leukocytes from healthy donors. We now report by means of a complementary DNA-based reverse transcription PCR that differential levels of SRIH mRNA were expressed in 9 of 11 human astrocytoma cell lines tested but were undetectable in activated peripheral blood mononuclear leukocytes as well as in a variety of human lymphocyte and monocyte cell lines. The synthesis and secretion of SRIH protein by astrocytoma cells that expressed SRIH transcripts were confirmed by specific radioimmunoassay of cell culture fluids. These findings support the notion that SRIH gene expression occurs in human astrocytoma cells but not in mature lymphoid cells of the immune system.

Differential effect of the immunomodulatory hormone somatostatin on replication of human immunodeficiency virus type 1 in CD4+ and CD8+ T lymphocytes

The long-acting somatostatin analog octreotide (SMS 201-995) possesses immunosuppressive properties and has been successfully used for the management of human immunodeficiency virus (HIV)-associated diarrhea, a condition commonly observed in the absence of known enteric pathogens. Since HIV type 1 (HIV-1) replication can occur in both CD4+ and CD8+ lymphocytes, we hypothesized that this benefit might be due to local effects on HIV-1 replication in these two T-cell subsets. As a model, we studied the effects of two synthetic molecules, SRIH 1-14 and SRIH 1-28, closely related to naturally occurring forms of somatostatin, as well as SMS 201-995 on HIV-1 replication in CD4+ and CD8+ cells derived from peripheral blood mononuclear cells (PBMC). We found that HIV-1 replication was inhibited in CD8+ cells but enhanced in infected CD4+ lymphocytes, as measured by p24 antigen levels in culture fluids. These differential effects were drug concentration dependent. We also observed that somatostatin inhibited the mitogen-induced proliferative responsiveness of both cell types. These effects on both HIV-1 replication and cell proliferation were independent of somatostatin gene expression, since somatostatin mRNAs were not detected in mitogen-stimulated PBMC, as determined by reverse transcription-PCR.

Somatostatin analogues suppress the inflammatory reaction in vivo

Somatostatin (Sms) and its agonist analogues inhibit the secretory activities of endocrine and neural cells. Recent studies have suggested that Sms has significant immunomodulatory properties. In this study, we examine the effects of two Sms octapeptide analogues on the inflammatory reaction in vivo. BIM 23014 (Somatulin) and Sandostatin were administered to male Sprague-Dawley rats subject to carrageenin-induced aseptic inflammation, at doses of 2-10 micrograms/rat, given either systemically or locally. Animals were killed 7 h after the induction of the inflammation, and the inflammatory exudates were aspirated and quantitated in terms of volume and leukocyte concentration. Sms analogues, administered via either route, significantly reduced the volume and the leukocyte concentration of the exudate in a time- and dose-dependent fashion. In corroboration of these, immunohistochemical evaluation of the levels of local inflammatory mediators, such as immunoreactive (Ir) TNF-alpha, Irsubstance P, and Ircorticotropin-releasing hormone, was inhibited significantly by Sms analogue treatment. These findings suggest that Sms analogues have significant antiinflammatory effects in vivo, associated with suppression of proinflammatory cytokines and neuropeptides. Furthermore, these data suggest that Sms agonists may be useful in the control of inflammatory reaction.

Evidence that somatostatin is localized and synthesized in lymphoid organs

Because several peptides originally found in the pituitary as within the central nervous system have been localized in lymphoid tissues and because somatostatin (somatotropin-release-inhibiting hormone, SRIH) can act on cells of the immune system, we searched for this peptide in lymphoid organs. We demonstrated that SRIH mRNA exists in lymphoid tissue, albeit in smaller levels than in the periventricular region of the hypothalamus, the brain region that contains the highest level of this mRNA. SRIH mRNA was found in the spleen and thymus of male rats and in the spleen, thymus, and bursa of Fabricius of the chicken. Its localization in the bursa indicates that the peptide must be present in B lymphocytes since this is the site of origin of B lymphocytes in birds. The SRIH concentration in these lymphoid organs as determined by radioimmunoassay was greater in the thymus than in the spleen of the rat. These concentrations were 50 times less than those found in the periventricular region of the hypothalamus, the site of the perikarya of SRIH-containing neurons. In the chicken, as in the rat, the concentration of SRIH was greater in the thymus than in the spleen; it was present in the bursa of Fabricius, also in higher concentration than in the spleen. Fluorescence immunocytochemistry revealed the presence of SRIH-positive cells in clusters inside the white pulp and more dispersed within the red pulp of the spleen of both the rat and the chicken. The thymus from these species also contained SRIH-positive cells within the medulla and around the corticomedullary junction. In the chicken, there were large clusters of SRIH-positive cells in the medullary portion of each nodule of the bursa of Fabricius. Preabsorption of the primary antiserum or replacing this antiserum with normal rabbit serum verified the specificity of staining. Sequential immunostaining of the same sections from rat spleen using first SRIH antibody and subsequently a monoclonal antibody against a rat B-cell surface antigen revealed the presence of SRIH immunoreactivity in some, but not all, B cells. Other cell types in spleen not yet identified also stained positively with the SRIH antibody but were not reactive to monoclonal antibodies to rat Thy-1.1, a marker for all the thymic T lymphocytes. The possibility that SRIH is present in other populations of cells in the spleen cannot be ruled out. Sequential immunostaining of the same sections of rat thymus revealed the presence of SRIH immunoreactivity in a small population of T lymphocytes in the medulla, as revealed by the Thy-1.1 marker. The SRIH-positive cells were nonimmunoreactive when exposed to the B-cell marker; however, the possibility that SRIH is present in other cells was not investigated. Thus, our results indicate that SRIH is synthesized and stored in cells of the immune system. SRIH may be secreted from these cells to exert paracrine actions that alter the function of immune cells in spleen and thymus.

Enhancement of human lymphocyte natural killer activity by somatostatin

The effect of somatostatin 1-14 (SS) on the natural killer (NK) activity of human peripheral blood lymphocytes was investigated. The NK activity was estimated by means of radioactive chromium (51Cr) assay with the use of human leukemia cells K 562 as targets. The previous exposure of lymphocytes obtained from healthy donors to somatostatin in the concentration of 10(-8) and 10(-6) M resulted in the enhancement of NK activity. The finding provides a further evidence of the immunomodulating somatostatin action.

Short-term somatostatin infusion affects T lymphocyte responsiveness in humans

Peripheral blood lymphocytes (PBL) function was analysed in 16 young men with duodenal ulcers after one-hour intravenous infusion of somatostatin (SMS) at a dose of 250 micrograms/h. Proliferative responses of PBL from SMS-treated patients were significantly diminished compared with pre-treatment values, after stimulation with PHA, PWM or Con A. Spontaneous IL-2R expression was moderately increased after SMS infusion but PHA-induced IL-2R expression was not affected by this drug. Alloantigen and autoantigen stimulation of PBL showed no significant changes in the proliferative response after SMS infusion. NK cell activity was similarly unaffected. These observations establish a link between SMS exposure and possible development of immune dysfunction.

Neuromodulation of hematopoiesis: a hypothesis

The possibility that Hematopoiesis is under CNS control has been discussed in the literature; but never been conclusively proved at the molecular level. Evidence in the form of antigenic similarity between murine brain and the Stem Cell, and the decline in the number of Stem Cells following the lesioning of the Locus Coeruleus Nucleus exists. The recent establishment of the CNS control of the Immune System which had been previously proposed on similar evidence is discussed. It is suggested that the advances in the study of Colony Stimulating Factors (CSFs) provide an opportunity to test the role of Neurotransmitters on the proliferation/differentiation of Stem Cells. A series of experiments using CSF-dependent and CSF-independent cell lines and the biochemical basis of CSF activity to detect neurological influences are proposed. A model of a CNS regulatory mechanism is proposed, and used to explain many unexplained features of the in vivo activity of the CSFs.

Effect of neuropeptides and monoamines on lymphocyte activation

Neuropeptides and monoamines have been found in tissues where immune reactions are initiated such as the skin, gut, and respiratory tract, and in these tissues neuropeptides and monoamines might be involved in the regulation of lymphocyte activation. Studies both in in vitro and in vivo showing that various neuropeptides and monoamines may influence reactions such as T lymphocyte proliferation, B lymphocyte proliferation, and antibody synthesis, lymphocyte migration, and cytotoxicity will be reviewed.

A molecular basis for interactions between the immune and neuroendocrine systems

Homeostatic and psychologic alterations associated with infections and tumors are very interesting yet poorly understood pathophysiologic responses. Numerous anecdotal and indirect examples suggest that these responses occur through a link between the central nervous and immune systems (for review see Blalock, Bost, & Smith, 1985; Spector & Korneva, 1981; Maestroni & Pierpaoli, 1981; Felton et al., 1985; Jankovic, 1985). Interactions between the two systems are just now being described. One possible mechanism is direct modulation of the immune system by the sympathetic nervous system. This could occur in innervated immune organs such as spleen, thymus, and bone marrow (Felton et al., 1985). The evidence for this is that sympathectomy and lesioning of specific regions of the brain can be shown to both enhance and/or suppress immune responses (Miles et al., 1985; Roszman et al., 1985). Also, the firing rate of hypothalamic neurons is altered during an immune response (Besedovsky et al., 1977). Alternatively, hormonal involvement in immune reactions has been known for some time, in particular the immunosuppressive effects of glucocorticoids (for review see Cupps & Fauci, 1982). Recently, we and others found that neuroendocrine peptide hormones will modulate T and B lymphocytes plus other immunocyte responses (Besedovsky et al., 1977; Cupps & Fauci, 1982; Johnson et al., 1982; Wybran et al., 1979; Hazum, Chang & Cuatrecasas, 1979; O'Dorisio et al., 1981; Gilman et al., 1982; McCain et al., 1982; Mathews et al., 1983; Plotnikoff et al., 1985; Johnson et al., 1984). Furthermore, lymphocytes themselves can synthesize biologically active neuroendocrine hormones (Blalock & Smith, 1980; O'Dorisio et al., 1980; Smith & Blalock, 1981; Smith et al., 1983; Lolait et al., 1984; Ruff & Pert, 1984), as well as possess specific hormone receptors (Blalock et al., 1985; Johnson et al., 1982; Wybran et al., 1979; Hazum et al., 1979; O'Dorisio et al., 1981; Lopker et al., 1980; Payan, Brewster & Goetzl, 1984; Pert et al., 1985). Immune responses (Besedovsky, del Rey & Sorkin, 1981), thymic hormones (Healy et al., 1983), and lymphokines (Lotze et al., 1985; Woloski et al., 1985) have all been shown to exert hormonal effects. Thus, another method for communication between the immune and neuroendocrine systems seems to be through soluble factors such as neuroendocrine hormones. This review will concentrate on the latter topic, in particular on work this laboratory has done over the past few years to show the lymphocyte production and immunoregulatory actions of neuroendocrine hormones.

Neuropeptide regulation of immediate and delayed hypersensitivity

Peptide mediators of sensory nerves that are released in tissues by noxious stimuli or inflammatory reactions rapidly elicit local and systemic responses similar to those of immediate hypersensitivity. These sensory neuropeptides affect functions of smooth muscles, blood vessels, leukocytes, and epithelial glands both directly and indirectly, through the actions of mediators released from mast cells stimulated by the peptides. Stereospecific receptors transduce the effects of neuropeptides of the peripheral nervous system (PNS) and central nervous system (CNS) on diverse functions of human, murine and guinea pig mononuclear and polymorphonuclear leukocytes, mast cells, and basophils in vitro and in vivo. Stimulatory and inhibitory effects of neuropeptides on leukocytes are attained in vitro at concentrations which are similar to those in the circulation and in tissues. The dissociation constant (KD) for the binding of a neuropeptide to its leukocyte receptor is within the range of concentrations that evoke cellular responses critical to immunity and hypersensitivity. Neuropeptides exhibit both cellular and stimulus specificities, as exemplified by the greater potency of substance P in activating mucosal than connective tissue mast cells and the capacity of somatostatin to inhibit the release of mediators from basophils challenged by IgE-dependent mechanisms, but not by basic peptides or ionophores. The selective release of distinct neuropeptides from different subsets of sensory nerve endings, the specificity of neuropeptide recognition by mast cells, basophils, lymphocytes, and other target cells, and the diversity of relevant activities of the neuropeptides suggest that the nervous system may initiate and modulate immediate and delayed hypersensitivity by unique mechanisms.

Neuropeptide modulation of the immune response in gut associated lymphoid tissue

The neuropeptides vasoactive intestinal peptide (VIP), substance P, and somatostatin are found in high concentrations in both the central nervous system and the gastrointestinal tract. Specific high affinity receptors for VIP, substance P and somatostatin have been identified on both human and murine lymphocytes, suggesting a role for each of these neuropeptides in a neuroimmune axis. These peptides may be important modulators of mucosal immunity regulating lymphocyte proliferation and trafficking in gut associated lymphoid tissue, synthesis of IgA, and histamine release. Somatostatin antagonism of both VIP and substance P effects has been observed in the immune system. Though the mechanisms by which these neuropeptides modulate immune function have not been completely delineated, current evidence supports the hypothesis that VIP modulates immune function via cAMP dependent pathways while substance P regulation of the immune response involves phospholipid metabolism. Somatostatin inhibition of both cAMP dependent and phospholipid dependent effects has been documented in endocrine tissues. Delineation of the role of these peptide-peptide interactions in modulation of the immune response promises to be a fruitful area for further investigation.

Distribution of somatostatin receptors on murine spleen and Peyer's patch T and B lymphocytes

Recent evidence suggests that the neuropeptide somatostatin (SOM) plays an immunoregulatory role. We demonstrated previously that SOM inhibits concanavalin A-induced cell proliferation and immunoglobulin synthesis by murine Peyer's patch and splenic lymphocytes. Available data suggest that these effects are in part mediated by specific SOM receptors expressed by lymphocytes, but as yet these receptors have not been characterized. Using cytofluorimetry we investigated the distribution and specificity of binding of fluorescent SOM (SOM*) to murine Peyer's patch and splenic T- and B-lymphocyte subpopulations. The specificity of binding was confirmed by radioassay. T and B cells from both organs showed specific binding of SOM*. In Peyer's patches, approximately 50% of all cell populations studied (whole, T- and B-cell-enriched) bound SOM specifically and this was significantly higher than the corresponding splenic lymphocyte populations. Eighty to eighty-four percent of Peyer's patch Thy1.2+, Lyt1+, or L3T4+ cells and 94% of Lyt2+ cells bound SOM. Greater than 80% of B cells from this organ bound SOM (sIgA+ = sIgM+ greater than sIgG+ cells). In spleen, approximately 30% of Thy1.2+, Lyt1+, or L3T4+ cells bound SOM and this was significantly less than the proportion of Lyt2+ cells (53%) which did so. More sIgA+ (89%) than sIgG+ (66%) than sIgM+ (55%) B cells bound SOM*. Although we have previously shown that the effect of SOM on immunoglobulin synthesis was relatively isotype-specific (IgA synthesis was predominantly affected, especially in Peyer's patches) this cannot be explained solely on the basis of preferential expression of SOM receptors by distinct lymphocyte subsets. Instead, it is probably the result of the specific immunological microenvironment in which the lymphocytes reside.

Substance P recognition by a subset of human T lymphocytes

The interaction of substance P with human blood T-lymphocytes, which stimulates T-lymphocyte proliferation, was quantified by both flow cytometric and direct binding assays. Fluorescence-detection flow cytometry recorded the binding of dichlorotriazinylamino-fluorescein-labeled substance P to 21 +/- 10% (mean +/- SD, n = 6) and 35 +/- 8% (n = 2) of human blood T-lymphocytes before and after stimulation with 10 micrograms/ml of phytohemagglutinin, respectively. The suppressor-cytotoxic (leu 2a) and helper-inducer (leu 3a) subsets identified by phycoerythrin-labeled monoclonal antibodies contained substance P-reactive T-lymphocytes at respective mean frequencies of 10 and 18%. [3H]substance P bound rapidly and reversibly to a mean of 7035 +/- 2850 sites/T-lymphocyte, which exhibited a dissociation constant (KD) of 1.85 +/- 0.70 X 10(-7) M (mean +/- SD, n = 5). [D-Pro2,D-Phe7,D-Trp9]substance P inhibited the binding of dichlorotriazinylamino-fluorescein-labeled substance P and [3H]substance P to T-lymphocytes at concentrations that suppressed the proliferative response to substance P. Substance P, eledoisin, and substance K (alpha-neurokinin), which all share with substance P the carboxy-terminal substituent -Gly-Leu-Met-NH2, were more potent than substance P in inhibiting the binding of [3H]substance P to T-lymphocytes, suggesting the importance of this sequence in the interaction. Purified human blood B-lymphocytes, monocytes, polymorphonuclear leukocytes, and platelets, and cultured Hut 78 cutaneous lymphoma T-cells, Jurkat cells, Molt-4 lymphoblasts, and HL-60 and U-937 monocyte-like cells all showed only minimal specific binding of [3H]substance P. The recognition of substance P by T-lymphocytes provides one mechanism for selective modulation of immunity by sensory nerves.

Inhibition by somatostatin of the proliferation of T-lymphocytes and Molt-4 lymphoblasts

The proliferation of Molt-4 lymphoblasts and phytohemagglutinin-stimulated human T lymphocytes in vitro was inhibited significantly by 10(-12) to 10(-10) M to 10(-13) to 10(-9) M somatostatin, as assessed by the uptake of [3H]thymidine and [3H]leucine, respectively. The inhibitory effect of somatostatin was not attributable to cytotoxicity and was associated with a mean degradation of 52 and over 95% of immunoreactive somatostatin, respectively, after 3 and 24 hr of incubation at 37 degrees C. The specific suppression of Molt-4 lymphoblasts and T lymphocytes by somatostatin represents a distinct mechanism for the specific regulation of immunological responses by neuropeptides of the peripheral nervous system and gastrointestinal tract.

Failure of somatostatin to influence experimental tumor cell growth in vivo and in vitro

The influence of somatostatin on tumor cell growth was studied in vivo in mice (sarcoma 180 ascites tumor and Lewis lung tumor) and in vitro on nontransformed and polyoma-transformed cell lines. 4 or 20 micrograms/100 g of cyclic somatostatin and 4 micrograms/100 g of linear protamin Zn-bound somatostatin were injected s.c. twice daily in the in vivo study. Cyclic somatostatin (1, 4 or 10 micrograms/ml) was added twice daily to the cell cultures. Somatostatin administration influenced neither the survival of animals nor the growth rate of cultured cell lines.