Opioid peptides as cytokines in T cell activation

Increased splenocyte proliferative response and cytokine production in beta-endorphin-deficient mice

We used beta-endorphin-deficient mice as a novel approach to confirm the physiological role that opioid peptides play in the development or regulation of the immune system. We found that mice lacking beta-endorphin possessed an enhanced immune response, measured in terms of splenocyte proliferation and interleukin (IL)-2 mRNA levels, in vitro production of the splenic macrophage inflammatory cytokines IL-6 and Tumor Necrosis Factor (TNF)-alpha and plasma IL-6 following lipopolysaccharide (LPS) administration. beta-Endorphin-deficient mice had attenuated increases of plasma ACTH and corticosterone levels in response to LPS. These results are consistent with a postulated inhibitory role of endogenous beta-endorphin on the immune system at multiple levels.

Light and immunomodulation

The immune system is susceptible to a variety of stresses. Recent work in neuroimmunology has begun to define how mood alteration, stress, the seasons, and daily rhythms can have a profound effect on immune response through hormonal modifications. Central to these factors may be light through an eye-brain hormonal modulation. In adult primates, only visible light (400-700 nm) is received by the retina. This photic energy is then transduced and delivered to the visual cortex and, by an alternative pathway, to the suprachiasmatic nucleus (SCN), the hypothalamic region that directs circadian rhythm. Visible light exposure also modulates the pituitary and pineal glands, leading to neuroendocrine changes. Melatonin, norepinephrine, and acetylcholine decrease with light activation, whereas cortisol, serotonin, GABA, and dopamine levels increase. The synthesis of vasoactive intestinal polypeptide (VIP), gastrin releasing peptide (GRP), and neuropeptide Y (NPY) in rat SCN has been shown to be modified by light. These induced neuroendocrine changes can lead to alterations in mood and circadian rhythm as well as immune modulation. An alternative pathway for immune modulation by light is through the skin. Visible light (400-700 nm) can penetrate epidermal and dermal layers of the skin and may directly interact with circulating lymphocytes to modulate immune function. In contrast to visible light, in vivo exposure to UV-B (280-320 nm) and UV-A (320-400 nm) radiation can alter normal human immune function only by a skin-mediated response. It is therefore important, when reporting neuroendocrine immune findings, to control the intensity, timing and wavelength of ambient light.

Leucine enkephalin degradation in allergopathic versus normal human plasma

The enzyme hydrolysis of labelled leu-enkephalin in the presence of plasma enzymes was studied by kinetic and chromatographic techniques in a group of allergopathic patients in the acute and quiescent stage; data obtained have been compared with those obtained with normal controls. Results shown indicate that in the quiescent stage substrate degradation is reduced, and that the pattern of the hydrolysis by-products is modified with respect to the controls. In the acute as compared to the quiescent stage, enkephalin hydrolysis is further reduced, and the pattern of hydrolysis by-products is further modified. ANOVA analysis of these data indicates that the dependency of hydrolysis reduction upon the grouping of subjects (i.e., controls, quiescent and acute stage) is statistically very significant. Reduced substrate hydrolysis, and modified hydrolysis pattern, appears to be associated with decreased activity of the enzymes involved and more significantly with increased activity of the low molecular weight plasma inhibitors. The combination of these two factors appears to define a hydrolysis pattern characteristic of the allergopathic subjects, similar in the quiescent and acute phase, and different from that observed in the controls.

Hepatic inactivation of Leu-enkephalin

Leu-enkephalin radiolabelled at the N-terminal tyrosine by two different methods was presented to isolated perfused rat livers. Approximately 10% of a pulse of tritiated Leu-enkephalin was taken up first-pass; this was increased to 62% when the peptide was iodinated with Bolton and Hunter reagent. Uptake of both forms of radiolabelled Leu-enkephalin was inhibited by taurocholate in a concentration-dependent manner. The proportion of internalised radioactivity secreted into bile also differed but in both cases showed a very rapid time-course similar to that of [24-(14)C]taurocholate and suggestive of non-endocytic transfer via membrane transport proteins. Pre-perfusion with the aminopeptidase inhibitor bestatin increased uptake of 3H-labelled Leu-enkephalin from 10% to 23%; no further increase occurred when the endopeptidase 24.11 inhibitor thiorphan was also present. On infusion of the native peptide into rat livers, 80% of Leu-enkephalin immunoreactivity was lost between the pre- and post-hepatic perfusate; this was reduced to 65% in the presence of 10(-5) M bestatin. The almost total release of the N-terminal tyrosine from 3H-labelled Leu-enkephalin which escaped first-pass uptake confirmed that substantial sinusoidal metabolism had occurred. Low levels of aminopeptidase N were visualised in the sinusoidal membrane using a specific monoclonal antibody coupled to peroxidase staining. Thus, hepatic inactivation of Leu-enkephalin is primarily via hydrolysis mediated by cell surface peptidase (including aminopeptidases) whilst uptake of the intact peptide, probably by a bile salt transport protein, is quantitatively minor unless the N-terminus is blocked by Bolton and Hunter reagent or peptidase inhibitors are present.

A secreted peptidase involved in T cell beta-endorphin metabolism

Beta-endorphin metabolism by CD4+ and CD8+ T cells, and the thymoma cell line, EL4, was investigated. In all three cell types, extracellular beta-endorphin was metabolized exclusively by a secreted, metal-dependent, thiol peptidase. The enzyme activity is expressed constitutively in EL4 cells and following activation of CD4+ and CD8+ T cells with anti-CD3 antibody. The enzyme is not one of the proteinases associated with cytolytic T cells and does not appear to be identical with any previously described beta-endorphin metabolizing enzyme. The enzyme cleaves beta-endorphin at approximately equal rates at either of two sites to yield beta-endorphin(1-17) (which is gamma-endorphin), beta-endorphin(1-18), beta-endorphin(18-31) and beta-endorphin(19-31). Evidence in the literature indicates that these N- and C-terminal peptides which contain, respectively, the opioid and non-opioid receptor binding domains of beta-endorphin, are biologically active. Thus, it is likely that this new T cell peptidase has important immunoregulatory activity.

Pretreatment of human peripheral blood lymphocytes with interleukin-2 or dexamethasone does not alter their response to Met-Enkephalin in a NK-cytotoxic assay

The effect of Met-Enkephalin (MENK; 10(-12) - 10(-8) M) on NK-activity of peripheral blood lymphocytes (PBL) after in vitro treatment (18 h, 37 degrees C) was examined in 30 young, healthy male donors. In the group as a whole (n = 30), no significant effect of MENK was detected. At the individual level, 18 of 30 donors (60%) responded to MENK either by mild enhancement (up to 8%, 8 responders), or by mild attenuation (up to 12%, 10 responders) of the basal NK-activity. The effect of MENK was donor-related regarding the dose-response, E/T ratio, and direction of MENK action. The influence of pretreatment of PBL (1 h) with either graded doses of interleukin-2 (IL-2; 3, 25, 50 U/ml) or dexamethasone (Dex; 2.5 x 10(-9), 2.5 x 10(-8), 2.5 x 10(-7) M), on the effect of MENK was also tested. The idea was that pretreatment of PBL would result in predictable, and/or stronger response to MENK. In the group as a whole again no significant effect of MENK was detected on the NK-activity of PBL prestimulated by IL-2 (n = 16), or inhibited by Dex (n = 12). Further, pretreatment of PBL with IL-2/Dex did not significantly alter the intensity of modulation by MENK, which was generally mild. The data obtained have shown that pretreatment of PBL with IL-2 or Dex, regardless of their concentrations, did not significantly alter the frequency of responders to MENK being 50%, 62.5% and 64.3% with 3, 25 or 50 U/ml IL-2, respectively, and 50% with all concentration of Dex used, as compared to that observed with resting PBL (60%). However, at the individual level physiological concentrations of MENK (10(-12) - 10(-9) M) induced enhancement or/and attenuation of the NK-activity pretreated with IL-2/Dex, respectively. The effect of MENK at the individual level was donor-related regarding the dose-response, E/T ratio, and direction of MENK action. Thus, pretreatment of PBL with graded concentrations of IL-2/Dex did not alter the effect of MENK on NK-activity, regarding the frequency and intensity, as well as the direction of modulation: it remained bidirectional, of low intensity, and independent of the grade of PBL preactivation/inhibition, therefore unpredictable.

Positive and negative immunomodulation by opioid peptides

The data that follow review part of the existing evidence concerning the neuroimmune functions mediated by opioid peptides, with particular regard to dual immunomodulatory effects. Limited references to substances other than opioid peptides are included, mainly to emphasize the possible similarities in the mediation of neuroimmune interactions by different informational substances, while the interactions directed from the immune to the nervous system have deliberately been omitted.

Visible light induced changes in the immune response through an eye-brain mechanism (photoneuroimmunology)

The immune system is susceptible to a variety of stresses. Recent work in neuroimmunology has begun to define how mood alteration, stress, the seasons, and daily rhythms can have a profound effect on immune response through hormonal modifications. Central to these factors may be light through an eye-brain hormonal modulation. In adult primates, only visible light (400-700 nm) is received by the retina. This photic energy is then transduced and delivered to the visual cortex and by an alternative pathway to the suprachiasmatic nucleus (SCN). The SCN is a part of the hypothalamic region in the brain believed to direct circadian rhythm. Visible light exposure also modulates the pituitary and pineal gland which leads to neuroendocrine changes. Melatonin, norepinephrine and acetylcholine decrease with light activation, while cortisol, serotonin, gaba and dopamine levels increase. The synthesis of vasoactive intestinal polypeptide (VIP), gastrin releasing peptide (GRP) and neuropeptide Y (NPY) in rat SCN has been shown to be modified by light. These induced neuroendocrine changes can lead to alterations in mood and circadian rhythm. All of these neuroendocrine changes can lead to immune modulation. An alternative pathway for immune modulation by light is through the skin. Visible light (400-700 nm) can penetrate epidermal and dermal layers of the skin and may directly interact with circulating lymphocytes to modulate immune function. However, even in the presence of phototoxic agents such as eosin and rose bengal, visible light did not produce suppression of contact hypersensitivity with suppresser cells. In contrast to visible light, in vivo exposure to UV-B (280-320 nm) and UV-A (320-400 nm) radiation can only alter normal human immune function by a skin mediated response. Each UV subgroup (B, A) induces an immunosuppressive response but by differing mechanisms involving the regulation of differing interleukins and growth factors. Some effects observed in humans are: inhibition of allergic contact dermatitis; inhibition of delayed hypersensitivity to an injected antigen; prolongation of skin-graft survival and induction of a tumor-susceptible state. The following article will review much of the progress in this field and explore possible areas of future research.

Methionine enkephalin metabolism by murine macrophage ectopeptidase(s)

Ectopeptidases which hydrolyze opioid and other neuropeptides have been identified in brain, kidney and intestine. In this study, identification of the enzymes metabolizing the opioid peptide methionine enkephalin (YGGFM) in murine macrophages was undertaken. Incubation of methionine enkephalin with intact murine peritoneal macrophages results in five products identified as Y, F, FM, GFM and GGFM by amino acid analysis and peptide microsequencing after fractionation by HPLC. The spectrum of metabolites results from at least two distinct aminopeptidase activities. The enzyme hydrolyzing YGGFM to GGFM is identified as the membrane-anchored aminopeptidase N (ApN; EC 3.4.11.2) based on its substrate specificity and inhibitor profile. A distinct bestatin and amastatin sensitive aminopeptidase catalyzes hydrolysis of GGFM to GFM. The macrophage ApN protein has a larger mass and is antigenically distinct from murine kidney ApN, which is suggested to result from glycosylation differences rather than expression of a distinct protein. The ApN catalytic activity and mRNA levels are increased in thioglycollate-elicited as compared to resident peritoneal macrophages. RT-PCR analysis identified a 0.7 kb fragment of the ApN coding sequence which was identical in mouse kidney and thioglycollate-elicited peritoneal macrophages and which has 89% identity with the corresponding rat kidney ApN cDNA sequence.