The interaction between gamma-type endorphins and HLA class I antigens

Homotypic and heterotypic in cis associations of MHC class I molecules at the cell surface

Through the presentation of peptide antigens to cytotoxic T lymphocytes, major histocompatibility complex (MHC) class I molecules mediate the adaptive immune response against tumors and viruses. Additional non-immunological functions include the heterotypic association of class I molecules with cell surface receptors, regulating their activities by unknown mechanisms. Also, homotypic associations resulting in class I dimers and oligomers - of unknown function - have been related to pathological outcomes. In this review, we provide an overview of the current knowledge about the occurrence, biochemical nature, and dynamics of homotypic and heterotypic associations of class I molecules at the cell surface with special focus on the molecular species that take part in the complexes and on the evidence that supports novel biological roles for class I molecules. We show that both heterotypic and homotypic class I associations reported in the literature describe not one but several kinds of oligomers with distinctive stoichiometry and biochemical properties.

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Major histocompatibility complex class I molecules form homotypic and heterotypic associations at the cell surface.

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Associations show distinctive stoichiometry and biochemical properties.

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Associations might regulate immunological and non-immunological processes.

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Heterotypic association with cell surface receptors might regulate receptor's activity.

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Homotypic associations have been related to pathological outcomes.

Characterization of signaling function and expression of HLA class I molecules in medulloblastoma

Although known for the important function in the immune system, MHC class I molecules are increasingly ascribed an alternative role in modifying signal transduction. In medulloblastoma, HLA class I molecules are associated with poor prognosis, and can induce ERK1/2 activation upon engagement with ligands that bind to incompletely assembled complexes (so called open conformers). We here demonstrate that ERK1/2 activation in medulloblastoma can occur in the absence of endogenously synthesized β2m, formally excluding involvement of closed HLA class conformation. In addition, several experimental observations suggest that heterogeneity of HLA class I expression may be a reflection of the status of original cells before transformation, rather than a consequence of immune-based selection of HLA-loss mutants. These results contribute to our understanding of an immune system-independent role of HLA class I in the pathology of medulloblastoma, and cancer in general.

Open conformers: the hidden face of MHC-I molecules

A pool of MHC-I molecules present at the plasma membrane can dissociate from the peptide and/or the light chain, becoming open MHC-I conformers. Whereas peptide-bound MHC-I molecules have an important role in regulating adaptive and innate immune responses, through trans-interactions with T cell and NK cell receptors, the function of the open MHC-I conformers is less clear but seems to be related to their inherent ability to cis-associate, both with themselves and with other receptors. Here, we review data indicating the open MHC-I conformers as regulators of ligand-receptor interactions and discuss the biological implications for immune and non-immune cells. The likelihood that the MHC-I heavy chains have hidden functions that are determined by the amino acid sequence of the alpha1 and alpha2 domains are discussed.

Effects of opioid substances on cAMP response to the beta-adrenergic agonist isoproterenol in human mononuclear leukocytes

The effects of different opioid substances on isoproterenol and forskolin-stimulated cyclic AMP (cAMP) intracellular accumulation, and on the binding of 125I-pindodol (IPIN) to beta 2-adrenoceptors were studied in human mononuclear leukocytes (MNL). The opioids used were alpha-endorphin, beta-endorphin, tau-endorphin, DAGO (a mu receptor agonist), dermenkephalin (a delta receptor agonist and morphine. Only morphine was able to increase the cAMP response to isoproterenol. The EC50 of isoproterenol for cAMP accumulation was shifted leftward by morphine; this effect was blocked by naloxone. On the contrary, the cAMP response to forskolin, direct activator of adenylate cyclase, was similar in the control test with respect to the experiments with morphine. The five opioid peptides induced no changes in the dose-response curves with isoproterenol and forskolin. Furthermore, none of the opioids induced changes in the IPIN binding. Our data show that morphine is able to exert a significant enhancement of the response of beta 2-adrenergic receptors to isoproterenol in human mononuclear leukocytes. This effect seems to be mediated by mu opioid receptors and seems to involve G protein.

Proximity measurements between H-2 antigens and the insulin receptor by fluorescence energy transfer: evidence that a close association does not influence insulin binding

Reports based on coprecipitation experiments have suggested that major histocompatibility complex class I products are complexed with the insulin receptor on the cell surface. Using an independent method that avoids the creation of immunoprecipitation artifacts during membrane solubilization, we have studied insulin receptor-class I product associations by determining the proximity between these class I products and the insulin receptor on intact cells with the use of fluorescence energy transfer. Significant energy transfer was seen between the insulin receptor and both murine H-2K- as well as H-2D-end products, indicating close proximity (e.g., within 10 nm). This cell-surface association is not from the relatively high class I density in that no significant energy transfer was measured between H-2K- vs. H-2D-end proteins. To extend these observations, we also tested whether class I products influence insulin-receptor binding and postbinding events as a result of their physical association. Using related cell lines positive and negative for class I expression, we found no correlation between insulin-receptor density or binding affinity with H-2 product expression. The class I-null variant, however, demonstrated an increase in insulin-mediated insulin-receptor internalization to suggest that if major histocompatibility complex class I products directly affect insulin-receptor function through specific cell-surface interactions, they may do so after ligand binding.

Internalization of MHC class I molecules is a prerequisite for endocytosis of endorphin by lymphocytes

The nature of the interaction between gamma-type endorphins and the HLA class I molecules was studied by immunoelectronmicroscopy. The HLA molecules were not involved in the actual binding of endorphin to the cell. In contrast, for the endocytosis of gamma-endorphin, co-internalization of the HLA class I molecules is essential. The internalization process starts with clustering of gamma-endorphin and HLA class I molecules in coated pits. Cells that do not carry HLA class I molecules (Daudi) or do not internalize HLA class I molecules (EBV-transformed B cells) bind but do not internalize gamma-endorphin. On the basis of these observations, we suggest that the MHC class I molecules may function as transport molecules. Whether it is a general phenomenon that non-immunological ligands use the HLA class I molecules to get into the cell and immunological ligands (viral proteins) to reach the cell surface, remains to be established.

Luteinization of human granulosa cells in vivo is associated with expression of MHC class II antigens

We previously described the presence of MHC class II (HLA-DR) antigens, structurally similar to those on lymphoid cells and bearing the genetically-appropriate allotypic determinants, on human adrenocortical cells in the zona reticularis of normal glands. We now report a similar expression by granulosa-lutein cells (GLC) in corpora lutea (CL) of normal ovaries, as detected by indirect immunofluorescence techniques with the use of mouse monoclonal antibodies (MAb). In some cases, GLC were also positive for HLA-DQ and -DP antigen expression. Neither granulosa nor theca interna cells in large antral follicles of the same ovaries showed any detectable expression of MHC class II antigens. Moreover, theca-lutein (paralutein) cells, identified by their reactivity with specific human autoantibodies in 5 of the 7 human CL examined, were also negative. Similarly, GLC, but not paralutein cells, in rhesus monkey CL showed significant cross-reactivity with anti-HLA-DR MAb. In contrast, lutein cells in ovaries from either cycling or 7-day-pregnant rats were negative for MHC class II (Ia) antigen expression. Expression of MHC class II antigens by human granulosa cells after their luteal transformation confirms the normal inducibility of certain human steroidogenic cells at the time of their further functional differentiation and enhanced biosynthetic activity, and suggests that these molecules may have additional functions beyond serving as restriction elements in the immune response.

Failure to find interference between anti-HLA antibodies and chlorpromazine

Chlorpromazine has been reported to interfere with the action of alloantibodies directed against HLA-A1. We attempted to replicate this finding using peripheral blood lymphocytes from 3 healthy donors in a complement-mediated lymphocytotoxicity assay. We were unable to find evidence of interference between chlorpromazine and the anti-HLA sera tested. Possible reasons for the difference between our finding and the previous report, as well as the implications for schizophrenia, are discussed.

Reversal of natural killing susceptibility in target cells expressing transfected class I HLA genes

A number of studies have suggested that resistance of target cells to natural killing (NK) may be correlated with their level of expression of major histocompatibility complex (MHC) class I antigens. To examine this hypothesis directly, a NK-sensitive class I-deficient human B-cell line was transfected with MHC class I genes. The expression of transfected HLA, but not H-2, class I gene products resulted in loss of susceptibility to human NK-mediated conjugation and cytolysis. Furthermore, this protection did not extend to cytotoxicity mediated by interleukin 2-stimulated human NK effector cells.

Opioid peptides modulate immune functions. A review

In the past few years it has become evident that neuropeptides may be direct mediators in the modulation of the immune response and the unspecific defense by the brain. Lymphocytes have been thought to have opioid receptors and to respond to opioids with an increase in blastogenesis, cytotoxicity and factor release. Lymphocytes are said to release various neuropeptides. Furthermore, there are some unexplained effects of morphine on the immune system and of the immune system on morphine withdrawal. The purpose of this paper is to review what has been previously published in this field. The well established modulation of phagocyte functions by opioids will only be scanned.

Persistence and selectivity of the immune phagocytosis inhibition by major histocompatibility complex antibodies

Major histocompatibility complex (MHC) antibodies induce immune phagocytosis inhibition (IPI) which lasts for at least 7 days. IPI-inducing antibodies do not inhibit the phagocytosis mediated by the beta-glucan receptor. This corresponds well to recent findings that these antibodies do not interfere with the phagocytosis of deactivated saccharomyces, mediated by the mannose-fucose receptor, or polyacrylic acid particles, also mediated by non-Fc receptors. Substances that interact with certain MHC antigens or with Fc receptors, certain toxins that inhibit surface molecule mobility, and ciclosporin do not cause IPI and do not suppress the induction of IPI by MHC antibodies. These substances are: opioid peptides, insulin, penicillin G, immune complexes, aggregated IgG, Fc fragments, ciclosporin, botulinum C2 toxin, sodium azide. Some lectins and EDTA are inhibitory in a non-selective fashion, since the Fc receptor independent phagocytosis is also abrogated.

Effect of two synthetic alpha-gliadin peptides on lymphocytes in celiac disease: identification of a novel class of opioid receptors

Two synthetic peptides containing residues 43-47 and 43-49 of alpha-gliadin were tested for inhibition of leukocyte migration in 47 patients with celiac disease. In nineteen patients, all on a normal diet, leukocyte migration was inhibited by the peptides and naloxone blocked this effect. In twenty-eight patients (24 of whom were on strict gluten-free diet) leukocyte migration was not affected by the peptides. Our results suggest that alpha-gliadin-(43-49), Tyr-Pro-Gln-Pro-Gln-Pro-Phe, is closely related to the active fragment, or to one of the active fragments of alpha-gliadin, and that it interacts with receptors that are similar to but not identical with the known opiate receptors.