Introduction
Beginning in 1975, a robust literature has accumulated to support the conclusion that opioids modulate immune responses. Overwhelmingly, these studies have shown that opioids are immunosuppressive. However, with the publication of a paper in 2005, the hypothesis was formulated that opioids are pro-inflammatory, and that the resulting opioid-induced inflammation mediates addiction.
Early Discovery That Opioids Are Immunosuppressive
In 1979, a paper was published in the Journal of Immunology by Wybran et al., which had a major impact on our understanding of the physiological effect of opioids on the immune system (1). It was reported that morphine inhibited the rosetting of human peripheral blood T cells with sheep red blood cells (SRBCs). Prior to wide-spread use of flow cytometry, the fortuitous ability of SRBCs to adhere to human T cells, but not B cells, was used as the technique for distinguishing the numbers of these two cell populations in a blood sample. What Wybran observed was that the morphine-induced inhibition of rosetting could be blocked by pretreatment with naloxone, an antagonist at opioid receptors (Figure 1).
Further, it was shown that the endogenous opioid peptide, met-enkephalin, enhanced the red blood cell rosetting, a phenomenon which was also blocked by naloxone.
In 1982 the AIDS (Acquired Immunodeficiency Syndrome) epidemic emerged. It was recognized that patients suffered from a severe depression in T cell numbers, but the viral cause was not identified until 1986. At the beginning of the outbreak, epidemiologic evidence showed that one third of AIDS patients were intravenous drug abusers, mainly of heroin (2). Based on the Wybran paper, the question arose as to whether the opioids were in some way mediating the immune suppression observed in AIDS patients.
Opioid Pharmacology as Background
Three major receptors were discovered that were designated mu, kappa and delta (3–5). In 1993, these receptors were cloned (6–8). Currently, they are termed mu opioid receptor (MOR), kappa opioid receptor (KOR), and delta opioid receptor (DOR). The endogenous ligands for these receptors are the neuropeptides: β-endorphin (MOR), dynorphin (KOR), and methionine-enkephalin (DOR), although these proteins have significant cross-affinity for the other receptors. Morphine has greatest affinity for the MOR, but it can bind to a lesser degree to the other receptors. The analgesic and psychoactive effects of morphine, as well as the adverse effects of respiratory depression and inhibition of gastric transit, are mediated through the MOR.
Naloxone and naltrexone are major antagonists of the opioid receptors (Figure 1). Both synthetic compounds bind to all three opioid receptors.
Opioids and Suppression of Natural Killer (NK) Cell Activity
Among the earliest studies testing the effect of opioids on immune function were those that examined NK cell activity, a measure of innate immunity.
Yeager et al. administered morphine intravenously for 24 h to normal, non-opioid abusing volunteers in the hospital, and obtained NK cells from peripheral blood by venipuncture before administration of the opioid, and 2 and 24 h later. Morphine administration resulted in a significant depression in NK cell activity at both time points compared to baseline (22). The studies cited above support the conclusion that morphine suppresses NK cell activity in rats, mice and humans, and that the mechanism of the immunosuppression is through the MOR. However, for suppression of NK cell cytotoxicity the effect of morphine does not appear to be direct, but rather is mediated by signals from the neural system.
Opioids and Suppression of Antibody Production
Opioids Given in vivo and Immunosuppression
The first paper showing that morphine inhibited antibody responses by mouse spleen cells to SRBCs as the antigen was published in 1975 (31).
A lingering question in the field as these results on opioid suppression of antibody responses emerged was whether the immunosuppression was due to a direct effect of the opioid on cells of the immune system or whether the effects were indirect through opioid-induced activation of other physiological systems. Pruett et al. argued that part of the suppression of the humoral immune response was due to activation of the HPA axis (41). As noted above in the section on opioids and NK cells, the sympathetic nervous system, the adrenergic system, dopaminergic pathways and Neuropeptide Y have all been implicated in mediating the effects of morphine. As a counter to this hypothesis is the definitive evidence that opioids added in vitro to immune cells suppress antibody responses in vitro.
Opioids Applied in vitro and Immunosuppression of Antibody Responses
In these experiments, spleen cells were taken from opioid naïve mice and were placed in culture where experimental wells received an opioid and control wells received medium. Other neurotransmitter systems of the body cannot be involved in immunosuppression observed by adding opioids to purified cells of the immune system in vitro.
Opioids and Depression of T Cell Mediated Adaptive Immune Responses
Several laboratories have reported that morphine administration in vivo blocks adaptive T cell responses.
Morphine and Depression of Cellularity, Induction of Apoptosis, and Inhibition of Cell Growth
Early studies using morphine slow-release pellets showed that drug administration resulted in splenic and thymus cell atrophy (23, 47). Later investigation of this phenomenon provided evidence that morphine added to human peripheral blood mononuclear cells (PBMCs) (48) or to human monocytes (49) in vitro induced apoptosis, in the monocytes by induction of nitric oxide. In a study of morphine-induced apoptosis, Yin et al. confirmed that morphine administration to mice in vivo resulted in reduced cellularity in the spleen, and showed that the opioid induced Fas in the spleen, heart and lung (50).
Zhang et al. carried out a more detailed study of the effect of morphine pellets on lymphocyte subsets in the spleen and lymph nodes of mice 7 days after pellet implantation and found reduced numbers of B cells and CD4 and CD8 T cells (55).
These studies provide evidence that morphine depresses numbers of major classes of lymphocytes when given in vivo or when added to cells in vitro. There is still controversy as to whether the in vivo effects are mediated by corticosteroids (57).
Effects of Opioids on Cell Subsets and Cytokines
Opioids Suppress Phagocytosis and Microbicidal Activity of Phagocytes and Enhance Viral Replication
An essential aspect of innate immunity is the ability of the neutrophils and macrophages to ingest and kill microbes. The literature on this subject overwhelmingly indicates that morphine suppresses phagocytosis and microbicidal activity of phagocytes.
It was shown that morphine depressed phagocytosis and fungicidal activity of macrophages which correlated with reduced production of superoxide anion (58).
Effect of Opioids on Macrophages, Microglia, and Macrophage/Microglial-Derived and T Cell-Derived Cytokines and Other Inflammatory Associated Mediators
Peterson and colleagues reported that in human PBMCs, morphine blocked production of the reactive oxygen intermediates, superoxide and peroxide, that are involved in microbicidal mechanisms of phagocytes in response to opsonized zymosan (69). Interferon-γ and TNF-α were also depressed by morphine (70, 71). Evidence was presented that the immunosuppressive cytokine, transforming growth factor-beta (TGF-β), produced by lymphocytes, was mediating the morphine-induced down-regulation of reactive oxygen intermediates (72).
These results support the conclusion that morphine depressed either macrophage numbers or production of macrophage pro-inflammatory cytokines (34). Wang et al. reported that heroin added to human macrophage cultures inhibited both IFN-α and IFN-β, which are antiviral molecules (67).
Opioids, Cell Movement, Chemokines, and Chemokine Receptors
Effects of Opioids on Cell Movement
Morphine slow-release pellets have been shown to depress leukocyte sticking and rolling along blood vessels in response to oxidized low-density lipoprotein, as visualized using intravital fluorescence microscopy of cells in dorsal skin-fold windows implanted in mice (90).
Effects of Opioids on Chemotaxis and Levels of Cytokines and Chemokines
A much larger literature exists describing effects of morphine added in vitro to chemotaxis by phagocytic cells in the periphery, and microglia and astrocytes in the CNS. Chao et al. reported that morphine inhibited primary human fetal microglial cells from chemotaxis in response to C5a, with an IC50 value of 1 fM (94).
Molecular Mechanisms of Opioid-Mediated Immunosuppression
As this field has progressed, investigators have dissected the mechanisms of opioid-induced immunosuppression. In Roy's laboratory, it was shown that murine peritoneal macrophages treated with micromolar doses of morphine had depressed NF-κB levels, but nanomolar doses of morphine led to NF-κB activation (83).
Opioids and Potentiation of Infection and Sepsis
The literature reviewed above shows that, with few exceptions, opioids are immunosuppressive. It is well-recognized in the clinical literature that opioid addicts have increased rates of infection, and the intersection of HIV infection with intravenous drug abuse is well-established (130–135). There are also studies showing increased infection rates, particularly for pneumonia, in patients who were not abusing opioids, but receiving them long-term for treatment of pain (136–138). As reviewed by Plein and Rittner (139), there is also a contradictory study by W. Hauser et al. (140) that did not find a correlation of increased infection with opioid use. In clinical studies there are many confounding factors.
Are Opioids Pro-inflammatory via Triggering of Toll-Like Receptor 4 (TLR4)?
Do Opioids Mediate Opioid Dependence by Being Pro-inflammatory?
The reader must appreciate from the literature cited above, that almost all of the studies carried out on effects of morphine and heroin on the immune system have concluded that opioids are immunosuppressive. Starting in the last decade, an unexpected hypothesis has emerged, namely, that morphine activates glia, and that glial activation via release of pro-inflammatory cytokines mediates morphine tolerance (166).
The DeLeo laboratory (181, 182) observed that morphine given chronically i.t. resulted in microglial activation, and that blocking glial activation with propentofylline reinstated the effectiveness of acute morphine (182). However, the role of TLR4 in microglial activation by morphine was not tested in these studies.
There are some reports that morphine can activate microglia and the hypothesis is being tested that pro-inflammatory cytokines released by these cells may mediate the addiction process or be involved in withdrawal symptoms.
In summary, there is little support from other laboratories for the hypothesis that morphine signals through the TLR4 receptor. However, there is still the possibility that cytokines and chemokines may be involved in addiction pathways. Valentinova et al. reported that morphine withdrawal induces TNF-a in the lateral habenula that is involved in behavioral modulation (191). A more comprehensive coverage of the role of inflammation in addictive processes and the cellular origin of inflammatory mediators that do not involve morphine triggering of TLR4 is beyond the scope of the present review.
Can the Two Lines of Evidence Be Reconciled?
This review has covered almost the entire literature on opioids and the immune response. If morphine bound to TLR4 and activated it, releasing pro-inflammatory cytokines, then one would predict that a literature would have developed showing that opioids up-regulate the immune system. As TLR4 is abundantly expressed on macrophages and dendritic cells in the periphery, it would be expected that giving morphine systemically would result in immune activation. Also, many experiments were carried out with macrophages that were exposed to morphine in vitro and in most cases macrophage phagocytic and microbicidal activity was suppressed, and cytokine production was depressed. As macrophages are the primary cells expressing TLR4, if morphine bound to TLR and activated this receptor, one would have predicted the opposite result. The molecular studies that are cited above also are almost all consistent with morphine-mediated inhibition of transcription factors and induction of miRNAs that down-regulate inflammatory responses. There should be no argument that procedures that induce pain frequently induce inflammation with production of pro-inflammatory cytokines. Blocking these cytokines alleviates pain, which is not surprising. It also makes sense that the analgesic effect of morphine might be mitigated by blocking the cytokines which are augmenting pain signals.
In reviewing the literature, one must take into account the findings from studies by other laboratories using mice genetically deficient in the MOR, which is a definitive approach to dissecting the effect of morphine on the immune system. In almost every case, mice lacking the MOR failed to show immunomodulatory effects of morphine. These animals all had intact TLR4 genes and receptors (177). In contrast, mice lacking a functional TLR4 gene still showed morphine mediated immunosuppression (33, 188). If TLR4 were the receptor through which morphine is activating the immune system, one would expect to observe immune activation in most experiments. Clearly, from the extensive review above, that is not the case. Virtually all of the literature shows morphine to be immunosuppressive.
Referência :
Front Immunol. 2019 Dec 20;10:2904.
