Quercetin inhibition of the induction and function of cytotoxic T lymphocytes

A review of immune modulators and immunotherapy in infectious diseases

The human immune system responds to harmful foreign invaders frequently encountered by the body and employs defense mechanisms to counteract such assaults. Various exogenous and endogenous factors play a prominent role in maintaining the balanced functioning of the immune system, which can result in immune suppression or immune stimulation. With the advent of different immune-modulatory agents, immune responses can be modulated or regulated to control infections and other health effects. Literature provides evidence on various immunomodulators from different sources and their role in modulating immune responses. Due to the limited efficacy of current drugs and the rise in drug resistance, there is a growing need for new therapies for infectious diseases. In this review, we aim to provide a comprehensive overview of different immune-modulating agents and immune therapies specifically focused on viral infectious diseases.

The potential of Senolytics in transplantation

Older organs provide a substantial unrealized potential with the capacity to close the gap between demand and supply in organ transplantation. The potential of senolytics in improving age-related conditions has been shown in various experimental studies and early clinical trials. Those encouraging data may also be of relevance for transplantation. As age-differences between donor and recipients are not uncommon, aging may be accelerated in recipients when transplanting older organs; young organs may, at least in theory, have the potential to 'rejuvenate' old recipients. Here, we review the relevance of senescent cells and the effects of senolytics on organ quality, alloimmune responses and outcomes in solid organ transplantation. This article is part of the Special Issue - Senolytics - Edited by Joao Passos and Diana Jurk.

Immunomodulatory Effects of Flavonoids: Possible Induction of T CD4+ Regulatory Cells Through Suppression of mTOR Pathway Signaling Activity

The increasing rate of autoimmune disorders and cancer in recent years has been a controversial issue in all aspects of prevention, diagnosis, prognosis and treatment. Among dietary factors, flavonoids have specific immunomodulatory effects that might be of importance to several cancers. Over different types of immune cells, T lymphocytes play a critical role in protecting the immune system as well as in the pathogenesis of specific autoimmune diseases. One of the important mediators of metabolism and immune system is mTOR, especially in T lymphocytes. In the current review, we assessed the effects of flavonoids on the immune system and then their impact on the mTOR pathway. Flavonoids can suppress mTOR activity and are consequently able to induce the T regulatory subset.

The flavones luteolin and apigenin inhibit in vitro antigen-specific proliferation and interferon-gamma production by murine and human autoimmune T cells

Plant-derived flavonoids are inhibitors of various intracellular processes, notably phosphorylation pathways, and potential inhibitors of cellular autoimmunity. In this study, the inhibiting effects of various flavonoids on antigen-specific proliferation and interferon-gamma (IFN-gamma) production by human and murine autoreactive T cells were evaluated in vitro. T-cell responses were evaluated for the human autoantigen alpha B-crystallin, a candidate autoantigen in multiple sclerosis, and for the murine encephalitogen proteolipid protein peptide PLP (139-151). The flavones apigenin and luteolin were found to be strong inhibitors of both murine and human T-cell responses while fisitin, quercitin, morin and hesperitin, members of the subclasses of flavonoles and flavanones, were ineffective. Antigen-specific IFN-gamma production was reduced more effectively by flavones than T-cell proliferation, suggesting that the intracellular pathway for IFN-gamma production in T cells is particularly sensitive to flavone inhibition. These results indicate that flavones but not flavanoles or flavanones are effective inhibitors of the potentially pathogenic function of autoreactive T cells. The effects of flavones were the same for human and murine autoreactive T cells, stressing the usefulness of animal models of autoimmunity for further studies on the effects of flavonones on autoimmune diseases.

The biochemistry and medical significance of the flavonoids

Flavonoids are plant pigments that are synthesised from phenylalanine, generally display marvelous colors known from flower petals, mostly emit brilliant fluorescence when they are excited by UV light, and are ubiquitous to green plant cells. The flavonoids are used by botanists for taxonomical classification. They regulate plant growth by inhibition of the exocytosis of the auxin indolyl acetic acid, as well as by induction of gene expression, and they influence other biological cells in numerous ways. Flavonoids inhibit or kill many bacterial strains, inhibit important viral enzymes, such as reverse transcriptase and protease, and destroy some pathogenic protozoans. Yet, their toxicity to animal cells is low. Flavonoids are major functional components of many herbal and insect preparations for medical use, e.g., propolis (bee's glue) and honey, which have been used since ancient times. The daily intake of flavonoids with normal food, especially fruit and vegetables, is 1-2 g. Modern authorised physicians are increasing their use of pure flavonoids to treat many important common diseases, due to their proven ability to inhibit specific enzymes, to simulate some hormones and neurotransmitters, and to scavenge free radicals.

Comparison of analgesic and anti-inflammatory activity of meloxicam gel with diclofenac and piroxicam gels in animal models: pharmacokinetic parameters after topical application

Meloxicam, a non-steroidal anti-inflammatory drug, is a preferential inhibitor of cyclooxygenase-2 and has demonstrated potent analgesic and anti-inflammatory activity after oral administration. The present work was carried out to elucidate the anti-inflammatory and analgesic activity of a newer topical gel formulation of meloxicam (1% w/w gel) and compare it with 0.5% w/w piroxicam and 1% w/w diclofenac gels in experimental animal models. The study was also extended to determine the pharmacokinetic profile of a newer formulation of meloxicam gel after topical application on depilated skin of rats. The anti-inflammatory activities of meloxicam, piroxicam and diclofenac gels were compared using carrageenan-induced acute paw oedema and complete Freund's adjuvant-induced chronic paw oedema in rats. Meloxicam gel showed increased protection against inflammation as compared to piroxicam and diclofenac gels. Acetic acid-induced writhing and formalin-induced phase I and phase II pain models were used to compare their analgesic activity. Meloxicam gel showed significant protection in formalin-induced phase II pain whereas its analgesic activity was less as compared to diclofenac and piroxicam gels in writhing test and formalin-induced phase I pain. The pharmacokinetic studies showed peak plasma drug concentration (C(max)) of 48.48 +/- 6.57 microg/ml at 2 h (T(max)) after topical application of 500 mg of meloxicam gel formulation. The area under the curve as calculated from 0 to 6 h was found to be 114.18 +/- 4.23 and 194.13 +/- 3.78 microg x h/ml for 0 to infinity. The results indicate that topical preparation of meloxicam could be an effective alternative to diclofenac and piroxicam gels in inflammatory conditions and its associated pain with the possibility of less systemic side-effects.

Inhibitory effects of baicalein and wogonin on lipopolysaccharide-induced nitric oxide production in macrophages

In order to elucidate the mechanism of the antiinflammatory action of baicalein and wogonin, flavonoids from the root of Scutellaria baicalensis, the effects of these compounds were investigated on lipopolysaccharide-induced nitric oxide production in a macrophage-derived cell line, RAW 264.7. Baicalein (5-25 microM) and wogonin (5-50 microM) inhibited lipopolysaccharide-induced nitric oxide generation in a concentration-dependent manner. The inhibitory effect of these compounds was observed only when they were added at the start of cell incubation soon after the stimulation with lipopolysaccharide. Baicalein (25 microM) and wogonin (25 microM) also inhibited protein expression of inducible nitric oxide synthase. This inhibitory effect of wogonin was stronger than that of baicalein, which agrees with the result that wogonin showed stronger inhibition of nitric oxide production than baicalein. These results suggest that baicalein and wogonin attenuate lipopolysaccharide-stimulated nitric oxide synthase induction in macrophages and thus may help to explain the antiinflammatory action of these flavonoid compounds.

Effects of naturally occurring flavonoids on mitogen-induced lymphocyte proliferation and mixed lymphocyte culture

In this investigation, 34 structurally different flavonoids including derivatives of chalcone, flavanone, flavan-3-ol, flavone, flavonol, and their glycosides were evaluated for in vitro suppression of mitogen-induced lymphocyte proliferation and mixed lymphocyte culture from mouse spleen. Flavonoids, mainly derivatives of flavone and flavonol, clearly demonstrated the suppressive effects on lymphocyte proliferation at higher than 10(-6) M depending on the structures of flavonoid molecules, although their suppressive activities were less than that of cyclosporin A or prednisolone. Various glycosidic substitutions to A- and/or C-ring of the flavonoid aglycones were found to eliminate the suppressive activities of their aglycones, regardless of sugar compositions and positions of substitutions. In concanavalin A-induced lymphocyte proliferation, derivatives of flavone and flavonol having 2,3-unsaturation and at least 1 hydroxyl group showed the suppressive activity. In lipopolysaccharide-induced lymphocyte proliferation, only myricetin was active among flavonoids tested at the concentrations up to 10(-5) M. In mixed lymphocyte culture, some derivatives of flavone and flavonol with 2,3-unsaturation were active and especially flavone derivatives showed the higher suppressive activities than those of the flavonol derivatives.

Effects of flavonoids on immune and inflammatory cell functions

No doubt can remain that the flavonoids have profound effects on the function of immune and inflammatory cells as determined by a large number and variety of in vitro and some in vivo observations. That these ubiquitous dietary chemicals may have significant in vivo effects on homeostasis within the immune system and on the behavior of secondary cell systems comprising the inflammatory response seems highly likely but more work is required to strengthen this hypothesis. Ample evidence indicates that selected flavonoids, depending on structure, can affect (usually inhibit) secretory processes, mitogenesis, and cell-cell interactions including possible effects on adhesion molecule expression and function. The possible action of flavonoids on the function of cytoskeletal elements is suggested by their effects on secretory processes. Moreover, evidence indicates that certain flavonoids may affect gene expression and the elaboration and effects of cytokines and cytokine receptors. How all of these effects are mediated is not yet clear but one important mechanism may be the capacity of flavonoids to stimulate or inhibit protein phosphorylation and thereby regulate cell function. Perhaps the counterbalancing effect of cellular protein tyrosine phosphatases will also be found to be affected by flavonoids. Some flavonoid effects can certainly be attributed to their recognized antioxidant and radical scavenging properties. A potential mechanism of action that requires scrutiny, particularly in relation to enzyme inhibition, is the redox activity of appropriately configured flavonoids. Finally, in a number of cell systems it seems that resting cells are not affected significantly by flavonoids but once a cell becomes activated by a physiological stimulus a flavonoid-sensitive substance is generated and interaction of flavonoids with that substance dramatically alters the outcome of the activation process.

The effects of the bioflavonoid quercetin on squamous cell carcinoma of head and neck origin

Quercetin exhibits antitumor activity. We investigated the effect of quercetin on the in vitro and in vivo growth of two squamous cell carcinoma cell lines and a normal human lung fibroblast-like cell line. The in vivo effect was evaluated using implantable cell growth chambers implanted subcutaneously in immunocompetent rats. Quercetin was injected intraperitoneally, and multiple dosages were tested. Cells were counted on days 1, 3, 5, and 7, and growth curves were constructed. Quercetin caused inhibition of growth in both squamous cell carcinoma lines. Effect on the fibroblast-like human lung cells was noted only at the maximum concentration. Significant growth inhibition of squamous cell carcinoma was observed in implantable cell growth chambers retrieved 3 days after quercetin treatment. Quercetin appears to possess a cytotoxic effect on squamous cell carcinoma of head and neck origin both in vivo and in vitro. The inhibitory effect on malignant cells appears to be selective and dose-dependent.

Some effects of flavonoids on lymphocyte proliferative responses

A number of representative flavonoids reversibly inhibit human lymphocyte proliferative responses in a concentration-dependent manner. The flavonoids quercetin and tangeretin are most effective when added during the early phase of exposure of lymphocytes to the mitogenic stimuli but become progressively less effective when added after increasing lengths of time following stimulation, suggesting an early flavonoid-sensitive step(s) in cell activation. In the proliferative response to phytomitogens, they do not act by inhibiting the early increase in calcium influx. They do not augment cellular cyclic-AMP concentration in basal or phytomitogen-stimulated lymphocytes nor reduce its increment in the presence of inhibitors of phosphodiesterase. At concentrations inhibitory to the proliferative response, quercetin (but not tangeretin) inhibits the calcium-activated, phospholipid-dependent protein kinase (C kinase). Certain flavonoids powerfully inhibit the uptake of thymidine into phytomitogen-stimulated lymphocytes but do not directly affect incorporation of already transported thymidine into newly synthesized DNA.

Comparison of the effects of quercetin with those of other flavonoids on the generation and effector function of cytotoxic T lymphocytes

In previous studies (Schwartz et al., 1982) we showed that the naturally occurring plant flavonoid quercetin can inhibit both the in vitro generation and effector function of alloantigen specific cytotoxic T lymphocytes (CTL). In the present studies, several additional flavonoids of different chemical classes were tested similarly to determine whether structure-function relationships exist. We have found that some other flavonoids, e.g. apigenin , fisetin , hesperetin and chalcone also can inhibit both CTL generation and effector function, with the effective concentration varying with the specific flavonoid tested. On the other hand, flavonoids such as rutin, naringin and catechin were inactive in both systems. Taxifolin ( dihydroquercetin ) differed from all the other flavonoids in that it was a relatively active inhibitor of CTL generation, but was essentially unable to inhibit CTL effector function. The presence of a double bond at position C-2-3 in the flavone and flavonol aglycones, a keto group at C-4, B ring hydroxylation and/or a free hydroxyl group at C-3 may be associated with activity. We also show that the effects of some, but not all, of the flavonoids active in our systems can be blocked by Cu2+ ions. Therefore, chelation of divalent cations such as Cu2+ cannot explain the function of all flavonoids in these systems.

Inhibition by quercetin of thyroid hormone stimulation in vitro of human red blood cell Ca2+-ATPase activity

Human red blood cell membrane Ca2+-ATPase activity is stimulated in vitro by physiological concentrations of thyroid hormone. Quercetin, a flavonoid that inhibits several membrane-linked ATPases, suppressed thyroid hormone action on red cell Ca2+-ATPase activity and also interfered with binding of the hormone by red cell membranes. These effects of quercetin were dose-dependent over a range of concentrations (1-50 microM). In contrast, in the absence of thyroid hormone, quercetin at low concentrations stimulated Ca2+-ATPase activity and at 50 microM inhibited the enzyme. The effects of quercetin at low concentrations (1-10 microM), namely, stimulation of Ca2+-ATPase and inhibition of membrane-binding of thyroid hormone, mimic those of thyroid hormone and are consistent with the thyronine-like structure of quercetin. At high concentrations, quercetin is generally inhibitory of Ca2+-ATPase activity. Chalcone, fisetin, hesperetin and tangeretin are other flavonoids shown to reduce susceptibility of membrane Ca2+-ATPase to hormonal stimulation.

Effect of quercetin on human polymorphonuclear leukocyte lysosomal enzyme release and phospholipid metabolism

Quercetin inhibited in a concentration-dependent manner the release of beta-glucuronidase from human polymorphonuclear leukocytes stimulated with zymosan-activated serum. 3H-arachidonic acid-prelabelled polymorphonuclear leukocytes released 3H-arachidonic acid upon stimulation with zymosan-activated serum and this was associated with a decrease of radioactivity in the phospholipid fraction as determined by thin layer chromatography. Quercetin inhibited the release of 3H-arachidonic acid. These observations suggest that the zymosan-activated serum stimulus activates phospholipase A2 and that phospholipase A2 is inhibited by quercetin. Thus, quercetin alters polymorphonuclear leukocyte phospholipid metabolism and responses to stimulation.