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Immunology and Cell Biology (2000) 78, 31–39 Special Feature Modulatory effects of dietary lipids on immune system functions M A N U E L A D E PA B L O a n d G E R A R D O Á LVA R E Z D E C I E N F U E G O S School of Experimental Sciences, Department of Health Sciences, University of Jaén, Jaén, Spain Summary Dietary lipid manipulation may affect a great number of immune parameters, such as lymphocyte proliferation, cytokine synthesis, natural killer (NK) cell activity, phagocytosis and so on. The immunomodulation induced by dietary fatty acids may be applied in the amelioration of inflammatory disorders, such as autoimmune diseases. However, the mechanisms that participate in these processes are still poorly understood. It is probable that modulation of immune system by fatty acids of the diet may occur by alteration of membrane fluidity, lipid peroxide formation, eicosanoid production or regulation of gene expression. However, recent studies have reported the effects of several free fatty acids on apoptosis induction of in vitro cultures. In fact, a possible explanation of the effects that fatty acids promote on the immune system cells could be associated with an apoptotic process performed in an irreversible way. In vivo studies have demonstrated the ability of fatty acids to alter the survival of animals fed diets containing oils and infected with a pathogenic bacterium. Experimental infection in animals fed dietary lipids produces a modification of resistance to micro-organisms. The present review analyses all of these parameters that dietary fatty acids are capable of altering in order to modify the immune response. Further studies will be needed to establish the mechanisms involved in immune system regulation, reduction of symptoms derived from autoimmune pathologies and so on. Key words: cytokine, dietary lipid, fish oil, immunomodulation, lymphocyte, macrophage/monocyte, monounsaturated fatty acid, n-3 polyunsaturated fatty acid, natural killer cell, olive oil. Introduction According to current understanding, the immune response of both humans and animals may be influenced by several essential nutrients, which modify the immune system functions. In fact, it is generally accepted that many of the important infections of human populations have been associated with a nutritional deficiency. From this point of view, we can affirm that dietary fatty acids may be able to modulate the immune system through several mechanisms that include reduction of lymphocyte proliferation, reduction of cytokine synthesis, phagocytic activity increase, modification of natural killer (NK) cell activity and so on. Such modulation of immune functions may be produced as a consequence of several factors, but in general the main event involved in this process may be associated with changes in the cell membrane due to dietary fatty acid manipulation. Fatty acids may be incorporated into the plasma membrane after dietary lipid administration, so that the composition of lipids in this cellular structure will reflect the composition of dietary lipids.1 Because of this incorporation, the phospholipid profiles associated with plasma membrane of lymphocytes, monocytes/macrophages or polymorphonuclear cells may be altered by dietary lipids.2,3 However, phospholipid changes in the plasma membrane depend on many factors (which affect the modulation of the immune response by fatty acids), such as the amount, type or time of dietary lipid administration; sex; and the species of animals or humans fed dietary lipids. Correspondence: Dr Manuel Antonio de Pablo Martínez, University of Jaén, School of Experimental Sciences, Department of Health Sciences, E-23071-Jaén, Spain. Email: [email protected] Received 26 August 1999; accepted 26 August 1999. Dietary lipids or free fatty acids not only affect the immune system directly, but also interfere in the production of other substances. When fatty acids are released from the plasma membranes or when they are incorporated from the extracellular medium, they may suffer biochemical degradation through the eicosanoid pathways. In fact, fatty acids may also influence eicosanoid production, because they modify the activity of enzymes involved in the synthesis of these substances, which participate as secondary messengers within the cytoplasm. Overall, these events will be responsible for the modulation of immune system functions by fatty acid action. In this scenario, the main substances that participate are the fatty acids, which are divided into two great families. These are classified as essential fatty acids for mammalian cells and should be administered in the diet. Essential fatty acids include the n-3 series, derived from linolenic acid, and the n-6 series, derived from linoleic acid. Both polyunsaturated fatty acids are not interconvertible in animals. Likewise, another family of fatty acids (not essential fatty acids), such as the n-9 series derived from oleic acid (monounsaturated fatty acid), also seems to play an important role in the immunomodulatory process.4,5 As mentioned previously, dietary fatty acid manipulation is involved in immune system modulation and a large number of studies has demonstrated the potential benefits of dietary unsaturated fatty acids on the immune system of both humans and animals. However, it is noteworthy that the n-3 polyunsaturated fatty acids are present in marine oils, because fish oils have been well documented due to their immunosuppressive effects. Classical studies have reported that the n-3 polyunsaturated fatty acid family is able to 32 MA de Pablo and G Álvarez de Cienfuegos reduce lymphocyte proliferation, neutrophil chemotaxis and secretion of several cytokines, such as IL-1, IL-2, TNF and so on.6,7 Subsequently, these and other studies have indicated that several parameters of the immune response are modulated by other unsaturated fatty acids and hence they may be applied in the prevention of several disorders. Thus, an epidemiological study carried out among Greenland Eskimos, who consume large amounts of fish oil in their diets, has demonstrated the effects of n-3 polyunsaturated fatty acids on the immune response. The results from this study indicate a low incidence of inflammatory and autoimmune diseases in this population.8 Therefore, the present review will focus on the current status of the relationship between dietary fatty acids and the immune system, the molecular mechanisms by which dietary lipids or free fatty acids exert their effects on the immune system and the possible application of dietary fatty acids contained in the oils as substances capable of acting in the prevention of autoimmune disorders or as agents that promote the modulation of the immune system in order to efficiently eliminate different micro-organisms or biological agents involved in infectious processes. Modulatory effects of lipids on immune cell functions Influence of fatty acids on lymphocyte proliferation Several studies have demonstrated the ability of unsaturated fatty acids to reduce the proliferation of T lymphocytes. Such studies have been performed in both animals and humans fed dietary lipids or have been carried out in cellular cultures incubated with free fatty acids. Therefore, it should be noted that diets containing polyunsaturated fatty acids, such as eicosapentaenoic or docosahexaenoic acids, suppress the mitogenic response of lymphocytes to a greater extent than diets rich in saturated fatty acids.9 However, another fatty acid, oleic acid (a monounsaturated fatty acid), also plays an important role in this process.4 Despite the fact that oleic acid (the most important fatty acid in olive oil) is involved in the immunomodulatory process, its regulatory effect on immune functions is lower than that carried out by n-3 polyunsaturated fatty acids, such as eicosapentaenoic or docosahexaenoic acids (the most important fatty acids in fish oil).9 Mitogen-stimulated lymphocytes from mice fed a diet containing olive oil show a significant reduction in lymphocyte proliferation in comparison with mitogen-stimulated lymphocytes from mice fed sunflower oil or hydrogenated coconut oil,4 but this reduction is lower than that found from diets containing eicosapentaenoic or docosahexaenoic fatty acids.9 Neverthless, a recent study has examined the effects of dietary docosahexaenoic acid (in absence of eicosapentaenoic acid) on the immune response of human volunteers and has reported that docosahexaenoic acid consumption does not inhibit many of the lymphocyte functions modified by fish oil consumption.10 This fact indicates that a diet containing fish oil modulates the immune response by the action of eicosapentaenoic acid rather than docosahexaenoic acid. This argument suggests that it is necessary to find the lipid component responsible for immune system modulation from diets containing different fatty acids. The effect of dietary lipids on lymphocyte proliferation also depends on lymphocyte subpopulations. A comparative study in rats fed diets containing unsaturated fatty acids has shown that there is a significant difference between the lymphocyte suppression found in spleens isolated from rats and suppression observed in lymphocytes from thymi of rats, in which lymphocyte proliferation is much lower.11 The fatty acid composition of lymphocytes that recirculate by the lymphatic system is affected by lipids and the immunological function of these lymphocytes is also influenced by dietary lipid manipulation.12 Ex vivo studies have suggested that the effect of fatty acids on the immune system depends on the fatty acid concentration. In fact, several studies have reported a significant reduction of lymphocyte proliferation in mitogen-stimulated lymphocytes isolated from both humans and animals fed diets containing a high concentration of unsaturated fatty acids belonging to the n-3 or n-9 fatty acid series.4–6,11,13 Diets containing a high concentration of olive oil have been administered to experimental animals and the hypothetical effects of olive oil on immune cell function previously reported could be related first to the oleic acid action and second to other components isolated from the olive oil. However, it has been reported recently that the immunosuppressive effects of olive oil incorporated into diets are most likely related to oleic acid action rather than to other components of olive oil.5 Regardless of the effect of fatty acid concentration on lymphocyte function, other authors have pointed out that the reduction of lymphocyte proliferation by n-3 polyunsaturated fatty acids, such as eicosapentaenoic or docosahexaenoic acid, has been shown to be equipotent,14 although sometimes the results obtained depend on the type of serum used in the cellular cultures.11,15 Hence, many authors have reported contradictory results, because the previously explained effects have been demonstrated when cells were cultured in autologous serum.16 However, when cells were cultured in foetal calf serum, contradictory results appeared or no effect at all was found.17 Table 1 reflects the discrepancies found regarding immunomodulation and dietary fatty acids in both humans and animals. It is interesting to remark here on the biological consequences in the clinical application of the lymphocyte proliferation regulation by fatty acids. As we will detail in the last section of the present review, these processes seem to play a fundamental role in the amelioration of patients suffering from autoimmune diseases who are characterized by an overactivation of the immune system, such as rheumatoid arthritis,18 psoriasis,19 multiple sclerosis20 and so on. Effects of fatty acids on cytokine production Fatty acids may regulate cytokine production and in fact cytokine modulation by fatty acids seems to be responsible for the reduction of lymphocyte proliferation in both animals and humans. This affirmation is based on the study carried out by Soyland et al., who found that the inhibitory effects of n-3 polyunsaturated fatty acids may, at least in part, be mediated by the inhibition of the expression of the CD25 molecule that constitutes an IL-2 receptor.21 However, other studies have reported that the mechanisms by which n-3 fatty acids exert suppression of cytokine synthesis remain unknown. Cytokines such as IL-1 and TNF are important mediators of Dietary fatty acids and immunomodulation Table 1 33 Influence of different families of fatty acids on immune system functions in both experimental animals and humans Immune functions Dietary fatty acids n-6 n-3 Lymphocyte proliferation IL-1 production IL-2 production TNF production NK cell activity ↓ Humans and animals4,6,11,13 ↓ Patients suffering from rheumatoid arthritis18 ↓ Human volunteers6,13 ↓ Humans and animals13,21,87 ↓ Humans6 ↑ Macrophages from mice24 ↓ Rats16 ↓ Humans37,38 NE Rats12,16 NE Rats28 ↑ Mice87 ↓ Rats28 ↓ Rats35 n-9 ↓ Rats5 ↓ Patients suffering from rheumatoid arthritis18 ↓ Balb/c mice23 NE Rats87 ↓ Mice23 NE Balb/c mice23 ↓ Balb/c mice4 ↓ Rats35 ↑, Increase; ↓, decrease; NE, no effect. inflammation and dietary fatty acids have been demonstrated to be substances capable of reducing the pro-inflammatory response induced by IL-1 and TNF.6,22,23 The effects promoted by n-3 (present in fish oil) fatty acids on IL-1 and TNF production in human volunteers fed a diet containing these fatty acids have demonstrated a significant reduction in these cytokines. Moreover, the levels of these biological substances returned to baseline values after stopping administration of the diet.6 In contrast, the reduction of cytokine production, such as IL-1 and IL-2, promoted by a diet containing olive oil depends on the time of dietary administration.23 Nevertheless, many studies have reported contradictory effects of dietary fatty acids on pro-inflammatory cytokine production22–25 and these discrepancies may be associated with the different polyunsaturated fatty acid actions on cell populations of various species.26 The mechanisms involved in the modification of cytokine synthesis remain unclear as yet, but a possible explanation could be found in the regulation at the transcriptional level, that is, reduction of cytokine mRNA production by polyunsaturated fatty acids.27 Animal studies have suggested that the effects of dietary fatty acids on production of macrophage-derived cytokines are different and that these depend on several factors, such as duration of feeding, the species of experimental animal, the type and state of activation of macrophages and so on.25,28–31 For example, fish oil and olive oil are capable of suppressing IL-1 production after 4 weeks of dietary supplementation in rats, whereas olive oil enhances IL-1 production after 8 weeks of dietary administration.31 The regulation of other cytokines by dietary fatty acids is also very important. In fact, the cytokine IL-10 suppresses LPS-induced production of pro-inflammatory cytokines. In this way, a recent study has demonstrated low TNF production in mice fed a diet containing coconut oil, whereas IL-10 production is significantly increased.32 Dietary fats also modulate IFN-γ production in assays in vivo during active infection with Listeria monocytogenes in fish oil-fed mice. Results from this study have described a higher concentration of this cytokine in serum compared with results from mice fed soybean oil or lard,33 but in vitro studies have not demonstrated a significant change in IFN-γ production. This discrepancy may be explained by modification of ΙFN-γ receptors, which have been reported recently to be reduced by dietary n-3 polyunsaturated fatty acids.34 Modulation of natural killer cell activity by fatty acids Natural killer cells participate in protection against virus, intracellular bacteria or tumoral cells. Activity of NK cells is also modulated by dietary fatty acids.4,35 Previous studies in animals have demonstrated that dietary lipids reduce the activity of these cells. In fact, these authors have described that a decrease in the total fatty acid consumption in the diet produces an increase in NK cell activity.36 Animal studies have revealed that diets containing fish oil or olive oil produce the greatest percentage of NK cell activity inhibition in comparison with diets rich in saturated fatty acids or n-6 polyunsaturated fatty acids.4,16,35 However, few studies have investigated the influence of dietary lipids on human NK cell activity, although it has been demonstrated that intravenous injection of triacylglycerol containing eicosapentaenoic acid is related to the suppression of peripheral blood NK cell activity from human volunteers.37 In addition, a diet containing docosahexaenoic acid has been demonstrated to reduce NK cell activity from humans, as reported by a recent study.38 Several studies have suggested different mechanisms by which fatty acids modulate NK cell activity, reporting that NK cell activity is modulated by production of eicosanoids such as prostaglandins,39 leukotrienes40 or lipoxins.41 However, the mechanism involved in the fatty acid modulation of NK cell activity remains unknown, because a significant reduction of prostaglandin E 2 and leukotriene B4 production has been recently reported in cells from humans fed a diet containing docosahexaenoic acid, which produced a significant decrease in NK cell activity.38 Effects of fatty acids on phagocytic activity and on inflammatory response Phagocytosis is an important mechanism in many cells for the elimination of micro-organisms or foreign particles, mediated by production of endocytic vesicles. Therefore, membrane fluidity plays an important role in this process. Plasma membranes of macrophages and other phagocytic 34 MA de Pablo and G Álvarez de Cienfuegos Figure 1 Schematic representation of different mechanisms of regulation of the immune system by dietary fatty acids. NFκB, nuclear factor kappa B; DCm, mitochondrial transmembrane potential. cells may be altered by dietary lipids. In fact, several studies have reported that unsaturated fatty acids increase phagocytosis.23,42 Peritoneal macrophages stimulated with zymosan particles have shown a significant increase of phagocytosis in cells isolated from mice fed diets enriched in olive oil23 and polyunsaturated fatty acids as eicosapentaenoic or docosahexaenoic acids.42 This effect may be due to the increase of hydrogen peroxide production, which enhances superoxide release.43 Again, eicosanoid production could be involved in this process; however, in vitro studies have suggested that eicosanoid inhibitors have no effect on alterations of chemiluminescence of peritoneal macrophages incubated with either arachidonic or eicosapentaenoic acid.44 Animal studies offer some results that reveal the effects of n-3 fatty acids on phagocytosis. Dietary n-3 fatty acids inhibit45 or do not affect46 phagocytosis promoted by macrophages, although this function has not been demonstrated in humans. In monocytes, a reduction of stearic and arachidonic acid and an increase of eicosapentaenoic and docosahexaenoic acid content have been associated with a reduction in superoxide production, as well as the action of other free radicals.47 Chemotaxis has also been reduced by dietary n-3 polyunsaturated fatty acids in a study carried out for long-term supplementation.48 In contrast, short-term supplementation and a low concentration of n-3 polyunsaturated fatty acids do not show any significant effect on monocyte chemotaxis.49 This effect seems to be attributed to a deficiency of docosahexaenoic acid,49 although, as previously mentioned, a recent study has revealed that this polyunsaturated fatty acid does not promote any inhibitory effect on different functions of immune system.10 Mechanisms involved in fatty acid modulation As discussed previously, the quantity and nature of lipids are important factors in the process of immune system modulation. Several mechanisms have been involved in the modulation of the immune system by fatty acids: (i) membrane fluidity; (ii) production of lipid peroxides; (iii) eicosanoid synthesis; and (iv) influence on gene regulation. It is possible that fatty acid immunomodulation occurs not only by the individual, but also by the collective action of these factors. Figure 1 shows the different mechanisms proposed by which dietary lipid manipulation or free fatty acids added into cellular cultures may modulate the immune system. Membrane fluidity As a consequence of changes in the phospholipid fatty acid composition due to dietary lipid manipulation, the fluidity of the cell membrane may change. Fatty acids from dietary lipids may be incorporated into any of the different phospholipids within the plasma membrane and they are clearly altered by the availability of dietary lipids. The changes in fatty acid composition of this structure have great importance because of the alteration of plasma membrane characteristics. This fact may be attributed to changes produced in the activity of proteins associated with the membrane, which act as receptors, form ion channels or are related to enzymatic functions. As a result, binding of cytokines to their respective receptors on the cell membrane surface may be altered.50,51 Specific macrophage functions may also be altered by lipids, mainly due to changes in membrane fluidity. In fact, unsaturated fatty acid incorporation is associated with an increase in the phagocytosis of zymosan particles and the correlation between phagocytosis and membrane phospholipid has been demonstrated to be very high.42 It is possible that the changes in the expression of surface proteins may be due to a vertical displacement of the membrane proteins by lipid action.52 Expression of surface molecules such as adhesion and major histocompatibility molecules from human monocytes are inhibited by eicosapentaenoic acid.53 Instead, a significant increase in the expression of human lymphocyte antigen (HLA) is observed in monocytes incubated with docosahexaenoic acid.53 Dietary fatty acids and immunomodulation Lipid peroxidation The inhibitory effects of fatty acids on cellular proliferation may be related to lipid peroxidation. Lipid peroxides are toxic to cells; in fact, polyunsaturated fatty acids derived from the n-3 family are incorporated into the cell membrane and increase the requirements for antioxidant nutrients, such as tocopherol, which protects membrane polyunsaturated fatty acids from lipid peroxidation.54 Several reports, however, have suggested that the lipid peroxidation is not responsible for the inhibitory effects of fatty acids on lymphocytes because the addition of antioxidant substances does not avoid the suppression of mitogen lymphocyte proliferation performed by fatty acids.55,56 Dietary fish oil has been described as enhancing antioxidant enzyme mRNA levels and decreasing free radical-induced tissue damage in vivo.57 Hence, this argument (which points out the possibility of increasing the activity of antioxidant enzymes) and the importance of incorporating an antioxidant, such as tocopherol, in diets containing fish oil produce the reduction of thiobarbituric acid-reactive substance (TBARS) production.58 In addition, polyunsaturated fatty acids are more sensitive to lipid peroxidation than are monounsaturated or saturated fatty acids. Incorporation of polyunsaturated fatty acids into membranes increases lipid peroxidation and this affects the expression of surface molecules. This fact may be related to the suppression of HLA-DR expression due to free radical production.59 This argument is confirmed by a recent study reporting that dietary supplementation with β-carotenes increases the expression of histocompatibility and adhesion molecules on human peripheral blood monocytes.60 Eicosanoid production Classical studies have described that several members belonging to the eicosanoid family derived from arachidonic acid participate in inflammatory processes and are also related to immunomodulatory effects.61 Fatty acids are released from the phospholipids that compose the plasma membrane and undergo enzymatic degradation to yield eicosanoids, which act as lipid mediators. For example, prostaglandin E2 or leukotriene B4 (derived from arachidonic acid) exert a great number of functions, such as modulation of lymphocyte proliferation,62 cytokine production63,64 and cytotoxicity.40 However, dietary n-3 polyunsaturated fatty acids reduce the levels of arachidonic acid in the cell membrane and as a consequence, the levels of prostaglandins derived from arachidonic acid are reduced. The products derived from n-3 fatty acids (as eicosapentaenoic acid) show different biological properties in comparison with those derived from arachidonic acid. Prostaglandin E3 is derived from eicosapentaenoic acid and several studies have demonstrated that in vitro mitogen-stimulated lymphocyte proliferation is inhibited by the action of this prostaglandin.62 Similarly, leukotriene B5 is also derived from eicosapentaenoic acid and it is generally assumed that its biological activities are lower than leukotrienes derived from arachidonic acid. However, leukotriene B5, as well as prostaglandin E3, are more potent than eicosanoids derived from arachidonic acid in inhibiting lymphocyte proliferation.62 In contrast, assays carried out with inhibitors of eicosanoid synthesis have 35 reported that these substances are unable to retain the inhibitory effect of fatty acids exerted on lymphocyte proliferation. Based on this fact, several authors have suggested that unsaturated fatty acids inhibit lymphocyte proliferation by a mechanism independent of eicosanoid synthesis.65,66 Regardless of these observations, eicosanoids regulate a great number of inflammatory effects, such as induction of fever, vasodilatation and production of macrophage- and lymphocyte-derived cytokines64 and their effects depend on each precursor. This is because, as discussed previously, eicosanoids produced from eicosapentaenoic acid show different biological activities than eicosanoids produced from arachidonic acid. Regulation of gene expression In addition to participation in the production of eicosanoids (which also modulate signal transduction through an indirect pathway), fatty acids are of major relevance because they are key determinants of membrane-bound enzyme and receptor expression. The influence of fatty acids on gene expression is still poorly understood. Nevertheless, fatty acids released from membrane phospholipids are important second messengers or substitute for the classical second messengers, such as inositide phospholipid or cyclic AMP signal transduction pathways, because they act in a reversible manner at a precise intracellular location for a very short time in order to amplify, attenuate or deviate a signal in a direct or indirect (by conversion from arachidonic acid to eicosanoids) pathway.67 Several studies have shown that administration of fish oil in the diet of autoimmune disease-prone mice produces an increase in the levels of mRNA for several cytokines, such as IL-2, IL-4 and transforming growth factor (TGF)-β.68 In the same way, dietary fish oil reduces mRNA production of IL-1, IL-6 and TNF-α.69 These findings confirm the important role of fatty acids as substances capable of modifying and influencing signal transduction pathways, mainly through modulation of protein kinase C (PKC) activity.70 In vitro and in vivo studies of fatty acids as inducers of apoptosis Irrespective of the data that support the important role of the previously cited mechanisms involved in the modulation of the immune system by fatty acids, recent studies have reported the effects of several fatty acids on apoptosis induction of in vitro cultured cells. Palmitic acid (a saturated fatty acid) induces cell death by dissipation of the mitochondrial transmembrane potential (∆ψm), the process that precedes nuclear apoptosis. These data show that palmitate-induced apoptosis involves a direct effect on mitochondria and an important regulation by anti-apoptotic proto-oncogene product Bcl-2 occurs.71 In fact, a cell-free system assay has demonstrated that the induction of apoptosis occurs via a direct effect on mitochondria, so that palmitate causes the release of soluble factors capable of inducing the apoptosis of isolated nuclei.71 Another study has demonstrated that palmitic and stearic acid induce apoptosis by de novo synthesis of ceramide and, in fact, etomoxir (an inhibitor of mitochondrial protein as carnitine palmitoyl transferase, CPT I) enhances palmitate-induced cell death. As a consequence of 36 MA de Pablo and G Álvarez de Cienfuegos this fact, an increase of ceramide was reported,72 although some discrepancies have been observed because, as mentioned previously, it is probable that palmitate-induced apoptosis occurs by a direct effect on mitochondria (not mediated via ceramide).71 Another fatty acid, eicosapentaenoic acid, has been studied as an apoptosis inducer. This fatty acid promotes apoptosis induction in monocytic U937-1 cells.73 In contrast, docosahexaenoic acid reduces the apoptotic effects induced by sphingosine (an important second messenger). In this case, the inhibition of cytosolic phospholipase A2 could be involved in the inhibitory activity of this polyunsaturated fatty acid.74 In addition to influence on in vitro cultures, dietary fatty acids may induce apoptosis, because a recent study has demonstrated that splenic cells from mice fed hyperlipidic diets containing saturated fatty acids were more susceptible to death by apoptosis than splenic cells from mice fed unsaturated fatty acids.75 It is noteworthy that these findings may provide another mechanism by which fatty acids promote immune system modulation. In this discipline, further studies will be needed in order to know more about the implication of fatty acids as inducers of apoptosis, as well as the molecular mechanisms involved in these processes. Modulation of the immune system by fatty acids modifies the response to bacterial or viral infection Clinical studies have demonstrated that experimental animals fed with diets containing a high concentration of unsaturated fatty acids show a reduced resistance to different infections or improve their immune response to micro-organisms. Survival studies have suggested a direct implication of several fatty acids in modulation of the immune response in order to produce an amelioration effect for both humans and animals suffering from inflammatory disorders. However, these effects were inefficient in the treatment of infectious diseases. Experimental infection has been carried out in animals fed dietary lipids in order to determine the ability of each oil to promote changes in the modulation of the immune response. In these studies, contradictory results have been found again. Whereas eicosapentaenoic, docosahexaenoic or oleic fatty acids may be applied to the amelioration of autoimmune disorders,18–21 several fatty acids have been shown to be inefficient because they impair resistance to experimental infection.33,76 Diets supplemented with eicosapentaenoic and docosahexaenoic acids (present in fish oil) affect resistance to Mycobacterium tuberculosis.76 Dietary fish oil reduces survival against L. monocytogenes and it also impairs the clearance of this bacterium from the spleen,33 while diets containing saturated fatty acids help to eliminate micro-organisms such as L. monocytogenes more efficiently than diets containing unsaturated fatty acids.33 In the same way, infection of mice fed a diet rich in fish oil with Salmonella typhimurium has been reported to be more severe, because of a reduction in these mice in resistance to infection by this bacterium.77 In contrast, other studies have reported that a fish oil-enriched diet increases mice survival following Klebsiella pneumoniae infection25,78 or that a reduction in development of parasites as Eimeria tenella occurs in chickens fed a diet containing n-3 fatty acids.79 Therefore, as we have mentioned earlier, several studies have observed that dietary polyunsaturated fatty acids may promote host resistance against certain infectious pathologies,33,76 whereas others have suggested that polyunsaturated fatty acids participate in the amelioration of these diseases.25,78 Finally, many studies have reported the effects of n-3 fatty acids on patients infected with human immunodeficiency virus. These fatty acids have been applied as coadjuvant treatment administered to these patients. Results from this study have demonstrated an increase in survival and a reduction in inflammatory responses.80 In the same way, a fish oil-enriched diet has also been described as able to increase significantly survival of murine acquired immunodeficiency syndrome in mice fed this diet.81 Dietary fatty acids and inflammatory disorders Activated T lymphocytes play an important role in the pathogenesis of many inflammatory diseases. The inflammatory process is the physiological response of the organism to stimuli promoted by micro-organisms or immunological mechanisms. Measurements of IL-2 production derived from overactivity of T lymphocytes, B lymphocyte activation and an increase in IL-2 receptor levels have produced contradictory results in patients suffering from these pathologies.82,83 Patients suffering from autoimmune disorders have been fed diets containing eicosapentaenoic, docosahexaenoic or oleic acid. The results obtained have revealed a significant amelioration of the inflammatory processes in these patients. In fact, patients suffering from rheumatoid arthritis fed with n-3 polyunsaturated fatty acids or olive oil showed a great amelioration of typical symptomatology caused by this disease.18 Psoriasis is another autoimmune disease characterized by elevated levels of arachidonic acid and products derived from 5-lipoxygenase. Therefore, the dietary supplementation of n-3 polyunsaturated fatty acids should reduce the levels of these substances and, in particular, eicosapentaenoic acid has been demonstrated as an effective subtance in the treatment of this disease.19,55 In patients affected by multiple sclerosis, abnormalities in the fatty acid intake have been described84 and a controlled trial in multiple sclerosis patients treated with n-3 polyunsaturated fatty acids for a very long period has shown some beneficial effects for these patients.20 Clinical studies have also demonstrated the importance of the dietary supplementation of n-3 fatty acids in patients suffering from systemic lupus erythematosus. A significant reduction in the symptomatology of lupus was reported after dietary lipid administration.82,83 Knowledge of how fatty acids promote immune regulation and the function of cells involved in host defence will lead to the development of treatments that may be applied to patients affected by diseases characterized by an overactivation of the immune system. However, the beneficial aspect of dietary fatty acids on the reduction of inflammatory disorders in these patients must be balanced by the fact that at the same time they may impair the ability to efficiently eliminate infectious micro-organisms. Concluding remarks The nutritional status of both humans and animals has been related to alteration of the immune response. Thus, numerous Dietary fatty acids and immunomodulation studies have described that several dietary fatty acids or free fatty acids are involved in the modulation of the immune system through mechanisms that modify the immune response. The immune parameters susceptible to modification by fatty acids supplied in the diet or free fatty acids added into cellular cultures are lymphocyte proliferation,9,13 cytokine production,4,6,22,23 activity of NK cells,4,35,38 phagocytosis,23,42,47 expression of markers in the surface of the cells52 and so on (Table 1). The immune system modulation induced by fatty acids depends on several biological and methodological factors, such as the type and concentration of fatty acids, cell types, species of experimental animals, serum used in the ex vivo or in vitro cultures and so on. Thus, the effects of dietary fatty acids or free fatty acids on the immune response of ex vivo or in vitro cultures are complex, because they depend on many factors and several discrepant results have been reported.85,86 Actually, many mechanisms involved in modulation of the immune system by fatty acids are unknown or poorly understood, although current understanding allows us to point out several factors that seem to participate in this process. These factors are: (i) membrane fluidity, which is modified as a consequence of changes in phospholipid composition; (ii) formation of lipid peroxides, which are toxic to the cells; (iii) eicosanoid production from long-chain fatty acids, showing different biological properties depending on their precursors; and (iv) influence on the regulation of the expression of genes encoding proteins that participate in cellular responses (Fig. 1). The mentioned immunomodulation associated with fatty acids may be applied to the amelioration and prevention of diseases characterized by an overactivation of the immune system, such as inflammatory or autoimmune disorders.18–21 In addition to these applications, unsaturated fatty acids may be used to reduce the susceptibility against bacterial infections due to the possibility of several lipids playing important roles in the efficient elimination of different micro-organisms or reducing host resistance against an infectious process due to the immunosuppression promoted by dietary unsaturated fatty acids. It should be noted that immunosuppression promoted by different dietary lipids has to be balanced, because this process may lead to an impairment of host response33,76 and increase the risk of infections.35 Recently, several reports have described the important role of fatty acids as apoptosis inducers or as substances capable of reducing cell death in cellular cultures.71–75 It is very important to determine the role of free fatty acids in programmed cell death as a possible mechanism of action in the modulatory activity attributable to fatty acids. Overall, the studies carried out show that fatty acids play an important role in immune system regulation through complex mechanisms and therefore they may be classified as substances capable of participating in the modulation of immune response. These substances could be used as therapy for the treatment of inflammatory disorders. References 1 Clamp AG, Ladha S, Clark DC, Grimble RF, Lund EK. 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