The Impact of Macrophage Membrane Lipid Composition on Innate Immune Response Mechanisms

Most microorganisms that are encountered in the daily life of a healthy individual are detected and destroyed within minutes or hours by the defence mechanisms of the innate immunity. Macrophages play a key role in innate immunity because they can recognize, ingest and destroy many pathogens without the aid of an adaptive immune response. Macrophages recognize pathogens via cell-surface receptors that can discriminate between the surface molecules displayed by pathogens and those of the host (Bryant & Fitzgerald, 2009). Binding of bacteria to macrophage receptors stimulates phagocytosis and uptake of pathogens into intracellular vesicles, where they are destroyed (Groves et al., 2008). Upon phagocytosis macrophages produce a variety of toxic products that help to kill the engulfed microorganisms. The most important of these are antimicrobial peptides, nitric oxide (NO), the superoxide anion (O2-), hydrogen peroxide (H2O2), the hydroxyl radical (·OH), hypochlorite (OCl-) and hypobromide (OBr-), which are directly toxic to bacteria (Pourova et al., 2010; Robinson, 2009). In addition, activation of macrophage receptors triggers the production of pro-inflammatory cytokines and chemokines (including IL-1┚, IL-6, TNF-┙ and CXCL8) (Hamilton et al., 1999) and the expression of co-stimulatory molecules such as B7.1 and B7.2 (Glaros et al. 2009), which orchestrates immune responses. Fatty acids are indispensable to life for all organisms. They serve as a source of metabolic energy providing twice the energy density compared to carbohydrates or proteins. In addition, fatty acids are an integral part of cellular membranes. The heterogeneity of fatty acids in the membrane contributes to membrane fluidity as well as to the physical and chemical properties of various membrane domains (Stillwell & Wassall, 2003; Wassall & Stillwell, 2008). Numerous cellular functions critically rely on the dynamic characteristics of the membranes. Thus, fatty acids are important for signalling mechanisms as well as catalytic processes by membrane-associated enzymes (Stillwell & Wassall, 2003; Wassall & Stillwell, 2008). The oxygenated derivatives of some fatty acids include prostaglandins, prostacyclins, thromboxanes and leukotrienes, lipoxins, resolvins, protectins and marensins (Bannenberg & Serhan, 2010; Kohli & Levy, 2009; Löffler et al., 2007; Norling & Serhan, 2010). This group of biological messenger substances mediate numerous and diverse actions especially in the immune system (Bannenberg & Serhan, 2010; Kohli & Levy, 2009; Löffler et al., 2007; Norling & Serhan, 2010).

orchestrating the immune response (Russel et al., 2009).At this, macrophages trigger the initiation of inflammatory processes via secretion of pro-inflammatory signal proteins, which activates further immune cells (Hamilton et al., 1999).A special function of macrophages is the clearance of the body from dead cells and cell debris (Russel et al., 2009).Macrophages express several cell-surface receptors, which can discriminate numerous bacterial components as bacterial carbohydrates (mannose receptor, glucan receptor), bacterial lipids (LPS receptors) as well as other structures typically found on pathogen surfaces (Toll-like receptors, scavenger receptors) (Bryant & Fitzgerald, 2009).The binding of a bacterium to a macrophage receptor stimulates the engulfment of the pathogen into intracellular vesicles, a process known as phagocytosis (Groves et al., 2008) (figure 2).The phagocytosis is an active process.Initially, the receptor-bound pathogen is surrounded by the membrane of the phagocyte.This is followed by an internalisation of the pathogen into an intracellular vesicle, the so called phagosom.The destruction of the microorganism in the phagosom occurs via lowering of the pH (Haas, 2007).Furthermore, the phagosom fuses with lysosomes, which enables the killing of the pathogen via lysosomal enzymes (Haas, 2007).Macrophages produce a variety of toxic substances to kill engulfed microorganisms (figure 3).This includes antimicrobial peptides, nitric oxide (NO), the superoxide anion (O 2 -) and hydrogen peroxide (H 2 O 2 ) (Pourova et al., 2010;Robinson, 2009).Nitric oxide is generated by the inducible nitric oxide synthase (iNOS) (Pourova et al., 2010;Robinson, 2009).Superoxide is synthesized by a multicomponent, membrane-associated NADPH oxidase in a process known as the respiratory burst, and further converted by the enzyme superoxide dismutase into H 2 O 2 (Pourova et al., 2010;Robinson, 2009).Based on H 2 O 2 a range of toxic chemicals, including the hydroxyl radical ( • OH), hypochlorite (OCl -) and hypobromide (OBr -) are produced by chemical and enzymatic reactions (Pourova et al., 2010;Robinson, 2009).Moreover, the activation of macrophages receptors promotes the expression of costimulating molecules as B7.1 and B7.2 (Glaros et al. 2009).

Inflammation and bacterial resistance 2.1 Characteristics of an inflammation
An inflammation is defined as a local accumulation of fluid, plasma proteins and leucocytes caused by physical injuries, infections or locale immune reactions (Johnston et al., 2007).Characteristics of an inflammation are redness, swelling, heat and pain.During an acute inflammation the host actively fights invading microorganisms.This includes the inhibition of infection spread by means of physical barriers and the activation of the complement system (Johnston et al., 2007).The process is triggered by macrophage secreted cytokines (Johnston et al., 2007).

Bacterial resistance mechanisms
Several pathogens have evolved resistance mechanism to escape the immune defence (Johnston et al., 2007).Such persistent microorganisms are the cause of chronic infections.A chronic infection is a protracted process.Characteristics are an ongoing tissue damage accompanied with permanent proliferation processes (Germolec et al., 2010).Bacterial resistance mechanisms are important factors of virulence, which have a significant impact on infection outcome.Via interaction with the immune system pathogens are able to modulate the defence mechanisms of the host thereby influencing the severity of disease (Veesenmeyer et al., 2009).Both the rate of pathogen elimination as well as the scale of tissue damage can be manipulated by the microorganisms.There is a multitude of bacterial resistance mechanisms.Typical examples are listed in table 1.The impermeability of the membrane and the active transport of antimicrobial substances out of the cell impair the accumulation of antibiotics in the pathogens inside.Bacterial resistances against -lactam antibiotics, fluoroquinolones, tetracyclines and aminoglycosides have been reported to be based on these mechanisms (Strateva & Yordanov, 2009).(Veesenmeyer et al., 2009;Strateva & Yordanov, 2009) Via biofilm formation bacteria are able to colonise on all types of animal tissue, plants and inert surfaces (Boyen et al., 2009;Veesenmeyer et al., 2009).Furthermore, the resistance of the microorganisms against environmental conditions is improved.Biofilm formation hampers the killing of the pathogens by immune defence mechanisms or antimicrobial substances (Boyen et al., 2009;Veesenmeyer et al., 2009).As a consequence there are chronic infections, which are hard to treat by the means of antibiotics (Boyen et al., 2009;Veesenmeyer et al., 2009).Quorum sensing is a communication mechanism, which is used by bacteria for collective coordination (Boyen et al., 2009).The microorganisms produce chemical signalling molecules as polypeptides (auto-inducing polypeptides.AIPs), N-acetyl-homoserine-lactone (auto inducer-1, AI-1) or furanosyl-borate-ester (auto inducer-2, AI-2) and secret them into the environment (Boyen et al., 2009).When exceeding a critical concentration the signal molecules modulate gene expression of the bacteria thus synchronising the behaviour of the whole population (Boyen et al., 2009).By this way quorum sensing promotes biofilm formation and manipulates virulence, mobility and sporulation of the microorganisms (Boyen et al., 2009).

Examples 2.3.1 Rhodococcus equi
Infections of immunocompetent humans with R. equi are rare.However, in case of immunesuppressing conditions as transplantations, cancer treatment and steroid medication pneumonia caused by R. equi have been observed (Kamboj, et al., 2005;Prescott, 1991).HIV positive people are particularly endangered.The mortality rate of infected AIDS patients is reported to be about 55% (Bell et al., 1998).The facultative intracellular bacterium is known to survive extreme environmental conditions as low pH (Benoit, 2000) or oxidative stress (Benoit, 2002).In addition, R. equi blocks the process of phagosom maturation thus surviving and proliferating within macrophages (Fernandez-Mora et al., 2005).Internalised viable R. equi organisms prevent the fusion of their phagosom with lysosomes leading to an arrested phagosom neutral in pH and without lysosomal contents (Fernandez-Mora et al., 2005).The underlying mechanisms are not known, so far.

Pseudomonas aeruginosa
P. aeruginosa is one of the leading hospital bugs in the world.According to estimates the microorganism accounts for 10 to 15% of all nosocomial infections (Blanc et al., 1998).P. aeruginosa colonises burns and wounds as well as the respiratory tract and the urinary tract of immunocompromised individuals (Trautmann et al., 2005).Due to the natural resistance of the opportunistic pathogen against numerous antibiotics P. aeruginosa infections are often hard to treat (Strateva & Yordanov, 2009).Moreover, the microorganism has a remarkably ability to acquire further resistance mechanisms against antimicrobial substances by the means of mutations (Strateva & Yordanov, 2009).Another notorious characteristic of P. aeruginosa is the biofilm formation (Boyen et al., 2009).The colonisation of surfaces of surgical implants, endotracheal tubes, catheters as well as the respiratory tract of individuals suffering from cystic fibrosis becomes an increasing medical problem.Further virulence factors of P. aeruginosa include the secretion of a number of toxins and the expression of flagella and type IV pili, which are of importance for surface attachment (Veesenmeyer et al., 2009).Two quorum sensing mechanisms of P. aeruginosa have been identified, so far: Las and Rhl (Veesenmeyer et al., 2009).A Pseudomonas quinolone signal (PQS) acts as mediator between the two systems (Pesci et al., 1999).Quorum sensing controls about 350 genes of P. aeruginosa (approximately 6% of the entire genome) modulating toxin synthesis and biofilm formation (Veesenmeyer et al., 2009).

Fatty acids
Fatty acids are composed of a hydrocarbon chain and a carboxyl group.Most naturally occurring fatty acids have a chain of an even number of carbon atoms.Fatty acids may be saturated or unsaturated depending on the existence of double bonds.In almost every unsaturated fatty acid the double bonds are in cis configuration (Löffler et al., 2007).This means that the adjacent hydrogen atoms are on the same side of the double bond resulting in a rigid binding, which restricts the conformational freedom of the fatty acid.The more cis double bonds a chain has, the less flexible it is.Thus, cis bonds limit the ability of a fatty acid to be closely packed (Stillwell & Wassall, 2003).Since fatty acids are parts of triglycerides in lipid droplets as well as of phospholipids in lipid bilayers the number of cis bonds of a fatty acid has an impact on basic physical properties of fat or biological membranes (Stillwell & Wassall, 2003).Depending on the position of the double bonds unsaturated fatty acids are classified in several fatty acid families: n3, n6, n7 and n9.This terminology is based on the first double bond relative to the end of the hydrocarbon chain.For example, the term n3 signifies that the first unsaturation exist at the third carbon-carbon bond from the terminal methyl end of the chain.A transformation of a fatty acid from one family to another is not possible (Löffler et al., 2007).Animals and humans lack the ability to introduce double bonds in fatty acids beyond carbon 9. So, the fatty acids linoleic acid (C18:2n6) and linolenic acid (C18:3n3) are essential to the animal and human organisms (Löffler et al., 2007).Important examples of the n3, the n6, the n7 and the n9 family are listed in table 2. Fatty acids are of great importance as membrane compound and in energy metabolism in the animal and human organisms.They can be found both as free fatty acids and as parts of acylglycerols, phospholipids, sphingolipids and cholesterol esters.Furthermore, some fatty acids serve as precursors of eicosanoid synthesis.Table 2. Important examples of the n3, the n6, the n7 and the n9 fatty acid family

Cellular membranes
Cellular membranes are essential to life for all living beings.In prokaryotic organisms the plasma membrane separates the inner of the cell from the environment.In eukaryotic organisms beyond that there are additional intracellular membranes which encompass the cell organelles leading to a compartmentalisation of the cell.Furthermore, in eukaryotic organisms there are numerous membrane-enclosed vesicles, which ensure the directed exchange of materials and membrane components between the compartments of the cell as well as between the cell and the environment.Membranes serve diverse functions in prokaryotic and eukaryotic cells.The plasma membrane is selectively-permeable to ions and organic molecules thus controlling the movement of substances in and out the cell.Biological membranes are involved in a variety of cellular processes as cell adhesion, ion conductivity and cell signalling.Moreover, membranes serve as an attachment surface both extracellular and intracellular.
The basic structure of biological membranes is a lipid bilayer, which is composed of phospholipids and sphingolipids.Further components of cellular membranes are cholesterol and proteins.Cholesterol crucially impacts the viscosity of the membrane.The hydrophobic interactions between the sterol scaffold of the cholesterol and the acyl chains of the phospholipids reduce the deformability of the lipid bilayer as well as the permeability of the membrane for small hydrophilic molecules (Quinn & Wolf, 2009).The proteins are the key to the overall functions of the membranes.They are either embedded into the membrane (integral proteins) or associated to it (peripheral and lipid-anchored proteins).Actions of membrane proteins include cell-cell contact, surface recognition, cytoskeleton contact, signalling, enzymatic activity and the transport of substances across the membrane.Beside cholesterol the physical and chemical properties of biological membranes are determined by the fatty acid pattern.The packing density of saturated fatty acids differs widely from the packing density of unsaturated fatty acids (Stillwell & Wassall, 2003).An increase in the proportion of unsaturated fatty acids therefore results in a weakening in the intermolecular hydrophobic interactions (Stillwell & Wassall, 2003).In this way basal properties of biological membranes are modulated including the assembly of the acyl chains, the fluidity, permeability and compressibility of the membrane, melting and flip-flop mechanisms as well as the function of membrane proteins (Stillwell & Wassall, 2003;Wassall & Stillwell, 2008).

Lipid rafts
The phospholipids and sphingolipids are not distributed evenly in cellular membranes.Via lateral interactions among one another as well as with proteins specific micro domains are formed, the so called lipid rafts.Lipid rafts are structurally and functionally distinct domains that can be distinguished within the cell membrane due to their specific lipid compositions (Henderson et al., 2004;Lingwood & Simons, 2010;Pike, 2003;Tresset, 2009).Sphingolipids, cholesterol and saturated fatty acids predominate in the rafts (Pike, 2003).This allows for a tight packing of the acyl chains.The remaining liquid-ordered domain is characterized by a reduced fluidity compared to the surrounding plasma membrane.Lipid rafts have been defined as "small (10-200 nm), heterogeneous, highly dynamic, steroland sphingolipid-enriched domains that compartmentalise cellular processes" (Pike, 2006).A variety of proteins has been shown to be enriched in membrane rafts such as GPIanchored proteins, flotillin, receptor tyrosine kinases and G protein-coupled receptors (Pike, 2003).The spatial proximity of the proteins in the rafts enables a better coordination and an increased efficiency of proceeding reactions.Taking the dynamic nature of membrane domains into account the rafts have been supposed to scaffold certain molecules while excluding others thus function as a unique signalling platform (Ye et al., 2010).Further cellular processes lipid rafts have been implicated include membrane trafficking, molecular sorting, internalisation processes as well as membrane-cytoskeleton interactions (Simons & Toomre, 2000).A number of macrophage functions have been shown to interrelate with membrane rafts.Examples include the endotoxin-mediated activation of macrophages, the MHCII-mediated presentation of antigens, phagocytosis as well as the production of pro-inflammatory cytokines (Gaus et al., 2005).Furthermore, lipid rafts have been shown to be used as target by a wide variety of pathogens to invade host cells (Hartlova et al., 2010).Bacterial toxins have been demonstrated to enter cells via certain toxin-associated receptors, which are known to be concentrated in membrane rafts (Van & Leo, 2002).Thus, the domains play an important role in infectious biology.

Fatty acid dependent modulation of the immune system
4.1 Fatty acids and the immune system Fatty acids, in particular polyunsaturated fatty acids (PUFAs), possess immune modulating properties.PUFAs are shown to modulate the proliferative activity of neutrophil granulocytes (Prescott, 1984), macrophages (Hughes & Pinder, 2000), T cells (Anderson & Fritsche, 2004) and dendritic cells (Zeyda et al., 2005).Furthermore, impacts on the respiratory burst, the production of cytokines as well as the expression of adhesion molecules have been described (Calder, 1998;Calder, 2006a).Of note, the effects triggered by the fatty acids depend on the fatty acid family.N3 fatty acids such as eicosapentaenoic acid (C20:5n3; table 2) and docosahexaenoic acid (C22:6n3; table 2) are thought to act antiinflammatory (Calder, 2006a;Schmitz & Ecker, 2008).In contrast, fatty acids from the n6 family such as linoleic acid (C18:2n6; table 2) and arachidonic acid (C20:4n6; table 2) are described to act pro-inflammatory (Calder, 2006a;Schmitz & Ecker, 2008).The interaction between fatty acids and the immune system is based on three known mechanisms (Benatti et al., 2004;Capkin et al, 2009): First, some fatty acids, e.g.eicosapentaenoic acid, docosahexaenoic acid and arachidonic acid, serve as precursors for the production of immune-modulating effectors.Second, fatty acids directly interact with intracellular receptors such as peroxisome proliferator-activated receptors (PPARs) or retinoid X receptors (RXRs) as well as G protein-coupled receptors (GPR120) (Oh et al., 2010).Third, the fatty acid composition of cell membranes modulates basic properties of the membrane thus influencing the activity of immune cells.A molecular model displaying the functional mechanisms, which underlay the interaction of fatty acids and the immune system, is shown in figure 4.

Eicosanoids, lipoxins, resolvins, protectins and marensins
Unsaturated fatty acids serve as precursors for the synthesis of several signal molecules and tissue hormones.These lipid mediators include the eicosanoids, the lipoxins, the resolvins, the protectins and the marensins.There are four families of eicosanoids: prostaglandins, prostacyclins, thromboxanes and leukotrienes (Löffler et al., 2007).Each of these families can further be divided into separate series depending on the fatty acid family they derive from.The eicosanoid series differ widely in their biological activity.In general, n6 eicosanoids act pro-inflammatory, n3 eicosanoids are much less inflammatory (Lee, 1984;Lee, 1988).
Eicosanoids are synthesized in almost every animal and human tissue modulating numerous hormonal and other stimuli.The several physiological effects are triggered by specific membrane receptors of target cells and target tissues (Löffler et al., 2007).Acting as immune-modulators and neurotransmitters among others they influence the contraction of smooth muscle cells (vasoconstriction / vasodilatation), the experience of pain and the platelet aggregation (Löffler et al., 2007).Moreover, eicosanoids play an important role in hypersensitivity reactions and inflammatory processes (Benatti et al., 2004).An overview on the biological effects of major eicosanoids is shown in table 3.  3. Biological effects of major eicosanoids (Löffler et al., 2007) The half-life of eicosanoids is about seconds to minutes.Thus, eicosanoids are considered as tissue hormones acting in an autocrine and paracrine manner (Löffler et al., 2007).The effect profile of eicosanoids therefore crucially depends on the type of eicosanoid receptors, which are expressed in close proximity to the eicosanoid-producing cell.In general, eicosanoid receptors are seven transmembrane receptors, which are also known as G protein-coupled receptors (Löffler et al., 2007).Subject to the type of receptor ligand binding results either in the modulation of the adenylate cyclase, which is followed by increased or decreased intracellular cAMP concentrations, or in an activation of the phosphatidylinositol cycle, which manipulates the concentration of cellular calcium (Löffler et al., 2007).Depending on the fatty acid, which is used as precursor, eicosanoid synthesis results in discrete mediators.The n6 PUFA arachidonic acid is the substrate for production of series 2 prostaglandins and series 4 leukotrienes (Benatti et al., 2004).Starting from the n3 PUFA eicosapentaenoic acid series 3 prostaglandins and series 5 leukotrienes are formed (Benatti et al., 2004).In comparison to the arachidonic acid derivates series 3 prostaglandins and series 5 leukotrienes are notably less inflammatory.As an example: the leukotriene B 5 has been reported to have a 10 to 100 times reduced biological activity than the leukotriene B 4 (Lee et al., 1984;Lee et al., 1988).

Substance
Beside eicosanoids there is a genus of specialized pro-resolving mediators (SPM).The several families of these distinct local mediators include the lipoxins, the resolvins, the protectins and the marensins (Bannenberg & Serhan, 2010;Kohli & Levy, 2009;Norling & Serhan, 2010).SPM play a key role in the endogenous down-regulation of inflammation.For a long time inflammation resolution has been thought to occur passively by a dilution of pro-inflammatory signals and mediators.Only recently it has emerged, that instead it is an active process, which is orchestrated by a distinct set of chemical effectors (Serhan et al., 2008).In resolving tissues an alteration in PUFA metabolism has been observed.A class switching in lipid mediator generation occurs, that leads to a change in enzymatic PUFA conversion from pro-inflammatory eicosanoids to pro-resolving mediators (Levy et al., 2001).SPM are derived from the n6 PUFA arachidonic acid and the n3 PUFAs eicosapentaenoic acid and docosahexaenoic acid as well (Bannenberg & Serhan, 2010;Kohli & Levy, 2009;Norling & Serhan, 2010).They exert influence at even picomolar to nanomolar concentrations (Serhan, 2005;Spite, 2009).Biological actions of SPM include the downregulation of cell adhesion molecules, the reduction of chemotaxis, transendothelial migration and neutrophil activation, the stimulation non-phlogistic phagocytosis of apoptotic neutrophils and macrophages as well as the removal of inflammatory leukocytes (Bannenberg & Serhan, 2010).In table 4 there is an overview regarding the biological activities of major SPM.
The first SPM identified are the lipoxins (Serhan, 2005).Lipoxins are enzymatically generated by lipoxygenase from the n6 PUFA arachidonic acid, a process, which is triggerd by aspirin (Chiang, 2004).The potent anti-inflammatory effectors are special important in slowing down neutrophil-mediated tissue injury.They limit the recruitment and the adhesion of neutrophils to the site of inflammation.Furthermore, lipoxins force the phagocytosis of apoptotic neutrophils (Bannenberg & Serhan, 2010;Norling & Serhan, 2010).Major lipoxins are Lipoxin A 4 (LXA 4 ) and Lipoxin B 4 (LXB 4 ).Resolvins are bioactive metabolites, which are synthesized from the enzymatic conversion of the n3 PUFA eicosapentaenoic acid and docosahexaenoic acid.The term resolvin is derived from 'resolution phase interaction products' (Serhan & Chiang, 2008).Depending on the fatty acid they are produced from resolvins are categorized as either E-series (from eicosapentaenoic acid) or D-series (from docosahexaenoic acid) (Serhan & Chiang, 2008).Aspirin-triggered forms have been described for each family (Serhan & Chiang).The Eseries resolvins currently comprise Resolvin E1 (RvE1) and Resolvin E2 (RvE2).The D-series resolvins include Resolvin D1 (RvD1), Resolvin D2 (RvD2), Resolvin D3 (RvD3) and Resolvin D4 (RvD4).In general, resolvins efficiently block the synthesis of pro-inflammatory mediators, regulate the entry of neutrophils to inflammatory sites and help to clear neutrophils from mucosal surfaces (Norling & Serhan, 2010).At this, the bioactivity of the anti-inflammatory effectors has been shown to be highly stereoselective both in vitro and in vivo (Levy, 2010).The n3 PUFA docosahexaenoic acid serves also as precursor for the synthesis of protectins.

Interaction of fatty acids with nuclear receptors and G protein-coupled receptors
Fatty acids regulate gene expression via interaction with nuclear receptors and G proteincoupled receptors (GPCRs).Nuclear receptors are defined as intracellular ligand-inducible transcription factors, which modulate gene expression in response to hydrophobic endogenous and exogenous chemicals (Khan & Heuvel, 2003).Functions affected include the fatty acid metabolism, the reproductive development and the detoxification of substances (Khan & Heuvel, 2003).In general, nuclear receptors consist of distinct functional domains.
The N-terminal A/B domain, has a weak transactivation activity (activation function 1 (AF-1)) (Bordoni et al., 2006;Khan & Heuvel, 2003).Adjacent there is the C domain, which is important for DNA binding (Bordoni et al., 2006;Khan & Heuvel, 2003).The DNA binding domain is the most conserved region of the nuclear receptor superfamily.It is composed of two zinc fingers, which bind to response elements (NREs) in their target promoters (Bordoni et al., 2006;Khan & Heuvel, 2003).The flanking D domain is a hinge region.This region may allow for conformational changes in the receptor structure following ligand binding (Bordoni et al., 2006;Khan & Heuvel, 2003).The C-terminal E/F domain contains the ligand binding site, which in every nuclear receptor is comprised of 10 to 13 -helixes that form a hydrophobic binding cavity (Bordoni et al., 2006;Khan & Heuvel, 2003).At the extreme Cterminus there is the ligand-dependend transactivation function-2 (AF-2) (Bordoni et al., 2006;Khan & Heuvel, 2003).Furthermore, this region also contains nuclear localisation signals and protein interaction surfaces for dimerization with heat shock proteins, co-regulators and other transcription factors (Bordoni et al., 2006;Khan & Heuvel, 2003).So far, five transcription factor families have been described to be modulated in their activity by fatty acids: peroxisome proliferator-activated receptors (PPARs), retinoid X receptors (RXRs), liver X receptors (LXRs), the hepatic nuclear factor 4 (HNF-4 ) and sterol regulatory element binding proteins (SREBPs) (Bordoni et al., 2006).The interaction between fatty acids and nuclear receptors is of importance in regulating lipid and glucose metabolism at this modulating the expression of specific enzymes as well as of fatty acid transporters (Bordoni et al., 2006).Beside, based on PPARs there is also a regulation of inflammation and immune response (Bishop-Bailey & Bystrom, 2009).PPARs were identified 1990 as transcription factors (Issemann & Green, 1990).In 1992 linoleic acid (C18:2n6; table 2) and arachidonic acid were demonstrated to activate PPARs (Gottlicher et al., 1992).There are three PPAR family members: PPAR (NR1C1), PPAR / (NR1C2; NUC1; FAAR fatty acid-activated receptor) and PPAR (NR1C3).The isotypes are distinguished from each other by their tissue distribution and their differential activation (Grimaldi, 2001).PPARs are implicated in a number of biological processes, such as the regulation of lipid metabolism (uptake, activation, oxidation and esterification of fatty acids), development, cell proliferation and differentiation as well as inflammatory responses Escher & Wahli, 2000).PPARs exert their effects on gene expression as heterodimers with RXRs (Kliewer et al., 1992).The PPAR/RXR heterodimer binds to specific PPAR response elements (PPRE), which located in the promoter of target genes (Issemann et al., 1993).The consensus sequence consists of a direct repeat of the hexamer AGGTCA separated by single-nucleotide spacer, the so called direct repeat (DR-1) (Ijpenberg et al., 1997).The conformational change that occurs upon ligand binding leads to the dissociation of co-repressors as nuclear receptor co-repressor (NCoR) and silencing mediator for retinoid-and thyroid-hormone receptors (SMRT) (Feige et al., 2006).In addition, activated PPARs fascilitate the recruitment of co-activators, chromatin remodelling factors and of the transcription machinery (Bishop-Bailey & Bystrom, 2009).
Beside nuclear receptors recently five G protein-coupled receptors have been described, which can be activated by free fatty acids: GPR40 binding long-chain fatty acids, GPR41 binding short-chain fatty acids, GPR43 binding short-chain fatty acids, GPR84 binding medium-chain fatty acids and GPR120 binding long-chain fatty acids (Oh et al., 2010).From the perspective of immune modulation GPR120 emerges as a receptor of particular interest.
In contrast to the other receptors of the family of fatty acid sensing GPCRs GPR120 is the only one, which is highly expressed in adipose tissue and macrophages (Oh et al., 2010).Moreover, ligand stimulation of GPR120 by the n3 PUFA DHA and EPA has been reported to result in potent anti-inflammatory effects, including inhibition of IL-6, TNF-and MCP-1 mRNA expression and secretion (Oh et al., 2010).Activation of GPR120 by n3 fatty acids leads to a coupling of GPR120 to the adaptor protein -arrestin2, which is followed by receptor and -arrestin2 internalization (Oh et al., 2010).In the cytoplasm the GPR120/ -arrestin2 complex can associate with the TGF-beta activated kinase 1 binding protein 1 (TAB1) at this blocking the association of TAB1 with the TGF-beta activated kinase 1 (TAK1) (Oh et al., 2010).Thus, there is an inhibition of TAK1 activation and downstream signalling to the IKK /NFκB and JNK/AP1 system resulting in the inhibition of Toll-like receptor 2/3 and 4 as well as TNF-action (Oh et al., 2010).

Fatty acid composition of cellular membranes
The lipid composition of cellular membranes is known to depend on the availability of fatty acids (Calder et al., 1994).Supplementation of cells with PUFAs results in an incorporation of the fatty acids into membrane phospholipids (Schmutzler et al., 2010;Schumann & Fuhrmann, 2010;Walloschke et al., 2010).Beyond that, the PUFAs were metabolized leading to an increase of the desaturation and elongation products of the fatty acids added (Schmutzler et al., 2010;Schumann & Fuhrmann, 2010;Walloschke et al., 2009).The enhanced proportion of unsaturated fatty acids in the cell membrane is accompanied by a heightening of the Methylene Bridge Index (MBI) of the lipid bilayer and an increasing of membrane fluidity (Schmutzler et al., 2010;Schumann & Fuhrmann, 2010;Walloschke et al., 2009).The MBI is calculated based on the ratio of a fatty acid (weight %) and the number of its bis-allyl-methylene positions (Kelley et al., 1995).The higher the MBI, the more fluid the membrane.Moreover, membranes with a heightened MBI are predicted to have an increased susceptibility against radical reactions (Kelley et al., 1995).The modulation of membrane fluidity makes an impact on the activity of membrane-bound enzymes and also on the function of membrane receptors thus affecting signal transduction (Benatti et al., 2004;Calder et al., 1994).Many key proteins of signal transduction, as Tolllike receptors or Nod-like receptors, are localized in lipid rafts (Pike, 2003).After binding of the ligand, the activated receptor complexes are compartmentalized in the lipid rafts (Jury et al., 2007).At this, the lipid rafts facilitate the association of signal transduction molecules (Jury et al., 2007;Pike, 2003).The membrane fluidity necessary for an optimal cell response is assumed to fall within particular boundaries (Calder et al., 1994).Changes in the lipid composition of the plasma membrane therefore directly affect cellular reactions on signals from the environmental.
Accordingly, there is a correlation between the activity of immune cells and the fatty acid pattern of their cell membrane.Of note, the availability of fatty acids is modulated by the diet.The connection between the dietary intake of fatty acids and inflammation was first drawn in the late 1970s.Epidemiological observation showed that native Greenland Eskimos (Dyerberg & Bang, 1979) and Japanese (Hirai et al, 1980), which have a high intake of n3 PUFA from seafood, have a low incidence for myocardial infarction, chronic inflammation and autoimmune disorders.To date, there are numerous studies, which have investigated the effects of the amount and the type of fat in the diet on cellular physiology (Calder, 2006b;Galli & Calder, 2009).It is now well accepted that the fatty acid composition of body cells, including immune cells, is sensitive to alteration according to the fatty acid composition of the diet (Calder, 2001).In particular, proportions of PUFAs of the n3 and the n6 family are readily modified thus providing a link between dietary PUFA intake, inflammation and immunity (Calder, 2001).The findings from these studies led health care professionals to encourage the general population to consume more n3 fatty acids.

Conclusion
The importance of lipid bilayers for the overall processes of life is increasingly realized.Plasma membranes are just more than compartmentalization elements separating the inside of a cell from the surrounding environment.They have numerous functions in cell adhesion, ion conductivity and cell signalling acting as an attachment surface for intracellular and extracellular.
Infection processes and immune defence greatly depend on cellular membrane interactions.
In an initial step of infection pathogens, as bacteria and viruses, as well as bacterial toxins, need to get inside the host cell (Murphy et al., 2008).The internalization enables the pathogens to proliferate and to poison the cell (Murphy et al., 2008).Thus, the attachment of pathogens to cellular surfaces and the binding to a cell-surface receptor are critical in the course of infection processes.Moreover, the mutual communication between immune cells crucially depends on signal molecules binding to cellular receptors as well as on direct membrane-membrane interactions.Stimulation of membrane receptors via cytokines and chemokines and direct cell-cell contacts are of importance in activating immune cells thus triggering immune defence (Murphy et al., 2008).Likewise, completion of the immune response after successful extinction of the pathogens is mediated by membrane-dependent signal transduction processes (Murphy et al., 2008).A further mechanism in immune defence, which involves the interaction between the plasma membranes of both pathogen and host, is the phagocytosis of pathogens by macrophages and dendritic cells.Of note, membranes are very flexible in their overall composition.Depending on cell type and tissue the proportion of phospholipids and sphingolipids varies greatly.The same is the case with membrane proteins.There are numerous highly specialised mechanisms that allow for the particular allocation of membrane lipids and membrane proteins according to the requirements and functions of a cell.Beyond that, the lipid bilayer is characterized by an asymmetry in the distribution of phospholipids and sphingolipids between the inner and the outer leaflet (Löffler et al., 2007).This asymmetry is determined by the balance of specific transport proteins (Löffler et al., 2007).The disruption of the lipid asymmetry is a piece in the substrate recognition of macrophages (Löffler et al., 2007).
However, the lipid composition of cellular membranes, including immune cells, can be modulated depending on the availability of fatty acids, providing a link between the diet and the immune defence (Calder et al., 1994).In particular, highly unsaturated fatty acids are immediately incorporated into the lipid bilayer (Schumann & Fuhrmann, 2010).The physical properties of membranes are highly reliant on the fatty acid pattern.Features modulated include the fatty acid chain order, the phase behaviour, the elastic compressibility, the ion permeability, the fusion, the rapid flip-flop and several protein functions thus affecting cell signal transduction (Stillwell & Wassall, 2003;Wassall & Stillwell, 2008).Moreover, the unsaturated fatty acids of membrane phospholipids serve as substrate for the synthesis of lipid signal molecules, as eicosanoids, lipoxins, resolvins, protectins and marensins, which are known to efficiently modulate immune defence mechanisms (Bannenberg & Serhan, 2010;Kohli & Levy, 2009;Löffler et al., 2007;Norling & Serhan, 2010).Beside, the fatty acids also act as ligands for several nuclear receptors and G protein-coupled receptors thus directly influencing the immune system (Bordoni et al., 2006).This key position of fatty acids, in particular of PUFAs of the n3 and the n6 family, makes them particular promising in the supportive therapy of chronic diseases linked to persistent pathogens.

Fig. 4 .
Fig. 4. Molecular model displaying the functional mechanisms, which underlay the interaction of fatty acids and the immune system , Inducing of inflammation, fever and pain, Activation of osteoclasts, Inhibition of chloride secretion in the stomach and of lipolysis in fat tissue Prostaglandin D 2 Bronchoconstriction, Promotion of sleep Prostaglandin F 2 Bronchoconstriction, Vasoconstriction, Constriction of smooth muscle cells Thromboxane A 2 Bronchoconstriction, Vasoconstriction, Platelet aggregation Prostacyclin I 2 Vasodilatation, Increasing of vascular permeability, Inhibition of platelet aggregation, Inducing of inflammation Table infiltration and pro-inflammatory signalling; Stimulation of phagocytosis of apoptotic neutrophils by macrophages; Protection from tissue damage and loss of function Table

Table 1 .
Bacterial resistance mechanisms