The immune system is a large network consisting of an innate and an adaptive part. Both systems work together and intertwine with each other to defend the body from germs or foreign substances (Figure 1) (Yatim and Lakkis, 2015; Hillion et al., 2020). The skin as part of the innate immune system is the first defence against pathogens and protects our body from foreign entries. When pathogens passed this barrier the innate immune system then activates a cascade of signalling pathways and immune cells from both, the innate and the adaptive, immune systems to fight off the danger (Smith et al., 2019).

Pathogens can be neutralized or directly killed by cells of the innate immune system (Macrophages, Natural killer cells etc.), but the scope of function of the innate immune cells goes beyond. Bacterial cell wall components or viral particles are processed by cells of the innate system and presented as antigens to the adaptive immune system. Furthermore, the innate immune cells recruit adaptive immune cells from the blood to the site of inflammation by secretion of chemokines and cytokines.

The adaptive immune system consists of T lymphocytes, B lymphocytes and the antibodies that are produced by the B cells (Buchholz et al., 2016; Bradley and Thomas, 2019; Wang et al., 2020). B cells come from and mature in the bone marrow. They are activated by a subpopulation of T cells (T helper cells) via antigens that bind at their surface immunoglobulin (Ig). Upon activation B cells then release soluble antibodies. The antibody release enables the humoral immune response consisting of pathogen neutralisation, opsonization and complement activation. The complement system enzymatically facilitates lysis and phagocytosis of pathogens (Merle et al., 2015). Antibodies also prevent bacterial adherence including binding to the pathogen (Forthal, 2014).

T cells derive from the bone marrow and are then moved through the blood for maturation in the thymus. They have a broad spectrum of functions that are determined by their surface expression pattern that divides these cells into different subpopulations (Romagnani, 2014). The activation and maturation of those cells is mediated via the T Cell Receptor (TCR). They have to undergo a checkpoint to ensure only non-self-antigen responsive cells are selected (Fife and Bluestone, 2008). Negative selection of T cells removes self-reactive cells from the body (Palmer, 2003), positive selection assures that only T cells with a TCR that can recognize foreign antigens via MHC stimulation will mature (Klein et al., 2014). This process takes places in the thymus and occurs with cells that already express the TCR and both co-receptors Cluster of Differentiation (CD) 4 and 8.

Under normal circumstances, the process of inflammation is a mechanism to defend the body from foreign danger like pathogens, toxins or damaged cells. Usually this process is managed within days and finished by resolution of this acute inflammation (Varela et al., 2018). Inflammatory cells, such as macrophages, neutrophils, B cells and T cells, are recruited and send to the site of pathogen infiltration to act against the those pathogens to heal the injury and thereby restoring tissue homeostasis (Figure 1) (Schnoor et al., 2016).

Local, acute inflammation in the tissue is characterized by Galen’s prominent five properties: rubor (redness), tumor (swelling), dolor (pain), calor (heat) and function laesa (impaired function). These characteristics are triggered by the immune cell response to the infectious or sterile damage. Receptors on the surface of immune cells can recognize harmful stimuli via different receptors. Infectious stimuli are so called PAMPs (pathogen-associated molecular patterns) that activate a variety of PRRs (pattern-recognition receptors) in immune and non-immune cells, but additionally endogenous signals also known as DAMPs (danger-associated molecular patterns) can be responsible for inflammatory responses (Walsh et al., 2013; Eppensteiner et al., 2019; Zindel and Kubes, 2020; Murao et al., 2021). Toll-like receptors (TLRs) are one class of these PRR. When they get activated, different inflammatory signalling cascades in immune or non-immune cells are triggered, resulting in the translocation of the nuclear factor 'kappa-light-chain-enhancer' of activated B-cells (NF-κB), activator protein 1 (AP-1) or interferon regulatory factor 3 (IRF3) from cytosol to the nucleus, thereby leading to a transcriptional regulation of the immune response (Kawai and Akira, 2007; Liu W. et al., 2009). The activation of these pathways leads to the release of pro-inflammatory cytokines like Interferon-Gamma (IFN-γ), Interleukin 1β (IL1β) or IL6 and pro-inflammatory lipid mediators like Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero3-phosphocholine), leukotriens, prostanoids or sphingolipids (Lawrence, 2009; Schauberger et al., 2016; Srivastava and Baig, 2018; Yanai et al., 2018). Interactions of cytokines with their receptors on immune cells activate signalling pathways including NF-κB, mitogen-activated protein kinase (MAPK), januskinase- signal transducers and activators of transcription (JAK-STAT) that promote further cytokine production, proliferation and differentiation of immune cell to specific subtypes (Moens et al., 2013; Fang et al., 2018; Salas et al., 2020). PAF is produced by various cells including neutrophils, eosinophils, endothelial cells and fibroblasts and induces platelet aggregation and leukocyte degranulation and adhesion (Pałgan and Bartuzi, 2015). Leukotrienes (Cysteinyl leukotrienes (CystLTs), leukotriene B4 (LTB4), leukotriene C4 (LTC4)) are produced by leukocytes such as macrophages, eosinophils, basophils and are important for leukocyte influx and vascular permeability (Rådmark et al., 2015; Schauberger et al., 2016). Prostanoids are produced by almost every cell type and are crucial for chemotaxis, activation of immune cells, act on platelet aggregation and vascular smooth muscle cells (Schauberger et al., 2016). S1P (sphingosine-1 phosphate) is the best known pro-inflammatory sphingolipid and involved in chemotaxis as well as activation of various immune cells (see also chapter 6.4) (Sukocheva et al., 2020).

To restore the pre-inflammation status of the tissue, immune cells must be removed from the inflammatory site, inflammatory signalling processes have to be resolved and chemokine gradients have to be diluted to prevent further migration of immune cells (Figure 2). Chemokines at the site of inflammation are cleaved by proteolysis and sequestration (Ye, 2013; Chongsathidkiet et al., 2018; Tao et al., 2019; Sallenave and Guillot, 2020). Neutrophils are the first cells to be egressed from the tissue (Greenlee-Wacker, 2016). They undergo Fas-ligand induced apoptosis via tumor necrosis factor (TNF) receptor interactions that lead to phosphoinositide 3-kinase (PI3K)-activation, they produce reactive oxygen species (ROS) (Salamone et al., 2001; Croker et al., 2011). Macrophages are responsible for the release of Fas-ligand and TNF and the clearance of neutrophils that went apoptotic (Brown and Savill, 1999). During this efferocytosis the pro-inflammatory expression pattern (M1) of macrophages change to an anti-inflammatory pattern (M2) (Kohno et al., 2021). M2 macrophages are characterized by their ability to release immunosuppressive cytokines like IL10, transforming growth factor β (TGFβ) and vascular endothelial growth factor (VEGF) (Orecchioni et al., 2019).

Anti-inflammatory signals are beneficial for tissue homeostasis. Stromal cells, macrophages and progenitor or stem cells must work together in order to impede scar formation or fibrosis (Figure 2). TGFβ released from macrophages promote fibroblast differentiation into myofibroblasts through regulation of ECM remodelling (Caja et al., 2018). VEGF signalling leads to angiogenesis and recovery of normal oxygen supply (Shibuya, 2011). Also mesenchymal stem cells (MSC) affect inflammation by the production of growth factors and chemokines that influence macrophage polarization, maturation and migration (Zhao et al., 2020). MSCs promote phagocytosis of macrophages and their differentiation by effecting the metabolism of the macrophages via a prostaglandin E2 (PGE2) dependant mechanism (Vasandan et al., 2016). Polarization of M1 type macrophages to a M2 type can also be enhanced by MSCs (Liao et al., 2020).

Dysfunction or -regulation of these processes can shift acute inflammation to a chronic status and are the cause of many diseases like inflammatory bowel disease (IBD), chronic airway inflammation, stromal keratitis, rheumatoid arthritis and even cancer.

T cells have a broad spectrum of functions. CD8 expressing T cells are also called cytotoxic T cells because of their capability of killing pathogens and cancer cells. Cytotoxic T cells have to be stimulated by antigen-presenting cells (APC) via MHC class I in order to mature (Kratky et al., 2011). This happens in the nature of an infection and causes these cells to proliferate and fight off intracellular dangers (Cox and Zajac, 2010). CD8 cells are capable of direct killing of infected cells via induction of apoptosis in those cells through the secretion of perforin and granzymes (Voskoboinik et al., 2015). Perforins form pores into the target’s cell membrane and enable the entrance of granzymes (Osińska et al., 2014). Granzymes are a group of serine proteases that cleave viral and cellular proteins of the infected target cells thereby killing it (Trapani, 2001). Apoptotic target cells are then phagocytized by macrophages (Kourtzelis et al., 2020). Another possibility of target cell killing is the induction of apoptosis via Fas-FasL interactions (Yamada et al., 2017). Upon Fas activation a signalling cascade is activated and caspase proteases are induced following apoptosis of these cells (Aouad et al., 2004; Sobrido-Cameán and Barreiro-Iglesias, 2018).

CD4 cells are often characterized as T helper cells (Th). Those cells regulate immune and non-immune cells via the production and release of cytokines and recognize antigens via MHC class II molecules. There is a variety of different T helper cell subsets, that is, described by their cytokine release and transcriptional profile. On the one hand, Th1 cells are prominent to produce IFNγ, TNFβ, TNFα and IL2 and they also express the transcription factor T-bet (T-box expressed in T cells) (Viallard et al., 1999; Szabo et al., 2000; Hwang et al., 2005; Solomou et al., 2006; Kisuya et al., 2019; Artham et al., 2020). On the other hand, TH2 cells do not release IFNγ, but IL4, IL5 and IL13 and they produce the transcription factors GATA binding protein 3 (GATA-3) and c-Maf (musculoaponeurotic fibrosarcoma) (McKenzie et al., 1998; Kanhere et al., 2012; Paul, 2015; Imbratta et al., 2020). Both subpopulations express different chemokine receptors, Th1 cells express C-C chemokine receptor type 5 (CCR5) and CXCR3, TH2 cells express CCR4 and CCR8 (Zingoni et al., 1998; Turner et al., 2007; Yoshie and Matsushima, 2015; Watanabe et al., 2020). Th1 cells are mainly responsible for the recruitment and activation of macrophages by IFNγ release and TH2 cells activate B cells via IL4 and IL13, but also eosinophils via IL5.

There is one additional major subpopulation among the CD4 effector cells, that is, called Th17 cells. Those cells mainly release IL17A to activate macrophages, endothelial and epithelial cells and fibroblasts, as well as CXCL8 to stimulate neutrophils and exhibit pro-inflammatory functions (Pelletier et al., 2010; Li J. et al., 2013; Brembilla et al., 2013; Tabarkiewicz et al., 2015; Zhang et al., 2021; Muench et al., 2022). RAR-related orphan receptor gamma (ROR(γ)t) is a transcription factor, that is, solely expressed by Th17 cells (Steinmetz et al., 2011).

Besides their effector functions in inflammatory processes, there is one specialized CD4 subset, that has a more regulatory function. Regulatory T cells (Tregs) are a subpopulation of CD4 expressing cells that additionally express high amounts of CD25, the alpha-subunit of the IL2 receptor (Kmieciak et al., 2009). They are also often categorized by their expression of the transcription factor forkhead box P3 (FOXP3) which is a prominent Treg marker (Rudensky, 2011). It is important for both, regulatory and developmental pathways in Tregs. Tregs are known for their immune-suppressive function and behaviour (Wan, 2010). The production of the anti-inflammatory cytokine IL10 is one of the main tasks of Tregs and a key player in suppressive function (Ng et al., 2013). The process of self-tolerance is tightly regulated by Tregs and disturbances in this processes lead to autoimmune diseases (Sakaguchi et al., 2008; Yang et al., 2015). Because of their immunosuppressive function, Tregs are a key mediator in resolution of inflammation. They promote macrophage efferocytosis and modulate monocyte macrophage differentiation thereby driving the resolution of inflammation (Weirather et al., 2014; Proto et al., 2018).

The TCR is a heterodimer complex that has two TCR chains and six CD3 chains also connected with different components like Co-receptors, kinases and ligands. Most T cells express the α- and β- TCR chain and are referred to as normal T cells, whereas T cells that express γ- and δ- TCR chain receptors are called γδ- T cells (Nanno et al., 2007). They only make up 0.5–5% of the T lymphocytes (Carding and Egan, 2002). The variable domains of the α and β, γ δ respectively, are responsible for the specificity of the TCR and distinguish the different antigens presented by the Major Histocompatibility complex (MHC). TCRα/TCRβ and TCRγ/TCRδ heterodimers form complexes with the CD3γ/ε and/or CD3δ/ε heterodimer molecules, as well as CD3ζ homodimers (Birnbaum et al., 2014).

T cells can be generally classified via their cell surface molecules CD4 and 8, respectively. Both markers are co-stimulatory proteins of the TCR. They are important mediators of the MHC peptide recognition cascade and they are also important for the maturation process of T cells in the thymus (Tikhonova et al., 2012). They are single molecules with an extracellular and a transmembrane region and an intracellular tail (Leahy, 1995). The intracellular tail enables interaction with the lymphocyte-specific protein tyrosine kinase (LCK) which initiates the signalling cascade upon activation of the TCR (Artyomov et al., 2010). CD4 can recognize MHC class I molecules, whereas CD8 cells bind to MHC class II molecules (Li Y. et al., 2013).

The activation of TCR is only possible when not only CD3 or CD4/8 are stimulated but also CD28 (Linsley and Ledbetter, 1993). It is a cell surface monomeric protein that binds to CD80 and CD86 of APCs in order to activate phosphoinositide-3-kinases (PI3K) (Garçon et al., 2008). PI3K catalyzes the conversion of phosphatdiylinositol-4,5-bisphosphate (PIP2) to phosphatidylinositol-3,4,5-trisphosphate (PIP3) which then activates phospholipase C-γ (PLC-γ) in the TCR activation machinery (Falasca et al., 1998). CD28 also enhances the production of IL4 and IL10 via TCR activation and CD40L binding (Hünig et al., 2010).

Upon stimulation of the TCR CD4/8 mediated LCK activation leads to the cascade of ZAP-70 phosphorylation and subsequent activation of the linker activator of T cells (LAT) (Lo et al., 2018). LAT as well as CD28, activate PI3K to form PIP2, but LAT can also activate PLC-γ directly (Balagopalan et al., 2015). Signalling pathways of TCR lead to NFκB, AP-1 and NFAT activation, which then promotes cell proliferation, differentiation and cell survival (Smith-Garvin et al., 2009).

The organization of the TCR’s activation takes place in so-called lipid rafts, often also called micro-domains or detergent-resistant membranes (DRM) (Zumerle et al., 2017). Lipid rafts are specialized compartments in the cell membrane with a high density of sphingomyelins, cholesterols and glycosphingolipids (Lingwood and Simons, 2010; Simons and Sampaio, 2011; Regen, 2020). They are cellular signalling hubs, bringing receptors closely together, kinases and second messengers thereby transforming a signal from the membrane to intracellular signalling cascades. The assembly of TCR/CD3, CD28, PKCθ and LCK is defined as supramolecular activation cluster (SMAC) with a central region (c-SMAC) and a peripheral region (p-SMAC) (Zumerle et al., 2017).

As lipid rafts are characterized as structures enriched in sphingomyelins and glycosphingolipids, the role of sphingolipids (Sph) is very crucial (Bieberich, 2018). Sphs are one of the most important classes of membrane lipids. They owe their name to the mythological sphinx after they were first discovered in brain tissue in the 1870s (Thudichum, 1962). Sphs commonly consist of a sphingoid base, like sphingosine or sphinganine, as backbone, at which via an amid-bound a second fatty acyl chain of different chain length is added. Further, the C1-hydroxygroup could be connected to different charged groups like phosphate, glucose/galactose or phosphocholine, resulting in Ceramide-1-phosphate, Glucosyl-Ceramide (GluCer), Galactosyl-Ceramide (GalCer) or sphingomyelin (SM). GluCer can further be processed to Lactosyl-Ceramide (LacCer) and complex Gangliosides (GMs). A nice overview of the sphingolipid pathway is given in (Rajasagi and Rouse, 2018). The de novo synthesis of ceramides takes place in the Endoplasmatic Reticulum (ER) afterwards ceramides are transported to the Golgi apparatus where the synthesis of complex sphingolipids takes place (Figure 3). The first step of the sphingolipid de novo synthesis is mediated by the serine palmitoyltransferase (SPT) which condensates serine and palmitoyl-CoA to generate 3-ketosphinganine, that is, directly reduced to sphinganine by the 3-ketophinganine reductase. Six isoforms of ceramide synthases (CerS1-6) add a second acyl-chain of different chain length to sphinganine/sphingosine to generate dihydro-ceramides or ceramides, respectively. CerS1 and CerS4 add C18/C20 fatty acids, CerS2 C22-C26, CerS3 C18-C32 and CerS5 and CerS6 use mainly C14/C16 fatty acids as substrate (Figure 3). Ceramide synthases are regulated and expressed tissue specific and each ceramide has definite cellular effects (Brachtendorf et al., 2019 und Wegner et al., 2016). Sphingolipids could also be recycled in the salvage pathway, where sphingosine can be obtained from constitutive degradation of sphingomyelin or glycosphingolipids either at the plasma membrane or in the late endosomes and lysosomes by the action of sphingomyelinases, glycosidases and ceramidases. Sphingosine is then either N-acetylated by CerS to form ceramides, phosphorylated by sphingosine-kinases to sphingosine-1-phosphate (S1P), or degraded to phosphatidylethanolamine and fatty aldehyde by lyases. S1P regulates many cellular functions via binding to G protein-coupled receptors S1P1-5. Those receptors reside in different cell types and tissues and upon their stimulation trigger signalling cascades for proliferation, survival, migration and vaso-regulative pathways (Schwab and Cyster, 2007; Obinata and Hla, 2019). The impact of membrane located sphingolipids on cells depends on their basic structure. Ceramides, for example, can affect cellular pathways and diseases differentially through the length of their fatty acid chains (Grösch et al., 2012). On the one hand, the amphilic properties of short chain ceramides result in membrane permeability and enable them to translocate freely into the cytosol. On the other hand, long chain ceramides stay tightly bound to the membrane and cannot pass as freely. Very long chain ceramides are responsible for the epidermal barrier function and can be found in the skin (Li et al., 2007). Specific lipid and protein interactions in membranes are crucial for signal transduction and can be influenced also by the chain length, saturation of the acyl chain or their polar head groups (Quinn, 2014; Uche et al., 2021). Therefore, the biophysical properties of membrane lipids can facilitate or impede receptor signalling.

Sphs control and influence major cellular events such as apoptosis, cell growth and proliferation, signal transduction and differentiation (Cerbón et al., 2018; Xie et al., 2019; Hadas et al., 2020; Jakobi et al., 2020; Loberto et al., 2020; Mignard et al., 2020; Muthusamy et al., 2020; Monasterio et al., 2021; Capolupo et al., 2022). Therefore, a dysregulation of these processes have also an impact on chronic inflammation which we want to highlight in the next sections.

In this chapter we want to give an overview how T cells affect resolution of inflammation and how sphingolipids can influence these mechanisms. As mentioned above, T cells have different functions in inflammation and can either enhance or contribute to the resolution of inflammatory lesions. While T helper cells seem to increase and sustain chronic inflammation, Tregs display an important factor in the resolution process.

The resolution of inflammation gained a lot of attention in the past few years as an active process of re-establishing tissue-homeostasis and resolving inflammatory processes. Tregs regulate macrophage responses, immunosuppressive cytokine release and restoration of tissue homeostasis whereas pro-inflammatory Th1/2/17 cells promote inflammation. In this review we highlighted how sphingolipids can interfere with the differentiation and activation process of T cells. A particular sphingolipid composition of the membrane seems to be required for the different subsets of T cells and the formation of variable immunological synapses. For this reason sphingolipids influence diverse chronic inflammatory diseases that are driven by Th1 or Th2 cell response, and an imbalance between Th17 and Tregs such as in rheumatoid arthritis (Tsukuda et al., 2012) or IBD patients (Komuro et al., 2022) and are important for the resolution of inflammation. So, in future studies enzymes of the sphingolipid pathway as well as sphingolipids themselves should be considered as new possible targets to boost resolving processes and to dampen pro-inflammatory mechanisms. Ongoing clinical trials with various sphingolipid modulators might shed light on the utility of these substances in different inflammatory diseases. However, to proof if anti-inflammatory effects are mediated by the impact of these substances on T cells, future studies should also investigate the T cell subtypes in blood after treatment of patients with sphingolipid modulators. Additionally, investigating the effect of sphingolipid inhibitors on primary human blood cells in vitro using lipidomics, transcriptomics and characterizing released cytokines by cytometric bead array assay (CBA), might give a hint how these substances influence T cell subtypes and which signaling pathways are involved in the resolution of inflammation.