A role for S1P and S1P1 in immature-B cell egress from mouse bone marrow

B-cell biology and development

B cells develop from hematopoietic precursor cells in an ordered maturation and selection process. Extensive studies with many different mouse mutants provided fundamental insights into this process. However, the characterization of genetic defects causing primary immunodeficiencies was essential in understanding human B-cell biology. Defects in pre-B-cell receptor components or in downstream signaling proteins, such as Bruton tyrosine kinase and B-cell linker protein, arrest development at the pre-B-cell stage. Defects in survival-regulating proteins, such as B-cell activator of the TNF-α family receptor (BAFF-R) or caspase recruitment domain-containing protein 11 (CARD11), interrupt maturation and prevent differentiation of transitional B cells into marginal zone and follicular B cells. Mature B-cell subsets, immune responses, and memory B-cell and plasma cell development are disturbed by mutations affecting Toll-like receptor signaling, B-cell antigen receptor coreceptors (eg, CD19), or enzymes responsible for immunoglobulin class-switch recombination. Transgenic mouse models helped to identify key regulatory mechanisms, such as receptor editing and clonal anergy, preventing the activation of B cells expressing antibodies recognizing autoantigens. Nevertheless, the combination of susceptible genetic backgrounds with the rescue of self-reactive B cells by T cells allows the generation of autoreactive clones found in patients with many autoimmune diseases and even in those with primary immunodeficiencies. The rapid progress of functional genomic research is expected to foster the development of new tools that specifically target dysfunctional B lymphocytes to treat autoimmunity, B-cell malignancies, and immunodeficiency.

Phospholipids: "greasing the wheels" of humoral immunity

Phospholipids are major structural components of all cellular membranes. In addition, certain phospholipids execute regulatory activities that affect cell behavior, function and fate in critically important physiological settings. The influence of phospholipids is especially obvious in the adaptive immune system, where these macromolecules mediate both intrinsic and extrinsic effects on B and T lymphocytes. This review article highlights the action of lysophospholipid sphingosine-1-phosphate as a lymphocyte chemoattractant, the function of phosphatidylinositol phosphates as signaling conduits in lymphocytes and the role of phospholipids as raw materials for membrane assembly and organelle biogenesis in activated B lymphocytes. Special emphasis is placed on the means by which these three processes push humoral immune responses forward. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Sphingosine-1-phosphate activates chemokine-promoted myeloma cell adhesion and migration involving α4β1 integrin function

Myeloma cell adhesion dependent on α4β1 integrin is crucial for the progression of multiple myeloma (MM). The α4β1-dependent myeloma cell adhesion is up-regulated by the chemokine CXCL12, and pharmacological blockade of the CXCL12 receptor CXCR4 leads to defective myeloma cell homing to bone marrow (BM). Sphingosine-1-phosphate (S1P) regulates immune cell trafficking upon binding to G-protein-coupled receptors. Here we show that myeloma cells express S1P1, a receptor for S1P. We found that S1P up-regulated the α4β1-mediated myeloma cell adhesion and transendothelial migration stimulated by CXCL12. S1P promoted generation of high-affinity α4β1 that efficiently bound the α4β1 ligand VCAM-1, a finding that was associated with S1P-triggered increase in talin-β1 integrin association. Furthermore, S1P cooperated with CXCL12 for enhancement of α4β1-dependent adhesion strengthening and spreading. CXCL12 and S1P activated the DOCK2-Rac1 pathway, which was required for stimulation of myeloma cell adhesion involving α4β1. Moreover, in vivo analyses indicated that S1P contributes to optimizing the interactions of MM cells with the BM microvasculture and for their lodging inside the bone marrow. The regulation of α4β1-dependent adhesion and migration of myeloma cells by CXCL12-S1P combined activities might have important consequences for myeloma disease progression.

Integrated signaling in developing lymphocytes: the role of DNA damage responses

Lymphocyte development occurs in a stepwise progression through distinct developmental stages. This ordered maturation ensures that cells express a single, non-autoreactive antigen receptor, which is the cornerstone of a diverse adaptive immune response. Expression of a mature antigen receptor requires assembly of the antigen receptor genes by the process of V(D)J recombination, a reaction that joins distant gene segments through DNA double-strand break (DSB) intermediates. These physiologic DSBs are generated by the recombinase-activating gene (RAG) -1 and -2 proteins, and their generation is regulated by lymphocyte and developmental stage-specific signals from cytokine receptors and antigen receptor chains. Collectively, these signals ensure that V(D)J recombination of specific antigen receptor genes occurs at discrete developmental stages. Once generated, RAG-induced DSBs activate the ataxia-telangiectasia mutated (ATM) kinase to orchestrate a multifaceted DNA damage response that ensures proper DSB repair. In response to RAG DSBs, ATM also regulates a cell type-specific transcriptional response, and here we discuss how this genetic program integrates with other cellular cues to regulate lymphocyte development.

An emerging link in stem cell mobilization between activation of the complement cascade and the chemotactic gradient of sphingosine-1-phosphate

Under steady-state conditions, hematopoietic stem/progenitor cells (HSPCs) egress from bone marrow (BM) and enter peripheral blood (PB) where they circulate at low levels. Their number in PB, however, increases significantly in several stress situations related to infection, organ/tissue damage, or strenuous exercise. Pharmacologically mediated enforced egress of HSPCs from the BM microenvironment into PB is called "mobilization", and this phenomenon has been exploited in hematological transplantology as a means to obtain HSPCs for hematopoietic reconstitution. In this review we will present the accumulated evidence that innate immunity, including the complement cascade and the granulocyte/monocyte lineage, and the PB plasma level of the bioactive lipid sphingosine-1-phosphate (S1P) together orchestrate this evolutionarily conserved mechanism that directs trafficking of HSPCs.

Chronic alcohol consumption impairs distribution and compromises circulation of B cells in B16BL6 melanoma-bearing mice

Accumulating research indicates that B cells are involved in anti-tumor immunity. Chronic alcohol consumption is associated with decreased survival of cancer patients. The effect of alcohol consumption on B cells in tumor-bearing hosts is unknown. Results in melanoma-bearing mice showed that chronic alcohol consumption did not alter the percentage and number of B cells in bone marrow, spleen, and lymph nodes but dramatically decreased B cells in the peripheral blood. Alcohol consumption did not alter the development of B cells in the bone marrow and did not affect follicular B cells in the spleen; however, it increased T1 B cells and decreased marginal zone B cells in the spleen. Alcohol consumption also decreased mature B cells in the blood. It did not alter the chemotactic capacity of plasma to facilitate migration of splenocytes or the chemotactic response of splenocytes to CXCL13 and CCL21. However, the response of splenocytes to sphingosine-1-phosphate was impaired in alcohol-consuming, melanoma-bearing mice. The expression of sphingosine-1-phosphate receptor-1 (S1PR1) and sphingosine-1-phosphate lyase-1 (SPL1) in splenocytes was downregulated. Taken together, these results indicate that chronic alcohol consumption decreases peripheral blood B cells by compromising B cell egress from the spleen. The downregulation of S1PR1 and SPL1 expression in alcohol-consuming, melanoma-bearing mice could be associated with compromised egress of B cells from the spleen.

The expanding family of bone marrow homing factors for hematopoietic stem cells: stromal derived factor 1 is not the only player in the game

The α-chemokine stromal derived factor 1 (SDF-1), which binds to the CXCR4 and CXCR7 receptors, directs migration and homing of CXCR4⁺ hematopoietic stem/progenitor cells (HSPCs) to bone marrow (BM) and plays a crucial role in retention of these cells in stem cell niches. However, this unique role of SDF-1 has been recently challenged by several observations supporting SDF-1-CXCR4-independent BM homing. Specifically, it has been demonstrated that HSPCs respond robustly to some bioactive lipids, such as sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P), and migrate in response to gradients of certain extracellular nucleotides, including uridine triphosphate (UTP) and adenosine triphosphate (ATP). Moreover, the responsiveness of HSPCs to an SDF-1 gradient is enhanced by some elements of innate immunity (e.g., C3 complement cascade cleavage fragments and antimicrobial cationic peptides, such as cathelicidin/LL-37 or β2-defensin) as well as prostaglandin E2 (PGE2). Since all these factors are upregulated in BM after myeloblative conditioning for transplantation, a more complex picture of homing emerges that involves several factors supporting, and in some situations even replacing, the SDF-1-CXCR4 axis.

The role of sphingosine-1-phosphate transporter Spns2 in immune system function

Sphingosine-1-phosphate (S1P) is lipid messenger involved in the regulation of embryonic development, immune system functions, and many other physiological processes. However, the mechanisms of S1P transport across cellular membranes remain poorly understood, with several ATP-binding cassette family members and the spinster 2 (Spns2) member of the major facilitator superfamily known to mediate S1P transport in cell culture. Spns2 was also shown to control S1P activities in zebrafish in vivo and to play a critical role in zebrafish cardiovascular development. However, the in vivo roles of Spns2 in mammals and its involvement in the different S1P-dependent physiological processes have not been investigated. In this study, we characterized Spns2-null mouse line carrying the Spns2(tm1a(KOMP)Wtsi) allele (Spns2(tm1a)). The Spns2(tm1a/tm1a) animals were viable, indicating a divergence in Spns2 function from its zebrafish ortholog. However, the immunological phenotype of the Spns2(tm1a/tm1a) mice closely mimicked the phenotypes of partial S1P deficiency and impaired S1P-dependent lymphocyte trafficking, with a depletion of lymphocytes in circulation, an increase in mature single-positive T cells in the thymus, and a selective reduction in mature B cells in the spleen and bone marrow. Spns2 activity in the nonhematopoietic cells was critical for normal lymphocyte development and localization. Overall, Spns2(tm1a/tm1a) resulted in impaired humoral immune responses to immunization. This study thus demonstrated a physiological role for Spns2 in mammalian immune system functions but not in cardiovascular development. Other components of the S1P signaling network are investigated as drug targets for immunosuppressive therapy, but the selective action of Spns2 may present an advantage in this regard.

Type II membrane protein CD69 regulates the formation of resting T-helper memory

Memory T-helper (Th) lymphocytes are crucial for the maintenance of acquired immunity to eliminate infectious pathogens. We have previously demonstrated that most memory Th lymphocytes reside and rest on stromal niches of the bone marrow (BM). Little is known, however, regarding the molecular basis for the generation and maintenance of BM memory Th lymphocytes. Here we show that CD69-deficient effector CD4 T lymphocytes fail to relocate into and persist in the BM and therefore to differentiate into memory cells. Consequently, CD69-deficient CD4 T cells fail to facilitate the production of high-affinity antibodies and the generation of BM long-lived plasma cells in the late phase of immune responses. Thus, CD69 is critical for the generation and maintenance of professional memory Th lymphocytes, which can efficiently help humoral immunity in the late phase. The deficit of immunological memory in CD69-deficient mice also highlights the essential role of BM for the establishment of Th memory.

The sphingosine-1-phosphate transporter Spns2 expressed on endothelial cells regulates lymphocyte trafficking in mice

The bioactive lysophospholipid mediator sphingosine-1-phosphate (S1P) promotes the egress of newly formed T cells from the thymus and the release of immature B cells from the bone marrow. It has remained unclear, however, where and how S1P is released. Here, we show that in mice, the S1P transporter spinster homolog 2 (Spns2) is responsible for the egress of mature T cells and immature B cells from the thymus and bone marrow, respectively. Global Spns2-KO mice exhibited marked accumulation of mature T cells in thymi and decreased numbers of peripheral T cells in blood and secondary lymphoid organs. Mature recirculating B cells were reduced in frequency in the bone marrow as well as in blood and secondary lymphoid organs. Bone marrow reconstitution studies revealed that Spns2 was not involved in S1P release from blood cells and suggested a role for Spns2 in other cells. Consistent with these data, endothelia-specific deletion of Spns2 resulted in defects of lymphocyte egress similar to those observed in the global Spns2-KO mice. These data suggest that Spns2 functions in ECs to establish the S1P gradient required for T and B cells to egress from their respective primary lymphoid organs. Furthermore, Spns2 could be a therapeutic target for a broad array of inflammatory and autoimmune diseases.

S1P promotes murine progenitor cell egress and mobilization via S1P1-mediated ROS signaling and SDF-1 release

The mechanisms of hematopoietic progenitor cell egress and clinical mobilization are not fully understood. Herein, we report that in vivo desensitization of Sphingosine-1-phosphate (S1P) receptors by FTY720 as well as disruption of S1P gradient toward the blood, reduced steady state egress of immature progenitors and primitive Sca-1(+)/c-Kit(+)/Lin(-) (SKL) cells via inhibition of SDF-1 release. Administration of AMD3100 or G-CSF to mice with deficiencies in either S1P production or its receptor S1P(1), or pretreated with FTY720, also resulted in reduced stem and progenitor cell mobilization. Mice injected with AMD3100 or G-CSF demonstrated transient increased S1P levels in the blood mediated via mTOR signaling, as well as an elevated rate of immature c-Kit(+)/Lin(-) cells expressing surface S1P(1) in the bone marrow (BM). Importantly, we found that S1P induced SDF-1 secretion from BM stromal cells including Nestin(+) mesenchymal stem cells via reactive oxygen species (ROS) signaling. Moreover, elevated ROS production by hematopoietic progenitor cells is also regulated by S1P. Our findings reveal that the S1P/S1P(1) axis regulates progenitor cell egress and mobilization via activation of ROS signaling on both hematopoietic progenitors and BM stromal cells, and SDF-1 release. The dynamic cross-talk between S1P and SDF-1 integrates BM stromal cells and hematopoeitic progenitor cell motility.

Lymphocyte development: integration of DNA damage response signaling

Lymphocytes traverse functionally discrete stages as they develop into mature B and T cells. This development is directed by cues from a variety of different cell surface receptors. To complete development, all lymphocytes must express a functional nonautoreactive heterodimeric antigen receptor. The genes that encode antigen receptor chains are assembled through the process of V(D)J recombination, a reaction that proceeds through DNA double-stranded break (DSB) intermediates. These DSBs are generated by the RAG endonuclease in G1-phase developing lymphocytes and activate ataxia-telangiectasia mutated (ATM), the kinase that orchestrates cellular DSB responses. The canonical DNA damage response includes cell cycle arrest, DNA break repair, and apoptosis of cells when DSBs are not repaired. However, recent studies have demonstrated that ATM activation in response to RAG DSBs also regulates a transcriptional program including many genes with no known function in canonical DNA damage responses. Rather, these genes have activities that would be important for lymphocyte development. Here, these findings and the broader concept that signals initiated by physiologic DNA DSBs provide cues that regulate cell type-specific processes and functions are discussed.

Immune regulation by sphingosine 1-phosphate and its receptors

It is well established that the lysophospholipid and signalling molecule sphingosine 1-phosphate (S1P) has many important functions in immune surveillance. S1P is produced from sphingosine by two distinct sphingosine kinases, SphK1 and SphK2, and acts as an intracellular messenger and as an extracellular ligand of five G protein-coupled cell surface receptors designated S1P(1)-S1P(5). S1P not only regulates peripheral lymphocyte circulation, but also influences their differentiation, activation, infiltration, and local positioning. The therapeutic value of modulating S1P metabolism and S1P receptor function is currently tested in clinical trials and holds great promise for treatment of different autoimmune diseases. Despite its obvious contribution to immune regulation, the analysis of S1P is still challenging. A major obstacle is the difficulty to analyze S1P locally in tissues and within cells due to its high metabolic turnover and the limited resolution of current analytical techniques like liquid chromatography and mass spectrometry. This review focuses on recent advancements to our understanding how different sources of S1P contribute to immune function, and how changes in production, secretion, and degradation of S1P can influence immune responses.

Sphingosine-1-phosphate and lymphocyte egress from lymphoid organs

Much has been learned about how cells enter lymphoid tissues. But how do they leave? Sphingosine-1-phosphate (S1P) has emerged over the past decade as a central mediator of lymphocyte egress. In this review, we summarize the current understanding of how S1P promotes exit from the secondary lymphoid organs and thymus. We review what is known about additional requirements for emigration and summarize the mostly distinct requirements for exit from the bone marrow. Egress from lymphoid organs is limited during immune responses, and we examine how this regulation works. There is accumulating evidence for roles of S1P in directing immune cell behavior within lymphoid tissues. How such actions can fit together with the egress-promoting role of S1P is discussed. Finally, we examine current understanding of how FTY720, a drug that targets S1P receptors and is approved for the treatment of multiple sclerosis, causes immune suppression.

A novel perspective on stem cell homing and mobilization: review on bioactive lipids as potent chemoattractants and cationic peptides as underappreciated modulators of responsiveness to SDF-1 gradients

Hematopoietic stem progenitor cells (HSPCs) respond robustly to α-chemokine stromal-derived factor-1 (SDF-1) gradients, and blockage of CXCR4, a seven-transmembrane-spanning G(αI)-protein-coupled SDF-1 receptor, mobilizes HSPCs into peripheral blood. Although the SDF-1-CXCR4 axis has an unquestionably important role in the retention of HSPCs in bone marrow (BM), new evidence shows that, in addition to SDF-1, the migration of HSPCs is directed by gradients of the bioactive lipids sphingosine-1 phosphate and ceramide-1 phosphate. Furthermore, the SDF-1 gradient may be positively primed/modulated by cationic peptides (C3a anaphylatoxin and cathelicidin) and, as previously demonstrated, HSPCs respond robustly even to very low SDF-1 gradients in the presence of priming factors. In this review, we discuss the role of bioactive lipids in stem cell trafficking and the consequences of HSPC priming by cationic peptides. Together, these phenomena support a picture in which the SDF-1-CXCR4 axis modulates homing, BM retention and mobilization of HSPCs in a more complex way than previously envisioned.

The actin-bundling protein L-plastin is essential for marginal zone B cell development

B cell development is exquisitely sensitive to location within specialized niches in the bone marrow and spleen. Location within these niches is carefully orchestrated through chemotactic and adhesive cues. In this article, we demonstrate the requirement for the actin-bundling protein L-plastin (LPL) in B cell motility toward the chemokines CXCL12 and CXCL13 and the lipid chemoattractant sphingosine-1-phosphate, which guide normal B cell development. Impaired motility of B cells in LPL(-/-) mice correlated with diminished splenic maturation of B cells, with a moderate (40%) loss of follicular B cells and a profound (>80%) loss of marginal zone B cells. Entry of LPL(-/-) B cells into the lymph nodes and bone marrow of mice was also impaired. Furthermore, LPL was required for the integrin-mediated enhancement of Transwell migration but was dispensable for integrin-mediated lymphocyte adhesion. These results suggest that LPL may participate in signaling that enables lymphocyte transmigration. In support of this hypothesis, the phosphorylation of Pyk-2, a tyrosine kinase that integrates chemotactic and adhesive cues, is diminished in LPL(-/-) B cells stimulated with chemokine. Finally, a well-characterized role of marginal zone B cells is the generation of a rapid humoral response to polysaccharide Ags. LPL(-/-) mice exhibited a defective Ab response to Streptococcus pneumoniae, indicating a functional consequence of defective marginal zone B cell development in LPL(-/-) mice.

Conditioning for hematopoietic transplantation activates the complement cascade and induces a proteolytic environment in bone marrow: a novel role for bioactive lipids and soluble C5b-C9 as homing factors

We have observed that conditioning for hematopoietic transplantation by lethal irradiation induces a proteolytic microenvironment in the bone marrow (BM) that activates the complement cascade (CC). As a result, BM is enriched for proteolytic enzymes and the soluble form of the terminal product of CC activation, the membrane attack complex C5b-C9 (MAC). At the same time, proteolytic enzymes induced in irradiated BM impair the chemotactic activity of α-chemokine stromal-derived factor-1 (SDF-1). As SDF-1 is considered a crucial BM chemoattractant for transplanted hematopoietic stem/progenitor cells (HSPCs), we sought to determine whether other factors that are resistant to proteolytic enzymes have a role in this process, focusing on proteolysis-resistant bioactive lipids. We found that the concentrations of sphingosine-1-phosphate (S1P) and ceramide-1-phosphate (C1P) increase in the BM after conditioning for transplantation and that both S1P and, as we show here for the first time, C1P are potent chemoattractants for HSPCs. Next, we observed that C5-deficient mice that do not generate MAC show impaired engraftment of HSPCs. In support of a role for MAC in homing and engraftment, we found that soluble MAC enhances in a CR3 (CD11b/CD18)-dependent manner the adhesion of HSPCs to BM stromal cells and increases the secretion of SDF-1 by BM stroma. We conclude that an increase in BM levels of proteolytic enzyme-resistant S1P and C1P and activation of CC, which leads to the generation of MAC, has an important and previously underappreciated role in the homing of transplanted HSPCs.

The outs and the ins of sphingosine-1-phosphate in immunity

The potent lipid mediator sphingosine-1-phosphate (S1P) is produced inside cells by two closely related kinases, sphingosine kinase 1 (SPHK1) and SPHK2, and has emerged as a crucial regulator of immunity. Many of the actions of S1P in innate and adaptive immunity are mediated by its binding to five G protein-coupled receptors, designated S1PR1-5, but recent findings have also identified important roles for S1P as a second messenger during inflammation. In this Review, we discuss recent advances in our understanding of the roles of S1P receptors and describe the newly identified intracellular targets of S1P that are crucial for immune responses. Finally, we discuss the therapeutic potential of new drugs that target S1P signalling and functions.

B cell homeostasis and plasma cell homing controlled by Krüppel-like factor 2

Krüppel-like factor 2 (KLF2) controls T lymphocyte egress from lymphoid organs by regulating sphingosin-1 phosphate receptor 1 (S1Pr1). Here we show that this is not the case for B cells. Instead, KLF2 controls homeostasis of B cells in peripheral lymphatic organs and homing of plasma cells to the bone marrow, presumably by controlling the expression of β(7)-integrin. In mice with a B cell-specific deletion of KLF2, S1Pr1 expression on B cells was only slightly affected. Accordingly, all splenic B cell subsets including B1 cells were present, but their numbers were increased with a clear bias for marginal zone (MZ) B cells. In contrast, fewer peyers patches harboring fewer B cells were found, and fewer B1 cells in the peritoneal cavity as well as recirculating B cells in the bone marrow were detected. Upon thymus-dependent immunization, IgG titers were diminished, and antigen-specific plasma cells were absent in the bone marrow, although numbers of antigen-specific splenic plasmablasts were normal. KLF2 plays also a role in determining the identity of follicular B cells, as KLF2-deficient follicular B cells showed calcium responses similar to those of MZ B cells and failed to down-regulate MZ B cell signature genes, such as CD21 and CXCR7.

Sphingosine-1-phosphate lyase deficiency produces a pro-inflammatory response while impairing neutrophil trafficking

Sphingosine-1-phosphate (S1P) lyase catalyzes the degradation of S1P, a potent signaling lysosphingolipid. Mice with an inactive S1P lyase gene are impaired in the capacity to degrade S1P, resulting in highly elevated S1P levels. These S1P lyase-deficient mice have low numbers of lymphocytes and high numbers of neutrophils in their blood. We found that the S1P lyase-deficient mice exhibited features of an inflammatory response including elevated levels of pro-inflammatory cytokines and an increased expression of genes in liver associated with an acute-phase response. However, the recruitment of their neutrophils into inflamed tissues was impaired and their neutrophils were defective in migration to chemotactic stimulus. The IL-23/IL-17/granulocyte-colony stimulating factor (G-CSF) cytokine-controlled loop regulating neutrophil homeostasis, which is dependent on neutrophil trafficking to tissues, was disturbed in S1P lyase-deficient mice. Deletion of the S1P4 receptor partially decreased the neutrophilia and inflammation in S1P lyase-deficient mice, implicating S1P receptor signaling in the phenotype. Thus, a genetic block in S1P degradation elicits a pro-inflammatory response but impairs neutrophil migration from blood into tissues.

Sphingosine-1-phosphate and immune regulation: trafficking and beyond

Sphingosine-1-phosphate (S1P) is a bioactive lipid with important functions in the immune system. S1P levels are regulated by the balance between its synthesis through sphingosine kinases and its degradation by S1P lyase. S1P signals through plasma membrane G-protein-coupled receptors (S1PR1-S1PR5) or acts directly on intracellular targets. Although it has long been known that the S1P-S1PR1 axis mediates T cell egress from lymphoid organs, recent studies have revealed intrinsic functions of S1P and its receptors in both innate and adaptive immune systems that are independent of immune cell trafficking. Here I summarize recent advances in understanding of the roles of S1P and S1P receptors in inflammatory and allergic responses and lymphocyte differentiation, which directly contribute to the regulation of inflammatory and autoimmune diseases. I also describe strategies to target S1P and S1P receptors for immune-mediated diseases, particularly the immunosuppressant FTY720 (fingolimod), which has recently become the first oral therapy for relapsing multiple sclerosis.

An update on sphingosine-1-phosphate and other sphingolipid mediators

Sphingolipids comprise a complex family of naturally occurring molecules that are enriched in lipid rafts and contribute to their unique biochemical properties. Membrane sphingolipids also serve as a reservoir for bioactive metabolites including sphingosine, ceramide, sphingosine-1-phosphate and ceramide-1-phosphate. Among these, sphingosine-1-phosphate has emerged as a central regulator of mammalian biology. Sphingosine-1-phosphate is essential for mammalian brain and cardiac development and for maturation of the systemic circulatory system and lymphatics. In addition, sphingosine-1-phosphate contributes to trafficking and effector functions of lymphocytes and other hematopoietic cells and protects against various forms of tissue injury. However, sphingosine-1-phosphate is also an oncogenic lipid that promotes tumor growth and progression. Recent preclinical and clinical investigations using pharmacological agents that target sphingosine-1-phosphate, its receptors and the enzymes required for its biosynthesis and degradation demonstrate the promise and potential risks of modulating sphingosine-1-phosphate signaling in treatment strategies for autoimmunity, cancer, cardiovascular disease and other pathological conditions.