Plasma cell S1P1 expression determines secondary lymphoid organ retention versus bone marrow tropism

Krüppel-like factors in lymphocyte biology

The Krüppel-like factor family of transcription factors plays an important role in differentiation, function, and homeostasis of many cell types. While their role in lymphocytes is still being determined, it is clear that these factors influence processes as varied as lymphocyte quiescence, trafficking, differentiation, and function. This review will present an overview of how these factors operate and coordinate with each other in lymphocyte regulation.

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.

The genetic network controlling plasma cell differentiation

Upon activation by antigen, mature B cells undergo immunoglobulin class switch recombination and differentiate into antibody-secreting plasma cells, the endpoint of the B cell developmental lineage. Careful quantitation of these processes, which are stochastic, independent and strongly linked to the division history of the cell, has revealed that populations of B cells behave in a highly predictable manner. Considerable progress has also been made in the last few years in understanding the gene regulatory network that controls the B cell to plasma cell transition. The mutually exclusive transcriptomes of B cells and plasma cells are maintained by the antagonistic influences of two groups of transcription factors, those that maintain the B cell program, including Pax5, Bach2 and Bcl6, and those that promote and facilitate plasma cell differentiation, notably Irf4, Blimp1 and Xbp1. In this review, we discuss progress in the definition of both the transcriptional and cellular events occurring during late B cell differentiation, as integrating these two approaches is crucial to defining a regulatory network that faithfully reflects the stochastic features and complexity of the humoral immune response.

Sphingosine-1-phospate receptor 4 (S1P₄) deficiency profoundly affects dendritic cell function and TH17-cell differentiation in a murine model

Although predominantly expressed on lymphocytic and hematopoietic cells, the role of sphingosine-1-phospate receptor 4 (S1P(4)) in immune homeostasis is still poorly understood. In this report, we used a S1P(4)-deficient murine model to characterize the biological role of S1P(4)-mediated S1P signaling in the immune system. S1p(4)(-/-) animals showed normal peripheral lymphocyte numbers and a regular architecture of secondary lymphoid organs. Interestingly, S1P(4) only marginally affects T-cell function in vivo. In contrast, dendritic cell (DC) migration and cytokine secretion are profoundly affected by S1P(4) deficiency. Lack of S1P(4) expression on DCs significantly reduces T(H)17 differentiation of T(H) cells. Furthermore, in various in vivo models of T(H)1- or T(H)2-dominated immune reactions, S1P(4) deficiency consistently increased the amplitude of T(H)2-dominated immune responses, while those depending on T(H)1-dominated mechanisms were diminished. Finally, S1p(4)(-/-) mice showed decreased pathology in a model of dextran sulfate sodium-induced colitis. In summary, for the first time, we show that S1P(4) signaling is involved in the regulation of DC function and T(H)17 T-cell differentiation. S1P(4)-mediated S1P signaling also modifies the course of various immune diseases in a murine model. We propose that S1P(4) may constitute an interesting target to influence the course of various autoimmune pathologies.

Mechanisms of fingolimod's efficacy and adverse effects in multiple sclerosis

Until recently, all approved multiple sclerosis (MS) disease treatments were administered parenterally. Oral fingolimod was approved in September 2010 by the US Food and Drug Administration to reduce relapses and disability progression in relapsing forms of MS. In the clinical trials that led to approval, fingolimod reduced not only acute relapses and magnetic resonance imaging lesion activity but also disability progression and brain volume loss, suggesting preservation of tissue. Fingolimod's mechanism of action in MS is not known with certainty. Its active form, fingolimod-phosphate (fingolimod-P), is a sphingosine 1-phosphate receptor (S1PR) modulator that inhibits egress of lymphocytes from lymph nodes and their recirculation, potentially reducing trafficking of pathogenic cells into the central nervous system (CNS). Fingolimod also readily penetrates the CNS, and fingolimod-P formed in situ may have direct effects on neural cells. Fingolimod potently inhibits the MS animal model, experimental autoimmune encephalomyelitis, but is ineffective in mice with selective deficiency of the S1P₁ S1PR subtype on astrocytes despite normal expression in the immune compartment. These findings suggest that S1PR modulation by fingolimod in both the immune system and CNS, producing a combination of beneficial anti-inflammatory and possibly neuroprotective/reparative effects, may contribute to its efficacy in MS. In clinical trials, fingolimod was generally safe and well tolerated. Its interaction with S1PRs in a variety of tissues largely accounts for the reported adverse effects, which were seen more frequently with doses 2.5 to 10x the approved 0.5 mg dose. Fingolimod's unique mechanism of action distinguishes it from all other currently approved MS therapies.

Fingolimod: a novel oral immunomodulatory treatment for multiple sclerosis

Fingolimod (FTY720), an immunomodulator that acts on sphingosine-1-phosphate receptors, is the first oral disease-modifying agent to be approved by the US FDA for the treatment of relapsing forms of multiple sclerosis (MS). Compared with other disease-modifying agents, fingolimod is unique in its mechanisms of action: it sequesters lymphocytes into lymph nodes without directly inhibiting effector functions, and it exerts pleiotropic actions on cultured oligodendrocytes and oligodendrocyte progenitors. Whether the latter contributes to the favorable response to this drug is currently being investigated. Results from Phase II and III clinical trials demonstrate that fingolimod is highly effective in relapsing–remitting MS. In this article, we review the background on MS therapy, the mechanisms and pharmacology of fingolimod and its benefit–risk profile as a novel therapy in MS.

CXCR3-dependent plasma blast migration to the central nervous system during viral encephalomyelitis

Immunoglobulin in cerebral spinal fluid and antibody secreting cells (ASC) within the central nervous system (CNS) parenchyma are common hallmarks of microbial infections and autoimmune disorders. However, the signals directing ASC migration into the inflamed CNS are poorly characterized. This study demonstrates that CXCR3 mediates CNS accumulation of ASC during neurotropic coronavirus-induced encephalomyelitis. Expansion of CXCR3-expressing ASC in draining lymph nodes prior to accumulation within the CNS was consistent with their recruitment by sustained expression of CXCR3 ligands during viral persistence. Both total and virus-specific ASC were reduced greater than 80% in the CNS of infected CXCR3(-/-) mice. Similar T cell CNS recruitment and local T cell-dependent antiviral activity further indicated that the ASC migration defect was T cell independent. Furthermore, in contrast to the reduction of ASC in the CNS, neither virus-specific ASC trafficking to bone marrow nor antiviral serum antibody was reduced relative to levels in control mice. Impaired ASC recruitment into the CNS of infected CXCR3(-/-) mice coincided with elevated levels of persisting viral RNA, sustained infectious virus, increased clinical disease, and mortality. These results demonstrate that CXCR3 ligands are indispensable for recruitment of activated ASC into the inflamed CNS and highlight their local protective role during persistent infection.

Discovery of a Potent, S1P3-Sparing Benzothiazole Agonist of Sphingosine-1-Phosphate Receptor 1 (S1P1)

Optimization of a benzofuranyl S1P1 agonist lead compound (3) led to the discovery of 1-(3-fluoro-4-(5-(2-fluorobenzyl)benzo[d]thiazol-2-yl)benzyl)azetidine-3-carboxylic acid (14), a potent S1P1 agonist with minimal activity at S1P3. Dosed orally at 0.3 mg/kg, 14 significantly reduced blood lymphocyte counts 24 h postdose and attenuated a delayed type hypersensitivity (DTH) response to antigen challenge.

Interferon-α accelerates murine systemic lupus erythematosus in a T cell-dependent manner

OBJECTIVE

To investigate the mechanism by which interferon-α (IFNα) accelerates systemic lupus erythematosus (SLE) in (NZB×NZW)F1 (NZB/NZW) mice.

METHODS

NZB/NZW mice were treated with an adenovirus expressing IFNα. In some mice, T cells were depleted with an anti-CD4 antibody. The production of anti-double-stranded DNA (anti-dsDNA) antibodies was measured by enzyme-linked immunosorbent assay and enzyme-linked immunospot assay. Germinal centers and antibody-secreting cells (ASCs) in spleens and IgG deposition and leukocyte infiltrates in kidneys were visualized by immunofluorescence staining. The phenotype of splenic cells was determined by flow cytometry. Finally, somatic hypermutation and gene usage in VH regions of IgG2a and IgG3 were studied by single-cell polymerase chain reaction.

RESULTS

IFNα-accelerated lupus in NZB/NZW mice was associated with elevated serum levels of IgG2 and IgG3 anti-dsDNA antibodies and accumulation of many IgG ASCs in the spleen, which did not develop into long-lived plasma cells. Furthermore, IgG2a and IgG3 antibodies in the mice were highly somatically mutated and used distinct repertoires of VH genes. The induction of SLE in the mice was associated with an increase in B cell Toll-like receptor 7 expression, increased serum levels of BAFF, interleukin-6 (IL-6), and tumor necrosis factor α, and induction of T cells expressing IL-21. Although IFNα drove a T cell-independent increase in serum levels of IgG, autoantibody induction and the development of nephritis were both completely dependent on CD4+ T cell help.

CONCLUSION

These findings demonstrate that, although IFNα activates both innate and adaptive immune responses in NZB/NZW mice, CD4+ T cells are necessary for IFNα-driven induction of anti-dsDNA antibodies and clinical SLE.

Evidence-based assessment of potential use of fingolimod in treatment of relapsing multiple sclerosis

Multiple sclerosis is an autoimmune inflammatory demyelinating disease of the central nervous system and represents one of the most common causes of chronic neurologic disability in young adults. All the current disease-modifying drugs are administered parenterally, and can be associated with varying degrees of injection site or infusion-related reactions. Together with other side effects, the parenteral route of administration is one of the key factors affecting adherence to therapy in multiple sclerosis. Fingolimod (FTY720) is an immunomodulator that acts on sphingosine 1-phosphate (S1P) receptors and is the first oral drug approved by the US Food and Drug Administration for the treatment of relapsing-remitting multiple sclerosis. Downmodulation of S1P receptor type 1 (S1P₁) slows the egress of lymphocytes from lymph nodes and recirculation to the central nervous system, reduces astrogliosis, and inhibits angiogenesis during chronic neuroinflammation. Fingolimod also regulates the migration of B cells and dendritic cells, and enhances endothelial barrier function. Results from Phase II and III clinical trials provide robust evidence of the efficacy of fingolimod in relapsing-remitting multiple sclerosis. While some caution should be exercised in terms of safety issues, the introduction of fingolimod represents a great advance in the treatment of relapsing-remitting multiple sclerosis. The pharmacologic data on fingolimod and its efficacy and safety in multiple sclerosis are reviewed in this paper.

Molecular pathogenesis of multiple myeloma: chromosomal aberrations, changes in gene expression, cytokine networks, and the bone marrow microenvironment

This chapter focuses on two aspects of myeloma pathogenesis: (1) chromosomal aberrations and resulting changes in gene and protein expression with a special focus on growth and survival factors of malignant (and normal) plasma cells and (2) the remodeling of the bone marrow microenvironment induced by accumulating myeloma cells. We begin this chapter with a discussion of normal plasma cell generation, their survival, and a novel class of inhibitory factors. This is crucial for the understanding of multiple myeloma, as several abilities attributed to malignant plasma cells are already present in their normal counterpart, especially the production of survival factors and interaction with the bone marrow microenvironment (niche). The chapter closes with a new model of pathogenesis of myeloma.

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.

Memory B and memory plasma cells

Vaccination provides a powerful means to control infections. It exploits and exemplifies the ability of the immune system to preserve the information that a specific pathogen has been encountered in the past. The cells and molecular mechanisms of immunological memory are still being discussed controversially. Here, we review the current concepts of memory B cells, the signals involved in their maintenance, and their role in enhanced secondary reactions. Memory plasma cells, secreting protective antibodies over lifetime, have been recognized only recently. Their characterization as cells resting in terms of proliferation and migration, and surviving in dedicated stromal niches, in the absence of antigen, has generated new concepts of how memory cells in general are organized by stroma cells, the 'resting memory'. In autoimmunity and chronic inflammation, memory B cells and memory plasma cells can be essential players, and they require special attention, as they do not respond to most conventional therapies. Their selective targeting will depend on a molecular understanding of their lifestyle.

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.

FTY720 regulates bone marrow egress of eosinophils and modulates late-phase skin reaction in mice

Eosinophilia in the blood and skin is frequently observed in patients with certain inflammatory skin diseases, such as atopic dermatitis. However, the mechanism underlying eosinophil circulation and the role of eosinophils in cutaneous immune responses remain unclear. In repeated hapten application-induced cutaneous responses in BALB/c mice, the administration of FTY720 before the last challenge decreased the number of skin-infiltrating eosinophils and reduced the late-phase reaction. A similar reduction of the late-phase reaction was observed by a sphingosine-1-phosphate G protein-coupled receptor (S1P1)-selective agonist, SEW2871. We monitored numerous alterations of eosinophils in the blood, spleen, bone marrow, and lymph nodes of interleukin-5 transgenic mice, used as an eosinophilia model, following FTY720 administration. The number of circulating eosinophils was significantly decreased after treatment with FTY720, and eosinophils accumulated in the bone marrow. In addition, eosinophils expressed S1P1, S1P3, and S1P4 mRNAs, and their chemotactic response to S1P was abolished by FTY720 as well as by SEW2871. These findings suggest that FTY720 affects the number of eosinophils in both the blood and skin by inhibiting the egress of eosinophils from the bone marrow and thus downmodulating the late-phase reaction.

Role of Kruppel-like factors in leukocyte development, function, and disease

The Krüppel-like transcription factor (KLF) family participates in diverse aspects of cellular growth, development, differentiation, and activation. Recently, several groups have identified new connections between the function of these factors and leukocyte responses in health and disease. Gene targeting of individual KLFs in mice has uncovered novel and unexpected physiologic roles among myeloid and lymphocyte cell lineage maturation, particularly in the bone marrow niche and blood. In addition, several KLF family members are downstream targets of stimuli and signaling pathways critical to T-cell trafficking, T regulatory cell differentiation or suppressor function, monocyte/macrophage activation or renewal, and B memory cell maturation or activation. Indeed, KLFs have been implicated in subtypes of leukemia, lymphoma, autoimmunity, and in acute and chronic inflammatory disease states, such as atherosclerosis, diabetes, and airway inflammation, raising the possibility that KLFs and their upstream signals are of therapeutic interest. This review focuses on the relevant literature of Krüppel-like factors in leukocyte biology and their implications in clinical settings.

Development and migration of plasma cells in the mouse lymph node

In this study, we imaged the differentiation and migratory behavior of nascent plasma cells (PCs) in mouse lymph nodes by intravital microscopy. Pre-PCs exhibited a unique migration pattern characterized by long, linear paths that were randomly oriented. Although chemotaxis via Galphai coupled-receptors has been implicated in PC migration, treatment with Pertussis toxin (Ptx), which ablates these signals, did not prevent movement of pre-PCs while it arrested other lymphocytes. In vitro, pre-PCs displayed processive amoeboid locomotion on surfaces coated with integrin ligand, whereas fully differentiated PCs moved slowly or were arrested. Both PC arrest and differentiation occurred in the medullary cords. Ptx treatment before PC differentiation blocked their accumulation in the medullary cords but pre-PCs still differentiated in other lymph node regions. Taken together, we suggest pre-PCs undergo a persistent random walk to find the medullary cords, where localized chemokines help retain these cells until they undergo differentiation and arrest in situ.

Cell-surface residence of sphingosine 1-phosphate receptor 1 on lymphocytes determines lymphocyte egress kinetics

The sphingosine 1-phosphate receptor 1 (S1P₁) promotes lymphocyte egress from lymphoid organs. Previous work showed that agonist-induced internalization of this G protein–coupled receptor correlates with inhibition of lymphocyte egress and results in lymphopenia. However, it is unclear if S1P₁ internalization is necessary for this effect. We characterize a knockin mouse (S1p1r^S5A/S5A) in which the C-terminal serine-rich S1P₁ motif, which is important for S1P₁ internalization but dispensable for S1P₁ signaling, is mutated. T cells expressing the mutant S1P₁ showed delayed S1P₁ internalization and defective desensitization after agonist stimulation. Mutant mice exhibited significantly delayed lymphopenia after S1P₁ agonist administration or disruption of the vascular S1P gradient. Adoptive transfer experiments demonstrated that mutant S1P₁ expression in lymphocytes, rather than endothelial cells, facilitated this delay in lymphopenia. Thus, cell-surface residency of S1P₁ on T cells is a primary determinant of lymphocyte egress kinetics in vivo.

S1P1 receptor directs the release of immature B cells from bone marrow into blood

S1P1 receptor expression is required for the egress of newly formed T cells from the thymus and exit of mature T and B cells from secondary lymphoid organs. In this study, we deleted the expression of the S1P1 receptor gene (S1pr1) in developing B cells in the bone marrow. Although B cell maturation within the bone marrow was largely normal in the B cell–specific S1pr1 knockout (B-S1pr1KO) mice, their newly generated immature B cells appeared in the blood at abnormally low numbers as compared with control mice. In the bone marrow of B-S1pr1KO mice, immature B cells in contact with the vascular compartment displayed increased apoptosis as compared with control mice. Forced expression of CD69, a negative regulator of S1P1 receptor expression, in developing bone marrow B cells also reduced the number of immature B cells in the blood. Attenuation of CXCR4 signaling, which is required for the proper retention of developing B cells in bone marrow, did not release immature B cells into the blood of B-S1pr1KO mice as effectively as in control mice. Our results indicate that the S1P1 receptor provides a signal necessary for the efficient transfer of newly generated immature B cells from the bone marrow to the blood.

Naive CD4+ T lymphocytes circulate through lymphoid organs to interact with endogenous antigens and upregulate their function

Naive T lymphocytes recirculate through the lymph-vascular system and enter and exit lymphoid organs. Using mice expressing the photoconvertible fluorescence protein Kaede, we demonstrated that naive T cells seek to interact with endogenous Ags after migrating to the lymphoid organs. The interaction with endogenous Ags transiently induces CD69 expression on T cells, which prolongs retention in the lymphoid organs. Cells that fail to express CD69 or lose CD69 expression migrate to other lymphoid organs. Functionally, CD69(+)-naive CD4(+) T cells exhibit faster and greater cytokine production than do CD69(-) naive CD4(+) T cells. These results indicate that CD4(+) T cells continuously migrate to interact with endogenous Ags, and such an interaction plays an important role in the Ag reactivity of naive CD4(+) T cells.

Molecular signatures of quiescent, mobilized and leukemia-initiating hematopoietic stem cells

Hematopoietic stem cells (HSC) are rare, multipotent cells capable of generating all specialized cells of the blood system. Appropriate regulation of HSC quiescence is thought to be crucial to maintain their lifelong function; however, the molecular pathways controlling stem cell quiescence remain poorly characterized. Likewise, the molecular events driving leukemogenesis remain elusive. In this study, we compare the gene expression profiles of steady-state bone marrow HSC to non-self-renewing multipotent progenitors; to HSC treated with mobilizing drugs that expand the HSC pool and induce egress from the marrow; and to leukemic HSC in a mouse model of chronic myelogenous leukemia. By intersecting the resulting lists of differentially regulated genes we identify a subset of molecules that are downregulated in all three circumstances, and thus may be particularly important for the maintenance and function of normal, quiescent HSC. These results identify potential key regulators of HSC and give insights into the clinically important processes of HSC mobilization for transplantation and leukemic development from cancer stem cells.

Involvement of Wnt signaling in dermal fibroblasts

Pachydermoperiostosis (PDP) is a rare disease characterized by unique phenotypes of the skin and bone, such as thick skin, implying that it may be caused by dysregulation of mesenchymal cells. The aim of this study is to examine the roles of dermal fibroblasts in the pathogenesis of pachydermia in association with Wnt signaling. The numbers of cultured fibroblasts were compared between healthy donors and PDP patients, and mRNA expression profiles in cultured dermal fibroblasts were examined by DNA microarray analysis and real-time reverse transcription-PCR. DKK1 and beta-catenin protein expressions were also evaluated by immunohistochemistry in the skin. To evaluate the in vivo roles of DKK1 in mice, DKK1 small interfering RNA was injected to the ears. We found that PDP fibroblasts proliferated more than control fibroblasts and that mRNA expression of a Wnt signaling antagonist, DKK1, was much lower in PDP fibroblasts than in normal ones. Consistently, decreased expression of DKK1 in fibroblasts and enhanced expression of beta-catenin were noted in PDP patients. Moreover, recombinant human DKK1 protein decreased the proliferation of dermal fibroblasts. In accord with the above human studies, intradermal injections of DKK1 small interfering RNA into mouse ears increased ear thickness as seen in PDP. Our findings suggest that enhanced Wnt signaling contributes to the development of pachydermia by enhancing dermal fibroblast functions.

Stromal cell contributions to the homeostasis and functionality of the immune system

A defining characteristic of the immune system is the constant movement of many of its constituent cells through the secondary lymphoid tissues, mainly the spleen and lymph nodes, where crucial interactions that underlie homeostatic regulation, peripheral tolerance and the effective development of adaptive immune responses take place. What has only recently been recognized is the role that non-haematopoietic stromal elements have in many aspects of immune cell migration, activation and survival. In this Review, we summarize our current understanding of lymphoid compartment stromal cells, examine their possible heterogeneity, discuss how these cells contribute to immune homeostasis and the efficient initiation of adaptive immune responses, and highlight how targeting of these elements by some pathogens can influence the host immune response.

Role of FTY720 on M1 and M2 macrophages, lymphocytes, and chemokines in 5/6 nephrectomized rats

Renal injury is accompanied by the presence of infiltrating inflammatory cells in the glomerulus and tubulointerstitium. FTY720 modifies lymphocyte migration into injured tissues by lymphocyte sequestration to secondary lymphoid organs. The purpose of this study was to examine the potential of FTY720 to influence the inflammatory response in a nonimmunological model of renal failure. Sham-operated and 5/6 nephrectomized (SNX) Sprague-Dawley rats received two different doses of FTY720 or vehicle orally for 14 wk. Treatment with FTY720 reduced glomerular and tubulointerstitial damage in SNX rats but failed to stabilize creatinine clearance. The increase in gene expression of chemokine receptors CCR1, CCR2, and CCR5 in kidneys of vehicle-treated SNX rats was significantly attenuated by high-dose FTY720. Treatment with high-dose FTY720 tended to normalize RANTES and MCP-1 renal gene expression. FTY720 affected not only glomerular and tubulointerstitial lymphocytes, but M1 and M2 phenotype macrophages were also reduced. FTY720 significantly reduced key mediators of renal inflammation and fibrosis. FTY720 also decreased immunoregulation of M2 macrophages, which are beneficial for tissue remodeling and repair.

Limited humoral immunoglobulin E memory influences serum immunoglobulin E levels in blood

The switch of B cells expressing membrane-bound Igs, which serve as antigen receptors, to antibody-secreting plasmablasts and finally to non-dividing, long-lived plasma cells (PCs) lacking an antigen receptor, marks the terminal differentiation of a B cell. Antibody-secreting PCs represent the key cell type for the maintenance of a proactive humoral immunological memory. Although some populations of long-lived PCs persist in the spleen, most of them return to their 'place of birth' and travel to the bone marrow or invade inflamed tissues, where they survive up to several months in survival niches as resident, immobile cells. Existing data strongly support the notion that isotype-specific receptor signalling influences the migration behaviour of plasmablasts to the bone marrow. The recent observation in the murine system that the immigration of plasmablasts and the final differentiation to long-lived PCs in the bone marrow is dependent on the expressed B-cell isotype and the related expression of chemokine receptors leads to the conclusion that during a T-helper type 2 (Th2)-mediated immune response in wild type mice, IgE plasmablasts do not have the same chance to contribute to long-lived PC memory as IgG1 plasmablasts. The overall limited humoral IgE memory additionally restricts the quantity of IgE Igs in the serum.

Ectopic germinal centers are rare in Sjogren's syndrome salivary glands and do not exclude autoreactive B cells

This study reports on the characterization of B cells of germinal center (GC)-like structures infiltrating the salivary glands (SGs) of patients with Sjögren's syndrome. Eight two-color combinations were devised to characterize the phenotype of these B cells in 11 SG specimens selected from biopsies obtained from 40 Sjögren's syndrome patients and three normal tonsils. The 9G4 mAb, which recognizes V4.34-encoded autoAbs, enabled us to identify autoreactive B cells. Quantitative RT-PCR was used to determine the level of mRNAs for activation-induced cytidine deaminase (AICDA), repressors and transcription factors. CD20(+)IgD(-)CD38(+)CD21(+)CD24(-) B cells, similar to those identified in tonsil GCs, were seen in the SGs of four patients and, and since they expressed AICDA, they were termed "real GCs". CD20(+)IgD(+)CD38(-)CD21(+)CD24(+) B cells, seen in aggregates from the remaining seven samples, were characteristically type 2 transitional B cells and marginal zone-type B cells. They lacked AICDA mRNAs and were termed "aggregates". Real GCs from SGs contained mRNAs for Pax-5 and Bcl-6, like tonsil GC cells, whereas aggregates contained mRNAs for Notch-2, Blimp-1, IRF-4, and BR3, similar to marginal zone B cells. Further experimental data in support of this dichotomy included the restriction of CXCR5 expression to real GC cells, while sphingosine 1-phosphate receptor 1 was expressed only in aggregates. In contrast, both types of B cell clusters expressed the idiotype recognized by the 9G4 mAb. Our data indicate that, in SGs, a minority of B cell clusters represent genuine GC cells, while the majority manifest features of being type 2 transitional B cells and marginal zone cells. Interestingly, both types of B cell aggregates include autoreactive B cells.

Human immunodeficiency virus type 1 gp120 reprogramming of CD4+ T-cell migration provides a mechanism for lymphadenopathy

Infection by human immunodeficiency virus type 1 (HIV-1) is associated with decreases in peripheral CD4(+) T cells and development of lymphadenopathy. The precise mechanisms by which HIV-1 induces these changes have not been elucidated. T-cell trafficking through lymphoid tissues is facilitated by CCL21-mediated entry and sphingosine-1-phosphate (S1P)-mediated egress. Having previously determined that HIV-1 envelop glycoprotein, gp120, directly alters T-cell migration, we investigated whether gp120 without HIV-1 infection could influence the responses of CD4(+) T cells to the signals involved in T-cell trafficking through lymph tissue. Incubation of normal human T cells with gp120 for 1 h resulted in reprogramming of CD4 T-cell migratory responses by increasing sensitivity to CCL20 and CCL21 and complete inhibition of migration to S1P. Incubation of human T cells with gp120 prior to injection into NOD.CB17-Prkdc(scid)/J mice resulted in increases in lymph node accumulation of CD4(+) T cells, with reciprocal decreases in blood and spleen compared to T cells not exposed to gp120. The effects of gp120 required CD4 signaling mediated through p56(lck). These findings suggest that gp120 alone can alter CD4(+) influx and efflux from lymph nodes in a fashion consistent with the development of lymphopenia and lymphadenopathy.

Dendritic cells and monocyte/macrophages that create the IL-6/APRIL-rich lymph node microenvironments where plasmablasts mature

IL-6 and APRIL influence the growth, differentiation, and survival of normal and neoplastic Ab-forming cells (AFC). In this study, we identify two subsets of myeloid cells that associate with the AFC and are the main producers of these factors during a T-dependent Ab response to alum-precipitated protein in mouse lymph nodes. First CD11c(+)CD8alpha(-) dendritic cells located in the perivascular area of the T zone provide about half of the IL-6 mRNA produced in the node together with significant amounts of APRIL mRNA. The number of these cells increases during the response, at least in part due to local proliferation. The second subset comprises Gr1(+)CD11b(+)F4/80(+) monocyte/macrophages. These colonize the medullary cords during the response and are the other main IL-6 mRNA producers and the greatest source of APRIL mRNA. This medullary cord monocyte/macrophage subset results in local increase of APRIL mRNA that mirrors the polarity of CXCL12 expression in the node. The distribution of these myeloid cell subsets correlates with a gradient of AFC maturation assessed by progressive loss of Ki67 as AFC pass from the B cell follicle along the perivascular areas to the medullary cords.

Sphingosine-1-phosphate and modulation of vascular tone

Sphingosine-1-phosphate (S1P) is a phosphorylated product of sphingosine, the core structure of the class of lipids termed sphingolipids. S1P is a naturally occurring lipid metabolite, and usually is present at a concentration of a few 100 nanomolar in human sera. S1P has been found to exert a diverse set of physiological and pathophysiological responses in mammalian tissues through the activation of heterotrimeric G-proteins that in turn modulate the activity of various downstream effecter molecules. In blood vessels, vascular endothelial cells and smooth muscle cells express specific receptors for S1P that modulate vascular tone. This article will provide a brief overview of S1P metabolism in the vasculature and will discuss some of the pathways whereby S1P regulates intracellular signal transduction pathways in endothelial and smooth muscle cells, leading to the activation of both vasorelaxation and vasoconstriction responses.

Migration of antibody secreting cells towards CXCL12 depends on the isotype that forms the BCR

Truncation of the cytoplasmic tail of membrane-bound IgE in vivo results in lower serum IgE levels, decreased numbers of IgE-secreting plasma cells and the abrogation of specific secondary immune responses. Here we present mouse strain KN1 that expresses a chimeric epsilon-gamma1 BCR, consisting of the extracellular domains of the epsilon gene and the transmembrane and cytoplasmic domains of the gamma1 gene. Thus, differences in the IgE immune response of KN1 mice reflect the influence of the "gamma1-mediated signalling" of mIgE bearing B cells. KN1 mice show an increased serum IgE level, resulting from an elevated number of IgE-secreting cells. Although the primary IgE immune response in KN1 mice is inconspicuous, the secondary response is far more robust. Most strikingly, IgE-antibody secreting cells with "gamma1-signalling history" migrate more efficiently towards the chemokine CXCL12, which guides plasmablasts to plasma cell niches, than IgE-antibody secreting cells with WT "epsilon-signalling history". We conclude that IgE plasmablasts have an intrinsic, lower chance to contribute to the long-lived plasma cell pool than IgG1 plasmablasts.

Incomplete inhibition of sphingosine 1-phosphate lyase modulates immune system function yet prevents early lethality and non-lymphoid lesions

Background

S1PL is an aldehyde-lyase that irreversibly cleaves sphingosine 1-phosphate (S1P) in the terminal step of sphingolipid catabolism. Because S1P modulates a wide range of physiological processes, its concentration must be tightly regulated within both intracellular and extracellular environments.

Methodology

In order to better understand the function of S1PL in this regulatory pathway, we assessed the in vivo effects of different levels of S1PL activity using knockout (KO) and humanized mouse models.

Principal Findings

Our analysis showed that all S1PL-deficient genetic models in this study displayed lymphopenia, with sequestration of mature T cells in the thymus and lymph nodes. In addition to the lymphoid phenotypes, S1PL KO mice (S1PL^−/−) also developed myeloid cell hyperplasia and significant lesions in the lung, heart, urinary tract, and bone, and had a markedly reduced life span. The humanized knock-in mice harboring one allele (S1PL^H/−) or two alleles (S1PL^H/H) of human S1PL expressed less than 10 and 20% of normal S1PL activity, respectively. This partial restoration of S1PL activity was sufficient to fully protect both humanized mouse lines from the lethal non-lymphoid lesions that developed in S1PL^−/− mice, but failed to restore normal T-cell development and trafficking. Detailed analysis of T-cell compartments indicated that complete absence of S1PL affected both maturation/development and egress of mature T cells from the thymus, whereas low level S1PL activity affected T-cell egress more than differentiation.

Significance

These findings demonstrate that lymphocyte trafficking is particularly sensitive to variations in S1PL activity and suggest that there is a window in which partial inhibition of S1PL could produce therapeutic levels of immunosuppression without causing clinically significant S1P-related lesions in non-lymphoid target organs.

The alliance of sphingosine-1-phosphate and its receptors in immunity

Sphingosine-1-phosphate (S1P) is a biologically active metabolite of plasma-membrane sphingolipids that is essential for immune-cell trafficking. Its concentration is increased in many inflammatory conditions, such as asthma and autoimmunity. Much of the immune function of S1P results from the engagement of a family of G-protein-coupled receptors (S1PR1-S1PR5). Recent findings on the role of S1P in immunosurveillance, the discovery of regulatory mechanisms in S1P-mediated immune-cell trafficking and new advances in understanding the mechanism by which S1P affects immune-cell function indicate that the alliance between S1P and its receptors has a fundamental role in immunity.

Biological roles of lysophospholipid receptors revealed by genetic null mice: an update

Two lysophospholipids (LPs), lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), are known to affect various cellular events. Their actions are mediated by binding to at least ten bona fide high-affinity G protein-coupled receptors referred to as LPA1-5 and S1P1-5. These LPs are expressed throughout the body and are involved in a range of biological activities including normal development, as well as functioning in most organ systems. A growing number of biological functions have been uncovered in vivo using single- or multiple-null mice for each LP receptor. This review will focus on findings from in vivo as well as in vitro studies using genetic null mice for the LP receptors, LPA1,2,3 and S1P1,2,3,5, and for the LP producing enzymes, autotaxin and sphingosine kinase 1/2.

The B-cell response to protein antigens in immunity and transplantation

The potential for B cells to play a role in transplantation has been recently increasingly recognized. Given that the field has focused almost exclusively on the T cell, here we review B-cell biology. We first describe the B-cell response to protein antigen, and then examine the antibody-independent effector functions of B cells, in particular antigen presentation to T cells. Finally we use autoimmunity as an analogy to focus on the potential role for B cells in transplantation. The direct evidence for this is then marshalled in the accompanying article by Zarkhin and Sarwal.

Ascomycete derivative to MS therapeutic: S1P receptor modulator FTY720

Fingolimod (FTY720) represents the first in a new class of immune-modulators whose target is sphingosine-1-phosphate (S1P) receptors. It was first identified by researchers at Kyoto University and Yoshitomi Pharmaceutical as a chemical derivative of the ascomycete metabolite ISP-1 (myriocin). Unlike its natural product parent, FTY720 does not interfere with sphingolipid biosynthesis. Instead, its best characterized mechanism of action upon in vivo phosphorylation, leading to the active principle FTY720-P, is the rapid and reversible inhibition of lymphocyte egress from peripheral lymph nodes. As a consequence of S1P1 receptor internalization, tissue-damaging T-cells can not recirculate and infiltrate sites of inflammation such as the central nervous system (CNS). Furthermore, FTY720-P modulation of S1P receptor signaling also enhances endothelial barrier function. Due to its mode of action, FTY720 effectively prevents transplant rejection and is active in various autoimmune disease models. The most striking efficacy is in the multiple sclerosis (MS) model of experimental autoimmune encephalomyelitis, which has now been confirmed in the clinic. FTY720 demonstrated promising results in Phase II trials and recently entered Phase III in patients with relapsing MS. Emerging evidence suggests that its efficacy in the CNS extends beyond immunomodulation to encompass other aspects of MS pathophysiology, including an influence on the blood-brain-barrier and glial repair mechanisms that could ultimately contribute to restoration of nerve function. FTY720 may represent a potent new therapeutic modality in MS, combined with the benefit of oral administration.

The vascular S1P gradient-cellular sources and biological significance

Sphingosine 1-phosphate (S1P), a product of sphingomyelin metabolism, is enriched in the circulatory system whereas it is estimated to be much lower in interstitial fluids of tissues. This concentration gradient, termed the vascular S1P gradient appears to form as a result of substrate availability and the action of metabolic enzymes. S1P levels in blood and lymph are estimated to be in the muM range. In the immune system, the S1P gradient is needed as a spatial cue for lymphocyte and hematopoietic cell trafficking. During inflammatory reactions in which enhanced vascular permeability occurs, a burst of S1P becomes available to its receptors in the extravascular compartment, which likely contributes to the tissue reactions. Thus, the presence of the vascular S1P gradient is thought to contribute to physiological and pathological conditions. From an evolutionary perspective, S1P receptors may have co-evolved with the advent of a closed vascular system and the trafficking paradigms for hematopoietic cells to navigate in and out of the vascular system.

Finding a way out: lymphocyte egress from lymphoid organs

The egress of lymphocytes from the thymus and secondary lymphoid organs into circulatory fluids is essential for normal immune function. The discovery that a small-molecule inhibitor of lymphocyte exit, FTY720, is a ligand for sphingosine 1-phosphate (S1P) receptors led to studies demonstrating that S1P receptor type 1 (S1P1) is needed in T cells and B cells for their egress from lymphoid organs. S1P exists in higher concentrations in blood and lymph than in lymphoid organs, and this differential is also required for lymphocyte exit. Transcriptional and post-translational mechanisms regulate S1P1 and thus the egress of lymphocytes. In this review we discuss the body of evidence supporting a model in which lymphocyte egress is promoted by encounter with S1P at exit sites. We relate this model to work examining the effects of S1P receptor agonists on endothelium.

Functions of sphingolipid metabolism in mammals--lessons from genetic defects

Much is known about the pathways that control the biosynthesis, transport and degradation of sphingolipids. During the last two decades, considerable progress has been made regarding the roles this complex group of lipids play in maintaining membrane integrity and modulating responses to numerous signals. Further novel insights have been provided by the analysis of newly discovered genetic diseases in humans as well as in animal models harboring mutations in the genes whose products control sphingolipid metabolism and action. Through the description of the phenotypic consequences of genetic defects resulting in the loss of activity of the many proteins that synthesize, transport, bind, or degrade sphingolipids, this review summarizes the (patho)physiological functions of these lipids.

BLIMP1 guides the fate of effector B and T cells

B-lymphocyte-induced maturation protein 1 (BLIMP1) is a transcriptional repressor, and its importance in controlling the terminal differentiation of antibody-secreting cells (ASCs) is well established. However, as we discuss in this Progress article, it has now become evident that the ASC programme consists of a discrete BLIMP1-independent initiation phase, followed by a second step in which BLIMP1 is absolutely required for the differentiation of fully mature ASCs. In addition, an important role for BLIMP1 in maintaining the homeostasis of effector T cells is emerging, suggesting intriguing parallels between the control of effector-cell fates in both B and T cells.

S1P1 receptor expression regulates emergence of NKT cells in peripheral tissues

The S1P1 receptor, on the surface of lymphocytes and endothelial cells, regulates the unique trafficking behavior of certain lymphocyte populations. We have examined whether the S1P1 receptor also dictates the distinctive tissue distribution of V alpha14-J alpha18 natural killer T (NKT) cells, whose trafficking pattern is not well understood. Mice (TCS1P1 KO) were established with a conditional deletion of the S1P1 receptor in thymocytes that included precursors of NKT cells. Within the thymus, NKT cells were found at normal or increased levels, indicating that S1P1 receptor expression was dispensable for NKT cell development. However, substantially reduced numbers of NKT cells were detected in the peripheral tissues of the TCS1P1 KO mice. Short-term S1P1 deletion after NKT cells had established residence in the periphery did not substantially alter their distribution in tissues, except for a partial decrease in the spleen. FTY720, a S1P1 receptor ligand that has potent effects on the trafficking of conventional T cells, did not alter the preexisting distribution of NKT cells within peripheral tissues of wild-type mice. Our results indicate that the S1P1 receptor expression on NKT cells is dispensable for development within thymus but is essential for the establishment of their tissue residency in the periphery.

Brain penetration of the oral immunomodulatory drug FTY720 and its phosphorylation in the central nervous system during experimental autoimmune encephalomyelitis: consequences for mode of action in multiple sclerosis

FTY720 [2-amino-2-[2-(4-octylphenyl) ethyl]propane-1,3-diol hydrochloride] is an oral sphingosine-1-phosphate receptor modulator under development for the treatment of multiple sclerosis (MS). The drug is phosphorylated in vivo by sphingosine kinase 2 to its bioactive form, FTY720-P. Although treatment with FTY720 is accompanied by a reduction of the peripheral lymphocyte count, its efficacy in MS and experimental autoimmune encephalomyelitis (EAE) may be due to additional, direct effects in the central nervous system (CNS). We now show that FTY720 localizes to the CNS white matter, preferentially along myelin sheaths. Brain trough levels of FTY720 and FTY720-P in rat EAE are of the same magnitude and dose dependently increase; they are in the range of 40 to 540 ng/g in the brain tissue at efficacious doses and exceed blood concentrations severalfold. In a rat model of chronic EAE, prolonged treatment with 0.03 mg/kg was efficacious, but limiting the dosing period failed to prevent EAE despite a significant decrease in blood lymphocytes. FTY720 effectiveness is likely due to a culmination of mechanisms involving reduction of autoreactive T cells, neuroprotective influence of FTY720-P in the CNS, and inhibition of inflammatory mediators in the brain.