Targeting sphingosine-1-phosphate for cancer therapy

This review summarises some important new findings that implicate sphingosine-1-phosphate (S1P) as a potent tumorigenic and angiogenic agent released from cancerous tumours into the tumour microenvironment. Also explored is the novel concept that bioactive lipid signalling molecules, like S1P, can themselves be targets for rational drug design, thereby opening up an entire class of lipidomic-based therapeutics for oncology and other human diseases.

The role of the lysophospholipid sphingosine 1-phosphate in immune cell biology

Sphingosine 1-phosphate (S1P) has been shown to be a bioactive lipid mediator intimately involved in mediating a variety of immunological processes. In particular, S1P regulates lymphocyte cell trafficking between the lymphatic system and the blood. The lysophospholipid signals mainly through five related G protein-coupled receptor subtypes, termed S1P(1) to S1P(5). S1P(1) seems to play an essential role in cell trafficking, as this receptor subtype promotes the egress of T and B cells from secondary lymphatic organs. This S1P(1)-mediated migratory response is a consequence of different S1P levels in the serum and lymphatic organs. In addition to its direct effects on lymphocyte motility, S1P strengthens cell barrier integrity in sinus-lining endothelial cells, thereby reducing lymphocyte egress out of lymph nodes. Furthermore, S1P modulates cytokine profiles in T and dendritic cells, resulting in an elevated differentiation of T helper-2 cells during the T cell activation process. It is of interest that the mode of molecular action of the novel immunomodulator FTY720 interferes with the signaling of S1P. After phosphorylation, FTY720 shares structural similarity with S1P, but in contrast to the natural ligand, phosphorylated FTY720 induces a prolonged internalization of S1P(1), resulting in an impaired S1P-mediated migration of lymphocytes.

Sphingosine kinase and sphingosine-1-phosphate regulate migration, endocytosis and apoptosis of dendritic cells

Dendritic cells (DC) are inducers of primary immune responses and represent an attractive vector for cancer immunotherapy. Sphingosine kinase (SphK) and its product sphingosine-1-phosphate (S1P) play an important role in the regulation of immune cells and cancer, affecting processes such as differentiation, growth or migration. We studied the role of SphK and S1P on migration of DC. RT-PCR showed mRNA expression of SphK in DC, declining from immature (iDC) to mature DC (mDC) to antigen-loaded mDC. Expression of S1P receptors was S1P(1) > S1P(2) = S1P(3), unrelated to maturation or antigen uptake. In transwell assays, iDC migrated towards SDF-1, MIP-1alpha, MCP and S1P, whereby S1P combined with a chemokine had a synergistic effect. mDC migrated towards 6Ckine and MIP-3beta, but not towards S1P. The SphK-inhibitor dihydro-sphingosine (DHS) reduced migration of iDC but not of mDC. In addition S1P(3)-inhibitor suramin inhibited DC migration in response to S1P. DHS had a reverse effect on endocytosis, enhancing the uptake of FITC dextran. We also observed an anti-apoptotic effect of S1P on mDC for the first time. This indicates that SphK/S1P may play a role in accumulation of peripheral iDC at the location of antigen and subsequent antigen-uptake. These findings may help to optimise DC-based cancer immunotherapy by modulation of SphK/S1P.

Comprehensive analysis of monoclonal antibodies against detergent-insoluble membrane/lipid rafts of HL60 cells

Glycosphingolipids and cholesterol are principal components of plasmamembrane microdomains, i.e. lipid rafts. Recent studies revealed the possible presence of a variety of microdomains that distinctly differ in terms of their molecular composition and functions. To understand their precise structures and functions, we produced monoclonal antibodies (MAbs) by immunizing mice to the microdomains prepared from a fraction of detergent-insoluble membrane (DIM) of HL60 cells. Biochemical characterization of the antigen epitopes led to classification of the MAbs into two groups. One group consists of MAbs that react with lipids such as phosphatidylglucoside, lysophosphatidylinositol, and gangliosides (GM1a and GD1b), and the other consists of MAbs that react with proteins such as annexin I, aminopeptidase N and acrogranin. Immunofluorescence staining of HL60 cells with the MAbs, except for the MAbs that recognize lysophosphatidylinositol or annexin I, resulted in patchy-like images of the cell membranes. Interestingly, MAbs belonging to the former group had the potential to induce cell proliferation/differentiation in vitro. Our MAbs against the DIM fraction of HL60 cells can be valuable tools for the study of membrane microdomains.

Validation of an anti-sphingosine-1-phosphate antibody as a potential therapeutic in reducing growth, invasion, and angiogenesis in multiple tumor lineages

S1P has been proposed to contribute to cancer progression by regulating tumor proliferation, invasion, and angiogenesis. We developed a biospecific monoclonal antibody to S1P to investigate its role in tumorigenesis. The anti-S1P mAb substantially reduced tumor progression and in some cases eliminated measurable tumors in murine xenograft and allograft models. Tumor growth inhibition was attributed to antiangiogenic and antitumorigenic effects of the antibody. The anti-S1P mAb blocked EC migration and resulting capillary formation, inhibited blood vessel formation induced by VEGF and bFGF, and arrested tumor-associated angiogenesis. The anti-S1P mAb also neutralized S1P-induced proliferation, release of proangiogenic cytokines, and the ability of S1P to protect tumor cells from apoptosis in several tumor cell lines, validating S1P as a target for therapy.

Targeting sphingosine-1-phosphate: a novel avenue for cancer therapeutics

Sphingosine-1-phosphate (S1P) is a pleiotropic lipid mediator that has been shown to regulate cell growth, cell survival, cell invasion, vascular maturation, and angiogenesis, processes that are important for cancer progression. In this issue of Cancer Cell, Visentin et al. demonstrate that a monoclonal antibody that binds S1P with extremely high affinity and specificity significantly slows tumor progression and associated angiogenesis in several animal models of human cancer. Their results suggest that S1P not only affects tumor cells themselves, but also is permissive or required for the actions of angiogenic factors, and thus may be a bona fide cancer target.

Anti-lysobisphosphatidic acid antibodies in patients with antiphospholipid syndrome and systemic lupus erythematosus

Lyso(bis)phosphatidic acid (LBPA) is a novel antigenic target in anti-phospholipid syndrome (APS) and antibodies directed against LBPA (aLBPA) have been detected in sera from APS patients. In this study we first evaluated aLBPA in comparison with the most widely used methods (i.e. anticardiolipin [(aCL)-enzyme-linked immunosorbent assay (ELISA)] and antibeta-2-glycoprotein-I antibodies (abeta(2)-GPI-ELISA) utilized to detect antiphospholipid antibodies in patients with primary or secondary APS, systemic lupus erythematosus, chronic HCV infection and healthy subjects. We then assessed the relationship between aLBPA, lupus anticoagulant (LAC) and the main clinical manifestations of APS. Finally, we evaluated the presence of 'pure' (i.e. beta(2)-GPI-independent) aLBPA in patients with APS and controls. The results indicate that aLBPA as well as abeta(2)-GPI display higher specificity but lower sensitivity for APS compared to aCL. Moreover, serum aLBPA correlate closely with aCL and abeta(2)-GPI in APS patients and are strictly associated with LAC positivity. We demonstrate that beta(2)-GPI binds to LBPA with affinity similar to CL, and antibodies able to react with phosholipid-protein complex exist; however, 'pure' aLBPA can also be detected in sera of APS patients. Altogether these data confirm that LBPA may be an antigenic target in APS and that aLBPA are serological markers of APS with similar sensitivity and specificity compared to abeta(2)-GPI. However, the clinical utility of aLBPA detection alone or in combination with aCL and/or abeta(2)-GPI remains to be elucidated in larger and longitudinal studies.

The S1P-analog FTY720 differentially modulates T-cell homing via HEV: T-cell-expressed S1P1 amplifies integrin activation in peripheral lymph nodes but not in Peyer patches

Sphingosine-1-phosphate (S1P) and its receptor S1P1 control T-cell egress from thymus and secondary lymphoid organs (SLOs). To further define the role of S1P1 in lymphocyte trafficking, we performed adoptive transfer experiments and intravital microscopy (IVM) using both S1P1-/- lymphocytes and recipient wild-type (WT) mice treated with FTY720, an immunosuppressant that downmodulates S1P receptors. S1P1 deficiency and FTY720 caused rapid disappearance of T cells from blood, prolonged retention in SLOs, and accumulation in bone marrow, but did not alter interstitial T-cell motility in peripheral lymph nodes (PLNs) as assessed by multiphoton IVM. However, S1P1-/- lymphocytes displayed reduced short-term homing to PLNs due to attenuated integrin-mediated firm arrest in high endothelial venules (HEVs). By contrast, S1P1-/- T cells homed normally to Peyer patches (PPs), whereas S1P1-/- B cells had a marked defect in homing to PPs and arrested poorly in PP HEVs. Therefore, S1P1 not only controls lymphocyte egress from SLOs, but also facilitates in a tissue- and subset-specific fashion integrin activation during homing. Interestingly, FTY720 treatment enhanced accumulation of both S1P1 sufficient and S1P1-/- T cells in PPs by enhancing integrin-mediated arrest in HEVs. Thus, FTY720 exerts unique effects on T-cell traffic in PPs that are independent of T-cell-expressed S1P1.

Modulation of adaptive immune responses by sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) has long been recognized as a mediator of a variety of cell functions. A growing body of evidence has accumulated demonstrating its role in cell migration and as a mediator of growth factor-induced events. In recent years, it has become apparent that S1P also mediates many cytokine and chemokine functions. Cells of the immune system function and migrate in response to a complex network of cytokines and chemokines, and the outcome is determined by the interplay of the effects of these molecules on the target cell. S1P may be a bona fide component of these networks and influence the responses of cells to these immune modulators.

Serum lysophosphatidic acid concentrations measured by dot immunogold filtration assay in patients with acute myocardial infarction

In this study the relation between lysophosphatidic acid (LPA) and myocardial infarction was investigated, the typical and simplified methods for measuring serum LPA concentration by dot immunogold filtration assay (DIFA) based on a polyclonal antibody to LPA were developed, and serum LPA concentrations were measured in 31 patients with acute myocardial infarction (AMI) and 12 controls (blood donors) by DIFA. Serum LPA levels were raised more than twofold 8 h after the onset of AMI. Maximal elevation (10.43 mg/L) was found at 48-72 h following onset and remained higher than the control concentration (1.66 mg/L) 7 days after AMI. The rise in serum LPA concentration in AMI patients suggests that LPA might be involved in AMI-related pathophysiology in the cardiovascular system. The simplified DIFA developed in the present study for measuring serum LPA concentration is convenient and highly sensitive.

Sphingosine 1-phosphate induces antimicrobial activity both in vitro and in vivo

Sphingosine 1-phosphate (S1P), a polar sphingolipid metabolite, is involved in a wide spectrum of biological processes, including Ca(++) mobilization, cell growth, differentiation, motility, and cytoskeleton organization. Here, we show a novel role of S1P in the induction of antimicrobial activity in human macrophages that leads to the intracellular killing of nonpathogenic Mycobacterium smegmatis and pathogenic M. tuberculosis. Such activity is mediated by host phospholipase D, which favors the acidification of mycobacteria-containing phagosomes. Moreover, when it was intravenously injected in mycobacteria-infected mice, S1P reduced mycobacterial growth and pulmonary tissue damage. These results identify S1P as a novel regulator of the host antimicrobial effector pathways.

Sphingosine 1-phosphate and its type 1 G protein-coupled receptor: trophic support and functional regulation of T lymphocytes

The lysophospholipid (LPL) growth factors sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are generated by macrophages, dendritic cells, mast cells, and platelets, which leads to lymph and plasma concentrations of 0.1-1 microM. Distinctive profiles of G protein-coupled receptors (GPCRs) for S1P and LPA are expressed by each type of immune cell and are regulated by cellular activation. At 1-100 nM, S1P signals T cells through their principal S1P(1) GPCRs with consequent protection from apoptosis, enhancement of chemotaxis, and facilitation of optimal regulatory activity of CD4(+)25(+) T cells. At 0.3-3 microM, S1P inhibits T cell chemotaxis and to a lesser extent other functions. These S1P-S1P(1) GPCR signals suppress homing of blood and spleen T cells to secondary lymphoid tissues. S1P(1) GPCR antagonists evoke lymphopenia by permitting blood T cells to enter lymph nodes and blocking S1P(1) GPCR-dependent T cell efflux from lymph nodes. Inversely, there is a decrease in lymphoid tissue traffic of T cells in transgenic mice, which overexpress lymphocyte S1P(1) GPCRs. The immunotherapeutic activity of S1P(1) GPCR antagonists, which limits T cell access to organ grafts and autoimmune antigens, does not reduce other functional capabilities of T cells. LPLs and their GPCRs thus constitute an immunoregulatory system of sufficient prominence for pharmacological targeting in transplantation, autoimmunity, and immunodeficiency.

Signaling and biological actions of sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P) is a product of sphingomyelin (SM) metabolism. It is present in most eukaryotic organisms; however, in higher vertebrates (bony fishes to mammals), it is utilized as a regulator of cell function by binding to extracellular receptors. Its role as a second messenger function was originally proposed; however, the unequivocal role of S1P as a second messenger in higher eukaryotes is lacking at present. As an extracellular mediator, S1P binds to a family of G-protein-coupled receptors named S1P(1-5), originally referred to as (EDG-1, 3, 5, 6 and 8). These receptors couple to multiple G-proteins and regulate intracellular signaling pathways. Most tissues express one or more S1P receptor subtypes; therefore, S1P is known as a multifunctional physiologic mediator. Its precise physiologic roles in multiple organ systems are just beginning to be elucidated; however, at present, regulation of cardiogenesis, vascular system formation, oocyte survival and immune cell trafficking has been recently reported. Currently, a S1P receptor agonist, is undergoing clinical trials as an agent to fight organ transplant rejection. Since S1P is a fundamental multifunctional mediator, better understanding of the biology of S1P holds great promise to develop novel tools to control various diseases.

Lysophospholipids and their G protein-coupled receptors in inflammation and immunity

Among the family of lipid-derived mediators, the group of lysophospholipids including lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) have growth-related and -unrelated effects on diverse cell types including lymphocytes, macrophages, smooth muscle cells, endothelial cells, and neuronal cells. This review summarizes the known effects of lysophospholipids and their G protein-coupled receptors (GPCRs) in inflammation and immunity. Lysophospholipids have the capacity to evoke and modulate immune responses by attracting and activating T-cells, B-cells and macrophages directly and influencing their interactions with other cell types. Immune cells express multiple subsets of lysophospholipid receptors, which are critical for specific cellular responses such as proliferation and migration that are fundamental to immunity. Investigation of the expression pattern of EDG-receptors on human T-cells revealed a dynamic transcriptional regulation influenced by both developmental stages and activation states. Other lipid mediators like psychosine and other GPCRs for lipid mediators like G2A also may be involved in the development of normal immune and inflammatory reactions and diseases. These observations suggest that agonists and antagonists for lysophospholipid receptors may influence immune responses.

Lysophospholipid mediators of immunity and neoplasia

Lysophosphatidic acid (LPA), sphingosine 1-phosphate (S1P) and some other structurally related lysophospholipids are active growth factors and stimuli for diverse cellular functions. LPA and S1P promote early T cell migration to tissue sites of immune responses and regulate T cell proliferation and secretion of numerous cytokines. Edg-4 (LPA2) LPA receptors, which are constitutively expressed by helper T cells, and Edg-2 (LPA1) LPA receptors, which are expressed only by activated helper T cells, transduce opposite effects of LPA on some T cell responses. A similar mechanism is observed for fine regulation of Edg R-mediated effects of LPA on ovarian cancer cells. Edg-4 (LPA2) R transduces proliferative responses, recruitment of autocrine protein growth factors, and migration of ovarian cancer cells, whereas Edg-2 (LPA1) R transduces inhibition of Edg-4 (LPA2) R-mediated responses and concurrently elicits apoptosis and anoikis of ovarian cancer cells. Edg-4 (LPA2) R is a distinctive functional marker for ovarian carcinoma, and is expressed both as the wild-type and a carboxyl-terminally extended gain-of-function mutant. Newly discovered non-lipid agonists and antagonists for individual Edg receptors will permit more sophisticated analyses of their respective contributions in human biology and pathophysiology, and may represent novel therapeutic modalities in immune disorders and cancer.

Evidence for anticoagulant activity and beta2-GPI accumulation in late endosomes of endothelial cells induced by anti-LBPA antibodies

This investigation was undertaken to test whether anti-LBPA antibodies and IgG from patients with APS interfere with intracellular beta2GPI distribution in EAhy926 endothelial cells and with the coagulation system. Cell incubation with anti-LBPA MoAb or with patients' IgG resulted in antibody binding to late endosomes and caused beta2GPI redistribution and accumulation within perinuclear vesicular structures reminiscent of late endosomes. This finding suggests that aPI may contribute to the pathogenic mechanisms of APS by modifying the intracellular traffic of proteins, by interactions between aPl and LBPA, beta2GPI and/or LBPA-beta2GPI complexes. The anticoagulant activity of anti-LBPA MoAb was analyzed in a sensitized activated partial thromboplastin time (aPTT) system and in a dilute Russell's viper venom time (dRVVT). A significant, concentration-dependent effect of the antibody on both aPTT and dRVVT prolongation was found. These observations suggest that LBPA is an important lipid target for aPl with potential functional implications for the immunopathogenesis of APS.

Localization of lysobisphosphatidic acid-rich membrane domains in late endosomes

Late endosomes accumulate internal membranes within the lumen of the organelle. These internal membranes are enriched in the late endosome specific phospholipid, lysobisphosphatidic acid (LBPA). The organization of LBPA-rich membrane domains is not well characterized. Using an LBPA-specific monoclonal antibody (6C4), we show that these membrane domains are not accessible from the cytoplasm. Using fluorescence correlation spectroscopy, we also show that 6C4 only binds sonicated, but not intact, late endosomes, presumably reflecting the release of internal membranes upon endosome rupture.

Production and application of LPA polyclonal antibody

By using colloidal gold as a hapten carrier, a kind of antibody against lysophosphatidic acid (LPA) was developed and used to successfully detect 500 ng/mL LPA in dot immunogold filtration assay. Such application of the LPA antibody could offer us a way to diagnose ovarian cancer at its early stage.

Cutting edge: differential constitutive expression of functional receptors for lysophosphatidic acid by human blood lymphocytes

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) from platelets and macrophages mediate T cell functions. Endothelial differentiation gene-encoded G protein-coupled receptors (Edg Rs) are specific for S1P (Edg-1, -3, -5, and -8 Rs) and LPA (Edg-2, -4, and -7 Rs). Human T cell tumors express many Edg Rs for both LPA and S1P. In contrast, human blood CD4+ T cells express predominantly Edg-4, and CD8+ T cells show only traces of Edg-2 and -5, by quantification of mRNA and Edg R Ags. LPA at 10-10-10-6 M suppressed significantly the secretion of IL-2 from anti-CD3 plus anti-CD28 Ab-challenged CD4+ T cells, but not CD8+ T cells. Monoclonal anti-Edg-4 R Ab, like LPA, suppressed stimulated IL-2 secretion from CD4+ T cells, but not CD8+ T cells. Constitutive expression of Edg-4 by CD4+, but not CD8+, human T cells accounts for differential functional responsiveness of the T cell subsets to LPA.

Sera from children with type 1 diabetes mellitus react against a new group of antigens composed of lysophospholipids

Several autoantibodies related to Type 1 diabetes mellitus and their corresponding autoantigens have been previously identified. While peptide antigens are more widely recognized, lipid antigens like sulfatides and gangliosides are also known epitopes for the diabetic humoral immune response. Islet cell antibodies (ICA) in Type 1 diabetes are heterogeneous immunoglobulins directed against selected antigens in the islets of Langerhans. Moreover, ICA may be the best predictive marker of disease in family members of patients with Type 1 diabetes. The aims of this study were: (1) to purify lipids from porcine pancreas that contain ICA epitopes; (2) to characterize these lipid antigens, and (3) to use the purified lipids in an assay to detect antibodies in patients with Type 1 diabetes. A unique family of 4 lysophospholipids, 1 fully characterized as lysophosphatidylmyoinositol, partially inhibited ICA staining, and therefore, were considered to be candidate antigens for an ICA immunoassay. Using a dot blot immunoassay, we detected antibodies directed against these phospholipids in 28 out of 46 (61%) diabetic sera, while detecting only 1 false positive out of 28 nondiabetic sera (3.6%; p < 0.0001 comparing diabetic vs. nondiabetic serum). Therefore, lysophospholipid immunoassay positivity is present in sera of Type 1 diabetic patients. Furthermore, we detected 15 out of 23 ICA-negative diabetic sera (65.2%), showing that our phospholipid immunoassay does not correlate with ICA positivity.

Interaction of anti-phospholipid antibodies with late endosomes of human endothelial cells

Anti-phospholipid antibodies (APLAs) are associated with thrombosis and/or recurrent pregnancy loss. APLAs bind to anionic phospholipids directly or indirectly via a cofactor such as beta(2)-glycoprotein 1 (beta(2)GPI). The lipid target of APLA is not yet established. Recently, we observed that APLAs in vitro can bind lysobisphosphatidic acid (LBPA). The internal membranes of late endosomes are enriched in this phospholipid. The current study was undertaken to determine to what extent binding of APLA to LBPA is correlated with binding to cardiolipin and to beta(2)GPI and to determine whether patient antibodies interact with late endosomes of human umbilical vein endothelial cells (HUVECs) and thus modify the intracellular trafficking of proteins. Binding of patient immunoglobulin G (n=37) to LBPA was correlated significantly with binding to cardiolipin. Although LBPA binding was correlated to a lesser extent with beta(2)GPI binding, we observed that beta(2)GPI binds with high affinity to LBPA. Immunofluorescence studies showed that late endosomes of HUVECs contain LBPA. Patient but not control antibodies recognized late endosomes, but not cardiolipin-rich mitochondria, even when we used antibodies that were immunopurified on cardiolipin. Incubation of HUVECs with patient plasma samples immunoreactive toward LBPA resulted in an accumulation of the antibodies in late endosomes and led to a redistribution of the insulinlike growth factor 2/mannose-6-phosphate receptor from the Golgi apparatus to late endosomes. Our results suggest that LBPA is an important lipid target of APLA in HUVECs. These antibodies are internalized by the cells and accumulate in late endosomes. By modifying the intracellular trafficking of proteins, APLA could contribute to several of the proposed pathogenic mechanisms leading to the antiphospholipid syndrome.

Cooperation of fibronectin with lysophosphatidic acid induces motility and transcellular migration of rat ascites hepatoma cells

We have previously shown that the transcellular migration of rat ascites hepatoma (AH130-MM1) cells through a cultured mesothelial cell monolayer (MCL) is triggered with lysophosphatidic acid (LPA) that stimulates actin polymerization and myosin light chain phosphorylation through the activation of Rho-ROCK (Rho-kinase) cascade. When, however, the motility of MM1 cells on a glass surface was tested by phagokinetic track motility assay, LPA failed to induce the motility. Nevertheless, when the glass had been coated with fibronectin (FN), LPA could induce phagokinetic motility which was accompanied by transformation of MM1 cells to fusiform-shape and assembly of focal adhesion. beta1 integrin, the counter receptor of FN, was expressed on MM1 cells. Anti-FN antibody, anti-beta1 integrin antibody and cyclo-GRGDSPA remarkably suppressed LPA-induced phagokinetic motility. These antibodies suppressed LPA-induced transcellular migration through MCL, as well. These results indicate that actin polymerization and phosphorylation of myosin light chain through Rho activation are insufficient for inducing motility but the cooperative FN/beta1 integrin-mediated adhesion is necessary for both the phagokinetic motility and transcellular migration of MM1 cells.

A lipid associated with the antiphospholipid syndrome regulates endosome structure and function

Little is known about the structure and function of membrane domains in the vacuolar apparatus of animal cells. A unique feature of late endosomes, which are part of the pathway that leads to lysosomes, is that they contain a complex system of poorly characterized internal membranes in their lumen. These endosomes are therefore known as multivesicular or multilamellar organelles. Some proteins distribute preferentially within these internal membranes, whereas others are exclusively localized to the organelle's limiting membrane. The composition and function of this membrane system are poorly understood. Here we show that these internal membranes contain large amounts of a unique lipid, and thus form specialized domains within endosomes. These specialized domains are involved in sorting the multifunctional receptor for insulin-like growth factor 2 and ligands bearing mannose-6-phosphate, in particular lysosomal enzymes. We also show that this unique lipid is a specific antigen for human antibodies associated with the antiphospholipid syndrome. These antibodies may act intracellularly by altering the protein-sorting functions of endosomes.