Fc gamma RIIB1/SHIP-mediated inhibitory signaling in B cells involves lipid rafts

The Membrane Phase Transition Gives Rise to Responsive Plasma Membrane Structure and Function

Several groups have recently reported evidence for the emergence of domains in cell plasma membranes when membrane proteins are organized by ligand binding or assembly of membrane proximal scaffolds. These domains recruit and retain components that favor the liquid-ordered phase, adding to a decades-old literature interrogating the contribution of membrane phase separation in plasma membrane organization and function. Here we propose that both past and present observations are consistent with a model in which membranes have a high compositional susceptibility, arising from their thermodynamic state in a single phase that is close to a miscibility phase transition. This rigorous framework naturally allows for both transient structure in the form of composition fluctuations and long-lived structure in the form of induced domains. In this way, the biological tuning of plasma membrane composition enables a responsive compositional landscape that facilitates and augments cellular biochemistry vital to plasma membrane functions.

Impact of Plasma Membrane Domains on IgG Fc Receptor Function

Lipid cell membranes not only represent the physical boundaries of cells. They also actively participate in many cellular processes. This contribution is facilitated by highly complex mixtures of different lipids and incorporation of various membrane proteins. One group of membrane-associated receptors are Fc receptors (FcRs). These cell-surface receptors are crucial for the activity of most immune cells as they bind immunoglobulins such as immunoglobulin G (IgG). Based on distinct mechanisms of IgG binding, two classes of Fc receptors are now recognized: the canonical type I FcγRs and select C-type lectin receptors newly referred to as type II FcRs. Upon IgG immune complex induced cross-linking, these receptors are known to induce a multitude of cellular effector responses in a cell-type dependent manner, including internalization, antigen processing, and presentation as well as production of cytokines. The response is also determined by specific intracellular signaling domains, allowing FcRs to either positively or negatively modulate immune cell activity. Expression of cell-type specific combinations and numbers of receptors therefore ultimately sets a threshold for induction of effector responses. Mechanistically, receptor cross-linking and localization to lipid rafts, i.e., organized membrane microdomains enriched in intracellular signaling proteins, were proposed as major determinants of initial FcR activation. Given that immune cell membranes might also vary in their lipid compositions, it is reasonable to speculate, that the cell membrane and especially lipid rafts serve as an additional regulator of FcR activity. In this article, we aim to summarize the current knowledge on the interplay of lipid rafts and IgG binding FcRs with a focus on the plasma membrane composition and receptor localization in immune cells, the proposed mechanisms underlying this localization and consequences for FcR function with respect to their immunoregulatory capacity.

Phagocytosis: Our Current Understanding of a Universal Biological Process

Phagocytosis is a cellular process for ingesting and eliminating particles larger than 0.5 μm in diameter, including microorganisms, foreign substances, and apoptotic cells. Phagocytosis is found in many types of cells and it is, in consequence an essential process for tissue homeostasis. However, only specialized cells termed professional phagocytes accomplish phagocytosis with high efficiency. Macrophages, neutrophils, monocytes, dendritic cells, and osteoclasts are among these dedicated cells. These professional phagocytes express several phagocytic receptors that activate signaling pathways resulting in phagocytosis. The process of phagocytosis involves several phases: i) detection of the particle to be ingested, ii) activation of the internalization process, iii) formation of a specialized vacuole called phagosome, and iv) maturation of the phagosome to transform it into a phagolysosome. In this review, we present a general view of our current understanding on cells, phagocytic receptors and phases involved in phagocytosis.

Dominant role of splenic marginal zone lipid rafts in the classical complement pathway against S. pneumoniae

Lipid rafts (LRs) play crucial roles in complex physiological processes, modulating innate and acquired immune responses to pathogens. The transmembrane C-type lectins human dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) and its mouse homolog SIGN-R1 are distributed in LRs and expressed on splenic marginal zone (MZ) macrophages. The DC-SIGN-C1q or SIGN-R1-C1q complex could mediate the immunoglobulin (Ig)-independent classical complement pathway against Streptococcus pneumoniae. Precise roles of LRs during this complement pathway are unknown. Here we show that LRs are indispensable for accelerating the DC-SIGN- or SIGN-R1-mediated classical complement pathway against S. pneumoniae, thus facilitating rapid clearance of the pathogen. The trimolecular complex of SIGN-R1-C1q-C4 was exclusively enriched in LRs of splenic MZ macrophages and their localization was essential for activating C3 catabolism and enhancing pneumococcal clearance, which were abolished in SIGN-R1-knockout mice. However, DC-SIGN replacement on splenic MZ macrophage’s LRs of SIGN-R1-depleted mice reversed these defects. Disruption of LRs dramatically reduced pneumococcal uptake and decomposition. Additionally, DC- SIGN, C1q, C4, and C3 were obviously distributed in splenic LRs of cadavers. Therefore, LRs on splenic SIGN-R1⁺ or DC-SIGN⁺ macrophages could provide spatially confined and optimal bidirectional platforms, not only for usual intracellular events, for example recognition and phagocytosis of pathogens, but also an unusual extracellular event such as the complement system. These findings improve our understanding of the orchestrated roles of the spleen, unraveling a new innate immune system initiated from splenic MZ LRs, and yielding answers to several long-standing problems, including the need to understand the profound role of LRs in innate immunity, the need to identify how such a small portion of splenic SIGN-R1⁺ macrophages (<0.05% of splenic macrophages) effectively resist S. pneumoniae, and the need to understand how LRs can promote the protective function of DC-SIGN against S. pneumoniae in the human spleen.

The Human FcγRII (CD32) Family of Leukocyte FcR in Health and Disease

FcγRs have been the focus of extensive research due to their key role linking innate and humoral immunity and their implication in both inflammatory and infectious disease. Within the human FcγR family FcγRII (activatory FcγRIIa and FcγRIIc, and inhibitory FcγRIIb) are unique in their ability to signal independent of the common γ chain. Through improved understanding of the structure of these receptors and how this affects their function we may be able to better understand how to target FcγR specific immune activation or inhibition, which will facilitate in the development of therapeutic monoclonal antibodies in patients where FcγRII activity may be desirable for efficacy. This review is focused on roles of the human FcγRII family members and their link to immunoregulation in healthy individuals and infection, autoimmunity and cancer.

The Rare Anaphylaxis-Associated FcγRIIa3 Exhibits Distinct Characteristics From the Canonical FcγRIIa1

FcγRIIa is an activating FcγR, unique to humans and non-human primates. It induces antibody-dependent proinflammatory responses and exists predominantly as FcγRIIa1. A unique splice variant, we designated FcγRIIa3, has been reported to be associated with anaphylactic reactions to intravenous immunoglobulins (IVIg) therapy. We aim to define the functional consequences of this FcγRIIa variant associated with adverse responses to IVIg therapy and evaluate the frequency of associated SNPs. FcγRIIa forms from macaque and human PBMCs were investigated for IgG-subclass specificity, biochemistry, membrane localization, and functional activity. Disease-associated SNPs were analyzed by sequencing genomic DNA from 224 individuals with immunodeficiency or autoimmune disease. FcγRIIa3 was identified in macaque and human PBMC. The FcγRIIa3 is distinguished from the canonical FcγRIIa1 by a unique 19-amino acid cytoplasmic insertion and these two FcγRIIa forms responded distinctly to antibody ligation. Whereas FcγRIIa1 was rapidly internalized, FcγRIIa3 was retained longer at the membrane, inducing greater calcium mobilization and cell degranulation. Four FCGR2A SNPs were identified including the previously reported intronic SNP associated with anaphylaxis, but in only 1 of 224 individuals. The unique cytoplasmic element of FcγRIIa3 delays internalization and is associated with enhanced cellular activation. The frequency of the immunodeficiency-associated SNP varies between disease populations but interestingly occurred at a lower frequency than previously reported. None-the-less enhanced FcγRIIa3 function may promote a proinflammatory environment and predispose to pathological inflammatory responses.

B cells interactions in lipid immune responses: implications in atherosclerotic disease

Atherosclerosis is considered as an inflammatory and chronic disorder with an important immunologic component, which underlies the majority of cardiovascular diseases; condition that belongs to a group of noncommunicable diseases that to date and despite of prevention and treatment approaches, they remain as the main cause of death worldwide, with 17.5 million of deaths every year. The impact of lipids in human health and disease is taking center stage in research, due to lipotoxicity explained by elevated concentration of circulating lipids, in addition to altered adipose tissue metabolism, and aberrant intracellular signaling. Immune response and metabolic regulation are highly integrated systems and the proper function of each one is dependent on the other. B lymphocytes express a variety of receptors that can recognize foreign, endogenous or modified self-antigens, among them oxidized low density lipoproteins, which are the main antigens in atherosclerosis. Mechanisms of B cells to recognize, remove and present lipids are not completely clear. However, it has been reported that B cell can recognize/remove lipids through a range of receptors, such as LDLR, CD1d, FcR and SR, which might have an atheroprotector or proatherogenic role during the course of atherosclerotic disease. Pertinent literature related to these receptors was examined to inform the present conclusions.

Lipid rafts in immune signalling: current progress and future perspective

Lipid rafts are dynamic assemblies of proteins and lipids that harbour many receptors and regulatory molecules and so act as a platform for signal transduction. They float freely within the liquid-disordered bilayer of cellular membranes and can cluster to form larger ordered domains. Alterations in lipid rafts are commonly found to be associated with the pathogenesis of several human diseases and recent reports have shown that the raft domains can also be perturbed by targeting raft proteins through microRNAs. Over the last few years, the importance of lipid rafts in modulating both innate and acquired immune responses has been elucidated. Various receptors present on immune cells like B cells, T cells, basophils and mast cells associate with lipid rafts on ligand binding and initiate signalling cascades leading to inflammation. Furthermore, disrupting lipid raft integrity alters lipopolysaccharide-induced cytokine secretion, IgE signalling, and B-cell and T-cell activation. The objective of this review is to summarize the recent progress in understanding the role of lipid rafts in the modulation of immune signalling and its related therapeutic potential for autoimmune diseases and inflammatory disorders.

Non-genomic oestrogen receptor signal in B lymphocytes: An approach towards therapeutic interventions for infection, autoimmunity and cancer

The non-genomic membrane bound oestrogen receptor (mER) regulates intracellular signals through receptor-ligand interactions. The mER, along with G-protein coupled oestrogen receptor GPR 30 (GPER), induces diverse cell signalling pathways in murine lymphocytes. The mER isoform ER-alpha46 has recently been demonstrated in human B and T lymphocytes as an analogue receptor for chemokine CCL18, the signalling events of which are not clearly understood. Ligand-induced mER and GPER signalling events are shared with BCR, CD19 mediated intracellular signalling through phospholipase C, PIP2/IP3/PI3 mediated activation of Akt, MAP kinase, and mTOR. Oestrogen has the ability to induce CD40-mediated activation of B cells. The complete signalling pathways of mER, GPR30 and their interaction with other signals are targeted areas for novel drug development in B cells during infection, autoimmunity and cancer. Therefore, an in depth investigation is critical for determining shared signal outputs during B cell activation. Here, we focus on the mode of action of membrane bound ER in B cells as therapeutic checkpoints.

Antibody modulation: Limiting the efficacy of therapeutic antibodies

Monoclonal antibodies (mAb) have revolutionised the way in which we treat disease. From cancer to autoimmunity, antibody therapy has been responsible for some of the most impressive clinical responses observed in the last 2 decades. A key component of this success has been their generally low levels of toxicity, and unique mechanisms of action. These two facets have allowed them to (a) be integrated rapidly into clinical practice in combination with conventional radio- and chemo-therapies and (b) to avoid the resistance mechanisms typically observed with classical small molecule drugs, such as upregulation of drug efflux transporters, dysregulation of apoptosis and mutations in key target enzymes/pathways. Although success with mAb therapies has been impressive, they are also subject to their own resistance mechanisms. In this perspective we discuss the various ways in which mAb therapeutics can be inhibited, concentrating mainly on the ways in which they can be removed from the target cell surface-a process called modulation. This can be achieved either in a cis-fashion on a single cell or in trans, precipitated by engagement with a second phagocytic cell. The evidence for each of these processes will be discussed, in addition to possible therapeutic strategies that might be employed to inhibit or reverse them.

Activatory and inhibitory Fcγ receptors augment rituximab-mediated internalization of CD20 independent of signaling via the cytoplasmic domain

Type I anti-CD20 mAb such as rituximab and ofatumumab engage with the inhibitory FcγR, FcγRIIb on the surface of B cells, resulting in immunoreceptor tyrosine-based inhibitory motif (ITIM) phosphorylation. Internalization of the CD20·mAb·FcγRIIb complex follows, the rate of which correlates with FcγRIIb expression. In contrast, although type II anti-CD20 mAb such as tositumomab and obinutuzumab also interact with and activate FcγRIIb, this interaction fails to augment the rate of CD20·mAb internalization, raising the question of whether ITIM phosphorylation plays any role in this process. We have assessed the molecular requirements for the internalization process and demonstrate that in contrast to internalization of IgG immune complexes, FcγRIIb-augmented internalization of rituximab-ligated CD20 occurs independently of the FcγRIIb ITIM, indicating that signaling downstream of FcγRIIb is not required. In transfected cells, activatory FcγRI, FcγRIIa, and FcγRIIIa augmented internalization of rituximab-ligated CD20 in a similar manner. However, FcγRIIa mediated a slower rate of internalization than cells expressing equivalent levels of the highly homologous FcγRIIb. The difference was maintained in cells expressing FcγRIIa and FcγRIIb lacking cytoplasmic domains and in which the transmembrane domains had been exchanged. This difference may be due to increased degradation of FcγRIIa, which traffics to lysosomes independently of rituximab. We conclude that the cytoplasmic domain of FcγR is not required for promoting internalization of rituximab-ligated CD20. Instead, we propose that FcγR provides a structural role in augmenting endocytosis that differs from that employed during the endocytosis of immune complexes.

IgG-effector functions: "the good, the bad and the ugly"

IgG-antibodies are potent and versatile mediators of host protection. They elicit their biological effects through specific interaction of the Fc-part with complement, specific cellular receptors, or both. Several factors should be taken into consideration when analyzing the nature and intensity of the immunological response elicited via IgG-effector functions, especially for the family of IgG-Fc receptors (FcγRs) exclusively expressed on immune cells. These include the various classes of leukocyte FcγR, expressed variably on different immune cells, each with distinct affinity for every IgG subclass, as well as genetic FcγR-polymorphisms affecting expression and affinity for IgG. Furthermore, various aspects of the IgG itself are also crucial for the outcome of the biological response. These include endogenously encoded IgG-polymorphisms, such as IgG3 polymorphisms, and post-transcriptional IgG-modifications, in particular IgG-Fc-glycosylation, affecting IgG effector functions through modified binding affinity to FcγR. These latter aspects concerning the variability in IgG3 on its half-life and placental transport and the clinical consequences of altered IgG-quality through glycosylation, will be the focus of this review.

Regulation of eNOS in caveolae

Caveolae are a specialized subset of lipid domains that are prevalent on the plasma membrane of endothelial cells. They compartmentalize signal transduction molecules which regulate multiple endothelial functions including the production of nitric oxide (NO) by the caveolae resident enzyme endothelial NO synthase (eNOS). eNOS is one of the three isoforms of the NOS enzyme which generates NO upon the conversion of L-arginine to L-citrulline and it is regulated by multiple mechanisms. Caveolin negatively impact eNOS activity through direct interaction with the enzyme. Circulating factors known to modify cardiovascular disease risk also influence the activity of the enzyme. In particular, high density lipoprotein cholesterol (HDL) maintains the lipid environment in caveolae, thereby promoting the retention and function of eNOS in the domain and it also causes direct activation of eNOS via scavenger receptor class B, Type I (SR-BI)-induced kinase signaling. Estrogen binding to estrogen receptors (ER) in caveolae also activates eNOS and this occurs through G protein coupling and kinase activation. Discrete domains within SR-BI and ER mediating signal initiation in caveolae have been identified. Counteracting the promodulatory actions of HDL and estrogen, C-reactive protein (CRP) antagonizes eNOS through FcγRIIB, which is the sole inhibitory receptor for IgG. Through their actions on eNOS, estrogen and CRP also regulate endothelial cell growth and migration. Thus, signaling events in caveolae invoked by known circulating cardiovascular disease risk factors have major impact on eNOS and endothelial cell phenotypes of importance to cardiovascular health and disease.

Effect of NKG2D ligand expression on host immune responses

Natural killer group 2, member D (NKG2D) is an activating receptor present on the surface of natural killer (NK) cells, some NKT cells, CD8(+) cytotoxic T cells, gammadelta T cells, and under certain conditions CD4(+) T cells. Present in both humans and mice, this highly conserved receptor binds to a surprisingly diverse family of ligands that are distant relatives of major histocompatibility complex class I molecules. There is increasing evidence that ligand expression can result in both immune activation (tumor clearance, viral immunity, autoimmunity, and transplantation) and immune silencing (tumor evasion). In this review, we describe this family of NKG2D ligands and the various mechanisms that control their expression in stressed and normal cells. We also discuss the host response to both membrane-bound and secreted NKG2D ligands and summarize the models proposed to explain the consequences of this differential expression.

FcgammaRI ligation leads to a complex with BLT1 in lipid rafts that enhances rat lung macrophage antimicrobial functions

Leukotriene (LT) B(4) is generated in response to engagement of the Fc gamma receptor (Fc gamma R) and potently contributes to Fc gamma R-mediated antimicrobial functions in pulmonary alveolar macrophages. In this study, we report that the LTB(4) receptor leukotriene B(4) receptor 1 (BLT1) redistributes from nonlipid raft (LR) to LR membrane microdomains upon immunoglobulin G-red blood cell, but not LTB(4), challenge. Cholesterol depletion to disrupt LRs abolished LTB(4)-induced enhancement of phagocytosis, microbicidal activity, and signaling. The dependence on LR integrity for BLT1 signaling correlated with formation of a complex consisting of BLT1, its primary coupled G protein G alpha i3, Src kinase, and Fc gamma RI within LRs. This association was dependent on Src-mediated phosphorylation of BLT1. These data identify a novel form of regulation in which engagement of a macrophage immunoreceptor recruits a stimulatory G protein-coupled receptor into a LR microdomain with resultant enhanced antimicrobial signaling.

Activated dectin-1 localizes to lipid raft microdomains for signaling and activation of phagocytosis and cytokine production in dendritic cells

Lipid rafts are plasma membrane microdomains that are enriched in cholesterol, glycosphingolipids, and glycosylphosphatidylinositol-anchored proteins and play an important role in the signaling of ITAM-bearing lymphocyte antigen receptors. Dectin-1 is a C-type lectin receptor (CLR) that recognizes beta-glucan in the cell walls of fungi and triggers signal transduction via its cytoplasmic hemi-ITAM. However, it is not known if similar to antigen receptors, Dectin-1 would also signal via lipid rafts and if the integrity of lipid raft microdomains is important for the physiological functions mediated by Dectin-1. We demonstrate here using sucrose gradient ultracentrifugation and confocal microscopy that Dectin-1 translocates to lipid rafts upon stimulation of dendritic cells (DCs) with the yeast derivative zymosan or beta-glucan. In addition, two key signaling molecules, Syk and PLCgamma2 are also recruited to lipid rafts upon the activation of Dectin-1, suggesting that lipid raft microdomains facilitate Dectin-1 signaling. Disruption of lipid raft integrity with the synthetic drug, methyl-beta-cyclodextrin (betamD) leads to reduced intracellular Ca2+ flux and defective Syk and ERK phosphorylation in Dectin-1-activated DCs. Furthermore, betamD-treated DCs have significantly attenuated production of IL-2, IL-10, and TNFalpha upon Dectin-1 engagement, and they also exhibit impaired phagocytosis of zymosan particles. Taken together, the data indicate that Dectin-1 and perhaps also other CLRs are recruited to lipid rafts upon activation and that the integrity of lipid rafts is important for the signaling and cellular functions initiated by this class of innate receptors.

Regulation of the Src homology 2 domain-containing inositol 5'-phosphatase (SHIP1) by the cyclic AMP-dependent protein kinase

Many agents that activate hematopoietic cells use phos pha tidyl ino si tol 3,4,5-trisphosphate (PtdIns 3,4,5-P(3)) to initiate signaling cascades. The SH2 domain-containing inositol 5' phosphatase, SHIP1, regulates hematopoietic cell function by opposing the action of phos pha tidyl ino si tol 3-kinase and reducing the levels of PtdIns 3,4,5-P(3). Activation of the cyclic AMP-de pend ent protein kinase (PKA) also opposes many of the pro-inflammatory responses of hematopoietic cells. We tested to see whether the activity of SHIP1 was regulated via phos pho ryl a tion with PKA. We prepared pure recombinant SHIP1 from HEK-293 cells and found it can be rapidly phos pho ryl a ted by PKA to a stoichiometry of 0.6 mol of PO(4)/mol of SHIP1. In (32)P-labeled HEK-293 cells transfected with SHIP1, stimulation with Sp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt hydrate (Sp-cAMPS) or activation of the beta-adrenergic receptor increased the phos pho ryl a tion state of SHIP1. Inhibition of protein phosphatase activity with okadaic acid also increased the phos pho ryl a tion of SHIP1. Phosphorylation of SHIP1 in vitro or in cells by PKA increased the 5' phosphatase activity of SHIP1 by 2-3-fold. Elevation of Ca(2+) in DT40 cells in response to B cell receptor cross-linking, an indicator of PtdIns 3,4,5-P(3) levels, was markedly blunted by pretreatment with Sp-cAMPS. This effect was absent in SHIP(-/-) DT40 cells showing that the effect of Sp-cAMPS in DT40 cells is SHIP1-de pend ent. Sp-cAMPS also blunted the ability of the B cell receptor to increase the phos pho ryl a tion of Akt in DT40 and A20 cells. Overall, activation of G protein-coupled receptors that raise cyclic AMP cause SHIP1 to be phosphorylated and stimulate its inositol phosphatase activity. These results outline a novel mechanism of SHIP1 regulation.

Selective regulation of autoreactive B cells by FcgammaRIIB

FcgammaRIIB is an inhibitory receptor which plays a role in limiting B cell and DC activation. Since FcgammaRIIB is known to dampen the signaling strength of the BCR, we wished to determine the impact of FcgammaRIIB on the regulation of BCRs which differ in their affinity for DNA. For these studies, FcgammaRIIB deficient BALB/c mice were bred with mice expressing the transgene-encoded H chain of the R4A anti-DNA antibody which gives rise to BCRs which express high, low or no affinity for DNA. The deletion of FcgammaRIIB in R4A BALB/c mice led to an alteration in the B cell repertoire, allowing for the expansion and activation of high affinity DNA-reactive B cells. By 6-8 months of age, R4A x FcgammaRIIB-/- BALB/c mice spontaneously developed anti-DNA antibody titers. These mice also displayed an induction of IFN-inducible genes and an elevation in levels of the B cell survival factor, BAFF. These data demonstrate that FcgammaRIIB preferentially limits activation of high affinity autoreactive B cells and can influence the activation of DC through an immune complex-mediated mechanism.

Lipid rafts and B cell signaling

B cells comprise an essential component of the humoral immune system. They are equipped with the unique ability to synthesize and secrete pathogen-neutralizing antibodies, and share with professional antigen presenting cells the ability to internalize foreign antigens, and process them for presentation to helper T cells. Recent evidence indicates that specialized cholesterol- and glycosphingolipid-rich microdomains in the plasma membrane commonly referred to as lipid rafts, serve to compartmentalize key signaling molecules during the different stages of B cell activation including B cell antigen receptor (BCR)-initiated signal transduction, endocytosis of BCR-antigen complexes, loading of antigenic peptides onto MHC class II molecules, MHC-II associated antigen presentation to helper T cells, and receipt of helper signals via the CD40 receptor. Here we review the recent literature arguing for a role of lipid rafts in the spatial organization of B cell function.

TLR4, ethanol, and lipid rafts: a new mechanism of ethanol action with implications for other receptor-mediated effects

Ethanol (EtOH) is the most widely abused substance in the United States, and it contributes to well-documented harmful (at high dosages) and beneficial (at low dosages) changes in inflammatory and immune responses. Lipid rafts have been implicated in the regulation and activation of several important receptor complexes in the immune system, including the TLR4 complex. Many questions remain about the precise mechanisms by which rafts regulate the assembly of these receptor complexes. Results summarized in this review indicate that EtOH acts by altering the LPS-induced redistribution of components of the TLR4 complex within the lipid raft and that this is related to changes in actin cytoskeleton rearrangement, receptor clustering, and subsequent signaling. EtOH provides an example of an immunomodulatory drug that acts at least in part by modifying lipid rafts, and it could represent a model to probe the relationships between rafts, receptor complexes, and signaling.

B cell conducts the lymphocyte orchestra

The interest for B cells has recently been revived. They normally play a role in the development, the regulation, as well as the activation of lymphoid architecture: they regulate dendritic cells and T-cell subsets function through cytokine production. Receptor editing is also essential in B cells and aids in preventing autoimmunity. Both abnormalities in the distribution of B-cell subsets and clinical benefit response to B-cell depletion in autoimmune states illustrate their importance. A new area has thus been reached, whereby B lymphocytes return as a significant contributor to autoimmune disorders.

Measles virus induces expression of SIP110, a constitutively membrane clustered lipid phosphatase, which inhibits T cell proliferation

Interference of measles virus (MV) with phosphatidyl-inositol-3-kinase (PI3K) activation in response to T cell receptor ligation was identified as important for the induction of T cell paralysis. We now show that MV exposure of unstimulated T cells induces expression of SIP110, an isoform of the lipid phosphatase SHIP145, which is translated from an intron-derived sequences containing mRNA. We found that MV contact can regulate stimulated exon inclusion into pre-mRNAs by targeting PI3K or MAPK-dependent nuclear translocation and activation of splicing regulatory serine-arginine rich (SR) and Sam68 proteins. Induction of SIP110 in resting T cells relied on MV-dependent interference with basal activity of the PI3K. SIP110 was cloned from MV-exposed T cells, and, when transiently expressed in primary or Jurkat T cells, localized into membrane clusters independently of T cell activation. Confirming that SIP110 is a catalytically active lipid phosphatase, its transgenic expression abolished basal and impaired PMA/ionomycin-stimulated phosphorylation of the Akt kinase which is important for T cell proliferation. Thus MV causes induction of SIP110 expression, which constitutively depletes the cellular phosphoinositol-3,4,5-phosphate pool suggesting that thereby the threshold for activation signals necessary for the induction of T cell proliferation is raised.

Fluorescence resonance energy transfer in living cells reveals dynamic membrane changes in the initiation of B cell signaling

B cell responses are initiated by the clustering of the B cell receptor (BCR) by the binding of multivalent antigens. Clustering leads to phosphorylation of tyrosines in the cytoplasmic domains of the BCR by the inner plasma membrane leaflet-associated Src-family kinase Lyn. At present, little is known about the earliest events after BCR clustering that precede the BCR's phosphorylation by Lyn. Here we use fluorescence resonance energy transfer (FRET) in living cells to detect the interaction of the BCR with a Lyn-based membrane-targeted reporter in the first several seconds after BCR clustering. The results showed that, within seconds of antigen binding, the BCR selectively and transiently associated with the Lyn construct and that this association preceded by several seconds the triggering of Ca2+ fluxes and could be prolonged by the engagement of the B cell coreceptor complex, CD19/CD21. Thus, FRET measurements in living B cells revealed highly dynamic and regulated antigen-induced changes in the plasma membrane, allowing association of the BCR with the earliest components of its signaling cascade.

Alemtuzumab induces caspase-independent cell death in human chronic lymphocytic leukemia cells through a lipid raft-dependent mechanism

Alemtuzumab is a humanized IgG1 kappa antibody directed against CD52, a glycosyl-phosphatidylinositol linked cell-membrane protein of unknown function. Herein, we demonstrate that alemtuzumab promotes rapid death of chronic lymphocytic leukemia (CLL) cells in vitro, in a complement and accessory cell free system. Using minimal detergent solubilization of CLL membranes, we found that CD52 colocalizes with ganglioside GM-1, a marker of membrane rafts. Fluorescence microscopy revealed that upon crosslinking CD52 with alemtuzumab+anti-Fc IgG, large patches, and in many cases caps, enriched in CD52 and GM-1 formed upon the CLL cell plasma membrane. Depletion of membrane cholesterol or inhibition of actin polymerization significantly diminished the formation of alemtuzumab-induced caps and reduced alemtuzumab-mediated CLL cell death. We compared alemtuzumab-induced direct cytotoxicity, effector cell-mediated toxicity and complement-mediated cytotoxicity of CLL cells to normal T cells. The direct cytotoxicity and observed capping was significantly greater for CLL cells as compared to normal T cells. Cell-mediated and complement-mediated cytotoxicity did not significantly differ between the two cell types. In summary, our data support the hypothesis that alemtuzumab can initiate CLL cell death by crosslinking CD52-enriched lipid rafts. Furthermore, the differential direct cytotoxic effect suggests that CD52 directed antibodies could possibly be engineered to more specifically target CLL cells.

FcγRIIB Ile232Thr transmembrane polymorphism associated with human systemic lupus erythematosus decreases affinity to lipid rafts and attenuates inhibitory effects on B cell receptor signaling

The B cell inhibitory receptor FcgammaRIIB plays crucial roles in the maintenance of self-tolerance. We have identified a polymorphism FCGR2B c.695T>C that results in the non-conservative replacement of 232Ile at the transmembrane helix to Thr and demonstrated the association of the polymorphism with susceptibility to systemic lupus erythematosus (SLE) in Asians. In this study, we examined the impact of FCGR2B c.695T>C on the functional properties of FcgammaRIIB by expressing each allele product in a human B cell line ST486 lacking endogenous FcgammaRIIB. FcgammaRIIB 232Thr was found to be significantly less potent than wild-type 232Ile in inhibiting B cell receptor (BCR)-mediated phosphatidylinositol-3,4,5-trisphosphate accumulation, Akt and PLCgamma2 activation and calcium mobilization, and to display decreased levels of tyrosine phosphorylation and SH2-containing 5'-inositolphosphate phosphatase recruitment compared with 232Ile after IgG Fc-mediated coligation with BCR. Notably, a quantitative analysis of the subcellular distribution of FcgammaRIIB using 125I-labeled anti-FcgammaRIIB revealed that FcgammaRIIB 232Thr is less effectively distributed to detergent-insoluble lipid rafts than 232Ile, findings in accordance with the importance of the transmembrane amino acid residues, in particular large hydrophobic amino acids including Ile, in the association of membrane proteins with lipid rafts. Given the crucial roles of lipid rafts in integrating BCR signaling, decreased association of FcgammaRIIB 232Thr could contribute to its impaired inhibitory potential. Collectively, the present findings indicate that the Ile232Thr substitution affects the localization and function of FcgammaRIIB and that the molecular mechanism may link the polymorphism and susceptibility to SLE.

B Lymphocytes Are Required for Development and Treatment of Autoimmune Diseases

Recent studies have revealed that B cells serve extraordinarily diverse functions within the immune system in addition to antibody production. These functions contribute to autoimmunity. They initiate the development of lymphoid architecture and regulate dendritic and T-cell function through cytokine production. Receptor editing is also essential to prevent autoimmunity. Both abnormalities in the distribution of B-cell subsets and the benefits of ablative B-cell therapy of autoimmune states confirm their importance. Results from transgenic models have demonstrated that the sensitivity of B cells to antigen receptor cross-linking correlates to autoimmunity, with particular reference to negative signaling by CD5 and CD22. These mechanisms maintain tolerance by recruiting src-homology 2 domain-containing protein tyrosine phosphatase-1. These findings open new prospects for immunotherapy of autoimmune diseases.

Differential signalling during B-cell maturation

The molecular mechanism by which the antigen receptors (BCR) on B cells can elicit differential maturation state-specific responses is one of the central problems in B-cell differentiation yet to be resolved. Indeed, many of the early signalling events detected following BCR ligation, such as activation of protein tyrosine kinases (PTK), phospholipase C (PLC), phosphoinositide-3-kinase (PI 3K), protein kinase C (PKC) and the RasMAPK (mitogen activating protein kinase) signalling cascades are observed throughout B-cell maturation. However, it is becoming clear that the differential functional responses of these BCR-coupled signals observed during B-cell maturation are dependent on a number of parameters including signal strength and duration, subcellular localisation of the signal, maturation-restricted expression of downstream signalling effector elements/isoforms and modulation of signal by co-receptors. Thus, the combined signature of BCR signalling is likely to dictate the functional response and act as a developmental checkpoint for B-cell maturation.

Decreased Lyn expression and translocation to lipid raft signaling domains in B lymphocytes from patients with systemic lupus erythematosus

OBJECTIVE

B lymphocytes from patients with systemic lupus erythematosus (SLE) are hyperactive and produce anti-double-stranded DNA (anti-dsDNA) autoantibodies. The cause or causes of B cell defects in SLE are unknown. In this study, we determined the level and subcellular distribution of Lyn protein, a key negative regulator of B cell receptor signaling, and assessed whether altered Lyn expression is characteristic of B cells in the setting of SLE.

METHODS

Negative selection was used to isolate B lymphocytes from blood. Lipid raft signaling domains were purified from B cells obtained from 62 patients with SLE, 15 patients with rheumatoid arthritis, and 31 healthy controls, by gradient ultracentrifugation. The total Lyn protein level was determined by Western blotting, confocal microscopy, and fluorescein-activated cell sorting (FACS). The distribution of Lyn into lipid raft and nonlipid raft domains was determined by Western blotting and confocal microscopy. Lyn content in B cell subpopulations was determined by FACS. In order to assess B lymphocyte activity, we used (3)H-thymidine incorporation and enzyme-linked immunosorbent assay to measure spontaneous proliferation and IgG and cytokine production by B cells.

RESULTS

This study revealed that B lymphocytes from a majority of patients with SLE have a reduced level of Lyn and manifest altered translocation to lipid rafts. An investigation into the mechanisms of Lyn reduction suggested that increased ubiquitination is involved. This was evident from increased ubiquitination of Lyn and translocation of c-Cbl into lipid rafts. Studies of B cell responses showed that altered Lyn expression was associated with heightened spontaneous proliferation, anti-dsDNA autoantibodies, and increased interleukin-10 production.

CONCLUSION

This study provides evidence for altered Lyn expression in B cells from a majority of patients with SLE. Altered Lyn expression in SLE may influence the B cell receptor signaling and B cell hyperactivity that are characteristic of the disease.

Phosphoinositide 3-kinase: diverse roles in immune cell activation

Cells of the immune system carry out diverse functions that are controlled by surface receptors for antigen, costimulatory molecules, cytokines, chemokines, and other ligands. A shared feature of signal transduction downstream of most receptors on immune cells, as in nonhematopoietic cell types, is the activation of phosphoinositide 3-kinase (PI3K). The mechanism by which this common signaling event is elicited by distinct receptors and contributes to unique functional outcomes is an intriguing puzzle. Understanding how specificity is achieved in PI3K signaling is of particular significance because altered regulation of this pathway is observed in many disease states, including leukemia and lymphoma. Here we review recent advances in the understanding of PI3K signaling mechanisms in different immune cells and receptor systems. We emphasize the concept that PI3K and its products are components of complex networks of interacting proteins and second messengers, rather than simple links in linear signaling cascades.

Exclusion of lipid rafts and decreased mobility of CD94/NKG2A receptors at the inhibitory NK cell synapse

CD94/NKG2A is an inhibitory receptor expressed by most human natural killer (NK) cells and a subset of T cells that recognizes human leukocyte antigen E (HLA-E) on potential target cells. To elucidate the cell surface dynamics of CD94/NKG2A receptors, we have expressed CD94/NKG2A-EGFP receptors in the rat basophilic leukemia (RBL) cell line. Photobleaching experiments revealed that CD94/NKG2A-EGFP receptors move freely within the plasma membrane and accumulate at the site of contact with ligand. The enriched CD94/NKG2A-EGFP is markedly less mobile than the nonligated receptor. We observed that not only are lipid rafts not required for receptor polarization, they are excluded from the site of receptor contact with the ligand. Furthermore, the lipid raft patches normally observed at the sites where FcepsilonR1 activation receptors are cross-linked were not observed when CD94/NKG2A was coengaged along with the activation receptor. These results suggest that immobilization of the CD94/NKG2A receptors at ligation sites not only promote sustenance of the inhibitory signal, but by lipid rafts exclusion prevent formation of activation signaling complexes.

In vivo oligomerization and raft localization of Ebola virus protein VP40 during vesicular budding

The matrix protein VP40 plays a critical role in Ebola virus assembly and budding, a process that utilizes specialized membrane domains known as lipid rafts. Previous studies with purified protein suggest a role for oligomerization of VP40 in this process. Here, we demonstrate VP40 oligomers in lipid rafts of mammalian cells, virus-like particles, and in the authentic Ebola virus. By mutagenesis, we identify several critical C-terminal sequences that regulate oligomerization at the plasma membrane, association with detergent-resistant membranes, and vesicular release of VP40, directly linking these phenomena. Furthermore, we demonstrate the active recruitment of TSG101 into lipid rafts by VP40. We also report the successful application of the biarsenic fluorophore, FlAsH, combined with a tetracysteine tag for imaging of Ebola VP40 in live cells.

A novel polymorphism in the Fcgamma receptor IIB (CD32B) transmembrane region alters receptor signaling

OBJECTIVE

The low-affinity receptor Fcgamma receptor IIb (FcgammaRIIb), with an immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic domain, down-regulates humoral immune responses and modulates the risk of autoimmunity in animal models. The transmembrane domain of FcgammaRIIb may also contribute to receptor signaling. Therefore, we investigated the biologic significance of single-nucleotide polymorphisms (SNPs) throughout the coding region.

METHODS

Discovery of SNPs in FCGR2B was performed by direct cycle sequencing of complementary DNA samples derived by reverse transcriptase-polymerase chain reaction. To assess the biologic significance of the nonsynonymous transmembrane SNP, we studied 3 functions influenced by the FcgammaRIIb transmembrane domain: tyrosine dephosphorylation of CD19, inhibition of B cell receptor (BCR)-induced calcium response, and modulation of BCR- or anti-Fas-induced apoptosis.

RESULTS

The nonsynonymous C-to-T transition in the first cytoplasmic exon, originally reported in the Raji cell line, was not found in either the African-American or the Caucasian population, but a nonsynonymous T-to-C transition at nucleotide 775 in exon 4 of FCGR2B, which changes isoleucine to threonine at residue 187 in the transmembrane domain, was significantly more common in African Americans. Using the FcgammaRIIb-negative mouse B cell line IIA1.6, we expressed both allelic forms as both full-length and truncated cytoplasmic domain constructs. The FCGR2B-187T allele mediated a higher level of CD19 dephosphorylation (P = 0.029) and a greater degree of inhibition of the calcium response (P = 0.003) when co-engaged with BCR than did FCGR2B-187I, independent of the presence of the ITIM. In contrast, FcgammaRIIb modulation of BCR-induced and anti-Fas antibody-induced cell death rates were similar in IIA1.6 cells expressing either the 187I or the 187T allelic form.

CONCLUSION

The differential activity of FCGR2B alleles suggests a novel mechanism of FcgammaRIIb regulation that may influence the risk of autoimmune disease.

Cross-talk between Fc receptors and integrins

Fc receptors (FcRs) and integrins are both key players of immune responses. These receptors are capable of independent signaling that leads to various cell functions. Recently, it is becoming clear that these receptors are also capable of modulating each other's responses. This modulation is achieved by direct physical interactions of the receptors on the cell surface, or by one receptor modifying the signaling pathway of the other receptor. Receptor co-localization and resonance energy transfer studies have shown that Fc receptors and integrins interact on the cell surface. Biochemical and functional studies have shown that signaling from Fc receptors modulates integrin adhesiveness through a process called "inside-out signaling". Other studies also show that the signaling from integrins modulates Fc receptor responses such as adhesion to immune complexes and cell mediated antibody-dependent cytotoxicity. This bi-directional cross-talk between Fc receptors and integrins is very important for the final cell function. Here, we review the latest information about interactions between Fc receptors and integrins.

Location is everything: lipid rafts and immune cell signaling

The cells of both the adaptive and innate immune systems express a dizzying array of receptors that transduce and integrate an enormous amount of information about the environment that allows the cells to mount effective immune responses. Over the past several years, significant advances have been made in elucidating the molecular details of signal cascades initiated by the engagement of immune cell receptors by their ligands. Recent evidence indicates that immune receptors and components of their signaling cascades are spatially organized and that this spatial organization plays a central role in the initiation and regulation of signaling. A key organizing element for signaling receptors appears to be cholesterol- and sphingolipid-rich plasma membrane microdomains termed lipid rafts. Research into the molecular basis of the spatial segregation and organization of signaling receptors provided by rafts is adding fundamentally to our understanding of the initiation and prolongation of signals in the immune system.

The B cell-specific major raft protein, Raftlin, is necessary for the integrity of lipid raft and BCR signal transduction

Recent evidence indicates that membrane microdomains, termed lipid rafts, have a role in B-cell activation as platforms for B-cell antigen receptor (BCR) signal initiation. To gain an insight into the possible functioning of lipid rafts in B cells, we applied liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) methodologies to the identification of proteins that co-purified with lipid rafts of Raji cells. Among these raft proteins, we characterized a novel protein termed Raftlin (raft-linking protein). Like the Src family kinase, Raftlin is localized exclusively in lipid rafts by fatty acylation of N-terminal Gly2 and Cys3, and is co-localized with BCR before and after BCR stimulation. Disruption of the Raftlin gene in the DT40 B-cell line resulted in a marked reduction in the quantity of lipid raft components, including Lyn and ganglioside GM1, while overexpression of Raftlin increased the content of raft protein. Moreover, BCR-mediated tyrosine phosphorylation and calcium mobilization were impaired by the lack of Raftlin and actually potentiated by overexpression of Raftlin. These data suggest that Raftlin plays a pivotal role in the formation and/or maintenance of lipid rafts, therefore regulating BCR-mediated signaling.

AKT1/PKBalpha is recruited to lipid rafts and activated downstream of PKC isotypes in CD3-induced T cell signaling

Protein kinase (PK) Ctheta and Akt/PKBalpha cooperate in T cell receptor/CD28-induced T cell signaling. We here demonstrate the recruitment of endogenous Akt1 and PKCtheta to lipid rafts in CD3-stimulated T cells. Further we show that Myr-PKCtheta mediates translocation of endogenous Akt1 to the plasma membrane as well as to lipid rafts, most likely explained by the observed complex formation of both protein kinases. In addition, in peripheral mouse T cells, the PKC inhibitor Gö6850 could partially block Akt1 activation in CD3-induced signaling, placing PKC isotype(s) upstream of Akt1. However, T cells derived from PKCtheta knockout mice were not impaired in CD3- or phorbol ester-induced Akt1 activity. Taken together, the results of this study give new insights into the functional link of Akt1 and PKCtheta in T cell signaling, demonstrating the co-recruitment of the two kinases and showing a novel pathway leading to Akt1 transactivation where PKC isotype(s) are involved but PKCtheta is not essential.

The roles of membrane microdomains (rafts) in T cell activation

Detergent-resistant membrane microdomains enriched in sphingolipids, cholesterol and glycosylphosphatidylinositol-anchored proteins play essential roles in T cell receptor (TCR) signaling. These 'membrane rafts' accumulate several cytoplasmic lipid-modified molecules, including Src-family kinases, coreceptors CD4 and CD8 and transmembrane adapters LAT and PAG/Cbp, essential for either initiation or amplification of the signaling process, while most other abundant transmembrane proteins are excluded from these structures. TCRs in various T cell subpopulations may differ in their use of membrane rafts. Membrane rafts also seem to be involved in many other aspects of T cell biology, such as functioning of cytokine and chemokine receptors, adhesion molecules, antigen presentation, establishing cell polarity or interaction with important pathogens. Although the concept of membrane rafts explains several diverse biological phenomena, many basic issues, such as composition, size and heterogeneity, under native conditions, as well as the dynamics of their interactions with TCRs and other immunoreceptors, remain unclear, partially because of technical problems.

Phosphorylation of FcgammaRIIA is required for the receptor-induced actin rearrangement and capping: the role of membrane rafts

Activation of Fcgamma receptor II (FcgammaRII) induces rearrangement of the actin-based cytoskeleton that serves as a driving force for FcgammaRII-mediated phagocytosis and FcgammaRII capping. To get insight into the signaling events that lead to the actin reorganization we investigated the role of raft-associated Src family tyrosine kinases in capping of FcgammaRII in U937 cells. After crosslinking, FcgammaRII was found to be recruited to detergent-resistant membrane domains (DRMs), rafts, where it coexisted with Lyn kinase and underwent tyrosine phosphorylation. Lyn was displaced from DRMs under the influence of DL-alpha-hydroxymyristic acid and 2-bromopalmitic acid, agents blocking N-terminal myristoylation and palmitoylation of proteins, respectively, and after disruption of DRM integrity by depletion of plasma membrane cholesterol with beta-cyclodextrin. Under these conditions, phosphorylation of the crosslinked FcgammaRII was diminished and assembly of FcgammaRII caps was blocked. The similar reduction of FcgammaRII cap formation correlated with inhibition of receptor phosphorylation was achieved with the use of PP1 and herbimycin A, specific inhibitors of Src family tyrosine kinases. Phosphorylation of FcgammaRIIA expressed in BHK cells, lacking endogenous FcgammaRs, was abolished by substitution of tyrosine 298 by phenylalanine in the ITAM of the receptor. The mutant receptor did not undergo translocation towards cap-like structures and failed to promote the receptor-mediated spreading of the cells, as compared to BHK cells transfected with the wild-type FcgammaRIIA. On the basis of these data, we suggest that tyrosine phosphorylation of activated FcgammaRIIA by raft-residing tyrosine kinases of the Src family triggers signaling pathways that control the rearrangement of the actin cytoskeleton required for FcgammaRII-mediated motility.

SH2-containing inositol phosphatase (SHIP-1) transiently translocates to raft domains and modulates CD16-mediated cytotoxicity in human NK cells

Membrane recruitment of the SH2-containing 5' inositol phosphatase 1 (SHIP-1) is responsible for the inhibitory signals that modulate phosphatidylinositol 3-kinase (PI3K)-dependent signaling pathways. Here we have investigated the molecular mechanisms underlying SHIP-1 activation and its role in CD16-mediated cytotoxicity. We initially demonstrated that a substantial fraction of SHIP-1-mediated 5' inositol phosphatase activity associates with CD16 zeta chain after receptor cross-linking. Moreover, CD16 stimulation on human primary natural killer (NK) cells induces the rapid and transient translocation of SHIP-1 in the lipid-enriched plasma membrane microdomains, termed rafts, where it associates with tyrosine-phosphorylated zeta chain and shc adaptor protein. As evaluated by confocal microscopy, CD16 engagement by reverse antibody-dependent cellular cytotoxicity (ADCC) rapidly induces SHIP-1 redistribution toward the area of NK cell contact with target cells and its codistribution with aggregated rafts where CD16 receptor also colocalizes. The functional role of SHIP-1 in the modulation of CD16-induced cytotoxicity was explored in NK cells infected with recombinant vaccinia viruses encoding wild-type or catalytic domain-deleted mutant SHIP-1. We found a significant SHIP-1-mediated decrease of CD16-induced cytotoxicity that is strictly dependent on its catalytic activity. These data demonstrate that CD16 engagement on NK cells induces membrane targeting and activation of SHIP-1, which acts as negative regulator of ADCC function.

Modulation of tumor growth by inhibitory Fc(gamma) receptor expressed by human melanoma cells

The efficacy of anti-tumor IgG reflects the balance between opposing signals mediated by activating and inhibitory Fc(gamma) receptors (Fc(gamma)Rs) expressed by effector cells. Here, we show that human malignant melanoma cells express the inhibitory low-affinity Fc(gamma) receptor Fc(gamma)RIIB1 in 40% of tested metastases. When melanoma cells were grafted in nude mice, a profound inhibition of Fc(gamma)RIIB1 tumor growth that required the intracytoplasmic region of the receptor was observed. IgG immune complexes (ICs) may be required for this inhibition, since sera from nude mice bearing tumors contained IgG that decreased the proliferation of Fc(gamma)RIIB1-positive cells in vitro, and tumor development of Fc(gamma)RIIB1-positive melanoma lines was not inhibited in antibody-defective severe combined immunodeficiency (SCID) mice. Passive immunization of SCID mice with anti-ganglioside G(D2) antibody resulted in significant inhibition of growth of Fc(gamma)RIIB1-positive tumors in an intracytoplasmic-dependent manner. Altogether, these data suggest that human melanoma cells express biologically active inhibitory Fc(gamma)RIIB1, which regulates their development upon direct interaction with anti-tumor antibodies. Therefore, Fc(gamma)R expression on human tumors may be one component of the efficacy of antibody-mediated therapies, and Fc(gamma)R-positive tumors could be the most sensitive candidates for such treatments.

On the mechanism of tolerance to the Rh D antigen mediated by passive anti-D (Rh D prophylaxis)

Anti-D prophylaxis is the most successful clinical application of antibody-mediated immune suppression. Passive IgG anti-D is given to Rh D-negative women to prevent immunisation to foetal Rh D-positive red blood cells (RBC) and subsequent haemolytic disease of the newborn. Despite its widespread use and efficacy, the mechanism of action of this therapy is unproven. The known facts about the antigen, antibody response, dose of anti-D, RBC clearance and effects of the passive anti-D on subsequent primary and secondary immune responses are discussed in relation to recent information on ways by which immune responses may be suppressed. Most Rh D antigen sites on RBC are not bound by passive anti-D, and thus epitope masking (which may occur in experimental murine models using xenogeneic RBC) is not the reason why anti-D responses are prevented by administration of prophylactic anti-D. It is hypothesised that although clearance and destruction of the antigenic RBC may be a contributing factor in preventing immunisation, down-regulation of antigen-specific B cells through co-ligation of B cell receptors and inhibitory IgG Fc receptors must also occur.

Lipid rafts and B-cell activation

The B-cell antigen receptor acts during B-cell activation both to initiate signalling cascades and to transport antigen into the cell for subsequent processing and presentation. Recent evidence indicates that membrane microdomains, termed lipid rafts, have a role in B-cell activation as platforms for B-cell receptor (BCR) signalling and might also act in antigen trafficking. Lipid rafts might facilitate the regulation of the BCR during B-cell development by B-cell co-receptors, and during viral infection. So, lipid rafts seem to be an important new piece of the B-cell signalling puzzle.