Cytoplasmic domain heterogeneity and functions of IgG Fc receptors in B lymphocytes

The function of Fcγ receptors in dendritic cells and macrophages

Dendritic cells (DCs) and macrophages use various receptors to recognize foreign antigens and to receive feedback control from adaptive immune cells. Although it was long believed that all immunoglobulin Fc receptors are universally expressed by phagocytes, recent findings indicate that only monocyte-derived DCs and macrophages express high levels of activating Fc receptors for IgG (FcγRs), whereas conventional and plasmacytoid DCs express the inhibitory FcγR. In this Review, we discuss how the uptake, processing and presentation of antigens by DCs and macrophages is influenced by FcγR recognition of immunoglobulins and immune complexes in the steady state and during inflammation.

CD19 and CD32b differentially regulate human B cell responsiveness

B cell activation is regulated by a variety of signals. CD19 positively regulates B cell activation, augmenting signals delivered through the BCR complex. In contrast, CD32b contains an ITIM and negatively regulates BCR signaling. Importantly, there are drugs currently in clinical trials and preclinical development that cross-link CD32b to molecules within the BCR complex. We wanted to address how single engagement versus cotargeting these molecules affects human B cell function. When B cells from healthy individuals were activated by signals that mimic a T cell response (IL-21 costimulation), ligation of CD32b, but not CD19, inhibited B cell expansion and plasma cell (PC) differentiation. In contrast, when B cells were activated through TLR, anti-CD19, but not anti-CD32b, blunted the response. However, when both CD19 and CD32b were coengaged by a bispecific anti-CD19×CD32b Ab, both types of stimuli were potently inhibited. Cross-linking CD19 with CD32b also inhibited Ab-independent functions of B cells, such as HLA upregulation, cytokine production, and the ability of B cells to prime CD4(+) T cells. Finally, although cross-linking CD19 and CD32b inhibited PC differentiation of primary B cells, it did not alter Ig production from pre-established PCs. These data elucidate the mechanism by which a complex set of signals determines the fate of B cell responsiveness. Although signals through CD19 influence TLR-driven activation, CD32b impacts the magnitude of the response following IL-21 costimulation. Therefore, simultaneous targeting of multiple surface molecules may be a necessary approach to comprehensively modulate B cell activation in vivo.

Fc receptors as adaptive immunoreceptors

Most biological activities of antibodies depend on their ability to engage Receptors for the Fc portion of immunoglobulins (FcRs) on a variety of cell types. As FcRs can trigger positive and negative signals, as these signals control several biological activities in individual cells, as FcRs are expressed by many cells of hematopoietic origin, mostly of the myeloid lineage, as these cells express various combinations of FcRs, and as FcR-expressing cells have different functional repertoires, antibodies can exert a wide spectrum of biological activities. Like B and T Cell Receptors (BCRs and TCRs), FcRs are bona fide immunoreceptors. Unlike BCRs and TCRs, however, FcRs are immunoreceptors with an adaptive specificity for antigen, with an adaptive affinity for antibodies, with an adaptive structure and with an adaptive signaling. They induce adaptive biological responses that depend on their tissue distribution and on FcR-expressing cells that are selected locally by antibodies. They critically determine health and disease. They are thus exquisitely adaptive therapeutic tools.

B-cell-mediated strategies to fight chronic allograft rejection

Solid organs have been transplanted for decades. Since the improvement in graft selection and in medical and surgical procedures, the likelihood of graft function after 1 year is now close to 90%. Nonetheless even well-matched recipients continue to need medications for the rest of their lives hence adverse side effects and enhanced morbidity. Understanding Immune rejection mechanisms, is of increasing importance since the greater use of living-unrelated donors and genetically unmatched individuals. Chronic rejection is devoted to T-cells, however the role of B-cells in rejection has been appreciated recently by the observation that B-cell depletion improve graft survival. By contrast however, B-cells can be beneficial to the grafted tissue. This protective effect is secondary to either the secretion of protective antibodies or the induction of B-cells that restrain excessive inflammatory responses, chiefly by local provision of IL-10, or inhibit effector T-cells by direct cellular interactions. As a proof of concept B-cell-mediated infectious transplantation tolerance could be achieved in animal models, and evidence emerged that the presence of such B-cells in transplanted patients correlate with a favorable outcome. Among these populations, regulatory B-cells constitute a recently described population. These cells may develop as a feedback mechanism to prevent uncontrolled reactivity to antigens and inflammatory stimuli. The difficult task for the clinician, is to quantify the respective ratios and functions of “tolerant” vs. effector B-cells within a transplanted organ, at a given time point in order to modulate B-cell-directed therapy. Several receptors at the B-cell membrane as well as signaling molecules, can now be targeted for this purpose. Understanding the temporal expansion of regulatory B-cells in grafted patients and the stimuli that activate them will help in the future to implement specific strategies aimed at fighting chronic allograft rejection.

Fcγ receptors enable anticancer action of proapoptotic and immune-modulatory antibodies

Diverse FcγR-dependent mechanisms mediate anticancer activity of proapoptotic and immunomodulatory antibodies.

Antibodies have important roles in controlling cellular immunity through interaction with activating or inhibitory Fcγ receptors (FcγRs). FcγR engagement can facilitate receptor cross-linking on target cells, or induce retrograde FcγR signals to stimulate or suppress antibody-dependent, cell-mediated depletion of antigen-bearing target cells. Recent studies uncover unexpectedly important roles for FcγRs in the anticancer action of antibodies designed to trigger tumor cell apoptosis or enhance antitumor immunity. Here, we outline a conceptual framework for understanding these findings and discuss their mechanistic and translational implications.

Autonomous phagosomal degradation and antigen presentation in dendritic cells

Phagocytosis plays a critical role in both innate and adaptive immunity. Phagosomal fusion with late endosomes and lysosomes enhances proteolysis, causing degradation of the phagocytic content. Increased degradation participates in both innate protection against pathogens and the production of antigenic peptides for presentation to T lymphocytes during adaptive immune responses. Specific ligands present in the phagosomal cargo influence the rate of phagosome fusion with lysosomes, thereby modulating both antigen degradation and presentation. Using a combination of cell sorting techniques and single phagosome flow cytometry-based analysis, we found that opsonization with IgG accelerates antigen degradation within individual IgG-containing phagosomes, but not in other phagosomes present in the same cell and devoid of IgG. Likewise, IgG opsonization enhances antigen presentation to CD4(+) T lymphocytes only when antigen and IgG are present within the same phagosome, whereas cells containing phagosomes with either antigen or IgG alone failed to present antigen efficiently. Therefore, individual phagosomes behave autonomously, in terms of both cargo degradation and antigen presentation to CD4(+) T cells. Phagosomal autonomy could serve as a basis for the intracellular discrimination between self and nonself antigens, resulting in the preferential presentation of peptides derived from opsonized, nonself antigens.

Impact on N-glycosylation profile of monoclonal anti-D antibodies as a way to control their immunoregulatory and cytotoxic properties

Prophylaxis of hemolytic disease of newborns is based on the ability of polyclonal anti-D antibodies for suppressing maternal immune response against D-positive fetal red blood cells. The immunosuppressive effect of anti-D antibody is mediated by interaction between its Fc-fragment and low-affinity IgG Fc-receptor (FcγR) on the immune cell. No clinically effective monoclonal anti-D antibody (mAb) that can replace polyclonal anti-D immunoglobulin has been developed yet. The goals of this study were comparison of structural and functional properties of human anti-D polyclonal and monoclonal Abs and assessment of the possibility to manipulate the effector properties of the mAb. N-Glycosylation and particularly the content of nonfucosylated glycans are crucial for affinity of mAb to FcγRIIIA, which plays the key role in the clearance of sensitized cells. We studied and compared glycoprofiles and FcγRIIIA-mediated hemolytic ability of human polyclonal antibodies and anti-D mAbs produced by human B-cell lines, human-rodent heterohybridomas, and a human non-lymphoid cell line PER.C6. Replacement of producing cell line and use of glycosylation modulators can convert an inert mAb into an active one. Nevertheless, rodent cell lines, as well as human non-lymphoid cells, distort natural glycosylation of human IgG and could lead to the loss of immunosuppressive properties. All of the anti-D mAbs secreted by human B-cell lines have a glycoprofile close to human serum IgG. Hence, the constant ratio of IgG glycoforms in human serum is predetermined by glycosylation at the level of the individual antibody-producing cell. The anti-D fraction of polyclonal anti-D immunoglobulin compared to the total human IgG contains more nonfucosylated glycans. Thus, only human transformed B-cells are an appropriate source for efficient anti-D mAbs that can imitate the action of polyclonal anti-D IgG.

Different partners, opposite outcomes: a new perspective of the immunobiology of indoleamine 2,3-dioxygenase

Indoleamine 2,3-dioxygenase (IDO), a metabolic enzyme that catalyzes tryptophan conversion into kynurenines, is a crucial regulator of immunity. Altered IDO activity is often associated with pathology, including neoplasia and autoimmunity. IDO is highly expressed in dendritic cells (DCs) that exploit the enzyme's activity and the production of tryptophan catabolites to regulate immune responses by acting on several cell types, including T lymphocytes, of which they promote a regulatory phenotype. IDO also contains immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that, once bound by distinct molecular partners, will either promote degradation or initiate signaling activity and self-maintenance of the enzyme. We here discuss how ITIM-dependent molecular events can affect the functional plasticity of IDO by modifying the protein half-life and its enzymic and nonenzymic functions.

Differences in endocytosis mediated by FcγRIIA and FcγRIIB2

An important function of Fcγ receptors is the removal of IgG-containing immune complexes from the circulation. The activating receptor FcγRIIA and inhibitory receptor FcγRIIB2 are both expressed on human myeloid cells, and are both capable of mediating endocytosis of immune complexes. We studied endocytosis of these two receptors expressed by transfection in ts20 Chinese hamster fibroblasts. We find that while FcγRIIA-mediated endocytosis requires the participation of the ubiquitin-conjugating system, the endocytosis of FcγRIIB2 does not. Little if any ubiquitylation of FcγRIIB2 was observed in response to immune complex binding. FcγRIIB2 mediates internalization of immune complexes at a faster rate than FcγRIIA, and facilitates the endocytosis of FcγRIIA upon co-engagement of both receptors. This may represent a novel mechanism by which the inhibitory receptor can reduce signalling from the activating Fcγ receptor.

Facilitated antigen uptake and timed exposure to TLR ligands dictate the antigen-presenting potential of plasmacytoid DCs

Subsets of antigen-presenting cDCs have a differential capacity to present exogenous and endogenous protein antigens to CD4(+) and/or CD8(+) T lymphocytes, depending on expression of antigen-uptake receptors, processing machinery, and microbial instruction. pDCs are also capable of antigen presentation, but the conditions under which they do this have not been systematically addressed. Highly purified cDCs and pDCs were exposed to exogenous, soluble OVA peptide or whole protein. Alternatively, they were made to express cytoplasmic or endosomal OVA by retroviral transduction or by infection with influenza virus containing OVA epitopes. Like cDCs, pDCs expressed the MHC I processing machinery and could present endogenous or cross-present exogenous OVA to CD8(+) T cells, provided they had been stimulated by CpG motif TLR9 ligands or by influenza. Unlike cDCs, the cross-priming activity of pDCs was enhanced, not decreased, by simultaneous TLR stimulation. Processing and presentation of exogenous OVA to CD4(+) T cells required TLR9 ligation prior to antigen encounter and addition of OVA-specific Igs. These stimuli up-regulated critical MHC II processing machinery and enhanced routing to acidic endosomal organelles in a FcγRII-dependent manner. Endogenous antigen was not presented to CD4(+) T cells when expressed in the cytoplasm of pDCs by retrovirus or contained in influenza, unless an Ii-chain-derived endosomal routing signal was present. Thus, timing of TLR ligation and facilitated antigen uptake dictate the potential of pDCs to present endogenous or exogenous antigen by influencing endosomal traffic and antigen-processing machinery.

Cutting edge: intravenous Ig inhibits invariant NKT cell-mediated allergic airway inflammation through FcγRIIIA-dependent mechanisms

Despite their increasing use in autoimmune, inflammatory, and allergic conditions, the mechanism of action of i.v. Igs (IVIg) is poorly understood. On the basis of the critical role of invariant NKT (iNKT) cells in allergic airway inflammation (AAI) and their constitutive expression of the low-affinity IgG receptor FcγRIIIA, we surmised that IVIg targets iNKT cells to exert their anti-inflammatory effect. We found that IVIg treatment significantly inhibited AAI in OVA-sensitized C57BL/6 mice and downregulated α-galactosylceramide-induced iNKT cell activation and cytokine production. Allergic responses were restored in iNKT cell-deficient mice by transferring iNKT cells from PBS- but not from IVIg-treated mice, suggesting that IVIg acts directly on activated iNKT cells that have a critical role in AAI. The inhibitory effects of IVIg on both iNKT cell activation/function and OVA-driven AAI were lost in FcγRIIIA(-/-) mice. Our data unravel an FcγRIIIA-dependent inhibitory effect of IVIg on activated iNKT cells that confers protection in AAI.

An Fcγ receptor-dependent mechanism drives antibody-mediated target-receptor signaling in cancer cells

Antibodies to cell-surface antigens trigger activatory Fcγ receptor (FcγR)-mediated retrograde signals in leukocytes to control immune effector functions. Here, we uncover an FcγR mechanism that drives antibody-dependent forward signaling in target cells. Agonistic antibodies to death receptor 5 (DR5) induce cancer-cell apoptosis and are in clinical trials; however, their mechanism of action in vivo is not fully defined. Interaction of the DR5-agonistic antibody drozitumab with leukocyte FcγRs promoted DR5-mediated tumor-cell apoptosis. Whereas the anti-CD20 antibody rituximab required activatory FcγRs for tumoricidal function, drozitumab was effective in the context of either activatory or inhibitory FcγRs. A CD40-agonistic antibody required similar FcγR interactions to stimulate nuclear factor-κB activity in B cells. Thus, FcγRs can drive antibody-mediated receptor signaling in target cells.

Immunohistochemical investigation of cells expressing CD21, membrane IgM, CD32 and a follicular dendritic cell marker in the lymphoid tissues of neonatal calves

Activation of B lymphocytes in the presence of passive maternal antibodies depends on expression of CD21, membrane IgM and CD32. On colligation with IgM, CD32 inhibits activation whereas CD21 enhances it. Recently, we assessed expression of CD21 and CD32 on IgM(+) cells from lymphoid tissues of newborn calves by flow cytometry, but this approach does not provide information about spatial distribution within lymphoid compartments. Therefore, histologic sections of lymphoid tissues from newborn and 7-month-old calves were examined using an immunoperoxidase technique. In all calves, CD21 and IgM stained cells were collocated in the cortex and paracortex of the retropharyngeal lymph node, in the marginal zone of the spleen and in lymphoid aggregates of palatine tonsils. Most CD32(+) cells were in the mantle zone of lymphoid follicles in 7-month-old calves, whereas only weak staining was observed in newborns. A few CD32(+) cells were also observed in the paracortex at both ages. Absence of CD32(+) cells in the center of follicles suggests that IgM(+)CD32(-) cells observed previously by flow cytometry were from germinal centers. Overall, there were few organized lymphoid aggregates within lymphoid tissues of newborn calves, and follicular dendritic cells were virtually undetectable. Their absence may be an important limitation for neonatal immunization.

Down regulation of Fc and complement receptors on B cells in rheumatoid arthritis

B cell tolerance is regulated by receptors that modulate B cell receptor signaling, such as Fc gamma receptor IIb (FcγRIIb; CD32b) and complement receptors (CR) 1 and 2. Deficiency in these receptors may contribute to autoimmunity. To address this we have investigated the receptor expression in healthy individuals in comparison with rheumatoid arthritis (RA) patients. In healthy subjects we found that women had overall lower FcγRIIb expression on B cells than men that significantly decreased with age. RA patients had fewer FcγRIIb, CR1 and CR2 positive B cells and decreased receptor expressions compared to healthy subjects. Further, the RA B cells displayed a significantly increased proliferative response when cultured with interleukin-2 in vitro. In summary, the dysregulated B cells in RA are associated with lower FcγRIIb, CR1 and CR2 levels. The reduced FcγRIIb expression on B cells in women may influence the increased frequency of autoimmunity in women.

Cell surface heparan sulfate proteoglycans influence MHC class II-restricted antigen presentation

Heparan sulfate proteoglycans (HSPGs) are glycoproteins ubiquitously distributed on the cell surface and in the extracellular matrix. Their heparan sulfate moieties often represent alternative attachment points for extracellular proteins that target specific receptors. Thus, HSPGs modulate ligand-receptor encounters and participate in numerous biological processes. In this study, we examined whether HSPGs can also influence MHC class II-restricted Ag presentation. We selected a heparan sulfate ligand derived from the HIV-1 Tat protein and coupled it to a model protein Ag. We showed that coupling of the Tat fragment makes the Ag capable of binding cells, including APCs, and increases its ability to stimulate specific T cells up to 180-fold. The boosting effect depends on Ag processing; it vanished in the presence of an excess of heparin or free Tat fragment, indicating that HSPGs can behave as receptors involved in MHC class II processing and presentation. Furthermore, with FcγRII-bearing APCs, immune complexes containing the coupled Ag stimulated T cells up to 700-fold more efficiently than Ag-containing immune complexes. This effect vanished in the presence of heparin and is not found with FcγRII(-) APCs, indicating that HSPGs can also behave as coreceptors during FcγRIIR-mediated Ag presentation. These results indicate that ubiquitous receptors, such as HSPGs, can influence MHC class II-restricted Ag presentation and suggest that proteins will be supported more efficiently by the immune system if they have the inherent capacity to bind heparan sulfate.

Divergent intracellular sorting of Fc{gamma}RIIA and Fc{gamma}RIIB2

The human low affinity FcγRII family includes both the activating receptor FcγRIIA and the inhibitory receptor FcγRIIB2. These receptors have opposing signaling functions but are both capable of internalizing IgG-containing immune complexes through clathrin-mediated endocytosis. We demonstrate that upon engagement by multivalent aggregated human IgG, FcγRIIA expressed in ts20 Chinese hamster fibroblasts is delivered along with its ligand to lysosomal compartments for degradation, while FcγRIIB2 dissociates from the ligand and is routed separately into the recycling pathway. FcγRIIA sorting to lysosomes requires receptor multimerization, but does not require either Src family kinase activity or ubiquitylation of receptor lysine residues. The sorting of FcγRIIB2 away from a degradative fate is not due to its lower affinity for IgG and occurs even upon persistent receptor aggregation. Upon co-engagement of FcγRIIA and FcγRIIB2, the receptors are sorted independently to distinct final fates after dissociation of co-clustering ligand. These results reveal fundamental differences in the trafficking behavior of different Fcγ receptors.

Metastatic melanomas express inhibitory low affinity fc gamma receptor and escape humoral immunity

Our research, inspired by the pioneering works of Isaac Witz in the 1980s, established that 40% of human metastatic melanomas express ectopically inhibitory Fc gamma receptors (FcγRIIB), while they are detected on less than 5% of primary cutaneous melanoma and not on melanocytes. We demonstrated that these tumoral FcγRIIB act as decoy receptors that bind the Fc portion of antimelanoma IgG, which may prevent Fc recognition by the effector cells of the immune system and allow the metastatic melanoma to escape the humoral/natural immune response. The FcγRIIB is able to inhibit the ADCC (antibody dependent cell cytotoxicity) in vitro. Interestingly, the percentage of melanoma expressing the FcγRIIB is high (70%) in organs like the liver, which is rich in patrolling NK (natural killer) cells that exercise their antitumoral activity by ADCC. We found that this tumoral FcγRIIB is fully functional and that its inhibitory potential can be triggered depending on the specificity of the anti-tumor antibody with which it interacts. Together these observations elucidate how metastatic melanomas interact with and potentially evade humoral immunity and provide direction for the improvement of anti-melanoma monoclonal antibody therapy.

Activating and inhibitory Fcgamma receptors in immunotherapy: being the actor or being the target

Membrane Fcgamma receptors (FcgammaRs) can act either as potent activators of effector cell functions or as inhibitors of receptor-mediated cell activation following engagement by IgG antibodies bound to their target molecules. The remarkable ability of activating FcgammaRs to trigger antibody-dependent cellular cytotoxicity, cytokine release and phagocytosis/endocytosis followed by antigen presentation has stimulated the development of a number of therapeutic monoclonal antibodies whose Fc regions have been engineered to optimize their effector functions, mostly their killing activities. Conversely, the demonstration that inhibitory FcgammaRs can block or downmodulate effector functions has led to the concept that targeting these receptors is of interest in a number of pathologies. The use of bispecific antibodies leading to the crosslinking of FcgammaRIIB with activating receptors could induce immunomodulation in autoimmune or allergic diseases. Alternatively, the use of cytotoxic/antagonist anti-FcgammaRIIB antibodies could kill FcgammaRIIB-positive tumor cells or prevent the downmodulation of activating receptors. Thus, antibodies engineered to preferentially target activating or inhibitory FcgammaRs are currently being designed for therapeutic use.

Expression of complement receptor 2 (CD21), membrane IgM and the inhibitory receptor CD32 (FcgammaRIIb) in the lymphoid tissues of neonatal calves

Limited active antibody responses in neonates following vaccination have been attributed to immaturity of the immune system and to the suppressive effects of maternal antibodies. The activating receptor CD21 (CR2), when co-ligated with membrane IgM (mIgM) by complement-bound antigen lowers the threshold for activation of B lymphocytes. The inhibitory receptor CD32 (FcgammaRII) when co-ligated with mIgM by antigen-antibody complexes raises the threshold for activation. Expression of these receptors, which potentially play roles in regulation of B cell responses in the presence of maternal antibodies in neonates, has been recently characterized in blood lymphocytes in neonatal calves. Little is known however about expression of these receptors in the lymphoid tissues, where immune responses are initiated. In this study, expression of CD21, mIgM and CD32 receptors by B lymphocytes was studied in a range of lymphoid tissues including spleen, lymph nodes and bone marrow from newborn and 7-week-old calves using flow cytometry. The proportion of naïve B lymphocytes in the lymphocyte gate was significantly lower in blood and spleen of newborn calves compared to 7-week-old calves. Over 90% of B lymphocytes expressed CD21 in the lymphoid tissues. In the lymph nodes and spleen, a lower proportion of mIgM(+) B lymphocytes expressed CD32 compared to blood. In addition, intensity of expression of CD32 on B cells in lymph nodes was significantly lower compared to that in blood, suggesting a lower potential for inhibitory signalling in B cells in the lymphoid microenvironment. Investigation of the CD5(+) B cell population (as an indicator of B1 B cells) suggested an increase in the proportion of IgM(+)CD5(+) cells with age in calves, in both blood and lymphoid tissue, in contrast to the situation in humans and mice. Overall, the majority of naïve B lymphocytes in lymphoid tissues in neonatal calves expressed both activating (CD21, mIgM) and inhibitory (CD32) receptors. These receptors may provide targets for novel adjuvants, to lower the threshold for activation of B cells in neonates, and enhance antibody responses.

FcgammaRIIB in autoimmunity and infection: evolutionary and therapeutic implications

FcgammaRIIB is the only inhibitory Fc receptor. It controls many aspects of immune and inflammatory responses, and variation in the gene encoding this protein has long been associated with susceptibility to autoimmune disease, particularly systemic lupus erythematosus (SLE). FcgammaRIIB is also involved in the complex regulation of defence against infection. A loss-of-function polymorphism in FcgammaRIIB protects against severe malaria, the investigation of which is beginning to clarify the evolutionary pressures that drive ethnic variation in autoimmunity. Our increased understanding of the function of FcgammaRIIB also has potentially far-reaching therapeutic implications, being involved in the mechanism of action of intravenous immunoglobulin, controlling the efficacy of monoclonal antibody therapy and providing a direct therapeutic target.

Effector cell recruitment with novel Fv-based dual-affinity re-targeting protein leads to potent tumor cytolysis and in vivo B-cell depletion

Bispecific antibodies capable of redirecting the lytic potential of immune effector cells to kill tumor targets have long been recognized as a potentially potent biological therapeutic intervention. Unfortunately, efforts to produce such molecules have been limited owing to inefficient production and poor stability properties. Here, we describe a novel Fv-derived strategy based on a covalently linked bispecific diabody structure that we term dual-affinity re-targeting (DART). As a model system, we linked an Fv specific for human CD16 (FcgammaRIII) on effector cells to an Fv specific for mouse or human CD32B (FcgammaRIIB), a normal B-cell and tumor target antigen. DART proteins were produced at high levels in mammalian cells, retained the binding activity of the respective parental Fv domains as well as bispecific binding, and showed extended storage and serum stability. Functionally, the DART molecules demonstrated extremely potent, dose-dependent cytotoxicity in retargeting human PBMC against B-lymphoma cell lines as well as in mediating autologous B-cell depletion in culture. In vivo studies in mice demonstrated effective B-cell depletion that was dependent on the transgenic expression of both CD16A on the effector cells and CD32B on the B-cell targets. Furthermore, DART proteins showed potent in vivo protective activity in a human Burkitt's lymphoma cell xenograft model. Thus, DART represents a biologically potent format that provides a versatile platform for generating bispecific antibody fragments for redirected killing and, with the selection of appropriate binding partners, applications outside of tumor cell cytotoxicity.

Dominant expression of the inhibitory FcgammaRIIB prevents antigen presentation by murine plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) are key regulators of the innate immune response, yet their direct role as APCs in the adaptive immune response is unclear. We found that unlike conventional DCs, immune complex (IC) exposed murine pDCs neither up-regulated costimulatory molecules nor activated Ag-specific CD4(+) and CD8(+) T cells. The inability of murine pDCs to promote T cell activation was due to inefficient proteolytic processing of internalized ICs. This defect in the IC processing capacity of pDCs results from a lack of activating FcgammaR expression (FcgammaRI, III, IV) and the dominant expression of the inhibitory receptor FcgammaRIIB. Consistent with this idea, transgenic expression of the activating human FcgammaRIIA gene, not present in the mouse genome, recapitulated the human situation and rescued IC antigenic presentation capacity by murine pDCs. The selective expression of FcgammaRIIB by murine pDCs was not strain dependent and was maintained even following stimulation with TLR ligands and inflammatory cytokines. The unexpected difference between the mouse and human in the expression of activating/inhibitory FcgammaRs has implications for the role of pDCs in Ab-modulated autoimmunity and anti-viral immunity.

Ligand binding but undetected functional response of FcR after their capture by T cells via trogocytosis

Intercellular transfer of cell surface proteins by trogocytosis is common and could affect T cell responses. Yet, the role of trogocytosis in T cell function is still elusive, and it is unknown whether a molecule, once captured by T cells, harbors the same biological properties as in donor APC. In this study, we showed that FcgammaR as well as the associated FcRgamma subunit could be detected at high levels on murine and human T cells after their intercellular transfer from FcgammaR-expressing APC. Capture of FcgammaR occurred during coculture of T cells with FcgammaR-expressing APC upon Ab- or Ag-mediated T cell stimulation. Once captured by T cells, FcgammaR were expressed in a conformation compatible with physiological function and conferred upon T cells the ability to bind immune complexes and to provision B cells with this source of Ag. However, we were unable to detect downstream signal or signaling-dependent function following the stimulation of FcgammaR captured by T cells, and biochemical studies suggested the improper integration of FcgammaR in the recipient T cell membrane. Thus, our study demonstrates that T cells capture FcgammaR that can efficiently exert ligand-binding activity, which, per se, could have functional consequences in T cell-B cell cooperation.

Studies on the mechanism by which antigen-specific IgG suppresses primary antibody responses: evidence for epitope masking and decreased localization of antigen in the spleen

Immunoglobulin (IgG) has the ability to suppress the Ab response against the Ag to which it binds. Although the mechanism remains unclear, this phenomenon has physiological relevance and is used clinically in Rh prophylaxis. As suppression works well in mice lacking the inhibitory FcgammaRIIB, the two most likely explanations are that IgG masks epitopes and/or that IgG increases the clearance of Ag. In the present study, mice were immunized with sheep red blood cells (SRBC) to which the hapten 5-iodo-4-hydroxyl-3-nitrophenacetyl (NIP) was conjugated at high or low density and the ability of IgG anti-NIP to suppress the Ab response to NIP and SRBC was assayed. Only the NIP-specific response was suppressed when mice were immunized with SRBC-NIP(low), whereas both NIP- and SRBC-specific responses were suppressed when SRBC-NIP(high) was used. This is best explained by epitope masking; at high epitope density, IgG also blocks neighbouring epitopes from recognition by B cells. We also examined the effects of IgG-mediated suppression on T-cell responses directly in vivo. While IgG anti-SRBC administered with sheep red blood cells ovalbumin (SRBC-OVA) almost completely suppressed the anti-SRBC and anti-OVA Ab responses, the OVA-specific T-cell response was still 50% of that observed in control mice. This is probably the result of decreased Ag exposure as IgG-bound SRBC were cleared faster from the bloodstream and were found at lower concentration in the spleen than unbound SRBC. These results suggest that both Ag clearance and epitope masking occurs during IgG-mediated suppression, but that under physiological circumstances epitope masking is the predominant mechanism.

Fcgamma receptor IIB and IIIB polymorphisms and susceptibility to systemic lupus erythematosus and lupus nephritis: a meta-analysis

The aim of this study was to explore whether polymorphisms of the Fcgamma receptors (FcgammaRs) IIB T/I232 and FcgammaRIIIB NA1/NA2, confer susceptibility to systemic lupus erythematosus (SLE) and lupus nephritis (LN). The authors conducted a meta-analysis on associations between the FcgammaRIIB T/I232 and FcgammaRIIIB NA1/NA2 polymorphisms and SLE and LN susceptibility as determined using 1) allele contrast, 2) recessive, 3) dominant models and 4) contrast of homozygotes. A total of 16 separate comparisons were considered, consisting of 2887 SLE patients and 3105 controls. Meta-analysis of the FcgammaRIIB T/I232 polymorphism showed a significant association between the FcgammaRIIB T allele and the risk of developing SLE compared with the FcgammaRIIB I allele (OR = 1.207, 95% CI = 1.061-1.373, P = 0.004). In subjects of Asian descent, a significant association was observed between the FcgammaRIIB T allele and SLE (OR = 1.332, 95% CI 1.138-1.558, P < 0.001). However, in Europeans no such association was found. In contrast, no association was found between SLE or LN and the FcgammaRIIIB NA1/NA2 polymorphism in all subjects, or in European and Asian populations. This meta-analysis shows that the FcgammaRIIB T/I232 polymorphism confers susceptibility to SLE, especially in Asian-derived populations.

Cloning and characterization of ovine immunoglobulin G Fc receptor II (FcgammaRII)

Immunoglobulin G (IgG) Fc receptors (FcgammaRs) bind to immune complexes through interactions with the Fc region of IgG to initiate or inhibit the defense mechanism of the leukocytes on which they are expressed. In this study, we describe the cloning, sequencing and characterization of ovine FcgammaRII. By screening a translated expression sequence tag (EST) database with the protein sequence of bovine IgG Fc receptor II, we identified a putative ovine homologue. Using rapid amplification of cDNA ends (RACE), we isolated the cDNA encoding ovine FcgammaRII from peripheral blood leucocyte RNA. The ovine FcgammaRII cDNA contains an 894bp open-reading frame, encoding a 297 amino acid transmembrane glycoprotein composed of two immunoglobulin-like extracellular domains, a transmembrane region and a cytoplasmic tail with an immunoreceptor tyrosine-based inhibitory motif (ITIM). The glycoprotein encoded by the cloned cDNA was then expressed on the surface of COS-7 cells and immunoglobulin-binding assays show that it binds ovine IgG1, but not IgG2. Identification of the ovine FcgammaRII will aid in the understanding of the molecular basis of IgG-FcgammaR interaction.

Cross-talk between TLR4 and FcgammaReceptorIII (CD16) pathways

Pathogen-pattern-recognition by Toll-like receptors (TLRs) and pathogen clearance after immune complex formation via engagement with Fc receptors (FcRs) represent central mechanisms that trigger the immune and inflammatory responses. In the present study, a linkage between TLR4 and FcgammaR was evaluated in vitro and in vivo. Most strikingly, in vitro activation of phagocytes by IgG immune complexes (IgGIC) resulted in an association of TLR4 with FcgammaRIII (CD16) based on co-immunoprecipitation analyses. Neutrophils and macrophages from TLR4 mutant (mut) mice were unresponsive to either lipopolysaccharide (LPS) or IgGIC in vitro, as determined by cytokine production. This phenomenon was accompanied by the inability to phosphorylate tyrosine residues within immunoreceptor tyrosine-based activation motifs (ITAMs) of the FcRgamma-subunit. To transfer these findings in vivo, two different models of acute lung injury (ALI) induced by intratracheal administration of either LPS or IgGIC were employed. As expected, LPS-induced ALI was abolished in TLR4 mut and TLR4(-/-) mice. Unexpectedly, TLR4 mut and TLR4(-/-) mice were also resistant to development of ALI following IgGIC deposition in the lungs. In conclusion, our findings suggest that TLR4 and FcgammaRIII pathways are structurally and functionally connected at the receptor level and that TLR4 is indispensable for FcgammaRIII signaling via FcRgamma-subunit activation.

Macrophage NADPH oxidase flavocytochrome B localizes to the plasma membrane and Rab11-positive recycling endosomes

Flavocytochrome b(558), the catalytic core of the phagocytic NADPH oxidase, mediates the transfer of electrons from NADPH to molecular oxygen to generate superoxide for host defense. Flavocytochrome b is a membrane heterodimer consisting of a large subunit gp91(phox) (NOX2) and a smaller subunit, p22(phox). Although in neutrophils flavocytochrome b has been shown to localize to the plasma membrane and specific granules, little is known about its distribution in macrophages. Using immunofluorescent staining and live cell imaging of fluorescently tagged gp91(phox) and p22(phox), we demonstrate in a Chinese hamster ovary cell model system and in RAW 264.7 and primary murine bone marrow-derived macrophages that flavocytochrome b is found in the Rab11-positive recycling endocytic compartment, as well as in Rab5-positive early endosomes and plasma membrane. Additionally, we show that unassembled p22(phox) and gp91(phox) subunits localize to the endoplasmic reticulum, which redistribute to the cell surface and endosomal compartments following heterodimer formation. These studies show for the first time that flavocytochrome b localizes to intracellular compartments in macrophages that recycle to the plasma membrane, which may act as a reservoir to deliver flavocytochrome b to the cell surface and phagosome membranes.

Selective expression of inhibitory Fcgamma receptor by metastatic melanoma impairs tumor susceptibility to IgG-dependent cellular response

During melanoma progression, patients develop anti-tumor immunity including the production of anti-tumor antibodies. Although the strategies developed by malignant cells to escape anti-tumor cellular immunity have been extensively investigated, little is known about tumor resistance to humoral immunity. The main effect of IgG antibodies is to activate the immune response by binding to host Fc gamma receptors (FcgammaR) expressed by immune cells. We previously reported in a limited study that some human metastatic melanoma cells ectopically express the FcgammaRIIB1, an inhibitory isoform of FcgammaR. By analyzing a large panel of different types of human primary and metastatic solid tumors, we report herein that expression of FcgammaRIIB is restricted to melanoma and is acquired during tumor progression. We show that FcgammaRIIB expression prevents the lysis of human metastatic melanoma cells by NK cell-mediated antibody-dependent cellular cytotoxicity (ADCC) in vitro, independently of the intracytoplasmic region of FcgammaRIIB. Using experimental mouse models, we demonstrate that expression of FcgammaRIIB protects B16F0 melanoma tumors from the ADCC induced by monoclonal and polyclonal anti-tumor IgG in vivo. Thus, our results identify FcgammaRIIB as a marker of human metastatic melanoma that impairs the tumor susceptibility to FcgammaR-dependent innate effector responses.

Negative signaling by inhibitory receptors: the NK cell paradigm

Receptors carrying immunoreceptor tyrosine-based inhibition motifs (ITIMs) in their cytoplasmic tail control a vast array of cellular responses, ranging from autoimmunity, allergy, phagocytosis of red blood cells, graft versus host disease, to even neuronal plasticity in the brain. The inhibitory function of many receptors has been deduced on the basis of cytoplasmic ITIM sequences. Tight regulation of natural killer (NK) cell cytotoxicity and cytokine production by inhibitory receptors specific for major histocompatibility complex class I molecules has served as a model system to study the negative signaling pathway triggered by an ITIM-containing receptor in the physiological context of NK-target cell interactions. Advances in our understanding of the molecular details of inhibitory signaling in NK cells have provided a conceptual framework to address how ITIM-mediated regulation controls cellular reactivity in diverse cell types.

Regulation of hematopoietic cell function by inhibitory immunoglobulin G receptors and their inositol lipid phosphatase effectors

Numerous autoimmune and inflammatory disorders stem from the dysregulation of hematopoietic cell activation. The activity of inositol lipid and protein tyrosine phosphatases, and the receptors that recruit them, is critical for prevention of these disorders. Balanced signaling by inhibitory and activating receptors is now recognized to be an important factor in tuning cell function and inflammatory potential. In this review, we provide an overview of current knowledge of membrane proximal events in signaling by inhibitory/regulatory receptors focusing on structural and functional characteristics of receptors and their effectors Src homology 2 (SH2) domain-containing tyrosine phosphatase 1 and SH2 domain-containing inositol 5-phosphatase-1. We review use of new strategies to identify novel regulatory receptors and effectors. Finally, we discuss complementary actions of paired inhibitory and activating receptors, using Fc gammaRIIA and Fc gammaRIIB regulation human basophil activation as a prototype.

Recruitment of Rab27a to phagosomes controls microbial antigen cross-presentation by dendritic cells

Polyreactive immunoglobulins (Ig) and complement components are present in tissues and blood of healthy individuals. They facilitate pathogen uptake and inactivation in lysosomes of phagocytes and thereby provide rapid protection against infection. Dendritic cells (DCs) are phagocytes that can acquire peptides from phagocytosed antigen to elicit cytotoxic immune responses by CD8(+) T lymphocytes. The mechanisms that select peptides for cross-presentation are not fully resolved. Here we investigated the role of polyreactive Ig and complement in directing phagosomal antigen processing for cross-presentation. Phagocytosis facilitated by serum opsonization required the presence of Ig for effective antigen cross-presentation of microbe-derived antigen. The presence of complement C3 in serum promoted phagocytosis, yet phagosomes were defective in antigen degradation. The small GTPase Rab27a was recently implicated in antigen cross-presentation and was rapidly recruited to phagosomes only when Ig was present. Our data suggest that prebinding of antigen by polyreactive Ig potentiates the efficiency of antigen cross-presentation to CD8(+) T cells through recruitment of Rab27a.

A novel subset of NK cells expressing high levels of inhibitory FcgammaRIIB modulating antibody-dependent function

NK cells can kill antibody-coated target cells following engagement of FcgammaRIIIA, the major activating FcgammaR expressed by these cells. The presence of FcgammaRIIC (CD32C) has also been reported, but its contribution to the FcgammaR-dependent effector functions of NK cells remains debated. We demonstrate here that inhibitory FcgammaRIIB is also expressed by a small subset of CD56+/NKp46+ NK cells and can efficiently down-modulate their FcgammaR-dependent effector function. Immunofluorescence analyses of NK cells from 52 healthy donors showed the presence of CD56bright/FcgammaRII(-) (5.2%+/-3.4), CD56dim/FcgammaRII(lo/-) (94.1%+/-3.4), and CD56dim/FcgammaRIIbright (0.64%+/-0.72) cells. QRT-PCR and protein analyses performed on isolated FcgammaRIIbright NK cells indicated that FcgammaRIIB is strongly expressed by these cells but not by FcgammaRII(lo/-) cells. In addition, FcgammaRIIbright cells showed a weaker antibody-dependent degranulation when incubated with IgG-coated target cells compared with FcgammaRII(lo/-) NK cells, although a strong FcgammaRIIIA expression was detected in both cells. Furthermore, the addition of anti-FcgammaRII Fab paralleled a higher degranulation of FcgammaRIIbright NK cells, indicating a direct role for FcgammaRIIB in this down-modulating effect. Thus, it is proposed that FcgammaRIIBbright NK cells represent a new NK cell compartment able to down-modulate NK cell functions triggered by the engagement of activating FcgammaR.

Immune opsonins modulate BLyS/BAFF release in a receptor-specific fashion

TNF ligand superfamily member 13B (B lymphocyte stimulator (BLyS), B cell activating factor (BAFF)) promotes primary B cell proliferation and Ig production. While the soluble form of BLyS/BAFF is thought to be the primary biologically active form, little is known about the regulation of its cleavage and processing. We provide evidence that Fcgamma receptor cross-linking triggers a rapid release of soluble, biologically active BLyS/BAFF from myeloid cells. Surprisingly, this function is primarily mediated by FcgammaRI, but not FcgammaRIIa as defined by specific mAb, and can be initiated by both IgG and C reactive protein as ligands. The generation of a B cell proliferation and survival factor by both innate and adaptive immune opsonins through engagement of an Fcgamma receptor, which can also enhance Ag uptake and presentation, provides a unique opportunity to facilitate Ab production. These results provide a mechanism by which Fcgamma receptors can elevate circulating BLyS levels and promote autoantibody production in immune complex-mediated autoimmune diseases.

Inhibition of B cell receptor-mediated activation of primary human B cells by coengagement of CD19 and FcgammaRIIb with Fc-engineered antibodies

The humoral immune response requires antigen-specific B cell activation and subsequent terminal differentiation into plasma cells. Engagement of B cell antigen receptor (BCR) on mature B cells activates an intracellular signaling cascade, including calcium mobilization, which leads to cell proliferation and differentiation. Coengagement by immune complex of BCR with the inhibitory Fc receptor FcgammaRIIb, the only IgG receptor expressed on B cells, inhibits B cell activation signals through a negative feedback loop. We now describe antibodies that mimic the inhibitory effects of immune complex by high-affinity coengagement of FcgammaRIIb and the BCR coreceptor complex on human B cells. We engineered the Fc domain of an anti-CD19 antibody to generate variants with up to approximately 430-fold greater affinity to FcgammaRIIb. Relative to native IgG1, the FcgammaRIIb binding-enhanced (IIbE) variants strongly inhibited BCR-induced calcium mobilization and viability in primary human B cells. Inhibitory effects involved phosphorylation of SH2-containing inositol polyphosphate 5-phosphatase (SHIP), which is known to be involved in FcgammaRIIb-induced negative feedback of B cell activation by immune complex. Coengagement of BCR and FcgammaRIIb by IIbE variants also overcame the anti-apoptotic effects of BCR activation. The use of a single antibody to suppress B cell functions by coengagement of BCR and FcgammaRIIb may represent a novel approach in the treatment of B cell-mediated autoimmune diseases.

Fc receptor genes and the systemic lupus erythematosus diathesis

Fc receptors represent a distinct group of hematopoeitic cell surface glycoproteins that have a characterized role in affecting the efficiency of the mononuclear phagocyte system to clear IgG immune complexes. Functional genetic variations in this family of receptors have been identified as heritable susceptibility factors for SLE and lupus nephritis across diverse populations. In this review, we describe the roles of the classical Fc receptors for IgG (Fc gamma) and non-classical Fc-like receptors (FCR1-FCRL6L), Fc receptors for IgE (Fc epsilon RI) and IgA and IgM (Fc alpha/mu R) in SLE diathesis. The combined effects of these genes on SLE pathogenesis, either via linkage disequilibrium or epistasis with additional genetic or environmental factors, provide a challenge for future investigations. The pursuit of a polygenic SLE-profile that includes longitudinal evaluations of SLE and markers involved in the protean clinical manifestations associated with SLE will facilitate our understanding of the cascade of inflammatory events associated with the diathesis.

Fcgamma receptors as regulators of immune responses

In addition to their role in binding antigen, antibodies can regulate immune responses through interacting with Fc receptors (FcRs). In recent years, significant progress has been made in understanding the mechanisms that regulate the activity of IgG antibodies in vivo. In this Review, we discuss recent studies addressing the multifaceted roles of FcRs for IgG (FcgammaRs) in the immune system and how this knowledge could be translated into novel therapeutic strategies to treat human autoimmune, infectious or malignant diseases.

Receptor-mediated endocytosis of immune complexes in rat liver sinusoidal endothelial cells is mediated by FcgammaRIIb2

Liver sinusoidal endothelial cells (LSECs) display a number of receptors for efficient uptake of potentially injurious molecules. The receptors for the Fc portion of immunoglobulin G (IgG) antibodies (FcgammaRs) regulate a number of physiological and pathophysiological events. We used reverse transcription polymerase chain reaction (RT-PCR) and Western blotting to determine the expression of different types of FcgammaRs in LSECs. Biochemical approaches and immunofluorescence microscopy were used to characterize the FcgammaR-mediated endocytosis of immune complexes (ICs). FcgammaRIIb2 was identified as the main receptor for the efficient uptake of ICs in LSECs. The receptor was shown to use the clathrin pathway for IC uptake; however, the association with lipid rafts may slow the rate of its internalization. Moreover, despite trafficking through lysosomal integral membrane protein-II (LIMP-II)-containing compartments, the receptor was not degraded. Finally, it was shown that the receptor recycles to the cell surface both with and without IC.

CONCLUSION

FcgammaRIIb2 is the main receptor for endocytosis of ICs in rat LSECs. Internalized ICs are degraded with slow kinetics, and IC internalization is not linked to receptor downregulation. After internalization, the receptor recycles to the cell surface both with and without ICs. Thus, FcgammaRIIb2 in rat LSECs is used as both a recycling receptor and a receptor for efficient IC clearance.

IgE Enhances Specific Antibody and T-cell Responses in Mice Overexpressing CD23

IgE administered with its specific antigen in vivo induces enhanced proliferation of specific T cells as well as enhanced production of specific antibodies. Both effects are dependent on the low-affinity receptor for IgE (CD23) and the underlying mechanism is thought to be increased antigen presentation following uptake of IgE/antigen complexes via CD23(+) B cells. By contrast, CD23 negatively regulates antibody responses to antigens administered with alum, i.e. without IgE. This effect has been observed as low IgG1 and IgE responses in transgenic mice overexpressing CD23 (CD23Tg). The present study was designed to test whether IgE could enhance antibody and T-cell responses in CD23Tg animals or whether CD23's downregulatory effect precludes IgE-mediated enhancement. IgE-anti-TNP administered with OVA-TNP enhances the OVA-specific antibody responses in wild-type (wt) and CD23Tg mice equally well. Interestingly, the total magnitude of antibody responses to IgE + OVA-TNP and to uncomplexed OVA-TNP, as well as to sheep erythrocytes and keyhole limpet haemocyanine, were lower in the CD23Tg mice. IgE induced proliferation of OVA-specific CD4(+) T cells to the same degree in wt and CD23Tg mice. The effect on T cells was dependent on CD23(+) B cells as demonstrated in in vitro proliferation assays. In conclusion, CD23 does indeed have dual immunoregulatory effects in the same animal. The receptor mediates enhancement of antibody and T-cell responses to IgE-complexed antigen, most likely via increased presentation of complexed antigen, while it negatively regulates the total antibody response to a variety of antigens.

Fc gamma receptor IIB on dendritic cells enforces peripheral tolerance by inhibiting effector T cell responses

The uptake of immune complexes by FcRs on APCs augments humoral and cellular responses to exogenous Ag. In this study, CD11c+ dendritic cells are shown to be responsible in vivo for immune complex-triggered priming of T cells. We examine the consequence of Ab-mediated uptake of self Ag by dendritic cells in the rat insulin promoter-membrane OVA model and identify a role for the inhibitory FcgammaRIIB in the maintenance of peripheral CD8 T cell tolerance. Effector differentiation of diabetogenic OT-I CD8+ T cells is enhanced in rat insulin promoter-membrane OVA mice lacking FcgammaRIIB, resulting in a high incidence of diabetes. FcgammaRIIB-mediated inhibition of CD8 T cell priming results from suppression of both DC activation and cross-presentation through activating FcgammaRs. Further FcgammaRIIB on DCs inhibited the induction of OVA-specific Th1 effectors, limiting Th1-type differentiation and memory T cell accumulation. In these MHC II-restricted responses, the presence of FcgammaRIIB only modestly affected initial CD4 T cell proliferative responses, suggesting that FcgammaRIIB limited effector cell differentiation primarily by inhibiting DC activation. Thus, FcgammaRIIB can contribute to peripheral tolerance maintenance by inhibiting DC activation alone or by also limiting processing of exogenously acquired Ag.

Expression profile of FcgammaRIIb on leukocytes and its dysregulation in systemic lupus erythematosus

FcgammaRIIb (CD32B, Online Mendelian Inheritance in Man 604590), an IgG FcR with a tyrosine-based inhibitory motif, plays a critical role in the balance of tolerance and autoimmunity in murine models. However, the high degree of homology between FcgammaRIIb and FcgammaRIIa in humans and the lack of specific Abs to differentiate them have hampered study of the normal expression profile of FcgammaRIIb and its potential dysregulation in autoimmune diseases such as systemic lupus erythematosus (SLE). Using our newly developed anti-FcgammaRIIb mAb 4F5 which does not react with FcgammaRIIa, we found that FcgammaRIIb is expressed on the cell surface of circulating B lymphocytes, monocytes, neutrophils, myeloid dendritic cells (DCs), and at very low levels on plasmacytoid DCs from some donors. Normal donors with the less frequent 2B.4 promoter haplotype have higher FcgammaRIIb expression on monocytes, neutrophils, and myeloid DCs similar to that reported for B lymphocytes, indicating that FcgammaRIIb expression on both myeloid and lymphoid cells is regulated by the naturally occurring regulatory single nucleotide polymorphisms in the FCGR2B promoter. FcgammaRIIb expression in normal controls is up-regulated on memory B lymphocytes compared with naive B lymphocytes. In contrast, in active SLE, FcgammaRIIb is significantly down-regulated on both memory and plasma B lymphocytes compared with naive and memory/plasma B lymphocytes from normals. Similar down-regulation of FcgammaRIIb on myeloid-lineage cells in SLE was not seen. Our studies demonstrate the constitutive regulation of FcgammaRIIb by natural gene polymorphisms and the acquired dysregulation in SLE autoimmunity, which may identify opportunities for using this receptor as a therapeutic target.