Immunoglobulin G-mediated inflammatory responses develop normally in complement-deficient mice

Efficacy of opsonic and nonopsonic serotype 3 pneumococcal capsular polysaccharide-specific monoclonal antibodies against intranasal challenge with Streptococcus pneumoniae in mice

Serotype-specific antibodies to pneumococcal capsular polysaccharide (PPS) are a critical component of vaccine-mediated immunity to Streptococcus pneumoniae. In this study, we investigated the in vitro opsonophagocytic activities of three PPS-specific mouse immunoglobulin G1 monoclonal antibodies (MAbs), 1E2, 5F6, and 7A9, and determined their in vivo efficacies against intranasal challenge with WU2, a serotype 3 pneumococcal strain, in normal and immunodeficient mice. The MAbs had different in vitro activities in a pneumococcal killing assay: 7A9 enhanced killing by mouse neutrophils and J774 cells in the presence of a complement source, whereas 5F6 promoted killing in the absence, but not the presence, of complement, and 1E2 did not promote killing under any conditions. Nonetheless, all three MAbs protected normal and complement component 3-deficient mice from a lethal intranasal challenge with WU2 in passive-immunization experiments in which 10 mug of the MAbs were administered intraperitoneally before intranasal challenge. In contrast, only 1E2 protected Fcgamma receptor IIB knockout (FcgammaRIIB KO) mice and mice that were depleted of neutrophils with the MAb RB6, whereas 7A9 and 5F6 required neutrophils and FcgammaRIIB to mediate protection. Conversely, 7A9 and 5F6 protected FcgammaR KO mice, but 1E2 did not. Hence, the efficacy of 1E2 required an activating FcgammaR(s), whereas 5F6 and 7A9 required the inhibitory FcgammaR (FcgammaRIIB). Taken together, our data demonstrate that both MAbs that do and do not promote pneumococcal killing in vitro can mediate protection in vivo, although their efficacies depend on different host receptors and/or components.

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.

Fc-receptors as regulators of immunity

Receptors for immunoglobulins [Fc-receptors (FcRs)] are widely expressed throughout the immune system. By binding to the antibody Fc-portion, they provide a link between the specificity of the adaptive immune system and the powerful effector functions triggered by innate immune effector cells. By virtue of coexpression of activating and inhibitory FcRs on the same cell, they set a threshold for immune cell activation by immune complexes (ICs). Besides their involvement in the efferent phase of an immune response, they are also important for modulating adaptive immune responses by regulating B cell and dendritic cell (DC) activation. Deletion of the inhibitory FcR leads to the loss of tolerance in the humoral immune system and the development of autoimmune disease. Uptake of ICs by FcRs on DCs and the concommitant triggering of activating and inhibitory signaling pathways will determine the strength of the initiated T-cell response. Loss of this balanced signaling results in uncontrolled responses that can lead to the damage of healthy tissues and ultimately to the initiation of autoimmune processes. In this chapter, we will discuss how coexpression of different activating and inhibitory receptors on different immune cells of the innate and adaptive immune system modulates cell activity. Moreover, we will focus on exogenous factors that can influence the balanced triggering of activating and inhibitory FcRs, such as the cytokine milieu and the role of differential antibody glycosylation.

Extravasations and emigration of neutrophils to the inflammatory site depend on the interaction of immune-complex with Fcgamma receptors and can be effectively blocked by decoy Fcgamma receptors

Extravasation and emigration of neutrophils to the site of inflammation are essential early steps in the initiation of many antibody-mediated autoimmune diseases. The Fc domains of cell bound autoantibodies or immune-complexes (IC) are capable of triggering the neutrophil emigration via complement and FcgammaRs-mediated mechanisms. To define the clinical relevance and the relative contribution of these 2 pathways in IC-mediated neutrophil emigration, we have neutralized the FcgammaR-binding activity of IC with a recombinant dimeric Fc receptor, CD16A-Ig, and investigated the early events of IC-induced inflammation in mice. Systemic administration of purified CD16A-Ig blocked IC-induced inflammation, mast- cell degranulation, and extravasation of neutrophils in a reversed Arthus reaction. Although the binding of CD16A-Ig to IC did not alter the complement-activating properties of IC, no evidence for complement-dependent neutrophil emigration was observed. These results suggest that interaction of IC with cells expressing FcgammaRs at the inflammatory site results in the secretion of chemoattractants, which mediate complement-independent emigration of neutrophils in this cutaneous acute inflammation model. Furthermore, blocking the interaction of IC to FcgammaRs expressed on inflammatory cells by administering high-avidity Fc fusion dimers of low-affinity FcgammaRs is an effective way of preventing IC-induced acute inflammation in autoimmune diseases.

Superoxide anion production by neutrophils is associated with prevalent clinical manifestations in systemic lupus erythematosus

To determine the relation between neutrophil function and the clinical characteristics of systemic lupus erythematosus (SLE), the superoxide anion (O2-) production by neutrophils, mediated by FcgammaR and FcgammaR/CR cooperation, was studied in 64 SLE patients classified according to their prevalent clinical manifestations. Three clinically distinct patterns were designated: (1) manifestations associated with the occurrence of cytotoxic antibodies (SLE-I group); (2) manifestations associated with circulating immune complexes (IC; SLE-II group), and (3) manifestations associated with IC and cytotoxic antibodies (SLE-III group). O2- production was evaluated by a lucigenin-dependent chemiluminescent assay in neutrophils stimulated with IC-IgG opsonized or not with complement. No difference in O2- production was observed when neutrophil responses from healthy controls were compared to the unclassified patients. However, when the SLE patient groups were considered, the following differences were observed: (1) SLE-I neutrophils showed lower O2- production mediated by the IgG receptor (FcgammaR) with the cooperation of complement receptors (FcgammaR/CR) than observed in the SLE-II, SLE-III, and healthy groups; (2) neutrophils from the SLE-II group showed a decreased [Formula: see text] production mediated by FcgammaR/CR compared to the SLE-III group, (3) SLE-III neutrophils produced more O(2)(-) than neutrophils from the SLE-II and control groups, and (4) CR showed inefficiency in mediating the O2- production by neutrophils from the SLE-I group. Comparative experiments on the kinetics of chemiluminescence (CL; Tmax and CLmax) disclosed differences only for the SLE-I group. Taken together, these results suggest that differences in oxidative metabolism of neutrophils mediated by FcgammaR/CR may reflect an acquired characteristic of disease associated with distinct clinical manifestations.

Complement in glomerular injury

In recent years, research into the role of complement in the immunopathogenesis of renal disease has broadened our understanding of the fragile balance between the protective and harmful functions of the complement system. Interventions into the complement system in various models of immune-mediated renal disease have resulted in both favourable and unfavourable effects and will allow us to precisely define the level of the complement cascade at which a therapeutic intervention will result in an optimal effect. The discovery of mutations of complement regulatory molecules has established a role of complement in the haemolytic uremic syndrome and membranoproliferative glomerulonephritis, and genotyping for mutations of the complement system are already leaving the research laboratory and have entered clinical practice. These clinical discoveries have resulted in the creation of relevant animal models which may provide crucial information for the development of highly specific therapeutic agents. Research into the role of complement in proteinuria has helped to understand pathways of inflammation which ultimately lead to renal failure irrespective of the underlying renal disease and is of major importance for the majority of renal patients. Complement science is a highly exciting area of translational research and hopefully will result in meaningful therapeutic advances in the near future.

Experimental anti-GBM disease as a tool for studying spontaneous lupus nephritis

Lupus nephritis is an immune-mediated disease, where antibodies and T cells both play pathogenic roles. Since spontaneous lupus nephritis in mouse models takes 6-12 months to manifest, there is an urgent need for a mouse model that can be used to delineate the pathogenic processes that lead to immune nephritis, over a quicker time frame. We propose that the experimental anti-glomerular basement membrane (GBM) disease model might be a suitable tool for uncovering some of the molecular steps underlying lupus nephritis. This article reviews the current evidence that supports the use of the experimental anti-GBM nephritis model for studying spontaneous lupus nephritis. Importantly, out of about 25 different molecules that have been specifically examined in the experimental anti-GBM model and also spontaneous lupus nephritis, all influence both diseases concordantly, suggesting that the experimental model might be a useful tool for unraveling the molecular basis of spontaneous lupus nephritis. This has important clinical implications, both from the perspective of genetic susceptibility as well as clinical therapeutics.

Inflammation and bone erosion are suppressed in models of rheumatoid arthritis following treatment with a novel Syk inhibitor

Spleen tyrosine kinase (Syk), a key mediator of immunoreceptor signaling in inflammatory cells, is essential for immune complex-mediated signal transduction initiated by activated receptors for immunoglobulin G. In collagen-induced arthritis, R788/R406, a novel and potent small molecule Syk inhibitor suppressed clinical arthritis, bone erosions, pannus formation, and synovitis. Serum anti-collagen type II antibody levels were unaltered, while the half-life of exogenous antibody was extended when co-administered with R406. Expression of the targeted kinase (Syk) in synovial tissue correlated with the joint level of inflammatory cell infiltrates and was virtually undetectable in treated rats. Syk inhibition suppressed synovial cytokines and cartilage oligomeric matrix protein (COMP) in serum, suggesting a sensitive and reliable biomarker for R406 activity. These results highlight the role of activating Fcgamma receptors in inflammatory synovitis and suggest that interruption of the signaling cascade with a novel Syk inhibitor may be a useful addition to immunosuppressive disease-modifying anti-rheumatic drugs currently used in the treatment of human autoimmune diseases such as rheumatoid arthritis.

Antibodies, Fc receptors and cancer

Since the first report of the successful use of a monoclonal antibody for the treatment of human B cell lymphoma in 1982, several antibodies have become incorporated into standard treatment protocols for cancer. One of the most important factors that determine antibody activity in vivo is the efficient interaction with cellular Fc-receptors on innate immune effector cells. It has become clear that the outcome of this interaction is influenced by several factors, such as the antibody isotype-specific affinity to activating and inhibitory receptors, the level of inhibitory FcgammaRIIB expression, and the composition of the sugar side chain attached to the antibody Fc-portion. These novel insights into antibody FcR interactions might be useful to produce the next generation of improved immunotherapeutic molecules.

Systemic rheumatoid vasculitis: a review

OBJECTIVES

To review the most recent information on the incidence, clinical course, pathology, pathogenesis, diagnosis, and treatment of rheumatoid vasculitis (RV), including the still scanty data on the use of biologics.

METHODS

PubMed and MEDLINE databases (1950-2006) were searched for the key words "vasculitis" and "rheumatoid arthritis"; and "rheumatoid arthritis" and "extra-articular manifestations." All relevant articles in English and French were reviewed. Additional words used in follow-up research include "anti-TNF," "rituximab," "IL-1 receptor antagonists," and "CTLA-4 Ig," all in conjunction with "vasculitis." Pertinent secondary references were also retrieved.

RESULTS

RV is an inflammatory condition of the small- and medium-sized vessels that affects a subset of patients with established rheumatoid arthritis (RA) (approximately 1 to approximately 5%). It has a vast array of clinical manifestations with a predilection for the skin (peripheral gangrene, deep cutaneous ulcers) and the peripheral nervous system (mononeuritis multiplex). Because of the lack of specific signs and symptoms, the diagnosis relies on the exclusion of other causes of similar lesions (diabetes, atherosclerosis, drug reactions, infection, neoplasias) and, ideally, on the histopathological demonstration of necrotizing vasculitis. Despite the availability of a host of promising new drugs for the treatment of RA, no clinical trials have tested their efficacy in RV; therefore, its management remains largely empirical.

CONCLUSIONS

Although RV has apparently been decreasing over the last 2 decades, possibly as a consequence of the more energetic approach to the management of RA currently used, it remains an important complication of RA that needs to be promptly recognized and treated.

The antiinflammatory activity of IgG: the intravenous IgG paradox

How high doses of intravenous IgG (IVIG) suppress autoimmune diseases remains unresolved. We have recently shown that the antiinflammatory activity of IVIG can be attributed to a minor species of IgGs that is modified with terminal sialic acids on their Fc-linked glycans. Here we propose that these Fc-sialylated IgGs engage a unique receptor on macrophages that, in turn, leads to the upregulation of an inhibitory Fcgamma receptor (FcgammaR), thereby protecting against autoantibody-mediated pathology.

FcR-bearing myeloid cells are responsible for triggering murine lupus nephritis

Lupus glomerulonephritis is initiated by deposition of IgG-containing immune complexes in renal glomeruli. FcR engagement by immune complexes (IC) is crucial to disease development as uncoupling this pathway in FcRgamma(-/-) abrogates inflammatory responses in (NZB x NZW)F1 mice. To define the roles of FcR-bearing hemopoietic cells and of kidney resident mesangial cells in pathogenesis, (NZB x NZW)F1 bone marrow chimeras were generated. Nephritis developed in (NZB x NZW)F1 mice expressing activating FcRs in hemopoietic cells. Conversely, recipients of FcRgamma(-/-) bone marrow were protected from disease development despite persistent expression of FcRgamma in mesangial cell populations. Thus, activating FcRs on circulating hemopoietic cells, rather than on mesangial cells, are required for IC-mediated pathogenesis in (NZB x NZW)F1. Transgenic FcRgamma(-/-) mice expressing FcRgamma limited to the CD11b+ monocyte/macrophage compartment developed glomerulonephritis in the anti-glomerular basement disease model, whereas nontransgenic FcRgamma(-/-) mice were completely protected. Thus, direct activation of circulating FcR-bearing myeloid cells, including monocytes/macrophages, by glomerular IC deposits is sufficient to initiate inflammatory responses.

Activating and inhibitory FcgammaRs in autoimmune disorders

Autoimmune disorders are characterized by the destruction of self-tissues by the immune system. Multiple checkpoints are in place to prevent autoreactivity under normal circumstances. Coexpression of activating and inhibitory Fc receptors (FcR) represents such a checkpoint by establishing a threshold for immune cell activation. In many human autoimmune diseases, however, balanced FcR expression is disturbed. Analysis of murine model systems provides strong evidence that aberrant FcR expression can result in uncontrolled immune responses and the initiation of autoimmune disease. This review will summarize this data and explain how this information might be used to better understand human autoimmune diseases and to develop novel therapeutic strategies.

Fcgamma receptor-dependent effector mechanisms regulate CD19 and CD20 antibody immunotherapies for B lymphocyte malignancies and autoimmunity

Immunotherapy using Rituximab, an unconjugated CD20 monoclonal antibody, has proven effective for treating non-Hodgkin's lymphoma and autoimmune disease. CD19 antibody immunotherapy is also effective in mouse models of lymphoma and autoimmunity. In both cases, mouse models have demonstrated that effector cell networks effectively deplete the vast majority of circulating and tissue B lymphocytes through Fcgamma receptor-dependent pathways. In mice, B cell depletion is predominantly, if not exclusively, mediated by monocytes. CD20 mAbs rapidly deplete circulating and tissue B cells in an antibody isotype-restricted manner with a hierarchy of antibody effectiveness: IgG2a/c > IgG1 > IgG2b >> IgG3. Depending on antibody isotype, mouse B cell depletion is regulated by FcgammaRI-, FcgammaRII-, FcgammaRIII-, and FcgammaRIV-dependent pathways. The potency of IgG2a/c mAbs for B cell depletion in vivo results from FcgammaRIV interactions, with likely contributions from high-affinity FcgammaRI. IgG1 mAbs induce B cell depletion through preferential, if not exclusive, interactions with low-affinity FcgammaRIII, while IgG2b mAbs interact preferentially with intermediate-affinity FcgammaRIV. By contrast, inhibitory FcgammaRIIB-deficiency significantly increases CD20 mAb-induced B cell depletion at low mAb doses by enhancing monocyte function. Thus, isotype-specific mAb interactions with distinct FcgammaRs contribute significantly to the effectiveness of CD20 mAbs in vivo. These results provide a molecular basis for earlier observations that human FcgammaRII and FcgammaRIII polymorphisms correlate with the in vivo effectiveness of CD20 antibody therapy. That the innate monocyte network depletes B cells through FcgammaR-dependent pathways during immunotherapy has important clinical implications for CD19, CD20, and other antibody-based therapies for the treatment of diverse B cell malignancies and autoimmune disease.

Multiple Innate Inflammatory Responses Induced after Systemic Adenovirus Vector Delivery Depend on a Functional Complement System

Excessive complement activation can result in extreme tissue damage and systemic inflammatory responses, similar to innate immune responses rapidly elicited after systemic adenovirus (Ad) injections. To determine if Ad interactions with the complement system impact upon Ad-induced innate immune responses, we injected Ad into complement-deficient, C3-knockout mice (C3-KO) or wild-type mice (WT) and quantitatively compared multiple anti-Ad innate immune responses in both strains of mice. In Ad-treated WT mice, we noted rapid increases in plasma KC levels (1 h post injection), followed by increases in IL-6, IFN-gamma, RANTES, IL-12(p40), IL-5, G-CSF, and GM-CSF and subsequently thrombocytopenia. Conversely, in Ad-treated C3-KO mice, many of these inflammatory responses were significantly blunted, including the avoidance of Ad-induced thrombocytopenia. Global liver transcriptome responses in Ad-treated WT mice were assessed by RT-PCR-validated gene array analysis and were found to be also significantly affected by the lack of complement activity in Ad-treated C3-KO mice. Finally, our results confirmed the ability of high dose Ads to transduce hepatocytes despite a lack of complement activity. In summary, Ad interactions with the mammalian complement system are significant and likely initiate and/or exacerbate many of the inflammatory responses noted after systemic Ad injections.

Molecular and cellular basis for pathogenicity of autoantibodies: lessons from murine monoclonal autoantibodies

The pathogenesis of autoantibody-mediated cellular and tissue lesions in autoimmune diseases is most straightforwardly attributable to the combined action of self-antigen binding properties and effector functions associated with the Fc regions of the different immunoglobulin (Ig) isotypes. The analysis of two different sets of monoclonal autoantibodies derived from lupus-prone mice revealed remarkable differences in the pathogenic potentials of different IgG subclasses: (1) the IgG2a and IgG2b subclasses of anti-red blood cell (RBC) autoantibodies are the most pathogenic and efficiently activate two classes of activating IgG Fc receptors (FcgammaRIII and FcgammaRIV) and complement; (2) the IgG3 subclass is less pathogenic and activate only complement; and (3) the IgG1 subclass is the least pathogenic and interact only with FcgammaRIII. In addition, because of the unique property of IgG3 to form self-associating complexes and generate cryoglobulins, this subclass of rheumatoid factor and anti-DNA autoantibodies became highly pathogenic and induced lupus-like nephritis and/or vasculitis. Since the switch to IgG2a and IgG3 is promoted by Th1 cytokine interferon gamma, these results strongly suggest that Th1 autoimmune responses could be critically involved in the generation of more pathogenic autoantibodies in systemic lupus erythematosus. This finding is consistent with the observation that the progression of murine lupus nephritis is correlated with the relative dominance of Th1 autoimmune responses. Finally, the analysis of IgG glycosylation pattern revealed that more sialylated IgG autoantibodies remained poorly pathogenic because of limited Fc-associated effector functions and loss of cryoglobulin activity. This suggests that the terminal sialylation of the oligosaccharide side chains of IgG could be a significant factor determining the pathogenic potential of autoantibodies. Our results thus underline the importance of subpopulations of autoantibodies, induced by the help of Th1 cells, in the pathogenesis of autoantibody-mediated cellular and tissue injuries.

Generation of C5a in the absence of C3: a new complement activation pathway

Complement-mediated tissue injury in humans occurs upon deposition of immune complexes, such as in autoimmune diseases and acute respiratory distress syndrome. Acute lung inflammatory injury in wild-type and C3-/- mice after deposition of IgG immune complexes was of equivalent intensity and was C5a dependent, but injury was greatly attenuated in Hc-/- mice (Hc encodes C5). Injury in lungs of C3-/- mice and C5a levels in bronchoalveolar lavage (BAL) fluids from these mice were greatly reduced in the presence of antithrombin III (ATIII) or hirudin but were not reduced in similarly treated C3+/+ mice. Plasma from C3-/- mice contained threefold higher levels of thrombin activity compared to plasma from C3+/+ mice. There were higher levels of F2 mRNA (encoding prothrombin) as well as prothrombin and thrombin protein in liver of C3-/- mice compared to C3+/+ mice. A potent solid-phase C5 convertase was generated using plasma from either C3+/+ or C3-/- mice. Human C5 incubated with thrombin generated C5a that was biologically active. These data suggest that, in the genetic absence of C3, thrombin substitutes for the C3-dependent C5 convertase. This linkage between the complement and coagulation pathways may represent a new pathway of complement activation.

Genetic evidence for the involvement of Fcγ receptor III in experimental autoimmune myasthenia gravis pathogenesis

Immune complexes and classical complement pathway play vital roles in experimental autoimmune myasthenia gravis (EAMG). To analyze the role of immune complex receptors in EAMG, FcgammaRIII knockout (KO) mice were immunized with AChR and were found out to be resistant to EAMG induction. This was associated with reduced neuromuscular junction deposits, lymph node cell (LNC) IL-6 production and serum complement levels. EAMG resistance of anti-C1q Ab-administered mice was also associated with reduced LNC IL-6 production and neuromuscular junction deposits, indicating C1q involvement in EAMG resistance. The data provide the first direct genetic evidence for Fcgamma receptor involvement in EAMG pathogenesis.

Divergent immunoglobulin g subclass activity through selective Fc receptor binding

Subclasses of immunoglobulin G (IgG) display substantial differences in their ability to mediate effector responses, contributing to variable activity of antibodies against microbes and tumors. We demonstrate that the mechanism underlying this long-standing observation of subclass dominance in function is provided by the differential affinities of IgG subclasses for specific activating IgG Fc receptors compared with their affinities for the inhibitory IgG Fc receptor. The significant differences in the ratios of activating-to-inhibitory receptor binding predicted the in vivo activity. We suggest that these highly predictable functions assigned by Fc binding will be an important consideration in the design of therapeutic antibodies and vaccines.

Fc receptors and their interaction with complement in autoimmunity

Genetic studies in mice indicate a crucial role for Fc receptors (FcR) in antibody-mediated autoimmune diseases. Like other immune regulatory receptor pairs, the FcR system is constituted by activating and inhibitory receptors that bind the same ligand, the Fc portion of Ig. Analyses of animal models have shown that the inhibitory Fc receptor, FcgammaRIIB can suppress antibody-mediated autoimmunity, whereas activating-type FcR, such as FcgammaRIII promote disease development. This review summarizes recent advances of FcR, as obtained from gene deletion studies in mice, and highlights the importance of factors that interact with FcR in autoimmunity. There is emerging evidence for an indispensable role of the complement component C5a in the regulation of FcR and the sensing of FcR-dependent effector cell responses. On the other hand, FcR might be alternatives to serum complement in the generation of C5a at sites of inflammation. Thus, FcR and complement interact with each other at the level of C5a by linking regulatory events with effector cell activities in autoimmunity. This connecting pathway is now proposed to be a promising new therapeutic target for the treatment of inflammation and autoimmune disease in both mice and humans.

A novel human CD32 mAb blocks experimental immune haemolytic anaemia in FcgammaRIIA transgenic mice

A fully human IgG1 kappa antibody (MDE-8) was generated, which recognised Fc-gamma receptor IIa (FcgammaRIIa) molecules on CD32 transfectants, peripheral blood monocytes, polymorphonuclear cells and platelets. This antibody blocked FcgammaRIIa ligand-binding via its F(ab')(2) fragment. Overnight incubation of monocytes with F(ab')(2) fragments of MDE-8 leads to a c. 60% decrease in cell surface expression of FcgammaRIIa. MDE-8 whole antibody induced a concomitant c. 30% decrease of FcgammaRI on THP-1 cells and monocytes. In humans FcgammaRIIa plays an important role in the clearance of antibody-coated red blood cells in vivo. As an equivalent of FcgammaRIIa does not exist in mice, the in vivo effect of MDE-8 was studied in an FcgammaRIIa transgenic mouse model. In these mice, antibody-induced anaemia could readily be blocked by MDE-8. These data document a new human antibody that effectively blocks FcgammaRIIa, induces modulation of both FcgammaRIIa and FcgammaRI from phagocytic cells, and ameliorates antibody-induced anaemia in vivo.

FcγRIV: A Novel FcR with Distinct IgG Subclass Specificity

Mouse IgG subclasses display a hierarchy of in vivo activities, with IgG2a and IgG2b showing the greatest protective and pathogenic properties. These enhanced activities result, in part, from their ability to bind to a novel, gamma chain-dependent, activating IgG Fc receptor, FcgammaRIV. FcgammaRIV maps in the 75 kb genomic interval between FcgammaRII and FcgammaRIII; its expression is restricted to myeloid lineage cells, and it binds to IgG2a and IgG2b with intermediate affinity. No binding to IgG1 or IgG3 was observed. Blocking FcgammaRIV binding to pathogenic anti-platelet antibodies is sufficient to protect mice from antibody-induced thrombocytopenia. Thus, the FcgammaR system has evolved distinct activation receptors displaying selectivity for IgG subclasses, with IgG1 antibodies exclusively dependent on FcgammaRIII, whereas IgG2a and IgG2b show preferential dependence on FcgammaRIV activation. These distinct binding affinities for the IgG subclasses to FcgammaRs account for their differential protective and pathogenic activities in vivo.

Macrophages induce the inflammatory response in the pulmonary Arthus reaction through G alpha i2 activation that controls C5aR and Fc receptor cooperation

Complement and FcgammaR effector pathways are central triggers of immune inflammation; however, the exact mechanisms for their cooperation with effector cells and their nature remain elusive. In this study we show that in the lung Arthus reaction, the initial contact between immune complexes and alveolar macrophages (AM) results in plasma complement-independent C5a production that causes decreased levels of inhibitory FcgammaRIIB, increased levels of activating FcgammaRIII, and highly induced FcgammaR-mediated TNF-alpha and CXCR2 ligand production. Blockade of C5aR completely reversed such changes. Strikingly, studies of pertussis toxin inhibition show the essential role of G(i)-type G protein signaling in C5aR-mediated control of the regulatory FcgammaR system in vitro, and analysis of the various C5aR-, FcgammaR-, and G(i)-deficient mice verifies the importance of Galpha(i2)-associated C5aR and the FcgammaRIII-FcgammaRIIB receptor pair in lung inflammation in vivo. Moreover, adoptive transfer experiments of C5aR- and FcgammaRIII-positive cells into C5aR- and FcgammaRIII-deficient mice establish AM as responsible effector cells. AM lacking either C5aR or FcgammaRIII do not possess any such inducibility of immune complex disease, whereas reconstitution with FcgammaRIIB-negative AM results in an enhanced pathology. These data suggest that AM function as a cellular link of C5a production and C5aR activation that uses a Galpha(i2)-dependent signal for modulating the two opposing FcgammaR, FcgammaRIIB and FcgammaRIII, in the initiation of the inflammatory cascade in the lung Arthus reaction.

Immune complexes as therapy for autoimmunity

For several decades, intravenous Ig has been used as treatment for a variety of immune-related diseases, including immune thrombocytopenic purpura (ITP), autoimmune neuropathies, systemic lupus erythematosus, myasthenia gravis, Guillain-Barré syndrome, skin blistering syndromes, and Kawasaki disease. Despite years of use, its mechanism of immunomodulation is still unclear. Recent studies using mouse models of ITP and arthritis, including one reported in this issue of the JCI, now provide some insights into this mechanism and the rationale for the development of Fcgamma receptor-targeted therapeutics.

Blood and endothelium in immune complex-mediated tissue injury

Antigen-antibody complexes can be formed both intravascularly and perivascularly and damage tissues by inducing inflammatory mechanisms. Recent studies have characterized a definite sequence of steps involved in these inflammatory mechanisms, and identified the predominance of particular chemical mediator(s) in each step. The lesions associated with this type of inflammation are characterized by the early development of plasma leakage, followed by the recruitment of polymorphonuclear leukocytes mediated by chemokines generated by FcgammaR-dependent mechanisms. The development of these lesions is modulated by endothelial cell-derived paracrine mediators, and activation of the coagulation system can ensue. The activation of platelets and coagulation, if not properly counterbalanced by fibrinolysis, might be a major factor for the late development of fibrotic changes and organ remodeling.

C5a initiates the inflammatory cascade in immune complex peritonitis

Immune complex (IC)-induced inflammation is integral to the pathogenesis of several autoimmune diseases. ICs activate the complement system and interact with IgG FcgammaR. In this study, we demonstrate that activation of the complement system, specifically generation of C5a, initiates the neutrophilic inflammation in IC peritonitis. We show that ablation of C5a receptor signaling abrogates neutrophil recruitment in wild-type mice and prevents the enhancement of neutrophil migration seen in FcgammaRIIB(-/-) mice, suggesting that C5aR signaling is the crucial initial event upstream of FcgammaR signaling. We also provide evidence that C5a initiates the inflammatory cascade both directly, through C5aR-mediated effector functions on infiltrating and resident peritoneal cells, and indirectly, through shifting the balance between activating and inhibitory FcgammaRs on resident cells toward an inflammatory phenotype. We conclude that complement activation and C5a generation are prerequisites for IC-induced inflammation through activating FcgammaR, which amplifies complement-induced inflammation in autoimmunity.

The innate mononuclear phagocyte network depletes B lymphocytes through Fc receptor-dependent mechanisms during anti-CD20 antibody immunotherapy

Anti-CD20 antibody immunotherapy effectively treats non-Hodgkin's lymphoma and autoimmune disease. However, the cellular and molecular pathways for B cell depletion remain undefined because human mechanistic studies are limited. Proposed mechanisms include antibody-, effector cell-, and complement-dependent cytotoxicity, the disruption of CD20 signaling pathways, and the induction of apoptosis. To identify the mechanisms for B cell depletion in vivo, a new mouse model for anti-CD20 immunotherapy was developed using a panel of twelve mouse anti-mouse CD20 monoclonal antibodies representing all four immunoglobulin G isotypes. Anti-CD20 antibodies rapidly depleted the vast majority of circulating and tissue B cells in an isotype-restricted manner that was completely dependent on effector cell Fc receptor expression. B cell depletion used both FcgammaRI- and FcgammaRIII-dependent pathways, whereas B cells were not eliminated in FcR common gamma chain-deficient mice. Monocytes were the dominant effector cells for B cell depletion, with no demonstrable role for T or natural killer cells. Although most anti-CD20 antibodies activated complement in vitro, B cell depletion was completely effective in mice with genetic deficiencies in C3, C4, or C1q complement components. That the innate monocyte network depletes B cells through FcgammaR-dependent pathways during anti-CD20 immunotherapy has important clinical implications for anti-CD20 and other antibody-based therapies.

The dark side of C5a in sepsis

Sepsis is a major clinical problem for which therapeutic interventions have been largely unsuccessful, in spite of promising strategies that were successful in animals, especially rodents. There is new evidence that sepsis causes excessive activation of the complement system and that this induces paralysis of innate immune functions in phagocytic cells due to effects of the powerful complement-activation product, C5a. This review describes our present understanding of how and why sepsis is a life-threatening condition and how it might be more effectively treated.

Overexpression of complement inhibitor Crry does not prevent cryoglobulin-associated membranoproliferative glomerulonephritis

BACKGROUND

Mice overexpressing thymic stromal lymphopoietin (TSLP) develop mixed cryoglobulinemia with renal disease closely resembling human cryoglobulin-associated membranoproliferative glomerulonephritis (MPGN), including glomerular deposits of immunoglobulins and complement. We assessed the effect of complement inhibition through overexpression of Crry (complement receptor-1 related gene/protein Y), which blocks the classic and alternative pathway of complement activation through inhibition of the C3 convertase, in cryoglobulinemia-associated immune complex glomerulonephritis.

METHODS

TSLP transgenic mice were crossbred with animals overexpressing Crry. Mice were sacrificed after 50 days (females) or 120 days (males), and kidneys, blood, and urine were collected from seven mice of each experimental group (wild type, Crry transgenic, TSLP transgenic, and Crry/TSLP doubly transgenic).

RESULTS

TSLP/Crry doubly transgenic animals demonstrated expected serum levels of Crry. Renal involvement, both in TSLP transgenic and TSLP/Crry doubly transgenic animals, was characterized by glomerular matrix expansion, macrophage influx, activation of mesangial cells, and deposition of immunoglobulins and complement. Overexpression of Crry did not result in significant improvement of renal pathology or laboratory findings. Expression of recombinant soluble Crry was confirmed by enzyme-linked immunosorbent assay (ELISA) in Crry transgenic animals. However, formation of the membrane attack complex C5b-9 as a marker of terminal active complement components and represented by glomerular C9 staining could not be inhibited in Crry transgenic TSLP mice.

CONCLUSION

These results indicate that overexpression of Crry was not sufficient to prevent renal injury in TSLP transgenic mice. We suggest that the inhibitory capacity of Crry may be overwhelmed by chronic complement activation. Further studies need to address the role of complement in cryoglobulinemic glomerulonephritis before therapeutic complement inhibition can be attempted.

The role of complement in the modulation by fluid-phase IgG of the production of reactive oxygen species by polymorphonuclear leukocytes stimulated with IgG immune complexes

The production of reactive oxygen species (ROS) by polymorphonuclear leukocytes (PMN) can be induced by immune complexes and is an important component of phagocytosis in the killing of microorganisms, but can also be involved in inflammatory reactions when immune complexes are deposited in tissues. We have observed that fluid-phase IgG can inhibit the generation of ROS by rabbit PMN stimulated with precipitated immune complexes of IgG (ICIgG) in a dose-dependent manner, acting as a modulatory factor in the range of physiological IgG concentrations. This inhibitory effect is compatible with the known affinity (Kd) of monomeric IgG for the receptors involved (FcRII and FcRIII). The presence of complement components in the immune complexes results in a higher stimulation of ROS production. In this case, however, there is no inhibition by fluid-phase IgG. The effect of complement is strongly dependent on the presence of divalent cations (Ca2+ or Mg2+) in the medium, whereas the stimulation of ICIgG (without complement) does not depend on these cations. We have obtained some evidence indicating that iC3b should be the component involved in the effect of complement through interaction with the CR3 receptor. The absence of the inhibitory effect of fluid-phase IgG in ROS production when complement is present in the immune complex shows that complement may be important in vivo not only in the production of chemotactic factors for PMN, but also in the next phase of the process, i.e., the generation of ROS.

Roles of B cells in rheumatoid arthritis

B lymphocytes play several critical roles in the pathogenesis of rheumatoid arthritis. They are the source of the rheumatoid factors and anticitrullinated protein antibodies, which contribute to immune complex formation and complement activation in the joints. B cells are also very efficient antigen-presenting cells, and can contribute to T cell activation through expression of costimulatory molecules. B cells both respond to and produce the chemokines and cytokines that promote leukocyte infiltration into the joints, formation of ectopic lymphoid structures, angiogenesis, and synovial hyperplasia. The success of B cell depletion therapy in rheumatoid arthritis may depend on disruption of all these diverse functions.

Predominant role of FcgammaRIII in the induction of accelerated nephrotoxic glomerulonephritis

BACKGROUND

Nephrotoxic glomerulonephritis is induced by the administration of antibody against the glomerular basement membrane (GBM). We demonstrated previously that Fc receptors for immunoglobulin G (IgG) (FcgammaR) play crucial roles in the induction of accelerated nephrotoxic glomerulonephritis by using FcRgamma-deficient (-/-) mice. Since FcRgamma-/- mice lack the cell surface expression of two activating FcgammaRs, FcgammaRI and FcgammaRIII. The present study aims to identify the FcgammaR responsible for the induction of nephrotoxic glomerulonephritis.

METHODS

Accelerated anti-GBM glomerulonephritis was induced in FcgammaRI-/-, FcgammaRIII-/-, and FcRgamma-/- mice by preimmunization with rabbit IgG followed by inoculation of rabbit anti-GBM antibody. Histologic analysis and immunostaining of renal sections were performed.

RESULTS

FcgammaRI-/- mice as well as wild-type mice showed severe glomerulonephritis with hypernitremia by the administration of anti-GBM antibody. In contrast, FcgammaRIII-/- mice showed much milder renal involvement, similar to FcRgamma-/- mice. Histologically, FcgammaRI-/- mice showed intracapillary proliferation, glomerular thrombosis, and crescent formation, whereas FcgammaRIII-/- mice showed only glomerular hypercellular changes. The depositions of anti-GBM antibodies, autologous antibodies and complement C3 along the GBM were equally observed among all three FcR-/- mouse types by immunostaining.

CONCLUSIONS

Accelerated nephrotoxic glomerulonephritis is induced predominantly through FcgammaRIII but not FcgammaRI.

Modulation of autoantibody production by mycophenolate mofetil: effects on the development of SLE in (NZB x NZW)F1 mice

BACKGROUND

Mycophenolate mofetil (MMF) has been successfully used to improve or prevent the development of systemic lupus erythematosus (SLE) in both humans and in several lupus-prone mice. In the present study, we evaluated mechanisms through which MMF may exert its therapeutic effect on the development of systemic autoimmunity.

METHODS

(NZBxNZW)F(1) female mice were continuously treated with 100 mg/kg/day (high dose) or 30 mg/kg/day (low dose) MMF beginning at 3 months of age. The development of an autoimmune syndrome was evaluated by measuring immunoglobulin (Ig) isotypes of autoantibodies and their levels, as well as by evaluating immunopathological kidney abnormalities and mortality curves.

RESULTS

At both doses, MMF efficiently modulated the development of SLE. Although the higher dose of MMF directly inhibited the production of autoantibodies, 30 mg/kg/day MMF promoted qualitative but not quantitative changes in autoantibodies in (NZB x NZW)F(1) female mice. These qualitative changes were manifested as a selective reduction in total or antigen-specific IgG2a antibody levels.

CONCLUSIONS

The mechanisms through which MMF controls the development of SLE in (NZB x NZW)F(1) females is highly dependent upon immunosuppressor dose. Interestingly, lower dose MMF selectively reduced IgG2a antibody levels, suggesting that this dose may modulate T(H1) CD4+ activity.

Skewed balance in basal expression and regulation of activating v inhibitory Fcgamma receptors in macrophages of collagen induced arthritis sensitive mice

BACKGROUND

Recently, it has been found that collagen type II arthritis susceptible mouse strains are hyperreactive to immune complexes (ICs), locally deposited into their knee joints.

OBJECTIVE

To investigate whether this strain specific knee joint hyperreactivity is related to a disturbed regulation of activatory and inhibitory FcgammaR on their macrophages before and after stimulation with ICs.

METHODS

Macrophages from collagen induced arthritis susceptible strains (DBA/1 and B10.RIII) and non-susceptible strains (C57BL/6 and BALB/c) were compared. FcgammaR levels on macrophages were detected at protein level by flow cytometric analysis and at mRNA level by reverse transcriptase-polymerase chain reaction. Macrophages were stimulated with ICs, and production of cytokines and enzymes was measured at different times.

RESULTS

On synovial and peritoneal macrophages of DBA/1 mice a higher basal FcgammaRII and III expression was found, which was skewed towards the activating FcgammaRIII. In B10.RIII macrophages, however, FcgammaRIII levels were much lower. Regulation of FcgammaR mRNA levels in macrophages was tested after stimulation with ICs for one and three days. DBA/1 and B10.RIII macrophages showed a prolonged up regulation of activating FcgammaRI and III, whereas the inhibiting FcgammaRII was significantly down regulated compared with non-susceptible strains. In line with this, DBA/1 and B10.RIII macrophages showed a higher interleukin 1 (IL1) and matrix metalloproteinase (MMP) production after IC exposure, whereas IL6 production was significantly reduced.

CONCLUSIONS

This study indicates that macrophages derived from collagen type II arthritis susceptible mice show a disregulated FcgammaR expression before, and even more clearly, after activation by ICs involved in inflammation and cartilage degradation, resulting in prolonged expression of activatory FcgammaRI and III, down regulation of inhibitory FcgammaRII and increased release of IL1 and MMP.

Essential role of Fc gamma receptors in anti-type II collagen antibody-induced arthritis

Anti-type II collagen (anti-CII) Ab is a well-known autoantibody observed in patients with rheumatoid arthritis. Injection of anti-CII Ab and LPS induces arthritis in mice in which anti-CII Ab as well as inflammatory cytokines, IL-1beta and TNF-alpha, play critical roles. We investigated the involvement of IgG FcRs (FcgammaRs) in this arthritis model. BALB/c mice injected with the F(ab')(2) of anti-CII Ab showed no signs of arthritis. Arthritis development was not observed in FcRgamma(-/-) mice and was partially suppressed in FcgammaRIII(-/-) mice despite the binding of anti-CII Ab and C3 to cartilage surface. Surprisingly, BALB/c mice lacking FcgammaRIIB, which is known as an inhibitory FcgammaR, developed arthritis with no exacerbation in arthritis score compared with wild-type (WT) mice, and only slight exacerbation was observed in the histopathological analysis. In contrast, aged FcgammaRIIB(-/-) BALB/c mice developed arthritis without LPS injection, suggesting an augmented susceptibility to arthritis in aged FcgammaRIIB(-/-) mice. No significant difference was observed among BALB/c-WT, -FcRgamma(-/-), and -FcgammaRIIB(-/-) mice on cytokine production induced by anti-CII Ab and LPS injection. Severe arthritis developed in BALB/c-WT and -FcgammaRIIB(-/-) mice, but not in BALB/c-FcRgamma(-/-) mice, after the injection of anti-CII Ab and inflammatory cytokines. These results suggest that the reason behind the nondevelopment of arthritis in FcRgamma(-/-) BALB/c mice is not due to a disorder in transient cytokine production, but to an irregularity downstream of cytokine production.

Administration of a soluble recombinant complement C3 inhibitor protects against renal disease in MRL/lpr mice

Complement receptor 1-related gene/protein y (Crry) in rodents is a potent membrane complement regulator that inhibits complement C3 activation by both classical and alternative pathways. To clarify the role of complement in lupus nephritis, MRL/lpr mice were given Crry as a recombinant protein (Crry-Ig) from 12 to 24 wk of age. Control groups were given saline or normal mouse IgG. Sera and urine were collected biweekly. Only 1 of 20 (5%) Crry-Ig-treated mice developed renal failure (BUN > 50 mg/dl) compared with 18 of 38 (47.4%) mice in control groups (P = 0.001). BUN levels at 24 wk were reduced from 68.8 +/- 9.7 mg/dl in control groups to 38.5 +/- 3.9 mg/dl in the Crry-Ig-treated group (P < 0.01). Urinary albumin excretion at 24 wk was also significantly reduced from 5.3 +/- 1.4 mg/mg creatinine in the control groups to 0.5 +/- 0.2 mg/mg creatinine in the Crry-Ig-treated group (P < 0.05). Of the histologic data at 24 wk, there was a significant reduction in scores for glomerulosclerosis and C3d, IgG, IgG3, and IgA staining intensity in glomeruli in complement-inhibited animals. Crry-Ig-treated animals were also protected from vasculitic lesions. Although there was no effect on relevant autoimmune manifestations such as anti-double stranded DNA titers or cryoglobulin IgG3 levels, circulating immune complex levels were markedly higher in complement-inhibited animals. Thus, inhibition of complement activation with Crry-Ig significantly reduces renal disease in MRL/lpr lupus mice. The data support the strategy of using recombinant complement C3 inhibitors to treat human lupus nephritis.

Fc receptor-independent development of autoimmune glomerulonephritis in lupus-prone MRL/lpr mice

OBJECTIVE

To determine the role of Fc receptors (FcR), which play crucial roles in antibody and immune complex-mediated inflammation and autoimmunity, including glomerulonephritis (GN), in the development of autoimmune GN and vasculitis in MRL/lpr mice, one of the most widely used lupus-prone mouse models.

METHODS

FcRgamma(-/-) MRL/lpr mice were generated by backcrossing for 8 generations. The development of GN and vasculitis of various sized vessels was analyzed histopathologically in the kidney, lung, and skin. Autoantibody and immune complex levels were determined biochemically at 16-24 weeks of age and compared with the findings in FcRgamma(+) MRL/lpr mice. The lifespan of the mice was also recorded.

RESULTS

Diffuse proliferative GN, with deposition of IgG and C3, developed in both FcRgamma(-/-) and FcRgamma(+) MRL/lpr mice. There was no difference in the survival rate and degree of proteinuria between FcRgamma(+) and FcRgamma(-/-) MRL/lpr mice. Regardless of the level of FcR expression, there were no significant differences in the levels of serum IgG, anti-DNA antibody, or circulating immune complexes between the two types of mice. Necrotizing vasculitis in medium-sized arteries of the kidneys and lungs as well as small-vessel vasculitis in the skin was observed in both in FcRgamma(+) and FcRgamma(-/-) MRL/lpr mice. In contrast, the Arthus reaction was induced in FcRgamma(+) MRL/lpr mice, but not in FcRgamma(-/-) MRL/lpr mice.

CONCLUSION

Unlike (NZB x NZW)F(1), the other strain of lupus-prone mice that develops GN in an FcR-dependent manner, the development of autoimmune GN and vasculitis in MRL/lpr mice was FcR-independent, implying heterogeneity of the contribution of FcR to the development of autoimmune disease.

Complement-mediated clearance of erythrocytes: mechanism and delineation of the regulatory roles of Crry and DAF. Decay-accelerating factor

The role of complement in the pathogenesis of autoimmune hemolytic anemia (AIHA) has been controversial and may depend on a number of factors, including the affinity and isotype of the pathogenic antibodies involved. We have recently shown that mouse erythrocytes deficient in the membrane C3 regulatory protein, complement receptor 1-related gene/protein y (Crry), but not decay-accelerating factor (DAF), were spontaneously eliminated in vivo by complement. Here, by generating a mouse deficient in both DAF and Crry, we further delineated the roles of Crry and DAF in regulating alternative and classical pathway C3 activation. By using immunoglobulin-, Fcgamma receptor (FcgammaR)-, C3-, C4-, and C5-deficient mice, we also determined the mechanism by which membrane C3 regulator-deficient erythrocytes are cleared from the circulation. Finally, we evaluated the relative importance of the Fc receptor versus the complement pathway in disposing antibody-opsonized DAF/Crry-deficient erythrocytes. We conclude that (1) Crry plays a more dominant role than DAF in regulating the alternative pathway of complement, whereas DAF and Crry are equally effective in preventing antibody-induced runaway complement activation on mouse erythrocytes; (2) DAF/Crry-deficient erythrocytes are eliminated by the alternative pathway of complement via complement receptor-mediated erythrophagocytosis in the spleen; and (3) when opsonized with an immunoglobulin G2a (IgG2a) autoantibody, Crry/DAF-deficient erythrocytes are eliminated more rapidly by complement than by the Fc receptor pathway. These results shed new light on the relative activities of Crry and DAF and underscore the critical roles of membrane C3 regulators in preventing spontaneous and antibody-induced erythrocyte damage in vivo.

Development of inflammation in proteoglycan-induced arthritis is dependent on Fc gamma R regulation of the cytokine/chemokine environment

FcgammaRs are specialized cell surface receptors that coordinately regulate immune responses. Although FcgammaR expression is a prerequisite for the development of several immune complex-mediated diseases, the mechanism responsible for FcgammaR-dependent regulation in autoimmunity remains unclear. Therefore, we assessed FcgammaR-dependent regulation of inflammation in proteoglycan-induced arthritis (PGIA) using FcgammaR(-/-) mice. FcgammaRIIb(-/-) mice developed arthritis at an earlier time point and with a greater severity than wild-type (WT) mice. In gamma-chain(-/-) (FcgammaRI(-/-) and FcgammaRIII(-/-)) mice, no clinical or histological evidence of inflammation was observed. Exacerbation of arthritis in FcgammaRIIb(-/-) mice correlated with enhanced PG-specific Ab production, but did not significantly affect PG-specific T cell priming. In gamma-chain(-/-) mice, the absence of arthritis did not correlate with serum Ab responses, as PG-specific Ab production was normal. Although PG-specific T cell proliferation was diminished, spleen cells from gamma-chain(-/-) mice successfully adoptively transferred arthritis into SCID mice. Our studies indicated that the mechanism responsible for FcgammaR regulation of PGIA development was at the level of inflammatory cytokine and beta-chemokine expression within the joint. FcgammaRIIb regulated the development of PGIA by controlling the initiation of cytokine and chemokine expression within the joint before the onset of arthritis, whereas the expression of FcgammaRI and or FcgammaRIII controlled cytokine and chemokine expression late in the development of PGIA during the onset of disease. These results suggest that FcgammaRs are critical for the development of inflammation during PGIA, possibly by maintaining or enhancing inflammatory cytokine and beta-chemokine production.

Fc gamma R-mediated phagocytosis stimulates localized pinocytosis in human neutrophils

Engulfment of IgG-coated particles by neutrophils and macrophages is an essential component of the innate immune response. This process, known as phagocytosis, is triggered by clustering of FcgammaR at sites where leukocytes make contact with the opsonized particles. We found that phagocytosis is accompanied by a burst of fluid phase pinocytosis, which is largely restricted to the immediate vicinity of the phagosomal cup. FcgammaR-induced pinocytosis preceded and appeared to be independent of phagosomal sealing. Accordingly, fluid phase uptake was accentuated by actin depolymerization, which precludes phagocytosis. Stimulation of pinocytosis required phosphatidylinositol 3-kinase activity and was eliminated when changes in the cytosolic free Ca(2+) concentration were prevented. Because stimulation of FcgammaR also induces secretion, which is similarly calcium and phosphatidylinositol 3-kinase dependent, we studied the possible relationship between these events. Neutrophil fragments devoid of secretory granules (cytoplasts) were prepared by sedimentation through Ficoll gradients. Cytoplasts could perform FcgammaR-mediated phagocytosis, which was not accompanied by activation of pinocytosis. This observation suggests that granule exocytosis is required for stimulation of pinocytosis. Analysis of the cytosolic Ca(2+) dependence of secretion and pinocytosis suggests that primary (lysosomal) granule exocytosis is the main determinant of pinocytosis during FcgammaR stimulation. Importantly, primary granules are secreted in a polarized fashion near forming phagosomes. Focal pinocytosis during particle engulfment may contribute to Ag processing and presentation and/or to retrieval of components of the secretory machinery. Alternatively, it may represent an early event in the remodeling of the phagosomal membrane, leading to phagosomal maturation.

Constitutive versus activation-dependent cross-presentation of immune complexes by CD8(+) and CD8(-) dendritic cells in vivo

Murine splenic dendritic cells (DCs) can be divided into two subsets based on CD8alpha expression, but the specific role of each subset in stimulation of T cells is largely unknown. An important function of DCs is the ability to take up exogenous antigens and cross-present them in the context of major histocompatibility complex (MHC) class I molecules to CD8(+) T cells. We previously demonstrated that, when cell-associated ovalbumin (OVA) is injected into mice, only the CD8(+) DC subset cross-presents OVA in the context of MHC class I. In contrast to this selectivity with cell-associated antigen, we show here that both DC subsets isolated from mice injected with OVA/anti-OVA immune complexes (OVA-IC) cross-present OVA to CD8(+) T cells. The use of immunoglobulin G Fc receptor (Fc(gamma)R) common gamma-chain-deficient mice revealed that the cross-presentation by CD8(-) DCs depended on the expression of gamma-chain-containing activating FcgammaRs, whereas cross-presentation by CD8(+) DCs was not reduced in gamma-chain-deficient mice. These results suggest that although CD8(+) DCs constitutively cross-present exogenous antigens in the context of MHC class I molecules, CD8(-) DCs only do so after activation, such as via ligation of Fc(gamma)Rs. Cross-presentation of immune complexes may play an important role in autoimmune diseases and the therapeutic effect of antitumor antibodies.

Structure-function relationships for human antibodies to pneumococcal capsular polysaccharide from transgenic mice with human immunoglobulin Loci

To investigate the influence of antibody structure and specificity on antibody efficacy against Streptococcus pneumoniae, human monospecific antibodies (MAbs) to serotype 3 pneumococcal capsular polysaccharide (PPS-3) were generated from transgenic mice reconstituted with human immunoglobulin loci (XenoMouse mice) vaccinated with a PPS-3-tetanus toxoid conjugate and their molecular genetic structures, epitope specificities, and protective efficacies in normal and complement-deficient mice were determined. Nucleic acid sequence analysis of three MAbs (A7, 1A2, and 7C5) revealed that they use two different V(H)3 genes (A7 and 1A2 both use V3-15) and three different V(kappa) gene segments. The MAbs were found to have similar affinities for PPS-3 but different epitope specificities and CDR3 regions. Both A7 and 7C5 had a lysine at the V(H)-D junction, whereas 1A2 had a threonine. Challenge experiments with serotype 3 S. pneumoniae in BALB/c mice revealed that both 10- and 1- micro g doses of A7 and 7C5 were protective, while only a 10- micro g dose of 1A2 was protective. Both A7 and 7C5 were also protective in mice lacking either an intact alternative (FB(-/-)) or classical (C4(-/-)) complement pathway, but 1A2 was not protective in either strain. Our data suggest that PPS-3 consists of epitopes that can elicit both highly protective and less protective antibodies and that the superior efficacies of certain antibodies may be a function of their structures and/or specificities. Further investigation of relationships between structure, specificity, and efficacy for defined MAbs to PPS may identify antibody features that might be useful surrogates for antibody (and vaccine) efficacy.