Cell surface recycling of internalized antigen permits dendritic cell priming of B cells

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.

The regulation of autoreactive B cells during innate immune responses

Systemic lupus erythematosus (SLE) highlights the dangers of dysregulated B cells and the importance of initiating and maintaining tolerance. In addition to central deletion, receptor editing, peripheral deletion, receptor revision, anergy, and indifference, we have described a new mechanism of B cell tolerance wherein dendritic cells (DCs) and macrophages (MPhis) regulate autoreactive B cells during innate immune responses. In part, DCs and MPhis repress autoreactive B cells by releasing IL-6 and soluble CD40L (sCD40L). This mechanism is selective in that IL-6 and sCD40L do not affect Ig secretion by naïve cells during innate immune responses, allowing immunity in the absence of autoimmunity. In lupus-prone mice, DCs and MPhis are defective in secretion of IL-6 and sCD40L and cannot effectively repress autoantibody secretion suggesting that defects in DC/MPhi-mediated tolerance may contribute to the autoimmune phenotype. Further, these studies suggest that reconstituting DCs and MPhis in SLE patients might restore regulation of autoreactive B cells and provide an alternative to immunosuppressive therapies.

Dendritic cell subsets in health and disease

The dendritic cell (DC) system of antigen-presenting cells controls immunity and tolerance. DCs initiate and regulate immune responses in a manner that depends on signals they receive from microbes and their cellular environment. They allow the immune system to make qualitatively distinct responses against different microbial infections. DCs are composed of subsets that express different microbial receptors and express different surface molecules and cytokines. Our studies lead us to propose that interstitial (dermal) DCs preferentially activate humoral immunity, whereas Langerhans cells preferentially induce cellular immunity. Alterations of the DC system result in diseases such as autoimmunity, allergy, and cancer. Conversely, DCs can be exploited for vaccination, and novel vaccines that directly target DCs in vivo are being designed.

In vivo depletion of CD11c+ cells impairs scrapie agent neuroinvasion from the intestine

Following oral exposure, some transmissible spongiform encephalopathy (TSE) agents accumulate first upon follicular dendritic cells (DCs) in the GALT. Studies in mice have shown that TSE agent accumulation in the GALT, in particular the Peyer's patches, is obligatory for the efficient transmission of disease to the brain. However, the mechanism through which TSE agents are initially conveyed from the gut lumen to the GALT is not known. Studies have implicated migratory hemopoietic DCs in this process, but direct demonstration of their involvement in vivo is lacking. In this study, we have investigated the contribution of CD11c(+) DCs in scrapie agent neuroinvasion through use of CD11c-diptheria toxin receptor-transgenic mice in which CD11c(+) DCs can be specifically and transiently depleted. Using two distinct scrapie agent strains (ME7 and 139A scrapie agents), we show that when CD11c(+) DCs were transiently depleted in the GALT and spleen before oral exposure, early agent accumulation in these tissues was blocked. In addition, CD11c(+) cell depletion reduced susceptibility to oral scrapie challenge indicating that TSE agent neuroinvasion from the GALT was impaired. In conclusion, these data demonstrate that migratory CD11c(+) DCs play a key role in the translocation of the scrapie agent from the gut lumen to the GALT from which neuroinvasion subsequently occurs.

Transfusion-induced autoantibodies and differential immunogenicity of blood group antigens: a novel hypothesis

Blood bank serology has identified hundreds of red blood cell (RBC) antigens contained within numerous blood group systems. Although most blood group antigens are defined by amino acid polymorphisms in the extracellular domain of membrane proteins, it is also possible that additional nonexofacial polymorphisms (NEPs) may exist within cytoplasmic or transmembrane domains. To assess this possibility, we analyzed several blood group molecules by searching the SNPper database for nonsynonymous single-nucleotide polymorphisms. We report the identification of a number of NEPs in the Kell, Kidd, and Duffy molecules. Because the identified NEPs are not exposed on the surface of intact RBCs and are, thus, not accessible to recipient antibodies, they would neither be detected by blood bank serology in vitro, nor would they be recognized targets in hemolytic transfusion reactions in vivo. The presentation of peptides containing NEPs by recipient MHC Class II molecules, however, would nevertheless produce helper T-cell epitopes. In addition to identifying NEPs in human blood group molecules, we explore a novel hypothesis that the presence of NEPs contributes to the immunogenicity of blood group antigens. We further hypothesize that NEPs provide a mechanism by which transfusion can lead to anti-RBC autoantibodies, which are known to occur in humans after transfusion. The scientific basis, existing evidence, approaches to testing, and predicted biology of this hypothesis are presented.

Mechanisms of killing by anti-CD20 monoclonal antibodies

CD20 is a cell-surface marker expressed on mature B cells and most malignant B cells, but not stem or plasma cells. It is an ideal target for monoclonal antibodies (mAb), such as rituximab and ofatumumab, as it is expressed at high levels on most B-cell malignancies, but does not become internalized or shed from the plasma membrane following mAb treatment. This allows mAb to persist on the cell surface for extended periods and deliver sustained immunological attack from complement and FcR-expressing innate effectors, particularly macrophages. CD20 can also generate transmembrane signals when engaged by certain mAb which, although unproven, might provide an important element of the therapeutic success of anti-CD20 mAb. These favourable characteristics have led to anti-CD20 mAb being developed and exploited for use in immunotherapy, where they have proven remarkably efficacious in both the treatment of malignant disease and autoimmune disorders by deleting malignant or normal B cells, respectively. In this review, we discuss how these mAb have driven research in the immunotherapy field over the last decade, detail their likely modes of action and their limitations in terms of effector exhaustion, and explore ways in which they might be enhanced and further exploited in the future.

Neonatal FcR expression in bone marrow-derived cells functions to protect serum IgG from catabolism

The neonatal FcR (FcRn) is a receptor that protects IgG from catabolism and is important in maintaining high serum Ab levels. A major site of expression of FcRn is vascular endothelial cells where FcRn functions to extend the serum persistence of IgG by recycling internalized IgG back to the surface. Because FcRn is expressed in other tissues, it is unclear whether endothelial cells are the only site of IgG protection. In this study, we used FcRn-deficient mice and specific antiserum to determine the tissue distribution of FcRn in the adult mouse. In addition to its expression in the vascular endothelium of several organs, we found FcRn to be highly expressed in bone marrow-derived cells and professional APCs in different tissues. Experiments using bone marrow chimeras showed that FcRn expression in these cells acted to significantly extend the half-life of serum IgG indicating that in addition to the vascular endothelium, bone marrow-derived phagocytic cells are a major site of IgG homeostasis.

Autoimmunity stimulated by adoptively transferred dendritic cells is initiated by both alphabeta and gammadelta T cells but does not require MyD88 signaling

Vaccination of nonautoimmune prone mice with syngeneic dendritic cells (DC) readily induces anti-DNA autoantibodies but does not trigger systemic disease. We observed that anti-DNA autoantibody generation absolutely required alphabeta T cells and that gammadelta T cells also contributed to the response, but that regulatory T cells restrained autoantibody production. Although both NZB/W F(1) mice and DC vaccinated C57/BL6 mice produced autoantibodies against dsDNA, vaccinated mice had higher levels of Abs against H1 histone and lower levels of antinucleosome Abs than NZB/W F(1) mice. Despite a 100-fold increase in IL-12 and Th1 skewing to a foreign Ag, OVA, synergistic TLR activation of DC in vitro failed to augment anti-DNA Abs or promote class switching beyond that induced by LPS alone. TLR stimulation was not absolutely required for the initial loss of B cell tolerance because anti-DNA levels were similar when wild-type (WT) or MyD88-deficient DC were used for vaccination or WT and MyD88-deficient recipients were vaccinated with WT DC. In contrast, systemic administration of LPS, augmented anti-DNA Ab levels and promoted class switching, and this response was dependent on donor DC signaling via MyD88. LPS also augmented responses in the MyD88-deficient recipients, suggesting that LPS likely exerts its effects on both transferred DC and host B cells in vivo. These results indicate that both the alphabeta and gammadelta subsets are necessary for promoting autoantibody production by DC vaccination, and that although TLR/MyD88 signaling is not absolutely required for initiation, this pathway does promote augmentation, and Th1-mediated skewing, of anti-DNA autoantibodies.

Augmented HER-2 specific immunity during treatment with trastuzumab and chemotherapy

PURPOSE

Passive immunotherapy with antitumor antibodies has the potential to induce active tumor immunity via the opsonic enhancement of immunogenicity of tumor antigen. We have assessed whether immune sensitization to the HER-2/neu tumor antigen occurs during treatment with the anti-HER-2/neu monoclonal antibody trastuzumab.

EXPERIMENTAL DESIGN

Twenty-seven patients treated with trastuzumab and chemotherapy were assessed for the induction of HER-2/neu-specific immunity. Sera and peripheral blood mononuclear cells obtained before and after trastuzumab therapy were compared for the presence of anti-HER-2/neu endogenous Iglambda antibodies and HER-2/neu-specific CD4 responses by ELISA and enzyme-linked immunospot, respectively.

RESULTS

Anti-HER-2/neu antibodies were detectable in 8 of 27 (29%) patients before trastuzumab treatment and in 15 of 27 (56%) patients during trastuzumab treatment. In the overall study population, anti-HER-2/neu humoral responses significantly increased during therapy (P < 0.001) and were not associated with development of an anti-idiotypic response. In 10 evaluable individuals, 6 showed augmented HER-2/neu-specific CD4 T-cell responses during therapy. Of the 22 individuals treated for metastatic disease, those patients showing objective clinical responses exhibited more frequent (P = 0.004) and larger (P = 0.006) treatment-associated anti-HER-2/neu humoral responses.

CONCLUSION

Humoral immune sensitization occurs during treatment with chemotherapy and trastuzumab. Further studies are warranted to investigate whether augmented anti-HER-2/neu humoral and cellular immunity contributes mechanistically to clinical outcome.

Antigen presentation and transfer between B cells and macrophages

B cells play an active role in directing immunity against specific proteins in part because of their capacity to sequester antigen via B cell receptor (BCR). Our prior findings indicate that B cells can initiate an immune response in vivo to self proteins independent of other antigen-presenting cells (APC). However, these studies also demonstrated that both dendritic cells and macrophages are important in the ongoing immune response. The present work illustrates a mechanism by which antigen acquired by B cells through BCR is specifically transferred to other APC, in particular, macrophages. The transfer of antigen is dependent on the specificity of BCR and requires direct contact between the cells, but does not require MHC compatibility between the cells and is independent of the activation state of macrophages. Antigen transfer is functional, in that macrophages, which received B cell derived-antigen, can activate CD4 T cells. Overall, these results define a novel mechanism by which B cells can focus immunity toward a specific antigen and transfer the ability to activate CD4 T cells to other APC.

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.

Intrinsic and cooperative antigen-presenting functions of dendritic-cell subsets in vivo

Dendritic cells (DCs) comprise several subsets, and their roles in the presentation of antigens derived from pathogens, vaccines and self tissues are now beginning to be elucidated. Differences in location, life cycle and intrinsic abilities to capture, process and present antigens on their MHC class I and class II molecules enable each DC subset to have distinct roles in immunity to infection and in the maintenance of self tolerance. Unexpected interactions among DC subsets have also been revealed. These interactions, which allow the integration of the intrinsic abilities of different DC types, enhance the ability of the DC network to respond to multiple scenarios of infection.

FcRn: the neonatal Fc receptor comes of age

The neonatal Fc receptor for IgG (FcRn) has been well characterized in the transfer of passive humoral immunity from a mother to her fetus. In addition, throughout life, FcRn protects IgG from degradation, thereby explaining the long half-life of this class of antibody in the serum. In recent years, it has become clear that FcRn is expressed in various sites in adults, where its potential function is now beginning to emerge. In addition, recent studies have examined the interaction between FcRn and the Fc portion of IgG with the aim of either improving the serum half-life of therapeutic monoclonal antibodies or reducing the half-life of pathogenic antibodies. This Review summarizes these two areas of FcRn biology.

The role of integrins and coreceptors in refining thresholds for B-cell responses

Despite compelling evidence that a large proportion of antigens encountered in vivo by B cells are membrane bound, the general view is that B cells are mainly activated by soluble antigens. This notion may have been biased somewhat over the years because the high affinity of the B-cell receptor (BCR) for soluble intact ligands allows efficient B-cell stimulation in vitro. In vivo, however, even soluble antigens are likely to be deposited on the surface of antigen-presenting cells, either by complement or Fc receptors in the form of immune complexes, thus becoming more potent stimulators of B-cell activation. In this framework, the BCR works in a complex environment of integrins and coreceptors, as well as the B-cell cytoskeleton. Over the last few years, we have focused on B-cell membrane-bound antigen recognition. Here, we discuss some of our findings in the context of what is currently known in this exciting new field.

Definition of germinal-center B cell migration in vivo reveals predominant intrazonal circulation patterns

Proliferation, mutation, and selection in the germinal center (GC) are thought to occur in distinct microanatomical compartments-the dark zone (DZ) and the light zone (LZ). Thus, affinity maturation has been posited to require frequent trafficking between zones. Here we report the use of multiphoton in vivo microscopy to determine migration patterns of GC B cells. Analysis of time-resolved images revealed unexpected patterns of movement as well as GC B cell morphology. Though frequent movement between the DZ and LZ was anticipated, few cells were observed to cross the interface between the two compartments. Moreover, cell-track trajectories indicated that cell movement in this region is predominantly parallel to the interface, suggesting that B cells circulate within individual LZ and DZ compartments. The results suggest a revision to our views of B cell circulation within GCs and the functional relationship of its two major compartments.

Subcapsular encounter and complement-dependent transport of immune complexes by lymph node B cells

The mechanism of B cell-antigen encounter in lymphoid tissues is incompletely understood. It is also unclear how immune complexes are transported to follicular dendritic cells. Here, using real-time two-photon microscopy we noted rapid delivery of immune complexes through the lymph to macrophages in the lymph node subcapsular sinus. B cells captured immune complexes by a complement receptor-dependent mechanism from macrophage processes that penetrated the follicle and transported the complexes to follicular dendritic cells. Furthermore, cognate B cells captured antigen-containing immune complexes from macrophage processes and migrated to the T zone. Our findings identify macrophages lining the subcapsular sinus as an important site of B cell encounter with immune complexes and show that intrafollicular B cell migration facilitates the transport of immune complexes as well as encounters with cognate antigen.

B cells acquire particulate antigen in a macrophage-rich area at the boundary between the follicle and the subcapsular sinus of the lymph node

Adaptive B cell responses are initiated by B cell receptor-antigen engagement. Despite its importance, the early stages of B cell interaction with antigen in vivo remain elusive. Using fluorescent particulate antigen in combination with multiphoton microscopy and immune staining, we identified the follicle-subcapsular sinus (SCS) boundary of the lymph node as a site for B cell-antigen acquisition. In this macrophage-rich region, antigen accumulated shortly after administration, where specific follicular B cells were retained for long periods of time. These B cells acquired antigen cumulatively and became the main antigen carriers inside the follicle before polarizing to the B cell-T cell border. These observations not only provide evidence of a B cell role as antigen transporters into the follicle, but also highlight the boundary between the follicle and the SCS as a site for initiation of B cell responses.

Critical role of monocytes to support normal B cell and diffuse large B cell lymphoma survival and proliferation

Large B cell lymphomas can comprise numerous CD14+ cells in the tumor stroma, which raises the question of whether monocytes can support B cell survival and proliferation. We show that the coculture of monocytes with B cells from peripheral blood or from diffuse large B cell lymphoma enabled prolonged B cell survival. Under these conditions, diffuse large lymphoma B cells proliferated, and addition of B cell-activating factor of the TNF family (BAFF) and IL-2 enhanced cell division. Monocytes and dendritic cells (DC) had similar antiapoptotic activity on healthy B cells but displayed differences with respect to B cell proliferation. Monocytes and cord blood-derived CD14+ cells promoted B cell proliferation in the presence of an anti-CD40 stimulus, whereas DC supported B cell proliferation when activated through the BCR. DC and CD14+ cells were able to induce plasmocyte differentiation. When B cells were activated via the BCR or CD40, they released the leukocyte attractant CCL5, and this chemokine is one of the main chemokines expressed in diffuse large B cell lymphoma. The data support the notion that large B cell lymphoma recruit monocytes via CCL5 to support B cell survival and proliferation.

Mucosal dendritic cells

The internal surfaces of the human body are covered by distinct types of epithelial cells and mucus-secreting cells. The mucosal surfaces serve many vital functions, such as respiration (nasal passage and lung), absorption (gastrointestinal tract), excretion (lung, urinary tract, large intestine), and reproduction (reproductive tract). In performing these functions, the host is inevitably exposed to environmental antigens, food particles, commensal flora, and pathogens. Mucosal surfaces contain specialized dendritic cells (DCs) capable of sensing these external stimuli and mounting appropriate local responses depending on the nature of the elements they encounter. In the absence of pathogens, mucosal DCs either ignore the antigen or induce regulatory responses. Upon recognition of microorganisms that invade the mucosal barrier, mucosal DCs mount robust protective immunity. This review highlights progress in our understanding of how mucosal DCs process external information and direct appropriate responses by mobilizing various cells of the innate and adaptive immune systems to achieve homeostasis and protection.

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.

Antibody-enhanced cross-presentation of self antigen breaks T cell tolerance

We have developed a model of autoimmunity to investigate autoantibody-mediated cross-presentation of self antigen. RIP-mOVA mice, expressing OVA in pancreatic beta cells, develop severe autoimmune diabetes when given OT-I cells (OVA-specific CD8(+) T cells) and anti-OVA IgG but not when given T cells alone. Anti-OVA IgG is not directly injurious to the islets but rather enhances cross-presentation of apoptotic islet antigen to the OT-I cells, leading to their differentiation into potent effector cells. Antibody-driven effector T cell activation is dependent on the presence of activating Fc receptors for IgG (FcgammaRs) and cross-priming DCs. As a consequence, diabetes incidence and severity was reduced in mice lacking activating FcgammaRs. An intact complement pathway was also required for disease development, as C3 deficiency was also partially protective. C3-deficient animals exhibited augmented T cell priming overall, indicating a proinflammatory role for complement activation after the T cell priming phase. Thus, we show that autoreactive antibody can potently enhance the activation of effector T cells in response to cross-presented self antigen, thereby contributing to T cell-mediated autoimmunity.

Dendritic cells internalise and re-present conformationally intact soluble MHC class I alloantigen for generation of alloantibody

Following organ transplantation soluble MHC class I is released from the graft and may contribute to alloimmunity. We determined in a well-established rat model whether DC are able to internalise soluble MHC class I alloantigen and then re-present intact alloantigen to B cells and T cells for generation of an alloantibody or CD8 T cell response. PVG.RT1(u) BM-derived DC internalised (via an active process) and retained intact a recombinant soluble form of RT1-A(a) (sRT1-A(a)). When PVG.RT1(u) rats were immunised with sRT1-A(a)-pulsed syngeneic DC, they developed a strong anti-sRT1-A(a) alloantibody response and showed accelerated rejection of RT1-A(a)-disparate PVG.R8 heart grafts. Alloantibody production and accelerated heart graft rejection were both dependent on immunisation with viable sRT1-A(a)-pulsed DC. The alloantibody response to sRT1-A(a)-pulsed DC was directed exclusively against conformational epitopes expressed by sRT1-A(a) and not epitopes expressed, for example, by non-conformational sRT1-A(a) heavy chain. Immunisation with sRT1-A(a)-pulsed syngeneic DC did not stimulate a CD8 T cell response. Our findings suggest a novel alloantigen recognition pathway whereby soluble MHC class I alloantigen released from an allograft may be taken up by recipient DC and presented in an intact unprocessed form to B cells for the generation of an alloantibody response.

The humoral immune response is initiated in lymph nodes by B cells that acquire soluble antigen directly in the follicles

The initial step in a humoral immune response involves the acquisition of antigens by B cells via surface immunoglobulin. Surprisingly, anatomic studies indicate that lymph-borne proteins do not have access to the follicles where naive B cells reside. Thus, it is unclear how B cells acquire antigens that drain to lymph nodes. By tracking a fluorescent antigen and a peptide:MHC II complex derived from it, we show that antigen-specific B cells residing in the follicles acquire antigen within minutes of injection, first in the region closest to the subcapsular sinus where lymph enters the lymph node. Antigen acquisition, presentation, and subsequent T cell-dependent activation did not require B cell migration through the T cell area or exposure to dendritic cells. These results indicate that the humoral response is initiated as soluble antigens diffuse directly from lymph in the subcapsular sinus to be acquired by antigen-specific B cells in the underlying follicles.

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.

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.

Dendritic Cell-Independent B Cell Activation During Acute Virus Infection: A Role for Early CCR7-Driven B-T Helper Cell Collaboration

This study provides a detailed spatiotemporal interaction analysis between B cells, Th cells, and dendritic cells (DC) during the generation of protective antiviral B cell immunity. Following vesicular stomatitis virus (VSV) infection, conditional ablation of CD11c-positive DC at the time-point of infection did not impair extrafollicular plasma cell generation and Ig class switching. In contrast, the generation of Th and B cell responses following immunization with recombinant VSV-glycoprotein was DC-dependent. Furthermore, we show that the CCR7-dependent interplay of the three cell-types is crucial for virus-neutralizing B cell responses in the presence of limiting amounts of Ag. An immediate event following VSV infection was the CCR7-mediated interaction of VSV-specific B and Th cells at the T cell-B cell zone border that facilitated plasma cell differentiation and Th cell activation. Taken together, these experiments provide evidence for a direct, CCR7-orchestrated and largely DC-independent mutual activation of Th cells and Ag-specific B cells that is most likely a critical step during early immune responses against cytopathic viruses.

Plasma cell differentiation in T-independent type 2 immune responses is independent of CD11c(high) dendritic cells

Dendritic cells (DC) play an important role as antigen-presenting cells in T cell stimulation. Interestingly, a number of recent studies also imply DC as critical accessory cells in B cell activation, isotype switching and plasma blast maintenance. Here we use the conditional in vivo ablation of CD11c(high) DC to investigate the role of these cells in T-independent type 2 immune responses. We show that CD11c(high) DC are dispensable for the initiation and maintenance of a primary immune response against the T-independent type 2 antigen (4-hydroxy-3-nirophenyl)acetyl-Ficoll. Our results suggest that support for plasma cell formation in T cell-independent immune responses can be provided by non-DC such as stromal cells, or is independent of external signals. Interestingly, we found plasma blasts to express CD11c and to be diphtheria toxin-sensitive in CD11c-diphtheria toxin receptor-transgenic mice, providing a unique tool for future analysis of in vivo aspects of plasma cell biology.

A recombinant allergen chimer as novel mucosal vaccine candidate for prevention of multi-sensitivities

BACKGROUND

As conventional immunotherapy is less efficacious in patients with allergic multi-sensitivities compared with mono-sensitized subjects, new intervention strategies are needed. Therefore, an allergen chimer was genetically engineered for treatment of multi-sensitization with birch and grass pollen on the basis of mucosal tolerance induction.

METHODS

The major birch pollen allergen Bet v 1 served as a scaffold for N- and C-terminal linkage of the immunodominant peptides of the grass pollen allergens Phl p 1 and Phl p 5 and this new construct was cloned and expressed in Escherichia coli. After purification, physicochemical and immunological characterization the chimer was used for intranasal tolerance induction prior to poly-sensitization with Bet v 1, Phl p 1 and Phl p 5.

RESULTS

The immunological characterization revealed that the conformation of Bet v 1 within the chimer was comparable to that of natural as well as recombinant Bet v 1. The chimer was immunogenic in mice for T and B cell responses to the three allergens. Intranasal application of the chimer prior to poly-sensitization significantly suppressed humoral and cellular allergen-specific Th2 responses and prevented development of airway inflammation upon allergen challenge. Moreover, local allergen-specific IgA antibodies were induced by the chimer. The mechanisms of poly-tolerance induction seemed to be mediated by regulatory cytokines, since TGF-beta and IL-10 mRNA in splenocytes were upregulated and tolerance was transferable with these cells.

CONCLUSION

The data indicate that such allergen chimers harboring several unrelated allergens or allergen peptides could serve as mucosal polyvalent vaccines for prevention of multi-sensitivities.

Expression of RAB4B, a protein governing endocytic recycling, is co-regulated with MHC class II genes

The small GTPase RAB4 regulates endocytic recycling, a process that contributes to Major Histocompatibility Complex (MHC)-mediated antigen presentation by specialized antigen presenting cells (APC) of the immune system. The gene encoding the RAB4B isoform of RAB4 was singled out by two complementary genome-wide screens. One of these consisted of a computer scan to identify genes containing characteristic MHC class II-related regulatory sequences. The second was the use of chromatin immunoprecipitation coupled to microarrays (ChIP-on-chip) to identify novel targets of a transcriptional co-activator called the MHC class II transactivator (CIITA). We show that the RAB4B gene is regulated by a typical MHC class II-like enhancer that is controlled directly by both CIITA and the multiprotein transcription factor complex known as the MHC class II enhanceosome. RAB4B expression is thus activated by the same regulatory machinery that is known to be essential for the expression of MHC class II genes. This molecular link between the transcriptional activation of RAB4B and MHC class II genes implies that APC boost their antigen presentation capacity by increasing RAB4-mediated endocytic recycling.

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.

Generation of gut-homing IgA-secreting B cells by intestinal dendritic cells

Normal intestinal mucosa contains abundant immunoglobulin A (IgA)-secreting cells, which are generated from B cells in gut-associated lymphoid tissues (GALT). We show that dendritic cells (DC) from GALT induce T cell-independent expression of IgA and gut-homing receptors on B cells. GALT-DC-derived retinoic acid (RA) alone conferred gut tropism but could not promote IgA secretion. However, RA potently synergized with GALT-DC-derived interleukin-6 (IL-6) or IL-5 to induce IgA secretion. Consequently, mice deficient in the RA precursor vitamin A lacked IgA-secreting cells in the small intestine. Thus, GALT-DC shape mucosal immunity by modulating B cell migration and effector activity through synergistically acting mediators.

Fc gamma receptors play a dominant role in protective tumor immunity against a virus-encoded tumor-specific antigen in a murine model of experimental pulmonary metastases

Simian virus 40 (SV40) large tumor antigen (Tag) represents a virus-encoded tumor-specific antigen expressed in many types of human cancers and a potential immunologic target for antitumor responses. Fc receptors are important mediators in the regulation and execution of host effector mechanisms against conditions including infectious diseases, autoimmunity, and cancer. By examining tumor protection in SV40 Tag-immunized wild-type BALB/c mice using an experimental pulmonary metastasis model, we attempted to address whether engagement of the immunoglobulin G Fc receptors (FcgammaRs) on effector cells is necessary to mediate antitumor responses. All immunized BALB/c FcgammaR-/- knockout mice developed anti-SV40 Tag antibody responses prior to experimental challenge with a tumorigenic cell line expressing SV40 Tag. However, all mice deficient in the activating FcgammaRI (CD64) and FcgammaRIII (CD16) were unable to mount protective immunologic responses against tumor challenge and developed tumor lung foci. In contrast, mice lacking the inhibitory receptor FcgammaRII (CD32) demonstrated resistance to tumorigenesis. These results underscore the importance of effector cell populations expressing FcgammaRI/III within this murine tumor model system, and along with the production of a specific humoral immune response, antibody-dependent cell-mediated cytotoxicity (ADCC) may be a functioning mechanism of tumor clearance. Additionally, these data demonstrate the potential utility of ADCC as a viable approach for targeting vaccination strategies that promote FcgammaRI/III scavenging pathways against cancer.

Checkpoints in memory B-cell evolution

We consider four sequential phases in the evolution and consolidation of high affinity B-cell memory as it is regulated in a cognate manner by antigen-specific T-helper (Th) cells. Sequential developmental checkpoints control cell fate in each phase of the pathway in ways that still remain poorly understood. The cellular composition and molecular attributes of each checkpoint are of great interest, but they may vary substantially depending on the nature of the immune stimulus. How this stimulus cascades through the innate and then the adaptive immune responses defines initial effector mechanisms in both Th and B-cell compartments. The germinal center reaction controls memory B-cell development with roles for antigen presentation and cognate Th cell regulation in the establishment of the memory B-cell compartment. Antigen re-challenge rapidly promotes effector responses from the memory compartments of both Th and B cells. Importantly, re-challenge also expands and consolidates immune memory at the serological and cellular levels. We review recent advances in our understanding of memory B-cell evolution with emphasis on the regulatory checkpoints that control lymphocyte fate at each developmental juncture.

Preferential Induction of CD4+ T Cell Responses through In Vivo Targeting of Antigen to Dendritic Cell-Associated C-Type Lectin-1

Targeting of Ags and therapeutics to dendritic cells (DCs) has immense potential for immunotherapy and vaccination. Because DCs are heterogeneous, optimal targeting strategies will require knowledge about functional specialization among DC subpopulations and identification of molecules for targeting appropriate DCs. We characterized the expression of a fungal recognition receptor, DC-associated C-type lectin-1 (Dectin-1), on mouse DC subpopulations and investigated the ability of an anti-Dectin-1 Ab to deliver Ag for the stimulation of immune responses. Dectin-1 was shown to be expressed on CD8alpha-CD4-CD11b+ DCs found in spleen and lymph nodes and dermal DCs present in skin and s.c. lymph nodes. Injection of Ag-anti-Dectin-1 conjugates induced CD4+ and CD8+ T cell and Ab responses at low doses where free Ag failed to elicit a response. Notably, qualitatively different immune responses were generated by targeting Ag to Dectin-1 vs CD205, a molecule expressed on CD8alpha+CD4-CD11b- DCs, dermal DCs, and Langerhans cells. Unlike anti-Dectin-1, anti-CD205 conjugates failed to elicit an Ab response. Moreover, when conjugates were injected i.v., anti-Dectin-1 stimulated a much stronger CD4+ T cell response and a much weaker CD8+ T cell response than anti-CD205. The results reveal Dectin-1 as a potential targeting molecule for immunization and have implications for the specialization of DC subpopulations.

Extrafollicular activation of lymph node B cells by antigen-bearing dendritic cells

In contrast to naïve T cells that recognize short antigen-derived peptides displayed by specialized antigen-presenting cells, immunoglobulin receptors of B lymphocytes primarily recognize intact proteins. How and where within a lymph node such unprocessed antigens become available for naïve B cell recognition is not clear. We used two-photon intravital imaging to show that, after exiting high-endothelial venules and before entry into lymph node follicles, B cells survey locally concentrated dendritic cells. Engagement of the B cell receptor by the dendritic cell (DC)-associated antigen leads to lymphocyte calcium signaling, migration arrest, antigen acquisition, and extrafollicular accumulation. These findings suggest a possible role for antigen-specific B-DC interactions in promoting T cell-dependent antibody responses in vivo.

Generation, migration and function of circulating dendritic cells

Tissue-resident dendritic cells (DCs) that migrate from peripheral sites to lymphoid organs are essential in the initiation of adaptive immune responses and for the maintenance of peripheral tolerance, and have been extensively studied. By contrast, blood-borne DCs represent a heterogeneous population, the origin, destination and function of which are still unclear. Recent studies have shown that circulating DCs capture blood-borne antigen and transport it into the extra-vascular space of lymphoid tissues for processing and presentation. Other findings suggest that a fraction of tissue-resident DCs might enter the blood after having acquired antigen in the periphery. Together, these studies imply that circulating DCs might modulate immune responses by translocating antigenic material from its point of origin to remote target tissues.

Imaging the function of regulatory T cells in vivo

Despite extensive research on regulatory T cells (Tregs) since their rebirth more than twenty years ago, the cellular and molecular mechanisms by which they act to suppress immune responses remain largely elusive. In vitro suppression assays are instrumental in the functional identification of these cells. However, suppressive mechanisms defined in in vitro assays might not be relevant to situations in vivo. Advances in live tissue and intravital imaging technologies combined with the ability to grow large numbers of Tregs for in vivo experimentation have created an opportunity to analyze Treg function in vivo in their native environment in real-time. Two-photon laser-scanning microscopic studies of Treg control of lymph node priming suggest that Tregs exert their function by limiting T helper (Th) cell access to dendritic cells (DCs). In the absence of Tregs, Th cells initially form transient interactions with DCs that lead to arrest of the Th cells and to the formation of stable conjugates between Th cells and DCs. In the presence of increasing number of Tregs, Th cell arrest and their prolonged contacts with DCs are progressively inhibited. The reduced DC contacts in the presence of Tregs are associated with suppressed proliferation and differentiation of Th cells. Expansion of such analysis to peripheral tissues together with the development of functional reporter mice will help to further elucidate the mode of operation of Tregs in vivo.

B cell recognition of membrane-bound antigen: an exquisite way of sensing ligands

B cell recognition of membrane-bound antigens leads to the formation of an immunological synapse and efficient B cell activation. Ongoing research has been directed towards identifying the parameters that regulate this process. Furthermore, considerable attention has also been focused on the cell types that mediate presentation of intact antigens to B cells, as well as on the mechanisms that underlie it. Whilst there are still many unanswered questions, recent developments in these areas begin to shed light on an emerging field.

B cell ligand discrimination through a spreading and contraction response

B cells recognize foreign antigens by virtue of cell surface immunoglobulin receptors and are most effectively activated by membrane-bound ligands. Here, we show that in the early stages of this process, B cells exhibit a two-phase response in which they first spread over the antigen-bearing membrane and then contract, thereby collecting bound antigen into a central aggregate. The extent of this response, which is both signaling- and actin-dependent, determines the quantity of antigen accumulated and hence the degree of B cell activation. Brownian dynamic simulations reproduce essential features of the antigen collection process and suggest a possible basis for affinity discrimination. We propose that dynamic spreading is an important step of the immune response.

B cell migration and interactions in the early phase of antibody responses

In the early phase of thymus-dependent antibody responses antigen-engaged B cells rapidly change their localization within the secondary lymphoid organs to access helper T cells. Central to this process is the tightly controlled distribution of chemokines, sphingosine-1-phosphate and other guidance cues within the lymphoid organ, determined in part by the stromal cells, and the changing responsiveness of activated lymphocytes to these cues. Studies that use the emerging technique of real-time two-photon imaging of intact lymphoid organs began to dissect the dynamics of B cell migration before and after antigen engagement in vivo. Recent studies also provided new insight into antigen transport mechanisms in lymphoid organs and examined signaling requirements for B lymphocyte positioning and motility. Taken together, these studies have provided a more detailed map of the steps involved in B cell migration to encounter antigen and helper T cells early during the adaptive immune response.

Langerhans cells--revisiting the paradigm using genetically engineered mice

Langerhans cells (LCs) are prominent dendritic cells (DCs) in epithelia, but their role in immunity and tolerance is poorly defined. 'Knockin' mice expressing enhanced green fluorescent protein (EGFP) under the control of the langerin (CD207) gene were recently developed in order to discriminate epidermal LCs from other DC subsets and at the same time to track their dynamics under steady-state or inflammatory conditions in vivo. Additional knockin mice expressing a diphtheria toxin receptor fused to EGFP were used to conditionally ablate LCs and assess their role in triggering hapten-specific T cell effectors through skin immunization. We review the insights that have been provided by these various knockin mice and discuss gaps in our knowledge of LCs that need to be filled.

Critical Role of ITIM-Bearing FcγR on DCs in the Capture and Presentation of Native Antigen to B Cells

The contribution of dendritic cells (DCs) to antibody responses through antigen-unspecific B cell costimulation is well documented. now demonstrate that the ITIM-bearing FcgammaRIIB allows DCs to recycle internalized native antigen (Ag) for presentation to B cells, a process that might be critical for the initiation of humoral responses.