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

The origins and functions of dendritic cells and macrophages in the skin

Immune cell populations in the skin are predominantly comprised of dendritic cells (DCs) and macrophages. A lack of consensus regarding how to define these cell types has hampered research in this area. In this Review, we focus on recent advances that, based on ontogeny and global gene-expression profiles, have succeeded in discriminating DCs from macrophages in the skin. We discuss how these studies have enabled researchers to revisit the origin, diversity and T cell-stimulatory properties of these cells, and have led to unifying principles that extend across tissues and species. By aligning the DC and macrophage subsets that are found in mouse skin with those that are present in human skin, these studies also provide crucial information for developing intradermal vaccines and for managing inflammatory skin conditions.

Fc receptor-targeting of immunogen as a strategy for enhanced antigen loading, vaccination, and protection using intranasally administered antigen-pulsed dendritic cells

Dendritic cells (DCs) play a critical role in the generation of adaptive immunity via the efficient capture, processing, and presentation of antigen (Ag) to naïve T cells. Administration of Ag-pulsed DCs is also an effective strategy for enhancing immunity to tumors and infectious disease organisms. Studies have also demonstrated that targeting Ags to Fcγ receptors (FcγR) on Ag presenting cells can enhance humoral and cellular immunity in vitro and in vivo. Specifically, our studies using a Francisella tularensis (Ft) infectious disease vaccine model have demonstrated that targeting immunogens to FcγR via intranasal (i.n.) administration of monoclonal antibody (mAb)-inactivated Ft (iFt) immune complexes (ICs) enhances protection against Ft challenge. Ft is the causative agent of tularemia, a debilitating disease of humans and other mammals and a category A biothreat agent for which there is no approved vaccine. Therefore, using iFt Ag as a model immunogen, we sought to determine if ex vivo targeting of iFt to FcγR on DCs would enhance the potency of i.n. administered iFt-pulsed DCs. In this study, bone marrow-derived DCs (BMDCs) were pulsed ex vivo with iFt or mAb-iFt ICs. Intranasal administration of mAb-iFt-pulsed BMDCs enhanced humoral and cellular immune responses, as well as protection against Ft live vaccine strain (LVS) challenge. Increased protection correlated with increased iFt loading on the BMDC surface as a consequence of FcγR-targeting. However, the inhibitory FcγRIIB had no impact on this enhancement. In conclusion, targeting Ag ex vivo to FcγR on DCs provides a method for enhanced Ag loading of DCs ex vivo, thereby reducing the amount of Ag required, while also avoiding the inhibitory impact of FcγRIIB. Thus, this represents a simple and less invasive strategy for increasing the potency of ex vivo-pulsed DC vaccines against chronic infectious diseases and cancer.

Rab5 and Rab4 regulate axon elongation in the Xenopus visual system

The elongation rate of axons is tightly regulated during development. Recycling of the plasma membrane is known to regulate axon extension; however, the specific molecules involved in recycling within the growth cone have not been fully characterized. Here, we investigated whether the small GTPases Rab4 and Rab5 involved in short-loop recycling regulate the extension of Xenopus retinal axons. We report that, in growth cones, Rab5 and Rab4 proteins localize to endosomes, which accumulate markers that are constitutively recycled. Fluorescence recovery after photo-bleaching experiments showed that Rab5 and Rab4 are recruited to endosomes in the growth cone, suggesting that they control recycling locally. Dynamic image analysis revealed that Rab4-positive carriers can bud off from Rab5 endosomes and move to the periphery of the growth cone, suggesting that both Rab5 and Rab4 contribute to recycling within the growth cone. Inhibition of Rab4 function with dominant-negative Rab4 or Rab4 morpholino and constitutive activation of Rab5 decreases the elongation of retinal axons in vitro and in vivo, but, unexpectedly, does not disrupt axon pathfinding. Thus, Rab5- and Rab4-mediated control of endosome trafficking appears to be crucial for axon growth. Collectively, our results suggest that recycling from Rab5-positive endosomes via Rab4 occurs within the growth cone and thereby supports axon elongation.

Transfer of antigen from human B cells to dendritic cells

The cooperation of B lymphocytes with other antigen presenting cells (APCs) is often necessary in the efficient processing and presentation of antigen. Herein, we describe a mechanism by which B cells physically interact with dendritic cells (DCs) resulting in the transfer of B cell receptor (BCR)-enriched antigen to these APCs. Antigen transfer involves direct contact between the two cells followed by the capture of B cell derived membrane and intracellular components. Strikingly, DCs acquire greater amounts of antigen by transfer from B cells than by endocytosis of free antigen. Blocking scavenger receptor A, a DC surface receptor involved in membrane acquisition, abrogates these events. We propose that antigen transfer from B cells to DCs results in a more focused immunologic response due to the selective editing of Ag by the BCR.

Delivery of polysaccharides using polymer particles: implications on size-dependent immunogenicity, opsonophagocytosis, and protective immunity

Bacterial capsular polysaccharides are components of many modern vaccines, but they are weakly immunogenic. Herein, we describe the delivery of pneumococcal capsular polysaccharide serotype-1 (PCP-1) in polylactide polymeric particles to enhance its immunogenicity. Immunization with PCP-1-entrapped particles elicited long-term memory antibody responses from a single intramuscular injection. PCP-1-entrapped nanoparticles (NPs) elicited significantly higher anti-PCP-1 IgG responses than that observed with soluble and microparticles (MPs) formulations. Delivering PCP-1 and pneumococcal proteins in same particles did not improve the IgG response. The sera of animals immunized with PCP-1-entrapped particles promoted efficient opsonophagocytosis of pneumococci by macrophages. Single-dose immunization with PCP-1-entrapped particles conferred a long-term serotype-specific protection against lethal pneumococcal challenge. The higher immunogenicity of PCP-1 nanoparticles showed correlation with enhanced uptake by antigen-presenting cells. The results highlight the potential of polymeric nanoparticles as an efficient means of presenting polysaccharide antigens to the immune system.

The function of Fcγ receptors in dendritic cells and macrophages

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

The antigen presenting cells instruct plasma cell differentiation

The professional antigen presenting cells (APCs), including many subsets of dendritic cells and macrophages, not only mediate prompt but non-specific response against microbes, but also bridge the antigen-specific adaptive immune response through antigen presentation. In the latter, typically activated B cells acquire cognate signals from T helper cells in the germinal center of lymphoid follicles to differentiate into plasma cells (PCs), which generate protective antibodies. Recent advances have revealed that many APC subsets provide not only “signal 1” (the antigen), but also “signal 2” to directly instruct the differentiation process of PCs in a T-cell-independent manner. Herein, the different signals provided by these APC subsets to direct B cell proliferation, survival, class switching, and terminal differentiation are discussed. We furthermore propose that the next generation of vaccines for boosting antibody response could be designed by targeting APCs.

Regulation of the mucosal phenotype in dendritic cells by PPARγ: role of tissue microenvironment

Mucosal DCs play a critical role in tissue homeostasis. Several stimuli can induce a mucosal phenotype; however, molecular pathways that regulate development of mucosal DC function are relatively unknown. This study sought to determine whether PPARγ contributes to the development of the "mucosal" phenotype in mouse DCs. Experiments demonstrated that PPARγ activation in BMDCs induced an immunosuppressive phenotype in which BMDCs had reduced expression of MHC class II and costimulatory molecules, increased IL-10 secretion, and reduced the ability to induce CD4 T cell proliferation. Activation of PPARγ enhanced the ability of BMDC to polarize CD4 T cells toward iTregs and to induce T cell expression of the mucosal homing receptor, CCR9. Activation of PPARγ increased the ability of BMDCs to induce T cell-independent IgA production in B cells. BMDCs from PPARγ(ΔDC) mice displayed enhanced expression of costimulatory molecules, enhanced proinflammatory cytokine production, and decreased IL-10 synthesis. Contrary to the inflammatory BMDC phenotype in vitro, PPARγ(ΔDC) mice showed no change in the frequency or phenotype of mDC in the colon. In contrast, mDCs in the lungs were increased significantly in PPARγ(ΔDC) mice. A modest increase in colitis severity was observed in DSS-treated PPARγ(ΔDC) mice compared with control. These results indicate that PPARγ activation induces a mucosal phenotype in mDCs and that loss of PPARγ promotes an inflammatory phenotype. However, the intestinal microenvironment in vivo can maintain the mucosal DC phenotype of via PPARγ-independent mechanisms.

Controlling immune responses by targeting antigens to dendritic cell subsets and B cells

Delivering antigens in vivo by coupling them to mAbs specific for unique receptors on antigen-presenting cells (APCs) is a promising approach for modulating immune responses. Antigen delivery to receptors found on myeloid dendritic cell (DC) subsets, plasmacytoid DCs and B cells has shown them all to be viable targets to stimulate either the cellular or humoral arms of the immune system. It is now evident that antigen-targeting approaches can also be used to invoke antigen-specific inhibition of immune responses. The outcome of activation versus inhibition is determined by a combination of factors that include the choice of APC, the receptor that is targeted, whether to include an adjuvant and, if so, which adjuvant to employ. In addition to their use as a means to modulate immune responses, antigen-targeting systems are also a useful method to investigate the function of DC subsets and the early mechanistic events that underlie the initiation of both cellular and humoral immune responses. In this review, we focus on the literature surrounding the control of B-cell responses when antigen is delivered to various APC subsets.

Sialylation of Campylobacter jejuni endotoxin promotes dendritic cell-mediated B cell responses through CD14-dependent production of IFN-β and TNF-α

Campylobacter jejuni is the most common bacterial cause of human gastroenteritis and often precedes development of Guillain-Barré syndrome (GBS), a life-threatening paralytic disease. The incorporation of the carbohydrate sialic acid into C. jejuni lipooligosaccharides (LOS) is associated with increased severity of gastroenteritis and with induction of GBS; however, the underlying mechanisms remain completely unknown. In this study, we demonstrate that sialic acids in C. jejuni endotoxin enhance the rapid production of IFN-β and TNF-α by human dendritic cells (DCs). Using neutralizing Abs and receptors it was shown that these DC-derived cytokines promote the proliferation of human mucosal B cells in a T cell-independent manner. The production of both IFN-β and TNF-α by DCs in response to LOS requires CD14, and the amplified response of DCs to sialylated C. jejuni LOS is CD14 dependent. Together, these results indicate that sialylation of C. jejuni LOS increases DC activation and promotes subsequent B cell responses through CD14-driven production of IFN-β and TNF-α. This enhanced DC/B cell response may explain the increased pathogenicity of sialylated C. jejuni and may be key to the initiation of B cell-mediated autoimmunity in GBS.

Overexpression of TLR7 promotes cell-intrinsic expansion and autoantibody production by transitional T1 B cells

Transgenic expression of TLR7 results in the expansion and hyperactivation of T1 B cells in response to endogenous RNA complexes, leading to increased autoantibody production.

Toll-like receptor (TLR), a ligand for single-stranded RNA, has been implicated in the development of pathogenic anti-RNA autoantibodies both in systemic lupus erythematous (SLE) patients and in murine models of lupus. It is still unclear, however, where and how TLR7-mediated interactions affect the development of autoreactive B cells. We found that overexpression of TLR7 in transgenic mice (TLR7.1Tg) leads to marked alterations of transitional (T1) B cells, associated with their expansion and proliferation within the splenic red pulp (RP). This phenotype was intrinsic to the T1 subset of B cells and occurred independently of type 1 IFN signals. Overexpression of RNase in TLR7.1Tg mice significantly limited the expansion and proliferation of T1 cells, indicating that endogenous RNA complexes are driving their activation. TLR7.1Tg T1 cells were hyper-responsive to anti-IgM and TLR7 ligand stimulation in vitro and produced high concentrations of class-switched IgG2b and IgG2c, including anti-RNA antibodies. Our results demonstrate that initial TLR7 stimulation of B cells occurs at the T1 stage of differentiation in the splenic RP and suggest that dysregulation of TLR7 expression in T1 cells can result in production of autoantibodies.

Engineered antibody Fc variant with selectively enhanced FcγRIIb binding over both FcγRIIa(R131) and FcγRIIa(H131)

Engaging inhibitory FcγRIIb by Fc region has been recently reported to be an attractive approach for improving the efficacy of antibody therapeutics. However, the previously reported S267E/L328F variant with enhanced binding affinity to FcγRIIb, also enhances binding affinity to FcγRIIa(R131) allotype to a similar degree because FcγRIIb and FcγRIIa(R131) are structurally similar. In this study, we applied comprehensive mutagenesis and structure-guided design based on the crystal structure of the Fc/FcγRIIb complex to identify a novel Fc variant with selectively enhanced FcγRIIb binding over both FcγRIIa(R131) and FcγRIIa(H131). This novel variant has more than 200-fold stronger binding affinity to FcγRIIb than wild-type IgG1, while binding affinity to FcγRIIa(R131) and FcγRIIa(H131) is comparable with or lower than wild-type IgG1. This selectivity was achieved by conformational change of the C(H)2 domain by mutating Pro to Asp at position 238. Fc variant with increased binding to both FcγRIIb and FcγRIIa induced platelet aggregation and activation in an immune complex form in vitro while our novel variant did not. When applied to agonistic anti-CD137 IgG1 antibody, our variant greatly enhanced the agonistic activity. Thus, the selective enhancement of FcγRIIb binding achieved by our Fc variant provides a novel tool for improving the efficacy of antibody therapeutics.

Recognition of higher order patterns in proteins: immunologic kernels

By applying analysis of the principal components of amino acid physical properties we predicted cathepsin cleavage sites, MHC binding affinity, and probability of B-cell epitope binding of peptides in tetanus toxin and in ten diverse additional proteins. Cross-correlation of these metrics, for peptides of all possible amino acid index positions, each evaluated in the context of a ±25 amino acid flanking region, indicated that there is a strongly repetitive pattern of short peptides of approximately thirty amino acids each bounded by cathepsin cleavage sites and each comprising B-cell linear epitopes, MHC–I and MHC-II binding peptides. Such “immunologic kernel” peptides comprise all signals necessary for adaptive immunologic cognition, response and recall. The patterns described indicate a higher order spatial integration that forms a symbolic logic coordinating the adaptive immune system.

Host-recognition of pathogens and commensals in the mammalian intestine

To peacefully coexist with the microbial inhabitants of the intestine, mammals have evolved elaborate and interconnected regulatory mechanisms to maintain immune homeostasis in the face of potential infection and tissue damage by pathogenic microorganisms. Physical barriers, antimicrobial factors and secretory antibodies act in concert to keep microbes at a distance from the epithelium and initiate repair mechanisms in the event of damage. Commensal bacteria are not ignored but dynamically controlled via many complex overlapping and intertwined mechanisms involving intestinal epithelial cells (IECs) and signals from the microbiota. Polarized IECs play a decisive role in homeostasis by regulating the expression and activity of the pattern-recognition receptors (PRRs), in different compartments of the intestine. The differential signaling and expression of receptors on apical and basal membranes of the epithelium also plays its part in distinguishing commensals from harmful invaders. In steady state conditions macrophages and dendritic cells (DCs) in the lamina propria (LP) are conditioned by environmental factors to induce immune tolerance. The distinction between pathogen and non-pathogen is linked to the ability of pathogens to invade and cause damage to the host cells and tissues. This induces local inflammatory responses and the attraction of capillary leukocytes by chemokines released from colonized and invaded epithelial cells. This bypasses the tolerogenic mechanisms controlling the responses of resident DCs and macrophages leading to pathogen killing and adaptive immune responses. Research on this topic has important implications for the development of novel therapeutic approaches to treat or prevent inflammatory bowel disease (IBD), inflammation-related cancer and other gut-related diseases and disorders.

B-lymphocyte lineage cells and the respiratory system

Adaptive humoral immune responses in the airways are mediated by B cells and plasma cells that express highly evolved and specific receptors and produce immunoglobulins of most isotypes. In some cases, such as autoimmune diseases or inflammatory diseases caused by excessive exposure to foreign antigens, these same immune cells can cause disease by virtue of overly vigorous responses. This review discusses the generation, differentiation, signaling, activation, and recruitment pathways of B cells and plasma cells, with special emphasis on unique characteristics of subsets of these cells functioning within the respiratory system. The primary sensitization events that generate B cells responsible for effector responses throughout the airways usually occur in the upper airways, tonsils, and adenoid structures that make up the Waldeyer ring. On secondary exposure to antigen in the airways, antigen-processing dendritic cells migrate into secondary lymphoid organs, such as lymph nodes, that drain the upper and lower airways, and further B-cell expansion takes place at those sites. Antigen exposure in the upper or lower airways can also drive expansion of B-lineage cells in the airway mucosal tissue and lead to the formation of inducible lymphoid follicles or aggregates that can mediate local immunity or disease.

Microfold (M) cells: important immunosurveillance posts in the intestinal epithelium

The transcytosis of antigens across the gut epithelium by microfold cells (M cells) is important for the induction of efficient immune responses to some mucosal antigens in Peyer's patches. Recently, substantial progress has been made in our understanding of the factors that influence the development and function of M cells. This review highlights these important advances, with particular emphasis on: the host genes which control the functional maturation of M cells; how this knowledge has led to the rapid advance in our understanding of M-cell biology in the steady state and during aging; molecules expressed on M cells which appear to be used as "immunosurveillance" receptors to sample pathogenic microorganisms in the gut; how certain pathogens appear to exploit M cells to infect the host; and finally how this knowledge has been used to specifically target antigens to M cells to attempt to improve the efficacy of mucosal vaccines.

Aberrant B cell selection and activation in systemic lupus erythematosus

The detrimental role of B lymphocytes in systemic lupus erythematosus (SLE) is evident from the high levels of pathogenic antinuclear autoantibodies (ANAs) found in SLE patients. Affirming this causative role, additional antibody-independent roles of B cells in SLE were appreciated. In recent years, many defects in B cell selection and activation have been identified in murine lupus models and SLE patients that explain the increased emergence and persistence of autoreactive B cells and their lowered activation threshold. Therefore, clinical trials with B cell depletion regimens in SLE patients were initiated but disappointingly the efficacy of B cell depleting agents proved to be limited. Remarkably however, a major breakthrough in SLE therapy was accomplished by blocking B cell survival factors rather then eliminating B cells. This surprising finding indicates that although SLE is a B cell-driven disease, the amplifying crosstalk between B cells and other cells of the immune system likely evokes the observed tolerance breakdown in B cells. Moreover, this implies that intelligent interception of pro-inflammatory loops rather then selectively silencing B cells will be key to the development of new SLE therapies. In this review, we will not only highlight the intrinsic B cell defects that facilitate the persistence of autoreactive B cells and their activation, but in addition we will focus on B cell extrinsic signals derived from T cells and innate immune cells that lower the activation threshold for B cells.

Endocytosis and recycling of immune complexes by follicular dendritic cells enhances B cell antigen binding and activation

Stromal-derived follicular dendritic cells (FDCs) are a major reservoir for antigen that are essential for formation of germinal centers, the site where memory and effector B cells differentiate. A long-standing question is how FDCs retain antigen in its native form for extended periods and how they display it to specific B cells. Here we found that FDCs acquired complement-coated immune complexes (ICs) from noncognate B cells via complement receptors 1 and 2 (CD35 and CD21, respectively) and rapidly internalized them by an actin-dependent pathway. ICs were retained intact within a nondegradative cycling compartment and were displayed periodically on the cell surface where they were accessible to antigen-specific B cells. This would explain how antigens are protected from damage and retained over long periods of time, while remaining accessible for B cells.

Dendritic cell-based tumor vaccinations in epithelial ovarian cancer: a systematic review

After decades of extensive research, epithelial ovarian cancer still remains a lethal disease. Multiple new studies have reported that the immune system plays a critical role in the growth and spread of ovarian carcinoma. This review summarizes the development of dendritic cell (DC) vaccinations specific for ovarian cancer. So far, DC-based vaccines have induced effective antitumor responses in animal models, but only limited results from human clinical trials are available. Although DC-based immunotherapy has proven to be clinically safe and efficient at inducing tumor-specific immune responses, its clear role in the therapy of ovarian cancer still needs to be clarified. The relatively disappointing low-response rates in early clinical trials point to the need for the development of more effective and personalized DC-based anticancer vaccines. This article reviews the basic mechanisms, limitations and future directions of DC-based anti-ovarian cancer vaccine development.

Copresentation of intact and processed MHC alloantigen by recipient dendritic cells enables delivery of linked help to alloreactive CD8 T cells by indirect-pathway CD4 T cells

In transplantation, direct-pathway CD8 T cells that recognize alloantigen on donor cells require CD4 help for activation and cytolytic function. The ability of indirect-pathway CD4 T cells to provide this help remains unexplained, because a fundamental requirement for epitope linkage is seemingly broken. The simultaneous presentation, by host dendritic cells (DCs), of both intact MHC class I alloantigen and processed alloantigen would deliver linked help, but has not been demonstrated definitively. In this study, we report that following in vitro coculture with BALB/c DCs, small numbers (~1.5%) of C57BL/6 (B6) DCs presented acquired H-2(d) alloantigen both as processed allopeptide and as unprocessed Ag. This represented class I alloantigen provides a conformational epitope for direct-pathway allorecognition, because B6 DCs isolated from cocultures and transferred to naive B6 mice provoked cytotoxic CD8 T cell alloimmunity. Crucially, this response was dependent upon simultaneous presentation of class II-restricted allopeptide, because despite acquiring similar amounts of H-2(d) alloantigen upon coculture, MHC class II-deficient B6 DCs failed to elicit cytotoxic alloimmunity. The relevance of this pathway to solid-organ transplantation was then confirmed by the demonstration that CD8 T cell cytotoxicity was provoked in secondary recipients by transfer of DCs purified from wild-type, but not from MHC class II-deficient, C57BL/6 recipients of BALB/c heart transplants. These experiments demonstrate that representation of conformationally intact MHC alloantigen by recipient APC can induce cytotoxic alloimmunity, but simultaneous copresentation of processed allopeptide is essential, presumably because this facilitates linked recognition by indirect-pathway CD4 Th cells.

Human FcγRII cytoplasmic domains differentially influence antibody-mediated dengue virus infection

Ab-dependent enhancement (ADE) of dengue virus (DENV) infection is mediated through the interaction of viral immune complexes with FcγRs, with notable efficiency of FcγRII. Most human dengue target cells coexpress activating (FcγRIIa) and inhibitory (FcγRIIb) isoforms, but their relative roles in ADE are not well understood. We studied the effects of FcγRIIa and FcγRIIb by transfecting cells to express each individual receptor isoform or through coexpression of both isoforms. We showed that although both isoforms similarly bind dengue-immune complexes, FcγRIIa efficiently internalized virus leading to productive cellular infection, unlike FcγRIIb. We next focused on the main discriminating feature of these isoforms: their distinct intracytoplasmic tails (FcγRIIa with an immunoreceptor tyrosine-based activation motif [ITAM] and FcγRIIb with an immunoreceptor tyrosine-based inhibitory motif [ITIM]). We engineered cells to express "swapped" versions of their FcγRII by switching the cytoplasmic tails containing the ITAM/ITIM motifs, leaving the remainder of the receptor intact. Our data show that both FcγRIIa and FcγRIIb comparably bind dengue immune complexes. However, wild type FcγRIIa facilitates DENV entry by virtue of the ITAM motif, whereas the swapped version FcγRIIa-ITIM significantly inhibited ADE. Similarly, replacing the inhibitory motif in FcγRIIb with an ITAM (FcγRIIb-ITAM) reconstituted ADE capacity to levels of the wild type activating counterpart, FcγRIIa. Our data suggest that FcγRIIa and FcγRIIb isoforms, as the most abundantly distributed class II Fcγ receptors, differentially influence Ab-mediated DENV infection under ADE conditions both at the level of cellular infection and viral production.

IL-7/anti-IL-7 mAb complexes augment cytokine potency in mice through association with IgG-Fc and by competition with IL-7R

Interleukin-7 (IL-7) is essential to T-cell survival as well as homeostatic proliferation, and clinical trials that exploit the mitogenic effects of IL-7 have achieved success in treating human diseases. In mice, the in vivo potency of IL-7 improves dramatically when it is administered as a complex with the anti-IL-7 neutralizing monoclonal antibody clone M25. However, the mechanism whereby M25 augments IL-7 potency is unknown. We have analyzed the discrete contributions of the antibody constant (Fc) and IL-7-binding (Fab) domains to the mechanism. By engaging the neonatal Fc receptor the Fc domain extends the in vivo lifespan of IL-7/M25 complexes and accounts for the majority of their activity. Unexpectedly, the IL-7-neutralizing Fab domain provides an additional, albeit smaller, contribution, possibly by serving as a cytokine depot. This study is the first to demonstrate that the neutralizing aspect of the monoclonal antibody is directly involved in enhancing the potency of a cytokine with a single form of receptor. Lessons from the mechanism of IL-7/M25 complexes inform the design of next-generation cytokine therapeutics.

Marginal zone B cells: virtues of innate-like antibody-producing lymphocytes

Protective responses to microorganisms involve the nonspecific but rapid defence mechanisms of the innate immune system, followed by the specific but slow defence mechanisms of the adaptive immune system. Located as sentinels at the interface between the circulation and lymphoid tissue, splenic marginal zone B cells rapidly respond to blood-borne antigens by adopting 'crossover' defensive strategies that blur the conventional boundaries of innate and adaptive immunity. This Review discusses how marginal zone B cells function as innate-like lymphocytes that mount rapid antibody responses to both T cell-dependent and T cell-independent antigens. These responses require the integration of activation signals from germline-encoded and somatically recombined receptors for microorganisms with helper signals from effector cells of the innate and adaptive immune systems.

Regulation of frontline antibody responses by innate immune signals

Mature B cells generate protective immunity by undergoing immunoglobulin (Ig) class switching and somatic hypermutation, two Ig gene-diversifying processes that usually require cognate interactions with T cells that express CD40 ligand. This T-cell-dependent pathway provides immunological memory but is relatively slow to occur. Thus, it must be integrated with a faster, T-cell-independent pathway for B-cell activation through CD40 ligand-like molecules that are released by innate immune cells in response to microbial products. Here, we discuss recent advances in our understanding of the interplay between the innate immune system and B cells, particularly "frontline" B cells located in the marginal zone of the spleen and in the intestine.

Prion disease and the innate immune system

Prion diseases or transmissible spongiform encephalopathies are a unique category of infectious protein-misfolding neurodegenerative disorders. Hypothesized to be caused by misfolding of the cellular prion protein these disorders possess an infectious quality that thrives in immune-competent hosts. While much has been discovered about the routing and critical components involved in the peripheral pathogenesis of these agents there are still many aspects to be discovered. Research into this area has been extensive as it represents a major target for therapeutic intervention within this group of diseases. The main focus of pathological damage in these diseases occurs within the central nervous system. Cells of the innate immune system have been proven to be critical players in the initial pathogenesis of prion disease, and may have a role in the pathological progression of disease. Understanding how prions interact with the host innate immune system may provide us with natural pathways and mechanisms to combat these diseases prior to their neuroinvasive stage. We present here a review of the current knowledge regarding the role of the innate immune system in prion pathogenesis.

The specialized roles of immature and mature dendritic cells in antigen cross-presentation

Exogenous antigen cross-presentation is integral to the stimulation of cytotoxic T-lymphocytes against viruses and tumors. Central to this process are dendritic cells (DCs), which specialize in cross-presentation. DCs may be considered to exist in two radically different states of activation, generally referred to as immature and mature. In each of these states, the cell has a series of separate and specialized abilities for the induction of T-cell immunity. In the immature state, the DC is adept in surveying the periphery, acquiring and storing antigen, but has a limited capacity for direct T-cell activation. During a brief and defined window of time following DC stimulation, nearly every aspect of antigen handling changes, as it transitions from an entity focused on protein preservation to one capable of efficient cross-presentation. It is this time period and the underlying molecular mechanisms active here, which form the core of our studies on cross-presentation.

B-cell selection and the development of autoantibodies

The clearest evidence that B cells play an important role in human autoimmunity is that immunotherapies that deplete B cells are very effective treatments for many autoimmune diseases. All people, healthy or ill, have autoreactive B cells, but not at the same frequency. A number of genes influence the level of these autoreactive B cells and whether they are eliminated or not during development at a central checkpoint in the bone marrow (BM) or at a later checkpoint in peripheral lymphoid tissues. These genes include those encoding proteins that regulate signaling through the B-cell receptor complex such as Btk and PTPN22, proteins that regulate innate signaling via Toll-like receptors (TLRs) such as MyD88 and interleukin-1 receptor-associated kinase 4, as well as the gene encoding the activation-induced deaminase (AID) essential for B cells to undergo class switch recombination and somatic hypermutation. Recent studies have revealed that TLR signaling elements and AID function not only in peripheral B cells to help mediate effective antibody responses to foreign antigens, but also in the BM to help remove autoreactive B-lineage cells at a very early point in B-cell development. Newly arising B cells that leave the BM and enter the blood and splenic red pulp can express both AID and TLR signaling elements like TLR7, and thus are fully equipped to respond rapidly to antigens (including autoantigens), to isotype class switch, and to undergo somatic hypermutation. These red pulp B cells may thus be an important source of autoantibody-producing cells arising particularly in extrafollicular sites, and indeed may be as significant a source of autoantibody-producing cells as B cells arising from germinal centers.

Microspectroscopy reveals mechanisms of lymphocyte activation

The immunological synapse (IS) regulates immune responses by integrating extracellular stimuli into intracellular signalling networks, which causes leukocyte differentiation and effector functions. The dynamic spatial organisation of molecules at the IS was initially characterised by wide-field fluorescence microscopy of cell conjugates and cells interacting with planar lipid bilayers. These methods showed stable supramolecular clusters of several microns in size, which were proposed to be responsible for sustained signalling and cell-cell adhesion. The recent emergence of microspectroscopy techniques with higher spatial and temporal resolution nonetheless reveals the complex dynamics of molecular reactions that mediate IS assembly and function. This review describes microspectroscopy-based in vitro experimental approaches for imaging the molecular dynamics at the IS, as well as their contributions and open questions in the field. It also describes experimental methods to obtain quantitative parameters of dynamic biochemical reactions in living cells, and discusses about the important role of quantitative imaging and theoretical science in our understanding of molecular mechanisms underlying lymphocyte activation.

Antigen vehiculization particles based on the Z protein of Junin virus

BACKGROUND

Arenavirus matrix protein Z plays an important role in virus budding and is able to generate enveloped virus-like-particles (VLPs) in absence of any other viral proteins. In these VLPs, Z protein is associated to the plasma membrane inner surface by its myristoyl residue. Budding induction and vesicle formation properties can be exploited to generate enveloped VLPs platform. These structures can be designed to carry specific antigen in the inner side or on the surface of VLPs.Vaccines based on VLPs are a highly effective type of subunit vaccines that mimic the overall structure of virus particles in absence of viral nucleic acid, being noninfectious.In this work we assayed the capacity of Junin Z protein to produce VLPs carrying the green fluorescent protein (eGFP), as a model antigen.

RESULTS

In this report the Junin Z protein ability to produce VLPs from 293T cells and its capacity to deliver a specific antigen (eGFP) fused to Z was evaluated. Confocal microscopy showed a particular membrane bending in cells expressing Z and a spot welded distribution in the cytoplasm. VLPs were detected by TEM (transmission electron microscopy) and were purified from cell supernatant. The proteinase protection assay demonstrated the VLPs integrity and the absence of degradation of the fused antigen, thus indicating its internal localization. Finally, immunization of mice with purified VLPs produced high titres of anti-eGFP antibodies compared to the controls.

CONCLUSIONS

It was proved that VLPs can be generated from cells transfected with a fusion Junin virus Z-eGFP protein in absence of any other viral protein, and the capacity of Z protein to support fusions at the C-terminal, without impairing its budding activity, allowing vehiculization of specific antigens into VLPs.

New helping friends for B cells

Over the past decade, a growing recognition of the importance of neutralizing antibodies in host defense combined with the success of B-cell depletion therapies in treating auto-immune disorders has led to an increased focus on better understanding the pathways underpinning B-cell antibody production. In general, B cells require cognate interaction with T helper cells in the germinal center of lymphoid follicles to generate protective antibodies. However, recent evidence shows that B cells receive additional help from invariant natural killer T cells, dendritic cells, and various granulocytes, including neutrophils, eosinophils, and basophils. These innate immune cells enhance T-cell-dependent antibody responses by delivering B-cell helper signals both in the germinal center and at postgerminal center lymphoid sites such as the bone marrow. In addition to enhancing and complementing the B-cell helper activity of canonical T cells, invariant natural killer T cells, dendritic cells, and granulocytes can deliver T cell-independent B-cell helper signals at the mucosal interface and in the marginal zone of the spleen to initiate rapid innate-like antibody responses. Here, we discuss recent advances in the role of adaptive and innate B-cell helper signals in antibody diversification and production.

Study of avidity of antigen-specific antibody as a means of understanding development of long-term immunological memory after Vibrio cholerae O1 infection

The avidity of antibodies to specific antigens and the relationship of avidity to memory B cell responses to these antigens have not been studied in patients with cholera or those receiving oral cholera vaccines. We measured the avidity of antibodies to cholera toxin B subunit (CTB) and Vibrio cholerae O1 lipopolysaccharide (LPS) in Bangladeshi adult cholera patients (n = 30), as well as vaccinees (n = 30) after administration of two doses of a killed oral cholera vaccine. We assessed antibody and memory B cell responses at the acute stage in patients or prior to vaccination in vaccinees and then in follow-up over a year. Both patients and vaccinees mounted CTB-specific IgG and IgA antibodies of high avidity. Patients showed longer persistence of these antibodies than vaccinees, with persistence lasting in patients up to day 270 to 360. The avidity of LPS-specific IgG and IgA antibodies in patients remained elevated up to 180 days of follow-up. Vaccinees mounted highly avid LPS-specific antibodies at day 17 (3 days after the second dose of vaccine), but the avidity waned rapidly to baseline by 30 days. We examined the correlation between antigen-specific memory B cell responses and avidity indices for both antigens. We found that numbers of CTB- and LPS-specific memory B cells significantly correlated with the avidity indices of the corresponding antibodies (P < 0.05; Spearman's ρ = 0.28 to 0.45). These findings suggest that antibody avidity after infection and immunization is a good correlate of the development and maintenance of memory B cell responses to Vibrio cholerae O1 antigens.

New concepts in the generation and functions of IgA

The intestinal mucosa contains the largest population of antibody-secreting plasma cells in the body, and in humans several grams of secretory immunoglobulin A (SIgA) are released into the intestine each day. In the gut lumen, SIgA serves as a first-line barrier that protects the epithelium from pathogens and toxins. Recently, next-generation sequencing has revolutionized our understanding of the nature of the intestinal microbiota and has also shed new light on the important roles of SIgA in the regulation of host-commensal homeostasis. Here, I discuss pathways of IgA induction in the context of SIgA specificity and function.

Antigen-specific suppression of humoral immunity by anergic Ars/A1 B cells

Autoreactive anergic B lymphocytes are considered to be dangerous because of their potential for activation and recruitment into autoimmune responses. However, they persist for days and constitute ∼5% of the B cell pool. We assessed their functional potential in the Ars/A1 transgene model, where anergic B cells express a dual-reactive Ag receptor that binds, in addition to a self-Ag, the hapten p-azophenylarsonate (Ars). When Ars/A1 B cells were transferred into adoptive recipients that were immunized with foreign proteins covalently conjugated with Ars, endogenous IgG immune responses to both were selectively and severely diminished, and the development of T helper cells was impaired. Approximately 95% inhibition of the anti-Ars response was attained with ∼4000 transferred Ars/A1 B cells through redundant mechanisms, one of which depended on their expression of MHC class II but not upon secretion of IL-10 or IgM. This Ag-specific suppressive activity implicates the autoreactive anergic B cell as an enforcer of immunological tolerance to self-Ags.

Regulation of B-cell responses by Toll-like receptors

The discovery of host-encoded gene products that sense molecular patterns in infectious microbes, and the demonstration of their role in triggering innate and adaptive immune responses, has been a key milestone in our understanding of immunology. Twenty-three years after Janeway first outlined the fundamental concepts of the 'pattern recognition' model, and 15 years since the identification of Toll-like receptors (TLRs) as pattern recognition receptors (PRRs), new insights continue to be revealed, and questions remain. For example, innate immune responses to microbes that are mediated by PRRs have historically been viewed as the domain of innate immune cell populations such as dendritic cells and macrophages. New evidence, however, has pointed to the role of B-cell-intrinsic TLR activation in shaping antibody responses. These studies have revealed that TLRs regulate a complex transcriptional network that controls multiple steps in the development of antigen-specific antibodies. This review covers these recent developments regarding the role of TLRs in B-cell gene expression and function in vitro and in vivo, and highlights the remaining challenges in the field, with particular emphasis on the role of TLRs in antibody responses to viral infection. A more complete understanding of how TLRs regulate antibody responses will lead to improved vaccine design.

Extrafollicular B cell activation by marginal zone dendritic cells drives T cell-dependent antibody responses

DCIR2⁺ DCs can initiate extrafollicular B cell responses to T cell–dependent antigen.

Dendritic cells (DCs) are best known for their ability to activate naive T cells, and emerging evidence suggests that distinct DC subsets induce specialized T cell responses. However, little is known concerning the role of DC subsets in the initiation of B cell responses. We report that antigen (Ag) delivery to DC-inhibitory receptor 2 (DCIR2) found on marginal zone (MZ)–associated CD8α⁻ DCs in mice leads to robust class-switched antibody (Ab) responses to a T cell–dependent (TD) Ag. DCIR2⁺ DCs induced rapid up-regulation of multiple B cell activation markers and changes in chemokine receptor expression, resulting in accumulation of Ag-specific B cells within extrafollicular splenic bridging channels as early as 24 h after immunization. Ag-specific B cells primed by DCIR2⁺ DCs were remarkably efficient at driving naive CD4 T cell proliferation, yet DCIR2-induced responses failed to form germinal centers or undergo affinity maturation of serum Ab unless toll-like receptor (TLR) 7 or TLR9 agonists were included at the time of immunization. These results demonstrate DCIR2⁺ DCs have a unique capacity to initiate extrafollicular B cell responses to TD Ag, and thus define a novel division of labor among splenic DC subsets for B cell activation during humoral immune responses.

Formation of BCR oligomers provides a mechanism for B cell affinity discrimination

B cells encounter antigen over a wide affinity range, from K(A)=10(5) M(-1) to K(A)=10(10) M(-1). The strength of B cell antigen receptor (BCR) signaling in response to antigen increases with affinity, a process known as "affinity discrimination". In this work, we use a computational simulation of B cell surface dynamics and membrane-proximal signaling to show that affinity discrimination can arise from the formation of BCR oligomers. It is known that BCRs form oligomers upon encountering antigen, and that the size and rate of formation of these oligomers both increase with affinity. In our simulation, we have introduced a requirement that only BCR-antigen complexes that are part of an oligomer can engage cytoplasmic signaling molecules such as Src-family kinases. Our simulation shows that as affinity increases, BCR signaling activity increases in addition to the number of collected antigen. Our results are also consistent with the existence of an experimentally-observed threshold affinity of activation at K(A)=10(5)-10(6) M(-1) (no signaling activity below this affinity value) and affinity discrimination ceiling of K(A)=10(10) M(-1) (no affinity discrimination above this affinity value). Comparison with experiments shows that the time scale of BCR oligomer formation predicted by our model (less than 10 s) is well within the time scale of experimentally observed association of BCR with Src-family kinases (10-20 s).

Prion pathogenesis and secondary lymphoid organs (SLO): tracking the SLO spread of prions to the brain

Prion diseases are subacute neurodegenerative diseases that affect humans and a range of domestic and free-ranging animal species. These diseases are characterized by the accumulation of PrP^Sc, an abnormally folded isoform of the cellular prion protein (PrP^C), in affected tissues. The pathology during prion disease appears to occur almost exclusively within the central nervous system. The extensive neurodegeneration which occurs ultimately leads to the death of the host. An intriguing feature of the prion diseases, when compared with other protein-misfolding diseases, is their transmissibility. Following peripheral exposure, some prion diseases accumulate to high levels within lymphoid tissues. The replication of prions within lymphoid tissue has been shown to be important for the efficient spread of disease to the brain. This article describes recent progress in our understanding of the cellular mechanisms that influence the propagation of prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. A thorough understanding of these events will lead to the identification of important targets for therapeutic intervention, or alternatively, reveal additional processes that influence disease susceptibility to peripherally-acquired prion diseases.

Activation of B cells by non-canonical helper signals

Cognate interaction between T and B lymphocytes of the adaptive immune system is essential for the production of high-affinity antibodies against microbes, and for the establishment of long-term immunological memory. Growing evidence shows that--in addition to presenting antigens to T and B cells--macrophages, dendritic cells and other cells of the innate immune system provide activating signals to B cells, as well as survival signals to antibody-secreting plasma cells. Here, we discuss how these innate immune cells contribute to the induction of highly diversified and temporally sustained antibody responses, both systemically and at mucosal sites of antigen entry.

Dendritic cells and their role in tumor immunosurveillance

Dendritic cells (DCs) comprise a heterogeneous population of cells that play a key role in initiating, directing and regulating adaptive immune responses, including those critically involved in tumor immunosurveillance. As a riposte to the central role of DCs in the generation of antitumor immune responses, tumors have developed various mechanisms which impair the immunostimulatory functions of DCs or even instruct them to actively contribute to tumor growth and progression. In the first part of this review we discuss general aspects of DC biology, including their origin, subtypes, immature and mature states, and functional plasticity which ensures a delicate balance between active immune response and immune tolerance. In the second part of the review we discuss the complex interactions between DCs and the tumor microenvironment, and point out the challenges faced by DCs during the recognition of tumor Ags. We also discuss the role of DCs in tumor angiogenesis and vasculogenesis.

The diverse roles of mononuclear phagocytes in prion disease pathogenesis

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are neurological diseases that can be transmitted through a number of different routes. A wide range of mammalian species are affected by the disease. After peripheral exposure, some TSE agents accumulate in lymphoid tissues at an early stage of disease prior to spreading to the nerves and the brain. Much research has focused on identifying the cells and molecules involved in the transmission of TSE agents from the site of exposure to the brain and several crucial cell types have been associated with this process. The identification of the key cells that influence the different stages of disease transmission might identify targets for therapeutic intervention. This review highlights the involvement of mononuclear phagocytes in TSE disease. Current data suggest these cells may exhibit a diverse range of roles in TSE disease from the transport or destruction of TSE agents in lymphoid tissues, to mediators or protectors of neuropathology in the brain.

Cancer immunotherapy via dendritic cells

Cancer immunotherapy attempts to harness the power and specificity of the immune system to treat tumours. The molecular identification of human cancer-specific antigens has allowed the development of antigen-specific immunotherapy. In one approach, autologous antigen-specific T cells are expanded ex vivo and then re-infused into patients. Another approach is through vaccination; that is, the provision of an antigen together with an adjuvant to elicit therapeutic T cells in vivo. Owing to their properties, dendritic cells (DCs) are often called 'nature's adjuvants' and thus have become the natural agents for antigen delivery. After four decades of research, it is now clear that DCs are at the centre of the immune system owing to their ability to control both immune tolerance and immunity. Thus, DCs are an essential target in efforts to generate therapeutic immunity against cancer.

Tolerogenic dendritic cells as a target for the therapy of immune thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disease in which platelets are destroyed by special antiplatelet autoantibodies produced by B cells. Dendritic cells (DCs) are professional antigen-presenting cells involved in humoral immunity and cellular immunity and among them DCs that induce autoimmune tolerance are called tolerogenic DCs (tDCs). As a promising immunotherapeutic strategy for ITP, tDCs have received increasing attention. In this review, we describe the significant role of DCs in regulating autoimmune balances, introduce the manipulation strategies to generate tDCs, summarize recent progress on the experimental application of tDCs for ITP therapy, and finally discuss the perspectives of tolerogenic vaccination for ITP treatment in the clinic.

Influence of dendritic cells on B-cell responses during HIV infection

Dendritic cells (DCs) modulate B-cell differentiation, activation, and survival mainly through production of growth factors such as B lymphocyte stimulator (BLyS/BAFF). DC populations have been reported to be affected in number, phenotype and function during HIV infection and such alterations may contribute to the dysregulation of the B-cell compartment. Herein, we reflect on the potential impact of DC on the pathogenesis of HIV-related B cell disorders, and how DC status may modulate the outcome of mucosal B cell responses against HIV, which are pivotal to the control of disease. A concept that could be extrapolated to the overall outcome of HIV disease, whereby control versus progression may reside in the host's capacity to maintain DC homeostasis at mucosal sites, where DC populations present an inherent capacity of modulating the balance between tolerance and protection, and are amongst the earliest cell types to be exposed to the virus.