A quantitative analysis of antigen-presenting cell function: activated B cells stimulate naive CD4 T cells but are inferior to dendritic cells in providing costimulation

Leishmaniasis: Immune Cells Crosstalk in Macrophage Polarization

Leishmaniasis is a complex infectious parasitic disease caused by protozoa of the genus Leishmania, belonging to a group of neglected tropical diseases. It establishes significant global health challenges, particularly in socio-economically disadvantaged regions. Macrophages, as innate immune cells, play a crucial role in initiating the inflammatory response against the pathogens responsible for this disease. Macrophage polarization, the process of differentiating macrophages into pro-inflammatory (M1) or anti-inflammatory (M2) phenotypes, is essential for the immune response in leishmaniasis. The M1 phenotype is associated with resistance to Leishmania infection, while the M2 phenotype is predominant in susceptible environments. Notably, various immune cells, including T cells, play a significant role in modulating macrophage polarization by releasing cytokines that influence macrophage maturation and function. Furthermore, other immune cells can also impact macrophage polarization in a T-cell-independent manner. Therefore, this review comprehensively examines macrophage polarization's role in leishmaniasis and other immune cells' potential involvement in this intricate process.

Challenges and opportunities in gene editing of B cells

B cells have long been an underutilized target in immune cell engineering, despite a number of unique attributes that could address longstanding challenges in medicine. Notably, B cells evolved to secrete large quantities of antibodies for prolonged periods, making them suitable platforms for long-term protein delivery. Recent advances in gene editing technologies, such as CRISPR-Cas, have improved the precision and efficiency of engineering and expanded potential applications of engineered B cells. While most work on B cell editing has focused on ex vivo modification, a body of recent work has also advanced the possibility of in vivo editing applications. In this review, we will discuss both past and current approaches to B cell engineering, and its promising applications in immunology research and therapeutic gene editing.

Therapeutic targets and biomarkers of tumor immunotherapy: response versus non-response

Cancers are highly complex diseases that are characterized by not only the overgrowth of malignant cells but also an altered immune response. The inhibition and reprogramming of the immune system play critical roles in tumor initiation and progression. Immunotherapy aims to reactivate antitumor immune cells and overcome the immune escape mechanisms of tumors. Represented by immune checkpoint blockade and adoptive cell transfer, tumor immunotherapy has seen tremendous success in the clinic, with the capability to induce long-term regression of some tumors that are refractory to all other treatments. Among them, immune checkpoint blocking therapy, represented by PD-1/PD-L1 inhibitors (nivolumab) and CTLA-4 inhibitors (ipilimumab), has shown encouraging therapeutic effects in the treatment of various malignant tumors, such as non-small cell lung cancer (NSCLC) and melanoma. In addition, with the advent of CAR-T, CAR-M and other novel immunotherapy methods, immunotherapy has entered a new era. At present, evidence indicates that the combination of multiple immunotherapy methods may be one way to improve the therapeutic effect. However, the overall clinical response rate of tumor immunotherapy still needs improvement, which warrants the development of novel therapeutic designs as well as the discovery of biomarkers that can guide the prescription of these agents. Learning from the past success and failure of both clinical and basic research is critical for the rational design of studies in the future. In this article, we describe the efforts to manipulate the immune system against cancer and discuss different targets and cell types that can be exploited to promote the antitumor immune response.

Intestinal Immune System and Amplification of Mouse Mammary Tumor Virus

Mouse mammary tumor virus (MMTV) is a virus that induces breast cancer in mice. During lactation, MMTV can transmit from mother to offspring through milk, and Peyer’s patches (PPs) in mouse intestine are the first and specific target organ. MMTV can be transported into PPs by microfold cells and then activate antigen-presenting cells (APCs) by directly binding with Toll-like receptors (TLRs) whereas infect them through mouse transferrin receptor 1 (mTfR1). After being endocytosed, MMTV is reversely transcribed and the cDNA inserts into the host genome. Superantigen (SAg) expressed by provirus is presented by APCs to cognate CD4⁺ T cells via MHCII molecules to induce SAg response, which leads to substantial proliferation and recruitment of related immune cells. Both APCs and T cells can be infected by MMTV and these extensively proliferated lymphocytes and recruited dendritic cells act as hotbeds for viral replication and amplification. In this case, intestinal lymphatic tissues can actually become the source of infection for the transmission of MMTV in vivo, which results in mammary gland infection by MMTV and eventually lead to the occurrence of breast cancer.

Canine peripheral blood mononuclear cell-derived B lymphocytes pretreated with lipopolysaccharide enhance the immunomodulatory effect through macrophage polarization

BACKGROUND

Preconditioning with lipopolysaccharide (LPS) is used to improve the secretion of anti-inflammatory agents in B cells. However, there are only a few studies on canine B cells.

OBJECTIVE

This study aimed to evaluate the immune regulatory capacity of canine peripheral blood mononuclear cell-derived B cells pretreated with LPS.

METHODS

Canine B cells were isolated from canine peripheral blood mononuclear cells, which were obtained from three healthy canine donors. The B cells were preconditioned with LPS, and then cell viability and the expression of the regulatory B cell marker were assessed. Finally, RNA extraction and immunofluorescence analysis were performed.

RESULTS

LPS primed B cells expressed the interleukin (IL)-10 surface marker and immunoregulatory gene expression, such as IL-10, programmed death-ligand 1, and transforming growth factor beta. Macrophages in the inflammatory condition cocultured with primed B cells were found to have significantly down-regulated pro-inflammatory cytokine, such as tumor necrosis factor-α, and up-regulated anti-inflammatory cytokines such as IL-10. Additionally, it was revealed that co-culture with primed B cells re-polarized M1 macrophages to M2 macrophages.

CONCLUSIONS

This study revealed that LPS-primed B cells have an anti-inflammatory effect and can re-polarize macrophages, suggesting the possibility of using LPS-primed B cells as a therapeutic agent for its anti-inflammatory effects and immune modulation.

B cells are sufficient to prime the dominant CD4+ Tfh response to Plasmodium infection

Arroyo and Pepper demonstrate that interactions with B cells, not dendritic cells, are required for the priming of the CD4⁺ T cell response during Plasmodium infection. This results in a Tfh-biased response as reported by others in both mice and humans.

CD4⁺ T follicular helper (Tfh) cells dominate the acute response to a blood-stage Plasmodium infection and provide signals to direct B cell differentiation and protective antibody expression. We studied antigen-specific CD4⁺ Tfh cells responding to Plasmodium infection in order to understand the generation and maintenance of the Tfh response. We discovered that a dominant, phenotypically stable, CXCR5⁺ Tfh population emerges within the first 4 d of infection and results in a CXCR5⁺ CCR7⁺ Tfh/central memory T cell response that persists well after parasite clearance. We also found that CD4⁺ T cell priming by B cells was both necessary and sufficient to generate this Tfh-dominant response, whereas priming by conventional dendritic cells was dispensable. This study provides important insights into the development of CD4⁺ Tfh cells during Plasmodium infection and highlights the heterogeneity of antigen-presenting cells involved in CD4⁺ T cell priming.

B cells as antigen-presenting cells in transplantation rejection and tolerance

Transplantation of fully allogeneic organs into immunocompetent recipients invariably elicits T cell and B cell responses that lead to the production of donor-specific antibodies (DSA). When immunosuppression is inadequate donor-specific T cell and B cell responses escape, leading to T cell-mediated rejection (TCMR), antibody mediated (ABMR) rejection, or mixed rejection (MR) exhibiting features of both TCMR and ABMR. Current literature suggests that ABMR is a major cause of late graft loss, and that new therapies to curtail the donor-specific humoral response are necessary. The majority of research into B cell responses elicited by allogeneic allografts in both preclinical models and clinical studies, has focused on the function of B cells as antibody-secreting cells and the pathogenic effects of DSA as mediators of ABMR. However, it has long been recognized that the DSA response to allografts is T cell-dependent, and that B cells engage in cognate interactions with T cells that provide "help" and promote B cell differentiation into antibody-secreting cells (ASCs). This review focusses the function of B cells as antigen-presenting cells (APCs) to T cells in lymphoid organs, how they may be critical APCs to T cell in the allograft, and the functional consequences of these interactions.

Co-expression of CD40L with CD70 or OX40L increases B-cell viability and antitumor efficacy

Activated B-cells are a promising alternative source of antigen-presenting cells. They can generally be obtained in sufficient numbers for clinical use, but in most instances produce weak immune responses and therapeutic effects that are suboptimal for use in therapeutic cancer vaccines. To improve the immunogenic potency and therapeutic efficacy of B-cell-based vaccines, ex vivo-activated B-cells were transduced with recombinant lentiviruses in order to express additional costimulatory ligands—CD40L, CD70, OX40L, or 4-1BBL—either individually or in pairs (CD70/CD40L, OX40L/CD40L, or 4-1BBL/CD40L). We observed that the expression of CD40L molecules on B-cells was crucial for T-cell priming and activation. Administration of B-cells co-expressing CD40L with the other costimulatory ligands provided substantial antigen-specific CD8 T-cell responses capable of provoking in vivo proliferation and potent cytolytic activities. Notably, expression of CD40L augmented B-cell viability by inhibiting apoptosis through upregulated expression of the anti-apoptotic molecules BCL2, Bcl-xL and Bax. B-cells co-expressing CD40L with CD70, OX40L, or 4-1BBL induced potent therapeutic antitumor effects in a B16 melanoma model. Moreover, the combination of genetically-modified B-cell vaccines with programmed cell death-1 blockade potentiated the therapeutic efficacy. These results suggest that B-cells endowed with additional costimulatory ligands enable the design of effective vaccination strategies against cancer.

The Deadly Dance of B Cells with Trypanosomatids

B cells are notorious actors for the host's protection against several infectious diseases. So much so that early vaccinology seated its principles upon their long-term protective antibody secretion capabilities. Indeed, there are many examples of acute infectious diseases that are combated by functional humoral responses. However, some chronic infectious diseases actively induce immune deregulations that often lead to defective, if not deleterious, humoral immune responses. In this review we summarize how Leishmania and Trypanosoma spp. directly manipulate B cell responses to induce polyclonal B cell activation, hypergammaglobulinemia, low-specificity antibodies, limited B cell survival, and regulatory B cells, contributing therefore to immunopathology and the establishment of persistent infections.

Epstein-Barr virus microRNAs reduce immune surveillance by virus-specific CD8+ T cells

Infection with Epstein-Barr virus (EBV) affects most humans worldwide and persists life-long in the presence of robust virus-specific T-cell responses. In both immunocompromised and some immunocompetent people, EBV causes several cancers and lymphoproliferative diseases. EBV transforms B cells in vitro and encodes at least 44 microRNAs (miRNAs), most of which are expressed in EBV-transformed B cells, but their functions are largely unknown. Recently, we showed that EBV miRNAs inhibit CD4⁺ T-cell responses to infected B cells by targeting IL-12, MHC class II, and lysosomal proteases. Here we investigated whether EBV miRNAs also counteract surveillance by CD8⁺ T cells. We have found that EBV miRNAs strongly inhibit recognition and killing of infected B cells by EBV-specific CD8⁺ T cells through multiple mechanisms. EBV miRNAs directly target the peptide transporter subunit TAP2 and reduce levels of the TAP1 subunit, MHC class I molecules, and EBNA1, a protein expressed in most forms of EBV latency and a target of EBV-specific CD8⁺ T cells. Moreover, miRNA-mediated down-regulation of the cytokine IL-12 decreases the recognition of infected cells by EBV-specific CD8⁺ T cells. Thus, EBV miRNAs use multiple, distinct pathways, allowing the virus to evade surveillance not only by CD4⁺ but also by antiviral CD8⁺ T cells.

Presentation of high antigen-dose by splenic B220(lo) B cells fosters a feedback loop between T helper type 2 memory and antibody isotype switching

Effective humoral immunity ensues when antigen presentation by B cells culminates in productive cooperation with T lymphocytes. This collaboration, however, remains ill-defined because naive antigen-specific B cells are rare and difficult to track in vivo. Herein, we used a defined transfer model to examine how B lymphocytes, as antigen-presenting cells, shape the development of T-cell memory suitable for generation of relevant antibody responses. Specifically, we examined how B cells presenting different doses of antigen during the initial priming phase shape the development of CD4 T-cell memory and its influence on humoral immunity. The findings indicate that B cells presenting low dose of antigen favour the development of T helper type 1 (Th1) type memory, while those presenting a high antigen dose yielded better Th2 memory cells. The memory Th2 cells supported the production of antibodies by effector B cells and promoted isotype switching to IgG1. Moreover, among the B-cell subsets tested for induction of Th2 memory, the splenic but not peritoneal B220(lo) cells were most effective in sustaining Th2 memory development as well as immunoglobulin isotype switching, and this function involved a tight control by programmed death 1-programmed death ligand 2 interactions.

Efficient induction of cross-presentating human B cell by transduction with human adenovirus type 7 vector

Although human autologous B cells represent a promising alternative to dendritic cells (DCs) for easy large-scale preparation, the naive human B cells are always poor at antigen presentation. The safe and effective usage record of human adenovirus type 7 (HAdV7) live vaccines makes it attractive as a promising vaccine vector candidate. To investigate whether HAdV7 vector could be used to induce the human B cells cross-presentation, in the present study, we constructed the E3-defective recombinant HAdV7 vector encoding green fluorescent protein (GFP) and carcinoembryonic antigen (CEA). We demonstrated that naive human B cells can efficiently be transduced, and that the MAPKs/NF-κB pathway can be activated by recombinant HAdV7. We proved that cytokine TNF-α, IL-6 and IL-10, surface molecule MHC class I and the CD86, antigen-processing machinery (APM) compounds ERp57, TAP-1, and TAP-2. were upregulated in HAdV7 transduced human B cells. We also found that CEA-specific IFNγ expression, degranulation, and in vitro and ex vivo cytotoxicities are induced in autologous CD8(+) T cells presensitized by HAd7CEA modified human B cells. Meanwhile, our evidences clearly show that Toll-like receptors 9 (TLR9) antagonist IRS 869 significantly eliminated most of the HAdV7 initiated B cell activation and CD8(+) T cells response, supporting the role and contribution of TLR9 signaling in HAdV7 induced human B cell cross-presentation. Besides a better understanding of the interactions between recombinant HAdV7 and human naive B cells, to our knowledge, the present study provides the first evidence to support the use of HAdV7-modified B cells as a vehicle for vaccines and immunotherapy.

Naive idiotope-specific B and T cells collaborate efficiently in the absence of dendritic cells

Anti-idiotope (anti-Id) Abs have a role in therapy against B cell lymphomas, as inhibitors of pathogenic autoantibodies, and as surrogate Ags for immunization. Despite these observations, the mechanism by which Id(+) Ig generates anti-Id Abs is essentially unknown. To address this issue, we generated a double knock-in mouse that expresses V regions of a somatically mutated anti-Id mAb with intermediate affinity (affinity constant [Ka] = 0.77 × 10(7) M(-1)) for the myeloma protein M315. The anti-Id mice have normal peripheral B cell populations, and allelic exclusion is efficient. Anti-Id B cells from BCR knock-in mice, together with Id-specific CD4(+) T cells from previously established TCR-transgenic mice, enabled us to study Id-specific T cell-B cell collaboration by dilution of transferred cells into syngeneic BALB/c recipients. We show that previously unstimulated (naive) Id-specific B and T cells collaborate efficiently in vivo, even at low frequencies and in the presence of low amounts of Id(+) Ig, resulting in germinal center formation, plasma cell development, and secretion of isotype-switched anti-Id Abs. We further demonstrate that Id-specific T cell-B cell collaboration occurs readily in the absence of adjuvant and is not dependent on Id-presentation by dendritic cells. The results underscore the potency of anti-Id B cells in MHC class II-restricted presentation of Id(+) Ig and suggest that Id-specific T cell-B cell collaboration is of physiological relevance.

Transient depletion of CD4+ CD25+ regulatory T cells results in multiple autoimmune diseases in wild-type and B-cell-deficient NOD mice

Approximately 80% of female wild-type non-obese diabetic (WT NOD) mice spontaneously develop diabetes, whereas B-cell-deficient (B(-/-)) NOD mice are resistant to diabetes. B(-/-) mice are also resistant to other spontaneous and experimentally induced autoimmune diseases, including arthritis, systemic lupus erythematosus, Sjögren syndrome and thyroiditis. Under normal conditions, activation of self-reactive T cells in the periphery is limited by CD4(+) CD25(+) natural regulatory T (Treg) cells. B(-/-) NOD.H-2h4 mice, normally resistant to spontaneous autoimmune thyroiditis (SAT), develop SAT when Treg cells are depleted, suggesting that Treg cells are preferentially activated when autoantigen is initially presented by non-B-cell antigen-presenting cells. To test the hypothesis that increased Treg cell activity in B(-/-) mice contributes to their resistance to other autoimmune diseases, WT and B(-/-) NOD mice were given anti-CD25 to transiently deplete CD4(+) CD25(+) Treg cells. The WT and B(-/-) NOD mice given anti-CD25 developed diabetes much earlier than WT mice given rat IgG, whereas rat IgG-treated B(-/-) mice did not develop diabetes. Treg-cell-depleted mice had increased lymphocyte infiltration of the pancreas, salivary glands and thyroid compared with controls given rat IgG. These results are consistent with the hypothesis that resistance of B-cell-deficient NOD mice to several autoimmune diseases is due to the activity of Treg cells.

B cell infection and activation by rabies virus-based vaccines

Replication-deficient rabies viruses (RABV) are promising rabies postexposure vaccines due to their prompt and potent stimulation of protective virus neutralizing antibody titers, which are produced in mice by both T-dependent and T-independent mechanisms. To promote such early and robust B cell stimulation, we hypothesized that live RABV-based vaccines directly infect B cells, thereby activating a large pool of antigen-presenting cells (APCs) capable of providing early priming and costimulation to CD4(+) T cells. In this report, we show that live RABV-based vaccine vectors efficiently infect naive primary murine and human B cells ex vivo. Infection of B cells resulted in the significant upregulation of early markers of B cell activation and antigen presentation, including CD69, major histocompatibility complex class II (MHC-II), and CD40 in murine B cells or HLA-DR and CD40 in human B cells compared to mock-infected cells or cells treated with an inactivated RABV-based vaccine. Furthermore, primary B cells infected with a live RABV expressing ovalbumin were able to prime and stimulate naive CD4(+) OT-II T cells to proliferate and to secrete interleukin-2 (IL-2), demonstrating a functional consequence of B cell infection and activation by live RABV-based vaccine vectors. We propose that this direct B cell stimulation by live RABV-based vaccines is a potential mechanism underlying their induction of early protective T cell-dependent B cell responses, and that designing live RABV-based vaccines to infect and activate B cells represents a promising strategy to develop a single-dose postexposure rabies vaccine where the generation of early protective antibody titers is critical.

Engineering B cells with mRNA

Ex vivo activated B cells are an alternative source of antigen presenting cells (APC). However, the ability of ex vivo activated B cells to function as potent APCs has been a concern especially when compared to dendritic cells (DC). Herein, we introduce a strategy to modulate antigen presentation and immune stimulation functions of activated B cells by co-transfection with multiple mRNAs encoding costimulatory molecules (OX40L, 4-1BBL, and CD80), cytokines (IL-12p35 and IL-12p40) and antigen. These activated B cells modified to express immune stimulatory molecules can be a potent alternative to DCs in immunotherapy.

B-cell targeted therapeutics in clinical development

B lymphocytes are the source of humoral immunity and are thus a critical component of the adaptive immune system. However, B cells can also be pathogenic and the origin of disease. Deregulated B-cell function has been implicated in several autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis, and multiple sclerosis. B cells contribute to pathological immune responses through the secretion of cytokines, costimulation of T cells, antigen presentation, and the production of autoantibodies. DNA-and RNA-containing immune complexes can also induce the production of type I interferons, which further promotes the inflammatory response. B-cell depletion with the CD20 antibody rituximab has provided clinical proof of concept that targeting B cells and the humoral response can result in significant benefit to patients. Consequently, the interest in B-cell targeted therapies has greatly increased in recent years and a number of new biologics exploiting various mechanisms are now in clinical development. This review provides an overview on current developments in the area of B-cell targeted therapies by describing molecules and subpopulations that currently offer themselves as therapeutic targets, the different strategies to target B cells currently under investigation as well as an update on the status of novel therapeutics in clinical development. Emerging data from clinical trials are providing critical insight regarding the role of B cells and autoantibodies in various autoimmune conditions and will guide the development of more efficacious therapeutics and better patient selection.

The role of B cells and humoral immunity in Mycobacterium tuberculosis infection

Tuberculosis (TB) remains a serious threat to public health, causing 2 million deaths annually world-wide. The control of TB has been hindered by the requirement of long duration of treatment involving multiple chemotherapeutic agents, the increased susceptibility to Mycobacterium tuberculosis infection in the HIV-infected population, and the development of multi-drug resistant and extensively resistant strains of tubercle bacilli. An efficacious and cost-efficient way to control TB is the development of effective anti-TB vaccines. This measure requires thorough understanding of the immune response to M. tuberculosis. While the role of cell-mediated immunity in the development of protective immune response to the tubercle bacillus has been well established, the role of B cells in this process is not clearly understood. Emerging evidence suggests that B cells and humoral immunity can modulate the immune response to various intracellular pathogens, including M. tuberculosis. These lymphocytes form conspicuous aggregates in the lungs of tuberculous humans, non-human primates, and mice, which display features of germinal center B cells. In murine TB, it has been shown that B cells can regulate the level of granulomatous reaction, cytokine production, and the T cell response. This chapter discusses the potential mechanisms by which specific functions of B cells and humoral immunity can shape the immune response to intracellular pathogens in general, and to M. tuberculosis in particular. Knowledge of the B cell-mediated immune response to M. tuberculosis may lead to the design of novel strategies, including the development of effective vaccines, to better control TB.

Two paradoxes and a surprise on the road to an understanding of systemic lupus erythematosus

Whereas systemic lupus erythematosus (SLE) as normally encountered results from the coming together of a complex mix of genetic and environmental factors, SLE also develops in virtually all those rare people who lack a functional gene for the first component of complement (C1q). The pathogenic IgG antibodies against double-stranded DNA characteristic of the disease are made in response to nucleosomes - the package of DNA and histone molecules forming the unit structure of chromatin - which are present in apoptotic cells. Analysis of the C1q phenomenon illuminates the arrangements that are normally in place to ensure tolerance is maintained to nucleosomal antigens. Surprisingly in view of the high level of apoptosis occurring in the thymus, it appears that anti-histone helper T cells, which are likely to be required for IgG anti-DNA production, are not deleted in the thymus. It seems rather that tolerance is maintained by non-availability of antigen brought about by the highly efficient C1q-dependent phagocytosis of apoptotic cells. This 'immunological ignorance' may be backed up by mechanisms of peripheral tolerance if antigen does become available. Idiopathic SLE may arise when apoptotic cell clearance is sub-optimal, making clearance a promising target for therapy.

APCs expressing high levels of programmed death ligand 2 sustain the development of CD4 T cell memory

The role APCs play in the transition of T cells from effector to memory remains largely undefined. This is likely due to the low frequency at which long-lived T cells arise, which hinders analysis of the events involved in memory development. In this study, we used TCR transgenic T cells to increase the frequency of long-lived T cells and developed a transfer model suitable for defining the contribution of APCs to the development of CD4 T cell memory. Accordingly, naive TCR transgenic T cells were stimulated in vitro with Ag presented by different types of APCs and transferred into MHC class II-deficient mice for parking, and the hosts were later analyzed for long-lived T cell frequency or challenged with suboptimal dose of Ag, and the long-lived cells-driven memory responses were measured. The findings indicate that B cells and CD8alpha(+) dendritic cells sustained elevated frequencies of long-lived T cells that yielded rapid and robust memory responses upon rechallenge with suboptimal dose of Ag. Furthermore, both types of APCs had significant programmed death (PD) ligand 2 expression prior to Ag stimulation, which was maintained at a high level during presentation of Ag to T cells. Blockade of PD ligand 2 interaction with its receptor PD-1 nullified the development of memory responses. These previously unrecognized findings suggest that targeting specific APCs for Ag presentation during vaccination could prove effective against microbial infections.

Ex VivoProgramming of Antigen-Presenting B Lymphocytes: Considerations on DNA Uptake and Cell Activation

Plasmids used in DNA vaccination not only serve as a source of antigen, but also have an important adjuvant effect. This review focuses on recent advancements made in understanding how cells internalize DNA, and how internalized DNA activates immune response pathways. We also comment on the role of B cells in both of these processes.

PARP-1 deficiency increases the severity of disease in a mouse model of multiple sclerosis

Poly(ADP-ribose) polymerase-1 (PARP-1) has been implicated in the pathogenesis of several central nervous system (CNS) disorders. However, the role of PARP-1 in autoimmune CNS injury remains poorly understood. Therefore, we studied experimental autoimmune encephalomyelitis (EAE), a model for multiple sclerosis in mice with a targeted deletion of PARP-1. We identified inherent physiological abnormalities in the circulating and splenic immune composition between PARP-1(-/-) and wild type (WT) mice. Upon EAE induction, PARP-1(-/-) mice had an earlier onset and developed a more severe EAE compared with WT cohorts. Splenic response was significantly higher in PARP-1(-/-) mice largely because of B cell expansion. Although formation of Th1 and Th17 effector T lymphocytes was unaffected, PARP-1(-/-) mice had significantly earlier CD4+ T lymphocyte and macrophage infiltration into the CNS during EAE. However, we did not detect significant differences in cytokine profiles between PARP-1(-/-) and WT spinal cords at the peak of EAE. Expression analysis of different PARP isozymes in EAE spinal cords showed that PARP-1 was down-regulated in WT mice and that PARP-3 but not PARP-2 was dramatically up-regulated in both PARP-1(-/-) and WT mice, suggesting that these PARP isozymes could have distinct roles in different CNS pathologies. Together, our results indicate that PARP-1 plays an important role in regulating the physiological immune composition and in immune modulation during EAE; our finding identifies a new aspect of immune regulation by PARPs in autoimmune CNS pathology.

Murine dendritic cell antigen-presenting cell function is not altered by burn injury

Severe injury disrupts normal immune regulation causing a transient hyperinflammatory reaction and suppressed adaptive immune function. This report addresses the potential contribution of dendritic cells (DC) to changes in adaptive immune function after injury by specifically measuring injury-induced changes in splenic DC numbers and subsets, cell-surface markers, TLR responses, and APC function. Using a mouse burn injury model, we found that injury did not markedly alter the relative percentage of lymphoid, myeloid, or plasmacytoid DC in the spleens of burn-injured mice. Moreover, we did not observe a significant reduction in cell-surface expression of several major costimulatory molecules, CD40, CD80, CD86, programmed death 1 ligand, ICOS ligand, and B7-H3, on DC. Instead, we observed increased cell-surface expression of CD86 at 1 day after injury with no significant changes in costimulatory molecule expression at 7 days after injury, suggesting that burn injury causes an early activation of DC. In addition, injury did not suppress DC reactivity to TLR2, TLR4, or TLR9 agonists. Most important, DC prepared from injured mice were able to present peptide antigen to naive OTII TCR transgenic CD4+ T cells as efficiently and effectively as DC from sham-injured mice. We also found that CD4 T cells stimulated with antigen presented by DC from sham or burn mice showed similar levels of IL-2, IFN-gamma, IL-10, and IL-13 production. Taken together, these findings support the conclusion that DC do not acquire a suppressive phenotype following severe injury in mice.

Activated B cells modified by electroporation of multiple mRNAs encoding immune stimulatory molecules are comparable to mature dendritic cells in inducing in vitro antigen-specific T-cell responses

Ex-vivo-activated B cells are an alternative source of antigen-presenting cells (APCs) and a potential replacement for dendritic cells (DCs) in immunotherapy. However, the ability of ex-vivo-activated B cells to function as potent APCs has been a concern, especially when compared to DCs. Our study investigated whether modification of activated B cells with immune stimulatory molecules could enhance the ability of activated B cells to stimulate T cells. We show that murine splenic B cells, activated with a combination of Toll-like receptor agonist and agonistic anti-CD40, stimulated antigen-specific CD8+ T cells more efficiently than cells activated with Toll-like receptor agonist or anti-CD40 alone, probably by down-regulation of the immune regulatory cytokine interleukin-10 (IL-10). However, the activated B cells were still poor T-cell stimulators compared to mature DCs. Therefore, we modified the activated B cells by simultaneous electroporation of multiple messenger RNAs encoding costimulatory molecules (OX40L and 4-1BBL), cytokines (IL-12p35 and IL-12p40) and antigen. We found that de novo expression or overexpression of OX40L, 4-1BBL and IL-12p70 on activated B cells synergistically enhanced proliferation as well as IL-2 and interferon-gamma production by CD8+ T cells. Furthermore, the RNA-modified activated B cells induced antigen-specific cytotoxic T lymphocyte responses as efficiently as mature DCs in vitro. Unexpectedly, modified activated B cells were inferior to mature DCs at in vivo induction of CD8+ T-cell responses. In summary, activated B cells modified to express immune stimulatory molecules are a potent alternative to DCs in immunotherapy.

The modulatory effects of lipopolysaccharide-stimulated B cells on differential T-cell polarization

Lipopolysaccharide (LPS) is a major component of environmental microbial products. Studies have defined the LPS dose as a critical determining factor in driving differential T-cell polarization but the direct effects of LPS on individual antigen-presenting cells is unknown. Here, we investigated the effects of LPS doses on naive B cells and the subsequent modulatory effects of these LPS-activated B cells on T-cell polarization. The LPS was able to induce a proliferative response starting at a dose of 100 ng/ml and was capable of enhancing antigen internalization at a dose of 1 microg/ml in naive B cells. Following LPS stimulation, up-regulation of the surface markers CD40, CD86, I-Ad, immunoglobulin M, CD54 and interleukin-10 production, accompanied by down-regulation of CD5 and CD184 (CXCR4) were observed in a LPS dose-dependent manner. Low doses (<10 ng/ml) of LPS-activated B cells drove T helper type 2 polarization whereas high doses (>0.1 microg/ml) of LPS-activated B cells resulted in T regulatory type 1 cell polarization. In conclusion, LPS-activated B cells acquire differential modulatory effects on T-cell polarization. Such modulatory effects of B cells are dependent on the stimulation with LPS in a dose-dependent manner. These observations may provide one of the mechanistic explanations for the influence of environmental microbes on the development of allergic diseases.

Immune regulation by B cells and antibodies a view towards the clinic

B lymphocytes contribute to immunity in multiple ways, including production of antibodies, presentation of antigen to T cells, organogenesis of secondary lymphoid organs, and secretion of cytokines. Recent clinical trials have shown that depleting B cells can be highly beneficial for patients with autoimmune diseases, implicating B cells and antibodies as key drivers of pathology. However, it should be kept in mind that B cell responses and antibodies also have important regulatory roles in limiting autoimmune pathology. Here, we analyze clinical examples illustrating the potential of antibodies as treatment for immune-mediated disorders and discuss the underlying mechanisms. Furthermore, we examine the regulatory functions of activated B cells, their involvement in the termination of some experimental autoimmune diseases, and their use in cell-based therapy for such pathologies. These suppressive functions of B cells and antibodies do not only open new ways for harnessing autoimmune illnesses, but they also should be taken into account when designing new strategies for vaccination against microbes and tumors.

B cells are important as antigen presenting cells for induction of MHC-restricted arthritis in transgenic mice

Rheumatoid arthritis and its animal model, collagen-induced arthritis, are known as a T and B cell dependent disease. To analyze the role of B cells in arthritis, we generated B cell deficient (microMT) mice carrying HLA-DQ8 as transgene, Abetao.DQ8.micromt mice. HLA-DQ8 transgenic mice (Abetao.DQ8) are susceptible to collagen induced arthritis, an animal model for inflammatory arthritis. Deletion of IgM gene led to the absence of B cells while T cells were comparable to Abetao.DQ8 mice. Arthritis and autoantibodies was completely abrogated in B cell deficient DQ8 mice. T cell response and proinflammatory cytokine production in response to type II collagen and its derived peptides in vitro was significantly decreased despite an increased number of Mac-1 positive cells in DQ8.micromt mice compared to DQ8 mice suggesting B cells could be important for antigen presentation as well. In vitro substitution of B cells from wild type mice restored the response in DQ8.micromt mice. B cells could also present CII-derived peptides to antigen-specific DQ8-restricted hybridomas reinforcing the role of B cells in presentation of antigens to T cells. The data suggest that B cells can be involved in pathogenesis of arthritis by producing autoantibodies and antigen presentation.

Critical role of dendritic cells in mouse mammary tumor virus in vivo infection

Mouse mammary tumor virus (MMTV) is a milk-transmitted betaretrovirus that causes mammary tumors in mice. Although mammary epithelial cells are the ultimate targets of MMTV, the virus utilizes components of the host immune system to establish infection. Previous studies indicated that dendritic cells play a role in MMTV infection. Here we show that dendritic cells are the first cells to be infected by MMTV in vivo and that they are capable of producing infectious virus that can be transmitted to other cell types. Moreover, upon contact with the virus, dendritic cells became more mature and migrated in response to the chemokine macrophage inflammatory protein 3beta. Finally, we demonstrate that targeted ablation of dendritic cells in vivo dramatically attenuated MMTV infection. These data indicate that MMTV infection of dendritic cells is critical to initial propagation of the virus in vivo.

Activated B cells mediate efficient expansion of rare antigen-specific T cells

Potent professional antigen-presenting cells (APC) are essential tools to activate and expand antigen-specific T cells in vitro for use in adoptive immunotherapy. CD40-activated B cells can be easily generated and propagated from human donors and have been successfully used to generate antigen-specific T-cell cultures. Here we show that CD40-activated B cells strongly and specifically expand rare populations of antigen-specific CD8 T cells, with frequencies of less than 1 in 20,000 CD8 T cells in peripheral blood. We focused on T cells recognizing an epitope from the human papillomavirus 16 (HPV-16) E7 protein. In 6 of 6 healthy donors, epitope-specific CD8+ T cells were found to be "rare" by this criterion, as shown by staining with human leukocyte antigen (HLA)/peptide multimers. Using peptide-loaded CD40-activated B cells, epitope-specific T cells could be selectively expanded in all donors up to 10(6) fold, and the resulting T-cell cultures contained up to 88% specific T cells. These results strongly encourage the use of CD40-stimulated B cells as APCs in immunotherapy.

B cells drive early T cell autoimmunity in vivo prior to dendritic cell-mediated autoantigen presentation

Both B cells and dendritic cells (DCs) have been implicated as autoantigen-presenting cells in the activation of self-reactive T cells. However, most self-proteins are ubiquitously and/or developmentally expressed, making it difficult to determine the source and the exposure of autoantigens to APCs in a controlled manner. In this study, we have used an Ig transgenic mouse model to examine the mechanisms by which B cells and other APCs acquire and present lupus autoantigens in vivo. Targeting a lupus autoantigen, the small nuclear ribonucleoprotein particle D protein, to the BCR activates autoreactive T cells in the periphery. Our in vivo studies demonstrate that autoantigen-specific B cells, when present in the repertoire, are the first subset of APCs to capture and present self-proteins for activating T cells. Thereafter, DCs acquire self-Ag and become effective APCs for stimulating the same subsets of autoreactive T cells. This mechanism provides one explanation of how early steps in autoimmunity can focus responses, via BCR, at a small group of self-proteins among the total milieu of intracellular self-proteins. Subsequently, DCs and other professional APCs may then amplify and perpetuate the autoimmune T cell response.

Primary T cell expansion and differentiation in vivo requires antigen presentation by B cells

B cells are well documented as APC; however, their role in supporting and programming the T cell response in vivo is still unclear. Studies using B cell-deficient mice have given rise to contradictory results. We have used mixed BM chimeric mice to define the contribution that B cells make as APC. When the B cell compartment is deficient in MHC class II, while other APC are largely normal, T cell clonal expansion is significantly reduced and the differentiation of T cells into cytokine-secreting effector cells is impaired (in particular, Th2 cells). The development of the memory T cell populations is also decreased. Although MHC class II-mediated presentation by B cells was crucial for an optimal T cell response, neither a B cell-specific lack of CD40 (influencing costimulation) nor lymphotoxin alpha (influencing lymphoid tissue architecture) had any effect on the T cell response. We conclude that in vivo B cells provide extra and essential Ag presentation capacity over and above that provided by dendritic cells, optimizing expansion and allowing the generation of memory and effector T cells.

B cells as antigen presenting cells

Several characteristics confer on B cells the ability to present antigen efficiently: (1) they can find T cells in secondary lymphoid organs shortly after antigen entrance, (2) BCR-mediated endocytosis allows them to concentrate small amounts of specific antigen, and (3) BCR signaling and HLA-DO expression direct their antigen processing machinery to favor presentation of antigens internalized through the BCR. When presenting antigen in a resting state, B cells can induce T cell tolerance. On the other hand, activation by antigen and T cell help converts them into APC capable of promoting immune responses. Presentation of self antigens by B cells is important in the development of autoimmune diseases, while presentation of tumor antigens is being used in vaccine strategies to generate immunity. Thus, detailed understanding of the antigen presenting function of B cells can lead to their use for the generation or inhibition of immune responses.

Concerted antigen presentation by dendritic cells and B cells is necessary for optimal CD4 T-cell immunity in vivo

The relative contributions of different types of antigen presenting cells to T-cell activation, expansion and induction of effector functions are still not fully understood. In order to evaluate the roles of dendritic versus B cells during these phases of a CD4 T-cell response in vivo, we adoptively transferred major histocompatibility complex class II restricted, T-cell receptor-transgenic CD4+ T cells into transgenic mice expressing selectively the T-cell restricting class II molecules on either dendritic cells, B cells or both. Upon immunization with peptide antigen, we observed that dendritic cells were sufficient to induce activation, expansion, interleukin-2 production and germinal centre migration of antigen-specific T cells, independently of other antigen-presenting cells. In contrast, neither resting nor activated B cells had similar antigen-presenting capacities in vivo. However, in double transgenic mice where both B cells and dendritic cells were capable of presenting antigen, T cells showed increased proliferation, expansion and cytokine production in vivo. Moreover, higher antigen-specific CD4 T-cell numbers accumulated in germinal centres. Our data demonstrate that dendritic cells are sufficient to activate naive CD4 T cells in vivo, but B cells subsequently can enhance CD4 T-cell expansion further.

Both Dendritic Cells and Macrophages Can Stimulate Naive CD8 T Cells In Vivo to Proliferate, Develop Effector Function, and Differentiate into Memory Cells

The generation of T cell immunity requires the acquisition and presentation of Ag on bone marrow-derived APCs. Dendritic cells (DC) are believed to be the most potent bone marrow-derived APCs, and the only ones that can stimulate naive T cells to productively respond to Ags. Because macrophages (Mphi) are bone marrow-derived APCs that are also found in tissues and lymphoid organs, can acquire and present Ag, and can express costimulatory molecules, we have investigated their potential to stimulate primary T cell responses in vivo. We find that both injected Mphi and DCs can migrate from peripheral tissues or blood into lymphoid organs. Moreover, injection of peptide-pulsed Mphi or DCs into mice stimulates CD8 T cells to proliferate, express effector functions including cytokine production and cytolysis, and differentiate into long-lived memory cells. Mphi and DCs stimulate T cells directly without requiring cross-presentation of Ag on host APCs. Therefore, more than one type of bone marrow-derived APC has the potential to prime T cell immunity. In contrast, another bone marrow-derived cell, the T lymphocyte, although capable of presenting Ag and homing to the T cell areas of lymphoid organs, is unable to stimulate primary responses. Because Mphi can be very abundant cells, especially at sites of infection and inflammation, they have the potential to play an important role in immune surveillance and the initiation of T cell immunity.

CD8 T cell priming by B lymphocytes is CD4 help dependent

While it is generally accepted that B lymphocytes can present antigen and activate CD4 T cells, priming of CD8 T cells by B lymphocytes remains controversial. Recently, we showed that mice injected with genetically programmed B lymphocytes generate antigen specific CD4 and CD8 T cell responses in vivo that could also be induced in mice lacking functional dendritic cells. To gain further insights into the requirements for T cell priming by antigen-presenting B lymphocytes, in vitro experiments were performed using ovalbumin (OVA) and OVA-specific TCR-transgenic CD4 and CD8 T cells. We found that while B lymphocytes can directly prime CD4 T cells, the activation of CD8 T cells requires T cell help. Transfer experiments show that help can either be contact dependent or be mediated by soluble factors in the supernatants of activated OVA-specific CD4 T cells. Furthermore, the effect of activated CD4 T cells can be replaced by soluble recombinant IL-4. Collectively, the data show the existence of different requirements for priming of CD4 and CD8 T cells and point to the previously unappreciated fact that the induction of CD8 T cell responses by B lymphocytes requires T cell help.

B cells can prime naive CD4+ T cells in vivo in the absence of other professional antigen-presenting cells in a CD154-CD40-dependent manner

The role of B cells as APC is well established. However, their ability to prime naive T cells in vivo has been difficult to examine because of the presence of dendritic cells. The current studies were undertaken to examine this issue in a model of adoptive transfer of antigen-specific B cells and T cells into histoincompatible Rag2(-/-) mice. By means of this system, we were able to demonstrate that antigen-specific B cells are competent APC for naive CD4(+) T cells specific for the same antigen. In vivo antigen presentation resulted in expansion of both CD4(+) T cells and B cells. The antigen-presenting function of the transferred B cells was dependent on the CD154-CD40 interaction, as transfer of CD154-deficient antigen-specific CD4(+) T cells or CD40-deficient B cells failed to induce T and B cell expansion in response to immunization. These results indicate that antigen-specific B cells have the capacity to induce primary T cell responses in the absence of other competent APC.

Human B cells that hyperexpress a triad of costimulatory molecules via avipox-vector infection: an alternative source of efficient antigen-presenting cells

Dendritic cells (DCs) are the most potent of the antigen-presenting cells (APCs). Preparation of sufficient numbers of mature DCs, however, is both costly and time-consuming. We have examined here the possibility of using an alternative source of APCs that would be easier to obtain, would not require extensive culture, and thus would be more applicable to human immunotherapy protocols. We show here that freshly isolated human B cells can be efficiently infected by a replication-defective fowlpox recombinant vector, designated rF-TRICOM (TRIad of COstimulatory Molecules), to markedly increase surface expression of the human costimulatory molecule B7-1 and moderately increase expression of intercellular adhesion molecule-1 (ICAM-1) and leukocyte function-associated antigen-3 (LFA-3). Peptide-pulsed rF-TRICOM-infected B cells were highly efficient in activating antigen-specific human T cells and shown to be superior to the use of CD40L in enhancing APC potency. Moreover, when infection of freshly isolated B cells with rF-TRICOM was combined with CD40L, a still further marked enhancement of the antigen-presenting potency was observed. Ex vivo-generated antigen-specific T cells activated in this manner might be applied to experimental protocols or used for adoptive transfer in immunotherapy protocols.

Induction of protective immunity by primed B-1 cells in Toxoplasma gondii -infected B cell-deficient mice

We examined the role of B-1 cells in protection against Toxoplasma gondii infection using B cell-deficient mice (muMT mice). We found that primed but not naïve B-1 cells from wild-type C57BL/6 mice protected B cell-deficient recipients from challenge infection. All muMT mice transferred with primed B-1 cells survived more than 5 months after T. gondii infection, whereas 100% of muMT mice transferred with naïve B-1 cells succumbed by 18 days after infection. Additionally, high expression of both T help (Th) 1- and Th2-type cytokines and a high level of nitric oxide production were observed in T. gondii-infected muMT mice transferred with primed B-1 cells. Thus, it was clearly demonstrated that B-1 cells play an important role in host protection against T. gondii infection in muMT mice.

Whole-body autoradiography reveals that the Peptostreptococcus magnus immunoglobulin-binding domains of protein L preferentially target B lymphocytes in the spleen and lymph nodes in vivo

Protein L is an immunoglobulin (Ig)-binding protein produced by the Gram-positive bacterium Peptostreptococcus magnus that interacts with the variable region of Ig kappa light chains. The Ig light chain-binding capacity of protein L gives it the potential to interact with cells expressing surface Ig such as B cells. The present study was performed to address the in vivo trafficking of protein L at both the organ and the cellular level. Using the powerful technique of whole-body autoradiography in a murine model system, we demonstrate specific targeting of protein L to secondary lymphoid tissues in whole-animal analysis. The observed targeting depends on the capacity to interact with murine Ig, as tissue targeting was not apparent in mice given protein H, an Ig-binding protein produced by Streptococcus pyogenes with affinity for human but not murine Ig. Tissue targeting data were combined with flow cytometry analysis, which demonstrated the capacity of protein L to target and activate B lymphocytes in vivo. B cells targeted by protein L had increased surface expression of CD86 and MHC-II, and protein L was present in vacuolar compartments of B cells. Protein L did not bind T cells or natural killer cells but had some capacity to target dendritic cells and macrophages. The data show that protein L preferentially targets secondary lymphoid organs, and activates and is internalized by B cells in vivo. Furthermore, the observed tissue and cell targeting properties require an affinity for murine Ig. These data support the potential use of this Ig-binding protein as a targeting approach to deliver agents to defined cell populations in vivo.

T cell immunity using transgenic B lymphocytes

Adaptive immunity exists in all vertebrates and plays a defense role against microbial pathogens and tumors. T cell responses begin when precursor T cells recognize antigen on specialized antigen-presenting cells and differentiate into effector cells. Currently, dendritic cells are considered the only cells capable of stimulating T lymphocytes. Here, we show that mature naïve B lymphocytes can be genetically programmed by using nonviral DNA and turned into powerful antigen-presenting cells with a dual capacity of synthesis and presentation of antigen to T cells in vivo. A single i.v. injection of transgenic lymphocytes activates T cell responses reproducibly and specifically even at very low cell doses (approximately 10(2)). We also demonstrate that T cell priming can occur in the absence of dendritic cells and results in immunological memory with protective effector functions. These findings disclose aspects in the regulation of adaptive immunity and indicate possibilities for vaccination against viruses and cancer in humans.

Neuroblastoma and dendritic cell function

Neuroblastoma, the most common extracranial solid tumor of childhood, remains a challenge for clinicians and investigators in pediatric surgical oncology. The absence of effective conventional therapies for most patients with neuroblastoma justifies the application of novel, biology-based, experimental approaches to the treatment of this deadly disease. The observation that some aggressive neuroblastomas, particularly in infants, may spontaneously regress suggested that immune-mediated mechanisms may be important in the biology of this disease. Advances in the understanding of the cognate interactions between T cells, antigen-presenting cells and tumors have demonstrated the sentinel role of dendritic cells (DC), the most potent antigen presenting cells, in initiating the cellular immune response to cancer. Until recently the function of DC in pediatric solid tumors, especially neuroblastoma, had not been extensively studied. This review discusses the role of DC in initiating and coordinating the immune response against cancer, the ability of neuroblastoma to induce DC dysregulation at multiple levels by inhibiting DC maturation and function, and the current vaccine strategies being designed to employ the unique ability of DC to promote neuroblastoma regression.

Marginal Zone, but Not Follicular B Cells, Are Potent Activators of Naive CD4 T Cells

The early involvement of marginal zone (MZ) B lymphocytes in T-independent immune responses is well established. In this study we compared the abilities of MZ and follicular (FO) B cells to collaborate with T cells. After immunization with soluble hen egg lysozyme, both MZ and FO B cells captured Ag and migrated to T cell areas in the response to hen egg lysozyme. MZ B cells were far superior to FO B cells in inducing CD4+ T cell expansion both in vitro and in vivo. MZ, but not FO, B cells, after interaction with T cells, differentiated into plasma cells, and in addition they stimulated Ag-specific CD4+ T cells to produce high levels of Th1-like cytokines upon primary stimulation in vitro. These results indicate that MZ B cells rapidly and effectively capture soluble Ag and activate CD4+ T cells to become effector T cells. The enhanced capacity of MZ B cells to prime T cells in this study appeared to be intrinsic to MZ B cells, as both MZ and FO B cell populations express an identical Ag receptor.

The role of B cells and alloantibody in the host response to human organ allografts

Some human organ transplants deteriorate slowly over a period of years, often developing characteristic syndromes: transplant glomerulopathy (TG) in kidneys, bronchiolitis obliterans in lungs, and coronary artery disease in hearts. In the past, we attributed late graft deterioration to "chronic rejection", a distinct but mysterious immunologic process different from conventional rejection. However, it is likely that much of chronic rejection is explained by conventional T-cell-mediated rejection (TMR), antibody-mediated rejection (AMR), and other insults. Recently, criteria have emerged to now permit us to diagnose AMR in kidney transplants, particularly C4d deposition in peritubular capillaries and circulating antibody against donor human leukocyte antigens (HLA). Some cases with AMR develop TG, although the relationship of TG to AMR is complex. Thus, a specific diagnosis of AMR in kidney can now be made, based on graft damage, C4d deposition, and donor-specific alloantibodies. Criteria for AMR in other organs must be defined. Not all late rejections are AMR; some deteriorating organs probably have smoldering TMR. The diagnosis of late ongoing AMR raises the possibility of treatment to suppress the alloantibody, but efficacy of the available treatments requires further study.

Incomplete activation of CD4 T cells by antigen-presenting transitional immature B cells: implications for peripheral B and T cell responsiveness

B cells leave the bone marrow as transitional B cells. Transitional B cells represent a target of negative selection and peripheral tolerance, both of which are abrogated in vitro by mediators of T cell help. In vitro, transitional and mature B cells differ in their responses to B cell receptor ligation. Whereas mature B cells up-regulate the T cell costimulatory molecule CD86 (B7.2) and are activated, transitional B cells do not and undergo apoptosis. The ability of transitional B cells to process and present Ag to CD4 T cells and to elicit protective signals in the absence of CD86 up-regulation was investigated. We report that transitional B cells can process and present Ag as peptide:MHC class II complexes. However, their ability to activate T cells and elicit help signals from CD4-expressing Th cells was compromised compared with mature B cells, unless exogenous T cell costimulation was provided. A stringent requirement for CD28 costimulation was not evident in interactions between transitional B cells and preactivated CD4-expressing T cells, indicating that T cells involved in vivo in an ongoing immune response might rescue Ag-specific transitional B cells from negative selection. These data suggest that during an immune response, immature B cells may be able to sustain the responses of preactivated CD4(+) T cells, while being unable to initiate activation of naive T cells. Furthermore, the ability of preactivated, but not naive T cells to provide survival signals to B cell receptor-engaged transitional immature B cells argues that these B cells may be directed toward activation rather than negative selection when encountering Ag in the context of a pre-existing immune response.

Antigen presentation to naive CD4 T cells in the lymph node

Although the presentation of peptide-major histocompatibility complex class II (pMHC class II) complexes to CD4 T cells has been studied extensively in vitro, knowledge of this process in vivo is limited. Unlike the in vitro situation, antigen presentation in vivo takes place within a complex microenvironment in which the movements of antigens, antigen-presenting cells (APCs) and T cells are governed by anatomic constraints. Here we review developments in the areas of lymph node architecture, APC subsets and T cell activation that have shed light on how antigen presentation occurs in the lymph nodes.

Microbial colonization drives lymphocyte accumulation and differentiation in the follicle-associated epithelium of Peyer's patches

Peyer's patches (PPs) are lined by follicle-associated epithelium (FAE) with Ag-transporting M cells. To investigate the spatial relationships of B cells, T cells, and dendritic cells (DCs) in PPs during microbial colonization, their in situ redistribution was examined in germfree (GF) rats exposed to a conventional pathogen-free microflora (conventionalized, CV). Although occasional B and T cells occurred in the FAE of GF rats, it contained mainly immature DCs (CD4(+)CD86(-)), whereas mature DCs (CD86(high)) were seen in the interfollicular zones even under GF conditions. In CV rats, DCs had disappeared from the FAE, which instead contained clusters by B and T cells associated with induction of putative M cell pockets. CD86 was seen neither in the FAE nor in the follicles under GF conditions, but it became apparent on intraepithelial B cells 5 wk after colonization. The level of CD86 on these B cells was comparable to that on germinal center B cells, although the B cell follicles did not show direct contact with the M cell areas. B cells in the follicular mantles acquired Bcl-2 after 12 wk in CV rats, whereas B cells in the FAE did not express Bcl-2 at a substantial level throughout the experimental period. The cellular redistribution patterns and phenotypic characteristics observed after colonization suggested that immature DCs, but not B cells, are involved in Ag presentation during primary immune responses against intestinal bacteria. However, the spatial cellular relationships sequentially being established among DCs, B cells, and T cells in PPs, are most likely important for the induction of post-germinal center B cells subsequently residing within the M cell pockets.

Hodgkin and Reed-Sternberg cell-associated autoantigen CLIP-170/restin is a marker for dendritic cells and is involved in the trafficking of macropinosomes to the cytoskeleton, supporting a function-based concept of Hodgkin and Reed-Sternberg cells

Little is known about the distribution in normal cells of CLIP-170, a linkage mediator between endocytic vesicles and microtubules, and restin, a splice variant encoded by the same gene and marker for Hodgkin and Reed-Sternberg (HRS) cells of Hodgkin disease. Although only trace amounts of CLIP-170/restin are present in peripheral blood mononuclear cell subpopulations, monocyte-derived dendritic cells (DCs) and interleukin-4 (IL-4) + CD40L-activated B cells express high levels of CLIP-170/restin. CLIP-170/restin colocalizes preferentially with membranes of intermediate macropinocytic vesicles, suggesting a new function of CLIP-170/restin in the trafficking of macropinosomes to the cytoskeleton, which is a crucial step in antigen presentation. The strong expression of CLIP-170/restin in HRS cells, DCs, and activated B cells underscores their functional similarities supporting a function-based concept of HRS cells as professional antigen-presenting cells.