In vivo CD40-gp39 interactions are essential for thymus-dependent humoral immunity. I. In vivo expression of CD40 ligand, cytokines, and antibody production delineates sites of cognate T-B cell interactions

Developments in kidney xenotransplantation

The search for kidney xenografts that are appropriate for patients with end-stage renal disease has been ongoing since the beginning of the last century. The major cause of xenograft loss is hyperacute and acute rejection, and this has almost been overcome via scientific progress. The success of two pre-clinical trials of α1,3-galactosyltransferase gene-knockout porcine kidneys in brain-dead patients in 2021 triggered research enthusiasm for kidney xenotransplantation. This minireview summarizes key issues from an immunological perspective: the discovery of key xenoantigens, investigations into key co-stimulatory signal inhibition, gene-editing technology, and immune tolerance induction. Further developments in immunology, particularly immunometabolism, might help promote the long-term outcomes of kidney xenografts.

Nanocell COVID-19 vaccine triggers a novel immune response pathway producing high-affinity antibodies which neutralize all variants of concern

Most current anti-viral vaccines elicit a humoral and cellular immune response via the pathway of phagocytic cell mediated viral antigen presentation to B and T cell surface receptors. However, this pathway results in reduced ability to neutralize S-protein Receptor Binding Domains (RBDs) from several Variants of Concern (VOC) and the rapid waning of memory B cell response requiring vaccine reformulation to cover dominant VOC S-proteins and multiple boosters. Here we show for the first time in mice and humans, that a bacterially derived, non-living, nanocell (EDV; EnGeneIC Dream Vector) packaged with plasmid expressed SARS-CoV-2 S-protein and α-galactosyl ceramide adjuvant (EDV-COVID-αGC), stimulates an alternate pathway due to dendritic cells (DC) displaying both S-polypeptides and αGC thereby recruiting and activating iNKT cells with release of IFNγ. This triggers DC activation/maturation, activation of follicular helper T cells (T_FH), cognate help to B cells with secretion of a cytokine milieu promoting B cell maturation, somatic hypermutation in germinal centers to result in high affinity antibodies. Surrogate virus neutralization tests show 90-100% neutralization of ancestral and early VOC in mice and human trial volunteers. EDV-COVID-αGC as a third dose booster neutralized Omicron BA. 4/5. Serum and PBMC analyses reveal long lasting S-specific memory B and T cells. In contrast, control EDVs lacking αGC, did not engage the iNKT/DC pathway resulting in antibody responses unable to neutralize all VOCs and had a reduced B cell memory. The vaccine is lyophilized, stored and transported at room temperature with a shelf-life of over a year.

Young but not older adults exhibit an expansion of CD45RA+CCR7+CD95+ T follicular helper cells in response to tetanus vaccine

A subset of CD4⁺ T cells, known as T follicular helper (Tfh), provides co-stimulating signals required to establish long-term humoral immunity. Recent studies have shown a reduced frequency and functionality of this population in older adults in comparison to young adults, in response to vaccination. To evaluate whether memory generation of circulating Tfh (cTfh) cells contributes to this phenomenon, the memory subpopulations of cTfh, and their activation degree, were evaluated both ex-vivo and in-vitro, in response to the model antigen tetanus toxoid (TT) after the first dose of tetanus vaccine. Here, we report a lower frequency of cTfh after vaccination in older adults compared to young adults. Moreover, whereas cTfh from older adults preferably expanded with an effector memory phenotype, young adults experienced a temporal increase of CCR7⁺CD45RA⁺ cTfh cells, which also displayed higher levels of CD95, CD40L, CXCR3, and Bcl-6 upon antigen re-encounter. This phenotype was confirmed using automatized algorithm. In conclusion, our results suggest that an age-related loss of heterogeneity and an expansion of more differentiated memory cells within the cTfh compartment could affect the responsiveness of older individuals to vaccines, making this phenotype a characteristic feature of immunosenescence.

A Posttranscriptional Pathway of CD40 Ligand mRNA Stability Is Required for the Development of an Optimal Humoral Immune Response

CD40 ligand (CD40L) mRNA stability is dependent on an activation-induced pathway that is mediated by the binding complexes containing the multifunctional RNA-binding protein, polypyrimidine tract-binding protein 1 (PTBP1) to a 3' untranslated region of the transcript. To understand the relationship between regulated CD40L and the requirement for variegated expression during a T-dependent response, we engineered a mouse lacking the CD40L stability element (CD40LΔ5) and asked how this mutation altered multiple aspects of the humoral immunity. We found that CD40LΔ5 mice expressed CD40L at 60% wildtype levels, and lowered expression corresponded to significantly decreased levels of T-dependent Abs, loss of germinal center (GC) B cells and a disorganized GC structure. Gene expression analysis of B cells from CD40LΔ5 mice revealed that genes associated with cell cycle and DNA replication were significantly downregulated and genes linked to apoptosis upregulated. Importantly, somatic hypermutation was relatively unaffected although the number of cells expressing high-affinity Abs was greatly reduced. Additionally, a significant loss of plasmablasts and early memory B cell precursors as a percentage of total GL7⁺ B cells was observed, indicating that differentiation cues leading to the development of post-GC subsets was highly dependent on a threshold level of CD40L. Thus, regulated mRNA stability plays an integral role in the optimization of humoral immunity by allowing for a dynamic level of CD40L expression on CD4 T cells that results in the proliferation and differentiation of pre-GC and GC B cells into functional subsets.

B Cell Activation and Escape of Tolerance Checkpoints: Recent Insights from Studying Autoreactive B Cells

Autoreactive B cells are key drivers of pathogenic processes in autoimmune diseases by the production of autoantibodies, secretion of cytokines, and presentation of autoantigens to T cells. However, the mechanisms that underlie the development of autoreactive B cells are not well understood. Here, we review recent studies leveraging novel techniques to identify and characterize (auto)antigen-specific B cells. The insights gained from such studies pertaining to the mechanisms involved in the escape of tolerance checkpoints and the activation of autoreactive B cells are discussed. In addition, we briefly highlight potential therapeutic strategies to target and eliminate autoreactive B cells in autoimmune diseases.

Frontline Science: CD40 signaling restricts RNA virus replication in Mϕs, leading to rapid innate immune control of acute virus infection

Many acute viral infections target tissue Mϕs, yet the mechanisms of Mϕ-mediated control of viruses are poorly understood. Here, we report that CD40 expressed by peritoneal Mϕs restricts early infection of a broad range of RNA viruses. Loss of CD40 expression enhanced virus replication as early as 12-24 h of infection and, conversely, stimulation of CD40 signaling with an agonistic Ab blocked infection. With peritoneal cell populations infected with the filovirus, wild-type (WT) Ebola virus (EBOV), or a BSL2 model virus, recombinant vesicular stomatitis virus encoding Ebola virus glycoprotein (rVSV/EBOV GP), we examined the mechanism conferring protection. Here, we demonstrate that restricted virus replication in Mϕs required CD154/CD40 interactions that stimulated IL-12 production through TRAF6-dependent signaling. In turn, IL-12 production resulted in IFN-γ production, which induced proinflammatory polarization of Mϕs, protecting the cells from infection. These CD40-dependent events protected mice against virus challenge. CD40-/- mice were exquisitely sensitive to intraperitoneal challenge with a dose of rVSV/EBOV GP that was sublethal to CD40+/+ mice, exhibiting viremia within 12 h of infection and rapidly succumbing to infection. This study identifies a previously unappreciated role for Mϕ-intrinsic CD40 signaling in controlling acute virus infection.

Germinal Center Cells Turning to the Dark Side: Neoplasms of B Cells, Follicular Helper T Cells, and Follicular Dendritic Cells

Gaining knowledge of the neoplastic side of the three main cells—B cells, Follicular Helper T (Tfh) cells, and follicular dendritic cells (FDCs) —involved in the germinal center (GC) reaction can shed light toward further understanding the microuniverse that is the GC, opening the possibility of better treatments. This paper gives a review of the more complex underlying mechanisms involved in the malignant transformations that take place in the GC. Whilst our understanding of the biology of the GC-related B cell lymphomas has increased—this is not reviewed in detail here—the dark side involving neoplasms of Tfh cells and FDCs are poorly studied, in great part, due to their low incidence. The aggressive behavior of Tfh lymphomas and the metastatic potential of FDCs sarcomas make them clinically relevant, merit further attention and are the main focus of this review. Tfh cells and FDCs malignancies can often be misdiagnosed. The better understanding of these entities linked to their molecular and genetic characterization will lead to prediction of high-risk patients, better diagnosis, prognosis, and treatments based on molecular profiles.

Higher Cytokine and Opsonizing Antibody Production Induced by Bovine Serum Albumin (BSA)-Conjugated Tetrasaccharide Related to Streptococcus pneumoniae Type 3 Capsular Polysaccharide

A number of studies have demonstrated the limited efficacy of S. pneumoniae type 3 capsular polysaccharide (CP) in the 13-valent pneumococcal conjugate vaccine against serotype 3 invasive pneumococcal diseases and carriage. Synthetic oligosaccharides (OSs) may provide an alternative to CPs for development of novel conjugated pneumococcal vaccines and diagnostic test systems. A comparative immunological study of di-, tri-, and tetra-bovine serum albumin (BSA) conjugates was performed. All oligosaccharides conjugated with biotin and immobilized on streptavidin-coated plates stimulated production of IL-1α, IL-2, IL-4, IL-5, IL-10, IFNγ, IL-17A, and TNFα, but not IL-6 and GM-CSF in monocultured mice splenocytes. The tetrasaccharide-biotin conjugate stimulated the highest levels of IL-4, IL-5, IL-10, and IFNγ, which regulate expression of specific immunoglobulin isotypes. The tetra-BSA conjugate adjuvanted with aluminum hydroxide elicited high levels of IgM, IgG1, IgG2a, and IgG2b antibodies (Abs). Anti-CP-induced Abs could only be measured using the biotinylated tetrasaccharide. The tetrasaccharide ligand possessed the highest binding capacity for anti-OS and antibacterial IgG Abs in immune sera. Sera to the tetra-BSA conjugate promoted greater phagocytosis of bacteria by neutrophils and monocytes than the CRM197-CP-antisera. Sera of mice immunized with the tetra-BSA conjugate exhibited the highest titer of anti-CP IgG1 Abs compared with sera of mice inoculated with the same doses of di- and tri-BSA conjugates. Upon intraperitoneal challenge with lethal doses of S. pneumoniae type 3, the tri- and tetra-BSA conjugates protected mice more significantly than the di-BSA conjugate. Therefore, it may be concluded that the tetrasaccharide ligand is an optimal candidate for development of a semi-synthetic vaccine against S. pneumoniae type 3 and diagnostic test systems.

CXCR5+PD-1+ follicular helper CD8 T cells control B cell tolerance

Many autoimmune diseases are characterized by the production of autoantibodies. The current view is that CD4⁺ T follicular helper (Tfh) cells are the main subset regulating autoreactive B cells. Here we report a CXCR5⁺PD1⁺ Tfh subset of CD8⁺ T cells whose development and function are negatively modulated by Stat5. These CD8⁺ Tfh cells regulate the germinal center B cell response and control autoantibody production, as deficiency of Stat5 in CD8 T cells leads to an increase of CD8⁺ Tfh cells, resulting in the breakdown of B cell tolerance and concomitant autoantibody production. CD8⁺ Tfh cells share similar gene signatures with CD4⁺ Tfh, and require CD40L/CD40 and TCR/MHCI interactions to deliver help to B cells. Our study thus highlights the diversity of follicular T cell subsets that contribute to the breakdown of B-cell tolerance.

B cell response and antibody production are generally facilitated by CD4⁺ follicular helper (Tfh) cells. Here the authors identify a subset of CXCR5⁺PD1⁺CD8⁺ Tfh cells that is normally suppressed by STAT5 signaling, so that STAT5 deficiency in mice increases the number of these CD8⁺ Tfh cells and induces concomitant production of autoantibodies.

Co-stimulatory and co-inhibitory pathways in cancer immunotherapy

Cancer remains the leading cause of death worldwide. Traditional treatments such as surgery, radiation, and chemotherapy have had limited efficacy, especially with late stage cancers. Cancer immunotherapy and targeted therapy have revolutionized how cancer is treated, especially in patients with late stage disease. In 2013 cancer immunotherapy was named the breakthrough of the year, partially due to the established efficacy of blockade of CTLA-4 and PD-1, both T cell co-inhibitory molecules involved in tumor-induced immunosuppression. Though early trials promised success, toxicity and tolerance to immunotherapy have hindered long-term successes. Optimizing the use of co-stimulatory and co-inhibitory pathways has the potential to increase the effectiveness of T cell-mediated antitumor immune response, leading to increased efficacy of cancer immunotherapy. This review will address major T cell co-stimulatory and co-inhibitory pathways and the role they play in regulating immune responses during cancer development and treatment.

Circulating Follicular Helper and Follicular Regulatory T Cells Are Severely Compromised in Human CD40 Deficiency: A Case Report

Mutations in genes that control class switch recombination and somatic hypermutation during the germinal center (GC) response can cause diverse immune dysfunctions. In particular, mutations in CD40LG, CD40, AICDA, or UNG cause hyper-IgM (HIGM) syndrome, a heterogeneous group of primary immunodeficiencies. Follicular helper (Tfh) and follicular regulatory (Tfr) T cells play a key role in the formation and regulation of GCs, but their role in HIGM pathogenesis is still limited. Here, we found that compared to CD40 ligand (CD40L)- and activation-induced cytidine deaminase (AICDA)-deficient patients, circulating Tfh and Tfr cells were severely compromised in terms of frequency and activation phenotype in a child with CD40 deficiency. These findings offer useful insight for human Tfh biology, with potential implications for understanding the molecular basis of HIGM syndrome caused by mutations in CD40.

CD154 Costimulation Shifts the Local T-Cell Receptor Repertoire Not Only During Thymic Selection but Also During Peripheral T-Dependent Humoral Immune Responses

CD154 is a transmembrane cytokine expressed transiently on activated CD4 T cells upon T-cell receptor (TCR) stimulation that interacts with CD40 on antigen-presenting cells. The signaling via CD154:CD40 is essential for B-cell maturation and germinal center formation and also for the final differentiation of CD4 T cells during T-dependent humoral immune responses. Recent data demonstrate that CD154 is critically involved in the selection of T-cell clones during the negative selection process in the thymus. Whether CD154 signaling influences the TCR repertoire during peripheral T-dependent humoral immune responses has not yet been elucidated. To find out, we used CD154-deficient mice and assessed the global TCRβ repertoire in T-cell zones (TCZ) of spleens by high-throughput sequencing after induction of a Th2 response to the multiepitopic antigen sheep red blood cells. Qualitative and quantitative comparison of the splenic TCZ-specific TCRβ repertoires revealed that CD154 deficiency shifts the distribution of Vβ-Jβ genes after antigen exposure. This data led to the conclusion that costimulation via CD154:CD40 during the interaction of T cells with CD40-matured B cells contributes to the recruitment of T-cell clones into the immune response and thereby shapes the peripheral TCR repertoire.

CD40-CD40L cross-talk drives fascin expression in dendritic cells for efficient antigen presentation to CD4+ T cells

Fascin is an actin-bundling protein that, among immune cells, is restricted to expression in dendritic cells (DCs). Previous reports have suggested that fascin plays an important role in governing DC antigen presentation to CD4+ T cells. However, no report has clearly linked the receptor-ligand engagement that can direct downstream regulation of fascin expression. In this study, bone marrow-derived DCs from wild-type versus CD40-knockout C57BL/6 mice were used to elucidate the mechanisms of fascin expression and activity upon CD40-CD40 ligand (CD40L) engagement. These investigations now show that CD40 engagement governs fascin expression in DCs to promote CD4+ T-cell cytokine production. Absence of CD40 signaling resulted in diminished fascin expression in DCs and was associated with impaired CD4+ T-cell responses. Furthermore, the study found that loss of CD40-CD40L engagement resulted in reduced DC-T-cell contacts. Rescue by ectopic fascin expression in CD40-deficient DCs was able to re-establish sustained contacts with T cells and restore cytokine production. Taken together, these results show that cross-talk through CD40-CD40L signaling drives elevated fascin expression in DCs to support acquisition of full T-cell responses.

Advances in targeting co-inhibitory and co-stimulatory pathways in transplantation settings: the Yin to the Yang of cancer immunotherapy

In the past decade, the power of harnessing T-cell co-signaling pathways has become increasingly understood to have significant clinical importance. In cancer immunotherapy, the field has concentrated on two related modalities: First, targeting cancer antigens through highly activated chimeric antigen T cells (CAR-Ts) and second, re-animating endogenous quiescent T cells through checkpoint blockade. In each of these strategies, the therapeutic goal is to re-ignite T-cell immunity, in order to eradicate tumors. In transplantation, there is also great interest in targeting T-cell co-signaling, but with the opposite goal: in this field, we seek the Yin to cancer immunotherapy's Yang, and focus on manipulating T-cell co-signaling to induce tolerance rather than activation. In this review, we discuss the major T-cell signaling pathways that are being investigated for tolerance induction, detailing preclinical studies and the path to the clinic for many of these molecules. These include blockade of co-stimulation pathways and agonism of coinhibitory pathways, in order to achieve the delicate state of balance that is transplant tolerance: a state which guarantees lifelong transplant acceptance without ongoing immunosuppression, and with preservation of protective immune responses. In the context of the clinical translation of immune tolerance strategies, we discuss the significant challenge that is embodied by the fact that targeted pathway modulators may have opposing effects on tolerance based on their impact on effector vs regulatory T-cell biology. Achieving this delicate balance holds the key to the major challenge of transplantation: lifelong control of alloreactivity while maintaining an otherwise intact immune system.

T follicular helper cell programming by cytokine-mediated events

CD4⁺ T cells, or T helper cells, are critical mediators and coordinators of adaptive immunity. Unique effector T helper cell populations have been identified that perform distinct functions in response to pathogenic infection. The T follicular helper (Tfh) cells are one such subset, which has been identified as the primary T-cell population responsible for interacting with B cells to promote effective antibody-mediated immune responses. Since their initial description at the turn of the century, and subsequent classification as a distinct T helper cell subset, there has been substantial interest in elucidating the regulatory mechanisms that govern Tfh cell formation. The collective insight from this body of work has demonstrated that Tfh cell differentiation is a complex and multistage process regulated by a litany of cell-intrinsic and cell-extrinsic factors. As with the development of the other recognized T helper cell subsets, specific cytokines exercise prominent roles in both the positive and negative regulation of Tfh cell development. However, the exact composition of, and stage-specific requirements for, these environmental factors in the governance of Tfh cell differentiation remain incompletely understood. In this review, we summarize what is known regarding the role of cytokines in both the promotion and inhibition of Tfh cell differentiation and function.

The NIK of time for B cells

NF-κB-inducing kinase (NIK) is a key mediator of the noncanonical NF-κB signaling pathway, which is critical for B-cell development and function. Although complete deletion of NIK in mice has been shown to result in defective B cells and impaired secondary lymphoid organogenesis, the consequences of deleting NIK exclusively in B cells have not been determined. In this issue of the European Journal of Immunology, Hahn et al. [Eur. J. Immunol. 2016. 46: 732-741] describe mice in which the NF-κB2 pathway mediator, NIK, is deleted at different points in B-cell lineage differentiation and activation. The results show that the survival of mature peripheral B cells, as well as appropriate kinetics of germinal center reactions, rely on noncanonical NF-κB signaling. These findings confirm and extend prior observations implicating a nonredundant role for NF-κB2 downstream of BAFF signaling via BAFF-R, and prompt assessment of the growing literature regarding the relative roles of BCR and BAFF signals in B-cell homeostasis, as well as the downstream pathways responsible.

Functional Antagonism of Human CD40 Achieved by Targeting a Unique Species-Specific Epitope

Current clinical anti-CD40 biologic agents include both antagonist molecules for the treatment of autoimmune diseases and agonist molecules for immuno-oncology, yet the relationship between CD40 epitope and these opposing biological outcomes is not well defined. This report describes the identification of potent antagonist domain antibodies (dAbs) that bind to a novel human CD40-specific epitope that is divergent in the CD40 of nonhuman primates. A similarly selected anti-cynomolgus CD40 dAb recognizing the homologous epitope is also a potent antagonist. Mutagenesis, biochemical, and X-ray crystallography studies demonstrate that the epitope is distinct from that of CD40 agonists. Both the human-specific and cynomolgus-specific molecules remain pure antagonists even when formatted as bivalent Fc-fusion proteins, making this an attractive therapeutic format for targeting hCD40 in autoimmune indications.

Expression, purification and biological characterization of the extracellular domain of CD40 from Pichia pastoris

BACKGROUND

CD40, also called Bp50, is a novel member of the TNF receptor superfamily. Based on its important role in multiple physiological and pathological processes, the CD40 signaling pathway has become a vital target for treating transplantation, autoimmune diseases and cancers. This study generated a protein fragment that disrupts this signaling pathway.

RESULTS

A DNA fragment encoding the extracellular domain of CD40 (CD40-N) has been codon-optimized and cloned into pPIC9K to create a Pichia pastoris expression and secretion strain. SDS-PAGE and Western blotting assays using the culture media from methanol-induced expression strains showed that recombinant CD40-N, a 27 kDa glycosylated protein, was secreted into the culture broth. The recombinant protein was purified to more than 90 % using Sephadex G-50 size-exclusion chromatography and Q Sepharose Fast Flow ion exchange. Finally, 120 mg of the protein was obtained at a relatively high purity from 3 l supernatant. Binding assay (ITC200 assay) shown the direct interaction of CD40-N and CD40 agonist antibody (G28-5). The bioactivity of recombinant CD40-N was confirmed by its ability to disrupt non-canonical NF-κB signaling activated by CD40 agonist antibody or CD40 ligand and to inhibit ant-CD40 agonist antibody-induced TNF-alpha expression in BJAB cells in vitro. In addition, our data indicate that the protein has curative potential in treating dextran sulfate sodium (DSS)-induced colitis in vivo.

CONCLUSIONS

The results show that the experimental procedure we have developed using P. pastoris can be used to produce large amounts of active CD40-N for research and industrial purposes. The protein fragment we have acquired has potential to be used in research or even treating inflammation diseases such as colitis.

Control of Toll-like receptor-mediated T cell-independent type 1 antibody responses by the inducible nuclear protein IκB-ζ

Antibody responses have been classified as being either T cell-dependent or T cell-independent (TI). TI antibody responses are further classified as being either type 1 (TI-1) or type 2 (TI-2), depending on their requirement for B cell-mediated antigen receptor signaling. Although the mechanistic basis of antibody responses has been studied extensively, it remains unclear whether different antibody responses share similarities in their transcriptional regulation. Here, we show that mice deficient in IκB-ζ, specifically in their B cells, have impaired TI-1 antibody responses but normal T cell-dependent and TI-2 antibody responses. The absence of IκB-ζ in B cells also impaired proliferation triggered by Toll-like receptor (TLR) activation, plasma cell differentiation, and class switch recombination (CSR). Mechanistically, IκB-ζ-deficient B cells could not induce TLR-mediated induction of activation-induced cytidine deaminase (AID), a class-switch DNA recombinase. Retroviral transduction of AID in IκB-ζ-deficient B cells restored CSR activity. Furthermore, acetylation of histone H3 in the vicinity of the transcription start site of the gene that encodes AID was reduced in IκB-ζ-deficient B cells relative to IκB-ζ-expressing B cells. These results indicate that IκB-ζ regulates TLR-mediated CSR by inducing AID. Moreover, IκB-ζ defines differences in the transcriptional regulation of different antibody responses.

T-cell costimulatory blockade in organ transplantation

Before it became possible to derive T-cell lines and clones, initial experimentation on the activation requirements of T lymphocytes was performed on transformed cell lines, such as Jurkat. These studies, although technically correct, proved misleading as most transformed T cells can be activated by stimulation of the clonotypic T-cell receptor (TCR) alone. In contrast, once it became possible to study nontransformed T cells, it quickly became clear that TCR stimulation by itself is insufficient for optimal activation of naïve T cells, but in fact, induces a state of anergy. It then became clear that functional activation of T cells requires not only recognition of major histocompatibility complex (MHC) and peptide by the TCR, but also requires ligation of costimulatory receptors expressed on the cell surface.

Differential modulation by delta9-tetrahydrocannabinol (∆9)-THC) of CD40 ligand (CD40L) expression in activated mouse splenic CD4+ T cells

The anti-inflammatory activity of cannabinoids has been widely demonstrated in experimental animal models and in humans. CD40-CD40-ligand (L) interactions are among the most crucial initiators of inflammation. This study investigated the effects of ∆(9)-THC on CD40L expression in mouse splenic T cells after activation with various stimuli. Time course studies demonstrated that peak surface expression of CD40L by CD4(+) T cells after anti-CD3/CD28 or phorbol ester plus calcium ionophore (PMA/Io) occurred 8 h post activation. Peak CD40L mRNA levels were observed at 2 h post PMA/Io treatment and at 4 h post anti-CD3/CD28 treatment. Pretreatment with ∆(9)-THC significantly impaired the upregulation of CD40L induced by anti-CD3/CD28 at both the protein and mRNA level. By contrast, ∆(9)-THC did not affect PMA/Io-induced surface CD40L expression on CD4(+) T cells. Additionally, ∆(9)-THC also attenuated anti-CD3/CD28-induced CD40L expression on CD4(+) T cells derived from CB1(-/-)/CB2(-/-) mice. We investigated whether the mechanism by which ∆(9)-THC suppressed CD40L expression involved putative cannabinoid activation of the glucocorticoid receptor (GR). Although activation of GR resulted in suppression of CD40L induction by anti-CD3/CD28, no interaction between ∆(9)-THC and GR was observed by a glucocorticoid response element (GRE) luciferase reporter assay in HEK293T cells. Collectively, these results suggest that ∆(9)-THC targets proximal T cell receptor-associated signaling in a cannabinoid receptor- and glucocorticoid receptor-independent manner. These findings identify suppression of CD40L expression as a novel part of the mechanism by which ∆(9)-THC exerts anti-inflammatory activity.

B7h-expressing dendritic cells and plasma B cells mediate distinct outcomes of ICOS costimulation in T cell-dependent antibody responses

Background

The ICOS-B7h costimulatory receptor-ligand pair is required for germinal center formation, the production of isotype-switched antibodies, and antibody affinity maturation in response to T cell-dependent antigens. However, the potentially distinct roles of regulated B7h expression on B cells and dendritic cells in T cell-dependent antibody responses have not been defined.

Results

We generated transgenic mice with lineage-restricted B7h expression to assess the cell-type specific roles of B7h expression on B cells and dendritic cells in regulating T cell-dependent antibody responses. Our results show that endogenous B7h expression is reduced on B cells after activation in vitro and is also reduced in vivo on antibody-secreting plasma B cells in comparison to both naïve and germinal center B cells from which they are derived. Increasing the level of B7h expression on activated and plasma B cells in B-B7hTg mice led to an increase in the number of antibody-secreting plasma cells generated after immunization and a corresponding increase in the concentration of antigen-specific high affinity serum IgG antibodies of all isotypes, without affecting the number of responding germinal center B cells. In contrast, ICOS costimulation mediated by dendritic cells in DC-B7hTg mice contributed to germinal center formation and selectively increased IgG2a production without affecting the overall magnitude of antibody responses.

Conclusions

Using transgenic mice with lineage-restricted B7h expression, we have revealed distinct roles of ICOS costimulation mediated by dendritic cells and B cells in the regulation of T cell-dependent antibody responses.

Signaling by the tumor necrosis factor receptor superfamily in B-cell biology and disease

Members of the tumor necrosis factor receptor superfamily (TNFRSF) participate prominently in B-cell maturation and function. In particular, B-cell activating factor belonging to the TNF family receptor (BAFF-R), B-cell maturation antigen (BCMA), and transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) play critical roles in promoting B-cell survival at distinct stages of development by engaging a proliferation-inducing ligand (APRIL) and/or BAFF. CD40 is also essential for directing the humoral response to T-cell-dependent antigens. Signaling by the TNFRSF is mediated primarily, albeit not exclusively, via the TNFR-associated factor (TRAF) proteins and activation of the canonical and/or non-canonical nuclear factor-κB (NF-κB) pathways. Dysregulated signaling by TNFRSF members can promote B-cell survival and proliferation, causing autoimmunity and neoplasia. In this review, we present a current understanding of the functions of and distinctions between APRIL/BAFF signaling by their respective receptors expressed on particular B-cell subsets. These findings are compared and contrasted with CD40 signaling, which employs similar signaling conduits to achieve distinct cellular outcomes in the context of the germinal center response. We also underscore how new findings and conceptual insights into TNFRSF signaling are facilitating the understanding of B-cell malignancies and autoimmune diseases.

Cyclosporine-resistant, Rab27a-independent mobilization of intracellular preformed CD40 ligand mediates antigen-specific T cell help in vitro

CD40L is critically important for the initiation and maintenance of adaptive immune responses. It is generally thought that CD40L expression in CD4(+) T cells is regulated transcriptionally and made from new mRNA following Ag recognition. However, recent studies with two-photon microscopy revealed that most cognate interactions between effector CD4(+) T cells and APCs are too short for de novo synthesis of CD40L. Given that effector and memory CD4(+) T cells store preformed CD40L (pCD40L) in lysosomal compartments and that pCD40L comes to the cell surface within minutes of antigenic stimulation, we and others have proposed that pCD40L might mediate T cell-dependent activation of cognate APCs during brief encounters in vivo. However, it has not been shown that this relatively small amount of pCD40L is sufficient to activate APCs, owing to the difficulty of separating the effects of pCD40L from those of de novo CD40L and other cytokines in vitro. In this study, we show that pCD40L surface mobilization is resistant to cyclosporine or FK506 treatment, while de novo CD40L and cytokine expression are completely inhibited. These drugs thus provide a tool to dissect the role of pCD40L in APC activation. We find that pCD40L mediates selective activation of cognate but not bystander APCs in vitro and that mobilization of pCD40L does not depend on Rab27a, which is required for mobilization of lytic granules. Therefore, effector CD4(+) T cells deliver pCD40L specifically to APCs on the same time scale as the lethal hit of CTLs but with distinct molecular machinery.

CD180 functions in activation, survival and cycling of B chronic lymphocytic leukaemia cells

We previously showed that approximately 60% of B chronic lymphocytic leukaemia (B-CLL) cells express surface CD180, an orphan receptor of the Toll-like receptor family. Here we investigated the ability of anti-CD180 monoclonal antibody (mAb) to induce activation, cell cycling, survival and signalling in B-CLL cells and normal B cells. Upon addition of anti-CD180 mAb, alone or in combination with anti-CD40 mAb or recombinant IL-4 (rIL-4), expression of CD86, Ki-67, uptake of DiOC(6) , phosphorylation of signalling protein kinases and Ca(2+) flux were measured in B-CLL cells from untreated patients and normal B cells from age-matched volunteers. Normal B cells and approximately 50% of CD180(+) B-CLL clones responded to CD180 ligation by activation, cycling and increased survival comparable with, or superior to, those induced by anti-CD40 mAb or rIL-4 (Responder B-CLL). Non-responder CD180(+) B-CLL clones failed to respond to CD180 mAb and responded poorly to CD40 mAb and rIL-4. Anti-CD180 mAb induced phosphorylation of ZAP70/Syk, Erk, p38MAPK and Akt in normal B cells and Responder B-CLL cells. In contrast, Erk, p38MAPK and Akt were not phosphorylated in Non-responder B-CLL cells indicating a block in signalling and possible anergy. CD180 may provide powerful expansion and survival signals for Responder B-CLL cells and have an important prognostic value.

Enhanced immunogenicity of pneumococcal surface adhesin A (PsaA) in mice via fusion to recombinant human B lymphocyte stimulator (BLyS)

Background

B lymphocyte stimulator (BLyS) is a member of the tumor necrosis factor superfamily of ligands that mediates its action through three known receptors. BLyS has been shown to enhance the production of antibodies against heterologous antigens when present at elevated concentrations, supporting an immunostimulatory role for BLyS in vivo.

Methods

We constructed a fusion protein consisting of human BLyS and Pneumococcal Surface Adhesin A (PsaA) and used this molecule to immunize mice. The immunostimulatory attributes mediated by BLyS in vivo were evaluated by characterizing immune responses directed against PsaA.

Results

The PsaA-BLyS fusion protein was able to act as a co-stimulant for murine spleen cell proliferation induced with F(ab')₂ fragments of anti-IgM in vitro in a fashion similar to recombinant BLyS, and immunization of mice with the PsaA-BLyS fusion protein resulted in dramatically elevated serum antibodies specific for PsaA. Mice immunized with PsaA admixed with recombinant BLyS exhibited only modest elevations in PsaA-specific responses following two immunizations, while mice immunized twice with PsaA alone exhibited undetectable PsaA-specific serum antibody responses. Sera obtained from PsaA-BLyS immunized mice exhibited high titers of IgG1, IgG2a, IgG2b, and IgG3, but no IgA, while mice immunized with PsaA admixed with BLyS exhibited only elevated titers of IgG1 following two immunizations. Splenocytes from PsaA-BLyS immunized mice exhibited elevated levels of secretion of IL-2, IL-4 and IL-5, and a very modest but consistent elevation of IFN-γ following in vitro stimulation with PsaA. In contrast, mice immunized with either PsaA admixed with BLyS or PsaA alone exhibited modestly elevated to absent PsaA-specific recall responses for the same cytokines. Mice deficient for one of the three receptors for BLyS designated Transmembrane activator, calcium modulator, and cyclophilin ligand [CAML] interactor (TACI) exhibited attenuated PsaA-specific serum antibody responses following immunization with PsaA-BLyS relative to wild-type littermates. TACI-deficient mice also exhibited decreased responsiveness to a standard pneumococcal conjugate vaccine.

Conclusion

This study identifies covalent attachment of BLyS as a highly effective adjuvant strategy that may yield improved vaccines. In addition, this is the first report demonstrating an unexpected role for TACI in the elicitation of antibodies by the PsaA-BLyS fusion protein.

Reviewers

This article was reviewed by Jonathan Yewdell, Rachel Gerstein, and Michael Cancro (nominated by Andy Caton).

Molecular mechanism and function of CD40/CD40L engagement in the immune system

SUMMARY

During the generation of a successful adaptive immune response, multiple molecular signals are required. A primary signal is the binding of cognate antigen to an antigen receptor expressed by T and B lymphocytes. Multiple secondary signals involve the engagement of costimulatory molecules expressed by T and B lymphocytes with their respective ligands. Because of its essential role in immunity, one of the best characterized of the costimulatory molecules is the receptor CD40. This receptor, a member of the tumor necrosis factor receptor family, is expressed by B cells, professional antigen-presenting cells, as well as non-immune cells and tumors. CD40 binds its ligand CD40L, which is transiently expressed on T cells and other non-immune cells under inflammatory conditions. A wide spectrum of molecular and cellular processes is regulated by CD40 engagement including the initiation and progression of cellular and humoral adaptive immunity. In this review, we describe the downstream signaling pathways initiated by CD40 and overview how CD40 engagement or antagonism modulates humoral and cellular immunity. Lastly, we discuss the role of CD40 as a target in harnessing anti-tumor immunity. This review underscores the essential role CD40 plays in adaptive immunity.

Strategic nonmyeloablative conditioning: CD154:CD40 costimulatory blockade at primary bone marrow transplantation promotes engraftment for secondary bone marrow transplantation after engraftment failure

There is an increased risk of failure of engraftment following nonmyeloablative conditioning. Sensitization resulting from failed bone marrow transplantation (BMT) remains a major challenge for secondary BMT. Approaches to allow successful retransplantation would have significant benefits for BMT candidates living with chronic diseases. We used a mouse model to investigate the effect of preparative regimens at primary BMT on outcome for secondary BMT. We found that conditioning with TBI or recipient T cell lymphodepletion at primary BMT did not promote successful secondary BMT. In striking contrast, successful secondary BMT could be achieved in mice conditioned with anti-CD154 costimulatory molecule blockade at first BMT. Blockade of CD154 alone or combined with T cell depletion inhibits generation of the humoral immune response after primary BMT, as evidenced by abrogation of production of anti-donor Abs. The humoral barrier is dominant in sensitization resulting from failed BMT, because almost all CFSE-labeled donor cells were killed at 0.5 and 3 h in sensitized recipients in in vivo cytotoxicity assay, reflecting Ab-mediated cytotoxicity. CD154:CD40 costimulatory blockade used at primary BMT promotes allogeneic engraftment in secondary BMT after engraftment failure at first BMT. The prevention of generation of anti-donor Abs at primary BMT is critical for successful secondary BMT.

Therapeutic interventions targeting CD40L (CD154) and CD40: the opportunities and challenges

CD40 was originally identified as a receptor on B-cells that delivers contact-dependent T helper signals to B-cells through interaction with CD40 ligand (CD40L, CD154). The pivotal role played by CD40-CD40L interaction is illustrated by the defects in B-lineage cell development and the altered structures of secondary lymphoid tissues in patients and engineered mice deficient in CD40 or CD40L. CD40 signaling also provides critical functions in stimulating antigen presentation, priming of helper and cytotoxic T-cells and a variety of inflammatory reactions. As such, dysregulations in the CD40-CD40L costimulation pathway are prominently featured in human diseases ranging from inflammatory conditions to systemic autoimmunity and tissue-specific autoimmune diseases. Moreover, studies in CD40-expressing cancers have provided convincing evidence that the CD40-CD40L pathway regulates survival of neoplastic cells as well as presentation of tumor-associated antigens to the immune system. Extensive research has been devoted to explore CD40 and CD40L as drug targets. A number of anti-CD40L and anti-CD40 antibodies with diverse biological effects are in clinical development for treatment of cancer and autoimmune diseases. This chapter reviews the role of CD40-CD40L costimulation in disease pathogenesis, the characteristics of therapeutic agents targeting this pathway and status of their clinical development.

Pattern recognition by B cells: the role of antigen repetitiveness versus Toll-like receptors

Viruses induce excellent antibody responses due to several intrinsic features. Their repetitive, organised structure is optimal for the activation of the B cell receptor (BCR), leading to an increased humoral response and a decreased dependence on T cell help. Viruses also trigger Toll-like receptors (TLRs), which in addition to increasing overall Ig levels, drive the switch to the IgG2a isotype. This isotype is more efficient in viral and bacterial clearance and will activate complement, which in turn lowers the threshold of BCR activation. Exploiting these characteristics in vaccine design may help us to create vaccines which are as safe as a recombinant vaccine yet still as effective as a virus in inducing B cell responses.

Germinal center helper T cells are dual functional regulatory cells with suppressive activity to conventional CD4+ T cells

Germinal center (GC) reaction is a T cell-dependent process in which activated B cells mature to produce high-affinity Abs and differentiate into memory B cells. The GC microenvironment is almost exclusively reserved for the optimal Ag-specific B cell clonal expansion, selection, and maturation, but lack significant conventional CD4(+) T cell responses. The mechanisms that ensure such a focused B cell response in the GC are not known. In this study, we report that human CD4(+)CD57(+) T cells, which are the major helper T cells in GCs, actively suppress the activation of conventional CD4(+) T cells, particularly Th1 cells, via a direct contact-dependent mechanism and soluble mediators. Our findings demonstrate that GC T cells are unique regulatory cells that provide critical help signals for B cell response but suppress conventional effector T cells in the same local environment.

Association of gamma interferon and interleukin-17 production in intestinal CD4+ T cells with protection against rotavirus shedding in mice intranasally immunized with VP6 and the adjuvant LT(R192G)

Mucosal immunization of mice with chimeric, Escherichia coli-expressed VP6, the protein that comprises the intermediate capsid layer of the rotavirus particle, together with attenuated E. coli heat-labile toxin LT(R192G) as an adjuvant, reduces fecal shedding of rotavirus antigen by >95% after murine rotavirus challenge, and the only lymphocytes required for protection are CD4+ T cells. Because these cells produce cytokines with antiviral properties, the cytokines whose expression is upregulated in intestinal memory CD4+ T cells immediately after rotavirus challenge of VP6/LT(R192G)-immunized mice may be directly or indirectly responsible for the rapid suppression of rotavirus shedding. This study was designed to identify which cytokines are significantly upregulated in intestinal effector sites and secondary lymphoid tissues of intranasally immunized BALB/c mice after challenge with murine rotavirus strain EDIM. Initially, this was done by using microarray analysis to quantify mRNAs for 96 murine common cytokines. With this procedure, the synthesis of mRNAs for gamma interferon (IFN-gamma) and interleukin-17 (IL-17) was found to be temporarily upregulated in intestinal lymphoid cells of VP6/LT(R192G)-immunized mice at 12 h after rotavirus challenge. These cytokines were also produced in CD4+ T cells obtained from intestinal sites specific to VP6/LT(R192G)-immunized mice after in vitro exposure to VP6 as determined by intracellular cytokine staining and secretion of cytokines. Although genetically modified mice that lack receptors for either IFN-gamma or IL-17 remained protected after immunization, these results provide suggestive evidence that these cytokines may play direct or indirect roles in protection against rotavirus after mucosal immunization of mice with VP6/LT(R192G).

TLR9 Signaling in B Cells Determines Class Switch Recombination to IgG2a

Although IgG2a is the most potent Ab isotype in the host response to viral and bacterial infections, the regulation of class switch recombination to IgG2a in vivo is not yet well understood. Recognition of pathogen-associated molecular patterns by dendritic cells expressing TLRs, like TLR7, recognizing ssRNA, or TLR9, recognizing DNA rich in nonmethylated CG motifs (CpG), favors induction of Th1 responses. It is generally assumed that these Th1 responses are responsible for the TLR-mediated induction of IgG2a. Using virus-like particles loaded with CpGs, we show here that TLR9 ligands can directly stimulate B cells to undergo isotype switching to IgG2a. Unexpectedly, TLR9 expression in non-B cells did not affect isotype switching in the Ab response against virus-like particles. Thus, TLR9 can regulate isotype switching to IgG2a directly by interacting with B cells rather than indirectly by inducing Th1 responses.

CD4+ T Lymphocytes Expressing CD40 Ligand Help the IgM Antibody Response to Soluble Pneumococcal Polysaccharides via an Intermediate Cell Type

Streptococcus pneumoniae causes serious infections in children, the elderly, and immunocompromised patients. Protection against infections with S. pneumoniae is mediated through Abs against the capsular polysaccharides (caps-PS). We previously showed that the murine Ab response to caps-PS is dependent on CD40-CD40L interaction. In the present paper, we addressed the question of whether the CD40-CD40L-mediated modulation of the anti-caps-PS immune reaction is the result of a direct interaction between B lymphocytes and T lymphocytes or of an indirect interaction. SCID/SCID mice reconstituted with B lymphocytes from wild-type mice did not mount anti-caps-PS Abs. SCID/SCID mice reconstituted with B lymphocytes from wild-type mice and CD4+ T lymphocytes from wild-type mice but not CD4+ T lymphocytes from CD40L knockout mice stimulated the anti-caps-PS Ab response. This indicated that CD4+ T lymphocytes stimulated the anti-caps-PS Ab response in a CD40L-dependent manner. SCID/SCID mice reconstituted with B lymphocytes from CD40 knockout mice and CD4+ T lymphocytes from wild-type mice generated an anti-caps-PS Ab response that could be inhibited by MR1, a blocking anti-CD40L Ab. These data indicated that CD4+ T lymphocytes stimulated the anti-caps-PS Ab response in an indirect way. Finally, lethally irradiated CD40 knockout mice reconstituted with bone marrow from wild-type mice mounted an anti-caps-PS Ab response that was comparable to the Ab response in wild-type mice, revealing that the required CD40 was on hemopoietic cells. In conclusion, we provide evidence that CD4+ T lymphocytes expressing CD40L stimulate the Ab response to soluble caps-PS by interacting with CD40-expressing non-B cells.

Antagonist anti-human CD40 antibody inhibits germinal center formation in cynomolgus monkeys

Interactions between CD40 on APC and CD154 (CD40L) expressed by activated CD4(+) T cells are crucially involved in formation and function of germinal centers (GC), but mechanistic insight into these interactions remains limited. Functional studies have mostly been restricted to experimental immunization of young-adult inbred SPF rodents that are often genetically manipulated, while studies in humans disallow in vivo manipulation. Therefore, we asked whether a functional antagonist of CD40 interferes with natural GC formation in adult cynomolgus monkeys (Macaca fascicularis) exposed to the environmental antigens of their conventional housing in captivity. Animals were treated with different doses of a unique chimeric antagonist anti-CD40 mAb (ch5D12) and analyzed 1 week or 7 weeks after last injection. Detailed in situ analysis showed that high-dose anti-CD40 treatment increased the ratio of primary over secondary follicles compared to PBS or low-dose treatment, indicative of impairment of the CG reaction. This impairment was reversible since recovery animals, except those with residual anti-CD40 levels, had normalized ratios. Anti-CD40 treatment was associated with decreased antibody production and increased numbers of apoptotic cells in GC. These data demonstrate that CD40-CD154 interactions are pivotal in physiological GC formation in primates responding to environmental antigens, and they support immunotherapeutic strategies using antagonist anti-CD40.

Rapid Response of Marginal Zone B Cells to Viral Particles

Marginal zone (MZ) B cells are thought to be responsible for the first wave of Abs against bacterial Ags. In this study, we assessed the in vivo response of MZ B cells in mice immunized with viral particles derived from the RNA phage Qbeta. We found that both follicular (FO) and MZ B cells responded to immunization with viral particles. MZ B cells responded with slightly faster kinetics, but numerically, FO B cells dominated the response. B1 B cells responded similarly to MZ B cells. Both MZ and FO B cells underwent isotype switching, with MZ B cells again exhibiting faster kinetics. In fact, almost all Qbeta-specific MZ B cells expressed surface IgG by day 5. Histological analysis demonstrated that a population of activated B cells remain associated with the MZ, probably due to the elevated integrin levels expressed by these cells. Thus, both MZ and FO B cells respond with rapid proliferation to viral infection and both populations undergo isotype switching, but MZ B cells remain in the MZ and may be responsible for local Ab production, opsonizing pathogens entering the spleen.

Tracking CD40 signaling during germinal center development

Substantial evidence indicates that signaling through the CD40 receptor (CD40) is required for germinal center (GC) and memory B-cell formation. However, it is not fully understood at which stages of B-cell development the CD40 pathway is activated in vivo. To address this question, we induced CD40 signaling in human transformed GC B cells in vitro and identified a CD40 gene expression signature by DNA microarray analysis. This signature was then investigated in the gene expression profiles of normal B cells and found in pre- and post-GC B cells (naive and memory) but, surprisingly, not in GC B cells. This finding was validated in lymphoid tissues by showing that the nuclear factor-kappaB (NF-kappaB) transcription factors, which translocate to the nucleus upon CD40 stimulation, are retained in the cytoplasm in most GC B cells, indicating the absence of CD40 signaling. Nevertheless, a subset of centrocytes and B cells in the subepithelium showed nuclear staining of multiple NF-kappaB subunits, suggesting that a fraction of naive and memory B cells may be subject to CD40 signaling or to other signals that activate NF-kappaB. Together, these results show that GC expansion occurs in the absence of CD40 signaling, which may act only in the initial and final stages of the GC reaction.

Polarized type 1 cytokine response and cell-mediated immunity determine genetic resistance to mousepox

Ectromelia virus (ECTV), a natural mouse pathogen and an orthopoxvirus, has been used to investigate the correlation between polarized type 1 or type 2 immune responses and resistance to disease in poxvirus infections by using well defined resistant and susceptible mouse strains. Our data show that distinct differences exist in the cytokine profiles expressed in resistant and susceptible mice infected with ECTV. Resistant C57BL/6 mice generate a type 1 cytokine response [IFN-gamma, IL-2, and tumor necrosis factor (TNF)], within the first few days of infection, which is associated with strong cytotoxic T lymphocyte response (CTL) and recovery from ECTV infection. Susceptible strains of mice (BALB/c and A/J) on the other hand generate a type 2 cytokine response (IL-4 but little or no IFN-gamma and IL-2), which is associated with a weak or an absent CTL response, resulting in uncontrolled virus replication and death. Although deletion of IL-4 function alone did not change the outcome of infection in susceptible mice, the loss of IFN-gamma function in resistant mice abrogated natural killer (NK) cell and CTL effector functions resulting in fulminant disease and 100% mortality. Therefore, a clear link exists between the early production of specific type 1 cytokines, in particular, IFN-gamma, the nature of the cellular immune response, and disease outcome in this virus model. This finding in the mousepox model raises the possibility that inappropriate cytokine responses may result in increased susceptibility to smallpox in humans.

The human antibody response to pneumococcal capsular polysaccharides is dependent on the CD40-CD40 ligand interaction

Protection against infections with Streptococcus pneumoniae is mediated by antibodies against the capsular polysaccharides (caps-PS). Here we show that in in vitro experiments CD4+ T lymphocytes stimulate and CD8+ T lymphocytes inhibit the human anti-caps-PS antibody response. Using antagonistic anti-CD40 and antagonistic anti-CD40 ligand (CD40L) monoclonal antibodies, we showed that the CD4+ T lymphocyte-mediated stimulation is dependent on the CD40-CD40L interaction. The role of CD40L was further illustrated by the observation that CD4+ T lymphocytes obtained from a patient with hyper-IgM syndrome were unable to enhance the immune response to caps-PS. Furthermore, CD4+ T lymphocytes from cord blood, which did not express CD40L in response to stimulation with caps-PS, failed to stimulate the antibody response of adult B lymphocytes to caps-PS. These in vitro findings were confirmed by in vivo experiments in which SCID/SCID mice were reconstituted with human mononuclear cells. Furthermore, we showed that caps-PS induce production of IL-4, IL-6, IL-10, and IFN-gamma, and that this enhanced production was inhibited by blocking the CD40-CD40L interaction. This is the first demonstration that the human immune response to caps-PS, which is markedly regulated by T lymphocytes, is dependent on the CD40-CD40L interaction.

CD40 Stimulation of Human Peripheral B Lymphocytes: Distinct Response from Naive and Memory Cells

During secondary immune response, memory B lymphocytes proliferate and differentiate into Ig-secreting cells. In mice, the binding of CD40 by CD154 clearly enhances the activation and differentiation of memory B lymphocytes. In humans, the role of CD40-CD154 in the stimulation of memory B lymphocytes is not as obvious since in vitro studies reported positive and negative effects on their proliferation and differentiation in Ig-secreting cells. In this study, we examine the response of peripheral memory and naive cells in relation to the duration of CD40-CD154 interaction. We measured the proliferation and differentiation of both subsets stimulated with CD154 and IL-4 for short- (4-5 days) and long-term (>7 days) periods. Following short-term stimulation, memory B lymphocytes did not expand but represented the only subset differentiating into IgG- and IgM-secreting cells. A longer stimulation of this population led to cell death, while promoting naive B lymphocyte proliferation, expansion, and differentiation into IgM- or IgG-secreting cells. This prolonged CD40 stimulation also triggered naive B lymphocytes to switch to IgG and to express CD27 even in absence of somatic hypermutation, suggesting that these latter events could be independent. This study suggests that naive and memory B lymphocytes have distinct requirements to engage an immune response, reflecting their different roles in humoral immunity.

Fc-dependent depletion of activated T cells occurs through CD40L-specific antibody rather than costimulation blockade

Although the underlying mechanisms are not well understood, it is generally believed that antigen recognition by T cells in the absence of costimulation may alter the immune response, leading to anergy or tolerance. Further support for this concept comes from animal models of autoimmunity and transplantation, where treatments based on costimulation blockade, in particular CD40 ligand (CD40L)-specific antibodies, have been highly effective. We investigated the mechanisms of action of an antibody to CD40L and provide evidence that its effects are dependent on the constant (Fc) region. Prolongation of graft survival is dependent on both complement- and Fc receptor-mediated mechanisms in a major histocompatibility complex (MHC)-mismatched skin transplant model. These data suggest that antibodies to CD40L act through selective depletion of activated T cells, rather than exerting immune modulation by costimulation blockade as currently postulated. This finding opens new avenues for treatment of immune disorders based on selective targeting of activated T cells.