Rapid induction of a novel costimulatory activity on B cells by CD40 ligand

Molecular basis and therapeutic implications of CD40/CD40L immune checkpoint

The CD40 receptor and its ligand CD40L is one of the most critical molecular pairs of the stimulatory immune checkpoints. Both CD40 and CD40L have a membrane form and a soluble form generated by proteolytic cleavage or alternative splicing. CD40 and CD40L are widely expressed in various types of cells, among which B cells and myeloid cells constitutively express high levels of CD40, and T cells and platelets express high levels of CD40L upon activation. CD40L self-assembles into functional trimers which induce CD40 trimerization and downstream signaling. The canonical CD40/CD40L signaling is mediated by recruitment of TRAFs and NF-κB activation, which is supplemented by signal pathways such as PI3K/AKT, MAPKs and JAK3/STATs. CD40/CD40L immune checkpoint leads to activation of both innate and adaptive immune cells via two-way signaling. CD40/CD40L interaction also participates in regulating thrombosis, tissue inflammation, hematopoiesis and tumor cell fate. Because of its essential role in immune activation, CD40/CD40L interaction has been regarded as an attractive immunotherapy target. In recent years, significant advance has been made in CD40/CD40L-targeted therapy. Various types of agents, including agonistic/antagonistic monoclonal antibodies, cellular vaccines, adenoviral vectors and protein antagonist, have been developed and evaluated in early-stage clinical trials for treating malignancies, autoimmune diseases and allograft rejection. In general, these agents have demonstrated favorable safety and some of them show promising clinical efficacy. The mechanisms of benefits include immune cell activation and tumor cell lysis/apoptosis in malignancies, or immune cell inactivation in autoimmune diseases and allograft rejection. This review provides a comprehensive overview of the structure, processing, cellular expression pattern, signaling and effector function of CD40/CD40L checkpoint molecules. In addition, we summarize the progress, targeted diseases and outcomes of current ongoing and completed clinical trials of CD40/CD40L-targeted therapy.

CD40 Ligand Expression Correlates with Resolution of Peritonitis and Mononuclear Cell Recruitment

Background

CD40 belongs to the tumor necrosis factor receptor family and its ligation is a central event in major inflammatory and immune reactions. We have previously demonstrated that CD40 ligation upregulates the secretion of mononuclear chemokines from peritoneal mesothelial cells (PMC), and that blocking the CD40 ligand (CD154) reduced the mononuclear infiltrate in a model of peritonitis.

Objective

To characterize the kinetics of CD154 expression on peritoneal leukocytes and examine the correlation of this occurrence with the mononuclear transition at the resolution phase of peritonitis.

Methods

Leukocytes were collected from the effluent of 11 patients during episodes of peritonitis while undergoing peritoneal dialysis (PD). The effluent was then analyzed by flow cytometry to characterize CD154 expression.

Results

CD154 expression on peritoneal mononuclear cells gradually increased during the resolution phase of peritonitis, peaking first on T cells (CD4+ and CD8+ cells at 20 – 45 hours) and then on macrophages (CD14+ at 20 – 50 hours). The maximal expression of CD154 on macrophages, CD4+ cells, and CD8+ cells during peak hours reached values of 33% ± 23%, 4% ± 3%, and 24% ± 17%, respectively. The increase in CD154 expression was in negative correlation ( r=–0.44, p = 0.032) with total leukocyte numbers and in positive correlation ( r = 0.52, p = 0.009) with the increase of mononuclear cells. Deterioration of peritonitis was associated with a decrease in CD154 levels, while recurrence of peritonitis was related to high CD154 levels.

Conclusion

Our data, which show a positive correlation between CD154 levels and mononuclear dominance, suggest that CD40–CD154 ligation plays an important role in the transition to mononuclear predominance in the late phase of peritonitis.

LTβR and CD40: working together in dendritic cells to optimize immune responses

Generating an immune response tailored to destroy an infecting organism while limiting bystander damage involves guiding T-cell activation using a variety of cues taken from the immunogen (antigen type, dose, and persistence, accompanying danger signals) as well as the host (tissue environment, T-cell frequency, and affinity for antigen). Dendritic cells (DCs) serve as translators of much of this information and are critically required for effective pathogen and tumor clearance. Moreover, dysregulation of DC activation can lead to autoimmunity. Inhibition of the lymphotoxin (LT) and CD40 pathways has been shown to be effective at quieting inflammation in settings where DC-T-cell interactions are key instigators of disease progression. In this review, we compare and contrast the CD40 and LT pathways in the context of receptor/ligand expression, signal transduction, and DC biology. We provide evidence that these two pathways play complementary roles in DC cytokine secretion, thus indirectly shaping the nature of the CD8(+) T-cell response to foreign antigen. Given the distinct role of these pathways in the context of DC function, we propose that dual therapies targeted at both the CD40 and LTβ receptor may have therapeutic potential in silencing DC-driven autoimmunity or in promoting tumor clearance.

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.

CD154 Is Essential for Protective Immunity in ExperimentalSalmonellaInfection: Evidence for a Dual Role in Innate and Adaptive Immune Responses

CD40-CD154 interactions are of central importance in the induction of humoral and cellular immune responses. In the present study, CD154-deficient (CD154-/-) mice were used to assess the role of CD40-CD154 interactions in regulating the immune response to a systemic Salmonella infection. Compared with C57BL/6 (CD154+/+) controls, CD154-/- mice were hypersusceptible to infection by an attenuated strain of Salmonella enterica serovar Typhimurium (S. typhimurium), as evidenced by decreased survival rate and mean time to death, which correlated with increased bacterial burden and persistence in target organs. CD154-/- mice exhibited a defect both in the production of IL-12, IFN-gamma, and NO during the acute phase of the disease and in the generation of Salmonella-specific Ab responses and Ig isotype switching. Furthermore, when CD154-/- animals were administered a sublethal dose of attenuated S. typhimurium and subsequently challenged with a virulent homologous strain, all mice succumbed to an overwhelming infection. Similar treatment of CD154+/+ mice consistently resulted in > or =90% protection. The lack of protective immunity in CD154-/- mice correlated with a decreased T cell recall response to Salmonella Ags. Significant protection against virulent challenge was conferred to presensitized CD154-/- mice by transfer of serum or T cells from immunized CD154+/+ mice. For best protection, however, a combination of immune serum and T cells was required. We conclude that intercellular communications via the CD40-CD154 pathway play a critical role in the induction of type 1 cytokine responses, memory T cell generation, Ab formation, and protection against primary as well as secondary Salmonella infections.

CD40-induced aggregation of MHC class II and CD80 on the cell surface leads to an early enhancement in antigen presentation

Ligation of CD40 on B cells increases their ability to present Ag and to activate MHC class II (MHC-II)-restricted T cells. How this occurs is not entirely clear. In this study we demonstrate that CD40 ligation on Ag-presenting B cells (APC) for a short period between 30 min and 3 h has a rapid, augmenting effect on the ability of a B cell line and normal B cells to activate T cells. This is not due to alterations in Ag processing or to an increase in surface expression of CD80, CD86, ICAM-1, or MHC-II. This effect is particularly evident with naive, resting T lymphocytes and appears to be more pronounced under limiting Ag concentrations. Shortly after CD40 ligation on a B cell line, MHC-II and CD80 progressively accumulated in cholesterol-enriched microdomains on the cell surface, which correlated with an initial enhancement in their Ag presentation ability. Moreover, CD40 ligation induced a second, late, more sustained enhancement of Ag presentation, which correlates with a significant increase in CD80 expression by APC. Thus, CD40 signaling enhances the efficiency with which APC activate T cells by at least two related, but distinct, mechanisms: an early stage characterized by aggregation of MHC-II and CD80 clusters, and a late stage in which a significant increase in CD80 expression is observed. These results raise the possibility that one important role of CD40 is to contribute to the formation of the immunological synapse on the APC side.

CD40 is Necessary for Activation of Naive T Cells by a Dendritic Cell Line In Vivo but not In Vitro

The importance of CD40-CD40L interactions during CD4(+) T-cell activation has been extensively investigated over the years; however, it still remains questionable whether the interaction is a prerequisite for dendritic cell (DC)-mediated antigen-specific priming in vivo. Naïve CD4(+) T cells require two signals for proper activation and induction of differentiation: signal 1 is provided by peptide antigens in the context of the major histocompatibility complex (MHC) class II, while signal 2 is delivered by costimulatory molecules such as CD80 or CD86 present on the antigen-presenting cell (APC). It is well known that the expression of CD80/CD86 is upregulated after interaction between CD40 on APCs and CD40L expressed by at least partly activated T cells. We used a DC line, JawsII, to compare the importance of CD40 expression and downstream signalling in vitro and in vivo. JawsII cells represent pre-immature bone marrow-derived DCs expressing low levels of MHC molecules, low levels of B7 molecules and no CD40. We have previously shown that JawsII cells, despite the lack of CD40 expression, are capable of priming naïve allogeneic T cells in vitro. In correlation with the current literature, we present data showing that constitutive expression of CD40 significantly increases the priming capacity of JawsII cells in vitro. In addition, we show that CD40 expression is required for JawsII cell-dependent T-cell priming in vivo.

Clearance of Pneumocystis carinii in mice is dependent on B cells but not on P carinii-specific antibody

Both CD4(+) T cells and B cells are critical for defense against Pneumocystis carinii infection; however, the mechanism by which B cells mediate protection is unknown. We show that P. carinii-specific IgM is not sufficient to mediate clearance of P. carinii from the lungs since CD40-deficient mice produced normal levels of specific IgM, but were unable to clear the organisms. Using chimeric mice in which the B cells were deficient in CD40 (CD40KO chimeras) we found that clearance of P. carinii infection is delayed compared with wild-type controls. These CD40KO chimeric mice produced normal levels of P. carinii-specific IgM, but did not produce class-switched IgG or IgA. Similarly, clearance of P. carinii was delayed in mice deficient in FcgammaRI and III (FcgammaRKO), indicating that P. carinii-specific IgG partially mediates opsonization and clearance of P. carinii. Opsonization of organisms by complement did not compensate for the lack of specific IgG or FcgammaR, since C3-deficient and C3-depleted FcgammaRKO mice were still able to clear P. carinii. Finally, micro MT and CD40KO chimeric mice had reduced numbers of activated CD4(+) T cells in the lungs and lymph nodes compared with wild-type mice, suggesting that B cells are important for activation of T cells in response to P. carinii. Together these data indicate that P. carinii-specific IgG plays an important, but not critical, role in defense against P. carinii. Moreover, these data suggest that B cells also mediate host defense against P. carinii by facilitating CD4(+) T cell activation or expansion.

Differentiation of monocytic cell clones into CD8 alpha+ dendritic cells (DC) suggests that monocytes can be direct precursors for both CD8 alpha+ and CD8 alpha- DC in the mouse

Dendritic cells (DC) are the professional APCs that initiate T cell immune responses. DC can develop from both myeloid and lymphoid progenitors. In the mouse, the CD8alpha(+) DC had been designated as "lymphoid" DC, and CD8alpha(-) DC as "myeloid" DC until recently when it was demonstrated that common myeloid progenitors can also give rise to CD8alpha(+) DC in bone marrow chimera mice. However, it is still not clear which committed myeloid lineages differentiate into CD8alpha(+) DC. Because monocytes can differentiate into DC in vivo, the simplest hypothesis is that the CD8alpha(+) DC can be derived from the monocyte/macrophage. In this study we show that cell clones, isolated from CD8alpha(+) DC lymphoma but with a monocytic phenotype (CD11c(low/-)D11b(high)CD8alpha(-)I-A(low)), can redifferentiate into CD8alpha(+) DC either when stimulated by LPS and CD40L or when they migrate into the lymphoid organs. Maturation of DC in vivo correlated with strong priming of allogeneic T cells. Moreover, the monocytes from cultured splenocytes or peritoneal exudates macrophages of wild-type mice are also capable of differentiating into CD11c(+)CD8alpha(+) DC after their migration into the draining lymph nodes. Our results suggest that monocytes can be direct precursors for CD11c(+)CD8alpha(+) DC in vivo. In addition, the monocyte clones described in this study may be valuable for studying the differentiation and function of CD8alpha(+) DC that mediate cross-presentation of Ag to CD8 T cells specific for cell-associate Ags.

Selective requirement for CD40-CD154 in drug-induced type 1 versus type 2 responses to trinitrophenyl-ovalbumin

CD154 is transiently expressed by activated T cells and interacts with CD40 on B cells, dendritic cells, macrophages, and monocytes. This costimulatory receptor-ligand couple seems decisive in Ag-driven immune responses but may be differentially involved in type 1 vs type 2 responses. We studied the importance of CD40-CD154 in both responses using the reporter Ag popliteal lymph node assay in which selectively acting drugs generate clearly polarized type 1 (streptozotocin) or type 2 (D-penicillamine, diphenylhydantoin) responses to a constant coinjected Ag in the same mouse strain. Treatment of mice with anti-CD154 reduced characteristic immunological parameters in type 2 responses (B and CD4(+) T cell proliferation, IgG1 and IgE Abs, and IL-4 secretion) and only slightly affected the type 1 response (small decrease in IFN-gamma production, influx of CD11c(+) and F4/80(+) cells, and prevention of architectural disruption of the lymph node, but no effect on IgG2a Ab and TNF-alpha secretion or B and CD4(+) T cell proliferation). The findings indicate that the CD40-CD154 costimulatory interaction is a prerequisite in drug-induced type 2 responses and is only marginally involved in type 1 responses. The observed expression patterns of CD80 and CD86 on different APC (B cells in type 2 and dendritic cells in type 1) may be responsible for this discrepancy.

Vav-induced activation of the human IFN-gamma gene promoter is mediated by upregulation of AP-1 activity

The role of Vav in the transcriptional regulation of the human interferon-gamma (IFN-gamma) promoter was investigated. Overexpression of Vav in Jurkat-TAg cells enhanced T cell receptor (TCR)-induced activation of a luciferase (Luc) reporter gene construct driven by cis-regulatory element of the IFN-gamma gene (-346 to +7). Electrophoresis mobility shift and Luc reporter assays demonstrated that the DNA-binding and transcriptional activity of the proximal AP-1-dependent NFAT site (positions -172 to -138), the AP-1/Ying-Yang 1 (YY1)-binding site (-209 to -184), and a consensus AP-1-binding site were upregulated by Vav. Vav enhanced TCR-induced activation of c-Jun N-terminal kinase (JNK) and its upstream regulator, Rho family GTPases. Finally, coexpression of a dominant-negative Rac1 mutant suppressed Vav-mediated upregulation of the transcriptional and DNA-binding activity of the proximal NFAT/AP-1 site and the AP-1/YY1 site, as well as the complete IFN-gamma promoter activity. Vav activates the IFN-gamma promoter via upregulation of AP-1-binding through a Rac1/JNK pathway.

Analysis of the oligomeric requirement for signaling by CD40 using soluble multimeric forms of its ligand, CD154

We describe the construction of a novel soluble dodecameric form of CD154 (CD40 ligand) that is more effective than trimeric tCD154 in triggering B cell activation. Dodecameric surfactant protein (SP)-D-CD154 was more potent than tCD154 in inducing B cell proliferation over a wide range of concentrations. At saturating concentrations, the level of proliferation triggered by SP-D-CD154 was fourfold higher than that achieved with tCD154. Moreover, stimulation with dodecameric CD154 induced higher levels of the costimulatory molecules ICAM-1 and CD86. The higher activity of dodecameric CD154 when compared to trimeric CD154 is unlikely to be due to differences in their avidity for CD40, since both forms bound to CD40 strongly. Therefore, the extent of receptor clustering directly regulates signaling by CD40.

Inhibition of the function of the FcgammaRIIB by a monoclonal antibody to thymic shared antigen-1, a Ly-6 family antigen

Thymic shared antigen-1 (TSA-1) is a member of the Ly-6 family of glycosyl-phosphatidylinositol (GPI)-linked proteins. While it has been proposed that TSA-1 may play a role in thymic development, a physiological ligand for this antigen has not been identified. Here we report that a monoclonal antibody (mAb) to TSA-1, generated by immunizing a hamster with CD40 ligand (CD40L)-activated B cells, interferes with the function of FcgammaRIIB on splenic B cells and the B-cell lymphoma cell line, M12, by binding to TSA on the same cells. The interaction of anti-TSA with FcgammaRIIB resulted in an inhibition of the ability of the FcgammaRIIB to cross-link and/or aggregate soluble anti-CD3 or soluble anti-Cbeta T-cell receptor (TCR), leading to an inhibition of induction of expression of CD25 and CD69, interleukin (IL)-2 production and proliferation of naive T cells. Cross-blocking studies with mAbs strongly suggested that a physical association exists between TSA-1 and the FcgammaRIIB on the surface of activated B cells and favour the view that a functional intermolecular association exists between these two distinct membrane antigens.

Chronic antigen-specific immune-system activation may potentially be involved in the loosening of cemented acetabular components

Previous studies have attempted to determine whether aseptic loosening and osteolysis are caused by a T cell-mediated type IV hypersensitivity reaction or a nonspecific foreign body reaction involving phagocytic macrophages. The purpose of this study was to examine the role of the B7-CD28 costimulatory pathway (which is indicative of an activated immune response) in loosening and osteolysis of total joint replacements (TJRs). We harvested periprosthetic tissues from 24 loose, cemented, all polyethylene, acetabular components in patients undergoing revision total hip replacement surgery for aseptic loosening. Prostheses were classified radiographically as to whether ballooning, scalloping osteolysis was present or not. Monoclonal antibodies were used to identify macrophages, antigen presenting cells (APCs) expressing B7-1 or B7-2, total T lymphocytes, and T cells expressing CD28 or CTLA-4. The large numbers of positive cells, including macrophages, T cells, and APCs in both groups are substantially higher than previously reported. Macrophages constituted the predominant cell type, the majority of which were APCs. B7-1 was expressed by 18.3% of all cells, and B7-2 was expressed by 61.0% of cells. Despite the fact that there were no statistically significant differences in expression of proteins in the B7-CD28 pathway between the osteolytic and nonosteolytic groups, the magnitude of positive staining suggests that the process of aseptic loosening (not osteolysis) may involve proteins of the B7-CD28 pathway, particularly B7-2. One possible antigenic stimulus is protein-coated particulate wear debris from prosthetic materials.

Identification of a CD28 response element in the CD40 ligand promoter

Ligation of the T cell coreceptor CD28 or CD2 by its cognate ligands B7-1 or LFA-3, respectively, greatly aids the Ag-induced up-regulation of several genes, including IL-2 and CD40 ligand (CD40L). Using luciferase reporter constructs under the control of the 1.2 kb of 5' noncoding region of the human CD40L gene, we have found that stimulation through CD28 was required for a strong transcriptional activity of the CD40L promoter in response to TCR ligation, while the activity induced by CD2 was slightly lower than CD28. Deletion analysis demonstrated that the transcriptional elements mediating this effect were located within a 300-bp region upstream of the start site. Further dissection of this region and gel shift analyses demonstrated the presence of a CD28 response element in a region located between nucleotides -170 to -164 relative to the start site. Transcriptional studies with a CD40L enhancer-promoter carrying a mutation in this putative CD28 response element revealed that the activity was reduced by 80 and 70% after B7-1 and LFA-3 costimulation, respectively. The transcription factor complex bound to this site contained at least JunD, c-Fos, p50, p65, and c-REL:, but not c-Jun. Mutations introduced into the CD28RE also blocked the binding of this complex. These observations identify an important role for the CD28 signaling pathway in the regulation of CD40L promoter transcriptional activity.

Resting B lymphocytes as APC for naive T lymphocytes: dependence on CD40 ligand/CD40

Although resting B cells as APC are tolerogenic for naive T cells in vivo, we show here that they can provide all the costimulatory signals necessary for naive T cell proliferation in vivo and in vitro. In the absence of an activating signal through the B cell Ag receptor, T cell proliferation after Ag recognition on resting B cells depends on CD40 expression on the B cells, implying that naive T cells use the membrane-bound cytokine, CD40 ligand (CD154), to induce the costimulatory signals that they need. Induction of B7-1 (CD80) and increased or sustained expression of CD44H, ICAM-1 (CD54), and B7-2 (CD86) are dependent on the interaction of CD40 ligand with CD40. Transient expression (12 h) of B7-2 is T cell- and peptide Ag-dependent, but CD40-independent. Only sustained (>/=24 h) expression of B7-2 and perhaps increased expression of ICAM-1 could be shown to be functionally important in this system. T cells cultured with CD40-deficient B cells and peptide remain about as responsive as fresh naive cells upon secondary culture with whole splenic APC. Therefore, B cells, and perhaps other APC, may be tolerogenic not because they fail to provide sufficient costimulation for T cell proliferation, but because they are deficient in some later functions necessary for a productive T cell response.

CD4+ T Cell Responses to CD40-Deficient APCs: Defects in Proliferation and Negative Selection Apply Only with B Cells as APCs

During T-APC interactions in vivo, interfering with CD40-CD154 interactions leads to reduced T cell priming, defects in effector function, and, in some cases, T cell tolerance. As shown here, however, presentation of conventional peptide Ags by CD40-deficient spleen APC in vitro leads to normal CD4+ T cell proliferative responses. By contrast, responses to the same peptides presented by purified B cells were markedly reduced in the absence of CD40. Thus, the requirement for CD40-CD154 interactions appears to be strongly influenced by the type of APC involved. Analysis of responses to endogenous superantigens, which are known to be strongly dependent on B cells for presentation, indicated that CD4+ responses to strong Ags are less dependent on CD40 than are responses to weak Ags. Similar findings applied to negative selection in the thymus. Thus, deletion of potentially autoreactive cells depended on CD40 expression when B APC were involved, and this requirement was most pronounced when negative selection was directed to weak Ags.

Blocking CD40 - CD154 and CD80/CD86 - CD28 interactions during primary allogeneic stimulation results in T cell anergy and high IL-10 production

Although CD28 triggering provides an important co-stimulatory signal to T cells, blocking the CD80/CD86 - CD28 interaction with CTLA-4lg fusion protein is not sufficient for tolerance induction in vivo or in vitro. According to more recent data, interruption of the CD40 - CD154 interaction might complement the effect of CTLA-4lg and induce graft acceptance. We studied the effects of a blocking anti-CD40 monoclonal antibody (mAb) and/or blocking anti-CD80/anti-CD86 mAb in cultures of human peripheral blood mononuclear cells (PBMC) stimulated with allogeneic PBMC. T cells activated by alloantigens in the presence of anti-CD80, anti-CD86 and anti-CD40 entered a state of alloantigen-specific non-responsiveness as evidenced upon restimulation by lack of proliferation, cytotoxic activity, and IL-2, IL-5 and IL-13 production. IFN-gamma production during restimulation was less than in the control cultures, while the production of IL-10 was enhanced. Addition of recombinant IL-2 during the restimulation rescued alloantigen-specific activity. We conclude that the simultaneous blocking of the CD40 - CD154 and CD80/CD86 - CD28 interaction during allogeneic T cell activation induces T cell anergy. Since anergic cells induced by this treatment still produce high levels of IL-10, the latter could contribute to modulation of antigen-presenting cell activity and to bystander suppression of residual reactive T cells.

CD40 and CD154 in cell-mediated immunity

CD40-CD154-mediated contact-dependent signals between B and T cells are required for the generation of thymus dependent (TD) humoral immune responses. CD40-CD154 interactions are however also important in many other cell systems. CD40 is expressed by a large variety of cell types other than B cells, and these include dendritic cells, follicular dendritic cells, monocytes, macrophages, mast cells, fibroblasts, and endothelial cells. CD40- and CD154-knockout mice and antibodies to CD40 and CD154 have helped to elucidate the role of the CD40-CD154 system in immune responses. Recently published studies indicate that CD40-CD154 interactions can influence T cell priming and T cell-mediated effector functions; they can also upregulate costimulatory molecules and activate macrophages, NK cells, and endothelia as well as participate in organ-specific autoimmune disease, graft rejection, and even atherosclerosis. This review focuses on the role of the CD40-CD154 system in the regulation of many newly discovered functions important in inflammation and cell-mediated immunity.

CD40 ligand is not essential for induction of type 1 cytokine responses or protective immunity after primary or secondary infection with histoplasma capsulatum

The induction of type 1 immune responses (interleukin [IL]-12, interferon [IFN]-gamma) has been shown to be important in mediating protection against many intracellular infections including Histoplasma capsulatum. Costimulatory molecules such as CD40 ligand (CD40L) have been shown to be a central regulator of type 1 responses in vivo. To study the role of CD40L in mediating protection against infection with H. capsulatum, CD40L-deficient (CD40L-/-) and CD40L+/+ mice were infected with H. capsulatum and assessed for various parameters. After a lethal challenge of H. capsulatum, CD40L-/- mice were not substantially different from CD40L+/+ mice in terms of mortality, fungal burden, or production of IFN-gamma, IL-12, nitric oxide, or tumor necrosis factor alpha. Moreover, CD40L-/- mice treated with anti-IFN-gamma or anti-IL-12 at the time of infection had accelerated mortality, providing further evidence that IL-12 and IFN-gamma are produced in vivo in the absence of CD40L. In addition, CD40L-/- mice infected with a sublethal dose of H. capsulatum survived infection, whereas all mice infected with the same dose and treated with anti-IFN-gamma had accelerated mortality, demonstrating that IFN-gamma but not CD40L was essential for primary immunity to H. capsulatum infection. Interestingly, depletion of either CD4+ or CD8+ T cells resulted in accelerated mortality in CD40L-/- mice, suggesting a critical role for these cells in response to infection. Finally, CD40L-/- mice initially infected with a sublethal dose of H. capsulatum were protected from secondary infection with a lethal dose of H. capsulatum, demonstrating that CD40L is not required for the maintenance of memory immunity.

CD40 Ligand/CD40 Stimulation Regulates the Production of IFN-γ from Human Peripheral Blood Mononuclear Cells in an IL-12- and/or CD28-Dependent Manner

CD40 ligand (CD40L)/CD40 costimulation is an important regulator of Th1 responses. Two mechanisms by which CD40L/CD40 stimulation may enhance IFN-γ are via direct induction of IL-12 and augmentation of the expression of costimulatory molecules such as B7 from APCs. We examined the ability of CD40L/CD40 stimulation to regulate the production of IFN-γ through IL-12 and/or CD28 costimulation from human PBMCs stimulated with T cell-specific stimuli. The roles of exogenous and endogenous CD40L/CD40 stimulation were evaluated using a trimeric soluble CD40L agonist (CD40T) and an anti-CD40L Ab, respectively. The presence of CD40T in cultures increased the production of IL-12 and IFN-γ from PBMCs stimulated with varying amounts of PHA. The mechanism, however, by which CD40T enhanced IFN-γ varied according to the level of T cell activation. Under maximal stimulatory conditions (PHA, 1/100), an IL-12-dependent pathway was dominant. At relatively low levels of T cell stimulation (PHA, 1/500 and 1/1000), however, an additional IL-12-independent CD28-dependent pathway was elucidated. We further studied the role of exogenous CD28 stimulation in regulating the production of IFN-γ. The enhancement of IFN-γ production induced by direct CD28 stimulation was primarily dependent on endogenous IL-12 or CD40L/CD40 stimulation. Together, these data suggest that the production of IFN-γ involves a complex interaction between two interdependent, yet distinct, costimulatory pathways and provide evidence that CD40T may be an effective adjuvant for the enhancement of responses.

Antigen-Primed T Cells from B Cell-Deficient JHD Mice Fail to Provide B Cell Help

B cells are effective in presenting Ag to primed T cells, but a specific role for B cells in priming naive T cells has not been clearly established. In this report we demonstrate that Ag administration to B cell-deficient JHD mice primes T cells for Ag-specific proliferative responses, but such primed T cells fail to provide help for isotype switching and IgG production in B cells. Although T cells primed in the absence of B cells could proliferate in response to Ag presented by B cells and could induce Ag-specific IgM production, such T cells failed to produce high levels of IL-4 as are normally induced in T cells by Ag-presenting B cells. These findings suggest that while B cells are not absolutely required for T cell priming, they provide signals to T cells that are not replicated by other APCs and that influence the subsequent ability of T cells to interact with B cells.

Homotypic interaction of the heat-stable antigen is not responsible for its co-stimulatory activity for T cell clonal expansion

The heat-stable antigen (HSA) is an important co-stimulatory molecule on antigen-presenting cells (APC). However, the receptor on T cells that receives the co-stimulatory signal from HSA has not been identified. Because the HSA is transiently expressed on T cells after the T cell receptor/CD3 complex is engaged, and because it can bind to itself in a homotypic fashion, it has been proposed that homotypic interaction of HSA is responsible for its co-stimulatory activity. Here we test this hypothesis using mice that have a targeted mutation of the HSA gene, as well as novel transgenic mice that constitutively express HSA on T cells. We show that HSA-deficient T cells remain responsive to co-stimulation by HSA. Furthermore, constitutive expression of HSA does not enhance T cell response to co-stimulatory by HSA. Taken together, our results demonstrate that homotypic interaction of HSA is not responsible for co-stimulation mediated by HSA expressed on APC.

The CD40-CD154 system in anti-infective host defense

Research in the past few years has documented significant advances in our understanding of the CD40-CD40 ligand (CD154) system in diverse immune functions. This system influences many T cell mediated inflammatory immune responses and effector functions, unmasking a previously unexpected role for CD40-CD154 in cell mediated immunity. Manipulation of CD154 in animal models of infection by the use of CD154-deficient mice or anti-CD154 antibodies has shown the importance of this system in the initiation of the inflammatory response, in the activation of antigen-presenting cells and in resistance to infections.

CTLA-4-B7 interaction is sufficient to costimulate T cell clonal expansion

T cell costimulation, particularly by the B7 family members B7-1 and B7-2, plays a critical role in regulating T cell-mediated immunity. Two molecules on T cells, CD28 and CTLA-4, are known to bind to B7. It has been suggested that CD28-B7 interaction promotes T cell response, whereas B7-CTLA-4 interaction downregulates T cell clonal expansion. However, the proposed responses of individual receptors to B7 have not been verified directly. Here, we report that B7-1 promotes clonal expansion of CD28-deficient T cells, and that the CD28-independent costimulatory activity is mediated by CTLA-4, as it is completely blocked by intact and Fab of anti-CTLA-4 mAb. In addition, a mutant B7-1 molecule, B7W88 >A, which has lost binding to CD28 but retained significant CTLA-4 binding activity, promotes T cell clonal expansion. Furthermore, while presence of CD28 enhances T cell response to B7-1, such response is also completely blocked by anti-CTLA-4 mAb. Taken together, our results demonstrate that B7-CTLA-4 interaction promotes T cell clonal expansion, and that optimal T cell response to B7 is achieved when both CD28 and CTLA-4 interact with B7. These results establish an important function of CTLA-4 in promoting T cell activation, and suggest an alternative interpretation of the function of CTLA-4 in T cell activation.

Suppression of murine thyroiditis via blockade of the CD40-CD40L interaction

The CD40 ligand (gp39) is transiently expressed on activated CD4+ T cells and mediates cognate helper function by interacting with CD40 on B cells. Increasing evidence suggests, however, critical involvement of gp39 not only in antibody-mediated responses but also in the development of effector T cells. Here, we have investigated the effect of in vivo gp39 blockade on the induction of murine experimental autoimmune thyroiditis (EAT), a T-cell-mediated disease. Over a 5-week period, EAT was induced in SJL mice with thyroglobulin (Tg) and adjuvant. Concomitantly, mice received intraperitoneal (i.p.) injections of MR1, a gp39-specific hamster monoclonal antibody (mAb), at 4-day intervals. Control mice were challenged with Tg but received equivalent doses of hamster immunoglobulin (HIg). It was observed that the control mice developed severe thyroiditis whereas the MR1-treated mice exhibited very low levels of infiltration that were mostly focal in nature. Blockade of gp39 was effective since the Tg-specific IgG titres were low or undetectable in all MR1-treated animals compared with the controls. In addition, upon restimulation with Tg in vitro, lymph node cells (LNC) from Tg-primed, MR1-treated mice proliferated less strongly and secreted significantly lower amounts of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) than LNC from untreated or HIg-treated controls. These results strongly suggest that in vivo blockade of gp39 suppresses EAT by inhibiting the priming of inflammatory Tg-specific T-helper type 1 cells.

The CD40 ligand. At the center of the immune universe?

For several years, the primary function of CD40 ligand (CD40L) has been believed to be in regulation of contact-dependent, CD40-CD40L-mediated signals between B- and T-cells, which are essential for the regulation of thymus-dependent (TD) humoral immune responses. Recently, a flurry of reports indicate that CD40 is expressed by variety of cell types other than B-cells that include dendritic cells, follicular dendritic cells, monocytes, macrophages, fibroblasts, and endothelial cells. These studies show that CD40-CD40L interactions are important in inflammatory process. For the past few years, through the availability of CD40L-knockout mice, new data have emerged to support the belief that CD40L has many more functions than its role in TD humoral immunity. CD40L-deficient mice have provided significant information towards our understanding of the in vivo role of CD40L. The current picture that emerges indicates that CD40-CD40L interactions mediate many cell-mediated immune responses and T-cell-mediated effector functions that are required for proper functioning of the host defense system. This article focuses on the in vivo role of the CD40L in regulation of cell-mediated effector functions.

The role of CD40 ligand in costimulation and T-cell activation

It is clear by now that cell-to-cell interactions involving a variety of signals are required for effective immune response. The data reviewed here suggest that CD40-CD40L interactions are critical for development of CD4 T-cell-dependent effector functions. Lack of this important interaction results in greatly reduced activation of CD4 T cells, while successful interaction of these molecules results in full activation of these T cells. Consequently, the absence of CD40-CD40L interactions leads to impairment of T-cell effector such as help for B-cell differentiation and class switch, activation of monocytes and macrophages to produce cytokines and to kill intracellular pathogens, and activation of autoreactive T cells to mount an autoimmune response. The effector functions of T cells controlled by CD40-CD40L interactions in a successful immune response are given in Table I. Data presented so far suggest that CD40-CD40L interactions play a role in early signalling events, where interactions of this kind are required to induce expression of costimulatory molecules on APC. One possible sequence of events in that APC, like DC, take up antigens at the site of injury or infection and migrate to lymph nodes, where they present antigens complexed with MHC class II molecules to naive T cells. This results in expression of CD40L on T cells. Coupling of this newly expressed CD40L on T cells with CD40 on APC results in expression of the costimulatory activity of the APC. At this time the costimulatory signal provided by the APC is received by the T cells via CD28/CTLA-4, which drives the cell to enter into cell cycle and complete T cell activation. T cells thereby activated can now enter into secondary cognate CD40-CD40L-dependent effector recognition with B cells to switch Ig class, macrophages to produce cytokines and new DC carrying the same antigen to up-regulate costimulatory activity. A tight regulation of expression of CD40L on T cells and costimulatory activity on APC would prevent activation of unwanted bystander T cells. The coupling of activation of the APC primed with the cognate antigen to the activation of the T-cell specific for that antigen in this model provides an additional regulatory step in the initiation of the immune response. This also suggests that a limited number of T cells/APC will be activated, both of which will be specific in nature. This additional step may be important for safeguarding against an autoimmune response. In addition, the fact that CD40L uniquely seems to play this role suggests that selective immunotherapies to treat autoimmune disease and prevent graft rejection can be targeted on this molecule. On the other hand, CD40-directed approaches to up-regulate costimulatory activity on APC could be developed to fight tumor growth, contain infections and treat immunodeficiencies.

Requirement for CD40 ligand in costimulation induction, T cell activation, and experimental allergic encephalomyelitis

The mechanism of CD40 ligand (CD40L)-mediated in vivo activation of CD4(+) T cells was examined by investigation of the development of experimental allergic encephalomyelitis (EAE) in CD40L-deficient mice that carried a transgenic T cell receptor specific for myelin basic protein. These mice failed to develop EAE after priming with antigen, and CD4(+) T cells remained quiescent and produced no interferon-gamma (IFN-gamma). T cells were primed to make IFN-gamma and induce EAE by providing these mice with B7.1(+) antigen-presenting cells (APCs). Thus, CD40L is required to induce costimulatory activity on APCs for in vivo activation of CD4(+) T cells to produce IFN-gamma and to evoke autoimmunity.

CD40 ligand-dependent T cell activation: requirement of B7-CD28 signaling through CD40

The role of CD40 ligand (CD40L) in the primary activation of T cells is not clear. The cellular and humoral immune responses to adenoviral vectors in a murine model of liver-directed gene transfer were studied to define the mechanisms responsible for CD40L-dependent T cell priming. CD40L-deficient mice did not develop effective cytotoxic T cells to transduced hepatocytes, and T cell-dependent B cell responses were absent. Full reconstitution of cellular and humoral immunity was achieved in CD40L-deficient mice by administration of an activating antibody to CD40 that increased expression of B7.2 on spleen cells. Wild-type mice could be made nonresponsive to vector by administration of antibodies to B7. Thus, CD40L-dependent activation of T cells occurs through signaling of CD40 in the antigen-presenting cell to enhance requisite costimulatory pathways that include B7.

Identification of a costimulatory molecule rapidly induced by CD40L as CD44H

The interaction between CD40 ligand and CD40 is critical for activation of T and B cells in vivo. We have recently demonstrated that this interaction rapidly induces a novel costimulatory activity distinct from B7 and independent of CD28. To study the molecular basis of the costimulatory activity, we have produced a novel monoclonal antibody, TM-1, that binds an 85-kilodalton costimulatory molecule rapidly induced by CD40L. Expression cloning reveals that TM-1 binds CD44H. CD44H expressed on Chinese hamster ovary cells has potent costimulatory activity for clonal expansion of T cells isolated from both wild-type mice and these with a targeted mutation of CD28. Thus, CD44H costimulates T cell proliferation by a CD28-independent mechanism. These results revealed that CD44H is a costimulatory molecule rapidly induced by CD40L.

Mechanisms underlying T-cell tolerance

To understand how the immune system manages to disarm potentially harmful T cells with reactivity against self-antigens, many immunologists have turned to model systems in which transgenic expression of TCRs, foreign antigens, or both, allows visualization of otherwise undetectable mechanisms. Recent data have offered several new insights into the variables that dictate the mechanism for tolerance employed by peripheral T cells. Some of these mechanisms can also dampen ongoing immune responses against foreign antigens and provide a strikingly powerful supplement to the primary mechanism of tolerance, thymic clonal deletion.

Disruption of CD40-CD40 ligand interactions results in an enhanced susceptibility to Leishmania amazonensis infection

To study the role of CD40 ligand (CD40L) in the host immune responses against intracellular pathogens, we infected CD40L knockout (CD40L-/-) mice with Leishmania amazonensis. Although wild-type mice were susceptible to infection and developed progressive ulcerative lesions, tissue parasite burdens in CD40L-/- mice were significantly higher. This heightened susceptibility to infection was associated with an impaired T cell and macrophage activation and altered inflammatory response, as reflected by low levels of IFN gamma, lymphotoxin-tumor necrosis factor (LT-TNF), and nitric oxide (NO) production. Furthermore, CD40L-/- mice failed to generate a protective immune response after immunization. These results indicate an essential role of cognate CD40-CD40L interactions in the generation of cellular immune responses against an intracellular parasite.

T-cell costimulation: T cells themselves call the shots

The CD40L molecule expressed by CD4+ regulatory T lymphocytes is known to deliver signals that activate B cells and macrophages. It now appears that CD40L regulates T cells themselves, during both their development and their participation in adaptive immune responses.

Impairment of antigen-specific T-cell priming in mice lacking CD40 ligand

Lack of functional expression of CD40 ligand (CD40L) on T cells results in hyper-IgM syndrome (HIGM1), a human immunodeficiency associated with a severely impaired humoral immune response that is consistent with defects in B-cell responses. Patients also succumb to recurrent opportunistic infections such as Pneumocystis carinii and Cryptosporidial diarrhoea, suggesting that T-cell functions are also compromised in these individuals, but so far this has not been explained. We have previously shown that mice deficient for CD40L, like HIGM1 patients, show grossly abnormal humoral responses. Here we report that CD40L-deficient mice are defective in antigen-specific T-cell responses. Adoptively transferred antigen-specific CD4+ T cells lacking CD40L failed to expand upon antigen challenge of the recipients, showing that expression of CD40L on T cells is required for in vivo priming of CD4+ T cells and therefore for the initiation of specific T-cell immune responses.