Blockade of costimulation prevents infection-induced immunopathology in interleukin-10-deficient mice

Interleukin-10 (IL-10) is associated with inhibition of cell-mediated immunity and downregulation of the expression of costimulatory molecules required for T-cell activation. When IL-10-deficient (IL-10KO) mice are infected with Toxoplasma gondii, they succumb to a T-cell-mediated shock-like reaction characterized by the overproduction of IL-12 and gamma interferon (IFN-gamma) associated with widespread necrosis of the liver. Since costimulation is critical for T-cell activation, we investigated the role of the CD28-B7 and CD40-CD40 ligand (CD40L) interactions in this infection-induced immunopathology. Our studies show that infection of mice with T. gondii resulted in increased expression of B7 and CD40 that was similar in wild-type and IL-10KO mice. In vivo blockade of the CD28-B7 or CD40-CD40L interactions following infection of IL-10KO mice with T. gondii did not affect serum levels of IFN-gamma or IL-12, nor did it prevent death in these mice. However, when both pathways were blocked, the IL-10KO mice survived the acute phase of infection and had reduced serum levels of IFN-gamma and alanine transaminase as well as decreased expression of inducible nitric oxide synthase in the liver and spleen. Analysis of parasite-specific recall responses from infected IL-10KO mice revealed that blockade of the CD40-CD40L interaction had minimal effects on cytokine production, whereas blockade of the CD28-B7 interaction resulted in decreased production of IFN-gamma but not IL-12. Further reduction of IFN-gamma production was observed when both costimulatory pathways were blocked. Together, these results demonstrate that the CD28-B7 and CD40-CD40L interactions are involved in the development of infection-induced immunopathology in the absence of IL-10.     

CD40 plays a crucial role in lipopolysaccharide-induced acute lung injury

Activated alveolar macrophages (AMphi) are known to constitute a critical modulator of the lung inflammatory response through the production of various mediators. However, the role of activated AMphi in acute lung injury (ALI) and acute respiratory distress syndrome is less well known. To address this issue, we examined a lipopolysaccharide (LPS)-induced lung injury model for the role of activated AMphi in vivo, focusing on activation through CD40, which is one of the most important pathways for the activation of antigen-presenting cells. Without CD40, LPS-induced ALI was significantly reduced in its histological degree of injury and recruitment of neutrophils into the lung. In addition, the release in the lung of inflammatory mediators such as tumor necrosis factor-alpha, interleukin-1beta, macrophage inflammatory protein 2, or matrix metalloproteinase was significantly reduced in mice deficient in CD40 (CD40KO). To elucidate the mechanism of this attenuation of ALI in CD40KO mice, we studied the function of AMphi ex vivo. AMphi purified from CD40KO mice could not induce expression of inducible nitric oxide synthase (iNOS) by LPS, although iNOS in wild-type AMphi was induced by LPS independently of CD40-CD154 interaction. The loss of surface expression of CD40 was enough to interrupt the expression of iNOS in AMphi in response to LPS. Also based on the tissue nitrotyrosine staining, the reactive oxygen and nitrogen intermediates seemed to be reduced in tissue in CD40KO mice. These results indicated that activation of AMphi through CD40 might be involved not only in amplification by the interaction with CD154 but also in the development of ALI by CD40 itself, and that the functional blockade of CD40 would yield one of the targets for the treatment of LPS-induced ALI and acute respiratory distress syndrome.     

Marrow-derived CD40-positive cells are required for mice to clear Cryptosporidium parvum infection

To clear a Cryptosporidium parvum infection, mice need CD4+ T cells, major histocompatibility complex class II, and an intact CD40-CD154 signaling pathway. CD40 is constitutively expressed on marrow-derived cells such as dendritic cells and B lymphocytes and is induced by gamma interferon (IFN-gamma) on most somatic cells. To determine whether the CD40 needed to clear a C. parvum infection has to be on marrow-derived mononuclear cells or on the epithelial cells that normally harbor the parasite, we transplanted CD40-/- mice with CD40+/- bone marrow and then infected them with C. parvum. These chimeras cleared the C. parvum infection, while CD40+/- controls transplanted with CD40-/- marrow cells remained infected. CD40 expression on marrow-derived cells therefore suffices for a C. parvum infection to be cleared, while CD40 expression on intestinal epithelial cells is not sufficient. There was no difference between the acquisition of CD69 and CD154 by mesenteric lymph node T cells of C. parvum-infected animals with intact or disrupted CD40-CD154 pathways. CD4 T cells entered the intestinal laminae propriae of C. parvum-infected animals whether or not the CD40 genes of these recipients were intact. These results suggest that, for a C. parvum infection to be cleared, CD40 is not necessary for T-cell activation but may instead contribute to an effector pathway of marrow-derived cells.     

Role of CD40-CVD40L in mouse severe malaria

We explored the role of CD40-CD40L (CD154) in the severe malaria elicited by Plasmodium berghei anka infection in mice. Mortality was >90% by day 8 after infection in +/+ mice, but markedly decreased in CD40-/- or in CD40L-/- mice, as well as in +/+ mice treated with anti-CD40L monoclonal antibody. Parasitemia was similar in the different conditions. Breakdown of the blood-brain barrier was evident in infected +/+, but not in CD40-/- mice. Thrombocytopenia was less severe in CD40-/- mice than in the +/+ controls. Sequestration of macrophages in brain venules and alveolar capillaries was reduced in CD40-/- or in CD40L-/- mice, whereas sequestration of parasitized red blood cells or polymorphonuclear leukocytes in alveolar capillaries was CD40-CD40L-independent. CD40 mRNA was increased in the brain and lung of infected mice whereas CD40L was increased in the lung. Tumor necrosis factor plasma levels were similarly increased in infected +/+ or CD40-/- mice. Expression of CD54 and its mRNA levels in the brain were moderately decreased in CD40-deficient mice. Thus the mortality associated with severe malaria requires CD40-CD40L interaction that contributes to the breakdown of the blood-brain barrier, macrophage sequestration, and platelet consumption.     

CD154-CD40-independent up-regulation of B7-2 on splenic antigen-presenting cells and efficient T cell priming by staphylococcal enterotoxin A

It has been demonstrated that in vivo T cell priming requires CD154-CD40 interaction, which is suggested to be critical in the induction of co-stimulatory activities on antigen-presenting cells (APC). In the current study, we demonstrate that in vivo administration of a high dose of a superantigen, staphylococcal enterotoxin A (SEA), could up-regulate B7-2 on most splenic APC independently of the CD154-CD40 interaction, followed by efficient expansion of SEA-reactive V(beta)3(+) T cells in CD154- or CD40-deficient mice. However, the CD154-CD40 interaction may be involved in SEA-mediated T cell activation, since a contribution of the CD154-CD40 interaction was observed when a lower dose of SEA was injected. CD154-independent T cell priming by SEA appeared also independent of the TRANCE-RANK pathway, which was shown to be capable of mediating CD154-independent activation of naive T cells during the infection of some viruses. These results indicate that SEA, which provokes rapid and efficient T cell responses without adjuvant, could utilize multiple CD154/TRANCE-independent pathways, to prime T cells.     

Pneumocystis pneumonia increases the susceptibility of mice to sublethal hyperoxia

Patients with Pneumocystis pneumonia often develop respiratory failure after entry into medical care, and one mechanism for this deterioration may be increased alveolar epithelial cell injury. In vitro, we previously demonstrated that Pneumocystis is not cytotoxic for alveolar epithelial cells. In vivo, however, infection with Pneumocystis could increase susceptibility to injury by stressors that, alone, would be sublethal. We examined transient exposure to hyperoxia as a prototypical stress that does cause mortality in normal mice. Mice were depleted of CD4+ T cells and inoculated intratracheally with Pneumocystis. Control mice were depleted of CD4+ T cells but did not receive Pneumocystis. After 4 weeks, mice were maintained in normoxia, were exposed to hyperoxia for 4 days, or were exposed to hyperoxia for 4 days followed by return to normoxia. CD4-depleted mice with Pneumocystis pneumonia demonstrated significant mortality after transient exposure to hyperoxia, while all uninfected control mice survived this stress. We determined that organism burdens were not different. However, infected mice exposed to hyperoxia and then returned to normoxia demonstrated significant increases in inflammatory cell accumulation and lung cell apoptosis. We conclude that Pneumocystis pneumonia leads to increased mortality following a normally sublethal hyperoxic insult, accompanied by alveolar epithelial cell injury and increased pulmonary inflammation.     

In vivo clearance of an intracellular bacterium, Francisella tularensis LVS, is dependent on the p40 subunit of interleukin-12 (IL-12) but not on IL-12 p70

To determine the role of interleukin-12 (IL-12) in primary and secondary immunity to a model intracellular bacterium, we have comprehensively evaluated infection with Francisella tularensis LVS in three murine models of IL-12 deficiency. Mice lacking the p40 protein of IL-12 (p40 knockout [KO] mice) and mice treated in vivo with neutralizing anti-IL-12 antibodies survived large doses of primary and secondary LVS infection but never cleared bacteria and exhibited a chronic infection. In dramatic contrast, mice lacking the p35 protein (p35 KO mice) of heterodimeric IL-12 readily survived large doses of primary sublethal LVS infection as well as maximal secondary lethal challenge, with only a slight delay in clearance of bacteria. LVS-immune wild-type (WT) lymphocytes produced large amounts of gamma interferon (IFN-gamma), but p35 KO and p40 KO lymphocytes produced much less; nonetheless, similar amounts of NO were found in all cultures containing immune lymphocytes, and all immune lymphocytes were equally capable of controlling intracellular growth of LVS in vitro. Purified CD4(+) and CD8(+) T cells from both WT and p40 KO mice controlled intracellular growth, even though T cells from WT mice produced much more IFN-gamma than those from p40 KO mice, and p40 KO T cells did not adopt a Th2 phenotype. Thus, while IL-12 p70 stimulation of IFN-gamma production may be important for bacteriostasis, IL-12 p70 is not necessary for appropriate development of LVS-immune T cells that are capable of controlling intracellular bacterial growth and for clearance of primary or secondary LVS infection. Instead, an additional mechanism dependent on the IL-12 p40 protein, either alone or in another complex such as the newly discovered heterodimer IL-23, appears to be responsible for actual clearance of this intracellular bacterium.     

Enhanced IL-10 production by CD4+ T cells primed in IL-15Rα-deficient mice

In this study, we investigated the functional outcomes of CD4(+) T cells primed in the absence of IL-15 transpresentation. Compared with their WT counterparts primed in WT mice, IL-15Rα KO CD4(+) T cells primed in KO mice were found to exclusively overproduce IL-10 upon in vitro restimulation(.) The comparable expression of IL-4 and Foxp3 in CD4(+) T cells primed in the WT and IL-15Rα KO mice indicated that this was neither due to T(H) 2- nor Treg cell-differentiation. IL-10 overproduction was also observed when OVA-specific TCR transgenic CD4(+) T (OT-II) cells were primed in KO mice, excluding an intrinsic deficiency of KO CD4(+) T cells. To investigate the WT and KO microenvironment, DCs from both WT and IL-15Rα KO mice were compared. DCs from both backgrounds were indistinguishable in their steady-state survival and in their expression of MHC class II and costimulatory molecules CD80, CD86, and CD40. However, IL-15Rα KO DCs primed OT-II cells in vitro to produce higher levels of IL-10 upon their restimulation. Additionally, IL-15Rα KO DCs produced significantly more IL-10 upon activation, and IL-10 neutralization during DC-mediated in vitro priming abolished IL-10 overproduction by CD4(+) T cells. Thus, IL-15Rα KO DCs provide an IL-10-enriched environment that preferentially primes CD4(+) T cells for more IL-10 production, highlighting a regulatory role for IL-15 transpresentation in CD4(+) T-cell priming.     

Enforced and prolonged CD40 ligand expression triggers autoantibody production in vivo.

CD40, a glycoprotein expressed on B lymphocytes plays an important role in B cell development, growth and differentiation. The ligand for the CD40 is a 39-kDa glycoprotein (CD154) expressed on the surface of activated T lymphocytes and is essential for thymus-dependent humoral immunity. The expression of CD154 is tightly regulated and its transient expression reduces the chances of potentially deleterious bystander activation of B cells. Stimulation through CD40 has been studied in vitro by using antibodies against CD40, by membranes of activated T cells or lately, by CD154 transfected cells. In this work we have evaluated the outcome of CD40-CD40 ligand interaction in vitro and in vivo by using CD154-transfected L929 cells. In vitro assays showed that CD154-L929 cells can induce on B cells: IL-4-dependent proliferation, up-regulation of CD23, CD54 and class II molecules and can also rescue WEHI-231 B cell lymphoma from anti-IgM-induced apoptosis. Interestingly, in vivo assays revealed that when CD154-L929 cells were inoculated into the spleen, mice developed a strong but transient production of anti-erythrocyte autoantibodies. Through B lymphocyte activation with CD154-transfected L929 cells both in vitro and in vivo, our data reveal that enforced and prolonged expression of CD40 ligand overcomes the tightly regulated mechanisms of B cell activation, triggering the production of autoantibodies. This system might be used to evaluate the early steps of an autoimmune response and the role of CD40-CD154 in the induction of primary responses in vivo.     

Elevated gelatinase activity in pulmonary alveolar proteinosis: role of macrophage-colony stimulating factor

Pulmonary alveolar proteinosis (PAP) is an anti-granulocyte macrophage-colony stimulating factor (GM-CSF) autoimmune disease resulting in the accumulation of phospholipids in the alveoli. GM-CSF knockout (KO) mice exhibit a strikingly similar lung pathology to patients with PAP. The lack of functionally active GM-CSF correlates with highly elevated concentrations of M-CSF in the lungs of PAP patients and GM-CSF KO mice. M-CSF has been associated with alternative macrophage activation, and in models of pulmonary fibrosis, M-CSF also contributes to tissue resorption and fibrosis. Matrix metalloproteinase-2 (MMP-2) and MMP-9 have been implicated in extracellular matrix degradation in animal models of fibrosis and asthma. We show for the first time that the lungs of PAP patients contain highly elevated levels of MMP-2 and MMP-9. PAP broncholaveolar lavage (BAL) cells but not bronchial epithelial cells expressed increased MMP-2 and MMP-9 mRNA relative to healthy controls. Both MMPs were detectable as pro and active proteins by gelatin zymography; and by fluorometric global assay, PAP-MMP activity was elevated. BAL cells/fluids from GM-CSF KO mice also demonstrated significantly elevated MMP-2 and MMP-9 gene expression, protein, and activity. Finally, PAP patients undergoing GM-CSF therapy exhibited significantly reduced MMPs and M-CSF. These data suggest that in the absence of GM-CSF, excess M-CSF in PAP may redirect alveolar macrophage activation, thus potentially contributing to elevated MMP expression in the lung.     

The ICOS-ligand B7-H2, expressed on human type II alveolar epithelial cells, plays a role in the pulmonary host defense system

The mechanism of immune defense against pathogens in the lung, has so far been poorly understood. Here, we show that human type II alveolar epithelial cells play a key role in defense via interactions between B7 homolog (B7h), also known as ICOS ligand, and its receptor ICOS expressed on activated T cells. The A549 alveolar type II cell line abundantly expresses B7-H2, CD40 and B7-1, but not B7-2 or hGL50. TNF-alpha significantly induced B7-H2 and CD40 expression by A549 cells, but had no effect on B7-1 or B7-2 expression. TNF-alpha-deficient mice exhibited low B7-H2 expression on alveolar epithelial cells in comparison with wild-type mice. Co-culture of TNF-alpha pre-stimulated A549 cells with CD4+ T cells promoted CD154 expression, CD4+ T cell proliferation and cytokine production, especially IFN-gamma. Monocyte-derived TNF-alpha in combination with IFN-gamma and LPS markedly induced B7-H2 expression in A549 cells. This study thus identifies a unique costimulatory pathway via alveolar epithelial type II cells that preferentially affects T helper cell function, implying that alveolar epithelial type II cells play a crucial role in innate immunity in the lung by regulating IFN-gamma-synthesis via B7-H2/ICOS interactions.     

CD154-dependent priming of diabetogenic CD4(+) T cells dissociated from activation of antigen-presenting cells

We followed the fate of K(d)- or I-A(g7)-restricted beta cell-autoreactive T cells in monoclonal TCR-transgenic NOD mice expressing or lacking CD154. 8.3-NOD.RAG-2(-/-)/CD154(-/-) mice, which bear autoreactive CD8(+) T cells, developed diabetes with the same incidence and tempo as 8.3-NOD.RAG-2(-/-)/CD154(+) mice. Recruitment of CD154(-/-) 8.3-CD8(+) CTL was accelerated by CD154(+)CD4(+) T cells, by expression of a B7.1 transgene in beta cells or by treatment of the mice with CpG-DNA or an agonistic anti-CD40 antibody. In contrast, the autoreactive CD4(+) T cells maturing in 4.1-NOD.RAG-2(-/-) mice lost their diabetogenic potential if they lacked CD154, even in the presence of CD154(+)CD4(+) T cells, B7.1 molecules on beta cells, CpG-DNA treatment, or systemic CD40 ligation. These results demonstrate the existence of a novel, CD154-dependent pathway of CD4(+) T cell activation that is independent of CD40-mediated activation of APCs.     

Enhanced lung injury and delayed clearance of Pneumocystis carinii in surfactant protein A-deficient mice: attenuation of cytokine responses and reactive oxygen-nitrogen species

Surfactant protein A (SP-A), a member of the collectin family, selectively binds to Pneumocystis carinii and mediates interactions between pathogen and host alveolar macrophages in vitro. To test the hypothesis that mice lacking SP-A have delayed clearance of Pneumocystis organisms and enhanced lung injury, wild-type C57BL/6 (WT) and SP-A-deficient mice (SP-A(-/-)) with or without selective CD4(+)-T-cell depletion were intratracheally inoculated with Pneumocystis organisms. Four weeks later, CD4-depleted SP-A-deficient mice had developed a more severe Pneumocystis infection than CD4-depleted WT (P. carinii pneumonia [PCP] scores of 3 versus 2, respectively). Whereas all non-CD4-depleted WT mice were free of PCP, intact SP-A(-/-) mice also had evidence of increased organism burden. Pneumocystis infection in SP-A-deficient mice was associated histologically with enhanced peribronchial and/or perivascular cellularity (score of 4 versus 2, SP-A(-/-) versus C57BL/6 mice, respectively) and a corresponding increase in bronchoalveolar lavage (BAL) cell counts. Increases in SP-D content, gamma interferon, interleukin-4, interleukin-5, and tumor necrosis factor alpha in BAL fluid occurred but were attenuated in PCP-infected SP-A(-/-) mice compared to WT mice. There were increases in total BAL NO levels in both infected groups, but nitrite levels were higher in SP-A(-/-) mice, indicating a reduction in production of higher oxides of nitrogen that was also reflected in lower levels of 3-nitrotyrosine staining in the SP-A(-/-) group. We conclude that despite increases in inflammatory cells, SP-A-deficient mice infected with P. carinii exhibit an enhanced susceptibility to the organism and attenuated production of proinflammatory cytokines and reactive oxygen-nitrogen species. These data support the concept that SP-A is a local effector molecule in the lung host defense against P. carinii in vivo.     

Requirement for CD40-CD40 Ligand Interaction for Elimination of Cryptosporidium parvum from Mice

Mice with disrupted genes for CD40 and CD40 ligand (CD40L) are unable to clear infection with Cryptosporidium parvum and develop cholangitis. Parasites are present in the gut, gall bladder, and biliary tree, and biliary epithelial cells express CD40 on the cell surface. SCID mice infected with C. parvum for >1 month can clear the infection after reconstitution with spleen cells from CD40, but not CD40L, knockout mice. In an in vitro model, C. parvum-infected HepG2 cells were triggered to apoptosis when incubated with a CD40L-CD8 fusion protein. The requirement for CD40-CD40L interactions for immunity to C. parvum indicated by our results may entail the triggering of apoptosis in infected cells, in addition to the known role of CD40L-CD40 interactions in stimulating cytokine production and promoting T-cell responses.     

Estimation of the number of CD154 molecules in membrane extracts used as a source of CD40 stimulation of human B lymphocytes

The CD40-CD154 interaction is better exemplified by a rheostat than by an on-off switch, and variations in its intensity can play a role in the regulation of B lymphocyte activation following primary and/or secondary humoral immune response. The CD40-CD154 interaction is often studied in co-culture models using CD154⁺ adherent cells, which can be problematic when performing protein or gene analyses. The use of membrane extracts prepared from CD154⁺-transfected cells can eliminate possible interferences caused by the presence of contaminating feeder cells. Given the dose-response effect of CD154 on target B cells, it is important to measure the amount of CD154 when using soluble membranes. We hereby report a simple method, based on cytometry analysis, to estimate the relative number of CD154 molecules in membrane extracts, allowing reproducibility in human B-cell activation level.     

Natural killer T cells contribute to airway eosinophilic inflammation induced by ragweed through enhanced IL-4 and eotaxin production

Although NKT cells have been found to be capable of modulating immune responses in several model systems, the role of NKT cells in allergy remains unclear. Using CD1 gene knockout (KO) mice, which lack NKT cells, we examined the function of NKT cells in the development of allergic inflammation induced by a common airborne human allergen, ragweed. The data showed that airway eosinophilia and mucus overproduction induced by ragweed were significantly reduced in CD1 KO mice, which was correlated with significantly lower allergen-driven IL-4 production and lower eotaxin responses in the airways of CD1 KO mice. Moreover, both ragweed-specific and total serum IgE levels in CD1 KO mice were significantly lower than those in control BALB/c mice. The reduced allergic reaction in CD1 KO mice is not due to intrinsic deficiency because they showed normal levels of immune cells and function. In addition, in vivo stimulation of NKT cells using their natural ligand, alpha-galactosylceramide, enhanced ragweed-induced airway eosinophilia, IL-4, and eotaxin production in control, but not CD1 KO mice. These data provide in vivo evidence for the involvement of NKT cells in the allergic mechanisms responsible for allergen-driven cytokine and chemokine production and airway inflammation.     

CD40-CD154: A perspective from type 2 immunity

The interaction between CD40 and CD154 (CD40 ligand) is central in immunology, participating in CD4⁺ T cell priming by dendritic cells (DC), CD4⁺ T cell help to B cells and classical macrophage activation by CD4⁺ T cells. However, its role in the Th2 side of immunology including helminth infection remains incompletely understood. Contrary to viral and bacterial stimuli, helminth products usually do not cause CD40 up-regulation in DC, and exogenous CD40 ligation drives Th2-biased systems towards Th1. On the other hand, CD40 and CD154 are necessary for induction of most Th2 responses. We attempt to reconcile these observations, mainly by proposing that (i) CD40 up-regulation in DC in Th2 systems is mostly induced by alarmins, (ii) the Th2 to Th1 shift induced by exogenous CD40 ligation is related to the capacity of such ligation to enhance IL-12 production by myeloid cells, and (iii) signals elicited by endogenous CD154 available in Th2 contexts and by exogenous CD40 ligation are probably different. We stress that CD40-CD154 is important beyond cognate cellular interactions. In such a context, we argue that the proliferation response of B-cells to IL-4 plus CD154 reflects a Th2-specific mechanism for polyclonal B-cell amplification and IgE production at infection sites. Finally, we argue that CD154 is a general immune activation signal across immune polarization including Th2, and propose that competition for CD154 at tissue sites may provide negative feedback on response induction at each site.     

CD40 ligation releases immature dendritic cells from the control of regulatory CD4+CD25+ T cells

We report that disruption of CD154 in nonobese diabetic (NOD) mice abrogates the helper function of CD4+CD25- T cells without impairing the regulatory activity of CD4+CD25+ T cells. Whereas CD4+ T cells from NOD mice enhanced a diabetogenic CD8+ T cell response in monoclonal TCR-transgenic NOD mice, CD4+ T cells from NOD.CD154(-/-) mice actively suppressed it. Suppression was mediated by regulatory CD4+CD25+ T cells capable of inhibiting CD8+ T cell responses induced by peptide-pulsed dendritic cells (DCs), but not peptide/MHC monomers. It involved inhibition of DC maturation, did not occur in the presence of CD154+ T-helper cells, and could be inhibited by activation of DCs with LPS, CpG DNA, or an agonistic anti-CD40 mAb. Thus, in at least some genetic backgrounds, CD154-CD40 interactions and innate stimuli release immature DCs from suppression by CD4+CD25+ T cells.     

The role of CD40-CD154 interactions in autoimmunity and the benefit of disrupting this pathway

Many tissue injuries and immune mediated pathologies such as graft allo-rejections were found to involve CD40-CD40 ligand (CD40L, CD154) signaling pathway. The disruption of this pathway in many animal models led to the improvement of graft survival in these models. CD40-CD154 interactions were also shown to play a significant role in the maintenance of autoimmunity, and the production of auto-antibodies in systemic lupus erythematosus (SLE). High-level expression of CD154 has been detected on T cells from patients with active SLE, rheumatoid arthritis (RA) and other autoimmune diseases, indicating that such cells could account for the high-level expression of immune accessory molecules on B cells of patients with active disease. An increased serum level of soluble CD154 was also reported in SLE, RA, and Sjogren's disease in correlation with the relevant auto-antibodies and with the clinical disease activity. Anti-CD154 antibody therapy prevents auto-antibody production and renal immune complex deposition in lupus nephritis, indicating that disruption of this pathway could be a beneficial treatment in SLE. However, the etiology of the higher than expected number of thromboembolic events in anti-CD154 treated SLE patients should be investigated and preventive measures should be considered.