Regulatory defects in Cbl and mitogen-activated protein kinase (extracellular signal-related kinase) pathways cause persistent hyperexpression of CD40 ligand in human lupus T cells

Alterations in lipid raft composition and dynamics contribute to abnormal T cell responses in systemic lupus erythematosus

In response to appropriate stimulation, T lymphocytes from systemic lupus erythematosus (SLE) patients exhibit increased and faster intracellular tyrosine phosphorylation and free calcium responses. We have explored whether the composition and dynamics of lipid rafts are responsible for the abnormal T cell responses in SLE. SLE T cells generate and possess higher amounts of ganglioside-containing lipid rafts and, unlike normal T cells, SLE T cell lipid rafts include FcRgamma and activated Syk kinase. IgM anti-CD3 Ab-mediated capping of TCR complexes occurs more rapidly in SLE T cells and concomitant with dramatic acceleration of actin polymerization kinetics. The significance of these findings is evident from the observation that cross-linking of lipid rafts evokes earlier and higher calcium responses in SLE T cells. Thus, we propose that alterations in the lipid raft signaling machinery represent an important mechanism that is responsible for the heightened and accelerated T cell responses in SLE.

Altered lipid raft-associated signaling and ganglioside expression in T lymphocytes from patients with systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterized by abnormalities in T lymphocyte receptor-mediated signal transduction pathways. Our previous studies have established that lymphocyte-specific protein tyrosine kinase (LCK) is reduced in T lymphocytes from patients with SLE and that this reduction is associated with disease activity and parallels an increase in LCK ubiquitination independent of T cell activation. This study investigated the expression of molecules that regulate LCK homeostasis, such as CD45, C-terminal Src kinase (CSK), and c-Cbl, in lipid raft domains from SLE T cells and investigated the localization of these proteins during T cell receptor (TCR) triggering. Our results indicate that the expression of raft-associated ganglioside, GM1, is increased in T cells from SLE patients and LCK may be differentially regulated due to an alteration in the association of CD45 with lipid raft domains. CD45 tyrosine phosphatase, which regulates LCK activity, was differentially expressed and its localization into lipid rafts was increased in T cells from patients with SLE. Furthermore, T cells allowed to "rest" in vitro showed a reversal of the changes in LCK, CD45, and GM1 expression. The results also revealed that alterations in the level of GM1 expression and lipid raft occupancy cannot be induced by serum factors from patients with SLE but indicated that cell-cell contact, activating aberrant proximal signaling pathways, may be important in influencing abnormalities in T cell signaling and, therefore, function in patients with SLE.

Human lupus T cells resist inactivation and escape death by upregulating COX-2

Autoimmune T-helper cells drive pathogenic autoantibody production in systemic lupus erythematosus (SLE), but the mechanisms maintaining those T cells are unknown. Autoreactive T cells are normally eliminated by functional inactivation (anergy) and activation-induced cell death (AICD) or apoptosis through death receptor (Fas) signaling. However, mutations in the genes encoding Fas and its ligand (FasL) are rare in classical SLE. By gene microarray profiling, validated by functional and biochemical studies, we establish here that activated T cells of lupus patients resist anergy and apoptosis by markedly upregulating and sustaining cyclooxygenase-2 (COX-2) expression. Inhibition of COX-2 caused apoptosis of the anergy-resistant lupus T cells by augmenting Fas signaling and markedly decreasing the survival molecule c-FLIP (cellular homolog of viral FLICE inhibitory protein). Studies with COX-2 inhibitors and Cox-2-deficient mice confirmed that this COX-2/FLIP antiapoptosis program is used selectively by anergy-resistant lupus T cells, and not by cancer cells or other autoimmune T cells. Notably, the gene encoding COX-2 is located in a lupus-susceptibility region on chromosome 1. We also found that only some COX-2 inhibitors were able to suppress the production of pathogenic autoantibodies to DNA by causing autoimmune T-cell apoptosis, an effect that was independent of prostaglandin E(2) (PGE(2)). These findings could be useful in the design of lupus therapies.

Flow cytometric assessment of the signaling status of human B lymphocytes from normal and autoimmune individuals

Abnormalities in lymphocyte signaling cascades are thought to play an important role in the development of autoimmune disease. However, the large amount of cellular material needed for standard biochemical assessment of signaling status has made it difficult to evaluate putative abnormalities completely using primary lymphocytes. The development of technology to employ intracellular staining and flow cytometry to assess the signaling status of individual cells has now made it possible to delineate the perturbations that are present in lymphocytes from patients with autoimmune disease. As an example, human B cells from the Ramos B cell line and the periphery of systemic lupus erythematosus (SLE) patients or normal nonautoimmune controls were assessed for activation of the NF-κB and mitogen activated protein kinase (MAPK) signaling cascades by intracellular multiparameter flow cytometric analysis and biochemical Western blotting. In combination with fluorochrome conjugated antibodies specific for surface proteins that define B cell subsets, antibodies that recognize activated, or phosphorylated inhibitors of κB (IκB) as well as the extracellular regulated kinase (ERK), jun N-terminal kinase (JNK) or p38 MAPKs were used to stain fixed and permeabilized human B cells and analyze them flow cytometrically. Examination of the known signaling pathways following engagement of CD40 on human B cells confirmed that intracellular flow cytometry and Western blotting equivalently assay CD154-induced phosphorylation and degradation of IκB proteins as well as phosphorylation of the MAPKs ERK, JNK and p38. In addition, B cells from the periphery of SLE patients had a more activated status immediately ex vivo as assessed by intracellular flow cytometric analysis of phosphorylated ERK, JNK and p38 when compared with B cells from the periphery of normal, nonautoimmune individuals. Together, these results indicate that multiparameter intracellular flow cytometric analysis of signaling pathways, such as the NF-κB and MAPK cascades, can be used routinely to assess the activation status of a small number of cells and thus delineate abnormalities in signaling molecules expressed in primary lymphocytes from patients with autoimmune disease.

Defective Control of Latent Epstein-Barr Virus Infection in Systemic Lupus Erythematosus

EBV infection is more common in patients with systemic lupus erythematosus (SLE) than in control subjects, suggesting that this virus plays an etiologic role in disease and/or that patients with lupus have impaired EBV-specific immune responses. In the current report we assessed immune responsiveness to EBV in patients with SLE and healthy controls, determining virus-specific T cell responses and EBV viral loads using whole blood recall assays, HLA-A2 tetramers, and real-time quantitative PCR. Patients with SLE had an approximately 40-fold increase in EBV viral loads compared with controls, a finding not explained by disease activity or immunosuppressive medications. The frequency of EBV-specific CD69+ CD4+ T cells producing IFN-gamma was higher in patients with SLE than in controls. By contrast, the frequency of EBV-specific CD69+ CD8+ T cells producing IFN-gamma in patients with SLE appeared lower than that in healthy controls, although this difference was not statistically significant. These findings suggest a role for CD4+ T cells in controlling, and a possible defect in CD8+ T cells in regulating, increased viral loads in lupus. These ideas were supported by correlations between viral loads and EBV-specific T cell responses in lupus patients. EBV viral loads were inversely correlated with the frequency of EBV-specific CD69+ CD4+ T cells producing IFN-gamma and were positively correlated with the frequencies of CD69+ CD8+ T cells producing IFN-gamma and with EBV-specific, HLA-A2 tetramer-positive CD8+ T cells. These results demonstrate that patients with SLE have defective control of latent EBV infection that probably stems from altered T cell responses against EBV.

Costimulation Blockade in the Treatment of Rheumatic Diseases

The autoimmune response is executed via cognate interactions between effector immune cells and antigen presenting cells. Cognate interactions provide the immune effectors with specific signals generated through the antigen receptor as well as with second, non-specific signals, generated from the interaction of pairs of cell-surface molecules (costimulatory molecules) present on their plasma membrane. Disruption of this second, non-specific costimulatory signal results in the interruption of the productive (auto)immune response, leading to anergy, a state of immune unresponsiveness. The CD28:B7 families of molecules and the CD40:CD40L pair of molecules are considered as critical costimulatory elements. Disruption of the CD28:B7 interaction using a genetically engineered soluble form of the inhibitory molecule CTLA4 in vitro, as well as in experimental models of rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE), led to the inhibition of the autoimmune response. Similarly, promising data stem from the use of an anti-CD40L monoclonal antibody (mAb) in murine SLE. While such treatments prevent the development of autoimmunity in animal models, this preventive approach is inapplicable to human diseases. However, the rational bench-to-bedside approach led investigators to clinical trials of CTLA4-Ig and of two different humanized anti-human CD40L mAbs in patients with RA and SLE, respectively. Initial experience with the use of CTLA4-Ig in patients with RA is encouraging, since in one short-term trial the construct was well-tolerated and produced clinically meaningful improvement of the disease in a significant proportion of those treated. Surprisingly, the anti-CD40L mAb treatment approach in human lupus was not fruitful, since short-term administration of the anti-CD40L mAb ruplizumab in lupus nephritis was correlated with life-threatening prothrombotic activity despite initial encouraging data in the serology and renal function of the patients. Also, IDEC-131 anti-CD40L mAb treatment did not prove to be clinically effective in human SLE, despite being well tolerated. Precise tailoring of the administration schemes for these novel therapeutic modalities is awaited.Finally, conceptually different approaches to block costimulation by inhibiting the induced expression of costimulatory molecules or the transmission of their specific intracytoplasmic signal have already produced encouraging preliminary results.

T-lymphocyte signalling in systemic lupus erythematosus: a lipid raft perspective

In the last few years it has become clear that in cells of the immune system, specialized microdomains present in the plasma membrane, called lipid rafts, have been found to play a central role in regulating signalling by immune receptors. Recent studies have looked at whether lipid rafts may be connected to the abnormalities in signalling seen in T lymphocytes isolated from patients with systemic lupus erythematosus (SLE). These early findings show that in SLE T cells, the expression and protein composition of lipid rafts is different when compared with normal T cells. These results also demonstrate changes in the function and localization of critical signalling molecules such as the LCK tyrosine kinase and the CD45 tyrosine phosphatase.

Dendritic cells under investigation in autoimmune disease

Autoimmune disorders play an increasing role in public health, especially in light of the fact of the growing aged population, which primarily develop such diseases. A clear understanding of the mechanisms leading to the development of autoimmune responses and finally to autoimmune disease does not exist. Autoimmunity is characterized by the presence of autoantibodies and/or autoreactive T cells and the corresponding organ manifestation. Following the discovery of autoreactive T cells found in the periphery of mice and humans, the old immunological concept that autoreactive T cells are completely deleted in the thymus during evolution has been revised in recent years. Although antigen-presenting cells and particularly dendritic cells are known to play an important role in the regulation of immune responses and the activation of T cells, recent evidence suggests that the role of dendritic cells in the development of autoimmunity has been underestimated previously. This article aims to give a general overview on the basic immunological principles involved and gives a short review of the current literature on the functional relevance of dendritic cells in various human and murine autoimmune disorders.

Immunopathology and the gene therapy of lupus

Lupus is a chronic autoimmune inflammatory disease with complex clinical manifestations. In humans, lupus, also known as systemic lupus erythematosus (SLE), affects between 40 and 250 individuals, mostly females, in each 100 000 of the population. There are also a number of murine models of lupus widely used in studies of the genetics, immunopathology, and treatment of lupus. Human patients and murine models of lupus manifest a wide range of immunological abnormalities. The most pervasive of these are: (1) the ability to produce pathogenic autoantibodies; (2) lack of T- and B-lymphocyte regulation; and (3) defective clearance of autoantigens and immune complexes. This article briefly reviews immunological abnormalities and disease mechanisms characteristic of lupus autoimmunity and highlight recent studies on the use of gene therapy to target these abnormalities.

Increased ubiquitination and reduced expression of LCK in T lymphocytes from patients with systemic lupus erythematosus

OBJECTIVE

To explore regulation of proximal signaling and composition of lipid rafts in T lymphocytes from patients with systemic lupus erythematosus (SLE).

METHODS

The expression, phosphorylation, and degradation of lipid raft-associated signaling molecules in T lymphocytes from 50 patients with SLE compared with 28 healthy controls and 22 rheumatoid arthritis patients were investigated. Lipid raft and nonraft fractions from T cells were isolated by ultracentrifugation. Proteins in the lipid raft and nonraft fractions were analyzed by Western blotting and probed for phosphotyrosine activity and for LCK, LAT, and CD3 epsilon. Immunoprecipitation experiments were performed to assess protein ubiquitination in T cell lysates. T cell phenotype and levels of intracellular LCK were determined by flow cytometry.

RESULTS

LCK, an essential signaling molecule for T cell activation, was significantly reduced in both lipid raft and nonraft fractions of T lymphocytes from patients with active SLE compared with controls, and the reduction was independent of treatment. To identify the likely causes of reduced LCK, we explored the possibility that chronic activation of T lymphocytes underlies LCK degradation. The results revealed an increase in protein ubiquitination, and specifically LCK ubiquitination, in T cells from SLE patients. However, our findings suggest that the increase in ubiquitination is independent of T cell activation.

CONCLUSION

LCK is reduced in T cell lipid rafts from patients with SLE. This reduction appears to be independent of activation and may be associated with abnormal ubiquitin-mediated regulation mechanisms.

Major peptide autoepitopes for nucleosome-centered T and B cell interaction in human and murine lupus

The potential cross-reactivity of normal T and B cells to nuclear antigens is vast, probably due to their "education" by apoptotic cell antigens in generative lymphoid organs. Despite this "nucleocentric repertoire," as we call it, the peripheral immune system normally remains tolerant or ignorant of the products of apoptosis. However, the T helper (Th) cells, and also B cells of lupus, have a regulatory defect in the expression of CD40 ligand (CD40L). A sustained hyper-expression of CD40L by lupus T cells can be triggered by sub-threshold stimuli, and is associated with impaired phosphorylation of Cbl-b, a critical downregulatory molecule in T cell signal transduction. This CD40L hyper-expression abnormally prolongs co-stimulatory signals to autoimmune B cells, and it probably instigates APC (dendritic cells, resting anti-DNA B cells, and macrophages) to present apoptotic cell autoantigens in an immunogenic fashion. We have identified the dominant nucleosomal epitopes that are critical for cognate interactions between autoimmune Th cells and anti-DNA B cells in lupus. By scanning of overlapping synthetic peptides, and by mass spectrometry of naturally processed peptides, five major epitopes in nucleosomal histones were localized, namely H1'(22-42), H2B(10-33), H3(85-105), H4(16-39), and H4(71-94). The autoimmune T cells as well as B cells of lupus recognize these epitopes, and with age, autoantibodies against the peptide epitopes cross-react with nuclear autoantigens. Moreover, the peptide autoepitopes can be promiscuously presented and recognized by lupus T cells in the context of diverse MHC alleles. This cross-reactivity opens up the possibility of developing "universally" tolerogenic peptides for therapy of lupus in humans despite their MHC diversity. Indeed, tolerogenic therapy with a single histone peptide epitope can halt the progression of established glomerulonephritis in lupus-prone mice by "tolerance spreading" that inactivates a broad spectrum of autoimmune T and B cells in concert.

Intrinsic T cell defects in systemic autoimmunity

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by loss of T cell tolerance to nuclear antigens. Studies in mice and humans have demonstrated that T cells from individuals with lupus are abnormal. Here, we review the known T cell defects in lupus and their possible biochemical nature, genetic causes, and significance for lupus pathogenesis.

CD4+ T cells from lupus-prone mice avoid antigen-specific tolerance induction in vivo

Activated T cells in spontaneous lupus presumably bypass normal tolerance mechanisms in the periphery, since thymic tolerance appears intact. To determine whether such T cells indeed avoid in vivo peripheral tolerance mechanisms, we assessed their activation and recall responses after in vivo Ag stimulation in the absence of exogenously supplied costimulatory signals. Naive CD4(+) AND (transgenic mice bearing rearranged TCR specific for pigeon cytochrome c, peptides 88-104) TCR-transgenic T cells, specific for pigeon cytochrome c, from lupus-prone Fas-intact MRL/Mp+(Fas-lpr) and from H-2(k)-matched control CBA/CaJ and B10.BR mice (MRL.AND, CBA.AND, and B10.AND, respectively) were adoptively transferred into (MRL x CBA)F(1) or (MRL x B10)F(1) recipients transgenically expressing membrane-bound pigeon cytochrome c as a self-Ag. MRL.AND and control CBA.AND and B10.AND-transgenic T cells were activated and divided after transfer, indicating encounter with their cognate Ag; however, T cells from CBA.AND and B10.AND mice were impaired in their ability to proliferate and produce IL-2 after challenge with pigeon cytochrome c in ex vivo recall assays, a typical phenotype of anergized cells. By contrast, MRL.AND T cells proliferated more, and a significantly higher percentage of such cells produced IL-2, compared with control T cells. This observation that MRL T cells avoided anergy induction in vivo was confirmed in an in vitro system where the cells were stimulated with an anti-CD3 in the absence of a costimulatory signal. These experiments provide direct evidence that CD4(+) T cells from Fas-intact lupus-prone MRL mice are more resistant than nonautoimmune control cells to anergy induction. Anergy avoidance in the periphery might contribute to the characteristic finding in lupus of inappropriate T cell activation in response to ubiquitous self-Ags.

Defective activity of ERK-1 and ERK-2 mitogen-activated protein kinases in peripheral blood T lymphocytes from patients with systemic lupus erythematosus: potential role of altered coupling of Ras guanine nucleotide exchange factor hSos to adapter protein Grb2 in lupus T cells

The integrity of the Ras/Raf/mitogen-activated protein kinase (MAPK) cascade is critical for maintenance of T cell tolerance, a process that fails in patients with systemic lupus erythematosus (SLE). In this study we have examined the activity of mitogen-activated protein kinases ERK-1 and ERK-2 in resting and TCR-activated peripheral blood T lymphocytes from patients with SLE. We also examined the binding of Ras guanine nucleotide exchange factor, human Son of Sevenless (hSos), to cytosolic adapter protein growth factor receptor-bound protein 2. T cells from lupus patients showed diminished catalytic activity and TCR-driven dual phosphorylation of ERK-1 and ERK-2 upon stimulation through the TCR/CD3 receptor, a defect that may be related to altered translocation of hSos to the Ras/Raf membrane complex and diminished nuclear translocation of trans-acting factor AP-1. Defective MAPK activity triggered by TCR/ CD3 activation may alter the coordination of signals needed for normal interleukin-2 production and maintenance of tolerance in lupus T cells.

CD40L overexpression on T cells and monocytes from patients with systemic lupus erythematosus is resistant to calcineurin inhibition

To explore the regulatory defects underlying the overexpression of CD40 ligand (CD40L, CD154) in human lupus we studied the effects of cyclosporin-A (CsA), which blocks Ca2+/calcineurin-dependent CD40L gene expression, on peripheral blood-derived T cells and monocytes. In contrast to control subjects, CsA failed to inhibit the prolonged CD40L expression observed in vitro on anti-CD3-activated lupus T cells. Resistance to CsA was not restricted to CD4+ or CD8+ T cell subsets and was disease activity-independent. Experiments assessing the effects of dexamethasone on CD40L expression, as well as of CsA on the early activation marker CD69 expression and on surface CD40L cleavage, confirmed the unique regulation of CD40L in lupus T cells. On the other hand, co-culture with anti-CD3-activated T cells caused surface CD40L expression on monocytes, which was not an Fc receptor-mediated event. Lupus monocytes clearly overexpressed CD40L comparing to healthy and disease-control monocytes, and, similarly to lupus T cells, displayed a prominent resistance to CsA inhibitory effects. These findings indicate that, besides Ca2+/calcineurin-dependent mechanisms, other pathways are involved in the dysregulation of CD40L in SLE immune cells, dissection of which may have important therapeutic implications.

Constitutive up-regulated activity of MAP kinase is associated with down-regulated early p21Ras pathway in lymphocytes of SLE patients

Aberrant expression of the p21Ras proto-oncogene has been reported in lymphoid cells of SLE patients. We previously showed that the expression of the p21Ras stimulatory element, hSOS1, is reduced in PBMC from SLE patients with non-active disease. However, the significance of this finding regarding the regulation and function of the p21Ras pathway and its correlation to disease activity remained unclear. The expression, regulation and function of the p21Ras pathway were determined in 23 ambulatory SLE patients with active and non-active disease and eleven controls. Levels of p21Ras stimulatory element hSOS1 but not p21Ras and its inhibitory element p120GAP were significantly decreased in SLE patients. Early p21Ras signalling was down-regulated in SLE patients with active disease as indicated by the decreased membrane/cytoplasmic (M/C) ratios of the p21Ras regulatory elements hSOS1 and p120GAP and by the non-responsiveness of these ratios to cellular stimulation. Anchorage of p21Ras to the cellular membrane was also significantly decreased in these patients. In contrast, the late p21Ras signalling was up-regulated in SLE patients as indicated by the significantly higher constitutive activity of the p21Ras down stream key regulator enzyme MAP Kinase. Taken together, our data demonstrate for the first time a disease associated functional defect in p21Ras signalling in lymphocytes of SLE patients.

Aberrant expression of the costimulatory molecule CD40 ligand on monocytes from patients with systemic lupus erythematosus

CD40 ligand (CD40L, CD154) is overexpressed on T and B cells in systemic lupus erythematosus (SLE). Monocytes have been shown to contribute to immune-mediated pathology in SLE and to express CD40L under certain conditions. Therefore, we studied CD40L expression on lupus monocytes ex vivo, as well as after activation in vitro. A highly significant sevenfold increase in the frequency of CD40L-expressing peripheral monocytes from 23 SLE patients, compared to 16 healthy individuals (mean percentage of CD40L(+)CD14(+) among CD14(+) cells, 11.7 versus 1.6), was found by flow cytometry. Increased CD40L expression on monocytes correlated significantly with disease activity, elevated gamma-globulin serum levels, as well as increased CD40L expression on T cells. CD40L expression by lupus monocytes was verified at both the mRNA and protein levels, while LPS stimulation was found to upregulate CD40L mRNA accumulation and surface protein expression. CD40L expression on activated lupus monocytes within anti-CD3-stimulated, mononuclear cell cultures was also enhanced compared to control-derived monocytes. These novel findings underscore the multiplicity of pathways through which monocytes may contribute to SLE pathology and suggest that T cell-independent CD40L-mediated cell to cell interactions may be also involved in humoral immune activation in SLE.

Naturally processed chromatin peptides reveal a major autoepitope that primes pathogenic T and B cells of lupus

Major autoepitopes for pathogenic Th cells of lupus were previously found in core histones of nucleosomes by testing overlapping synthetic peptides. To detect other dominant epitopes, we eluted peptides from MHC class II molecules of a murine lupus APC line that was fed with crude chromatin. The eluted peptides were purified by reverse-phase HPLC and tested for their ability to stimulate autoimmune Th clones, and then analyzed by mass spectrometry. Amino acid sequences of stimulatory fractions revealed three new autoepitopes. Two of the epitopes were homologous to brain transcription factor BRN-3, whereas the third sequence was homologous to histone H1'(22-42). H1'(22-42) stimulated autoimmune Th cells to augment the production of pathogenic antinuclear Abs, and was much more potent than other nucleosomal epitopes in accelerating glomerulonephritis in lupus-prone (SWR x NZB)F(1) (SNF(1)) mice. Remarkably, a marked expansion of Th1 cells recognizing the H1'(22-42) epitope occurred spontaneously in SNF(1) mice very early in life. A significant proportion of H1'(22-42)-specific T cell clones cross-reacted with one or more core histone epitopes, but not with epitopes in other lupus autoantigens. The H1'(22-42) epitope was also recognized by autoimmune B cells, and with the onset of lupus nephritis, serum autoantibodies to the H1'(22-42) epitope become increasingly cross-reactive with nuclear autoantigens. Convergence of T and B cell epitopes in H1'(22-42) and its ability to elicit a cross-reactive response make it a highly dominant epitope that could be targeted for therapy and for tracking autoimmune T and B cells.

Regulation of CD40 ligand expression in systemic lupus erythematosus

Production of pathogenic autoantibodies in systemic lupus erythematosus (SLE) requires T cell help, along with ligation of the B cell surface immunoglobulin receptor by antigen. It is likely that macrophages, dendritic cells, and endothelial cells are also activated by interactions with T cells and contribute to lupus pathology. CD40 ligand (CD40L, CD154), a member of the tumor necrosis factor family of cell surface molecules, mediates these contact dependent signals delivered by CD4 + T helper cells to CD40 + target cells. Recent data from SLE patients and murine lupus models have demonstrated prolonged expression of CD40L on lupus T cells and its capacity to mediate excessive B cell activation. This review summarizes the current information regarding transcriptional and post-transcriptional regulation of CD40L expression in normal and SLE T cells. More complete characterization of the mechanisms that regulate the magnitude and duration of CD40L expression should suggest new approaches to modulate this promising therapeutic target.