C. van Kooten

CD40-CD40 ligand.

CD40 is a cell surface receptor that belongs to the tumor necrosis factor‐R (TNF‐R) family, and that was first identified and functionally characterized on B lymphocytes. Its critical role in T cell‐dependent humoral immune responses was demonstrated by patients with the hyper‐IgM syndrome, as well as by gene targeting in mice. However, in recent years it has become clear that CD40 is expressed much more broadly, including expression on monocytes, dendritic cells, endothelial cells, and epithelial cells. In addition, the CD40‐ligand (CD40‐L/CD154), a member of the TNF family, is also expressed more widely than activated CD4+ T cells only. Therefore it is now thought that CD40‐CD40‐L interactions play a more general role in immune regulation. Collectively these studies have culminated in pre‐clinical and clinical studies that are in progress. This article reviews recent developments in this field of research, with main emphasis on (1) structure and expression of CD40 and its ligand; (2) CD40 signal transduction; (3) in vitro function of CD40 on different cell types; and (4) in vivo functions of CD40/CD40‐L interactions. J. Leukoc. Biol. 67: 2–17; 2000.

Functional Role of CD40 and Its Ligand

CD40, a cell surface receptor which belongs to the TNF-R family, was first identified and functionally characterized on B lymphocytes. In recent years, CD40 has been found expressed on other cells, including monocytes, dendritic cells, endothelial cells and epithelial cells and is now thought to play a more general role in immune regulation. The present paper reviews recent developments about CD40, with main emphasis on: (1) structure and expression of CD40 and its ligand; (2) CD40 signal transduction; (3) in vitro function of CD40 on different cell types, and (4) in vivo functions of CD40/CD40L interactions.

Calcineurin inhibitors affect B cell antibody responses indirectly by interfering with T cell help

In general, humoral immune responses depend critically upon T cell help. In transplantation, prevention or treatment of humoral rejection therefore require drugs that ideally inhibit both B cell and T helper cell activity. Here, we studied the effects of commonly used immunosuppressive drugs [tacrolimus, cyclosporin, mycophenolic acid (MPA) and rapamycin] on T cell helper activity and on T cell‐dependent B cell responses. T cells were activated polyclonally in the presence of immunosuppressive drugs in order to analyse the effect of these drugs on T cell proliferation, co‐stimulatory ligand expression and cytokines. The impact of immunosuppressive drugs on T cell‐dependent immunoglobulin production by B cells was addressed in T–B cell co‐cultures. All drugs affected T cell proliferation and attenuated T cell co‐stimulatory ligand (CD154 and CD278) expression when T cells were activated polyclonally. Tacrolimus, cyclosporin and rapamycin also attenuated B cell stimulatory cytokine mRNA levels in T cells. As a consequence, a decrease in immunoglobulin levels was observed in autologous T–B cell co‐cultures, where T cell help is essential for immunoglobulin production. In contrast, when pre‐activated T cells were used to stimulate autologous B cells, calcineurin inhibitors failed to inhibit B cell immunoglobulin production, whereas MPA and rapamycin did show inhibition. From these studies, it is evident that calcineurin inhibitors affect the humoral immune response by interfering with T helper signals, but not by targeting B cells directly. Furthermore, our studies support the necessity of intervening in T cell helper function to attenuate humoral responses.

Glomerular deposition of C1q and anti-C1q antibodies in mice following injection of antimouse C1q antibodies.

Anti‐C1q autoantibodies are present in the serum of patients with different autoimmune diseases such as systemic lupus erythematosus (SLE). The occurrence of these autoantibodies correlates with renal involvement. In the present study we examined whether injection of rabbit antimouse C1q antibodies in mice leads to deposition in kidneys. Injection of healthy mice with a single dose of rabbit IgG antimouse C1q antibodies resulted in deposition of both C1q and IgG anti‐C1q in glomeruli. The pattern of deposition observed in the glomeruli of mice injected with antimouse C1q antibodies both at 24 h and 2 weeks was both glomerular basement membrane (GBM)‐associated and mesangial. Injection of control IgG did not have a detectable effect on circulating C1q levels, and no deposition of either C1q or rabbit IgG was seen at 24 h. The deposition of rabbit antimouse C1q and C1q in glomeruli resulted in complement activation, as assessed by C3 deposition, and influx of leucocytes associated with albuminuria in some, but not all mice. In none of the control mice was albuminuria observed. This report is the first to show that anti‐C1q antibodies deposit in the healthy glomerulus together with autologous C1q. This deposition is stable for at least 2 weeks, causes complement activation, leucocyte influx and can lead to mild albuminuria.

Mannan-binding lectin mediates renal ischemia/reperfusion injury independent of complement activation.

Ischemia/reperfusion injury (IRI) remains a major problem in renal transplantation. Clinical studies have identified that high serum levels of Mannan‐binding lectin (MBL), the initiator of the lectin pathway of complement activation, are associated with inferior renal allograft survival. Using a rat model, we identified an entirely novel role for MBL in mediating renal IRI. Therapeutic inhibition of MBL was protective against kidney dysfunction, tubular damage, neutrophil and macrophage accumulation, and expression of proinflammatory cytokines and chemokines. Following reperfusion, exposure of tubular epithelial cells to circulation‐derived MBL resulted in internalization of MBL followed by the rapid induction of tubular epithelial cell death. Interestingly, this MBL‐mediated tubular injury was completely independent of complement activation since attenuation of complement activation was not protective against renal IRI. Our identification that MBL‐mediated cell death precedes complement activation strongly suggests that exposure of epithelial cells to MBL immediately following reperfusion is the primary culprit of tubular injury. In addition, also human tubular epithelial cells in vitro were shown to be susceptible to the cytotoxic effect of human MBL. Taken together, these data reveal a crucial role for MBL in the early pathophysiology of renal IRI and identify MBL as a novel therapeutic target in kidney transplantation.

Maturation-resistant dendritic cells induce hyporesponsiveness in alloreactive CD45RA+ and CD45RO+ T-cell populations.

Dendritic cells (DCs) play a crucial role in the induction of antigen‐specific immunity and tolerance. Considering in vivo application of DCs prior to human organ transplantation, a protocol to develop tolerogenic DCs that not only induce unresponsiveness in naive (CD45RA+) T cells, but also in alloreactive memory (CD45RO+) T cells is required. The present study shows that dexamethasone (Dex) alters the differentiation of human monocyte‐derived DCs. DexDCs cocultured with allogeneic CD4+ T cells induced low proliferating and low IFNγ producing T cells. This is caused by lack of both costimulation via CD28 and hampered production of a soluble factor, as well as additional active suppression via B7‐H1 and IL‐10. T cells primed by DexDCs demonstrated hyporesponsiveness upon restimulation with mature DCs seemingly via the induction of anergy, since these cells showed no enhanced apoptosis and only a limited suppressive capacity. Interestingly, not only cocultures of allogeneic CD45RA+, but also of CD45RO+ T cells with DexDCs rendered T‐cell populations hyporesponsive to restimulation with mature DCs. The finding that also alloreactive memory T cells can be regulated supports the rationale of cell‐based therapies to obtain allograft‐specific tolerance in transplant recipients.

Production of complement components by cells of the immune system

The complement system is an important part of the innate immune defence. It contributes not only to local inflammation, removal and killing of pathogens, but it also assists in shaping of the adaptive immune response. Besides a role in inflammation, complement is also involved in physiological processes such as waste disposal and developmental programmes. The complement system comprises several soluble and membrane‐bound proteins. The bulk of the soluble proteins is produced mainly by the liver. While several complement proteins are produced by a wide variety of cell types, other complement proteins are produced by only a few related cell types. As these data suggest that local production by specific cell types may have specific functions, more detailed studies have been employed recently analysing the local and even intracellular role of these complement proteins. Here we review the current knowledge about extrahepatic production and/or secretion of complement components. More specifically, we address what is known about complement synthesis by cells of the human immune system.

Pancreas allograft biopsies with positive c4d staining and anti-donor antibodies related to worse outcome for patients.

C4d+ antibody‐mediated rejection following pancreas transplantation has not been well characterized. Therefore, we assessed the outcomes of 27 pancreas transplantation patients (28 biopsies), with both C4d staining and donor‐specific antibodies (DSA) determined, from a cohort of 257 patients. The median follow‐up was 50 (interquartile range [IQR] 8–118) months. Patients were categorized into 3 groups: group 1, patients with minimal or no C4d staining and no DSA (n = 13); group 2, patients with either DSA present but no C4d, diffuse C4d+ and no DSA or focal C4d+ and DSA (n = 6); group 3, patients with diffuse C4d+ staining and DSA (n = 9). Active septal inflammation, acinar inflammation and acinar cell injury/necrosis were significantly more abundant in group 3 than in group 2 (respective p‐values: 0.009; 0.033; 0.025) and in group 1 (respective p‐values: 0.034; 0.009; 0.002). The overall uncensored pancreas graft survival rate for groups 1, 2 and 3 were 53.3%, 66.7% and 34.6%, respectively (p = 0.044). In conclusion, recipients of pancreas transplants with no C4d or DSA had excellent long‐term graft survival in comparison with patients with both C4d+ and DSA present. Hence, C4d should be used as an additional marker in combination with DSA in the evaluation of pancreas transplant biopsies.

Accumulation of autoreactive effector T cells and allo-specific regulatory T cells in the pancreas allograft of a type 1 diabetic recipient

Aims/hypothesisSimultaneous kidney–pancreas transplantation is an established treatment for patients with type 1 diabetes and end-stage renal failure, even though restored beta cell function may become affected by recurrent islet autoimmunity or graft rejection. We characterised infiltrating lymphocytes isolated from a pancreatic graft with normal endocrine function in a kidney–pancreas recipient with type 1 diabetes.MethodsThe pancreas graft was removed due to recurrent graft pancreatitis of unknown cause. Pancreas-infiltrating lymphocytes and peripheral blood mononuclear cells (PBMC) were isolated and characterised phenotypically and functionally.ResultsCompared with PBMC, pancreas-infiltrating lymphocytes exhibited a distinct activation/memory phenotype and T cell receptor profile that were indicative of selective infiltration of the pancreas. Islet autoreactive CD8+ T cells could be detected in the pancreas and were increased in frequency compared with PBMC. Additionally, an augmentation of CD8+ CD28− regulatory T cells was observed in the pancreas; these induced expression of the inhibitory receptor immunoglobulin-like transcript-3 on antigen-presenting cells in a donor HLA class I-specific manner.Conclusions/interpretationThese data demonstrate the simultaneous presence of regulatory and effector T cells in the pancreas allograft of a recipient with type 1 diabetes. They also indicate that circulating islet autoreactive T cells may reflect immunological processes in pancreatic tissue, even though their frequency in the periphery may lead to underestimation of their presence in the pancreas. Additional specificities were also present in the pancreas that were undetectable in the circulation.

Anti-C1q autoantibodies in murine lupus nephritis

Autoantibodies against C1q can be found in the circulation of patients with several autoimmune diseases including systemic lupus erythematosus (SLE). In SLE there is an association between the occurrence of these antibodies and renal involvement. How anti‐C1q autoantibodies contribute to renal disease is currently unknown. Cohorts of MRL‐lpr mice, which are known to develop age‐dependent SLE‐like disease, were used to study the relationship between levels of anti‐C1q autoantibodies and renal disease. We collected serum, urine and renal tissue and analysed autoantibodies, complement levels and renal deposition as well as renal function. At 2 months of age all mice already had elevated levels of anti‐C1q autoantibodies, and elution of kidneys revealed the presence of these antibodies in renal immune deposits in MRL‐lpr mice and not in control MRL+/+ mice. In conclusion, anti‐C1q antibodies are already present in serum and immune deposits of the kidney early in life and therefore can play a role in nephritis during experimental SLE‐like disease in mice.