CD40 ligation enhances IL-15 production by tubular epithelial cells

Interleukin-15 in kidney disease and therapeutics

PURPOSE OF REVIEW

Interleukin 15 (IL-15) is a member of the IL-2 family of common gamma chain receptor cytokines with well described anti-inflammatory, pro-survival and pro-proliferative signaling properties. The cytoprotective role of IL-15 in the kidney is now coming into focus with recent reports of its beneficial actions in various forms of kidney disease. This review will summarize what is currently known about IL-15 signaling in the kidney and highlight recent evidence of its beneficial effects on kidney physiology.

RECENT FINDINGS

IL-15 and its heterotrimeric receptor are expressed throughout the kidney. Like all IL-2 family cytokines, IL-15 can activate signaling through the Janus Kinase (JAK)/Signal transducer of activated T-cells (STAT), phosphoinositol-3 kinase (PI-3K)/AKT and mitogen activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathways and recent evidence suggests that STAT5B is an essential transcriptional mediator of prosurvival signaling in glomerular visceral epithelial cells (i.e. podocytes). IL-15 has also been shown to suppress pro-apoptotic signaling in models of acute kidney injury and pro-fibrotic signaling in models of chronic kidney disease.

SUMMARY

The cytoprotective properties of IL-15 suggest that it may have potential as a nonimmunosuppresive therapeutic for kidney disease. A novel class of IL-15 immunotherapies has emerged for the treatment cancer and some have demonstrated efficacy in clinical trials. These well tolerated IL-15 agonists could possibly be repurposed for the treatment of kidney disease and warrant further exploration.

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.

Current understanding of the administration of mesenchymal stem cells in acute kidney injury to chronic kidney disease transition: a review with a focus on preclinical models

Incomplete recovery from acute kidney injury (AKI) can result in long-term functional deficits and has been recognized as a major contributor to chronic kidney disease (CKD), which is termed the AKI-CKD transition. Currently, an effective intervention for this disorder is still lacking. Principally, therapeutic strategies targeting the AKI-CKD transition can be divided into those reducing the severity of AKI or promoting the regenerative process towards beneficially adaptive repair pathways. Considering the fact that mesenchymal stem cells (MSCs) have the potential to address both aspects, therapeutic regimens based on MSCs have a promising future. In light of this information, we focus on the currently available evidence associated with MSC therapy involved in the treatment of the AKI-CKD transition and the underlying mechanisms. All of these discussions will contribute to the establishment of a reliable therapeutic strategy for patients with this problem, who can be easily ignored by physicians, and will lead to a better clinical outcome for them.

Immune receptors CD40 and CD86 in oral keratinocytes and implications for oral lichen planus

Lichen planus (LP) is a chronic T-cell-mediated mucocutaneous inflammatory disease that targets stratified epithelia, including those lining the oral cavity. The intraoral variant of LP (OLP) is associated with interferon (IFN)-γ production by infiltrating T lymphocytes; however, the role of epithelial cells in the etiopathogenesis OLP is not completely understood. There is however a growing body of evidence regarding the involvement of epithelial-derived cytokines, immune receptors, and costimulatory molecules in the pathobiological processes that promote and sustain OLP. In the present study, we used a reverse transcriptase-polymerase chain reaction assay to assess whether CD40-a receptor found mainly on antigen presenting cells-and the costimulatory molecule CD86 were expressed in oral keratinocytes (three strains of primary normal oral keratinocytes and the H357 cell line) in the presence or absence of IFN-γ. To further characterize the involvement of CD40 in OLP, expression and distribution of receptor and ligand (CD40/CD154) in tissues from OLP were evaluated by immunohistochemistry. The present results are the first to show that both CD40 and CD86 are constitutively expressed at low levels in oral keratinocytes and that their expression was enhanced by IFN-γ stimulation. The intensity of CD40 staining in OLP tissues was strong. Taken together, the results strongly suggest that CD40 and CD86 play a role in the pathophysiology of oral inflammatory diseases such as OLP.

Increasing the biological activity of IL-2 and IL-15 through complexing with anti-IL-2 mAbs and IL-15Rα-Fc chimera

IL-2 and IL-15 are structurally relative cytokines that share two receptor subunits, CD132 (γ(c) chain) and CD122 (β chain). However, the expression pattern and physiological role of IL-2 and IL-15 private receptor α chains CD25 and IL-15Rα, respectively, are strikingly different. CD25, together with CD122 and CD132, forms a trimeric high affinity IL-2 receptor that is expressed and functions on cells acquiring an IL-2 signal. Conversely, IL-15Rα is expressed and binds IL-15 with high affinity per se already in the endoplasmic reticulum of the IL-15 producing cells and it presents IL-15 to cells expressing CD122/CD132 dimeric receptor in trans. Thus, while IL-2 is secreted almost exclusively by activated T cells and acts as a free molecule, IL-15 is expressed mostly by myeloid cells and works as a cell surface-associated cytokine. Interestingly, the in vivo biological activity of IL-2 can be dramatically increased through complexing with certain anti-IL-2 mAbs; such IL-2/anti-IL-2 mAbs immunocomplexes selectively stimulate the proliferation of a distinct population of immune cells, depending on the clone of the anti-IL-2 mAb used. IL-2/S4B6 mAb immunocomplexes are highly stimulatory for CD122(high) populations (memory CD8(+) T and NK cells) and intermediately also for CD25(high) populations (Treg and activated T cells), while IL-2/JES6-1 mAb immunocomplexes enormously expand only CD25(high) cells. Although IL-2 immunocomplexes are much more potent than IL-2 in vivo, they show comparable to slightly lower activity in vitro. The in vivo biological activity of IL-15 can be dramatically increased through complexing with recombinant IL-15Rα-Fc chimera; however, IL-15/IL-15Rα-Fc complexes are significantly more potent than IL-15 both in vivo and in vitro. In this review we summarize and discuss the features and biological relevance of IL-2/anti-IL-2 mAbs and IL-15/IL-15Rα-Fc complexes, and try to foreshadow their potential in immunological research and immunotherapy.

KIR and HLA-C interactions promote differential dendritic cell maturation and is a major determinant of graft failure following kidney transplantation

BACKGROUND

HLA-C is an important ligand for killer immunoglobulin like receptors (KIR) that regulate natural killer (NK) cell function. Based on KIR specificity HLA-C molecules are allocated into two groups, HLA-C1 or HLA-C2; HLA-C2 is more inhibiting to NK cell function than HLA-C1. We studied the clinical importance of HLA-C genotypes on the long-term graft survival of 760 kidney transplants performed at our centre utilising a population based genetic study and cell culture model to define putative mechanisms.

METHODS AND FINDINGS

Genotyping was performed using conventional DNA PCR techniques and correlations made to clinical outcomes. We found that transplant recipients with HLA-C2 had significantly better long-term graft survival than transplant recipients with HLA-C1 (66% versus 44% at 10 years, log-rank p = 0.002, HR = 1.51, 95%CI = 1.16-1.97). In in-vitro NK and dendritic cell (DC) co-culture model we made several key observations that correlated with the population based genetic study. We observed that donor derived NK cells, on activation with IL-15, promoted differential HLA-C genotype dependent DC maturation. In NK-DC co-culture, the possession of HLA-C2 by DC was associated with anti-inflammatory cytokine production (IL-1RA/IL-6), diminished DC maturation (CD86, HLA-DR), and absent CCR7 expression. Conversely, possession of HLA-C1 by DC was associated with pro-inflammatory cytokine synthesis (TNF-α, IL-12p40/p70), enhanced DC maturation and up-regulation of CCR7 expression. By immunohistochemistry the presence of donor NK cells was confirmed in pre-transplant kidneys.

CONCLUSIONS

We propose that after kidney transplantation IL-15 activated donor derived NK cells interact with recipient DC with less activation of indirect allo-reactivity in HLA-C2 positive recipients than HLA-C1 positive recipients; this has implications for long-term graft survival. Early events following kidney transplantation involving NK-DC interaction via KIR and HLA-C immune synapse may have a central role in long-term kidney transplant outcomes.

In vivo therapeutic efficacy of intra-renal CD40 silencing in a model of humoral acute rejection

The humoral branch of the immune response has an important role in acute and chronic allograft dysfunction. The CD40/CD40L costimulatory pathway is crucial in B- and T- alloresponse. Our group has developed a new small interfering RNA (siRNA) molecule against CD40 that effectively inhibits its expression. The aim of the present study was to prevent rejection in an acute vascular rejection model of kidney transplant by intra-graft gene silencing with anti-CD40 siRNA (siCD40), associated or not with sub-therapeutic rapamycin. Four groups were designed: unspecific siRNA as control; sub-therapeutic rapamycin; siCD40; and combination therapy. Long-surviving rats were found only in both siCD40-treated groups. The CD40 mRNA was overexpressed in control grafts but treatment with siCD40 decreased its expression. Recipient spleen CD40+ B-lymphocytes were reduced in both siCD40-treated groups. Moreover, CD40 silencing reduced donor-specific antibodies, graft complement deposition and immune-inflammatory mediators. The characteristic histological features of humoral rejection were not found in siCD40-treated grafts, which showed a more cellular histological pattern. Therefore, the intra-renal effective blockade of the CD40/CD40L signal reduces the graft inflammation as well as the incidence of humoral vascular acute rejection, finally changing the type of rejection from humoral to cellular.

Epithelial cells as immune effector cells: the role of CD40

Through the expression of inflammatory mediators and immune-related molecules, epithelial cells function as immune effector cells in a wide variety of tissues; the expression of the CD40 receptor on these cells contributes this role. Engagement of CD40 activates epithelial cells and results in their release of pro- and anti-inflammatory mediators as well as pro-fibrotic molecules. As such, epithelial CD40 has been implicated in the pathogenesis of inflammatory disorders, generation of self-tolerance, and rejection of allografts.

Subcellular expression pattern and role of IL-15 in pneumococci induced lung epithelial apoptosis

Streptococcus pneumoniae is the leading causative agent of community-acquired pneumonia. Induction of apoptosis in pulmonary epithelial cells by bacteria during pneumonia might be harmful to the host. Interleukin-15 (IL-15) has been demonstrated as an effective inhibitor of apoptosis and is expressed in lung epithelium on the mRNA and protein level. Therefore, we characterized the sub-cellular expression pattern of the short and long IL-15 isoforms in lung epithelial cells in vitro as well as its role in pneumococci-related lung epithelial cell apoptosis. We found an expression pattern for both IL-15 signal peptides in the pulmonary epithelial cell lines A549 and Beas-2B. Moreover, a strong co-localization of IL-15 and IL-15Ralpha was detected on cell surfaces. Compared to pro-inflammatory cytokine stimulation, neither IL-15 nor its trimeric receptor complex was up-regulated after pneumococcal infection. However, overexpression of IL-15 isoforms revealed IL-15LSP and IL-15Vkl as inhibitors of pneumococci induced apoptosis in pulmonary epithelial cells. Thus, IL-15 may act as an anti-apoptotic molecule in pneumococci infection, thereby suggesting IL-15 as a benefical cytokine in pulmonary host defense against infection.

The interleukin-15/interleukin-15 receptor system as a model for juxtacrine and reverse signaling

Interleukin-15 (IL-15) is a pleiotropic cytokine of the 4 alpha-helix bundle family, which binds to a receptor complex that displays common elements with the IL-2 receptor and a unique high-affinity alpha chain. This review focuses on juxtacrine and reverse signaling levels in the IL-15/IL-15R system. Specifically, we discuss how agonistic stimulation of membrane-bound IL-15 induces phosphorylation of members of the MAP kinase family and of focal adhesion kinase (FAK), thereby upregulating processes including cytokine secretion, cell adhesion and migration. In addition, we explore IL-15 trans-presentation and intracellular signaling, and define promising molecular targets for future pharmacological intervention in infectious diseases and immunological disorders. These frontiers in IL-15/IL-15Ralpha research serve as highly instructive examples for key concepts, unsolved problems and therapeutic opportunities in juxtacrine and reverse signaling in general.

CD40: A Mediator of Pro- and Anti-Inflammatory Signals in Renal Tubular Epithelial Cells

Infiltration of immune cells into the renal interstitium is characteristic of chronic inflammatory kidney diseases. CD4+ T cells and platelets express CD40 ligand (CD40L) and are reported to mediate proinflammatory events in renal proximal tubular epithelial cells (RPTEC) via interaction with CD40. In other cell types, CD40 signals can also induce protective genes. Here, human RPTEC were treated with sCD40L to ligate CD40, and a significant increase in the generation of proinflammatory reactive oxygen species was found; however, CD40-activated cells did not undergo apoptosis. This suggests that CD40 signals may simultaneously induce antiapoptotic genes for cytoprotection of RPTEC. Heme oxygenase-1 (HO-1) expressed in RPTEC serves as a protective gene, but it is not known whether it is regulated by CD40. Next, RPTEC were transiently transfected with a full-length HO-1 promoter-luciferase construct and were treated with sCD40L. CD40 ligation was found to significantly increase HO-1 promoter activity. By electrophoretic mobility shift assay, it was confirmed that CD40 signaling induced the transcriptional activation of HO-1 through the binding of NF-kappaB to its promoter. By Western blot analysis, a marked increase in HO-1 protein expression following CD40 ligation was also found. These observations are of clinical significance because it was found that CD40 and HO-1 are induced in expression in vivo in inflamed rejecting kidney biopsies and co-expressed in renal tubules. Therefore, ligation of CD40 in RPTEC promotes both inflammatory and anti-inflammatory processes. Regulating the balance between these two events may be of importance in the prevention of tubular injury associated with renal disease.

PD-L1 is expressed by human renal tubular epithelial cells and suppresses T cell cytokine synthesis

T cell activation is affected by both stimulatory and inhibitory co-signaling. MHC class II-expressing renal tubular epithelial cells (TEC) can function as APC for T cells. To study the influence of inhibitory ligands on TEC-mediated T cell activation, we examined the expression of programmed death ligand-1 (PD-L1) on human TEC line HK-2 cells, as well as in normal and diseased kidney samples. RT-PCR, FACS, and immunocytochemistry showed that PD-L1 is constitutively expressed on HK-2 cells, and is dramatically upregulated by IFN-gamma. In situ hybridization and immunohistochemical staining revealed constitutive low expression of PD-L1 on proximal tubules at both mRNA and protein levels in normal kidneys, but much higher expression in kidneys with type IV lupus nephritis. In vitro, pretreatment of IFN-gamma-stimulated HK-2 cells with anti-PD-L1 significantly enhanced IL-2 secretion from cocultured, mitogen-activated Jurkat or human peripheral blood T cells. These results suggest that the PD-L1:PD-1 pathway negatively regulates T cell activation by TEC, and may play an inhibitory role in TEC-mediated immune activation and immunopathology in the kidney.

CD40 ligand increases complement C3 secretion by proximal tubular epithelial cells

Interstitial leukocyte infiltration is a major finding in tubulointerstitial damage (TID). Infiltrating lymphocytes interact with proximal tubular epithelial cells (PTEC) by means of secreted soluble factors and/or cell contact mechanisms. CD40 expressed onto PTEC can be engaged by CD40L present on T cells. PTEC are able to locally secrete complement C3, which may most likely promote TID. The aim of the study was to investigate the putative action of CD40 ligation on enhancement of C3 secretion by PTEC. Primary human PTEC and stabilized HK-2 cells were used in culture experiments. Cells were stimulated by soluble factors IL-1beta, IFN-gamma, and/or CD40L-expressing murine fibroblast L cells. Analysis of C3 gene expression was evaluated by reverse-transcription PCR and Northern blot. Secreted C3 was assayed by ELISA and a functional hemolytic test on supernatants. Intracellular events were explored by the NF-kappaB-specific inhibitor caffeic acid phenetyl ester (CAPE). Among soluble factors, IL-1beta and IFN-gamma increased C3 gene expression and secretion (two-fold to three-fold versus basal) on both HK-2 and PTEC. CD40 engagement by CD40L upregulated HK-2 C3 secretion by four-fold. IL-1beta did not further increase CD40-induced C3 secretion, whereas IFN-gamma associated with CD40L was the strongest stimulus (30-fold increase). Inhibition of NF-kappaB offset CD40L-induced C3 secretion by 70%. CD40 ligation is able to enhance C3 secretion by PTEC. This cell contact mechanism is in synergism with a T cell-derived soluble factor (IFN-gamma). C3 secretion induced by CD40L may represent a mechanism of amplification of TID associated with lymphocyte infiltration.

Mechanism of steroid action in renal epithelial cells

Renal tubular epithelial cells (TEC) are thought to play an active role in tubulointerstitial inflammation. Various immune and non-immune factors activate TEC to produce a variety of cytokines and chemokines, contributing to attraction of inflammatory cells to the kidney. The proinflammatory transcription factor nuclear factor-kappaB (NF-kappaB) appears to be a key player in these responses and tubular expression of NF-kappaB has been demonstrated in vitro and in vivo. Although glucocorticoids are known to inhibit NF-kappaB activation at different levels, the proinflammatory capacity of TEC was not inhibited. In contrast, glucocorticoids seemed to enhance the profibrotic response of TEC, emphasizing the cell-type specific characteristics of glucocorticoid action. We propose that specific inhibition of NF-kappaB activation in TEC might be an attractive strategy for therapeutic intervention in renal inflammation.

Major involvement of CD40 in the regulation of chemokine secretion from human peritoneal mesothelial cells

BACKGROUND

CD40 is a member of the tumor necrosis factor (TNF) family of receptors, whose ligand, CD154, is expressed by activated mononuclear cells. CD40 activation is a major immune regulatory pathway and is important for the regulation of chemokine and cytokine secretion. This study investigates the effect of CD40 ligation on the secretion of chemokines from human peritoneal mesothelial cells (HPMC).

METHODS

We activated CD40 in HPMC along with combinations of TNF-alpha, interleukin-1 (IL-1), and interferon gamma (IFN-gamma), and evaluated the mRNA levels and protein secretion of regulated upon activation, normal T-cell expressed and secreted (RANTES), monocyte chemoattractant protein-1 (MCP-1), and IL-8.

RESULTS

CD40 ligation had a small stimulatory effect on the secretion of all three chemokines, while TNF-alpha, IL-1 and IFN-gamma induced their secretion in a dose-dependent manner. The combination of CD40 ligation with either IL-1 or TNF-alpha increased chemokine secretion additively. IFN-gamma and CD40 ligation acted in synergy to induce the secretion of the mononuclear recruiting chemokines RANTES and MCP-1 (up to approximately 36-fold and approximately threefold, respectively), for which the combination of all three cytokines with CD40 ligation was extremely potent. In contrast, the secretion of the neutrophil chemoattractant IL-8, induced by CD40 ligation or by the combination of IL-1 and TNF-alpha, was reduced in the presence of IFN-gamma.

CONCLUSION

In light of our data, it is reasonable to suggest that in the mononuclear phase of peritonitis, IFN-gamma and CD154, expressed by activated mononuclear cells, diminish IL-8 secretion from HPMC and thus inhibit neutrophil recruitment. At the same time, the two act in synergy to induce the secretion of RANTES and MCP-1 from HPMC. Hence, by regulating chemokine secretion, CD40 may be involved in peritonitis and in the development of late phase mononuclear predominance.

Blockade of CD40-CD40 ligand protects against renal injury in chronic proteinuric renal disease

BACKGROUND

Interaction between CD40 and CD40 ligand (CD40L) is involved in both cognate and innate immune responses. Blockade of CD40-CD40L interactions reduces severity of renal injury in murine lupus nephritis and membranous nephropathy. We hypothesized that CD40-CD40L could contribute to renal injury in models that are not antibody-dependent, and that anti-CD40L could diminish inflammation and fibrosis in murine adriamycin nephropathy.

METHODS

Male BALB/c mice were divided into three groups (N = 6 per group): (1). saline-treated, age-matched control; (2). adriamycin only; and (3). MR1 + adriamycin. In group 3, mice were treated with intraperitoneal injections of anti-CD40L antibody (clone MR1, 0.4 mg per mouse) after the onset of proteinuria at days 5, 7, 9, and 11 after adriamycin treatment. Animal subgroups were compared at 14 and 42 days after induction of adriamycin nephropathy. Functional and pathologic markers of disease severity, cellular components of interstitial inflammation, and the degree of CD40 expression were assessed. Relative cortical RNA expression of the chemokine monocyte-chemoattractant protein-1 (MCP-1) and regulated on activation normal T cell expressed and secreted (RANTES) was also compared between animal groups.

RESULTS

CD40 was weakly expressed in tubules of normal mice but was expressed in tubules, interstitium, and glomeruli of mice with adriamycin nephropathy in a time-dependent manner. MR1 treatment resulted in a significant attenuation of the severity of adriamycin nephropathy at day 42 [e.g., glomerular sclerosis (%), group 3, 20.1 +/- 4.7 vs. group 2, 30.2 +/- 7.2, P < 0.001]. CD40L blockade significantly reduced tubulointerstitial injury as well [tubular diameter microm), group 3, 42.5 +/- 6.9 vs. group 2, 66.3 +/- 13.7, P < 0.001; and group 1, 37.3 +/- 5.7, P < 0.01; tubular cell height microm), group 3, 16.3 +/- 1.7 vs. group 2, 11 +/- 1.8, P < 0.01; and group 1, 18.2 +/- 1.9, P < 0.01; interstitial volume (%), group 3, 13.9 +/- 5.1 vs. group 2, 26.2 +/- 4.9, P < 0.001; and group 1, 1.3 +/- 0.7, P < 0.001; proteinuria (mg/24 hours), group 3, 1.8 +/- 0.6 vs. group 2, 4.3 +/- 0.8, P < 0.001; and group 1, 0.7 +/- 0.2, P < 0.05; and creatinine clearance microL/min), group 3, 75 +/- 4 vs. group 2, 35 +/- 2, P < 0.001; and group 1, 82 +/- 4, P < 0.01] were also improved by MR1. MR1 treatment also resulted in a significant reduction in the number of cortical macrophages at both 14 and 42 days after adriamycin (P < 0.01). Cortical expression of MCP-1 and RANTES was significantly reduced by MR1 treatment at 42 days after adriamycin (P < 0.01 and P < 0.05, respectively).

CONCLUSION

Blockade of CD40-CD40L interaction protects against renal structural and functional injury in this murine model of chronic proteinuric renal disease.

Tubulitis in renal allograft rejection: role of transforming growth factor-beta and interleukin-15 in development and maintenance of CD103+ intraepithelial T cells

BACKGROUND

Renal tubules normally show no lymphocyte infiltration, but tubulitis is a feature of renal allograft rejection with many intratubular T cells expressing CD8 and CD103 (the alphaEbeta7 integrin). We investigated the development and maintenance of allospecific CD103 T cells within the tubular microenvironment.

METHODS

Mixed lymphocyte cultures were supplemented with transforming growth factor (TGF)-beta1 to model the expression and function of CD103 observed in situ on intratubular lymphocytes. Immunocytochemical techniques were used to identify cells coexpressing CD8 and interleukin (IL)-15Ralpha, to enumerate proliferating intratubular T cells, and to quantify IL-15 expression within the tubules of control and rejection-graded transplant biopsy specimens. These results were compared with a parallel analysis of the phenotype and proliferation of allospecific T cells expanded in vitro in the presence of TGF-beta1 and IL-15.

RESULTS

TGF-beta1 only induced the expression of adhesive CD103 after at least one cycle of alloantigen-specific cell division in vitro. In the renal allograft, a similar proportion of intratubular T cells was observed to proliferate during and after acute rejection. Tubular epithelial cells expressed IL-15 constitutively, whereas intratubular CD8 T cells expressed IL-15 receptor alpha. IL-15 and TGF-beta1 synergized to promote expansion and survival of allospecific CD8 CD103 T cells in vitro, but IL-15 down-regulated perforin expression.

CONCLUSIONS

These results suggest that activated, allospecific CD8 T cells are recruited to tubules during acute rejection where they encounter TGF-beta, up-regulate CD103 expression, and bind E-cadherin. A proportion of these cells proliferates and is maintained in a state of low perforin expression by the combined action of TGF-beta and IL-15.

Post-transplant renal tubulitis: the recruitment, differentiation and persistence of intra-epithelial T cells

Tubulitis is used by the Banff protocol as a major criterion to grade acute renal allograft rejection. This review integrates results from in vitro and in vivo studies to develop a chronological model to explain the development and functions of tubular inflammation during the rejection process. Proteoglycan-immobilized chemokines are the primary motivators for the vectorial recruitment of specific immune cell populations from the blood, through the endothelium and interstitial tissues to the renal tubules. After penetration of the basement membrane, T cells encounter TGF-beta that can induce expression of the alphaEbeta7 integrin on proliferating cells. This allows adhesion to E-cadherin on the baso-lateral surfaces of tubular epithelial cells and provides an explanation for the epithelial-specific cytotoxicity observed during acute rejection. Tubular epithelium is also a rich source of IL-15 that can stimulate IL-15 receptor-expressing intratubular CD8+ T cells. This anti-apoptotic microenvironment may explain the long-term persistence of cycling T cells within intact tubules after episodes of acute rejection. These memory-like T cells may have local immunoregulatory properties, including the production of additional TGF-beta, but could also modify normal tubular homeostasis resulting in epithelial to mesenchymal transdifferentiation, tubulointerstitial fibrosis and, ultimately, graft failure.

IL-15 up-regulates iNOS expression and NO production by gingival epithelial cells

To investigate the biological activity of epithelial cells in view of host defense, we analyzed the mRNA expression of inducible NOS (iNOS) as well as NO production by human gingival epithelial cells (HGEC) stimulated with IL-15. RT-PCR analysis revealed that HGEC expressed IL-15 receptor alpha-chain mRNA. In addition, stimulation with IL-15 enhanced iNOS expression by HGEC through an increase of both mRNA and protein levels. Moreover, IL-15 up-regulated the production of NO(2)(-)/NO(3)(-), a NO-derived stable end product, from HGEC. The enhanced NO production by IL-15 was inhibited by AMT, an iNOS-specific inhibitor. These results suggest that IL-15 is a potent regulator of iNOS expression by HGEC and involved in innate immunity in the mucosal epithelium.

On CD28/CD40 ligand costimulation, common gamma-chain signals, and the alloimmune response

Activation and robust expansion of naive T cells often require T cell costimulatory signals and T cell growth factors. However, the precise growth and costimulation requirements for activation and expansion of CD4(+) and CD8(+) T cells in vivo in allograft response are still not clearly defined. In the present study, we critically examined the role of CD28/CD40 ligand (CD40L) costimulation and the common gamma-chain (gamma(c)) signals, a shared signaling component by receptors for all known T cell growth factors (i.e., IL-2, IL-4, IL-7, IL-9, IL-15, IL-21), in activation and expansion of CD4(+) and CD8(+) T cells in the allogeneic hosts. We found that CD28/CD40L costimulation and the gamma(c) signals are differentially involved in proliferation and clonal expansion of CD4(+) and CD8(+) T cells in response to alloantigen stimulation. CD8(+) T cells are highly dependent on the gamma(c) signals for survival, expansion, and functional maturation, whereas in vivo expansion of alloreactive CD4(+) T cells is largely gamma(c) independent. T cell costimulation via CD28 and CD40L, however, is necessary and sufficient for activation and expansion of CD4(+) T cells in vivo. In a skin transplant model, blocking both CD28/CD40L and the gamma(c) pathways induced prolonged skin allograft survival. Our study provides critical insights that the CD4 and CD8 compartments are most likely governed by distinct mechanisms in vivo, and targeting both costimulatory and gamma(c) signals may be highly effective in certain cytopathic conditions involving activation of both CD4(+) and CD8(+) T cells.

CD40 signaling and plaque instability

Today, multiple lines of evidence support the view of atherosclerosis as a chronic inflammatory disease and implicate components of the immune system in atherogenesis. Recent work has documented overexpression of the potent immune mediator CD40 and its counterpart CD40 ligand (CD40L) in experimental and human atherosclerotic lesions. Notably, interruption of CD40/CD40L interactions not only diminished the formation and progression of mouse atheroma, but also fostered changes in lesion biology and structure, which are associated in humans with "plaque stabilization." In accordance with the hypothesis that CD40 signaling promotes plaque instability, in vitro studies demonstrated that ligation of CD40 on atheroma-associated cell types, namely endothelial cells, smooth muscle cells, and macrophages, mediates functions considered crucial to the process of atherogenesis, such as the expression of cytokines, chemokines, growth factors, matrix metalloproteinases, and procoagulants. The combination of the broad gamut of proatherogenic biological responses triggered by ligation of CD40 on endothelial cells, smooth muscle cells, and macrophages in vitro and the results of in vivo studies of interruption of CD40 signaling suggests a central role for this receptor/ligand dyad during atherogenesis, proposing CD40/CD40L interactions as a novel potential therapeutic target for this prevalent human disease.

Interleukin-15 is the main mediator of lymphocyte proliferation in cultures mixed with human kidney tubular epithelial cells

BACKGROUND

Interleukin- (IL) 15 is a potent non-T cell-derived cytokine with IL-2-like activities. Elevated levels of IL-15 expression were observed in biopsies of acutely rejected human kidney grafts. We tested the role of IL-15 in mixed lymphocyte kidney reaction (MLKR) and the effects of immunosuppressive drugs on this reaction.

METHODS

Primary cultures of human tubular epithelial cells (TEC) were stimulated by interferon-gamma and treated with cyclosporin A (CsA, 10-1000 ng/ml), rapamycin (Rapa, 2.5-10 ng/ml), and dexamethasone (Dex, 10-10-10-7 M). IL-15 levels were measured by ELISA. To induce MLKR, we seeded OKT3-prestimulated allogenic human peripheral blood mononuclear cells (PBMC) on a monolayer of TEC in the presence of CsA (25-250 ng/ml), Rapa (0.25-1 ng/ml), and Dex (10-10-10-7 M). PBMC proliferation was quantified by 3H-thymidine incorporation.

RESULTS

CsA, Dex, and Rapa had no effect on IL-15 production by TEC. The presence of TEC induced marked proliferation of PBMC. Pretreatment of TEC with IFN-gamma enhanced MLKR in direct correlation with the increased IL-15 levels. MLKR was blocked by anti-IL-15, but not significantly by anti-IL-2 monoclonal antibody. Contrary to Rapa and Dex, CsA failed to inhibit MLKR CONCLUSIONS: IL-15 is a major mediator of lymphocyte proliferation in MLKR. Its production by TEC was unaffected by CsA, Rapa, and Dex. However, IL-15 activity is effectively inhibited by Rapa and Dex but not by CsA. The diversity in the effects of the various drugs is probably related to the different mechanisms. Our results support the possible involvement of renal IL-15 in graft rejection and suggest that resistance to CsA treatment is related to its failure to decrease IL-15 activity.