Thrombomodulin mutations in atypical hemolytic-uremic syndrome

BACKGROUND

The hemolytic-uremic syndrome consists of the triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The common form of the syndrome is triggered by infection with Shiga toxin-producing bacteria and has a favorable outcome. The less common form of the syndrome, called atypical hemolytic-uremic syndrome, accounts for about 10% of cases, and patients with this form of the syndrome have a poor prognosis. Approximately half of the patients with atypical hemolytic-uremic syndrome have mutations in genes that regulate the complement system. Genetic factors in the remaining cases are unknown. We studied the role of thrombomodulin, an endothelial glycoprotein with anticoagulant, antiinflammatory, and cytoprotective properties, in atypical hemolytic-uremic syndrome.

METHODS

We sequenced the entire thrombomodulin gene (THBD) in 152 patients with atypical hemolytic-uremic syndrome and in 380 controls. Using purified proteins and cell-expression systems, we investigated whether thrombomodulin regulates the complement system, and we characterized the mechanisms. We evaluated the effects of thrombomodulin missense mutations associated with atypical hemolytic-uremic syndrome on complement activation by expressing thrombomodulin variants in cultured cells.

RESULTS

Of 152 patients with atypical hemolytic-uremic syndrome, 7 unrelated patients had six different heterozygous missense THBD mutations. In vitro, thrombomodulin binds to C3b and factor H (CFH) and negatively regulates complement by accelerating factor I-mediated inactivation of C3b in the presence of cofactors, CFH or C4b binding protein. By promoting activation of the plasma procarboxypeptidase B, thrombomodulin also accelerates the inactivation of anaphylatoxins C3a and C5a. Cultured cells expressing thrombomodulin variants associated with atypical hemolytic-uremic syndrome had diminished capacity to inactivate C3b and to activate procarboxypeptidase B and were thus less protected from activated complement.

CONCLUSIONS

Mutations that impair the function of thrombomodulin occur in about 5% of patients with atypical hemolytic-uremic syndrome.

Reduced Nitric Oxide Bioavailability In a Baboon Model of Shiga Toxin Mediated Hemolytic Uremic Syndrome (HUS)

BACKGROUND

Although there is agreement that post-diarrheal hemolytic uremic syndrome (D+ HUS) is caused by Shiga toxin (Stx)-producing E. coli, little is known about factors that mediate the host response to these toxins and potentially contribute to pathogenesis. Nitric oxide (NO) is a candidate mediator by virtue of its antiplatelet and renal vasodilatory properties.

METHODS

We used a baboon model of HUS to measure plasma and urinary NO metabolites and expression of NO synthase (eNOS and iNOS) in renal tissue following the intravenous administration of Stx-1.

RESULTS

Plasma concentrations through 60 hours of observation did not differ significantly from controls. Urinary values (indexed against urinary creatinine) tended, however, to rise during the initial 12 hours following administration of Stx-1. This was followed by a sustained reduction that coincided with the development of hemolytic anemia (schistocytosis) and other features of HUS. However, immunohistochemical staining for eNOS and iNOS in tissue obtained immediately after death at a median of 59 hours showed similar levels in control and Stx-treated animals, despite the presence of a florid thrombotic microangiopathy and tubular injury in the Stx-treated group.

CONCLUSION

We propose that urinary NO metabolite reduction was due to NO inactivation subsequent to its avid binding to free hemoglobin released from lysed red blood cells, and that this contributed to the acute renal failure by facilitating vasoconstriction and platelet aggregation and adhesion within the renal microvasculature.

Proteasomal processing of albumin by renal dendritic cells generates antigenic peptides

The role of dendritic cells (DC) that accumulate in the renal parenchyma of non-immune-mediated proteinuric nephropathies is not well understood. Under certain circumstances, DC capture immunologically ignored antigens, including self-antigens, and present them within MHC class I, initiating an autoimmune response. We studied whether DC could generate antigenic peptides from the self-protein albumin. Exposure of rat proximal tubular cells to autologous albumin resulted in its proteolytic cleavage to form an N-terminal 24-amino acid peptide (ALB1-24). This peptide was further processed by the DC proteasome into antigenic peptides that had binding motifs for MHC class I and were capable of activating syngeneic CD8+ T cells. In vivo, the rat five-sixths nephrectomy model allowed the localization and activation of renal DC. Accumulation of DC in the renal parenchyma peaked 1 wk after surgery and decreased at 4 wk, concomitant with their appearance in the renal draining lymph nodes. DC from renal lymph nodes, loaded with ALB1-24, activated syngeneic CD8+ T cells in primary culture. The response of CD8+ T cells of five-sixths nephrectomized rats was amplified with secondary stimulation. In contrast, DC from renal lymph nodes of five-sixths nephrectomized rats treated with the proteasomal inhibitor bortezomib lost their capacity to stimulate CD8+ T cells in primary and secondary cultures. These data suggest that albumin can be a source of potentially antigenic peptides upon renal injury and that renal DC play a role in processing self-proteins through a proteasome-dependent pathway.

Pretransplant infusion of mesenchymal stem cells prolongs the survival of a semiallogeneic heart transplant through the generation of regulatory T cells

In this study, we investigated whether mesenchymal stem cells (MSC) had immunomodulatory properties in solid organ allotransplantation, using a semiallogeneic heart transplant mouse model, and studied the mechanism(s) underlying MSC tolerogenic effects. Either single (portal vein, day -7) or double (portal vein, day -7 and tail vein, day -1) pretransplant infusions of donor-derived B6C3 MSC in B6 recipients induced a profound T cell hyporesponsiveness and prolonged B6C3 cardiac allograft survival. The protolerogenic effect was abrogated when donor-derived MSC were injected together with B6C3 hematopoietic stem cells (HSC), suggesting that HSC negatively impact MSC immunomodulatory properties. Both the induction (pretransplant) and the maintenance phase (>100 days posttransplant) of donor-derived MSC-induced tolerance were associated with CD4(+)CD25(+)Foxp3(+) Treg expansion and impaired anti-donor Th1 activity. MSC-induced regulatory T cells (Treg) were donor-specific since adoptive transfer of splenocytes from tolerant mice prevented the rejection of fully MHC-mismatched donor-specific secondary allografts but not of third-party grafts. In addition, infusion of recipient-derived B6 MSC tolerized a semiallogeneic B6C3 cardiac allograft, but not a fully MHC-mismatched BALB/c graft, and expanded Treg. A double i.v. pretransplant infusion of recipient-derived MSC had the same tolerogenic effect as the combined intraportal/i.v. MSC infusions, which makes the tolerogenic protocol applicable in a clinical setting. In contrast, single MSC infusions given either peritransplant or 1 day after transplant were less effective. Altogether these findings indicate that MSC immunomodulatory properties require HSC removal, partial sharing of MHC Ags between the donor and the recipient and pretransplant infusion, and are associated with expansion of donor-specific Treg.

Polymorphisms of EDNRB, ATG, and ACE genes in salt-sensitive hypertension

Almost 50% of hypertensive individuals manifest blood pressure changes in response to salt depletion or repletion and are termed "salt sensitive" (SS). Blunted activity of the endothelin (ET) system and the renin-angiotensin-aldosterone system (RAAS) have been reported as possible mechanisms contributing to salt sensitivity. Data are available that endothelin receptor subtype B (ETBR)-deficient rats develop salt-sensitive hypertension when fed a high-salt diet. Whether the ETBR gene (EDNRB) is involved in genetic predisposition to human salt-sensitive hypertension has not been studied so far. We screened EDNRB in 104 hypertensive patients (49 salt sensitive and 55 salt resistant) and 110 normotensive controls. No new sequence variation was found, but genotype distribution of the common polymorphism G1065A revealed that the AA + GA genotypes were significantly more frequent in salt-resistant than in salt-sensitive individuals (p = 0.007), suggesting a protective role for the A allele. We also screened angiotensinogen gene AGT M235T and angiotensin-converting enzyme insertion/deletion polymorphism ACE I/D and found an association between TT genotype and hypertension. A possible synergistic effect to salt-sensitive hypertension was found by combining EDNRB GG with ACE DD/ID genotypes. In conclusion, our data confirm the role of ET system and RAAS in salt-sensitive hypertension.

Complement-mediated dysfunction of glomerular filtration barrier accelerates progressive renal injury

Intrarenal complement activation leads to chronic tubulointerstitial injury in animal models of proteinuric nephropathies, making this process a potential target for therapy. This study investigated whether a C3-mediated pathway promotes renal injury in the protein overload model and whether the abnormal exposure of proximal tubular cells to filtered complement could trigger the resulting inflammatory response. Mice with C3 deficiency were protected to a significant degree against the protein overload-induced interstitial inflammatory response and tissue damage, and they had less severe podocyte injury and less proteinuria. When the same injury was induced in wild-type (WT) mice, antiproteinuric treatment with the angiotensin-converting enzyme inhibitor lisinopril reduced the amount of plasma protein filtered, decreased the accumulation of C3 by proximal tubular cells, and protected against interstitial inflammation and damage. For determination of the injurious role of plasma-derived C3, as opposed to tubular cell-derived C3, C3-deficient kidneys were transplanted into WT mice. Protein overload led to the development of glomerular injury, accumulation of C3 in podocytes and proximal tubules, and tubulointerstitial changes. Conversely, when WT kidneys were transplanted into C3-deficient mice, protein overload led to a more mild disease and abnormal C3 deposition was not observed. These data suggest that the presence of C3 increases the glomerular filtration barrier's susceptibility to injury, ultrafiltered C3 contributes more to tubulointerstitial damage induced by protein overload than locally synthesized C3, and local C3 synthesis is irrelevant to the development of proteinuria. It is speculated that therapies targeting complement combined with interventions to minimize proteinuria would more effectively prevent the progression of renal disease.

Mutations in FN1 cause glomerulopathy with fibronectin deposits

Glomerulopathy with fibronectin (FN) deposits (GFND) is an autosomal dominant disease with age-related penetrance, characterized by proteinuria, microscopic hematuria, hypertension, and massive glomerular deposits of FN that lead to end-stage renal failure. The genetic abnormality underlying GFND was still unknown. We hypothesized that mutations in FN1, which encodes FN, were the cause of GFND. In a large Italian pedigree with eight affected subjects, we found linkage with GFND at the FN1 locus at 2q32. We sequenced the FN1 in 15 unrelated pedigrees and found three heterozygous missense mutations, the W1925R, L1974R, and Y973C, that cosegregated with the disease in six pedigrees. The mutations affected two domains of FN (Hep-II domain for the W1925R and the L1974R, and Hep-III domain for the Y973C) that play key roles in FN-cell interaction and in FN fibrillogenesis. Mutant recombinant Hep-II fragments were expressed, and functional studies revealed a lower binding to heparin and to endothelial cells and podocytes compared with wild-type Hep-II and an impaired capability to induce endothelial cell spreading and cytoskeletal reorganization. Overall dominant mutations in FN1 accounted for 40% of cases of GFND in our study group. These findings may help understanding the pathogenesis of proteinuria and glomerular FN deposits in GFND and possibly in more common renal diseases such as diabetic nephropathy, IgA nephropathy, and lupus nephritis. To our knowledge no FN1 mutation causing a human disease was previously reported.

The complement factor H R1210C mutation is associated with atypical hemolytic uremic syndrome

Mutations in the gene encoding complement factor H (CFH) that alter the C3b/polyanions-binding site in the C-terminal region impair the capacity of factor H to protect host cells. These mutations are also strongly associated with atypical hemolytic uremic syndrome (aHUS). Although most of the aHUS-associated CFH mutations seem "unique" to an individual patient or family, the R1210C mutation has been reported in several unrelated aHUS patients from distinct geographic origins. Five aHUS pedigrees and 7 individual aHUS patients were analyzed to identify potential correlations between the R1210C mutation and clinical phenotype and to characterize the origins of this mutation. The clinical phenotype of aHUS patients carrying the R1210C mutation was heterogeneous. Interestingly, 12 of the 13 affected patients carried at least one additional known genetic risk factor for aHUS. These data are in accord with the 30% penetrance of aHUS in R1210C mutation carriers, as it seems that the presence of other genetic or environmental risk factors significantly contribute to the manifestation and severity of aHUS in these subjects. Genotype analysis of CFH and CFHR3 polymorphisms in the 12 unrelated carriers suggested that the R1210C mutation has a single origin. In conclusion, the R1210C mutation of complement factor H is a prototypical aHUS mutation that is present as a rare polymorphism in geographically separated human populations.

Complement and the atypical hemolytic uremic syndrome in children

Over the past decade, atypical hemolytic uremic syndrome (aHUS) has been demonstrated to be a disorder of the regulation of the complement alternative pathway. Among approximately 200 children with the disease, reported in the literature, 50% had mutations of the complement regulatory proteins factor H, membrane cofactor protein (MCP) or factor I. Mutations in factor B and C3 have also been reported recently. In addition, 10% of children have factor H dysfunction due to anti-factor H antibodies. Early age at onset appears as characteristic of factor H and factor I mutated patients, while MCP-associated HUS is not observed before age 1 year. Low C3 level may occur in patients with factor H and factor I mutation, while C3 level is generally normal in MCP-mutated patients. Normal plasma factor H and factor I levels do not preclude the presence of a mutation in these genes. The worst prognosis is for factor H-mutated patients, as 60% die or reach end-stage renal disease (ESRD) within the first year after onset of the disease. Patients with mutations in MCP have a relapsing course, but no patient has ever reached ESRD in the first year of the disease. Half of the patients with factor I mutations have a rapid evolution to ESRD, but half recover. Early intensive plasmatherapy appears to have a beneficial effect, except in MCP-mutated patients. There is a high risk of graft loss for HUS recurrence or thrombosis in all groups except the MCP-mutated group. Recent success of liver-kidney transplantation combined with plasmatherapy opens this option for patients with mutations of factors synthesized in the liver. New therapies such as factor H concentrate or complement inhibitors offer hope for the future.

Translational mini-review series on complement factor H: therapies of renal diseases associated with complement factor H abnormalities: atypical haemolytic uraemic syndrome and membranoproliferative glomerulonephritis

Genetic and acquired abnormalities in complement factor H (CFH) have been associated with two different human renal diseases: haemolytic uraemic syndrome and membrano proliferative glomerulonephritis. The new genetic and pathogenetic findings in these diseases and their clinical implications for the management and cure of patients are reviewed in this paper.

Membrane cofactor protein mutations in atypical hemolytic uremic syndrome (aHUS), fatal Stx-HUS, C3 glomerulonephritis, and the HELLP syndrome

The hemolytic uremic syndrome (HUS) is a triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal impairment. Genetic studies demonstrate that heterozygous mutations of membrane cofactor protein (MCP;CD46) predispose to atypical HUS (aHUS), which is not associated with exposure to Shiga toxin (Stx). Among the initial 25 MCP mutations in patients with aHUS were 2, R69W and A304V, that were expressed normally and for which no dysfunction was found. The R69W mutation is in complement control protein module 2, while A304V is in the hydrophobic transmembrane domain. In addition to 3 patients with aHUS, the A304V mutation was identified in 1 patient each with fatal Stx-HUS, the HELLP (hemolysis, elevated liver enzymes, and low platelets) syndrome, and glomerulonephritis with C3 deposits. A major goal was to assess if these putative mutations lead to defective complement regulation. Permanent cell lines expressing the mutated proteins were complement "challenged," and membrane control of C3 fragment deposition was monitored. Both the R69W and A304V MCP mutations were deficient in their ability to control the alternative pathway of complement activation on a cell surface, illustrating the importance of modeling transmembrane proteins in situ.

Where next with atypical hemolytic uremic syndrome?

Hemolytic uremic syndrome (HUS) is a systemic disease characterized by damage to endothelial cells, erythrocytes and kidney glomeruli. A "typical" form of HUS follows gastrointestinal infection with enterohemorrhagic E. coli (e.g. O157:H7). Atypical HUS (aHUS) is not associated with gastrointestinal infections but is sporadic or familial in nature. Approximately 50% of aHUS cases are associated with a mutation in one or more genes coding for proteins involved in regulation or activation of the alternative pathway of complement. The link between the disease and the mutations shows the important balance of the alternative pathway between activation and regulation on host cell surfaces. It also demonstrates the power of this pathway in destroying cellular targets in general. In this review we discuss the current knowledge on pathogenesis, classification, diagnostics and management of this disease. We indicate a comprehensive diagnostic approach for aHUS based on the latest knowledge on complement dysregulation to gain both immediate and future patient benefit by assisting in choosing more appropriate therapy for each patient. We also indicate directions in which therapy of aHUS might improve and indicate the need to re-think the terminology and categorisation of the HUS-like diseases so that any advantage in the understanding of complement regulatory problems can be applied to patients accurately.

Characterization of mutations in complement factor I (CFI) associated with hemolytic uremic syndrome

Recent studies have identified mutations in the complement regulatory gene factor I (CFI) that predispose to atypical hemolytic uremic syndrome (aHUS). CFI is a two-chain serine protease in which the light chain carries the catalytic domain while the heavy chain's function is unclear. It downregulates the alternative and classical complement pathways by cleaving the alpha' chains of C3b and C4b in the presence of cofactor proteins (known as cofactor activity). Many CFI mutations in aHUS result in low CFI levels with a consequent quantitative defect in complement regulation. In others, the mutant protein is present in normal amounts but the presumed functional deficiency has not yet been defined. In this report we examine the nature of the functional defect in aHUS-associated CFI mutations. The I322T, D501N and D506V mutations reside in the serine protease domain of CFI and result in secreted proteins that lack C3b and C4b cofactor activity. The delTTCAC (1446-1450) mutant leads to a protein that is not secreted. The R299W mutant lies in a region of the CFI heavy chain of no known function. Our assessments demonstrate decreased C3b and C4b cofactor activity, providing evidence that this region is important for cofactor activity. In two other heavy chain mutants and one probable polymorphic variant, no functional deficiency was identified. These defective mutant proteins will result in an inability to appropriately control the complement cascade at sites of endothelial cell injury. The excessive complement activation for a given degree of damage may result in generation of a procoagulant state and aHUS.

DnIKK2-transfected dendritic cells induce a novel population of inducible nitric oxide synthase-expressing CD4+CD25- cells with tolerogenic properties

BACKGROUND

We previously documented that rat bone marrow-derived dendritic cells (DCs), transfected with an adenovirus encoding a dominant negative form of IKK2 (dnIKK2), have impaired allostimulatory capacity and generate CD4 T cells with regulatory function. Here we investigate the potency, the phenotype, and the mechanism of action of dnIKK2-DC-induced regulatory cells and we evaluated their tolerogenic properties in vivo.

METHODS

Brown Norway (BN) transfected dnIKK2-DCs were cultured with Lewis (LW) lymphocytes in primary mixed lymphocyte reaction (MLR). CD4 T cells were purified from primary MLR and incubated in secondary coculture MLR with LW lymphocytes. Phenotypic characterization was performed by fluorescence-activated cell sorting and real-time polymerase chain reaction. The tolerogenic potential of CD4 T cells pre-exposed to dnIKK2-DCs was evaluated in vivo in a model of kidney allotransplantation.

RESULTS

CD4 T cells pre-exposed to dnIKK2-DCs were CD4CD25 and expressed interleukin (IL)-10, transforming growth factor-beta, interferon-gamma, IL-2, and inducible nitric oxide synthase (iNOS). These cells (dnIKK2-Treg), cocultured (at up to 1:10 ratio) with a primary MLR, suppressed T-cell proliferation to alloantigens. The regulatory effect was cell-to-cell contact-independent since it was also observed in a transwell system. A nitric oxide synthase inhibitor significantly reverted dnIKK2-Treg-mediated suppression, whereas neutralizing antibodies to IL-10 and TGF-beta had no significant effect. DnIKK2-Treg given in vivo to LW rats prolonged the survival of a kidney allograft from BN rats (the donor rat strain used for generating DCs).

CONCLUSIONS

DnIKK2-Treg is a unique population of CD4CD25 T cells expressing high levels of iNOS. These cells potently inhibit T-cell response in vitro and induce prolongation of kidney allograft survival in vivo.

Role of thymic- and graft-dependent mechanisms in tolerance induction to rat kidney transplant by donor PBMC infusion

We previously demonstrated the presence of regulatory T cells (Tregs) in lymph nodes (LNs) from rats made tolerant to a kidney allograft by donor peripheral blood mononuclear cell (PBMC) infusion. Here, we investigated the origin of Treg and characterized their phenotype and mechanisms underlying their suppressive effect. At different points after PBMC infusion, thymus, LN, and graft-infiltrating -lymphocyte's (GIL) alloreactivity was evaluated in mixed lymphocyte reaction (MLR), coculture, and transwell experiments. GIL phenotype (by fluorescence-activated cell sorting and immunohistochemistry) and cytokines mRNA expression were analyzed. Before transplantation, CD4(+) thymocytes and LN cells from donor PBMC-infused rats showed a reduced anti-donor but a normal anti-third-party proliferation. Anti-donor hyporesponsiveness was reverted by interleukin (IL)-2. CD4(+) thymocytes had no regulatory activity on a naïve MLR. Treg appeared in LN at 60 days post-transplant. CD4(+)-GIL isolated early (5 days) and late post-transplant (days 60-80) were hyporesponsive and suppressed a naïve MLR. IL-10 mRNA was upregulated in GIL and an anti-IL-10 monoclonal antibody reverted their inhibitory effect. Cell-to-cell contact potentiated the suppressive activity of CD4(+)-GIL. We suppose that allograft tolerance in this model is mediated by pretransplant generation of anergic cells in the thymus, which may have a permissive role to prevent early graft disruption. The healed graft is a source of donor antigens, which led to early selection of Treg. In the late phase, tolerance is maintained by appearance of Treg in LN.

The interactive Factor H-atypical hemolytic uremic syndrome mutation database and website: update and integration of membrane cofactor protein and Factor I mutations with structural models

Atypical hemolytic uremic syndrome (aHUS) is a disease of hemolytic anemia, thrombocytopenia, and renal failure associated with defective alternative pathway (AP) complement control. Previously, we presented a database (www.FH-HUS.org) focusing on aHUS mutations in the Factor H gene (CFH). Here, new aHUS mutations are reported for the complement regulatory proteins Factor H (FH), Factor I (FI), and membrane cofactor protein (MCP). Additional mutations or polymorphisms within CFH have been associated with membranoproliferative glomerulonephritis (MPGN) and age-related macular degeneration (AMD). Accordingly, the database now includes substitutions that predispose to aHUS, MPGN, and AMD. For this, structural models for the domains in MCP and FI were developed using homology modeling. With this new database, patients with mutations in more than one gene can be displayed and interpreted in a coherent manner. The database also includes SNP polymorphisms in CFH, MCP, and IF. There are now a total of 167 genetic alterations, including 100 in CFH, 43 in MCP, and 24 in IF. The mutations characterize clinical outcomes that vary from several AMD-associated polymorphisms to those associated with aHUS, MPGN, or FI deficiency. A consensus short complement regulator (SCR) domain structure facilitated the interpretations of aHUS mutations. Specific locations within this consensus domain often correlate with the occurrence of clinical phenotypes. The AMD Tyr402His polymorphism is structurally located at a hotspot for several aHUS mutations. The database emphasizes the causative role of the alternative pathway of complement in disease and provides a repository of knowledge to assist future diagnosis and novel therapeutic approaches.

Genetic analysis of the complement factor H related 5 gene in haemolytic uraemic syndrome

Several mutations in the CFH gene have been described in non-Shiga-toxin-associated haemolytic uraemic syndrome (non-Stx-HUS), a rare syndrome characterized by haemolytic anaemia, thrombocytopenia and acute renal failure. Mutations in genes encoding other complement regulatory proteins, membrane cofactor protein (CD46) and complement factor I (CFI), were also involved in the pathogenesis of the disease. Anyway, mutations in the three genes account for no more than 50% of cases of non-Stx-HUS. Human complement factor H related 5 (CFHR5) is a recently characterised member of the human complement factor H (CFH) family that has been found as a component of immune deposits in human kidney with sclerotic lesions from different causes. CFHR5 possesses cofactor activity and has been proposed to play a role in complement regulation in the glomerulus. We screened CFHR5 gene for variations potentially involved in the aetiology of HUS. Forty-five patients with HUS and 80 controls were analysed. Altogether, 5 genetic variants in CFHR5 were found in overall 9/45 HUS patients and in 4/80 controls. Statistical analysis showed that allelic variants in CFHR5 were prefentially associated with HUS. Based on these data, we conclude that, though not causative, CFHR5 genetic alterations may play a secondary role in the pathogenesis of HUS.