Factor H and the pathogenesis of renal diseases

Kidney diseases

Dysregulation and accelerated activation of the alternative pathway (AP) of complement is known to cause or accentuate several pathologic conditions in which kidney injury leads to the appearance of hematuria and proteinuria and ultimately to the development of chronic renal failure. Multiple genetic and acquired defects involving plasma- and membrane-associated proteins are probably necessary to impair the protection of host tissues and to confer a significant predisposition to AP-mediated kidney diseases. This review aims to explore how our current understanding will make it possible to identify the mechanisms that underlie AP-mediated kidney diseases and to discuss the available clinical evidence that supports complement-directed therapies. Although the value of limiting uncontrolled complement activation has long been recognized, incorporating complement-targeted treatments into clinical use has proved challenging. Availability of anti-complement therapy has dramatically transformed the outcome of atypical hemolytic uremic syndrome, one of the most severe kidney diseases. Innovative drugs that directly counteract AP dysregulation have also opened new perspectives for the management of other kidney diseases in which complement activation is involved. However, gained experience indicates that the choice of drug should be tailored to each patient's characteristics, including clinical, histologic, genetic, and biochemical parameters. Successfully treating patients requires further research in the field and close collaboration between clinicians and researchers who have special expertise in the complement system.

Thrombotic microangiopathy in children

The syndrome of thrombotic microangiopathy (TMA) is a clinical-pathological entity characterized by microangiopathic hemolytic anemia, thrombocytopenia, and end organ involvement. It comprises a spectrum of underlying etiologies that may differ in children and adults. In children, apart from ruling out shigatoxin-associated hemolytic uremic syndrome (HUS) and other infection-associated TMA like Streptococcus pneumoniae-HUS, rare inherited causes including complement-associated HUS, cobalamin defects, and mutations in diacylglycerol kinase epsilon gene must be investigated. TMA should also be considered in the setting of solid organ or hematopoietic stem cell transplantation. In this review, acquired and inherited causes of TMA are described with a focus on particularities of the main causes of TMA in children. A pragmatic approach that may help the clinician tailor evaluation and management is provided. The described approach will allow for early initiation of treatment while waiting for the definitive diagnosis of the underlying TMA.

Streptococcal pneumonia-associated hemolytic uremic syndrome treated by T-antibody-negative plasma exchange in children: Two case reports

BACKGROUND

The occurrence of Streptococcus pneumoniae-associated hemolytic uremic syndrome (SP-HUS) is increasing. Thomsen-Friedenreich antigen activation is highly involved in the pathogenesis of SP-HUS, and T-antibody-negative plasma exchange (PE) may be effective in the treatment of severe cases of SP-HUS.

CASE SUMMARY

We retrospectively reviewed two pediatric patients with SP-HUS. Both clinical features and laboratory examination results of the children were described. T-antibody-negative PE was performed in both cases. Both children made a full recovery after repeated PE and remained well at a 2 year follow-up.

CONCLUSION

Streptococcal pneumonia continues to be an uncommon but important cause of HUS. The successful treatment of the presented cases suggests that T-antibody-negative PE may benefit patients with SP-HUS.

New insights into the pathogenesis of Streptococcus pneumoniae-associated hemolytic uremic syndrome

The purpose of this review is to describe Streptococcus pneumoniae-associated hemolytic uremic syndrome (P-HUS) with emphasis on new insights into the pathophysiology and management over the past 10 years. Even though awareness of this clinico-pathological entity has increased, it likely remains under-recognized. Recent observations indicate that although neuraminidase activity and exposure of the T-antigen are necessary for development of P-HUS, they are not sufficient; activation of the alternate pathway of complement may also contribute. It is unclear, however, whether or not eculizumab and/or plasmapheresis are of value.

Membranoproliferative glomerulonephritis and C3 glomerulopathy in children: change in treatment modality? A report of a case series

Background

Membranoproliferative glomerulonephritis (MPGN) with immune complexes and C3 glomerulopathy (C3G) in children are rare and have a variable outcome, with some patients progressing to end-stage renal disease (ESRD). Mutations in genes encoding regulatory proteins of the alternative complement pathway and of complement C3 (C3) have been identified as concausative factors.

Methods

Three children with MPGN type I, four with C3G, i.e. three with C3 glomerulonephritis (C3GN) and one with dense deposit disease (DDD), were followed. Clinical, autoimmune data, histological characteristics, estimated glomerular filtration rate (eGFR), proteinuria, serum C3, genetic and biochemical analysis were assessed.

Results

The median age at onset was 7.3 years and the median eGFR was 72 mL/min/1.73 m². Six children had marked proteinuria. All were treated with renin-angiotensin-aldosterone system (RAAS) blockers. Three were given one or more immunosuppressive drugs and two eculizumab. At the last median follow-up of 9 years after diagnosis, three children had normal eGFR and no or mild proteinuria on RAAS blockers only. Among four patients without remission of proteinuria, genetic analysis revealed mutations in complement regulator proteins of the alternative pathway. None of the three patients with immunosuppressive treatment achieved partial or complete remission of proteinuria and two progressed to ESRD and renal transplantation. Two patients treated with eculizumab revealed relevant decreases in proteinuria.

Conclusions

In children with MPGN type I and C3G, the outcomes of renal function and response to treatment modality show great variability independent from histological diagnosis at disease onset. In case of severe clinical presentation at disease onset, early genetic and biochemical analysis of the alternative pathway dysregulation is recommended. Treatment with eculizumab appears to be an option to slow disease progression in single cases.

Defining the role of pneumococcal neuraminidases and O-glycosidase in pneumococcal haemolytic uraemic syndrome

The host and bacterial factors that lead to development of pneumococcal haemolytic uraemic syndrome (pHUS) remain poorly defined; however, it is widely believed that pneumococcal exposure of the Thomsen-Friedenreich antigen (T-antigen) on host surfaces is a key step in pathogenesis. Two enzymatic activities encoded by pneumococci determine the level of T-antigen exposed. Neuraminidases cleave terminal sialic acid to expose the T-antigen which is subsequently cleaved by O-glycosidase Eng. While a handful of studies have examined the role of neuraminidases in T-antigen exposure, no studies have addressed the potential role of O-glycosidase. This study used 29 pHUS isolates from the USA and 31 serotype-matched controls. All isolates contained eng, and no significant correlation between enzymatic activity and disease state (pHUS and blood non-pHUS isolates) was observed. A prior study from Taiwan suggested that neuraminidase NanC contributes to the development of pHUS. However, we observed no difference in nanC distribution. Similar to previously published data, we found no significant correlation between neuraminidase activity and disease state. Accurate quantification of these enzymatic activities from bacteria grown in whole blood is currently impossible, but we confirmed that there were no significant correlations between disease state and neuraminidase and O-glycosidase transcript levels after incubation in blood. Genomic sequencing of six pHUS isolates did not identify any genetic elements possibly contributing to haemolytic uraemic syndrome. These findings support the hypothesis that while exposure of T-antigen may be an important step in disease pathogenesis, host factors likely play a substantial role in determining which individuals develop haemolytic uraemic syndrome after pneumococcal invasive disease.

Crucial genes and pathways in chicken germ stem cell differentiation

Male germ cell differentiation is a subtle and complex regulatory process. Currently, its regulatory mechanism is still not fully understood. In our experiment, we performed the first comprehensive genome and transcriptome-wide analyses of the crucial genes and signaling pathways in three kinds of crucial cells (embryonic stem cells, primordial germ cell, and spermatogonial stem cells) that are associated with the male germ cell differentiation. We identified thousands of differentially expressed genes in this process, and from these we chose 173 candidate genes, of which 98 genes were involved in cell differentiation, 19 were involved in the metabolic process, and 56 were involved in the differentiation and metabolic processes, like GAL9, AMH, PLK1, and PSMD7 and so on. In addition, we found that 18 key signaling pathways were involved mainly in cell proliferation, differentiation, and signal transduction processes like TGF-β, Notch, and Jak-STAT. Further exploration found that the candidate gene expression patterns were the same between in vitro induction experiments and transcriptome results. Our results yield clues to the mechanistic basis of male germ cell differentiation and provide an important reference for further studies.

Current treatment of atypical hemolytic uremic syndrome

Tremendous advances have been made in understanding the pathogenesis of atypical Hemolytic Uremic Syndrome (aHUS), an extremely rare disease. Insights into the molecular biology of aHUS resulted in rapid advances in treatment with eculizumab (Soliris(®), Alexion Pharmaceuticals Inc.). Historically, aHUS was associated with very high rates of mortality and morbidity. Prior therapies included plasma therapy and/or liver transplantation. Although often life saving, these were imperfect and had many complications. We review the conditions included under the rubric of aHUS: S. pneumoniae HUS (SpHUS), inborn errors of metabolism, and disorders of complement regulation, emphasizing their differences and similarities. We focus on the clinical features, diagnosis, and pathogenesis, and treatment of aHUS that results from mutations in genes encoding alternative complement regulators, SpHUS and HUS associated with inborn errors of metabolism. Mutations in complement genes, or antibodies to their protein products, result in unregulated activity of the alternate complement pathway, endothelial injury, and thrombotic microangiopathy (TMA). Eculizumab is a humanized monoclonal antibody that inhibits the production of the terminal complement components C5a and the membrane attack complex (C5b-9) by binding to complement protein C5a. This blocks the proinflammatory and cytolytic effects of terminal complement activation. Eculizumab use has been reported in many case reports, and retrospective and prospective clinical trials in aHUS. There have been few serious side effects and no reports of tachphylaxis or drug resistance. The results are very encouraging and eculizumab is now recognized as the treatment of choice for aHUS.

Mechanism of inflammation in age-related macular degeneration: an up-to-date on genetic landmarks

Age-related macular degeneration (AMD) is the most common cause of irreversible visual impairment among people over 50 years of age, accounting for up to 50% of all cases of legal blindness in Western countries. Although the aging represents the main determinant of AMD, it must be considered a multifaceted disease caused by interactions among environmental risk factors and genetic backgrounds. Mounting evidence and/or arguments document the crucial role of inflammation and immune-mediated processes in the pathogenesis of AMD. Proinflammatory effects secondary to chronic inflammation (e.g., alternative complement activation) and heterogeneous types of oxidative stress (e.g., impaired cholesterol homeostasis) can result in degenerative damages at the level of crucial macular structures, that is photoreceptors, retinal pigment epithelium, and Bruch's membrane. In the most recent years, the association of AMD with genes, directly or indirectly, involved in immunoinflammatory pathways is increasingly becoming an essential core for AMD knowledge. Starting from the key basic-research notions detectable at the root of AMD pathogenesis, the present up-to-date paper reviews the best-known and/or the most attractive genetic findings linked to the mechanisms of inflammation of this complex disease.

Investigating the role of pneumococcal neuraminidase A activity in isolates from pneumococcal haemolytic uraemic syndrome

Streptococcus pneumoniae diseases are a rare but increasingly recognized trigger of atypical haemolytic uraemic syndrome (HUS) in young children and associated with a higher mortality rate than diarrhoea-associated HUS. This study aimed to determine the importance of neuraminidase A (NanA) and genomic diversity in the pathogenesis of pneumococcal HUS (pHUS). We investigated the nanA gene sequence, gene expression, neuraminidase activity and comparative genomic hybridization of invasive pneumococcal disease (IPD) isolates from patients with pHUS and control strains matched by serotype and sequence type (ST), isolated from patients with IPD but not pHUS. The nanA sequence of 33 isolates was determined and mutations at 142 aa positions were identified. High levels of diversity were observed within the NanA protein, with mosaic blocks, insertions and repeat regions present. When comparing nanA allelic diversity with ST and disease profile in the isolates tested, nanA alleles clustered mostly by ST. No particular nanA allele was associated with pHUS. There was no significant difference in overall neuraminidase activity between pHUS isolates and controls when induced/uninduced with N-acetylneuraminic acid. Comparative genomic hybridization showed little difference in genetic content between the pHUS isolates and the controls. Results of gene expression studies identified 12 genes differentially regulated in all pHUS isolates compared with the control. Although neuraminidase enzyme activity may be important in pHUS progression and contribute to pathogenesis, the lack of a distinction between pHUS isolates and controls suggests that host factors, such as acquired abnormalities of the alternative complement cascade in young children, may play a more significant role in the outcome of pHUS.

Update on Streptococcus pneumoniae associated hemolytic uremic syndrome

PURPOSE OF REVIEW

Streptococcus pneumoniae associated hemolytic uremic syndrome (SpHUS) is defined by the occurrence of acute hemolytic anemia, thrombocytopenia and acute kidney injury in a patient with a S. pneumoniae infection. We review the pathophysiology, clinical course, treatment and prognosis for SpHUS. We also describe an expanded classification system that uses additional diagnostic criteria to identify more patients with a high likelihood of having SpHUS.

RECENT FINDINGS

SpHUS often may be underdiagnosed because of overlapping features with disseminated intravascular coagulation (DIC) and the lack of strict diagnostic criteria. The epidemiology has changed with the emergence of different pneumococcal serotypes as newer pneumococcal vaccines have been introduced.

SUMMARY

SpHUS accounts for 5-15% of all HUS cases. The majority of SpHUS patients have pneumonia and a low mortality rate in contrast to those with meningitis, who have a more severe clinical course. Although the pathogenesis of SpHUS remains unknown, the Thomsen-Friedenreich antigen seems to play a central role. S. pneumoniae produces neuraminidase, thereby exposing the Thomsen-Friedenreich antigen on the surface of cell membranes. Thomsen-Friedenreich antigen exposure can result in hemolysis and direct endothelial injury leading to HUS phenotype. Early identification of these patients is critical so that fresh frozen plasma may be avoided.

A rare case: childhood-onset C3 glomerulonephritis due to homozygous factor H deficiency

C3 glomerulopathy is a recently described pathological entity including dense deposit disease and C3 glomerulonephritis (C3GN). In some cases, C3 glomerulopathy is associated with defects or even complete deficiency of factor H. However, complete factor H deficiency among patients with C3GN is rare, and paediatric cases have not yet been described. Here, we report a child with homozygous factor H deficiency who presented with haematuria and minor proteinuria, together with undetectable plasma C3 levels, at the age of 10 years. Kidney biopsy demonstrated C3GN. Detailed complement analysis revealed complete factor H deficiency due to a homozygous CFH mutation. Furthermore, there was a complete deletion of CFHR-1/-3. During follow-up, the patient has had recurrent episodes of macro-haematuria and minor proteinuria, but during 4 years of follow-up, no deterioration of renal function has been observed. Mutations of factor H in C3GN have been described; however, complete CFH deficiency is rare in these patients. Furthermore, clinical presentation usually occurs in adulthood. Therefore, this case presents a rare manifestation of the disease and might contribute to the early detection of similar cases also in childhood.

Development and application of a next-generation-sequencing (NGS) approach to detect known and novel gene defects underlying retinal diseases

Background

Inherited retinal disorders are clinically and genetically heterogeneous with more than 150 gene defects accounting for the diversity of disease phenotypes. So far, mutation detection was mainly performed by APEX technology and direct Sanger sequencing of known genes. However, these methods are time consuming, expensive and unable to provide a result if the patient carries a new gene mutation. In addition, multiplicity of phenotypes associated with the same gene defect may be overlooked.

Methods

To overcome these challenges, we designed an exon sequencing array to target 254 known and candidate genes using Agilent capture. Subsequently, 20 DNA samples from 17 different families, including four patients with known mutations were sequenced using Illumina Genome Analyzer IIx next-generation-sequencing (NGS) platform. Different filtering approaches were applied to identify the genetic defect. The most likely disease causing variants were analyzed by Sanger sequencing. Co-segregation and sequencing analysis of control samples validated the pathogenicity of the observed variants.

Results

The phenotype of the patients included retinitis pigmentosa, congenital stationary night blindness, Best disease, early-onset cone dystrophy and Stargardt disease. In three of four control samples with known genotypes NGS detected the expected mutations. Three known and five novel mutations were identified in NR2E3, PRPF3, EYS, PRPF8, CRB1, TRPM1 and CACNA1F. One of the control samples with a known genotype belongs to a family with two clinical phenotypes (Best and CSNB), where a novel mutation was identified for CSNB. In six families the disease associated mutations were not found, indicating that novel gene defects remain to be identified.

Conclusions

In summary, this unbiased and time-efficient NGS approach allowed mutation detection in 75% of control cases and in 57% of test cases. Furthermore, it has the possibility of associating known gene defects with novel phenotypes and mode of inheritance.

Membranoproliferative pattern of glomerular injury associated with complement component 9 deficiency due to Arg95Stop mutation

BACKGROUND

Arg95Stop mutation of exon 4 in complement component 9 (C9) gene is common in individuals in Japan with C9 deficiency (C9D); however, understanding of the influences of C9D on human glomerulonephritis remains elusive.

METHODS

A total of 1288 patients with chronic kidney disease (CKD) were recruited from the hospitals in Niigata prefecture. They were screened for the Arg95Stop mutation of C9 gene by allele-specific PCR.

RESULTS

We identified two individuals with C9D among 1,288 CKD patients, a frequency comparable to that of the general Japanese population (0.16%). Case 1 involved a 44-year-old man presenting with nephrotic proteinuria. The hemolytic activity of CH50 was low, and the concentration of C9 was not detected. Sequencing of exon 4 of the C9 gene showed the Arg95Stop mutation. Renal biopsy revealed diffuse global mesangial proliferation with extensive duplication of glomerular capillary walls. Mesangial, subendothelial and subepithelial deposits were noticed with light and electron microscopy. Immunofluorescent study showed predominant mesangial IgA deposition. Case 2 involved a 62-year-old man presenting with proteinuria and hematuria. His CH50 level was decreased. Renal biopsy revealed diffuse global mesangial proliferation with extensive duplication of glomerular capillary walls. Immune deposits were also confirmed. The percentage of C9D among patients with mesangial proliferation and duplication of GBM in this study was 5.1%.

CONCLUSION

These results suggested that the lack of membrane attack complex because of an Arg95Stop mutation of the C9 gene predisposed patients to pathognomonic glomerulonephritis.

Complement control protein factor H: the good, the bad, and the inadequate

The complement system is an essential component of the innate immune system that participates in elimination of pathogens and altered host cells and comprises an essential link between the innate and adaptive immune system. Soluble and membrane-bound complement regulators protect cells and tissues from unintended complement-mediated injury. Complement factor H is a soluble complement regulator essential for controlling the alternative pathway in blood and on cell surfaces. Normal recognition of self-cell markers (i.e. polyanions) and C3b/C3d fragments is necessary for factor H function. Inadequate recognition of host cell surfaces by factor H due to mutations and polymorphisms have been associated with complement-mediated tissue damage and disease. On the other hand, unwanted recognition of pathogens and altered self-cells (i.e. cancer) by factor H is used as an immune evasion strategy. This review will focus on the current knowledge related to these versatile recognition properties of factor H.

Alternative complement pathway deregulation is correlated with dengue severity

BACKGROUND

The complement system, a key component that links the innate and adaptive immune responses, has three pathways: the classical, lectin, and alternative pathways. In the present study, we have analyzed the levels of various complement components in blood samples from dengue fever (DF) and dengue hemorrhagic fever (DHF) patients and found that the level of complement activation is associated with disease severity.

METHODS AND RESULTS

Patients with DHF had lower levels of complement factor 3 (C3; p = 0.002) and increased levels of C3a, C4a and C5a (p<0.0001) when compared to those with the less severe form, DF. There were no significant differences between DF and DHF patients in the levels of C1q, immunocomplexes (CIC-CIq) and CRP. However, small but statistically significant differences were detected in the levels of MBL. In contrast, the levels of two regulatory proteins of the alternative pathway varied widely between DF and DHF patients: DHF patients had higher levels of factor D (p = 0.01), which cleaves factor B to yield the active (C3bBb) C3 convertase, and lower levels of factor H (p = 0.03), which inactivates the (C3bBb) C3 convertase, than did DF patients. When we considered the levels of factors D and H together as an indicator of (C3bBb) C3 convertase regulation, we found that the plasma levels of these regulatory proteins in DHF patients favored the formation of the (C3bBb) C3 convertase, whereas its formation was inhibited in DF patients (p<0.0001).

CONCLUSION

The data suggest that an imbalance in the levels of regulatory factors D and H is associated with an abnormal regulation of complement activity in DHF patients.

Renal disease associated with inherited disorders of the complement system

The human complement system is vital for host defense and plays a role in a number of inflammatory disorders. Inherited deficiency or dysfunction of most of the individual complement components occurs uncommonly. The phenotype displayed by such patients varies with the specific component deficiency and ranges from recurrent infections to autoimmune diseases. Most of the latter are associated with glomerulonephritis. The onset of severe lupus erythematosus in a young child, with prominent cutaneous and renal manifestations, especially if a similar disorder is present in another family member, is a clue to the presence of a complement component deficiency. The distinguishing of acquired deficiencies from inherited deficiencies in complement components is sometimes difficult and may require sophisticated laboratory testing.

Complement factor H deficiency and posttransplantation glomerulonephritis with isolated C3 deposits

We report the first cases of atypical hemolytic and uremic syndrome associated with complement factor H (CFH) deficiency in native kidneys and glomerulonephritis with isolated C3 deposits after kidney transplantation. Two boys developed atypical hemolytic and uremic syndrome at 16 and 11 months of age, associated with low C3 and CFH levels. Both rapidly progressed to end-stage renal failure and received a kidney transplant. Patient 1 had combined CFH and complement factor I (CFI) heterozygous mutations and a membrane cofactor protein (gene symbol, CD46) gene polymorphism. Five years posttransplantation, an allograft biopsy specimen showed numerous mesangial and extramembranous C3 deposits, although the patient had no biological sign of glomerulopathy. Nine years after transplantation, he was well with stable kidney function. Patient 2, who had a homozygous CFH mutation, developed glomerulonephritis with isolated C3 deposits 5 months after kidney transplantation while he was treated for early recurrence of hemolytic anemia. Four years later, the second kidney transplant biopsy specimen showed recurrence of thrombotic microangiopathy. Six years posttransplantation, kidney function was stable and complete blood cell count was normal with regular plasma therapy. These observations suggest that constitutional dysregulation of the alternative pathway is associated with a wide spectrum of kidney diseases, and glomerulonephritis with isolated C3 deposits and thrombotic microangiopathy may be different expressions of the same condition. Several factors could influence the disease, such as degree of CFH haploinsufficiency and other complement alternative pathway regulator abnormalities, such as a membrane cofactor protein polymorphism.

Pathogenesis of thrombotic microangiopathies

Profound thrombocytopenia and microangiopathic hemolytic anemia characterize thrombotic microangiopathy, which includes two major disorders: thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). TTP has at least three types: congenital or familial, idiopathic, and nonidiopathic. The congenital and idiopathic TTP syndromes are caused primarily by deficiency of ADAMTS13, owing to mutations in the ADAMTS13 gene or autoantibodies that inhibit ADAMTS13 activity. HUS is similar to TTP, but is associated with acute renal failure. Diarrhea-associated HUS accounts for more than 90% of cases and is usually caused by infection with Shiga-toxin-producing Escherichia coli (O157:H7). Diarrhea-negative HUS is associated with complement dysregulation in up to 50% of cases, caused by mutations in complement factor H, membrane cofactor protein, factor I or factor B, or by autoantibodies against factor H. The incomplete penetrance of mutations in either ADAMTS13 or complement regulatory genes suggests that precipitating events or triggers may be required to cause thrombotic microangiopathy in many patients.

Dense deposit disease: new insights

PURPOSE OF REVIEW

Dense deposit disease is a rare but devastating disease primarily affecting children. This review focuses on new information regarding the pathophysiology of dense deposit disease, its appearance histopathologically, its relationship to other diseases including macular degeneration and acquired partial lipodystrophy and potential new therapies.

RECENT FINDINGS

The microscopic features of dense deposit disease have been separated into five patterns with only about 25% of patients showing membranoproliferative features. The subtle interplay between genetic changes in complement regulatory proteins and dysregulation of the alternative pathway of complement is now more evident. Haplotype mapping has shown at-risk phenotypes of complement factor H associated with the development of dense deposit disease. Treatment protocols are empiric and not very effective. New information on complement inhibitors and plasma exchange, however, has brought hope for new therapies in the near future.

SUMMARY

Understanding of the pathology and the pathophysiology of dense deposit disease has advanced rapidly in the last decade. New efforts in genetic mapping along with the development of novel inhibitors of the complement system will lead to improved care for patients afflicted with this uncommon condition.

Prognosis and pathological characteristics of five children with non-Shiga toxin-mediated hemolytic uremic syndrome

BACKGROUND

The three major signs of hemolytic uremic syndrome (HUS) are hemolytic anemia, thrombopenia and acute renal failure. HUS is classified into Shiga toxin-mediated HUS (Stx-HUS) and non-Shiga toxin-mediated HUS (nStx-HUS). The prognosis of nStx-HUS is reported to be less favorable than that of Stx-HUS. Although the association between the prognosis and pathological characteristics of HUS have been reported such that the prognosis was considered to be poor for thrombotic microangiopathy (TMA) with predominant arterial involvement (arterial TMA), good for TMA with predominant glomerular involvement (glomerular TMA) and dependent on the extent of necrosis in cases of renal cortical necrosis, it is not yet clear whether pathological findings are also related to the renal prognosis of nStx-HUS cases. Therefore the purpose of the present paper was to analyze renal biopsy findings and prognosis for five children with nStx-HUS.

METHODS

Clinical records of five cases of nStx-HUS among 74 cases of diagnosed HUS were reviewed, and information and data were summarized.

RESULTS

Histological examination of the kidney led to the diagnosis of arterial TMA in three cases, and glomerular TMA and severe renal cortical necrosis in one case each. Analysis of the relationship between renal histological findings and the prognosis found that three patients with arterial TMA and one patient with severe renal cortical necrosis later developed end-stage renal failure while one patient with glomerular TMA has continued to show normal renal function.

CONCLUSIONS

These findings indicate that pathological findings are closely related to the prognosis in cases of nStx-HUS.

MPGN II--genetically determined by defective complement regulation?

MPGN II is a rare disease which is characterized by complement containing deposits within the GBM. The disease is characterized by functional impairment of the GBM causing progressive loss of renal function eventually resulting in end stage renal disease. It now becomes evident that in addition to C3NeF, which inhibits the inactivation of the alternative C3 convertase C3bBb, different genetically determined factors are also involved in the pathogenesis of MPGN II. These factors though different from C3NeF also result in defective complement regulation acting either through separate pathways or synergistically with C3NeF. Following the finding of MPGN II in Factor H deficient animals, patients with MPGN II were identified presenting with an activated complement system caused by Factor H deficiency. Factor H gene mutations result in a lack of plasma Factor H or in a functional defect of Factor H protein. Loss of Factor H function can also be caused by inactivating Factor H autoantibodies, C3 mutations preventing interaction between C3 and Factor H, or autoantibodies against C3. Identification of patients with MPGN II caused by defective complement control may allow treatment by replacement of the missing factor via plasma infusion, thus possibly preventing or at least delaying disease progress.

An interactive web database of factor H-associated hemolytic uremic syndrome mutations: insights into the structural consequences of disease-associated mutations

Factor H (FH) is a central complement regulator comprised of 20 short complement repeat (SCR) domains. Nucleotide changes within this gene (CFH) have been observed in patients with hemolytic uremic syndrome (HUS), and also membranoproliferative glomerulonephritis and age-related macular degeneration. All parts of FH are affected, but many mutations are clustered in the C-terminal part of FH. Up to now, structural analyses of HUS have been based on SCR-20, a domain that is involved in FH interactions with C3b, heparin, and endothelial cells. In order to identify the structural and functional consequence of HUS mutations, further disease-associated mutations were analyzed in terms of homology and nuclear magnetic resonance (NMR) models for factor H SCR domains. An interactive web database of 54 human HUS-associated mutations and others was created from the literature (www.FH-HUS.org). This has comprehensive search and analysis tools, integrating phenotypic and genetic data with structural analysis. Each mutation can be highlighted on the SCR structure together with the patient FH and C3 levels where available. Two new insights were obtained from our collection of data. First, phenotypic data on FH clarify our previously-proposed classification of Type I and Type II disorders that both lead to HUS, where Type I affects FH secretion and folding, and Type II leads to expressed protein in plasma that is functionally defective. Second, the new mutations show more clearly that SCR domains from SCR-16 to SCR-19 are important for the ligand binding activities of FH as well as SCR-20. This FH web database will facilitate the interpretation of new mutations and polymorphisms when these are identified in patients, and it will clarify the functional role of FH.

Proteomic profiling of urinary protein excretion in the factor H-deficient mouse

BACKGROUND

Since the 1970s a variety of experimental techniques have been employed in an attempt to identify urinary biomarkers of renal injury. While these approaches have met with some success, modern proteomic tools now permit broad based high-throughput analysis of the urinary proteome.

METHODS

Using the ICAT isotopic labeling based LC/MS/MS approach, comparative urinary protein profiling was performed in a murine model of membranoproliferative glomerulonephritis. Paired samples were analyzed mice with a targeted deletion of the complement regulatory protein factor H (FH-/-) and control mice.

RESULTS

25 distinct urinary proteins were identified of which 7 were differentially expressed in the FH-/- mice. Two proteins were markedly altered in the urine of FH-/- mice compared to controls: uromodulin (5.5-fold lower) and the MHC class II molecule H2e (8.6-fold higher). Differential expression was confirmed by Western blot and RT-PCR. Immunofluorescent staining demonstrated a marked increased expression of H2e and a reduction of uromodulin expression in the tubular epithelium of FH-/- mice.

CONCLUSIONS

These findings provide insight into early complement-dependent alterations in tubular protein expression which may play critical roles in the development of tubulointerstitial disease, and provide experimental support for the use of urinary proteomic profiling in murine models of renal injury.

Deletion of Lys224 in regulatory domain 4 of Factor H reveals a novel pathomechanism for dense deposit disease (MPGN II)

We report a novel pathomechanism for membranoproliferative glomerulonephritis type II (MPGN II) caused by a mutant Factor H protein expressed in the plasma. Genetic analyses of two patients revealed deletion of a single Lys residue (K224) located within the complement regulatory region in domain 4 of Factor H. This deletion resulted in defective complement control: mutant protein purified from the plasma of patients showed severely reduced cofactor and decay-accelerating activity, as well as reduced binding to the central complement component C3b. However, cell-binding activity of the mutant protein was normal and comparable to wild-type Factor H. The patients are daughters of consanguineous parents. As both patients but also their healthy mother were positive for C3 nephritic factor, the mutant Factor H protein is considered relevant for unrestricted activation of the disease-causing activation of the alternative complement pathway. Replacement of functional Factor H by fresh frozen plasma (10-15 ml/kg/14 days) was well tolerated, prevented so far disease progression in both patients, and is in the long run expected to preserve kidney function.

Phenotypic expression of factor H mutations in patients with atypical hemolytic uremic syndrome

We investigated the phenotypic expression of factor H mutations in two patients with atypical hemolytic uremic syndrome (HUS). Factor H in serum was assayed by rocket immunoelectrophoresis, immunoblotting, and double immunodiffusion and in tissue by immunohistochemistry. Functional activity was analyzed by hemolysis of sheep erythrocytes and binding to endothelial cells. A homozygous mutation in complement control protein (CCP) domain 10 of factor H was identified in an adult man who first developed membranoproliferative glomerulonephritis and later HUS. C3 levels were very low. The patient had undetectable factor H levels in serum and a weak factor H 150 kDa band. Double immunodiffusion showed partial antigenic identity with factor H in normal serum owing to the presence of factor H-like protein 1. Strong specific labeling for factor H was detected in glomerular endothelium, mesangium and in glomerular and tubular epithelium as well as in bone marrow cells. A heterozygous mutation in CCP 20 of factor H was found in a girl with HUS. C3 levels were moderately decreased at onset. Factor H levels were normal and a normal 150 kDa band was present. Double immunodiffusion showed antigenic identity with normal factor H. Factor H labeling was minimal in the renal cortex. Factor H dysfunction was demonstrated by increased sheep erythrocyte hemolysis and decreased binding to endothelial cells. In summary, two different factor H mutations associated with HUS were examined: in one, factor H accumulated in cells, and in the other, membrane binding was reduced.

A case of atypical hemolytic uremic syndrome with a transient decrease in complement factor H

We report a case of sporadic atypical hemolytic uremic syndrome (HUS) with a transient decrease in complement factor H. Referred for hemolysis and azotemia without diarrhea prodrome, this 31-month-old boy showed a decreased complement 3 (C3) and complement factor H (FH) level. However, the factor H gene (HF1) mutation was missing. After the hemolysis was controlled with plasma infusion, the C3 and FH levels recovered. The patient's renal function fully recovered and remained normal, and there was no recurrence of the HUS.

Steroid-responsive idiopathic glomerular capillary endotheliosis: case report and literature review

Glomerular capillary endotheliosis is a lesion of endothelial cell injury. Morphological characteristics are endothelial swelling with glomerular hypertrophy and a reduction in capillary lumen size. This lesion commonly is found in patients with thrombotic microangiopathy, but similar histopathologic characteristics have been reported in patients with other diseases. A previously healthy 39-year-old woman presented with progressive lower-extremity swelling and arthralgias for 1 week. She had no other symptoms and denied prior illness. Her examination was remarkable for hypertension and pitting edema. Urine showed dysmorphic red blood cells and proteinuria. Serum creatinine level increased from 1.1 to 2.0 mg/dL (97 to 177 micromol/L) during several weeks. She did not meet criteria for systemic lupus erythematosus. Other test results included a negative pregnancy test and normal complement levels. Additional workup was negative for other causes of glomerular capillary endotheliosis. She underwent 2 renal biopsies. The first showed marked endothelial cell swelling, and the second biopsy 2 months later showed disease progression. Both were consistent with glomerular capillary endotheliosis. Proteinuria and serum creatinine level elevation responded to methylprednisolone therapy within 1 week, recurred after steroid doses were tapered, and responded again after restarting steroid therapy with monthly cyclophosphamide infusions. The differential diagnosis for glomerular capillary endotheliosis is limited. Various causes have been implicated, such as dysregulation of vascular endothelial growth factor, abnormal collagen production, and endothelial abnormalities. We did not identify prior cases of idiopathic glomerular capillary endotheliosis in the literature. Idiopathic glomerular capillary endotheliosis may be a newly recognized entity potentially responsive to steroid and cytotoxic regimens.

Successful plasma therapy for atypical hemolytic uremic syndrome caused by factor H deficiency owing to a novel mutation in the complement cofactor protein domain 15

Quantitative or functional deficiency of complement factor H results in uncontrolled complement activation. This leads to thrombotic microangiopathy and finally causes renal failure (atypical hemolytic uremic syndrome [aHUS]). By regular analysis of factor H in patients with aHUS, the authors found a complete factor H deficiency in an infant in whom aHUS developed at 8 months of age. Factor H was quantified by enzyme-linked immunosorbent assay and further analyzed by Western blot using a factor H-specific antibody. Complement activation was determined by measuring total hemolytic activity of the classical (CH50) and alternative (APH50) pathways, C3 and C3d. The sequence of factor H gene was determined. Serial factor H measurements after fresh frozen plasma infusion allowed calculation of a factor H half-life. Factor H was absent in plasma (<1 mug/mL), and the complement system was highly activated (CH50, APH50, C3 decreased; C3d increased). Genetic analysis identified a novel homozygous factor H mutation (T2770A; Y899Stop) in CCP domain 15, most likely causing defective protein secretion. Time course measurements of factor H after plasma infusion established a factor H half-life of about 6 days. By repetitive plasma infusions (20 mL/kg over about 2 to 3 hours) the authors were able to interrupt the vicious circle of thrombotic microangiopathy in a factor H-deficient patient with aHUS. Based on the measured factor H half-life of about 6 days, regular plasma infusions in 2-week intervals were given, which prevented further aHUS episodes and stopped the decline of kidney function.

Comparison of surface recognition and C3b binding properties of mouse and human complement factor H

Factor H (FH) is a central complement regulator both in plasma and on certain cellular and acellular surfaces that are in contact with plasma. Although FH deficiency has been shown to lead to similar diseases in man and mice (membranoproliferative glomerulonephritis or dense deposit disease) little is known about the similarity between the human and murine FH functions. We here characterize the interactions of murine FH (mFH) with C3b, glycosaminoglycans, and endothelial cells and compare these interactions with those of human FH (hFH). To achieve this we purified mFH and murine C3 from plasma, prepared murine C3b, and expressed recombinant mFH constructs containing domains 1-5 and 18-20 (mFH1-5 and mFH18-20). For comparisons, hFH, human C3b, and recombinant hFH1-5 and hFH18-20 were used. We demonstrate that mFH and mFH1-5 do act as cofactors for factor I-mediated cleavage of human C3b. Surface plasmon resonance analysis showed binding of mFH18-20 to murine C3b and weak binding to human C3b. The mFH18-20 construct bound to heparin in a manner comparable to hFH18-20. It was demonstrated by flow cytometry that mFH and mFH18-20 bind to human endothelial cells in a similar manner to hFH and hFH18-20. Taken together, locations of the key functions of mFH, i.e. complement regulation and surface recognition, are comparable to hFH. Recently, mutations in the carboxy-terminal end of hFH have been found to be associated with atypical hemolytic uremic syndrome (aHUS). Based on the results in this report it is conceptually attractive to establish a murine model for aHUS.

Shiga-toxin-producing Escherichia coli and haemolytic uraemic syndrome

Most cases of diarrhoea-associated haemolytic uraemic syndrome (HUS) are caused by Shiga-toxin-producing bacteria; the pathophysiology differs from that of thrombotic thrombocytopenic purpura. Among Shiga-toxin-producing Escherichia coli (STEC), O157:H7 has the strongest association worldwide with HUS. Many different vehicles, in addition to the commonly suspected ground (minced) beef, can transmit this pathogen to people. Antibiotics, antimotility agents, narcotics, and non-steroidal anti-inflammatory drugs should not be given to acutely infected patients, and we advise hospital admission and administration of intravenous fluids. Management of HUS remains supportive; there are no specific therapies to ameliorate the course. The vascular injury leading to HUS is likely to be well under way by the time infected patients seek medical attention for diarrhoea. The best way to prevent HUS is to prevent primary infection with Shiga-toxin-producing bacteria.

Factor H binds to washed human platelets

BACKGROUND

Factor H regulates the alternative pathway of complement. The protein has three heparin-binding sites, is synthesized primarily in the liver and copurifies from platelets with thrombospondin-1. Factor H mutations at the C-terminus are associated with atypical hemolytic uremic syndrome, a condition in which platelets are consumed. Objectives The aim of this study was to investigate if factor H interacts with platelets.

METHODS

Binding of factor H, recombinant C- or N-terminus constructs and a C-terminus mutant to washed (plasma and complement-free) platelets was analyzed by flow cytometry. Binding of factor H and constructs to thrombospondin-1 was measured by surface plasmon resonance.

RESULTS

Factor H bound to platelets in a dose-dependent manner. The major binding site was localized to the C-terminus. The interaction was partially blocked by heparin. Inhibition with anti-GPIIb/IIIa, or with fibrinogen, suggested that the platelet GPIIb/IIIa receptor is involved in factor H binding. Factor H binds to thrombospondin-1. Addition of thrombospondin-1 increased factor H binding to platelets. Factor H mutated at the C-terminus also bound to platelets, albeit to a significantly lesser degree.

CONCLUSIONS

This study reports a novel property of factor H, i.e. binding to platelets, either directly via the GPIIb/IIIa receptor or indirectly via thrombospondin-1, in the absence of complement. Binding to platelets was mostly mediated by the C-terminal region of factor H and factor H mutated at the C-terminus exhibited reduced binding.

Anti-Factor H autoantibodies associated with atypical hemolytic uremic syndrome

Several studies have demonstrated genetic predisposition in non-shigatoxin-associated hemolytic uremic syndrome (HUS), involving regulatory proteins of the complement alternative pathway: Factor H (FH) and membrane co-factor protein (CD46). Regarding the observations of thrombotic thrombocytopenic purpura patients, in whom a von Willebrand factor protease (ADAMST-13) deficiency may be inherited or acquired secondary to IgG antibodies, it was speculated that HUS might occur in a context of an autoimmune disease with the development of anti-FH antibodies leading to an acquired FH deficiency. The presence of FH autoantibodies was investigated by an ELISA method using coated purified human FH in a series of 48 children who presented with atypical HUS and were recruited from French university hospitals. Anti-FH IgG antibodies were detected in the plasma of three children who presented with recurrent HUS. The anti-FH specificity was conserved by the Fab'2 fraction. The plasma FH activity was found to be decreased, whereas plasma FH antigenic levels and FH gene analysis were normal, indicating that the presence of anti-FH antibodies led to an acquired functional FH deficiency. This report supports for the first time that HUS may occur in a context of an autoimmune disease with the development of anti-FH-specific antibody leading to an acquired FH deficiency. This new mechanism of functional FH deficiency may lead to the design of new approaches of diagnosis and treatment with a particular interest in plasma exchanges or immunosuppressive therapies.

Complement Factor H Limits Immune Complex Deposition and Prevents Inflammation and Scarring in Glomeruli of Mice with Chronic Serum Sickness

Factor H is the major complement regulator in plasma. Abnormalities in factor H have been implicated in membranoproliferative glomerulonephritis in both humans and experimental animals. It has been shown that factor H on rodent platelets functions analogously to human erythrocyte complement receptor 1 in its role to traffic immune complexes to the mononuclear phagocyte system. C57BL/6 factor H-deficient mice (Cfh(-/-)) and wild-type (wt) controls were immunized daily for 5 wk with heterologous apoferritin to study the chronic serum sickness GN model. Immunizations were started in 6- to 8-wk-old mice, which was before the development of spontaneous membranoproliferative glomerulonephritis in some Cfh(-/-) animals. Glomerular deposition of IgG immune complexes in glomeruli was qualitatively and quantitatively increased in Cfh(-/-) mice compared with wt mice. Consistent with the increase in glomerular immune complexes and possibly because of alternative pathway complement activation, Cfh(-/-) mice had increased glomerular C3 deposition. Wt mice developed no glomerular pathology. In contrast, Cfh(-/-) mice developed diffuse proliferative GN with focal crescents and glomerulosclerosis. In addition, there was significantly increased expression of collagen IV, fibronectin, and laminin mRNA in Cfh(-/-) glomeruli. These data show a role for platelet-associated factor H to process immune complexes and limit their accumulation in glomeruli. Once deposited in glomeruli, excessive complement activation can lead to glomerular inflammation and the rapid development of a scarring phenotype.

Clinical and laboratory evaluation of complement deficiency

The complement system provides innate defense against microbial pathogens and is a "complement" to humoral (antibody-mediated) immunity. Consisting of plasma and membrane proteins, this proinflammatory system works in part by a cascade involving limited proteolysis whereby one component activates the next, resulting in a dramatic amplification. The overall goal is deposition of complement fragments on pathologic targets for the purposes of opsonization, lysis, and liberation of peptides that promote the inflammatory response. Deficiencies of complement components predispose to infections and autoimmune syndromes. Even though total deficiency of a complement component is rare, patients presenting with certain bacterial infections and autoimmune syndromes, especially SLE, have a much greater incidence of deficiency. This review will summarize the clinical manifestations and pathophysiology of congenital and acquired complement deficiency diseases. We will also present an algorithm for laboratory diagnosis of complement deficiency and discuss current and future therapeutic options.

Effects of complement factor D deficiency on the renal disease of MRL/lpr mice

BACKGROUND

The alternative complement pathway (AP) is activated in individuals with lupus nephritis and in murine models of systemic lupus erythematosus, including MRL/lpr mice. A previous study from our laboratory evaluated the development of renal disease in MRL/lpr mice genetically deficient in factor B (Bf-/-), a protein necessary for AP activation. MRL/lpr Bf-/- mice developed less renal disease and had improved survival; however, these mice were also a different major histocompatibility complex (MHC) haplotype (H-2b) than their wild-type littermates (H-2k) due to the gene for Bf being located in the MHC gene complex. We undertook the current study to determine if the decreased renal disease in MRL/lpr Bf-/- mice was due to the lack of AP activation or the H-2b haplotype by studying the effects of factor D (Df) deficiency, a critical protein for AP activation, on disease development in MRL/lpr mice.

METHODS

Df-deficient mice were backcrossed with MRL/lpr mice for four to nine generations. MRL/lpr H-2k Df-/-, Df+/-, and Df+/+ littermates were evaluated for disease development. Lack of AP activation in MRL/lpr Df-/- mice was determined by the zymosan assay. Serum creatinine levels were measured using a creatinine kit. Proteinuria and autoantibody levels were determined by enzyme-linked immunosorbent assay (ELISA). Sections from one kidney were stained with fluorescein isothiocyanate (FITC) alpha-murine C3 or alpha-murine IgG to detect C3 and IgG deposition. The remaining kidney was cut in half with one half fixed, sectioned, and stained with hematoxylin and eosin and periodic acid-Schiff (PAS) to evaluate pathology and another half fixed in glutaraldehyde and examined via electron microscopy.

RESULTS

MRL/lpr Df-/- mice had similar glomerular IgG deposition, proteinuria and autoantibody levels, as Df+/+ and Df+/- littermates. However, glomerular C3 deposition, serum creatinine levels, and pathologic renal disease were significantly reduced in Df-/- mice. Despite the lack of renal disease in Df-/- mice, life span was not impacted by factor D deficiency.

CONCLUSION

The absence of Df and AP activation is protective against the development of proliferative renal disease in MRL/lpr mice suggesting the similar effect of Bf deficiency in MRL/lpr mice was also due to the lack of AP activation.

Rat glomerular epithelial cells produce and bear factor H on their surface that is up-regulated under complement attack

BACKGROUND

Factor H is a potent complement inhibitory molecule that is primarily produced by the liver and appears in plasma as a soluble protein. Yet there is evidence that other cells, including those in the kidney, can produce factor H, and that it can be cell-associated as well as present as a plasma protein. Here we studied factor H in rat glomerular epithelial cells (GEC).

METHODS

A polyclonal antibody to factor H was used to identify factor H protein. A polymerase chain reaction (PCR)-based strategy was utilized to clone the full-length cDNA of GEC factor H. The relative quantity of factor H mRNA was measured by quantitative reverse transcription (RT)-PCR in cultured GEC exposed to complement activation and in the passive Heymann nephritis (PHN) model of membranous nephropathy.

RESULTS

By immunofluorescence microscopy, factor H protein was present on the plasma membranes of cultured GEC. Based upon Western blot studies, this appeared to be the full-length 150 kD factor H protein. Factor H cDNA cloned from GEC was identical to the newly deposited sequence for rat liver factor H cDNA. In cultured GEC in which complement was activated, factor H mRNA increased over time. Similarly, in the PHN model in which complement was activated on GEC in vivo, factor H mRNA and protein also increased over time.

CONCLUSION

Cultured GEC and glomeruli express factor H mRNA and protein. As modeled both in vitro and in vivo in the rat, factor H is up-regulated in membranous nephropathy. This is likely to be a direct response of GEC to complement attack and may represent a protective response of this cell.

Research on complement: old issues revisited and a novel sphere of influence

Immunology in recent years has taken a somewhat surprising turn, expressed by a renewed interest in innate immunity. Especially intriguing is the regulatory role exerted by the innate components on the adaptive response, with Toll receptors and complement components being the most investigated. This function has been firmly established for complement protein CR2 (CD21) as part of the BCR co-receptor CD19/CD21/CD81. New findings are now providing a broader picture of complement and its tuning of the immune response; for example, complement proteins have been implicated in the control of T-cell-mediated responses. We will review some of these data here and summarize new discoveries in areas of research on more traditional topics within the complement literature, such as complement and renal diseases, and the therapeutic use of C1-Inhibitor. We cover papers selected from studies presented at the XIX International Complement Workshop, held in Palermo in September 2002, and published within the past six months.

Clostridium difficile colitis associated with hemolytic-uremic syndrome

The authors report the case of a 46-year-old woman who presented with vomiting and profuse bloody diarrhea. Laboratory studies were significant for a hematocrit of 27% and lactate dehydrogenase of 5,394 U/L (5,394 U/L). Her renal function deteriorated rapidly with a peak creatinine of 12.4 mg/dL (1,096.4 micromol/L), and platelet count dropped simultaneously to a nadir of 123,000/microL (123 x 10(9)/L]. Schistocytes were observed in peripheral blood smear. Stool was positive for Clostridium difficile toxin A by enzyme immunoassay (EIA). Stool assay for Shiga-like toxin was negative by EIA, and stool cultures returned negative for Escherichia coli O157:H7 and other enteric pathogens. A diagnosis of C difficile colitis associated with hemolytic-uremic syndrome was made; the patient received plasmapheresis and recovered with no relapse after 10 months of follow-up. This is the second reported case of C difficile colitis associated with hemolytic-uremic syndrome in an adult.

The molecular basis for hereditary porcine membranoproliferative glomerulonephritis type II: point mutations in the factor H coding sequence block protein secretion

Porcine membranoproliferative glomerulonephritis type II in piglets of the Norwegian Yorkshire breed is considered the first animal model of human dense deposit disease. Porcine dense deposit disease is caused by the absence of the complement regulator factor H in plasma. Here we report the molecular basis for this absence. Single nucleotide exchanges at position C1590G and T3610G in the coding region of the factor H gene result in amino acid exchanges at nonframework residues L493V and I1166R that are located within SCR 9 and SCR 20, respectively. Apparently the L493V mutation represents a polymorphism whereas the I1166R causes the physiological consequences a block in protein secretion. Expression analysis shows comparable mRNA levels for factor H in liver tissue derived from both affected and healthy animals. In affected piglets, factor H protein is detected in increased amounts in liver cells. Factor H accumulates inside the hepatocytes and is not released as shown by Western blot analysis and immunohistochemistry. These data demonstrate that single amino acid exchanges of two nonframework amino acids either alone or in combination block protein secretion of factor H. This observation is also of interest for other human diseases in which factor H is involved, such as human factor H-associated form of hemolytic uremic syndrome.