The genetics and pathogenesis of haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura

Overlapping Atypical Hemolytic Uremic Syndrome and C3 Glomerulopathy with Mutation in CFI in a Japanese Patient: A Case Report

A 34-year-old Japanese man presented with blurred vision, headache, nausea, anemia, thrombocytopenia, and severe renal dysfunction. Thrombotic microangiopathy was initially suspected to have been caused by malignant hypertension. Antihypertensive medications did not improve his thrombocytopenia or renal dysfunction, and other diseases causing thrombotic microangiopathy were ruled out. Therefore, the patient was diagnosed with atypical hemolytic uremic syndrome. A renal biopsy revealed an overlap of thrombotic microangiopathy and C3 glomerulopathy. Genetic testing revealed c.848A>G (p.Asp283Gly), a missense heterozygous variant in the gene encoding complement factor I. Overlapping atypical hemolytic uremic syndrome and C3 glomerulopathy with complement factor I mutation is very rare, especially in Japan.

Kidney Transplantation in Patients with Atypical Hemolytic Uremic Syndrome due to Complement Factor H Deficiency: Impact of Liver Transplantation

BACKGROUND

Atypical hemolytic uremic syndrome (aHUS) is a rare disease that is often associated with genetic defects. Mutations of complement factor H (CFH) are the most common genetic defects that cause aHUS and often result in end-stage renal disease. Since CFH is mainly produced in the liver, liver transplantation (LT) has been performed in patients with defective CFH.

METHODS

The clinical courses of four kidney allograft recipients who lost their native kidney functions due to aHUS associated with a CFH mutation were reviewed.

RESULTS

Subject A underwent kidney transplantation (KT) twice, aHUS recurred and the allograft kidney failed within a few years. Subject B received a KT and soon experienced a recurrence of aHUS coinciding with infection. Her allograft kidney function has worsened, and she remains on plasma infusion therapy. Subject C underwent LT followed by KT. She is doing well without plasma infusion therapy after combined LT-KT for 3 years. Subject D received KT following LT and is now recurrence-free from aHUS.

CONCLUSION

In patients with aHUS associated with a CFH mutation, KT without LT was complicated with a recurrence of aHUS, which might lead to allograft loss. Conversely, LT was successful in preventing the recurrence of aHUS and thus might be another option for a recurrence-free life for aHUS patients associated with CFH mutation.

Circulating DNA and myeloperoxidase indicate disease activity in patients with thrombotic microangiopathies

Thrombotic microangiopathies (TMAs) are a group of life-threatening disorders characterized by thrombocytopenia, fragmentation of erythrocytes, and ischemic organ damage. Genetic disorders, autoimmune disease, and cancer are risk factors for TMAs, but an additional, unknown trigger is needed to bring about acute disease. Recent studies suggest that DNA and histones are released during inflammation or infection and stimulate coagulation, thrombosis, thrombocytopenia, and organ damage in mice. We show that extracellular DNA and histones as well as markers of neutrophils are present in acute TMAs. Analysis of plasma from TMA patients of different clinical categories revealed elevated levels of DNA-histone complexes and myeloperoxidase (MPO) from neutrophil granules as well as S100A8/A9, a heterocomplex abundant in neutrophil cytosol. During therapy of thrombotic thrombocytopenic purpura, a subtype of TMAs often associated with severe ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs, member 13) deficiency, plasma DNA and MPO were inversely correlated with platelet counts, and their levels indicated amelioration or exacerbation of the disease. ADAMTS13 deficiency together with increased levels of plasma DNA and MPO were characteristic for acute thrombotic thrombocytopenic purpura. A minor infection often precedes acute TMA and extracellular DNA and histones released during the inflammatory response could provide the second hit, which precipitates acute TMA in patients with pre-existing risk factors.

The development of atypical hemolytic uremic syndrome depends on complement C5

Gene variants in the alternative pathway of the complement system strongly associate with atypical hemolytic uremic syndrome (aHUS), presumably by predisposing to increased complement activation within the kidney. Complement factor H (CFH) is the major regulator of complement activation through the alternative pathway. Factor H-deficient mice transgenically expressing a mutant CFH protein (Cfh(-/-).FHΔ16-20) that functionally mimics the CFH mutations reported in aHUS patients spontaneously develop thrombotic microangiopathy. To investigate the role of complement C5 activation in this aHUS model, we generated C5-deficient Cfh(-/-).FHΔ16-20 mice. Both C5-sufficient and C5-deficient Cfh(-/-).FHΔ16-20 mice had abnormal C3 deposition within the kidney, but spontaneous aHUS did not develop in any of the C5-deficient mice. Furthermore, although Cfh(-/-).FHΔ16-20 animals demonstrated marked hypersensitivity to experimentally triggered renal injury, animals with concomitant C5 deficiency did not. These data demonstrate a critical role for C5 activation in both spontaneous aHUS and experimentally triggered renal injury in animals with defective complement factor H function. This study provides a rationale to investigate therapeutic inhibition of C5 in human aHUS.

Variant-specific quantification of factor H in plasma identifies null alleles associated with atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS) is associated with complement alternative pathway defects in over half the cases. Point mutations that affect complement surface regulation are common in factor H (CFH); however, sometimes individuals have null mutations in heterozygosis. The latter are difficult to identify, although a consistently low plasma factor H (fH) concentration is suggestive; definitive proof requires demonstration that the mutant sequence is not expressed in vitro. Here, novel reagents and assays that distinguish and individually quantify the common factor H-Y402H polymorphic variants were used to identify alleles of the CFH gene, resulting in low or null expression of full-length fH and also normal or increased expression of the alternative splice product factor H-like-1 (FHL-1). Our assay identified three Y402H heterozygotes with low or absent fH-H402 but normal or increased FHL-1-H402 levels in a cohort of affected patients. Novel mutations explained the null phenotype in two cases, which was confirmed by family studies in one. In the third case, family studies showed that a known mutation was present on the Y allele. The cause of reduced expression of the H allele was not found, although the data suggested altered splicing. In each family, inheritance of low expression or null alleles for fH strongly associated with aHUS. Thus, our assays provide a rapid means to identify fH expression defects without resorting to gene sequencing or expression analysis.

Vaccination with attenuated Salmonella enterica Dublin expressing E coli O157:H7 outer membrane protein Intimin induces transient reduction of fecal shedding of E coli O157:H7 in cattle

Background

Escherichia coli serogroup O157:H7 has emerged as an important zoonotic bacterial pathogen, causing a range of symptoms from self-limiting bloody diarrhea to severe hemorrhagic colitis and hemolytic-uremic syndrome in humans. Beef and dairy cattle are considered the most important animal reservoirs for this pathogen. One of the important virulence characteristics of E. coli O157:H7 is the eaeA gene encoding the 97 kDa surface protein intimin. Intimin is required for attachment and effacement during the interaction of enterohemorrhagic E. coli with human and bovine neonatal enterocytes. The present study was undertaken to test the hypothesis that an adaptive mucosal immune response directed against intimin will reduce or prevent enteric colonization and fecal shedding of E. coli O157:H7 in cattle.

Results

Cattle were orally inoculated with either milk (control), milk with live attenuated Salmonella enterica serovar Dublin (vector), or milk with live attenuated recombinant S. Dublin expressing intimin (vaccinated) on days 0, 14 and 28. On day 98, all calves were challenged orally with E. coli O157:H7 to evaluate whether vaccination with the recombinant S. Dublin expressing intimin would reduce the level of E. coli O157:H7 fecal shedding.

During the first 28 days, vaccinated calves shed both the vector strain and the intimin-expressing S. Dublin strain at a similar level. The vector strain was shed for a significantly longer period as compared to the level of recombinant vaccine strain. Calves that received the intimin-expressed vaccine ceased shedding S. Dublin from day 28 to day 63. All calves were challenged with E. coli O157:H7 on day 98 to determine the effect on fecal shedding of E. coli O157:H7. The amount of E. coli O157:H7 in feces was measured for 30 days post-challenge. We observed a transient clearance of E. coli O157:H7 from the feces in the vaccinated calves. The magnitude of fecal E. coli O157:H7 shedding did not correlate with the presence of intimin-specific fecal IgA.

Conclusion

Oral vaccination with live attenuated recombinant S. Dublin expressing intimin reduced enteric colonization and fecal shedding of E. coli O157:H7. However, the transient clearance of E. coli O157:H7 was not associated with an enhanced IgA-mediated mucosal immune response.

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.

Unusual manifestation of diarrhea-associated haemolytic uraemic syndrome in an adult

Hemolytic-Uremic Syndrome (HUS) is an uncommon disease characterized by microangiopathic hemolytica anaemia, thrombocytopenia, and acute renal failure. There are two forms of HUS: diarrhoea (D+)- and non-diarrhoea (D-)-associated HUS. We report the case of a 21-year-old woman presented to our department with jaundice, anaemia, thrombocytopenia, and anuria, preceded by a diarrheal prodrome, secondary to infection with Escherichia coli O157:H7. The whole clinical and laboratory investigation led to the diagnosis of HUS. Her condition was complicated with cholestasis, liver dysfunction, bleeding from the vagina, and myocardial involvement. She was treated only with fresh frozen plasma transfusions and hemodialysis, and despite the long duration of anuria (22 days), the patient showed subsequent improvement over days until full recovery one month later.

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.

Case Studies

Ms A was a 28 year old woman in her second pregnancy (she had one previous termination of pregnancy) who booked-in at 19 weeks’ gestation. She was known to have sickle cell disease (specifically sickle cell anaemia [HbSS]), and her last crisis had occurred 3 years before this pregnancy.

Modeling how CD46 deficiency predisposes to atypical hemolytic uremic syndrome

Mutations in complement regulatory proteins predispose to the development of aHUS. Approximately 50% of patients bear a mutation in one of three complement control proteins, factor H, factor I, or membrane cofactor protein (MCP; CD46). Another membrane regulator that is closely related to MCP, decay accelerating factor (DAF; CD55) thus far has shown no association with aHUS and continues to be investigated. The goal of this study was to compare the regulatory profile of MCP and DAF and to assess how alterations in MCP predispose to complement dysregulation. We employed a model system of complement activation on Chinese hamster ovary (CHO) cell transfectants. The four regularly expressed isoforms of MCP and DAF inhibited C3b deposition by the alternative pathway. DAF, but not MCP, inhibited the classical pathway. Most patients with MCP-aHUS are heterozygous and express only 25-50% of the wild-type protein. We, therefore, analyzed the effect of reduced levels of wild-type MCP and found that cells with lowered expression levels were less efficient in inhibiting alternative pathway activation. Further, a dysfunctional MCP mutant, expressed at normal levels and identified in five patients with aHUS (S206P), failed to protect against C3b amplification on CHO cells, even if expression levels were increased 10-fold. Our results add new information relative to the necessity for appropriate expression levels of MCP and further implicate the alternative pathway in disease processes such as aHUS.

Implications of the initial mutations in membrane cofactor protein (MCP; CD46) leading to atypical hemolytic uremic syndrome

The hemolytic uremic syndrome is characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia and acute renal failure. There are two general types. One occurs in epidemic form and is diarrheal associated (D+HUS). It has a good prognosis. The second is a rare form known as atypical (aHUS), which may be familial or sporadic, and has a poor prognosis. aHUS is increasingly recognized to be a disease of defective complement regulation, particularly cofactor activity. Mutations in membrane cofactor protein (MCP; CD46) that predispose to the development of aHUS were first identified in 2003. MCP is a membrane-bound complement regulator that acts as a cofactor for the factor I-mediated cleavage of C3b and C4b deposited on host cells. More than 20 different mutations in MCP have now been identified in patients with aHUS. Many of these mutants have been functionally characterized and have helped to define the pathogenic mechanisms leading to aHUS development. Over 75% of the reported mutations cause a reduction in MCP expression, due to homozygous, compound heterozygous or heterozygous mutations. This deficiency of MCP leads to inadequate control of complement activation on endothelial cells after an initiating injury. The remaining MCP mutants are expressed, but demonstrate reduced ligand (C3b/C4b) binding capacity and cofactor activity of MCP. MCP mutations in aHUS demonstrate incomplete penetrance, indicating that additional genetic and environmental factors are required to manifest disease. MCP mutants as a cause of aHUS have a favorable clinical outcome in comparison to patients with factor H (CFH) or factor I (IF) mutations. In 90% of the renal transplants performed in patients with MCP-HUS, there has been no recurrence of the primary disease, whilst >50% of factor I or factor H deficient patients have had a prompt recurrence. This highlights the importance of defining and characterizing the underlying genetic defects in patients with aHUS.

Factor H genotype–phenotype correlations: lessons from aHUS, MPGN II, and AMD

Missense mutations in the C-terminal region of Factor H are associated with atypical hemolytic uremic syndrome, whereas homozygous Factor H deficiency is more frequently associated with membranoproliferative glomerulonephritis type II (MPGN II). The report of Licht et al. of a mutation in the complement-regulatory N-terminal region of Factor H in MPGN II provides additional insight into the pathogenesis of this condition.

Factor B of the alternative complement pathway regulates development of airway hyperresponsiveness and inflammation

Exposure to inhaled allergens leads to increases in airway hyperresponsiveness (AHR) and inflammation, associated with increased levels of biologically active fragments derived from the complement C3 and C5 family of proteins. Further, complement activation during allergen challenge in sensitized animals is necessary for the development of AHR and airway inflammation. To define the complement pathway involved, we studied mice deficient in complement factor 4 (C4-/-), a critical component of the classical pathway, or factor B (fB-/-), an essential protein in the alternative complement pathway. WT, C4-/-, and fB-/- mice were sensitized to ovalbumin and subsequently exposed to nebulized ovalbumin (1% in saline) on 3 consecutive days. After allergen sensitization and challenge, fB-/- mice demonstrated significantly lower airway responsiveness to methacholine and less airway inflammation. In contrast, C4-/- mice showed no reduction in AHR and airway inflammation compared with WT mice. Tissue inflammation, goblet cell hyperplasia, and IL-4, IL-5, and IL-13 levels in BAL fluid were significantly reduced in fB-/- mice compared with C4-/- and WT mice. The development of AHR and airway inflammation in sensitized fB-/- mice could be restored after intranasal administration of purified factor B before the airway challenge. In addition, administration of a neutralizing anti-factor B mAb to sensitized mice before airway challenge reduced the development of AHR and airway inflammation. These results demonstrate that in sensitized hosts complement activation through the alternative pathway after allergen exposure is critical to the development of AHR and airway inflammation.

Clinical aspects and molecular basis of primary deficiencies of complement component C3 and its regulatory proteins factor I and factor H

The complement system participates in both innate and acquired immune responses. Deficiencies in any of the protein components of this system are generally uncommon and require specialized services for diagnosis. Consequently, complement deficiencies are clinically underscored and may be more common than is normally estimated. As C3 is the major complement component and participates in all three pathways of activation, it is fundamental to understand all the clinical consequences observed in patients for which this protein is below normal concentration or absent in the serum. C3 deficiencies are generally associated with higher susceptibility to severe infections and in some cases with autoimmune diseases such as systemic lupus erythematosus. Here, we review the main clinical aspects and the molecular basis of primary C3 deficiency as well as the mutations in the regulatory proteins factor I and factor H that result in secondary C3 deficiencies. We also discuss the use of animal models to study these deficiencies.

Complement factor H-associated atypical hemolytic uremic syndrome in monozygotic twins: concordant presentation, discordant response to treatment

Hemolytic uremic syndrome not associated with diarrhea (diarrhea negative, atypical) is less common than the diarrhea-positive typical form, but frequently results in end-stage renal failure. Although there are anecdotal cases of successful treatment with fresh frozen plasma alone, the value of this treatment compared with plasma exchange (PE) is difficult to assess. We describe monozygotic female twins who presented at 5 years of age with factor H-related (c.3572 > T; Ser1191Leu) atypical hemolytic uremic syndrome within months of each other. In the first twin to present, 10 sessions of PE with fresh frozen plasma replacement (40 mL/kg) resulted in resolution of hemolysis and improvement in plasma creatinine level (1.9 to 1.5 mg/dL [166 to 137 micromol/L]). Subsequently, 17 infusions of fresh frozen plasma were administered during a 4-month period for recurrent thrombocytopenia. However, within 4 months, plasma creatinine level increased to 5.1 mg/dL (450 micromol/L), necessitating peritoneal dialysis. When the second twin presented with the same disease, an extended PE regimen was instituted. After 10 daily sessions, PE was continued once every 2 weeks. Two recurrences were treated successfully with daily PE for 7 days. After 44 months of follow-up, kidney function is normal (plasma creatinine, 0.6 mg/dL [53 micromol/L]; creatinine clearance, 119 mL/min/1.73 m2 [1.98 mL/s/1.73 m2]) on maintenance PE therapy. In conclusion, the response to treatment of these monozygotic twins suggests that long-term PE may have benefits over plasma infusion alone.

Does complement factor B have a role in the pathogenesis of atypical HUS?

Atypical haemolytic uraemic syndrome (aHUS) is a disorder of complement dysregulation. Because complement factor B (fB) carries the catalytic site of the alternative pathway convertase we examined it as both a potential candidate gene and modifier in the pathogenesis of aHUS. No factor B gene (BF) mutations were found in 20 patients with aHUS. There was no statistical difference between controls and aHUS patients in either BF allele or haplotype frequency. In conclusion, in this small series of aHUS patients we found no evidence that fB has a major role in the pathogenesis of aHUS.

Enterohaemorrhagic E. coli in veterinary medicine

The emergence of enterohaemorrhagic Escherichia coli (EHEC) as major human pathogens began with the identification of serotype O157:H7 in the early 1980s as the cause of various food-borne outbreaks of severe intestinal disease. The key virulence factors include verocytotoxins (Vt) and effectors and adhesins associated with type III secretion systems. Tracing the origins of human outbreaks reveals that the primary source of this organism is the ruminant gastro-intestinal tract and a variety of transmission routes to humans have been identified. The epidemiology of E. coli O157:H7 within cattle and other ruminants has been studied extensively and the prevalence of non-O157:H7 serotypes contrasts with the observed dominance of E. coli O157:H7 amongst human EHEC isolates. Although there is some evidence that EHEC cause disease in young animals, the high prevalence of Vt within healthy ruminants suggests that this is not a virulence factor within these species. An understanding of the mechanisms underpinning EHEC persistence within their natural reservoir hosts and the development of a molecular understanding of EHEC biology and evolution could eventually allow a reduction in the incidence of human disease and may reduce future threats. The use of animal models to replicate and study human EHEC pathogenesis is described.

Interplay between ADAMTS13 and von Willebrand factor in inherited and acquired thrombotic microangiopathies

The presence of unusually large multimers of von Willebrand factor (VWF) is thought to be a major pathogenic factor for thrombotic thrombocytopenic purpura (TTP). ADAMTS13 is a protease that regulates the multimeric size and function of VWF by cleaving VWF. Hence, congenital or acquired deficiency of ADAMTS13 causes life-threatening illness of TTP. Mutations in the ADAMTS13 gene cause inherited TTP, and the development of autoantibodies that inhibit ADAMTS13 activity frequently are associated with acquired TTP. ADAMTS13 consists of 1,427 amino acid residues and is composed of multiple structural and functional domains, containing a signal peptide, a propeptide, a reprolysin-like metalloprotease domain, a disintegrin-like domain, a thrombospondin type-1 (Tsp1) motif, a cysteine-rich domain, a spacer domain, seven additional Tsp1 repeats, and two CUB domains. In particular, the cysteine-rich/spacer domains are essential for VWF cleavage and are the principal epitopes recognized by autoantibodies in patients with acquired TTP. Therefore, it is likely that these domains are involved in the recognition and binding of ADAMTS13 to VWF. ADAMTS13 circulates in the blood in an active state, and efficiently cleaves unfold form of VWF induced under shear stress caused by blood flow, preventing the accumulation of pathogenic unusually large VWF multimers (ULVWF). Thus, ADAMTS13 helps maintain vascular homeostasis by preventing the excess thrombus formation.

Mutations in complement factor I predispose to development of atypical hemolytic uremic syndrome

Mutations in the plasma complement regulator factor H (CFH) and the transmembrane complement regulator membrane co-factor protein (MCP) have been shown to predispose to atypical hemolytic uremic syndrome (HUS). Both of these proteins act as co-factors for complement factor I (IF). IF is a highly specific serine protease that cleaves the alpha-chains of C3b and C4b and thus downregulates activation of both the classical and the alternative complement pathways. This study looked for IF mutations in a panel of 76 patients with HUS. Mutations were detected in two patients, both of whom had reduced serum IF levels. A heterozygous bp change, c.463 G>A, which results in a premature stop codon (W127X), was found in one, and in the other, a heterozygous single base pair deletion in exon 7 (del 922C) was detected. Both patients had a history of recurrent HUS after transplantation. This is in accordance with the high rate of recurrence in patients with CFH mutations. Patients who are reported to have mutations in MCP, by contrast, do not have recurrence after transplantation. As with CFH- and MCP-associated HUS, there was incomplete penetrance in the family of one of the affected individuals. This study provides further evidence that atypical HUS is a disease of complement dysregulation.

Innate autoimmunity

The adaptive immune system has evolved highly specific pattern recognition proteins and receptors that, when triggered, provide a first line of host defense against pathogens. Studies reveal that these innate recognition proteins are also self-reactive and can initiate inflammation against self-tissues in a similar manner as with pathogens. This specific event is referred to as "innate autoimmunity." In this review, we describe two classes of autoimmune responses, that is, reperfusion injury and fetal loss syndrome, in which the recognition and injury are mediated by innate immunity. Both disorders are common and are clinically important. Reperfusion injury (RI) represents an acute inflammatory response after a reversible ischemic event and subsequent restoration of blood flow. Findings that injury is IgM and complement dependent and that a single natural antibody prepared from a panel of B-1 cell hybridomas can restore injury in antibody-deficient mice suggest that RI is an autoimmune-type disorder. Fetal loss syndrome is also an antibody- and complement-dependent disorder. Although both immune and natural antibodies are likely involved in recognition of phospholipid self-antigens, inhibition of the complement pathway in rodent models can block fetal loss. As new innate recognition proteins and receptors are identified, it is likely that innate responses to self represent frequent events and possibly underlie many of the known chronic autoimmune disorders normally attributable to dysregulation of adaptive immunity.

Complement receptors and the shaping of the natural antibody repertoire

Complement and complement receptors have been known for several decades to play important roles in immune effector mechanisms related to pathogen elimination and tissue inflammation. In addition, studies over the last 10 years have clearly demonstrated a key role for the complement C3d activation fragment receptor designated CR2 (complement receptor type 2) in the switched-isotype, high-affinity and memory humoral immune responses to T-dependent foreign antigens. More recent studies have extended those observations to include a key role for CR2 and C3d in the humoral immune response to T-independent foreign antigens. Conversely, as these studies have proceeded, a parallel series of analyses have linked defects in expression or function of complement C4 and other classical pathway activation pathway proteins, as well as CR2 and the closely related CR1, to the loss of self tolerance to nuclear antigens such as double-stranded DNA and chromatin in systemic lupus erythematosus. With regard to the topic of this issue, it is now becoming increasingly clear that CR2 also plays a major role in the development of the natural antibody repertoire. Specifically, in the absence of this receptor natural IgM and IgG develop in the naïve animal that demonstrate clearly altered recognition patterns for specific natural antibody targets. This repertoire change is important physiologically in at least one setting because these CR2-dependent natural antibodies are necessary for the recognition of ischemic self tissues. In addition, it is possible that certain of the phenotypes manifest by CR2-deficient mice may be strongly influenced not only by effects on later stages of B cell activation and maturation, as commonly thought, but also by alterations in the pre-existing pool of natural antibodies that are influenced by this receptor. This review will examine the evidence that has accumulated over the last few years supporting these hypotheses.

Structure of vaccinia complement protein in complex with heparin and potential implications for complement regulation

Vaccinia virus complement control protein (VCP), a homolog of the regulators of the complement activation family of proteins, inhibits complement activation through mechanisms similar to human fluid-phase complement regulators factor H and C4b-binding protein. VCP has a heparin-binding activity that assists vaccinia in host interactions. Interaction with cell-surface polyanions like heparin is centrally important in the functioning of fluid-phase complement regulators and is the basis of host-target discrimination by the alternative pathway. We report the structure of VCP in complex with a heparin decasaccharide, which reveals changes in VCP that might be pertinent to complement regulation. Properties that VCP shares with fluid-phase complement regulators suggest that such conformational changes may be of relevance in the functioning of other complement regulators. Additionally, comparison of VCP-heparin interactions with potentially similar interactions in factor H might enable understanding of the structural basis of familial hemolytic uremic syndrome, attributed to mutational disruption of heparin and C3b binding by factor H.

Evaluation of the living kidney donor: current perspectives

Living kidney donation is increasing because of prolonged waiting times on the transplant list, as well as improved outcomes for recipients. In 2001, the number of living donors surpassed the number of deceased donors; this trend likely will continue with ever-increasing margins. Because of this increase, as well as changes in our society's health, it is time to re-review the guidelines for selecting living kidney donors established by Kasiske et al in 1995. A conference will be held this year to review updated literature on medical conditions that impact on renal health. From this, new guidelines for the medical evaluation of living renal donors will be constructed. This review discusses information known to date on the outcomes of individuals undergoing unilateral nephrectomy, the impact of lifestyle on renal function in the setting of nephrectomy, and advancements in the detection of genetically transmitted renal diseases that impact on today's decisions on living donation.

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.

Mutations in human complement regulator, membrane cofactor protein (CD46), predispose to development of familial hemolytic uremic syndrome

Membrane cofactor protein (MCP; CD46) is a widely expressed transmembrane complement regulator. Like factor H it inhibits complement activation by regulating C3b deposition on targets. Factor H mutations occur in 10-20% of patients with hemolytic uremic syndrome (HUS). We hypothesized that MCP mutations could predispose to HUS, and we sequenced MCP coding exons in affected individuals from 30 families. MCP mutations were detected in affected individuals of three families: a deletion of two amino acids (D237/S238) in family 1 (heterozygous) and a substitution, S206P, in families 2 (heterozygous) and 3 (homozygous). We evaluated protein expression and function in peripheral blood mononuclear cells from these individuals. An individual with the D237/S238 deletion had reduced MCP levels and approximately 50% C3b binding compared with normal controls. Individuals with the S206P change expressed normal quantities of protein, but demonstrated approximately 50% reduction in C3b binding in heterozygotes and complete lack of C3b binding in homozygotes. MCP expression and function was evaluated in transfectants reproducing these mutations. The deletion mutant was retained intracellularly. S206P protein was expressed on the cell surface but had a reduced ability to prevent complement activation, consistent with its reduced C3b binding and cofactor activity. This study presents further evidence that complement dysregulation predisposes to development of thrombotic microangiopathy and that screening patients for such defects could provide informed treatment strategies.