Functional Hemolytic Test for Complement Alternative Pathway Convertase Activity.

The complement system is a key part of innate immunity. However, if the system becomes dysregulated, damage to healthy host cells can occur, especially to the glomerular cells of the kidney. The convertases of the alternative pathway of the complement system play a crucial role in complement activation. In healthy conditions, their activity is strictly regulated. In patients with diseases caused by complement alternative pathway dysregulation, such as C3 glomerulopathy and atypical hemolytic uremic syndrome, factors can be present in the blood that disturb this delicate balance, leading to convertase overactivity. Such factors include C3 nephritic factors, which are autoantibodies against the C3 convertase that prolong its activity, or genetic variants resulting in a stabilized convertase complex. This chapter describes a method in which the activity and stability of the alternative pathway convertases can be measured to detect aberrant serum factors causing convertase overactivity.

Morphofunctional Effects of C5 Convertase Blockade in Immune Complex-Mediated Membranoproliferative Glomerulonephritis: Report of Two Cases with Evidence of Terminal Complement Activation.

A membranoproliferative pattern of glomerular injury is frequently observed in patients with complement-mediated disorders, such as C3 glomerulopathies (C3G) and primary immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN). The outcomes of C3G and -IC-MPGN are poor, independently of immunosuppressive therapy. However, two 48-week treatment periods with the anti-C5 monoclonal antibody eculizumab, divided by a -12-week washout period, achieved remission of proteinuria and stabilization/improvement of the glomerular filtration rate (GFR), measured through iohexol plasma clearance, in 3 of 10 patients with biopsy-proven MPGN, nephrotic syndrome and terminal complement complex sC5b-9 plasma levels >1,000 mg/mL, at inclusion. Baseline and end-of-study kidney biopsies were available for 2 patients with IC-MPGN, and their baseline characteristics were similar. However, in 1 patient proteinuria and GFR did not improve during the study, whereas in the other proteinuria decreased from 4.84 to 2.12 g/24-h and GFR increased from 91.5 to 142.7 mL/min/1.73 m2. Glomerular inflammation improved and median (interquartile range) glomerular staining for C5b-9 decreased in both cases: from 23.6 to 18.2% (p = 0.021) in the patient who achieved remission and from 15.8 to 10.7% (p = 0.019) in the patient with persistent proteinuria. Chronic glomerular lesions progressed and C3 glomerular staining and electron-dense deposits did not change appreciably in either case. However, in the patient who achieved remission, ultrastructural evaluation revealed features of glomerular microangiopathy at inclusion, which fully recovered posttreatment. Podocyte foot process effacement was observed in both patients at inclusion, but recovered only in the patient with microangiopathy. Thus, in 2 patients with -IC-MPGN, chronic glomerular changes progressed despite eculizumab-induced amelioration of glomerular inflammation and inhibition of sC5b-9 deposition, and independently of treatment effects on proteinuria and podocytes. The finding that the regression of microangiopathic changes was associated with improved clinical outcomes suggests that C5 blockade might have a therapeutic role in patients with IC-MPGN displaying microangiopathic endothelial injury.

Analysis of the Complement System in the Clinical Immunology Laboratory.

The complement system is a critical component of both the innate and adaptive immune systems that augments the function of antibodies and phagocytes. Antigen-antibody immune complexes, lectin binding, and accelerated C3 tick-over can activate this well-coordinated and carefully regulated process. The importance of this system is highlighted by the disorders that arise when complement components or regulators are deficient or dysregulated. This article describes the pathways involved in complement activation and function, the regulation of these various pathways, and the interpretation of laboratory testing performed for the diagnosis of diseases of complement deficiency, exuberant complement activation, and complement dysregulation.

Complement Regulation and Immune Evasion by Hepatitis C Virus.

A prominent role for complement has been identified in the linkage of innate and adaptive immunity. The liver is the main source of complement and hepatocytes are the primary sites for synthesis of complement components in vivo. We have discovered that hepatitis C virus (HCV) impairs C4 and C3 synthesis. Liver damage may diminish capacity of complement synthesis in patients. However, we observed that the changes in measured complement components in chronically HCV infected patients do not correlate with liver fibrosis or rheumatoid factor present in the blood, serum albumin, or alkaline phosphatase levels. Complement component C3 is of critical importance in B cell activation and T cell-dependent antibody responses. C3 activity is required for optimal expansion of CD8+T cells during a systemic viral infection. Deficiencies in complement may predispose patients to infections via ineffective opsonization, and defects in lytic activity via membrane attack complex. Interestingly, C9 is significantly reduced at the mRNA level in chronically HCV infected liver biopsy specimens, while many hepatocyte derived complement components (C6, C8, Factor B, MASP1, and MBL) and unrelated genes remain mostly unaffected. This implies an HCV specific effect, not a global effect from liver disease.

Insights into the Effects of Complement Factor H on the Assembly and Decay of the Alternative Pathway C3 Proconvertase and C3 Convertase.

The activated fragment of C3 (C3b) and factor B form the C3 proconvertase (C3bB), which is cleaved by factor D to C3 convertase (C3bBb). Older studies (Conrad, D. H., Carlo, J. R., and Ruddy, S. (1978)J. Exp. Med.147, 1792-1805; Pangburn, M. K., and Müller-Eberhard, H. J. (1978)Proc. Natl. Acad. Sci. U.S.A.75, 2416-2420; Kazatchkine, M. D., Fearon, D. T., and Austen, K. F. (1979)J. Immunol.122, 75-81) indicated that the complement alternative pathway regulator factor H (FH) competes with factor B for C3b binding; however, the capability of FH to prevent C3bB assembly has not been formally investigated. Moreover, in the few published studies FH did not favor C3bB dissociation. Whether FH may affect C3bBb formation from C3bB is unknown. We set up user-friendly assays based on combined microplate/Western blotting techniques that specifically detect either C3bB or C3bBb, with the aim of investigating the effect of FH on C3bB assembly and decay and C3bBb formation and decay. We document that FH does not affect C3bB assembly, indicating that FH does not efficiently compete with factor B for C3b binding. We also found that FH does not dissociate C3bB. FH showed a strong C3bBb decay-accelerating activity, as reported previously, and also exerted an apparent inhibitory effect on C3bBb formation. The latter effect was not fully attributable to a rapid FH-mediated dissociation of C3bBb complexes, because blocking decay with properdin and C3 nephritic factor did not restore C3bBb formation. FH almost completely prevented release of the smaller cleavage subunit of FB (Ba), without modifying the amount of C3bB complexes, suggesting that FH inhibits the conversion of C3bB to C3bBb. Thus, the inhibitory effect of FH on C3bBb formation is likely the sum of inhibition of C3bB conversion to C3bBb and of C3bBb decay acceleration. Further studies are required to confirm these findings in physiological cell-based settings.

A novel method for direct measurement of complement convertases activity in human serum.

Complement convertases are enzymatic complexes that play a central role in sustaining and amplification of the complement cascade. Impairment of complement function leads directly or indirectly to pathological conditions, including higher infection rate, kidney diseases, autoimmune- or neurodegenerative diseases and ischaemia-reperfusion injury. An assay for direct measurement of activity of the convertases in patient sera is not available. Existing assays testing convertase function are based on purified complement components and, thus, convertase formation occurs under non-physiological conditions. We designed a new assay, in which C5 blocking compounds enabled separation of the complement cascade into two phases: the first ending at the stage of C5 convertases and the second ending with membrane attack complex formation. The use of rabbit erythrocytes or antibody-sensitized sheep erythrocytes as the platforms for convertase formation enabled easy readout based on measurement of haemolysis. Thus, properties of patient sera could be studied directly regarding convertase activity and membrane attack complex formation. Another advantage of this assay was the possibility to screen for host factors such as C3 nephritic factor and other anti-complement autoantibodies, or gain-of-function mutations, which prolong the half-life of complement convertases. Herein, we present proof of concept, detailed description and validation of this novel assay.

The complement system is activated in a biphasic pattern after coronary artery bypass grafting.

BACKGROUND: The complement system is a key component in the inflammatory response after coronary artery bypass grafting (CABG). The routes of complement activation and deactivation after cardiac surgery are not clear. The aim of this study was to analyze routes of complement activation after uncomplicated CABG. METHODS: Complement components and activation products were measured in 20 nondiabetic adult patients undergoing elective CABG at several times postoperatively starting at admission to the intensive care unit. RESULTS: Complement activation after uncomplicated CABG showed a biphasic pattern. In the first 8 hours after admission to the intensive care unit, complement activation was initiated by the classical lectin pathway and augmented by the alternative pathway. Ultimately, this resulted in terminal pathway activation and formation of terminal complement complex. In the second phase, starting at 8 hours after the operation, complement was still activated by the classical lectin pathway, but there was no augmentation by the alternative pathway and no terminal complement complex formation. This implies that during this second stage, inhibitory mechanisms beyond C3b are engaged. CONCLUSIONS: Complement activation after cardiac surgery is regulated in a complex biphasic way, with additional inhibitory mechanisms engaged from 8 hours postoperatively onward.

C1 inhibitor, C3 activator, IgG, IgA, and IgM titers in Nigerian sickle cell disease patients with plasmodium falciparum.

BACKGROUND: Sickle cell disease (HbSS) is a major health problem in Nigeria and malaria has been implicated as a leading cause of morbidity/mortality in sickle cell disease patients. Few reasons were put forward to explain the observed morbidity/mortality of HbSS subjects due to Plasmodium falciparum (P. falciparum) malaria. OBJECTIVES: To determine the level of immunoglobulin classes (IgM, IgA, and IgG) and regulators of complement system (C1 inhibitor and C3 activator) in Nigerian HbSS patients with and without P. falciparum parasitemia. METHODS: A total of 64 subjects were considered, including 10 HbSS genotypic subjects with P. falciparum parasitemia (HbSS+PfM), 18 HbAA genotypic subjects with P. falciparum parasitemia (HbAA+PfM), 20 HbSS without P. falciparum parasitemia (HbSS-PfM), and 16 HbAA genotypic subjects without P. falciparum parasitemia (HbAA-PfM). IgM, IgA, IgG, C1 inhibitor, and C3 activator titers were quantified by single radial immunodiffusion method. RESULTS: The mean levels of IgG in HbSS+PfM (2373.90+/-1772.81mg/dl) and HbAA+PfM (1868.80+/-0.00mg/dl) were significantly higher compared with HbSS-PfM (644.55+/-171.15mg/dl) or HbAA-PfM (659.75+/-158.01mg/dl) patients. HbAA-PfM subjects had the lowest level of IgM (67.27+/-63.7mg/dl), though no significant difference was observed comparing mean levels of IgM between the four groups. IgA titer was significantly higher in HbSS-PfM patients (249.00+/-94.8mg/dl) compared with HbAA-PfM (p<0.05), HbAA+PfM (p<0.05), or HbSS+PfM (p<0.05). The mean values of C1 inhibitor were lower in HbSS+PfM and HbAA+PfM compared with HbSS-PfM or HbAA-PfM. However, HbAA+PfM had a significantly lower value of C1 inhibitor compared with HbAA-PfM (p<0.01). C3 activator was highest in HbSS-PfM (17.10+/-7.35mg/dl) and was significantly higher compared with HbSS+PfM (p<0.05). CONCLUSION: Increased C1 inhibitor and decreased C3 activator in HbSS+PfM compared with HbAA+PfM shows that deranged regulation of complement factors may be responsible for increased susceptibility of HbSS to P. falciparum malaria.

Mucous membrane pemphigoid: a retrospective study.

BACKGROUND: Mucous membrane pemphigoid (MMP) is a vesiculobullous, autoimmune disease that occurs primarily in older women. The objective of this study was to perform a retrospective analysis of the intraoral clinical signs, symptoms and diagnostic findings of MMP. STUDY DESIGN: The charts of 729 patients in a university-based dental referral practice were reviewed. RESULTS: Of 729 patients, a clinical diagnosis of MMP was rendered in 29 cases. Of these cases, 93 percent had only oral lesions at the time of presentation, whereas 7 percent had lesions at other sites in addition to the oral cavity. Sixty-eight percent were female and 83 percent of the patients were over 50 years at onset. Common sites of involvement were gingiva and buccal mucosa. Sixty-three percent exhibited erosive or ulcerative lesions. Thirty-five percent showed clinical evidence of epithelial separation. Eighty-eight percent of biopsied patients had histopathologic findings consistent with MMP. Seventy-seven percent of patients exhibited IgG and C3 in the basement membrane region, consistent with pemphigoid. Eighty-two percent of the 29 patients who had two or more lesions were treated with topical corticosteroids. CONCLUSIONS: The intraoral sites most commonly affected by MMP are the gingiva and buccal mucosa. Routine histopathologic evaluation is an effective diagnostic tool when used in conjunction with direct immunofluorescence.

Fluorochrome-linked immunoassay for functional analysis of the mannose binding lectin complement pathway to the level of C3 cleavage.

The humoral response to invading pathogens is mediated by a repertoire of innate immune molecules and receptors able to recognize pathogen-associated molecular patterns. Mannose binding lectin (MBL) and ficolins are initiation molecules of the lectin complement pathway (LCP) that bridge innate and adaptive immunity. Activation of the MBL-dependent lectin pathway, to the level of C3 cleavage, requires functional MASP-2, C2, C4 and C3, all of which have been identified with genetic polymorphisms that can affect protein concentration and function. Current assays for MBL and MASP-2 lack the ability to assess activation of all components to the level of C3 cleavage in a single assay platform. We developed a novel, low volume, fluorochrome linked immunoassay (FLISA) that quantitatively assesses the functional status of MBL, MASP-2 and C3 convertase in a single well. The assay can be used with plasma or serum. Multiple freeze/thaw cycles of serum do not significantly alter the assay, making it ideal for high throughput of large sample databases with minimal volume use. The FLISA can be used potentially to identify specific human disease correlations between these components and clinical outcomes in already established databases.

Deficient alternative complement pathway activation due to factor D deficiency by 2 novel mutations in the complement factor D gene in a family with meningococcal infections.

The complement system is an essential element in our innate defense against infections with Neisseria meningitidis. We describe 2 cases of meningococcal septic shock, 1 of them fatal, in 2 children of a Turkish family. In the surviving patient, alternative pathway activation was absent and factor D plasma concentrations were undetectable. Concentrations of mannose-binding lectin (MBL), C1q, C4 and C3, factor B, properdin, factor H, and factor I were normal. Mutation analysis of the factor D gene revealed a T638 > G (Val213 > Gly) and a T640 > C (Cys214 > Arg) mutation in the genomic DNA from the patient, both in homozygous form. The consanguineous parents and an unaffected sister had these mutations in heterozygous form. In vitro incubation of factor-D-deficient plasma of the boy with serogroup B N meningitidis showed normal MBL-mediated complement activation but no formation of the alternative pathway C3-convertase C3bBbP, and severely decreased C3bc formation and terminal complement activation. The defect was restored after supplementation with factor D. In conclusion, this is the second report of a factor D gene mutation leading to factor D deficiency in a family with meningococcal disease. This deficiency abolishes alternative-pathway dependent complement activation by N meningitidis, and leads to an increased susceptibility to invasive meningococcal disease.

The complement regulators C1 inhibitor and soluble complement receptor 1 attenuate acute lung injury in rabbits.

Because activation of the complement system plays a major role in the pathogenesis of acute lung injury, the availability of new specific complement inhibitors represents a promising therapeutic approach. In the present study we investigated pulmonary edema formation and pulmonary artery pressure (PAP) in acute complement-induced lung injury for possible therapeutic impact of the complement regulators C1 inhibitor and soluble complement receptor 1. Eighteen isolated and ventilated rabbit lungs were perfused with pooled normal human serum (NHS, final concentration 35%) in Krebs-Henseleit buffer in a recirculating system. Lung weight gain and PAP were continuously recorded. Complement activation was blocked by the addition of C1 inhibitor (1.0 U/mL, n = 6) or sCR 1 (2.0 microg/mL, n = 6). Lungs that received NHS without inhibitors served as controls (n = 6). This study was performed according to the Helsinki Declaration and approved by the local government. Application of NHS resulted in an increase of PAP within 20 min from 8+/-2 to 42+/-6 mmHg, which was significantly (P < 0.05) decreased by C1-Inh (25+/-5 mmHg) and sCRI (20 +/-3 mmHg). Moreover, pulmonary edema formation after NHS, as assessed by overall weight gain, was reduced by both C1-Inh and sCR1, compared with controls. These findings were paralleled with significantly decreased thromboxane release rates and reduced tissue deposition of C3c and C5b-9. C1 inhibitor and sCR1 attenuate the complement-induced pulmonary capillary leakage and PAP increase, indicating the protective effect of complement inhibition in isolated perfused rabbit lungs.

Membranoproliferative glomerulonephritis type III: association of glomerular deposits with circulating nephritic factor-stabilized convertase.

Deposits in the glomerular ultrastructure of 44 renal biopsy specimens from 21 patients with membranoproliferative glomerulonephritis (MPGN) type III have been compared with those in the ultrastructure of 34 biopsy specimens from 19 patients with MPGN type I. Previous studies have concluded that subepithelial deposits on the paramesangial portion of the glomerular basement membrane in MPGN types II and III are closely associated with circulating nephritic factor-stabilized convertase. In the present study, subendothelial deposits in MPGN type III were also found to be closely associated with nephritic factor; they were present in 14 of 26 (54%) biopsy specimens obtained during hypocomplementemia but in none of the 18 biopsy specimens obtained during normocomplementemia (P < 0.001). Subepithelial loop deposits in type III were also more frequent in biopsy specimens obtained during hypocomplementemia and are probably in some way also associated with circulating stabilized convertase. Taken together, the results of this and previous studies are compatible with the hypothesis that an excess of the C3b-dependent convertase in the circulation is basic to the pathogenesis of MPGN types II and III as well as of several other nephritides associated with factor H dysfunction. The half-life, structural complexity, and size of the convertases circulating in these nephritides increase in the following order: native convertase, convertase stabilized by the nephritic factor of the amplification loop (NFa), and convertase stabilized by nephritic factor of the terminal pathway (NFt). In the same order, the nephritides associated with these convertases more frequently manifest and have increasing amounts of glomerular deposit. This relationship of glomerular deposits with circulating convertase, however, is only circumstantial. There is no evidence that the convertase or a part thereof is a constituent of the deposits. The lesion that is the hallmark of MPGN type III is one in which interruptions of lamina densa are associated with subendothelial and subepithelial deposits, often confluent, and interspersed with multiple layers of new lamina densa-like material. This "type III lesion," which by implication is also associated with circulating nephritic factor, is the most persistent of the glomerular deposits. For reasons that are not yet clear, the type III lesion was absent in three patients who were severely hypocomplementemic, and the diagnosis of type III was made only after this lesion appeared in biopsy specimens obtained later. In MPGN type I, differing from type III, subendothelial deposits were present in 100% of biopsy specimens obtained during hypocomplementemia and in 47% of those obtained during normocomplementemia. Their persistence in type I may reflect rearrangement and condensation of the deposited material, shown by other investigators to be dependent on the presence of immunoglobulin G, which is largely absent from the deposits in type III. The comparison of deposits in types I and III indicates that relating the presence of subendothelial and paramesangial deposits to the C3 level at the time of biopsy can be helpful in distinguishing types I and III when the type III lesion is not present.

Identification of a highly encapsulated, genetically related group of invasive type III group B streptococci.

Type III group B streptococci (GBS) isolated from Tokyo and Salt Lake City were classified according to the similarity of HindIII and Sse83871 restriction digest patterns (RDPs) of bacterial DNA. The bacteria were clustered into three RDP types, with excellent correlation between subtyping based on the two enzymes. The majority (91%) of invasive isolates obtained from neonates were RDP type III-3. The mean sialic acid content of the III-3 strains was higher than that of other type III strains. Closely related isolates were concordant for expression of the bacterial enzyme C5a-ase, but invasive strains were no more likely to be C5a-ase positive than were strains isolated from the genitourinary tract of pregnant women. These data indicate that a group of genetically related organisms with increased capsule production causes the majority of invasive type III GBS disease.

Glomerular paramesangial deposits: association with hypocomplementemia in membranoproliferative glomerulonephritis types I and III.

Of 22 subjects previously reported with some form of factor H dysfunction, 12 had a glomerulonephritis that appeared to not be of immune complex origin. Factor H dysfunction results in elevated circulating levels of the C3b-dependent C3 convertase, C3b,Bb. Of the 12 cases with glomerulonephritis, the glomerular deposits in the six whose biopsy specimens were studied were predominately subepithelial on the paramesangial portion of the glomerular basement membrane. In a subsequent study, similar deposits were found in patients with membranoproliferative glomerulonephritis (MPGN) type II, also a nephritis that is probably not of immune complex origin. Paramesangial deposits were found in these patients only in biopsy specimens obtained when the C3 level was low, at which time convertase stabilized by nephritic factor would be present in the circulation. This association of paramesangial deposits with circulating convertase was further tested by correlating these deposits with the level of C3 at the time of biopsy in MPGN types I and III. The results in type III MPGN were similar to those in type II; paramesangial deposits were frequently present when the C3 level was low as a result of circulating nephritic factor of the terminal pathway, NFt, and were usually absent when the C3 level was in the upper two thirds of the normal range. Deposits persisted in those patients with C3 levels that had been low but that had increased during the year before biopsy to within the lower one third of the normal range. The persistence of paramesangial deposits in MPGN type III, as compared with MPGN type II, may be related to the differences in composition and function of the two NF stabilized convertases (C3bn,Bb,P,NFt and C3b,Bb,NFa, respectively) that circulate in these two disorders. In contrast to MPGN type III, the hypocomplementemia in MPGN type I is thought to be, for the most part, the result of classical pathway activation, which is not associated with elevated circulating convertase levels. In agreement with this, paramesangial deposits were found in only two of 34 biopsy specimens. At the time of those two biopsies, both patients had a complement profile indicating that the NFt was circulating, as in MPGN type III. In three other cases with profiles compatible with circulating NFt, paramesangial deposits were not found. In all patients with type I MPGN, electron microscopy and immunofluorescence of the glomeruli gave results typical of an immune complex nephritis. Thus, even though the complement profile in MPGN type I may at times indicate the presence of a nephritic factor, circulating immune complexes appear to be basic to pathogenesis. The observations support the hypothesis that elevated levels of the C3b-dependent convertase, as found in the "experiments of nature" with factor H dysfunction and in MPGN types II and III, are associated with paramesangial deposits. The nature of this association and the role of these deposits in producing the nephritis is not clear.

Nonbullous pemphigoid: prodrome of bullous pemphigoid or a distinct pemphigoid variant?

We describe two patients with pruritic, mainly urticarial or eczematous lesions associated with peripheral blood eosinophilia. No vesicles or blisters developed in either patient throughout the course of the disease (29 and 38 months, respectively). To characterize the clinicopathologic features of these patients we performed histopathologic studies, direct and indirect immunofluorescence, immunoelectron microscopy (patient 2), and immunoprecipitation of both patients' serum. Histopathologic examination revealed a moderate eosinophilic infiltrate partly arranged along the basement membrane zone and focally invading the epidermis. Linear deposits of immunoglobulin and C3 along the dermoepidermal junction were localized within the lamina lucida and over the hemidesmosomal plaques. Immunoprecipitation revealed the presence of circulating autoantibodies against the 230 kd bullous pemphigoid antigen. These findings suggest that our patients had a distinct, nonbullous variant of the pemphigoid spectrum.

Analysis of the effects of activation of the alternative pathway of complement on erythrocytes with an isolated deficiency of decay accelerating factor.

E from individuals with the Inab blood group phenotype have an isolated deficiency of decay accelerating factor (DAF, CD55). DAF is a glycosyl phosphatidylinositol anchored membrane protein that inhibits activation of both the classical and alternative pathways of complement. Deficiency of DAF from the E of paroxysmal nocturnal hemoglobinuria (PNH) is thought to contribute to their greater sensitivity to complement-mediated lysis. Unlike PNH E, however, Inab cells are not susceptible to acidified serum lysis, a process that is mediated through activation of the alternative pathway. This observation led us to hypothesize that membrane constituents other than DAF control susceptibility to acidified serum lysis. To investigate this hypothesis, Inab E were incubated in acidified serum, and hemolysis and C3 deposition (as a measure of alternative pathway activation) were quantitated. C3 deposition of Inab cells was approximately 20 times greater than normal, however, hemolysis was not observed. Inab E expressed a normal amount of membrane inhibitor of reactive lysis (MIRL, CD59), a glycosyl phosphatidylinositol anchored protein that is also deficient in PNH. When MIRL function was blocked with antibody, C3 deposition markedly increased, and 100% of the Inab cells hemolyzed in acidified serum. These studies demonstrate that susceptibility to acidified serum lysis is controlled primarily by MIRL, and that in addition to its regulatory affect on the membrane attack complex, MIRL also modulates the activity of the C3 convertase of the alternative pathway by a mechanism that remains to be determined.

Increased activation of the alternative complement pathway in sickle cell disease.

Complement proteins play an important role in host defenses against Streptococcus pneumoniae, a major cause of serious infections in sickle cell (SS) disease. Previous studies have suggested abnormalities of the alternative complement pathway in SS disease. We measured activation of the alternative pathway in sera from patients with SS disease utilizing an enzyme immunoassay which detects C3b,P complexes, derivative of the C3b,Bb,P alternative pathway convertase. In all, 89% of SS sera had elevated concentrations of C3b,P complexes, indicative of increased alternative pathway activation. Chronic activation of the alternative pathway may contribute to impaired host defense in SS patients.