Circulating Immune Complexes, Complement Activation Kinetics and Serum Sickness following Treatment with Heterologous Anti-snake Venom Globulin

Consecutive serum and plasma samples, from a patient receiving 100 ml polyvalent horse anti-venom globulin after a rattlesnake bite, were analysed for circulating immune complexes (IC) and activation of complement factors. IC were determined by two independent methods, a complement consumption assay and a Clq-binding assay. Rapidly rising levels of complement-fixing circulating IC were detected as early as 4--5 days after the serum treatment and distinct IC-activity was recorded in both assays on day 8 when clinical symptoms of serum sickness were observed. The IC remained in circulation for at least 5 weeks. Signs of intravascular C-activation in the form of low C3, C4 and C5 values was noted on day 1 after treatment. Factor B was demonstrable 3--4 days after the snake bite and this factor and C3c attained a peak around day 8, just before maximal suppression of native C3 and C4. 14 days after the globulin treatment C3c and B were declining rapidly while C3 and C4 approached normal values first 36 days after treatment. An increase in heterophilic antibodies to sheep erythrocytes was observed after treatment with anti-venom globulin.     

Three cases of factor I deficiency: the effect of treatment with plasma.

Three patients with congenital factor I deficiency associated with different clinical manifestations are described. Case 1 had one single episode of meningococcal disease, case 2 experienced four episodes of meningococcal disease and several other severe infections, whereas case 3, without known predisposition for infections, died from a subacute immune-complex mediated syndrome, resembling polyarteritis nodosa. Family studies in cases 1 and 2 revealed healthy individuals with factor I concentrations below the lower reference limit, indicating heterozygous carriers. The pedigree analyses were consistent with autosomal codominant inheritance. The estimated minimal frequency of the deficient gene was 0.002. Pedigree analysis was not performed in case 3 but the father and sister was found to be probable heterozygous carriers. Cases 2 and 3 were treated with infusions of freshly frozen plasma (FFP) (40 and 27 ml/kg bodyweight) during acute illness and the immunochemical complement profile was monitored. Following plasma infusion factor I was cleared from the circulation with a half-life of 29-45 h. The plasma infusions induced generation of C3d and C4d, increase in native factor B and C3 concentrations and disappearance of Ba split products. Native C3 and C4 increased to normal concentrations and remained normal till 16 days after the plasma infusions, whereas native factor B decreased to preinfusion levels 8 days after plasma infusion. It is concluded, that congenital factor I deficiency can present with different clinical manifestations and may be more prevalent than hitherto anticipated. Furthermore, infusion of blood products containing small amounts of functional factor I can partly normalize the complement profile, with a more prolonged effect on C3 and C4 than on factor B metabolism.     

Complement components, C1 activation and disease activity in SLE

Laboratory parameters were studied in 8 systemic lupus erythematosus patients during periods of high and low disease activity, mainly as defined by clinical criteria. Renal manifestations were present in 6 patients 5 of which showed antibodies to native DNA. C-reactive protein was raised in 3 patients. Only 1 of these showed a superimposed bacterial infection. Markedly high concentrations of C1r-C1s-C1 inactivator cOmplexes (C1r-C1s-Cl IA) in the sera provided direct evidence of C1 activation independent of disease activity. During active disease. C1r-C1s-C1 IA were correlated with C1q binding immune complexes as measured by solid phase, but not by fluid phase assay. Immunochemical concentrations of C1q, C4 and C3 and functional C2 were decreased in active SLE, consistent with sequential activation of the classical pathway. Discrepancies were noted between functional and immunochemical assay for C2 but not for factor B. Although essentially within the normal range, the levels of C1s, C4 binding protein, C5 and properdin were lower during active than during inactive disease. The concentrations of the factors B, I and H did not suggest involvement of the alternative pathway. 1 exceptional patient showed low factor B, a relative decrease of factor I and the presence of Bb fragments in plasma during active SLE. Markedly high factor D values were found. This could partly be explained by reduced renal function.     

Reduced Erythrocyte CR1 (CD 35) Receptor Function and Complement Opsonization in Factor I-Deficient Patients is Restored by Plasma Infusion

Erythrocytes (E) from three factor I-deficient patients were investigated for surface-bound complement factors and CR1 (CD 35) expression and function. The E were coated with C4b, C3b, and factor H. Following plasma infusion or in vitro incubation of the patients' E with normal human serum (NHS) or purified factor I, cell-bound C4b and C3b could no longer be detected. The E now expressed C3d, and factor H was unaffected, indicating that factor H was bound to the C3d part of the C3b molecules, providing the co-factor for effective cleavage of E-bound C3b when purified factor I was added. The binding of monoclonal anti-CR1 antibodies (M710) to the patients' E was markedly reduced compared with control E, and was not normalized by treatment with NHS, probably because covalently bound C3d/factor H interfered with the binding of M710. By contrast, the reduced ability of the patients' E-CR1 to bind complement-opsonized immune complexes (IC) was normalized after plasma infusion. This shows that the impaired CR1 function was acquired and emphasizes the importance of performing functional CR1 assays. Complement opsonization of IC for binding to normal E was severely compromised in the patients' sera due to consumption of factor B and C3. After plasma infusion the opsonization capacity of the patients' sera was restored. Thus, two mechanisms of importance for normal clearance of IC were compromised in factor I-deficient patients: (1) the opsonization of IC for binding to E-CR1, and (2) the capacity of E-CR1 to bind opsonized complexes. Both dysfunctions were temporarily corrected by plasma infusion.     

Complement-mediated solubilization of immune complexes. Solubilization inhibition and complement factor levels in SLE patients

Thirty-two of 36 serum samples from 19 SLE patients showed reduced capacity to mediate complement-dependent solubilization of immune complexes (IC). SLE patients with nephritis exerted the lowest complement-mediated solubilization capacity (CMSC) whereas sera from patients with inactive disease gave the highest CMSC values, with three out of four samples within the normal reference range. Thirty-five of the 36 serum samples showed inhibition of CMSC in a newly developed CMSC inhibition assay. The strongest CMSC inhibition was exerted by sera from newly discovered cases of SLE who received no medical treatment and the lowest inhibition by sera from patients with inactive disease. There was a significant negative correlation between CMSC and CMSC inhibition (r = -0.67, P less than 0.001). Sera with low concentrations of C1q, C3, factor B or high C3d levels showed markedly reduced CMSC values. Pronounced CMSC inhibition was observed only in samples with normal or high factor H values. No significant correlation was found between CMSC or CMSC inhibition and circulating IC levels, but pronounced CMSC inhibition was registered only in strongly IC positive sera.     

Factor D

Complement factor D (FD) is a serine protease that plays an essential role in the activation of the alternative pathway (AP) by cleaving complement factor B (FB) and generating the C3 convertases C3(H₂O)Bb and C3bBb. FD is produced mainly from adipose tissue and circulates in an activated form. On the contrary, the other serine proteases of the complement system are mainly synthesized in the liver. The activation mechanism of FD has long been unknown. Recently, a serendipitous discovery in the mechanism of FD activation has been provided by a generation of Masp1 gene knockout mice lacking both the serine protease MASP-1 and its alternative splicing variant MASP-3, designated MASP-1/3-deficient mice. Sera from the MASP-1/3-deficient mice had little-to-no lectin pathway (LP) and AP activity with circulating zymogen or proenzyme FD (pro-FD). Sera from patients with 3MC syndrome carrying mutations in the MASP1 gene also had circulating pro-FD, suggesting that MASP-1 and/or MASP-3 are involved in activation of FD. Here, we summarize the current knowledge of the mechanism of FD activation that was finally elucidated using the sera of mice monospecifically deficient for MASP-1 or MASP-3. Sera of the MASP-1-deficient mice lacked LP activity, but those of the MASP-3-deficient mice lacked AP activity with pro-FD. This review illustrates the pivotal role of MASP-3 in the physiological activation of the AP via activation of FD.     

Zinc Ions Inhibit Factor I-Mediated Release of CR1-Bound Immune Complexes and Degradation of Cell-Bound Complement Factors C3b and C4b

ZnCl2 exerted a dose-dependent inhibition of citrate-phosphate-dextrose (CPD) plasma-induced release of 125I-labelled BSA-anti-BSA immune complexes (IC) bound to complement receptor type 1 (CR1, CD35) in human whole blood. Maximal inhibition was observed at 10 mM of ZnCl2. Furthermore, the release of IC bound to erythrocyte (E)-CR1 by purified factor I, factor I-deficient serum plus purified factor I, or normal human serum was reduced by approximately 90%, 64%, and 52%, respectively, in the presence of 10 mM ZnCl2. The effect of ZnCl2 on factor I-mediated degradation of cell-bound C3b/C4b was also investigated employing CPD blood or E from a factor I-deficient donor. These cells expressed covalently bound C3b and C4b as demonstrated by a simple agglutination technique. Upon incubation of CPD whole blood with purified factor I, or of E with purified factor I or normal CPD plasma, the C-fragments were cleaved and the cells were no longer agglutinated by antibodies to C3c and C4c. The presence of ZnCl2 prevented this factor I-mediated degradation of C3b and C4b, as evidenced by the unaffected agglutination of the cells by the antibodies. We conclude that ZnCl2 inhibited factor I activity since: (1) release of complement-preopsonized IC from E-CR1 by purified factor I was markedly inhibited (90%) in the presence of ZnCl2, (2) preincubation of the cells with ZnCl2 caused only a moderate inhibition (32-38%) of the IC release, and (3) degradation by purified factor I of covalently cell-bound C3b and C4b was abrogated in the presence of 10 mM ZnCl2.     

Factor H autoantibodies in membranoproliferative glomerulonephritis

Factor H autoantibodies are found in ∼10% of aHUS patients. Most are associated with complete deficiency of factor H related proteins 1/3 and bind to the C terminal recognition domain. MPGN, like aHUS, is characterised by complement activation. In this study we, therefore, examined the hypothesis that factor H autoantibodies are associated with MPGN. We screened sera from 16 MPGN patients and 100 normal controls using ELISA and detected strongly positive IgG factor H autoantibodies in 2 patients. One patient had type II (DDD) MPGN (male aged 24 yrs) with C3NeF and the other type I (female aged 26 yrs) with no detectable C3NeF. We identified the binding site of the autoantibodies using small SCR domain fragments in the ELISA and showed that the autoantibodies in both patients bound predominately to the N terminal complement regulatory domain of factor H. We measured CFHR 1/3 copy number using MLPA and showed that both patients had 2 copies of CFHR1 and 3. Finally, we examined the functionality of detected factor H autoantibodies using purified patient IgG and observed increased haemolysis when purified IgG from both patients was added to normal human sera prior to incubation with rabbit red blood cells. Thus, in a cohort of MPGN patients we have found a high titre of functionally significant factor H autoantibodies in two patients with MPGN. Antibody depleting therapy may have a role in such patients and we suggest that screening for factor H autoantibodies should be undertaken in all patients with MPGN.     

Factor H autoantibodies in membranoproliferative glomerulonephritis

Factor H autoantibodies are found in ~10% of aHUS patients. Most are associated with complete deficiency of factor H related proteins 1/3 and bind to the C terminal recognition domain. MPGN, like aHUS, is characterised by complement activation. In this study we, therefore, examined the hypothesis that factor H autoantibodies are associated with MPGN. We screened sera from 16 MPGN patients and 100 normal controls using ELISA and detected strongly positive IgG factor H autoantibodies in 2 patients. One patient had type II (DDD) MPGN (male aged 24 yrs) with C3NeF and the other type I (female aged 26 yrs) with no detectable C3NeF. We identified the binding site of the autoantibodies using small SCR domain fragments in the ELISA and showed that the autoantibodies in both patients bound predominately to the N terminal complement regulatory domain of factor H. We measured CFHR 1/3 copy number using MLPA and showed that both patients had 2 copies of CFHR1 and 3. Finally, we examined the functionality of detected factor H autoantibodies using purified patient IgG and observed increased haemolysis when purified IgG from both patients was added to normal human sera prior to incubation with rabbit red blood cells. Thus, in a cohort of MPGN patients we have found a high titre of functionally significant factor H autoantibodies in two patients with MPGN. Antibody depleting therapy may have a role in such patients and we suggest that screening for factor H autoantibodies should be undertaken in all patients with MPGN.     

Complement fragments in patients with bronchial asthma]

In this study, we investigated the pharmacological reactions induced by ibudilast to the complement system with the aim of clarifying the functional relation of the complement system to allergic reactions and pathology in patients with bronchial asthma. Complement hemolytic activities (CH50 and ACH50), complement profile, anaphylatoxins (C3a and C5a) and complement fragments (Bb, iC3b and C4d) were measured in 20 patients with bronchial asthma. One of antiasthmatic activities induced by ibudilast was concluded to be brought about though inactivation of the alternative complement pathway working on type III allergic reaction. Ibudilast increased the complement fragment Bb in the patients' plasma with the fairly controlled bronchial asthma. This increase in circulating Bb was suspected to be a result of inactivation of intermediate complement complexes, for example C3b.Bb.P, because the amounts in plasma of C3 and C5 showed no changes, while those of factor, B, P, H and I were decreased by ibudilast administration in patients with fairly controlled bronchial asthma. This antiasthmatic ability of ibudilast was restrained in those patients whose peripheral leukocytopenia was advanced before ibudilast administration, and in those whom ibudilast did not provoke an increase in the plasma level of iC3b, or did not prevent the serum level of C5 from increasing. In those unfairly controlled cases, enough anaphylatoxins, especially C5a might be produced to make the margination of peripheral neutrophils to the lung and increase CR3 on neutrophils binding with iC3b.     

Cryoprecipitate of patients with cryoglobulinemic glomerulonephritis contains molecules of the lectin complement pathway

Serological and histological studies were carried out to explore the role of the lectin complement pathway in the pathogenesis of cryoglobulinemic glomerulonephritis. Sixteen patients with mixed cryoglobulinemia type II with glomerulonephritis (GN) were enrolled. All cases had hepatitis C virus (HCV) infection. The serum concentration of mannose-binding lectin (MBL) was significantly higher in the GN patients than in the normal controls according to ELISA (P < 0.01). IgG, IgM, C1q, C4d, HCV envelope antigen, MBL, and MBL-associated serine protease-1 (MASP-1) could be visualized in the cryoprecipitate of the 16 patients by Dot blot assay. Renal biopsy specimens obtained from 3 patients were examined by immunohistochemistry, and the glomeruli strongly stained for IgG, IgM, MBL, MASP-1, C4d, C3c, and C3d in a fringe-like pattern. The pattern of HCV constituent deposition was partially fringe-like. The complement profiles of the 16 cases were distinctive; briefly, the serum levels of C1q, C2, and C3 were reduced, although the levels of circulating regulatory proteins (C1-inhibitor, factor H, and factor I) were in the normal range. The serum C4 level was significantly reduced. These results indicate that immune complex formation involves molecules of the lectin pathway and leads to organ damage in cryoglobulinemic glomerulonephritis.     

The tick-over theory revisited: formation and regulation of the soluble alternative complement C3 convertase (C3(H2O)Bb)

The molecular interactions between the components of the C3 convertase of the alternative pathway (AP) of complement and its regulators, in both surface-bound and fluid-phase form, are still incompletely understood. The fact that the AP convertase is labile makes studies difficult to perform. According to the so called tick-over theory, hydrolyzed C3, called C3(H(2)O), forms the initial convertase in fluid phase together with factor B. In the present study, we have applied western blot analysis and ELISA together with fluorescence resonance energy transfer (FRET) to study the formation of the fluid-phase AP convertases C3(H(2)O)Bb and C3bBb and their regulation by factor H and factor I at specific time points and, with FRET, in real time. In our hands, factor B showed a higher affinity for C3(H(2)O) than for C3b, although in both cases it was readily activated to Bb. However, the convertase activity of C3bBb was approximately twice that of C3(H(2)O)Bb, as monitored by the generation of C3a. But in contrast, the C3(H(2)O)Bb convertase was more resistant to inactivation by factor H and factor I than was the C3bBb convertase. Under conditions that totally inactivated C3bBb, C3(H(2)O)Bb still retained approximately 25% of its initial activity.     

Proteolytic activity of the different fragments of factor B on the third component of complement (C3). Involvement of the N-terminal domain of Bb in magnesium binding

Kinetic experiments measuring the proteolytic activity of Bb and 33Kd fragment (the C-terminal domain of factor B) on C3 were performed in several conditions, in order to assess the role of factor B domains in the catalytic activity and magnesium binding. The experiments were carried out in fluid phase with 125I-C3 or C3(H2O) as substrates and in the presence of nonradioactive C3b as cofactor. The results indicate: (a) The C-terminal domain, 33Kd, possesses proteolytic activity on C3, which is Mg2(+)-independent, whereas proteolysis by Bb is enhanced in 5 mM Mg2+. (b) C3b behaves as cofactor of 33Kd proteolytic activity on C3 and factor H is able to inhibit this activity. (d) Kinetics of C3 proteolysis by 33Kd shows a lag phase which is also displayed by Bb in the absence but not in the presence of Mg2+. Taken together these data are consistent with the involvement of the N-terminal domain of Bb in Mg2+ binding, which results in an enhancement of the proteolytic activity on C3 of the adjacent C-terminal domain. A C3 convertase model accounting for these results is presented.     

Alternative pathway activation of complement in laparoscopic and open rectal surgery

The study was designed to investigate whether complement is activated in patients subject to rectal surgery and whether the choice of surgical technique (open or laparoscopic) has any impact on the activation of complement. Our hypothesis is that laparoscopic surgery leads to a lower-level activation of complement than open surgery. Patients (n = 24) subject to rectal surgery owing to rectal cancer were included. The study was prospective and randomized. The patients were randomized to either laparoscopic surgery (n = 12) or open surgery (n = 12). Blood samples for determination of complement activation (C4d, Bb, C3bc and the terminal C5b-9 complex TCC) were drawn before start of surgery (T0) and at the following time-points after start of surgery: 180 min (T1), 360 min (T2), 24 h (T3) and 3-5 days (T4). A significant increase in the alternative pathway activation product Bb and in the terminal pathway activation product TCC was seen over time in both groups (P < 0.001). Bb peaked early (T1) and returned to baseline levels post-operatively, whereas TCC increased steadily with maximum values in the late post-operative period. The plasma concentrations of C4d and C3bc decreased significantly in both groups at T1 and T2 and returned to baseline levels at T4. There was no significant difference between the groups. Rectal surgery causes activation of the complement system. Complement is activated through the alternative pathway. Results mostly showed no significant differences between laparoscopic and open rectal surgery apart from lower levels of factor Bb in the former group in the perioperative period.     

Complement-activating ability of leucocytes from patients with complement factor I deficiency.

Previous studies from this laboratory have shown that normal peripheral blood B cells are capable of activating complement via the alternative pathway (AP), that the activation is associated with complement receptor type 2 (CR2) expression, and that erythrocytes at normal blood levels partially inhibit the activation. The purpose of the present study was to investigate whether factor I (FI) deficiency, which leads to continued formation of the AP convertase (C3bBb) resulting in the consumption of factor B and C3 and large scale generation of C3b fragments, affects the phenotype and/or function of the patients' B cells. Using flow cytometry, peripheral blood leucocytes (PBL) from two FI-deficient patients were investigated for expression of complement receptors and complement regulatory proteins, in vivo-deposited C3 fragments and in vitro complement-activating ability. CR1 levels on B cells were significantly lower in FI-deficient patients than in normal individuals, whereas CR2 levels were found to be reduced, although not to a significant extent. CR1 levels on monocytes and polymorphonuclear leucocytes (PMN) were found to be normal or slightly raised. All leucocyte subpopulations were found to be covered in vivo with C3b fragments. AP activation on B cells from FI-deficient patients in homologous serum was significantly reduced compared with that for normal individuals, whereas no in vitro activation was seen in autologous serum. In addition, the in vivo-bound C3b fragments were degraded to C3d,g when the patients' PBL were incubated in homologous serum containing EDTA. Finally, the patients, erythrocytes failed to exert any inhibition on AP activation in homologous serum.     

Requirements for the production of high-titre C3 nephritic factor (NEF) antibody in vitro.

C3 nephritic factor (NEF) is an IgG autoantibody directed against neoantigenic determinants of the alternative C3 convertase (C3b.Bb). Structural and functional studies require important amounts of this antibody, which are difficult to obtain from patients' sera. We have developed a method for increasing NEF production in vitro. Epstein-Barr virus is a herpes virus which transforms B lymphocytes. Some authors were able to induce the production of NEF in vitro after infection with Epstein-Barr virus (EBV). These works were preformed without any previous cellular selection of B cells. We have performed a method of preselecting antigen-binding cells prior to EBV transformation. Non-preselected cells yielded 0.16 U/million cells in culture (U/M) of NEF antibody, whereas enriched cells for NEF antibody in eliminated 8 U/M (sheep erythrocytes coated with anti-IgG, A, M). Specific NEF synthesis can be increased, in peripheral blood mononuclear cells (PBMC) from patients by in vitro stimulation with the antigens recognized by NEF [C3b.Bb, 21,000 MW protein from patients' E membranes and 26,000 MW protein from sheep E membranes (ShE)]. The highest stimulation is induced by the C3b.Bb and by 26,000 MW protein, 21,000 MW protein had lowest stimulatory effect. In this work also we have shown that patients having NEF antibody in sera have an increase of the CD5-CD19 subset, when compared with the controls.     

Interaction of Complement-Solubilized Immune Complexes with CR1 Receptors on Human Erythrocytes. The Binding Reaction

The binding of complement (C)-solubilized ¹²⁵I bovine serum albumin (BSA) anti-BSA immune complexes (IC) to CR1 receptors on human red blond cells (RBC-CR1) was studied. The binding of IC to CRI was strongly dependent on the molar antigen lo antibody ratio, and IC formed in moderate antigen excess showed no binding. IC solubilized, in 50% human serum in the presence of autologous RBC bound rapidly lo RBC-CRI, with maximal binding within less than 1 min at 37°C. Release of CRI-hound IC under these conditions occurred slowly, requiring more than.30 min. Only binding of 'partially' solubilized, e.g., anti C3c (C4c) and conglutinin-reactive 1C occurred, whereas fully solubilized complexes (IC-C3dg. C4d) showed virtually no binding. Solubilization of IC in the presence of Mg-EGTA or in C2-deficient serum resulted in a markedly delayed binding of IC ti RBC, indicating the importance of an intact classical pathway in preparing the IC for binding to RBC-CR1. C-solobilized IC could be absorbed to solid-phase conglutinin or antibody to C3c abd C4c, and tgese kugabds were able to inhibit the binding of solubilized IC to RBC. Heparin also exerted a marked, dose-dependent inhibitory effect on the binding of presolubilized IC to RBC-CR1, whereas the binding was unaffected by the addition of monosaccharides or by the concentration of Ca²⁻ or Mg²⁻ ions.     

Partial H (β1H) deficiency and glomerulonephritis in two families

H (1H) controls the C3b amplification loop by its ability to displace Bb from the alternative pathway convertase, C3b,Bb, and acts as a cofactor with I (C3b inactivator) to produce inactive C3b. Serum C3 levels are dependent to a large extent on the levels of H and I. Partial H deficiency was found in two families. The index case in Family 1 had vasculitis, thrombocytopenia, proteinuria, and depressed serum H and C3 levels. The index case in Family 2 had depressed serum H and B (Factor B) levels and IgA nephropathy which progressed to renal failure. His sister also had IgA nephropathy and depressed serum H and C3 levels. The depressed serum C3 level, B level, and H level could be responsible for the development of the immune diseases found in some members of these families.     

Soluble immune complexes in lupus mice: clearance from blood and estimation of formation rates

Elevated levels of circulating immune complexes (IC) occur in patients with systemic lupus erythematosus (SLE) and in several strains of mice that spontaneously develop a lupus-like illness. An increase in circulating IC might occur as a result of increased IC formation or decreased IC clearance. Previous work with one murine lupus strain, NZB/W, demonstrated normal clearance of soluble IC. We studied the in vivo behavior of stable model IgG IC in two murine lupus strains. MRL/l and BXSB, and in two normal strains, C3H/HeN and BALB/c. IC were injected intravenously, and blood radioactivity was measured over 3 hr. A clearance curve was derived for each mouse using the Marquardt-Levenberg curve-fitting method. A formula is derived which predicts the formation rate of IC in each animal from the blood level of IC and the clearance curve parameters. All mice exhibited biphasic exponential clearance of IC over 3 hr. Clearance of IC was significantly slower in MRL/l mice than in normal strains (P less than 0.02), and clearance in BXSB mice was significantly faster than in normals (P less than 0.02). However, the derived formula suggests that the observed differences in IC clearance have only a small effect on the blood levels of IC in the mice studied, and therefore suggests that the major factor which accounts for increased blood levels of IC in lupus mice is an increase in the IC formation rate.     

Pattern of C3, iC3b, and C3d in patients hospitalized for acute asthma.

Twenty-three patients hospitalized for acute asthma were studied for a peripheral blood complement profile consisting of C3, C4, C3d, iC3b, C4d, and Bb concentrations. Compared with normals (n = 22) and patients (n = 10) with acute bacterial infections (ABI), asthmatic patients had significantly higher serum C3 concentrations (P less than .001). Plasma C3d levels and iC3b in asthmatic patients were both comparable to those observed in normal controls, whereas patients with ABI had significantly higher iC3b levels than both other groups. The ratio of iC3b to C3 concentrations were similar in asthmatic patients and controls, and iC3b levels were correlated with total serum C3 levels in asthmatic patients (r = .55, p less than .001) as well as in normal (r = .69, p less than .001). Both of these groups had significantly lower iC3b to C3 ratios compared with the ABI group (P less than .0001). Also observed in asthmatic patients were a significant correlation between serum C4 and C3 levels (r = .83, P less than .001) and a lower mean ratio of plasma C4d to C4 compared with normals (P less than .005). This profile of complement alterations is distinct from that observed in acute bacterial infection. These changes in asthmatic patients may relate to an acute phase reaction phenomenon affecting complement and/or complement regulatory proteins.