Complement system in pneumococcal infections

The role of complement in Streptococcus pneumoniae-associated haemolytic uraemic syndrome

BACKGROUND

Atypical forms of haemolytic uraemic syndrome (aHUS) include HUS caused by defects in the regulation of alternative complement pathway and HUS linked to neuraminidase-producing pathogens, such as Streptococcus pneumoniae. Increasing data support a pathogenic role of neuraminidase in the development of S. pneumoniae-associated haemolytic uraemic syndrome (SP-HUS), but the role of complement has never been clarified in detail. Therefore, we aimed to investigate whether the pathologic complement profile and genetic risk factors of aHUS are present in patients with SP-HUS.

METHODS

Enrolling five patients with SP-HUS classical and alternative pathway activity, besides C3, C4, factors H, B, I and anti-factor H autoantibody levels were determined. The coding regions of CFH, CFI, CD46 (MCP), THBD, C3 and CFB genes were sequenced and the copy number of CFI, CD46, CFH and related genes were also analyzed.

RESULTS

We found that in the acute phase samples of SP-HUS patients, complement components C4, C3 and activity of the classical and alternative pathways were decreased, indicating severe activation and complement consumption, but most of these alterations normalized later in remission. Three of the patients carried mutations and risk haplotypes in complement-mediated aHUS associated genes. The identified mutations include a previously published CFI variant (P50A) and two novel ones in CFH (R1149X) and THBD (T44I) genes.

CONCLUSIONS

Our results suggest that severe complement dysregulation and consumption accompany the progress of invasive pneumococcal disease (IPD)-associated SP-HUS and genetic variations of complement genes may contribute to the development of this complication in a proportion of the affected patients.

Pneumococcal endocarditis update: analysis of 10 cases diagnosed between 1974 and 1984

We analyzed the clinical characteristics of 10 patients with pneumococcal endocarditis hospitalized between 1974 and 1984. Patients with pneumococcal endocarditis were typically middle-aged men. Forty percent were alcoholic. They sought medical attention early in the course of their illness and were given appropriate antibiotics promptly. The aortic valve was involved in seven patients. Five patients developed signs of severe valvular insufficiency, and congestive heart failure was present at the time of admission in four patients. Only three patients were recognized to have endocarditis prior to death or to the occurrence of a major complication of their infection. The total in-hospital mortality rate among these patients was 50%. Thus pneumococcal endocarditis is generally an acute, left-sided endocarditis that is associated with rapid valvular destruction and a high mortality rate. Unfortunately, recent advances in diagnosis and treatment of bacterial endocarditis have not substantially improved the outcome of this devastating infection.

The role of complement in inflammation during experimental pneumococcal meningitis

The mechanism whereby an effective bactericidal inflammatory reaction develops in the subarachnoid space is not clearly defined. While normal cerebrospinal fluid is deficient in complement, immunoglobulin and leukocytes, these serum components appear in cerebrospinal fluid (CSF) during the course of bacterial meningitis. Using a rabbit model of pneumococcal meningitis we examined the role of the alternate complement pathway in three early events important to the defense of the subarachnoid space: leukocyte chemotaxis, phagocyte mediated bacterial killing, and clearance of bacterial components from the cerebrospinal fluid space. Rabbits treated with cobra venom factor to deplete complement were inoculated intracisternally with encapsulated (type II or XIX) pneumococci. Following complement depletion, there was a dramatic (at least 100-fold) decrease in the LD50 for these strains. Nevertheless, complement depletion did not affect the magnitude of CSF leucocytosis or the rate of clearance of bacterial particles from CSF. A short delay in the appearance of leukocytes in CSF was found in the absence of complement. The major effect of complement depletion, however, was to diminish the efficiency of leukocyte mediated killing of encapsulated bacteria in the CSF. Although the short delay in the onset of leukocytosis in the complement depleted animals is consistent with a chemotactic role of complement in the normal animal, the quantitatively normal leukocytosis in the complement depleted rabbits clearly indicates that important chemotaxins other than complement function in CSF. Inhibition of leukocytosis by indomethacin and diclofenac suggests that metabolite(s) of the arachidonic acid pathway may perform such a chemotactic role. A major role of complement in the defense of the subarachnoid space appears to be as an opsonin needed for the effective bactericidal activity of leukocytes. It is the lack of this function that best explains the greatly decreased LD50 value of encapsulated pneumococci in the complement depleted animal.

Relationships between alternative complement pathway activation, C-reactive protein, and pneumococcal infection

In the absence of specific antibody, opsonization of Streptococcus pneumoniae may be mediated by the alternative complement pathway (AP) or by C-reactive protein (CRP) via C1 binding. To determine the role of these mechanisms in pneumococcal (PNC) disease, we studied 19 patients with differing severities of PNC infection. C4 and CRP levels and zymosan-induced consumption of 50% hemolytic complement (CH50) were measured in specimens obtained acutely and then weekly. In patients with complicated illness, the modified mean CH50 in acute sera was 178 +/- 57 U/ml, significantly lower than the mean CH50 of 331 +/- 80 U/ml in patients with uncomplicated illness (P less than 0.05). The values of the two groups on a given day approximated each other on days 7, 14, and 23. Consumption of complement by zymosan was also lower in acute sera of patients with complicated illness, with a mean value of 19 +/- 18 U/ml compared with 58 +/- 30 U/ml in those with uncomplicated illness (P less than 0.05). This difference was also seen on day 7 (P less than 0.05). Disease involving lower-numbered PNC serotypes (less than 10) correlated with reduced availability of AP factors in acute sera, independent of illness severity. Mean CRP levels were inversely related to zymosan-induced complement activation in patients with complicated illness. These data suggest that in vivo depletion of AP factors is significantly greater in patients with complicated illness and is associated with high CRP levels. CRP may enhance AP activation via C3 convertase generation and function with it as a preantibody host defense mechanism.

Activation of human complement by the pneumococcal toxin pneumolysin

Highly purified pneumolysin (at a concentration of 10 micrograms/ml) caused significant activation of human complement, as measured by conversion of C3. Complement activation in the presence of pneumolysin was not observed in sera chelated with a combination of Mg2+ and ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid, and activation was only slight in C2-deficient sera. This suggests that the toxin is capable of activating the classical complement pathway. Treatment of normal human serum with pneumolysin also significantly reduced its opsonic activity for Streptococcus pneumoniae.

Gram-positive bacteria-induced granulocytopenia and pulmonary leukostasis in rabbits

Pneumococci have been shown to induce granulocytopenia and pulmonary leukostasis which might contribute to morbidity and mortality in pneumococcal sepsis. We studied whether other gram-positive species (groups A and B streptococci, Staphylococcus aureus, Bacillus cereus, and Clostridium perfringens) could also induce these phenomena. Rabbits were challenged with nonviable preparations of each species, and all five species induced profound granulocytopenia (mean decrease of 90%) and pulmonary leukostasis. In vitro studies of serum incubated with these species of bacteria showed a mean consumption of total hemolytic complement of 90%, a mean increase of chemotactic activity for granulocytes of 204%, and a mean augmentation of granulocyte adherence of 45% (compared with 18% for the control). Infusion of sonicate-exposed sera induced granulocytopenia in recipient rabbits. Thus, several nonviable gram-positive species can interact with serum to activate the complement system, generate C5a bioactivity, augment granulocyte adherence, and generate a neutropenia-inducing factor. These alterations may contribute to granulocytopenia or pulmonary leukostasis, which may play a role in the morbidity and mortality associated with gram-positive bacterial infections.

Evaluation of Streptococcus pneumoniae type XIV opsonins by phagocytosis-associated chemiluminescence and a bactericidal assay

The relative roles of serum factors required for opsonization of type XIV Streptococcus pneumoniae were investigated by means of luminol-enhanced chemiluminescence (CL), bactericidal, and immunofluorescence assays employing adult sera containing high (>1,000 ng of antibody nitrogen per ml) or low (<200 ng of antibody nitrogen per ml) antibody concentrations as determined by radioimmunoassay. Specific antibody concentration correlated directly with both total and heat-labile CL activity (P < 0.005) and with the bactericidal index (P < 0.05) at a serum concentration of 10%. The importance of specific antibody as an opsonin was confirmed by the abolition of CL activity and immunoglobulin immunofluorescence observed after absorption of heated sera with type XIV pneumococcal cells and by the dose response in CL and bactericidal activity observed with the addition of immunoglobulin G to hypogammaglobulinemic serum. A role for the classical complement pathway in opsonization was indicated by significantly greater CL integrals for high-antibody sera than for low-antibody sera depleted of factor D and by the bactericidal activity noted for untreated, but not magnesium ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid-chelated low-antibody sera. The alternative pathway contributed more than half of the CL activity of both high- and low-antibody sera. However, after magnesium ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid chelation, only sera with high antibody concentrations or agammaglobulinemic serum reconstituted with immunoglobulin G with high specific antibody levels supported significant bactericidal activity. Therefore, type-specific antibody and complement promote opsonization of type XIV S. pneumoniae, and this may occur via either complement pathway. These results suggest that CL is a suitable tool to delineate serum factors and their contribution to opsonization, but results must be related to other functional assays.

Complement in pneumococcal infections with varying degrees of severity

Complement component levels (Clq, Cls, C4, C3, factor B and properdin) and C1 subcomponent complexes (C1r-C1s, C1-r-C1-, C1-r-C1-s-C1 inactivator, 1A) were studied in 16 adults with pneumococcal infections varying severity. Patients with fulminant disease and signs of septic shock showed pronounced hypocomplementemia. In patients with pneumococcal pneumonia or meningitis elevated levels of C1-r-C1-s-C1- IA complexes indicated activation of C1, despite normal levels of C1q, C1s, C4 and C3. Moderately decreased properdin values suggested involvement of the alternative pathway. In adults with pneumococcal otitis no changes in the complement profile was found. In contrast, pronounced aberrations of the C1 subcomponents were earlier demonstrated in children with otitis.

Reduction of circulating granulocytes induced by type 1 pneumococcal cell walls in New Zealand white rabbits

Granulocytopenia seen during human pneumococcal disease is associated with a worsened prognosis. Streptococcus pneumoniae type 1 reduces circulating granulocytes and induces pulmonary leukostasis in rabbits. We studied which type 1 pneumococcal fraction(s) might be responsible for the reduction in circulating granulocytes. Rabbits were challenged intravenously with nonpyrogenic sterile saline, sonicated type 1 pneumococci, capsular polysaccharide from type 1 pneumococci, or cell walls from type 1 pneumococci. Nonviable pneumococci caused a mean decrease in blood granulocytes of 64% as compared with a mean increase of 124% in saline-injected controls, a difference significant at P less than 0.001. Pneumococcal cell walls induced significant decreases in circulating granulocytes at all doses tested when compared with saline-injected controls, whereas capsular polysaccharide induced no reduction in granulocytes. On a weight-for-weight basis, cell wall induced significantly more granulocyte reduction than did capsular polysaccharide at doses of either 10 mg (P less than 0.01) or 20 mg (P less than 0.005). A nonencapsulated pneumococcus also induced a profound granulocyte reduction (mean decrease in blood granulocytes, 88%) in the absence of detectable circulating polysaccharide. The cell wall fraction of S. pneumoniae type 1 was a more effective constituent in promoting the reduction in circulating granulocytes in rabbits.

Alterations in serum opsonic activity and complement levels in pneumococcal disease

Pneumococcal opsonic activity and concentrations of pneumococcal capsular polysaccharide antigen, C3, C4 factor B, C3 and factor B breakdown products were measured in the serum obtained acutely from 12 patients with serious pneumococcal disease. One patient showed markedly reduced pneumococcal opsonic activity, borderline-low C3, and the presence of C3 and factor B breakdown products and died. Although eight additional patients showed depressed levels of C3 or C4 or the presence of C3 or factor B breakdown products, none had reduced pneumococcal opsonic activity. All of the three remaining patients had normal opsonic activity and C3 and C4 levels. Covalescent serum was obtained from eight patients; six had normal C3 and C4 levels, and two had persistent C4 depression. These data show that complement is activated during pneumococcal disease and suggest that extensive complement activation may impair pneumococcal opsonic activity in certain patients and thereby compromise an important host defense mechanism.

Effect of intravenously injected killed pneumococci on leukocytes, complement, and phagocytosis in rabbits

A pneumococcal infection may be lethal in the absence of overwhelming pulmonary involvement, and death may occur even after the organisms have been killed with antibiotics. The mechanism of death is not understood but may be related to circulating pneumococcal products. For investigating the effects of nonviable pneumococci on several host defense mechanisms, rabbits were injected intravenously with 4 X 10(8) colony-forming units of killed sonified type 13 or type 29 pneumococci. Blood was sampled periodically for the next 24 h, and the following were measured: (i) circulating levels of leukocytes; (ii) activity of the classical and alternate complement pathways; and (iii) ability of the serum to opsonize pneumococci for ingestion and killing by polymorphonuclear leukocytes. Saline-injected control rabbits showed no change in any of the functions. Nonimmune rabbits injected with either pneumococcal serotype showed progressive and profound leukopenia, no change or an increase in classical and alternate complement pathway activity, and a profound reduction in the serum-opsonizing capcity for pneumococci of the same serotype as that used in the injection. The opsonizing capacity remained normal for the other serotype. When a previously immunized animal was injected, the opsonizing capacity for the homologous organism remained intact, but leukopenia nervertheless occurred.

Experimental type 25 pneumococcal pneumonia in rats: an electron-microscopic study

A self-healing lobar pneumonia was produced in rats by intratracheal instillation of Type 25 pneumococci. Sequential changes in the lung were examined by electron microscopy from the onset of acute inflammation through resolution. Manifestations of the infection included vascular endothelial changes of acute inflammation and leukocytic mobilization. The pulmonary alveolar epithelium showed only minimal changes throughout the infection. The significance of the findings is discussed in relation to the pathogenesis of pneumococcal lobar pneumonia.

Complement activation in pneumonia

Complement analyses performed on serially collected plasma samples from 10 patients during acute infectious pneumonia or bronchopneumonia showed normal or increased values for hemolytic activity and Clq, C4, C3, and factor B levels. However, the levels of C3 breakdown products (C3d) were significantly (greater than 2 SD) increased in six patients during the first three days of observation, suggesting that hypercatabolism of complement components may occur during the acute phase of infectious pneumonia concomitant with a hypersynthesis of some complement components. Evidence of circulating immune complexes was obtained only in two patients with the 125I-Clq Binding Test.

Efficacy of antimicrobial therapy in experimental rat pneumonia: effects of impaired phagocytosis

The importance of intact host defense mechanisms for successful antimicrobial therapy was investigated in an animal model. Recovery from lobar pneumococcal pneumonia as a result of penicillin therapy was studied in normal rats and in rats treated with cobra venom factor. This factor was used to selectively suppress the phagocytosis of pneumococci as a result of complement depletion. Although complete recovery from the infection occurred in normal rats after appropriate penicillin therapy, this was not the case in cobra venom factor-treated rats. Within the limitations of this study, evidence is presented for loss of antibiotic activity as a consequence of impaired phagocytosis.

Interaction of pneumococcal antigens with complement in rats

Complement activation with pneumococcal antigens was studied both in vitro and after injection of the antigens into rats. Whole pneumococci of various serotypes activated C3-C9 in rat serum treated with ethyleneglycol-bis (beta-aminoethyl ether)-N,N'-tetraacetic acid, although serotypes differed greatly in the extent of activation. Some purified pneumococcal capsular polysaccharides also activated C3-C9 in rat serum, but only when the antigens were present in concentrations of 500 to 1,000 mug/ml. Much of the activation with capsular polysaccharides was eliminated by the use of ethyleneglycol-bis (beta-aminoethyl ether)-N,N'-tetraacetic acid. Activation of C3-C9 by capsular polysaccharides did not correlate with the level of reactivity observed with whole organisms of the same serotypes. After injection of 5 x 10(9) pneumococci (type 3 or type 4) intravenously into rats, there was a transient decline in serum C3-C9 activity, but there was no decline in C3-C9 levels after intravenous injection of 1,000 mug of type 3 or type 4 capsular polysaccharides. As determined by immunofluorescence, circulating capsular polysaccharide was deposited in several tissues, including the vascular endothelium and glomerular mesangium of the kidney. C3 was not detectable in these deposits, and there was no histological evidence of an inflammatory response. Capsular polysaccharides appear to be only weak activators of complement. Other pneumococcal antigens may be more important in the pathogenesis of hypocomplementemia in pneumococcal infection.

Serum opsonic deficiency produced by Streptococcus pneumoniae and by capsular polysaccharide antigens

The opsonic requirements for phagocytosis of S. pneumoniae types 6, 7, 18, and 23 were determined in normal and C2 deficient serum, and in normal serum chelated with magnesium ethyleneglycoltetraacetic acid. All four strains were effectively opsonized via the alternative complement pathway, a finding suggesting that the capsular polysaccharides of these strains activated complement via the alternative pathway. Since bacteremic pneumococcal disease is often associated with circulating capsular polysaccharide, it was considered that this cellular component may activate complement in vivo and impair host defenses by producing an opsonic defect for pneumococci. To examine this hypothesis, serum was incubated with suspensions of whole S. pneumoniae types 6, 7, 18, or 23 or with purified capsular polysaccharide from each of these types, and residual complement activity and opsonic capacity were measured. Hemolytic C 3--9 complement activity and opsonic capacity for 3H-thymidine labeled Salmonella typhimurium, a species effectively opsonized via the alternative pathway, were reduced in serum following incubation. Polysaccharide concentrations as low as 1 microgram/ml inhibited serum opsonic capacity for salmonella. Whole pneumococci and pneumococcal capsular polysaccharide also inhibited the opsonic activity of human C2 deficient serum for salmonella, further evidence for activation of complement via the alternative pathway. Pneumococcal capsular polysaccharide markedly inhibited the opsonic capacity of normal serum for the homologous pneumoccal type. Thus, amounts of pneumococcal capsular polysaccharide, similar to those found in the serum of patients with pneumococcal disease, bring about decomplementation of serum via activation of the alternative pathway and inhibit pneumococcal opsonization.

Complement activating factor(s) of Trypanosoma lewisi: some physiochemical characteristics of the active components

Of the complement activating factors present in Trypanosoma lewisi, the major component, a carbohydrate containing substance was further investigated. This component was found to have a lag time of complete activation of 2 CH50 units of bovine complement of approximately 15 minutes while 1% trypsin (a known activator of complement, used as a control system) was capable of instant consumption of a similar quantity of complement. In addition, the complement activating factor of trypanosomes was observed to be stable at 100 degrees C for 15 minutes and over a pH range of 3.0 to 11.0. Thin layer chromatography studies suggested that at least part of the active component contained lipid, perhaps indicating that it may be glycolipid in nature.

Complement levels in pneumococcal pneumonia

Levels of complement proteins and functional activity of the alternate complement pathway were assessed in 39 patients with pneumococcal pneumonia. Mean levels of C3 and properdin and the functional activity of the alternate pathway in acute sera were significantly (P less than 0.05) below normal, whereas levels of components of the early classical pathway were normal. Although levels of factor B were in the normal range, they correlated significantly with C3 levels; there was no significant relation between C3 levels and C4 or C1q levels. The 19 patients iwth pneumococcal pneumonia and bacteremia had significantly lower mean values of properdin and factor B than the 20 patients without bacteremia, suggesting a more severe depression of the alternate complement pathway with bacteremia. During convalescence, complement levels were normal or elevated in most of the patients, but mean levels of properdin remained significantly below normal in bacteremic patients. Functional activity of the alternate pathway also remained below normal. These results indicate that there is a selective depression of the alternate pathway in patients with pneumococcal pneumonia, and they are consistent with the concept that the alternate pathway has an important role in host defenses in pneumococcal infection.

Classical and alternative complement pathway activation by pneumococci

Sixty-two strains of Streptococcus pneumoniae were studied for their abilities to consume selected components of classical and alternative complement pathways in human sera. The classical pathway was blocked by chelating calcium with ethyleneglycol-bios (beta-aminoethyl ether)-N,N-tetraacetic acid and by removing C4. The alternative pathway was blocked by removing factor B. Each strain's activation of the two pathways was compared with its nonimmune reactivity with the Fc region of immunoglobulin G (IgG). Activation of the classical complement pathway appeared to be independent of such Fc reactivity. Highly Fc-reactive strains, however, were shown to activate the alternative pathway more effectively than did less Fc-reactive strains. Since pneumococcal activation of the alternative pathway requires non-immunospecific IgG, these findings suggest that nonimmune binding of IgG on the pneumococcal surface endows it with complement-activating properties.

Effect of temperature on bacterial killing by serum and by polymorphonuclear leukocytes

Bacterial killing by serum alone and by polymorphonuclear )PMN) leukocytes was studied at 37 degrees C and compared with killing at 39 and 41 degrees C. The test organisms for serum killing were Staphylococcus aureus 502A (serum resistant) and Escherichia coli O14 (serum sensitive). The organisms used in PMN killing tests were Streptococcus pneumoniae type 29 and E. coli O86.S aureus was not killed by serum alone at any temperature. Changes in temperature did not affect the rate of serum killing of E. coli O14 for the first 60 min, but by 90 and 120 min there was a discrepancy with continued killing at 37 degrees C, but no further killing at 39 and 41 degrees C. PMN phagocytic killing of the pneumococcus was enhanced at 39 degrees C compared with 37 degrees C, and phagocytic killing of E. coli O86 was decreased at 41 degrees C when compared with 37 degrees C. Therefore, it appears that under certain circumstances fever may aid the host PMNs in destroying organisms, whereas under other circumstances it may interfere with such destruction.

C1 subcomponents in acute pneumococcal otitis media in children

Twenty children with acute pneumococcal otitis media were studied. In 6 children the infection ran a normal course and healed after the first episode and in 14 it relapsed. The serum levels of the immunoglobulins IgG, IgA and IgM were normal in all 20 children. Specific antibodies to pneumococcal polysaccharide were found in all cases, with no differences in the titers between the relapsed cases and those that healed. The complement components were quantitated with electroimmuno assay. G1q proved depressed in 60 per cent of the relapsed cases and in 16 per cent of the healed cases. C1r and C1s were disproportionally high compared with the C1q levels. Furthermore, crossed immunoelectrophoresis revealed abnormal complexes composed of C1r and C1s, and complexes composed of C1r, C1s and C1 IA. These complexes were more pronounced in sera from the children with relapsing otitis media.

Effects of pneumococcal mucopeptide and capsular polysaccharide on phagocytosis

Pneumoccal cell wall and capsular products from eight serotypes were tested for their ability to inhibit polymorphonuclear neutrophil killing of the same eight pneumococcal strains. Crude pneumococcal cell wall preparations from all serotypes inhibited phagocytic killing of several pneumococcal serotypes, and were just as effective with heterologous as with homologous strains. Phagocytosis dependent on heat-labile serum factors was inhibited, whereas phagocytosis not dependent on heat-labile factors was not significantly affected. These findings were compatible with inhibition of complement consumption. The inhibitory activity was found in a purified cell wall mucopeptide, whereas purified capsular polysaccharides failed to inhibit phagocytosis.