Homozygous C2 deficiency (C2D) is an important and fairly common form of complement deficiency in individuals of European descent23,52,74. The C2 gene is located in the middle of the major histocompatibility complex (MHC) class III region together with the genes for C4 and factor B75. Two principal variants of C2D have been distinguished. C2D type I is characterized by the absence of detectable C2 synthesis, while C2D type II is caused by a selective block of C2 secretion. The main cause of C2D type I is a 28-bp deletion in the C2 gene of the human leukocyte antigen-B*18,S042,DRB1*15 MHC haplotype10,34. This mutation is thought to account for more than 90% of all C2D cases. Rare C2D alleles have been identified in conjunction with other MHC haplotypes48.
The function of C2 in the complement cascade is to provide the catalytic subunit of the C3 convertase C4b2a72. C4b2a can be generated through the classical pathway (C1qr2s2, C4, C2), which is the principal mechanism for antibody-dependent activation of complement, and through the lectin pathway, which supports innate immunity. The lectin pathway uses mannan-binding lectin (MBL) and ficolins for recognition of defined carbohydrate structures41,71. MBL-associated serine proteases (MASPs) form complexes with MBL and ficolins and are activated when the recognition molecules bind to appropriate targets. The C1s moiety of the C1qr2s2 complex and MASP-2 in MBL/MASP or ficolin/MASP complexes specifically cleave C4 and C253. Thus, activation of complement through the classical pathway and lectin pathway is impaired in C2D. By contrast, the alternative activation pathway (factor B, factor D, and properdin) is intact72, and C1-dependent and MBL-dependent C2 bypass mechanisms may contribute to complement activation in C2D36,40.
C2-deficient persons may be healthy1, but C2D is primarily known to be associated with systemic lupus erythematosus (SLE) and SLE-like disease, and with susceptibility to invasive infections caused by encapsulated bacteria23,48,52,62,74. Information concerning disease associations in C2D is based mainly on descriptions in the literature of individual patients and families, which implies that conclusions may be biased at many levels3. Among 107 reported cases of C2D, 32% of the patients had SLE or SLE-like disease and 22% had at least 1 episode of invasive infection caused by encapsulated bacteria23,52. Other diseases have also been reported in conjunction with C2D.
This investigation concerns the clinical findings in 40 persons with C2D identified in Sweden between 1977 and 2002. Medical records were reviewed, giving long observation times for most patients. Although effects of patient selection were certainly operative, the identified cohort should provide an improved basis for discussion of the clinical consequences of C2D. The findings suggest that the importance of severe infection is underrated in C2D. Furthermore, the patients demonstrated an increased rate of cardiovascular disease that appeared to be independent of rheumatic disease manifestations.
PATIENTS AND METHODS
Screening for detection of complement deficiency by hemolysis in gel64,70 as a routine part of complement analysis was initiated at the Clinical Immunology Unit, University Hospital of Lund, Lund, Sweden, by the end of the 1970s. Since that time, screening has been performed with samples from about 40,000 consecutive patients covering a broad spectrum of clinical conditions. Between 1977 and 2002, 40 Swedish patients with C2D were identified (Tables 1 and 2). To our knowledge, no other Swedish patients with C2D were diagnosed during this period. The patients were retrieved from hospital departments of internal medicine, infectious diseases, rheumatology, dermatology, pediatrics, and otorhinolaryngology, and from general and private practice. Seventeen C2D patients were found in Scania, a province in southern Sweden for which the laboratory provides primary service with regard to complement analysis. Patients 1, 19, 20, 21, 24, 25, 27, and 30 were treated at the University Hospital of Lund; Patients 2, 3, 17, and 18 at the Hospital of Ängelholm; Patients 4 and 28 at the University Hospital of Malmö; Patients 8 and 9 at the Hospital of Kristianstad; and Patient 35 at the Hospital of Helsingborg.
From the rest of Sweden, samples either were sent directly to our laboratory for complement analysis or were referred from Clinical Immunology laboratories (Karolinska Hospital, Huddinge University Hospital, Sahlgrenska University Hospital, and the University Hospital of Örebro) following initial screening for detection of complement deficiency. In the Stockholm area, Patient 7 was treated at the Sachs Children's Hospital, Patient 11 at the South Hospital, Patient 23 at the Karolinska Hospital, and Patient 40 at the Astrid Lindgren Children's Hospital. Patient 36 was treated in private practice. Patients 14, 26, 31, 32, and 33 were treated at Sahlgrenska University Hospital, Gothenburg; Patients 37, 38, and 39 at Skövde Hospital; Patient 29 at Uddevalla Hospital; Patient 12 at Trollhättan Hospital; Patient 16 at Jönköping Hospital; Patient 34 at Växjö Hospital; Patient 6 at Norrköping Hospital; Patient 22 at Örebro Hospital; Patient 5 at Boden Hospital; Patient 10 at Umeå University Hospital; Patient 13 at Skellefteå Hospital; and Patient 15 at Härnösand Hospital.
Medical records were reviewed and discussed with patients' physicians. The multicenter study was approved by the Lund University Research Ethics Committee (LU 513-01), and ethics committees of the 6 other centers involved. The study was based on written informed consent. One patient did not permit review of his medical records beyond the age of 11 years.
Information concerning the number of inhabitants in the province of Scania and in Sweden was obtained from Sweden's Statistical Databases, Stockholm, Sweden. Data from the Swedish National Board of Health and Welfare registries of disease and causes of death were also used.
Blood samples were obtained from the patients and from first-degree relatives. Serum and EDTA plasma were stored in aliquots at −80°C. In 5 C2D patients, the small sample volumes available restricted extended analysis. Screening for detection of complement deficiency was performed with hemolytic gel assays using sensitized sheep erythrocytes for the classical pathway and guinea pig erythrocytes for the alternative pathway70. C2 and most other complement proteins were quantified by electroimmunoassay35,37. C3 and C4 were determined by turbidimetry (Cobas Mira, Roche Diagnostica, Basel, Switzerland). C2 concentrations were given in mg/L assuming that the pooled normal serum used for reference contained C2 at 26 mg/L17. The immunoglobulins IgM, IgG, and IgA were determined by turbidimetry using age-related reference areas6,67. Screening for antinuclear antibodies (ANA) was performed by indirect immunofluorescence with HEp-2 cells (Euroimmun, Lübeck, Germany) at patient serum dilutions 1/100 and 1/400. This corresponds to detection of ANA at 3.5 and 14 IU/mL (World Health Organization reference serum 66/233). The diagnostic ANA titer was 400, as established by determination of the 96.5 percentile for negativity in healthy blood donors (98 females, 98 males). DNA was extracted from whole blood according to standard procedures43. Detection of the 28-bp deletion associated with C2 deficiency type I was done by polymerase chain reaction (PCR) amplification69. DR typing was performed using a PCR-technique (Olerup SSP AB, Saltsjöbaden, Sweden). C4 was phenotyped according to established procedures69.
To assess the risk of acute myocardial infarction (AMI) in C2D, we had to take into account the known influence of age and gender. Therefore, persons in the C2D cohort between 30 and 79 years of age during the follow-up period 1940-2000 were used as the observed population (Figure 1). Twenty-five persons could be observed and attributed with person-time at risk during this period. The number of person-years at risk during the follow-up period was summarized for men, women, all persons, and those diagnosed with SLE. A person was considered as being at risk only until his or her first AMI was recorded. The 5 patients with AMI had their first AMI between 1975 and 1999. Age- and gender-specific data on mean AMI incidence available during 1987-2001 were used to obtain the number of cases with their first AMI that could be expected to occur in the C2D cohort during the follow-up period if the cohort had the same incidence of AMI as an imaginary cohort from the general Swedish population with the same size and the same age and gender distribution. Calculations of the standardized mortality/morbidity ratios were made with exact confidence intervals based on the Poisson distribution. Statistical significance is considered when the lower limit of the 95% CI is 1.0 or above54,55.
A cohort of 40 Swedish citizens with C2D belonging to 33 apparently unrelated white families was identified in the course of 25 years (see Tables 1 and 2): 23 females and 17 males. The mean age at the time C2D was diagnosed was 31 years (range, 17-76 yr; median, 34 yr). At the end of the study, the mean age was 52 years (range, 1-79 yr; median, 44 yr). Nine patients were younger than 18 years when C2D was diagnosed (range, 1-16 yr). Medical records were reviewed covering a total of 1560 person-years (see Figure 1) including laboratory results, radiologic findings, clinical physiology investigations, and autopsy reports. Medical records covering 96% of the accumulated person-years were reviewed, giving a mean observation time of 39 years (range, 1-77 yr).
C2 was not detectable in sera (<0.5 mg/L) from the 40 persons with C2D, while other complement proteins were present at essentially normal concentrations. Heterozygous first-degree relatives (n = 25) showed C2 in the range between 7.5 and 19.5 mg/L (reference area, 20.0-41.3 mg/L).
Thirty-three persons with C2D were found to be homozygous for the 28-bp deletion34 and were also homozygous for DRB1*15 and C4A*4 B*2. For 1 patient (Patient 39), who died of fulminant infection, DNA was not available. However, the 28-bp deletion was present in both parents, which implied that the patient was homozygous for this defect. Three persons with undetectable C2 (Patients 19, 37, 38) were heterozygous for the 28-bp deletion indicating compound heterozygosity for C2D involving a second mutation, distinct from the 28-bp deletion. Patient 19 showed DRB1 15, 11 and the C4 phenotype C4A 3,4 B 1,2, suggesting that the second mutation was present on a haplotype containing DRB1*11 and C4A*3 B*1. Patients 37 and 38 were brothers, and showed DRB1 1,15 and the C4 phenotype C4A4 B2. Thus, the second mutation was probably present on a haplotype containing DRB1*1 and C4A*4 B*2. The mutations in Patient 19 and in the brothers, Patients 37 and 38, did not appear to be part of MHC haplotypes that have been described previously in conjunction with C2 null genes48. In 3 deceased patients (Patients 1, 10, 14) investigations for the 28-bp deletion and MHC typing could not be performed.
The concentrations of IgM, IgG, and IgA were normal in 35 patients from whom appropriate samples were available. According to medical records the 5 remaining patients did not have hypogammaglobulinemia. ANA at a diagnostic level (titer ≥ 400) were found in 3 patients with SLE. ANA at a low level (titer 100) was found in 4 SLE patients.
The occurrence of infection was the most prominent clinical manifestation in the C2D cohort. In an attempt to describe the wide range of severity of infection in individual patients, we divided the 40 patients into 4 groups. One group (n = 10) consisted of patients (Patients 1, 8, 9, 10, 18, 20, 31, 32, 33, 38), who had a history of minor infections including recurrent otitis media, sinusitis, throat infections, and infections of the respiratory tract. A second group of patients (n = 7) (Patients 2, 3, 14, 19, 23, 27, 30) had documented minor infections and at least 1 episode of pneumonia. A third group (n = 11) (Patients 4, 5, 12, 21, 25, 26, 29, 35, 36, 39, 40) had 1 invasive infection combined with a history of pneumonia and other infections. The fourth group (n = 12) (Patients 6, 7, 11, 13, 15, 16, 17, 22, 24, 28, 34, 37) had at least 2 invasive infections.
Eleven female patients and 8 males had a history of pneumonia. In 6 patients the bacteriologic cause was established by blood culture demonstrating Streptococcus pneumoniae. Recurrent pneumonia (2 episodes or more) was documented in 7 female patients and 3 males. One patient (Patient 11) was treated for 57 episodes of pneumonia, requiring hospitalization 15 times.
A history of meningitis was obtained in 4 female patients and 6 males. Four of these patients had 2 episodes of meningitis (Table 3). The predominant cause of meningitis was S. pneumoniae (64%), followed by Neisseria meningitidis (14%), Streptococcus agalactiae (group B streptococci) (14%), and Haemophilus influenzae (7%). The mean age of the patients at the time of meningitis episode was 10 years (range, 3 wk-51 yr; median, 4 yr).
Septicemia was documented in 8 female patients and 10 males (Table 4). Two episodes or more were found in 5 patients. Positive blood cultures were obtained in 21 of the 23 episodes. The predominant etiologic agent was S. pneumoniae (52%), followed by Staphylococcus aureus (13%). The other bacteria found were S. agalactiae, N. meningitidis, H. influenzae type b, Kingella kingae, Stenotrophomonas maltophilia, and an Enterococcal species, each as a single isolate. The origin or infectious focus could be established in 17 of the 23 septic episodes. The mean age of the patients at the time of septicemia episode was 26 years (range, 1 mo-75 yr; median, 24 yr). In 3 patients, the precise age at the time of septicemia was not recorded.
The invasive infections among the 23 patients were not restricted to septicemia or meningitis (n = 21). Osteitis (Patients 5, 6, 11), pyelonephritis (Patients 13, 24), and peritonitis (Patient 26), were also classified as invasive infections. Some of these patients were also documented as having 1 or several episodes of septicemia or meningitis (Patients 6, 11, 13, 24).
Documented minor infections usually occurred during infancy (1-23 mo) and childhood (2-12 yr), and ceased during adolescence (13-18 yr). Nine of the 18 patients with pneumonia had their first pneumonia during infancy or childhood. Among the 18 patients with septicemia, the first episode occurred during infancy and childhood in 10 patients. Nine of the 10 patients with meningitis experienced the first episode during infancy and childhood. Eighty percent of the patients with recurrent episodes of septicemia or meningitis were under the age of 18 years. During adolescence, only 1 invasive infection (meningitis) was recorded. Among adults, invasive infections or pneumonia occurred in 18 patients.
Ten patients (7 female and 3 male) fulfilled 4 or more of the 1982 American College of Rheumatology (ACR) classification criteria for SLE. The mean age at onset of SLE was 34 years (median, 32 yr). ANA were demonstrated at diagnostic levels in 3 and at a low level in 4 of the SLE patients. In Patient 10, the presence of ANA was documented in the medical records. The low prevalence of ANA in SLE patients with C2D agrees with results of previous investigations48.
Four patients (Patients 5, 25, 27, 33) had undifferentiated connective tissue disease or incomplete SLE (<4 ACR criteria). Another 3 patients (Patients 24, 31, 34) had vasculitis with skin manifestations, verified by biopsy in 2 cases.
Severe infections occurred in some of the patients, and 1 patient with SLE (Patient 12) died of an invasive pneumococcal infection. In 4 SLE patients, serious infections predated the onset of SLE by several decades. There was no relationship between susceptibility to infections and the presence of rheumatologic disease (Table 5).
Cardiovascular disease occurred at a high rate. Three male patients (Patients 3, 10, 26) and 2 females (Patients 1, 5) had a total of 10 AMI episodes. Calculations concerning the risk for AMI in the C2D cohort showed a statistically significant increase that was about 4.0 times higher than that found in the general Swedish population (Table 6). Two AMI episodes were documented in a woman with SLE (Patient 1). AMI also occurred in a male patient with SLE (Patient 3). The increased AMI rate was statistically significant in the group of patients with SLE, undifferentiated connective tissue disease, or vasculitis, but not in the group with SLE or in patients without rheumatologic disease (see Table 6). None of the 5 patients with AMI was a tobacco smoker, or had diabetes or hyperlipemia11,16,32,42. Two patients (Patients 10, 26) had well-regulated hypertension11. The mean age at the first AMI was 58 years (range, 33-77 yr; median, 57 yr). A cerebrovascular accident occurred once in a female with SLE (Patient 1) and 4 times in a male without SLE (Patient 26). These 2 patients had additional cardiovascular manifestations including acute aorta dissection, AMI, congestive heart failure, and hypertension. Cardiac valve insufficiency was seen in 3 patients (Patients 5, 11, 12). Angina pectoris (Patient 5), atrial fibrillation (Patient 11), and venous thrombosis (Patient 34), were each seen once. Hypertension was found in 5 patients (Patients 10, 12, 14, 21, 26).
Other Disease Manifestations
Disease categories of clinical manifestations other than those related to infectious, cardiovascular, or rheumatologic diseases are summarized in Table 7. In the cohort, 1 male (Patient 35) and 2 female patients (Patients 8, 30) had asthma. The prevalence of allergy and eczema was low. One patient was diagnosed with pyoderma gangrenosum. Abdominal surgery had been performed in 7 patients (appendicitis, cholecystitis, and inguinal hernia). Six patients had gastroenterology-related manifestations: gastritis (2/40), pancreatitis (2/40), proctitis (1/40), and chronic diarrhea (1/40). Cancer was documented in 4 patients (Patients 11, 14, 26, 34).
Cause of Death
Nine C2D patients died during the observation period. Infections (pneumonia, meningitis, and septicemia) accounted for death in 5 patients. AMI was established as a cause of death in 3 patients, and 1 patient died of breast cancer. Of the 9 patients who died, 4 patients had SLE: 2 died of infection, 1 died of AMI, and 1 died of lung cancer.
Manifestations in First-Degree Relatives
Family studies were performed in 18 families, resulting in identification of 7 nonindex persons with C2D among the first-degree relatives (Table 8). A female SLE patient (Patient 2) had a C2-deficient brother (Patient 3), who had a history of minor infection and pneumonia at the time of the initial investigation, but developed SLE 10 years later. Another index patient (Patient 24) with severe pneumococcal pneumonia had a sister with a past history of meningococcal meningitis (Patient 25). A 5-year-old girl (Patient 20) was investigated for complement function due to transient hematuria, the cause of which was never established. Her history included recurrent otitis media. Her mother, who was also found to be C2 deficient (Patient 21), had a history of septicemia. Four additional nonindex cases of C2D had a history of minor infections, but no significant manifestations of disease. One of these was a 5-year-old child with a short observation time.
Family histories revealed the occurrence of severe infections in family members, most of whom were not investigated for complement function. Patients 6 and 26 had first-degree relatives who had died of meningitis. The father of Patient 20 was a heterozygous carrier and had a history of meningitis as an adult. The patient's grandfather died of meningitis.
With the exception of the father of Patient 20, none of the 25 heterozygous carriers identified had a history of invasive infection or rheumatologic disease conditions.
Even in the absence of immunosuppressive treatment, 79% of patients with SLE develop serious infections during the course of their disease51. Treatment with corticosteroids further increases the incidence of infections28. Two SLE patients (Patients 11, 12) were treated with corticosteroids at 2.5-20.0 mg per day at the time of the invasive infections. Patient 12 developed proliferative glomerulonephritis (World Health Organization grade IV) during his last year of life. Plasma exchange and intravenous pulse cyclophosphamide treatment were tried without success. He died of septicemia and meningitis shortly after. Patient 34 had received treatment with corticosteroids and cyclophosphamide for glomerulonephritis, but not at the time of the 2 septicemia episodes that he experienced. Patients 4 and 5 developed invasive infections during corticosteroid therapy.
The principal finding in the current study is that severe infection was the predominant clinical manifestation among C2D patients: 57% of the patients had a past history of invasive infections, and 30% had repeated infections of this kind. In addition, pneumonia was a frequent finding. To some extent, the predominance of infections in the current study compared with previously published data23,52 may be explained by patient selection at the clinical level and by long observation times. More likely, the importance of C2D as a basis for susceptibility to infection has not been appreciated fully in the literature. Effects of patient selection were probably stronger for SLE, which was present in 25% of the cohort, in contrast to the prevalence of 10% for SLE in C2D proposed in the year 200048. It is also noteworthy that 8 of our SLE patients were identified in 1977-1990 (see Table 1) and only 2 SLE patients were identified in 1993-2002 (see Table 2). Another 18% of our patients had undifferentiated connective tissue disease or vasculitis. The association we found between C2D and cardiovascular disease could hardly have been influenced by patient selection. Some clinical manifestations that have been reported in C2D and other complement deficiency states, such as anaphylactoid purpura23,52,65, acute glomerulonephritis27, and membranoproliferative glomerulonephritis23,52, were not observed among our patients.
Many persons with C2D are known to be essentially healthy or to have limited clinical problems with a questionable relationship to C2D. Consistent with this, we found that 4 of 7 first-degree relatives with C2D did not have major clinical problems, with reservation for a short period of observation in at least 1 of the nonindex cases. Severe infections were found in a few additional first-degree relatives. The data suggest that C2D might be associated with significant disease in perhaps 50% of the cases, and that the most important category that tends to be overlooked is the group of patients with severe infections. The absence of conditions such as classical rheumatoid arthritis in C2D62 might be due to protective effects of linked MHC genes2 or of the complement deficiency.
Estimates of the prevalence of C2D have been made by determining the allele frequency of C2 null genes, either by measurement of C2 or by detection of the 28-bp deletion of the C2 gene in western countries. The results have suggested prevalences of C2D in the approximate range between 1:13,500 and 1:40,00048. Of the 17 patients with C2D retrieved from Scania, 6 had SLE. Considering current diagnostic practices in Scania, it is possible that all or nearly all Scanian SLE patients with C2D were found. Pickering et al48 have proposed that development of SLE in C2D occurs in the order of 10%. With the finding of 6 SLE patients with C2D in Scania, this suggests that about 60 persons might have C2D in the province, which has 1.2 million inhabitants, corresponding to a C2D prevalence of about 1:20,000. Extrapolated to the Swedish population of about 9 million, prevalences ranging between 1:40,000 and 1:20,000 would correspond to 225-450 Swedish C2D cases. We found 40 patients, which further emphasizes that C2D is often overlooked.
The reasons for the development of SLE in classical pathway deficiencies have been subject to extensive study48,68. Current data favor the hypothesis that autoimmunity in the disease is triggered by impaired complement-dependent elimination of apoptotic cells. Impaired handling of immune complexes might also play an important pathogenetic role.
The role of complement in cardiovascular disease is ambiguous. Animal experiments suggest that deficiency of late complement components protects against the development of atherosclerosis8,26. Conversely, genetically engineered C3-deficient mice have been shown to develop atherosclerosis at an increased rate14. MBL deficiency has been reported to be implicated in the disease process, either through increased susceptibility to infection caused by Chlamydia pneumoniae or by involvement of the lectin pathway in inflammation39,57. In recent prospective studies, MBL deficiency has been shown7 to be associated with coronary artery disease in American Indians and with arterial thrombosis in SLE47. C2 participates in C3 activation through the lectin pathway71, which might be the cause for the development of atherosclerosis in C2D. The 4-fold increase of the risk for a first AMI found in the C2D cohort is comparable to the increased risk of a coronary event in tobacco smokers42, which is one of the strongest independent predictors of premature coronary heart disease15. Patients with established atherosclerotic disease have a 5- to 7-fold increased risk of recurrent AMI compared with the general population15. The frequent occurrence of cardiovascular disease in C2D was unexpected, and due to the design of the present study, analysis according to the Framingham coronary risk profile was not performed5,18.
Experimental studies in genetically engineered C1q-deficient mice have emphasized the importance of genetic background on disease expression of complement deficiencies44. Background genes are likely to contribute to the heterogeneity of the human C2D phenotype, but this has not yet been demonstrated. As C2D is nearly always caused by the homozygous presence of a C2 gene containing a specific mutation3,75, immunologic properties are probably more uniform in C2D than in other complement deficiencies due to the presence of strongly linked MCH genes69. C2D and other deficiencies of the classical pathway are associated with IgG subclass aberrations9 that do not function as markers for susceptibility to infection or other manifestations of C2D4. Common variable immunodeficiency61, IgG allotypes63,76, and impaired alternative pathway function45,58 have been discussed in relation to susceptibility to infection in C2D.
In accordance with previous studies22,23, S. pneumoniae was found to be the predominant cause of infection in C2D. N. meningitidis, H. influenzae type b, and other bacteria were identified in comparatively few patients. Two patients, 1 of whom has been reported before59, had neonatal infections with S. agalactiae. Another child with S. agalactiae infection and C2D has also been described20. It is noteworthy that repeated episodes of pyelonephritis occurred in 4 patients belonging to the group with a history of septicemia and meningitis.
The reasons for impaired immunity in C2D are not altogether clear. Conversely, immunity is evidently sustained by C2-independent mechanisms in many patients. Experiments with genetically engineered mouse strains suggest that the classical pathway supports innate immunity to S. pneumoniae and S. agalactiae13,73. Earlier studies of C4-deficient guinea-pigs suggested that innate immunity to S. pneumoniae is a function of the alternative pathway12. The lectin pathway, which involves activation of C3 by C4b2a might also contribute to innate immunity mechanisms71; deficiency of MBL has been reported to be associated with susceptibility to pneumococcal56 and meningococcal disease31. Furthermore, a recently described patient with MASP-2 deficiency had repeated severe pneumococcal infections66. On the other hand, the findings of Brown et al13 do not suggest that the lectin pathway contributes to defense against S. pneumoniae.
The frequent restriction of severe infections to infancy and childhood in C2D indicates that acquired immunity is operative, and in accord with this it has been suggested that vaccination might be helpful22,23. Acquired immunity in C2D could be accomplished by antibodies capable of recruiting the alternative pathway33,38,50,59. Target-bound IgG can serve as a protected site for assembly of alternative pathway C3 convertase25, and antibodies to capsular sialic acid of S. agalactiae have been shown to promote opsonization by blocking sialic acid-mediated down-regulation of the alternative pathway21. Antibodies could also contribute to immunity by C1-dependent C2-bypass activation of the alternative pathway36. On the other hand, the importance of the alternative pathway in acquired immunity to S. pneumoniae might be questioned, based on animal experimental data12.
Antibodies might also support immunity through complement-independent mechanisms. Among these, phagocyte Fc-receptors interacting with antibodies of the IgG2 isotype have been shown to be important in deficiencies of the late complement components24,49.
Impaired function of the classical pathway can limit antibody production46, which is explained by the adjuvant effect of C3d fragments on the immune response19. The significance of this for immune defense in C2D is not known. However, available evidence does not suggest that antibody responses to encapsulated bacteria are grossly impaired in C2D29,59,60.
In conclusion, retrospective analysis of Swedish patients with C2D revealed that invasive infections occurred at a high rate. We believe the importance of C2D as an immunodeficiency predisposing to severe infection is not fully recognized. The findings also confirm the well-known association between C2D and SLE, and provide novel evidence for a possible role of C2D in the development of atherosclerosis. C2D has approximately the same prevalence as common variable immunodeficiency30, and perhaps should be considered more often in the context of immunodeficiencies, as well as in atherosclerotic and inflammatory diseases.
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