Establishment of Gene Copy Number-Specific Normal Ranges for Serum C4 and Its Utility for Interpretation in Patients With Chronically Low Serum C4 Concentrations

Human Complement C4B Allotypes and Deficiencies in Selected Cases With Autoimmune Diseases

Human complement C4 is one of the most diverse but heritable effectors for humoral immunity. To help understand the roles of C4 in the defense and pathogenesis of autoimmune and inflammatory diseases, we determined the bases of polymorphisms including the frequent genetic deficiency of C4A and/or C4B isotypes. We demonstrated the diversities of C4A and C4B proteins and their gene copy number variations (CNVs) in healthy subjects and patients with autoimmune disease, such as type 1 diabetes, systemic lupus erythematosus (SLE) and encephalitis. We identified subjects with (a) the fastest migrating C4B allotype, B7, or (b) a deficiency of C4B protein caused by genetic mutation in addition to gene copy-number variation. Those variants and mutants were characterized, sequenced and specific techniques for detection developed. Novel findings were made in four case series. First, the amino acid sequence determinant for C4B7 was likely the R729Q variation at the anaphylatoxin-like region. Second, in healthy White subject MS630, a C-nucleotide deletion at codon-755 led to frameshift mutations in his single C4B gene, which was a private mutation. Third, in European family E94 with multiplex lupus-related mortality and low serum C4 levels, the culprit was a recurrent haplotype with HLA-A30, B18 and DR7 that segregated with two defective C4B genes and identical mutations at the donor splice site of intron-28. Fourth, in East-Asian subject E133P with anti-NMDA receptor encephalitis, the C4B gene had a mutation that changed tryptophan-660 to a stop-codon (W660x), which was present in a haplotype with HLA-DRB1*04:06 and B*15:27. The W660x mutation is recurrent among East-Asians with a frequency of 1.5% but not detectable among patients with SLE. A meticulous annotation of C4 sequences revealed clusters of variations proximal to sites for protein processing, activation and inactivation, and binding of interacting molecules.

New biomarkers in SLE: from bench to bedside

Biomarkers may have a diagnostic or monitoring value, or may predict response to therapy or disease course. The aim of this review is to discuss new serum and urinary biomarkers recently proposed for the diagnosis and management of SLE patients. Novel sensitive and specific assays have been proposed to evaluate complement proteins, ‘old’ biomarkers that are still a cornerstone in the management of this disease. Chemokines and lectins have been evaluated as surrogate biomarkers of IFN signature. Other cytokines like the B cell activating factor (BAFF) family cytokines are directly related to perturbations of the B cell compartment as key pathogenetic mechanism of the disease. A large number of urine biomarkers have been proposed, either related to the migration and homing of leukocytes to the kidney or to the local regulation of inflammatory circuits and the survival of renal intrinsic cells. The combination of traditional disease-specific biomarkers and novel serum or urine biomarkers may represent the best choice to correctly classify, stage and treat patients with SLE.

Deregulated cellular circuits driving immunoglobulins and complement consumption associate with the severity of COVID-19 patients

SARS‐CoV‐2 infection causes an abrupt response by the host immune system, which is largely responsible for the outcome of COVID‐19. We investigated whether the specific immune responses in the peripheral blood of 276 patients were associated with the severity and progression of COVID‐19. At admission, dramatic lymphopenia of T, B, and NK cells is associated with severity. Conversely, the proportion of B cells, plasmablasts, circulating follicular helper T cells (cTfh) and CD56^–CD16⁺ NK‐cells increased. Regarding humoral immunity, levels of IgM, IgA, and IgG were unaffected, but when degrees of severity were considered, IgG was lower in severe patients. Compared to healthy donors, complement C3 and C4 protein levels were higher in mild and moderate, but not in severe patients, while the activation peptide of C5 (C5a) increased from the admission in every patient, regardless of their severity. Moreover, total IgG, the IgG1 and IgG3 isotypes, and C4 decreased from day 0 to day 10 in patients who were hospitalized for more than two weeks, but not in patients who were discharged earlier. Our study provides important clues to understand the immune response observed in COVID‐19 patients, associating severity with an imbalanced humoral response, and identifying new targets for therapeutic intervention.

Complement Components, C3 and C4, and the Metabolic Syndrome

INTRODUCTION

Increased systemic inflammation plays a significant role in the development of adult cardiometabolic diseases such as insulin resistance, dyslipidemia, atherosclerosis, and hypertension. The complement system is a part of the innate immune system and plays a key role in the regulation of inflammation. Of particular importance is the activation of complement components C3 and C4. C3 is produced primarily by the liver but is also produced in adipocytes, macrophages and endothelial cells, all of which are present in adipose tissues. Dietary fat and chylomicrons stimulate C3 production. Adipocytes in addition to producing C3 also have receptors for activated C3 and other complement components and thus also respond to as well as produce a target for complement. C3adesArg, also known as acylation stimulation factor, increases adipocyte triglyceride synthesis and release. These physiological effects play a significant role in the development of metabolic syndrome. Epidemiologically, obese adults and non-obese adults with cardiometabolic disease who are not obese have been shown to have increased complement levels. C4 levels also correlate with body mass index. Genetically, specific C3 polymorphisms have been shown to predict future cardiovascular events and. D decreased C4 long gene copy number is associated with increased longevity.

CONCLUSION

Future research is clearly needed to clarify the role of complement in the development of cardiovascular disease and mechanisms for its action. The complement system may provide a new area for intervention in the prevention of cardiometabolic diseases.

Insights on the relationship between complement component C4 serum concentrations and C4 gene copy numbers in a Western Australian systemic lupus erythematosus cohort

The relationship between serum concentration of complement C4 ([C4]) and C4 gene copy number (GCN) was investigated in 56 systemic lupus erythematosus (SLE) patients and 33 age and sex-matched controls in a Western Australian population. C4A and C4B gene copy numbers (C4A & B GCN) together with the presence or absence of the ≈6.4-kb human endogenous retroviral element type K (hereafter HERV-K) in intron 9 were estimated by two TaqMan™ real-time PCR (RT-PCR) assays that measured total C4 and HERV-K GCNs, respectively. There was good correlation between the two methods; however, the HERV-K GCN method showed a positive bias (≈6%) relative to the C4A & B total GCN. Despite individual variation, excellent correlation between total C4 GCN and mean [C4] per GCN was observed for both the SLE and control cohorts ( R²= 88% and R²= 99%, respectively). It was noted that serum [C4] was significantly lower in the SLE patients than the controls ( p = 0.006) despite there being no difference between C4A and C4B GCN in both cohorts. The data therefore confirm previous reports that the C4A genes are preferentially associated with the presence of the HERV-K insertion relative to C4B genes and does not support the hypothesis that low [C4] in SLE is explained by low C4A GCNs. There was no evidence also that the presence of the HERV-K insertion in C4 genes influenced [C4]. This study supports the view that low [C4] in SLE patients is due to consumption rather than deficient synthesis related to lower C4A & B GCN.

Clinical features of patients with homozygous complement C4A or C4B deficiency

INTRODUCTION

Homozygous deficiencies of complement C4A or C4B are detected in 1-10% of populations. In genome-wide association studies C4 deficiencies are missed because the genetic variation of C4 is complex. There are no studies where the clinical presentation of these patients is analyzed. This study was aimed to characterize the clinical features of patients with homozygous C4A or C4B deficiency.

MATERIAL AND METHODS

Thirty-two patients with no functional C4A, 87 patients with no C4B and 120 with normal amount of C4 genes were included. C4A and C4B numbers were assessed with genomic quantitative real-time PCR. Medical history was studied retrospectively from patients' files.

RESULTS

Novel associations between homozygous C4A deficiency and lymphoma, coeliac disease and sarcoidosis were detected. These conditions were present in 12.5%, (4/32 in patients vs. 0.8%, 1/120, in controls, OR = 17.00, 95%CI = 1.83-158.04, p = 0.007), 12.5% (4/32 in patients vs. 0%, 0/120 in controls, OR = 1.14, 95%CI = 1.00-1.30, p = 0.002) and 12.5%, respectively (4/32 in patients vs. 2.5%, 3/120 in controls, OR = 5.571, 95%CI = 1.79-2.32, p = 0.036). In addition, C4A and C4B deficiencies were both associated with adverse drug reactions leading to drug discontinuation (34.4%, 11/32 in C4A-deficient patients vs. 14.2%, 17/120 in controls, OR = 3.174, 95%CI = 1.30-7.74, p = 0.009 and 28.7%, 25/87 in C4B-deficient patients, OR = 2.44, 95%CI = 1.22-4.88, p = 0.010).

CONCLUSION

This reported cohort of homozygous deficiencies of C4A or C4B suggests that C4 deficiencies may have various unrecorded disease associations. C4 gene should be considered as a candidate gene in studying these selected disease associations.

The role of complement inhibition in kidney transplantation

INTRODUCTION AND BACKGROUND

The complement system which belongs to the innate immune system acts both as a first line of defence against various pathogens and as a guardian of host homeostasis. The role of complement has been recently highlighted in several aspects of kidney transplantation: ischaemia-reperfusion, antibody-mediated rejection and native kidney disease recurrence.

SOURCES OF DATA

Experimental data, availability of complement-blocking molecules (mainly the anti-C5 monoclonal antibody, eculizumab) and several trials in human kidney transplant recipients has led to some areas of agreement and some disappointment.

AREAS OF AGREEMENT AND CONTROVERSIES

So far, eculizumab has shown great efficacy in treatment and prevention of atypical haemolytic and uraemic syndrome, some efficacy in the prevention of antibody-mediated and so far no efficacy in the prevention of delayed graft function.

GROWING POINTS

Among the numerous potentially available drugs potentially interfering with complement, recent focus has been made on C1 blockers in the setting of antibody-mediated rejection with promising results.

AREAS TIMELY FOR DEVELOPING RESEARCH

Complement is now recognized as a major player in transplant immunology, several targets are going to be tested to define precisely which ones may be potentially useful in clinical practice.

SLE and Serum Complement: Causative, Concomitant or Coincidental?

BACKGROUND

Systemic Lupus Erythematosus (SLE) is an incurable autoimmune disorder with complement activation playing a key role in the pathogenesis of immune-mediated tissue injury. While quantifying complement to monitor SLE disease activity has been the standard of care since the 1950s, decreased complement levels are not consistently associated with flares.

OBJECTIVE

We seek to clarify the SLE phenotype in which complement deficiency is causative, concomitant, or coincidental.

METHODS

A PUBMED literature review was conducted using key words 'complement,' 'SLE,' and 'SLE flares' in English-only journals from 1972-2017. Relevant clinical studies and review articles were found that examined the measurement of complement levels in SLE, and more specifically, interpretation of low serum complement levels regardless of disease activity.

CONCLUSION

Complement activation plays a key role in the pathophysiology of SLE and it is recommended to continue monitoring serum levels of C3 and C4 to assess for disease activity. However, it is important to note that decreased serum complement is not consistently associated with disease flares.It is clinically important to find novel ways to assess disease activity in SLE. Reduced serum levels of cell-bound complement activation products may more accurately reflect disease activity than conventional serum C3 and C4 monitoring.