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Partial deficiency of the fourth component of human complement (C4) and autoantibody directed against C4 in a patient
Insights
A severe lupus patient had low C4 levels, revealing a familial partial C4 deficiency. An IgM autoantibody targeting C4 was identified, impacting complement system function.
Area of Science:
- Immunology
- Genetics
- Complement System Biology
Background:
- Systemic lupus erythematosus (SLE) is an autoimmune disease often associated with complement component deficiencies.
- Complement component 4 (C4) plays a crucial role in the classical complement pathway, essential for immune complex clearance.
Observation:
- A 17-year-old patient with severe SLE presented with markedly reduced serum C4 levels.
- Genetic analysis of the patient's family identified a C4A3,C4BQo haplotype, indicating a partial C4 deficiency inherited from the mother.
Findings:
- The patient's serum contained an autoantibody with anti-C4 specificity, characterized as IgM and belonging to the immunoconglutinin family.
- This autoantibody was found to influence the formation and dissociation kinetics of the classical C3-convertase, a key enzyme in complement activation.
Implications:
- This study highlights a novel link between inherited C4 deficiency, autoantibody production against C4, and SLE pathogenesis.
- Understanding the interaction of anti-C4 autoantibodies with complement components may offer new therapeutic targets for SLE and related autoimmune disorders.
Abstract:
The finding of dramatically depressed levels of C4 in a 17-year old patient with severe systemic lupus erythematosus (SLE) prompted a genetic study of her family. This study revealed the existence of a partial C4 deficiency; we found the presence of a C4A3,C4BQo haplotype which was transmitted by the mother to each of her children. This patient possessed, in her serum, an autoantibody with anti-C4 specificity. The immunochemical characterization of this autoantibody revealed that it was IgM and belonged to the immunoconglutinin family. We have studied the effects of this autoantibody on the formation and dissociation kinetics of classical C3-convertase.