Beta chain deficiency in three patients with dysfunctional C8 molecules.

Area of Science:

• Immunology
• Complement System Biology

Background:

• Complement C8 is a crucial component of the terminal complement pathway, essential for cell lysis.
• Dysfunctional C8 can lead to increased susceptibility to certain infections.

Purpose of the Study:

• To investigate the structural and functional characteristics of a dysfunctional complement C8 molecule found in multiple individuals.
• To identify the specific subunit defect responsible for the loss of C8 hemolytic activity.

Main Methods:

• Immunoprecipitation of C8 from patient sera using antihuman C8 antiserum.
• Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis.
• Functional assays involving reconstitution of hemolytic activity with purified C8 subunits (alpha-gamma dimer and beta chain).
• Binding studies using EAC1-7 and soluble C5b-7 complexes.

Main Results:

• Sera from affected individuals contained a dysfunctional C8 molecule with partial immunologic identity to normal C8 but lacking hemolytic activity.
• SDS-PAGE analysis revealed the selective absence of a 62,000 mol. wt band in the dysfunctional C8 immunoprecipitates.
• Hemolytic activity was restored by adding the purified beta subunit to C8-deficient sera, but not by the alpha-gamma dimer.
• The dysfunctional C8 did not interfere with the binding of normal C8 to C5b-7 or the subsequent lytic process.

Conclusions:

• The dysfunctional C8 molecule is characterized by the absence of a specific subunit (62,000 mol. wt), likely the beta chain.
• The beta chain is essential for the functional activity of complement C8 in mediating cell lysis.
• This defect does not impair the assembly of the membrane attack complex but rather the functional execution of lysis.