Circulating immune complex-like materials which bind to heat inactivated C1q interfere with the C1q solid phase assay...

Area of Science:

• Immunology
• Biochemistry
• Clinical Chemistry

Background:

• Immune complexes (IC) play a role in various autoimmune diseases.
• Accurate detection of circulating immune complexes (CIC) is crucial for diagnosis and monitoring.
• The C1q binding assay is a common method for CIC detection, but can be affected by interfering substances.

Purpose of the Study:

• To develop and validate a C1q solid phase assay (C1q SP) for accurate detection of CIC.
• To differentiate true CIC from interfering CIC-like materials in patient sera.
• To assess the impact of interfering substances on traditional C1q binding assays.

Main Methods:

• Development of a C1q solid phase assay (C1q SP) utilizing native and heat-inactivated C1q.
• Gel filtration analysis to determine the molecular size of binding substances.
• Testing patient sera for CIC using C1q SP and comparing results with traditional methods.

Main Results:

• Aggregated human globulin (AHG) and IC bind to native C1q but not heat-inactivated C1q.
• CIC-like materials in patient sera bind to heat-inactivated C1q and are monomeric IgG size.
• True CIC bind only to native C1q and are found in heavier molecular fractions.
• Approximately two-thirds of CIC-positive results by C1q SP were false positives due to these CIC-like materials.

Conclusions:

• The C1q SP assay effectively distinguishes true CIC from interfering CIC-like materials.
• Heat-inactivated C1q can be used to identify non-specific binding.
• The presence of CIC-like materials significantly impacts the accuracy of standard CIC detection methods, leading to overestimation.