The binding properties of human complement component C1q. Interaction with mucopolysaccharides.

Area of Science:

• Immunology
• Biochemistry
• Molecular Biology

Background:

• The human complement system is a crucial part of innate immunity.
• Complement subcomponent C1q initiates the classical complement pathway.
• The interaction of C1q with various molecules is essential for immune regulation.

Purpose of the Study:

• To quantitatively investigate the binding of human complement subcomponent C1q to mucopolysaccharides, specifically heparin.
• To identify the binding sites and affinity of C1q for low molecular weight heparin glycosaminoglycan (LMW-Hep).
• To explore the functional implications of C1q-mucopolysaccharide interactions in complement activation.

Main Methods:

• Quantitative binding assays using 125I-labeled LMW-Hep to measure C1q-heparin interactions.
• Fluorescence polarization experiments to confirm binding interactions.
• Inhibition assays using various mucopolysaccharides to assess their effect on C1q binding.
• Hemolytic assays to evaluate the functional impact of LMW-Hep on complement component C1.

Main Results:

• Two classes of binding sites for LMW-Hep on C1q were identified with distinct affinities.
• The higher affinity binding site was localized to the collagenous region of C1q (C1q-c), while the globular region showed no binding.
• Heparan sulfate and dermatan sulfate were potent inhibitors of C1q-LMW-Hep interaction.
• LMW-Hep inhibited the assembly of functional complement component C1 and its hemolytic activity.

Conclusions:

• Mucopolysaccharides, particularly heparan sulfate and dermatan sulfate, interact with human complement subcomponent C1q.
• The collagenous region of C1q is responsible for high-affinity binding of LMW-Hep.
• These findings suggest a regulatory role for mucopolysaccharides in the initial phase of complement activation.