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Measuring Erythrocyte Complement Receptor 1 Using Flow Cytometry
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Complement Receptor 3 Forms a Compact High-Affinity Complex with iC3b.

Rasmus K Jensen1, Goran Bajic2,3,4, Mehmet Sen5

  • 1Department of Molecular Biology and Genetics, Aarhus University, Aarhus, Denmark.

Journal of Immunology (Baltimore, Md. : 1950)
|June 12, 2021
PubMed
Summary
This summary is machine-generated.

Complement receptor 3 (CR3) binds complement fragment iC3b with high affinity, utilizing interactions beyond the iC3b thioester domain. This explains efficient phagocytosis and aids therapeutic development for inflammatory diseases.

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Area of Science:

  • Immunology
  • Molecular Biology
  • Structural Biology

Background:

  • Complement receptor 3 (CR3), also known as Mac-1, is crucial for phagocytosis of complement-opsonized particles.
  • While CR3's interaction with the iC3b thioester domain is known, other CR3-iC3b binding sites remain unclear.

Purpose of the Study:

  • To investigate the detailed interaction between iC3b and the CR3 headpiece.
  • To elucidate the structural basis for CR3-mediated phagocytosis of iC3b-opsonized targets.

Main Methods:

  • Surface plasmon resonance (SPR) to quantify binding affinities.
  • Small-angle X-ray scattering (SAXS) to determine complex structure.
  • CR3-expressing cell binding assays.

Main Results:

  • CR3 headpiece exhibits high affinity (30 nM) for iC3b, significantly stronger than the iC3b thioester domain alone (515 nM).
  • CR3-iC3b interaction in cells shows an affinity of 50 nM.
  • iC3b undergoes a conformational change from extended to compact upon CR3 binding.

Conclusions:

  • The CR3-iC3b interaction is high-affinity and involves regions outside the iC3b thioester domain.
  • This interaction mechanism explains enhanced phagocytosis of iC3b compared to C3dg.
  • Findings support developing targeted therapeutics for inflammatory and neurodegenerative diseases.