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Complement-activating ability of leucocytes from patients with complement factor I deficiency

H V Marquart1, J M Rasmussen, R G Leslie

  • 1Department of Medical Microbiology, Odense University, Denmark.

Immunology
|July 1, 1997
PubMed
Summary
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Factor I deficiency impairs B cell complement activation via the alternative pathway (AP). This study found reduced CR1 on B cells and impaired AP function in patients with factor I (FI) deficiency.

Area of Science:

  • Immunology
  • Complement System
  • Cell Biology

Background:

  • Normal peripheral blood B cells activate the alternative pathway (AP) of complement.
  • This activation correlates with complement receptor type 2 (CR2) expression.
  • Erythrocytes normally inhibit this AP activation to some extent.

Purpose of the Study:

  • To investigate the impact of factor I (FI) deficiency on B cell phenotype and function.
  • FI deficiency leads to uncontrolled AP convertase formation, C3b fragment generation, and complement component consumption.

Main Methods:

  • Flow cytometry was used to analyze peripheral blood leukocytes (PBL) from two FI-deficient patients.
  • Analysis included complement receptor expression (CR1, CR2), complement regulatory proteins, in vivo C3 fragment deposition, and in vitro AP activation.

Related Experiment Videos

  • Erythrocyte inhibition assays were also performed.

    • Main Results:

    • FI-deficient patients showed significantly lower CR1 levels on B cells and monocytes/PMNs compared to normal individuals.
    • CR2 levels on B cells were reduced, though not significantly.
    • All leukocyte subpopulations in FI-deficient patients were coated in vivo with C3b fragments, which were degraded to C3d,g upon incubation in homologous serum with EDTA.
    • AP activation on B cells from FI-deficient patients was significantly reduced in homologous serum and absent in autologous serum.
    • Erythrocytes from FI-deficient patients failed to inhibit AP activation.

    Conclusions:

    • Factor I deficiency significantly alters B cell complement receptor expression and impairs alternative pathway activation.
    • The in vivo deposition of C3b fragments and subsequent degradation suggest dysregulation of complement homeostasis.
    • These findings highlight the critical role of factor I in regulating complement activation and maintaining B cell function.