Abstract: FR-OR063

The Potential Significance of Complement Factor D Bypass in fD-Targeted Treatment for C3G

Session Information

Category: Glomerular

  • 1002 Glomerular: Basic/Experimental Pathology

Authors

  • Zhang, Yuzhou, University of Iowa, Iowa City, Iowa, United States
  • Keenan, Adam C., University of Iowa, Iowa City, Iowa, United States
  • Lindorfer, Margaret A, University of Virginia School of Medicine, Charlottesville, Virginia, United States
  • Nester, Carla M., University of Iowa, Iowa City, Iowa, United States
  • Taylor, Ronald P., University of Virginia School of Medicine, Charlottesville, Virginia, United States
  • Lambris, John, University of Pennsylvania School of Medicine , Philadelphia, Pennsylvania, United States
  • Smith, Richard J., University of Iowa, Iowa City, Iowa, United States
Background

C3 glomerulopathy (C3G) is characterized by predominant deposition of complement C3 in the glomerulus in the absence or sparse presence of immunoglobulins. The underlying disease mechanism is uncontrolled activity of the C3 convertase of the alternative pathway (AP). Convertase activity is normally tightly controlled by the fluid-phase AP regulator factor H (fH). Mice with a targeted deletion of fH (Cfh-/-) develop features of C3G, with intense C3 deposition along glomerular capillary walls accompanied by subendothelial electron-dense deposits. The rate-limiting protease, fD, is the only enzyme known to cleave fB. Several AP inhibiting small-molecule antagonists against fD have been developed.

Methods

To assess fD-targeted therapy, we backcrossed Cfh-/- mice with Cfd-/- mice for >10 generations and evaluated complement dysregulation and renal pathology in Cfh-/- mice, Cfd-/- mice and Cfh-/-;Cfd-/- mice. We developed an in vitro fB-cleavage assay using a recombinant protein that contains the active catalytic domain of mannose-binding lectin-associated serine protease 3 (MASP-3).

Results

Cfh-/-;Cfd-/- mice have abundant mesangial and capillary deposition of C3 and C5b-9 with scant densities on electron microscopy consistent with ongoing complement dysregulation even in the absence of fD. Serum from Cfh-/- mice was devoid of complement activity due to depletion of complement proteins. Circulating C3 levels in Cfd-/- mice were 40% higher than in wild type mice however serum from Cfd-/- mice could not initiate AP activity without the addition of human fD. C3 levels in Cfh-/-;Cfd-/- and wild-type mice were comparable however even without fD serum from Cfh-/-;Cfd-/- mice was able to initiate AP activity based on hemolysis of rabbit erythrocytes and a positive C3-deposition assay. In the absence of fD, we found that the MASP-3 catalytic domain was capable of cleaving fB in the presence of C3b.

Conclusion

In the absence of fD, AP convertase assembly and activation can be initiated by the fB-cleavage activity of the lectin pathway serine protease MASP-3. This in vivo fD-bypass activation mechanism is always present but only becomes operational when the complement system is ‘pushed’ by the absence of both fH and fD. The implications of this bypass mechanism should be considered in fD-targeted treatment for C3G.

Funding

  • NIDDK Support