FHR3 Blocks C3d-Mediated Coactivation of Human B Cells

J Immunol. 2016 Jul 15;197(2):620-9. doi: 10.4049/jimmunol.1600053. Epub 2016 Jun 8.


The autoimmune renal disease deficient for complement factor H-related (CFHR) genes and autoantibody-positive form of hemolytic uremic syndrome is characterized by the presence of autoantibodies specific for the central complement regulator, factor H, combined with a homozygous deficiency, mostly in CFHR3 and CFHR1 Because FHR3 and FHR1 bind to C3d and inactivated C3b, which are ligands for complement receptor type 2 (CR2/CD21), the aim of the current study was to examine whether FHR3-C3d or FHR1-C3d complexes modulate B cell activation. Laser-scanning microscopy and automated image-based analysis showed that FHR3, but not FHR1 or factor H, blocked B cell activation by the BCR coreceptor complex (CD19/CD21/CD81). FHR3 bound to C3d, thereby inhibiting the interaction between C3d and CD21 and preventing colocalization of the coreceptor complex with the BCR. FHR3 neutralized the adjuvant effect of C3d on B cells, as shown by inhibited intracellular CD19 and Akt phosphorylation in Raji cells, as well as Ca(2+) release in peripheral B cells. In cases of CFHR3/CFHR1 deficiency, the FHR3 binding sites on C3d are occupied by factor H, which lacks B cell-inhibitory functions. These data provide evidence that FHR3, which is absent in patients with the autoimmune form of hemolytic uremic syndrome, is involved in B cell regulation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • B-Lymphocytes / immunology*
  • Blood Proteins / immunology*
  • Cell Separation
  • Complement C3d / immunology*
  • Enzyme-Linked Immunosorbent Assay
  • Flow Cytometry
  • Hemolytic-Uremic Syndrome / immunology*
  • Humans
  • Image Processing, Computer-Assisted
  • Lymphocyte Activation / immunology*
  • Microscopy, Confocal


  • Blood Proteins
  • CFHR3 protein, human
  • Complement C3d