Human Factor H-Related Protein 2 (CFHR2) Regulates Complement Activation

Hannes U Eberhardt,Peter Hortschansky,Sascha Böhm,Andrea Hartmann,Markus J Kemper,Peter F Zipfel,Teresia Hallström,Qian Chen,Reinhard Wallich,Denise Buhlmann,Christine Skerka

doi:10.1371/journal.pone.0078617

Abstract

Mutations and deletions within the human CFHR gene cluster on chromosome 1 are associated with diseases, such as dense deposit disease, CFHR nephropathy or age-related macular degeneration. Resulting mutant CFHR proteins can affect complement regulation. Here we identify human CFHR2 as a novel alternative pathway complement regulator that inhibits the C3 alternative pathway convertase and terminal pathway assembly. CFHR2 is composed of four short consensus repeat domains (SCRs). Two CFHR2 molecules form a dimer through their N-terminal SCRs, and each of the two C-terminal ends can bind C3b. C3b bound CFHR2 still allows C3 convertase formation but the CFHR2 bound convertases do not cleave the substrate C3. Interestingly CFHR2 hardly competes off factor H from C3b. Thus CFHR2 likely acts in concert with factor H, as CFHR2 inhibits convertases while simultaneously allowing factor H assisted degradation by factor I.

Highlights

The complement system serves as a central component of innate immunity, regulating hemostasis and cooperating with the adaptive immune response [1,2]
For comparison CFHR2, factor H and CRIg were added to Normal human serum (NHS) (20% in HEPES-EGTA buffer) incubated with LPS coated wells as described and complement activation was followed by C3b deposition and detection of C3a or Ba in the supernatant by ELISA Kits (Quidel)
Recombinant CFHR2 forms exclusively homodimers To identify the role of CFHR2 in complement activation we expressed CFHR2 and CFHR2 fragments (Fig. 1A) recombinantly in Pichia pastoris

Summary

Introduction

The complement system serves as a central component of innate immunity, regulating hemostasis and cooperating with the adaptive immune response [1,2]. Five factor H related proteins (CFHR1-CFHR5) circulate in human plasma, and three members of this group are described as complement regulators [3]. Homo- and heterodimerization of CFHR proteins was identified, mediated by a dimerization domain located in the N-terminal SCR domains of CFHR1, CFHR2 and CFHR5 [9]. Each single CFHR protein is encoded by a separate gene and all five CFHR genes are located on human chromosome 1q32 in the CFHR gene cluster [10], downstream of the factor H gene. We identify CFHR2 as a novel human alternative pathway complement regulator that inhibits the amplification loop of the AP at level of the C3 convertase and TCC assembly

Materials and Methods

Results and Discussion

Conclusions