Factor H-related proteins determine complement-activating surfaces

• The role of Factor H-related proteins (FHRs) is poorly understood and controversial.
• First considered complement inhibitors, new data show FHRs enhance activation.
• FHRs antagonize FH regulation; an activity denominated complement de-regulation.
• FHRs provide improved discrimination of the self and non-self/foreign surfaces.

Complement factor H-related proteins (FHRs) are strongly associated with different diseases involving complement dysregulation, which suggests a major role for these proteins regulating complement activation. Because FHRs are evolutionarily and structurally related to complement inhibitor factor H (FH), the initial assumption was that the FHRs are also negative complement regulators. Whereas weak complement inhibiting activities were originally reported for these molecules, recent developments indicate that FHRs may enhance complement activation, with important implications for the role of these proteins in health and disease. We review these findings here, and propose that FHRs represent a complex set of surface recognition molecules that, by competing with FH, provide improved discrimination of self and non-self surfaces and play a central role in determining appropriate activation of the complement pathway.