Complement activation plays an important role in pharmacokinetic and performance of
intravenously administered nanomedicines. Significant efforts have been directed toward
engineering of nanosurfaces with low complement activation, but due to promiscuity
of complement factors and redundancy of pathways, it is still a major challenge. Cell
membrane-anchored Decay Accelerating Factor (DAF, a.k.a. CD55) is an efficient membrane
bound complement regulator that inhibits both classical and alternative C3 convertases
by accelerating their spontaneous decay. Here we tested the effect of various short
consensus repeats (SCRs, “sushi” domains) of human CD55 on nanoparticle-mediated complement
activation in human sera and plasma. Structural modeling suggested that SCR-2, SCR-3
and SCR-4 are critical for binding to the alternative pathway C3bBb convertase, whereas
SCR-1 is dispensable. Various domains were expressed in E.coli and purified by an
affinity column. SCRs were added to lepirudin plasma or sera from different healthy
subjects, to monitor nanoparticle-mediated complement activation as well as C3 opsonization.
Using superparamagnetic iron oxide nanoworms (SPIO NWs), we found that SCR-2-3-4 was
the most effective inhibitor (IC 50 ~0.24 µM for C3 opsonization in sera), followed
by SCR-1-2-3-4 (IC 50 ~0.6 µM), whereas shorter domains (SCR-3, SCR-2-3, SCR-3-4)
were ineffective. SCR-2-3-4 also inhibited C5a generation (IC 50 ~0.16 µM in sera).
In addition to SPIO NWs, SCR-2-3-4 effectively inhibited C3 opsonisation and C5a production
by clinically approved nanoparticles (Feraheme, LipoDox and Onivyde). SCR-2-3-4 inhibited
both lectin and alternative pathway activation by nanoparticles. When added to lepirudin-anticoagulated
blood from healthy donors, it significantly reduced the uptake of SPIO NWs by neutrophils
and monocytes. These results suggest that soluble domains of membrane bound complement
inhibitors are potential candidates for preventing nanomedicine- mediated complement
activation in human subjects.