Deposition of complement factors (opsonisation) on nanoparticles may promote clearance from the blood by macrophages and trigger proinflammatory responses, but the mechanisms regulating the efficiency of complement activation are poorly understood. We previously demonstrated that opsonisation of superparamagnetic iron oxide nanoworms (SPIO NWs) with the third complement protein (C3) was dependent on the biomolecule corona. Here we show that natural antibodies play a critical role in C3 opsonisation of SPIO NWs and a range of clinically approved nanopharmaceuticals. The dependency of C3 opsonisation on immunoglobulin binding is predominantly universal and is observed regardless of the complement activation pathway. Few surface-bound immunoglobulin molecules trigger complement activation and opsonisation. While the total amount of nanoparticle-absorbed protein does not determine C3 deposition efficiency, the biomolecule corona per se enhances immunoglobulin binding to all nanoparticle types. We therefore show that natural antibodies represent a link between biomolecule corona and C3 opsonisation, and may determine individual complement responses to nanomedicines.

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 membraneanchored 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 *

Complement is one of the critical branches of innate immunity that determines the recognition of engineered nanoparticles by immune cells. Antibodytargeted iron oxide nanoparticles are a popular platform for magnetic separations, in vitro diagnostics, and molecular imaging. We used 60 nm cross-linked iron oxide nanoworms (CLIO NWs) modified with antibodies against Her2/neu and EpCAM, which are common markers of blood-borne cancer cells, to understand the role of complement in the selectivity of targeting of tumor cells in whole blood. CLIO NWs showed highly efficient targeting and magnetic isolation of tumor cells spiked in lepirudin-anticoagulated blood, but specificity was low due to high uptake by neutrophils, monocytes, and lymphocytes. Complement C3 opsonization in plasma was predominantly via the alternative pathway regardless of the presence of antibody, PEG, or fluorescent tag, but was higher for antibody-conjugated CLIO NWs. Addition of various soluble inhibitors of complement convertase (compstatin, soluble CD35, and soluble CD55) to whole human blood blocked up to 99% of the uptake of targeted CLIO NWs by leukocytes, which resulted in a more selective magnetic isolation of tumor cells. Using well-characterized nanomaterials, we demonstrate here that complement therapeutics can be used to improve targeting selectivity.