The Distribution of Complement Proteins in Soft and Hard Coronas Impacts Macrophage Uptake of Nanoparticles

Complement proteins, as key constituents of the protein corona, facilitate the elimination of exogenous nanoparticles by immune cells, thereby limiting their tumor accumulation and therapeutic efficacy. Current research on complement proteins primarily focuses on the hard corona (HC), whereas their distribution and biological relevance in the soft corona (SC)-the dynamic layer directly interfacing with biological systems-remain underexplored. In this study, analyses in pooled and individual sera across different nanoparticle types, including carboxylated polystyrene nanoparticles (PS-COOH), silica magnetic nanoparticles (SMNs), and liposomes (Lips), revealed that the third complement protein (C3) fragments associated with both HC and SC, with their distribution dictated by nanoparticle surface chemistry. For PS-COOH and SMNs, iC3b is preferentially enriched in the SC, whereas Lips surface favored iC3b deposition within the HC, likely due to exposed amino groups that facilitate C3 attachment via thioester-amine reactions. Functional assays demonstrate that macrophage uptake is primarily driven by HC-bound iC3b, while SC proteins attenuate this process by shielding HC-bound C3 fragments and limiting their accessibility to complement receptors. Moreover, SC does not effectively mediate nanoparticle-phagocyte interactions, as its dynamic nature prevents stable engagement with cellular receptors. Collectively, these findings provide new mechanistic insights into SC-mediated clearance of nanomedicines.