Complement activation and cell uptake responses toward polymer-functionalized protein nanocapsules.

Self-assembling protein nanocapsules can be engineered for various bionanotechnology applications. Using the dodecahedral scaffold of the E2 subunit from pyruvate dehydrogenase, we introduced non-native surface cysteines for site-directed functionalization. The modified nanoparticle's structural, assembly, and thermostability properties were comparable to the wild-type scaffold (E2-WT), and after conjugation of poly(ethylene glycol) (PEG) to these cysteines, the nanoparticle remained intact and stable up to 79.7 ± 1.8 °C. PEGylation of particles reduced uptake by human monocyte-derived macrophages and MDA-MB-231 breast cancer cells, with decreased uptake as PEG chain length is increased. In vitro C4-depletion and C5a-production assays yielded 97.6 ± 10.8% serum C4 remaining and 40.1 ± 6.0 ng/mL C5a for E2-WT, demonstrating that complement activation is weak for non-PEGylated E2 nanoparticles. Conjugation of PEG to these particles moderately increased complement response to give 79.7 ± 6.0% C4 remaining and 87.6 ± 10.1 ng/mL C5a. Our results demonstrate that PEGylation of the E2 protein nanocapsules can modulate cellular uptake and induce low levels of complement activation, likely via the classical/lectin pathways.