Proteins of the innate immune system crystallize on carbon nanotubes but are not activated.

The classical pathway of complement is an essential component of the human innate immune system involved in the defense against pathogens as well as in the clearance of altered self-components. Activation of this pathway is triggered by C1, a multimolecular complex comprising a recognition protein C1q associated with a catalytic subunit C1s-C1r-C1r-C1s. We report here the direct observation of organized binding of C1 components C1q and C1s-C1r-C1r-C1s on carbon nanotubes, an ubiquitous component in nanotechnology research. Electron microscopy imaging showed individual multiwalled carbon nanotubes with protein molecules organized along the length of the sidewalls, often over 1 μm long. Less well-organized protein attachment was also observed on double-walled carbon nanotubes. Protein-solubilized nanotubes continued to attract protein molecules after their surface was fully covered. Despite the C1q binding properties, none of the nanotubes activated the C1 complex. We discuss these results on the adsorption mechanisms of macromolecules on carbon nanotubes and the possibility of using carbon nanotubes for structural studies of macromolecules. Importantly, the observations suggest that carbon nanotubes may interfere with the human immune system when entering the bloodstream. Our results raise caution in the applications of carbon nanotubes in biomedicine but may also open possibilities of novel applications concerning the many biochemical processes involving the versatile C1 macromolecule.