Another peek into how carbon nanotubes may interfere with the human immune system
(Nanowerk Spotlight) The toxicity issues surrounding carbon nanotubes (CNTs) are highly relevant for two reasons: Firstly, as more and more products containing CNTs come to market, there is a chance that free CNTs get released during their life cycles, most likely during production or disposal, and find their way through the environment into the body. Secondly, and much more pertinent with regard to potential health risks, is the use of CNTs in biological and medical settings. CNTs interesting structural, chemical, electrical, and optical properties are explored by numerous nanomedicine research groups around the world with the goal of drastically improving performance and efficacy of biological detection, imaging, and therapy applications. In many of these envisaged applications, CNTs would be deliberately injected or implanted in the body. For instance, CNT-based intercellular molecular delivery vehicles have been developed for intracellular gene and drug delivery in vitro (see: "Nanotechnology based stem cell therapies for damaged heart muscles").
Some groups are using CNTs in research for vaccination as well as gene and cancer therapy. Here, the CNT applications are designed to interact directly with the immune system. Understanding the interplay between CNTs and immune proteins is therefore critical for both improving CNT applications in biology and medicine and avoiding potentially noxious immune responses.
New work by researchers in France addresses the interaction of carbon nanotubes with the human immune system, an interdisciplinary field that concerns both the positive and negative health aspects of nanotechnology.
"Taking a molecular approach to the immunological properties of CNTs, we studied the interaction of the complement activation protein C1 of the human innate immune system with CNTs," Wai Li Ling tells Nanowerk. "We show, for the first time, organized crystal-like binding and continued accumulation of human protein complexes along the full length of carbon nanotubes visualized by electron microscopy. We show, also for the first time, that CNTs fail to directly activate the C1 complex of complement."
The C1 complex is an important component of the complement system, a group of serum proteins forming part of the innate immune system. Besides foreign objects such as pathogens, C1 is involved in recognizing and clearing altered self-structures and is associated with apoptotic cell clearance, neurodegenerative and autoimmune diseases, among many vital bodily processes and disorder.
Cartoons of the C1 components, left to right: recognition unit C1q, globular head domains of C1q, and proenzyme C1s-C1r-C1r-C1s, and the organized arrays they form respectively on multi-walled carbon nanotube as visualized by electron microscopy. (Image: Wai Li Ling)
In their transmission electron microscopy studies, the research team analyzed the binding of C1 components on commercially available single-wall, double-wall, and multi-wall CNTs. Besides the ordered packing of the first monolayer on the CNTs, they found that proteins continued to stack on the fully covered CNTs, thereby depleting the proteins in the solution. Independent of the results of the binding experiments, the important observation is that all CNTs failed to activate the C1 complex in vitro.
"Our results that CNTs bind and accumulate C1 components yet fail to activate the complex suggest that carbon nanotubes will interfere with the immune system when entering the bloodstream" explains Ling. "This should raise warning flags with regard to applications of CNTs in biomedicine but, on the other hand, it also opens possibilities of novel CNT applications concerning the many biochemical processes involving the versatile C1 macromolecule."
From a scientific point of view, the organized binding of the protein complexes on CNTs demonstrated in this study offers insights into the adsorption mechanisms of macromolecules on CNT surfaces –something that still is poorly understood.
"From our experiments with various proteins and from studying published results, we propose that interaction among the macromolecules may be an important factor in determining whether a stable overlayer of macromolecules, which leads to the solubilization of CNTs, can occur on the nanotube surface" says Ling. "The binding results demonstrated in the study also present an encouraging step in using CNTs to solve protein structures. Proteins are nanomachines and their structures tell us how they function. Solving protein structures is also like finding out the shape of a piece of puzzle. From that, we can find out how the different pieces work together."
Due to the delicate and often flexible nature of proteins, solving their structure is far from trivial. For instance, despite years of effort, the two human immune proteins studied in this work have failed to crystallize in 3D, a pre-requisite for x-ray structural studies.
"The fact that they form organized arrays on the CNTs gives us hope that we may be able to use CNTs to form helical arrays for solving their structures," explains Ling. "It will be great if we get another tool in our bag to solve hard-to-solve protein structures."
The human immune system is a very complex system with many components and feedback systems. It will be important to employ both top-down and bottom-up approaches to study the immunological properties of nanoparticles. While clinical studies will be indispensable, it will also be important to find out how the different members in the immune system interact with the CNTs to fully understand their immunological properties.