It is the task of hazard research and of toxicology to uncover detrimental effects. Once a substantial knowledge has been gained, it is important to shift gears and try to look at the problem from the other side: What is safe? This change in perspective is necessary to allow focusing on genuinely problematic materials and not focus too much work on less worrisome entities, merely because there is large body of literature about them.
The group of Albert Duschl from The Paris-Lodron University of Salzburg (PLUS), one of NANORIGO project partner, has recently published a review that addresses this issue with respect to the immunosafety of nanomaterials: Himly M, Geppert M, Hofer S, Hofstätter N, Horejs-Höck J, Duschl A. 2020. When would immunologists consider a nanomaterial to be safe? Recommendations for planning studies on nanosafety. Small. DOI:10.1002/smll.201907483. The paper tries to answer which data are needed to consider a nanomaterial as – from an immunological point of view – rather harmless. Of course, this does not cover other issues like genotoxicity of developmental toxicity, but since the immune system is often the first and certainly the professional responder upon exposure to nanoparticles, it is an important part of safety assessment.
The authors recommend to fully cover three aspects in the design of a study:
- A stringent testing for contamination with Endotoxin / LPS is required to exclude that these powerful proinflammatory agents elicit reactions that can be falsely attributed to the particles tested. Due to assay interference it is necessary to use two different tests relying on different principles. The sensitivity of the test needs to match the sensitivity of the relevant cell, organ or species. Note that the sensitivity of human Dendritic cells is 0.2 EU/ml.
- The dose needs to be justified by applying the same dose in vitro that may also be acting in vivo. The dose considered here is the dose that reaches a cell – NP per cell would be good expression. This requires to calculate which dose of nanoparticles is “seen” by cells that e.g. adhere to the bottom of a well. Dose per m² would in this case be a good measure, but dose per m³ would be not. A second calculation is needed to estimate the dose to which e.g. a lung epithelial cell of a human is exposed, which again is not the same as particles/m³.
- If markers of cell stress or of inflammation are observed in absence of proinflammatory contaminants and at a dose that occurs in real life, then it needs to be checked whether this represents an immune reaction or merely a homeostatic fluctuation. It is easy to produce artifacts; for example, cells go into cell stress when they are handled due to the temperature shock. This analysis may require some help for experts, while the first two points can be addressed by any experimenter.
Within NANORIGO, we specifically address the second point, for example by producing an equivalence dose library for tissue-delivered dose, that will support the RGF and its users. Work with the partners should allow us to further improve this library and make it user-friendly.