How do nanotubes suppress T cells?
Alison Elder
Mice inhaling low levels of multiwalled carbon nanotubes show suppressed immune function. New studies suggest that this suppression originates from signals in the lungs.
Engineered nanomaterials have diverse physicochemical properties, and the question of whether their unique properties are correlated with adverse outcomes for the environment and our health is actively being investigated. With outstanding tensile and conducting properties, multiwalled carbon nanotubes (MWNTs) — concentric multiple hollow rolls of graphitic carbon that are several microns long and tens of nanometres in diameter — are poised to revolutionize many fields from engineering to medicine. However, concerns have been raised because they show similar shape (that is, high length to diameter ratio), biodurability and undesirable health effects to asbestos1,2. High production volumes and, consequently, their potential for occupational and incidental exposures during the manufacturing process have put them in the spotlight.
Previously, it was shown that MWNTs can suppress the immune system of mice when inhaled3. Following up on this study, on page 451 of this issue, Jacob McDonald and colleagues of the Lovelace Respiratory Research Institute in New Mexico report that this suppression originates from signals in the lung that turn on signals that directly affect the function of T cells in the spleen4. T cells are a class of white blood cells that coordinate the immune system to fight infections and diseases, and whether their suppression has long-term consequences remains unclear.
The New Mexico team explored mechanistic aspects of the immune response by perturbing the cyclooxygenase-2 pathway — a biochemical route to making compounds that are responsible for various inflammatory processes. By blocking the transmission of signals from the lungs to other organs with ibuprofen (a common anti-inflammatory pain reliever that blocks the cyclooxygenase pathway), it was possible to partially rescue the suppression of T cells in the spleen. Mice deficient in the cyclooxygenase-2 gene, which did not develop overt lung inflammation after inhaling the MWNTs, confirmed the importance of this signalling pathway.
The present findings are certainly intriguing and highlight what we might expect in terms of health outcomes following repeat exposures. What is most valuable about this study is the use of aerosolization technology to achieve exposure in a physiologically relevant manner. A lack of data for this type of exposure is a serious concern with many other studies — and not just those with carbon nanotubes — where the rate of dose delivery and/or the route of exposure are not realistic for humans.
The field has come a long way in thefew years since carbon nanotubes (instilled into the lungs as relatively large globs called agglomerates) were first reported to cause lethality in rodents5,6, raising the possibility that exposure of engineered nanomaterials might pose serious health risks to humans. More recent work has shown a variety of outcomes with carbon nanotubes that are delivered using either chemical or physical dispersants: from no response in the lungs to severe lung injury, and everything in between3,7–9.These disparities could be explained by differences in metal impurities, length, diameter, flexibility and surface defects of the MWNTs, or by the way the material is delivered to the animals.
The present work by McDonald and co-workers focuses on responses to MWNTs that were delivered at two aerosol concentrations, but the immunosuppressive effects were not present at the lower of these two dose levels. This is important because it suggests that if workplace exposures can be appropriately controlled for this type of nanomaterial, adverse health outcomes in humans could be avoided, provided that humans respond similarly to the mice. Mice are used routinely in biomedical and toxicological research and can provide useful information for investigating a variety of diseases and injuries that are similar to the human condition.
However, some major questions remain for those who are trying to assess the safety of engineered nanomaterials in a more generalized sense. Are these findings applicable to all carbon nanotubes? Are the exposure concentrations realistic? The authors appropriately provided estimates of human-equivalent aerosol concentrations, but we do not yet know enough about actual workplace exposures to determine if these estimates are adequate. Indeed, the limited data available includes only those from a simulated workplace10 and one actual workplace11, and the reported range is from 53 to 435 μg cm-3 of air. In both cases, it is likely that other components were also present in the aerosols as well as MWNTs.
The point is that it is difficult to place these results and those in other papers into a meaningful context when so little is known about real-world exposures, both in terms of concentrations and the physical and chemical properties of MWNTs. Although exposure assessment may seem mundane compared with teasing out the subtle changes in the function of different white blood cells and the signals that affect them, it is an essential component of risk assessment and should be a major goal over the next few years so that the information can be incorporated into the design of toxicology studies.
Inhalation exposures remain the gold standard for assessing the toxicity of airborne materials until a simpler proxy method has been validated. Therefore, it is essential that inhalation exposure studies — like the one highlighted here — are incorporated into the hazard characterization framework for engineered nanomaterials so that appropriate risk estimations can be performed from realistic exposures. .
Alison Elder is in the Department of Environmental Medicine, University of Rochester, Rochester, New York 14610, USA.
e.mail: Alison_Elder@urmc.rochester.edu
References
1. Poland, C. A. et al. Nature Nanotech. 3, 423–428 (2008).
2. Takagi, A. et al. J. Toxicol. Sci. 33, 105–116 (2008).
3. Mitchell, L. A. et al. Toxicol. Sci. 100, 203–214 (2007).
4. Mitchell, L. A. et al. Nature Nanotech. 4, 451–456 (2009).
5. Lam, C.-W., James, J. T., McCluskey, R. & Hunter, R. L.Toxicol. Sci. 77, 126–134 (2004).
6. Warheit, D. B. et al. Toxicol. Sci. 77, 117–125 (2004).
7. Li, J. G. et al. Environ. Toxicol. 22, 415–421 (2007).
8. Ryman-Rasmussen, J. P. et al. Am. J. Respir. Cell Mol. Biol.40, 349–358 (2009).
9. Shvedova, A. A. et al. Am. J. Physiol. 295, L552–L565 (2008).
10. Maynard, A. D. et al. J. Toxicol. Environ. Health A 67, 87–107 (2004).
11. Han, J. H. et al. Inhal. Toxicol. 20, 741–749 (2008).
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Evaluation of nanoparticle immunotoxicity
Marina a. dobrovolskaia1*, dori r. germolec2 and James l. Weaver3
The pharmaceutical industry is developing increasing numbers of drugs and diagnostics based on nanoparticles, and evaluating the immune response to these diverse formulations has become a challenge for scientists and regulatory agencies alike. An international panel of scientists and representatives from various agencies and companies reviewed the limitations of current tests at a workshop held at the National Cancer Institute in Frederick, Maryland. This article outlines practical strategies for identifying and controlling interferences in common evaluation methods and the implications for regulation.
Mechanisms for how inhaled multiwalled carbon nanotubes suppress systemic
immune function in mice
L. A. Mitchell1,2, F. T. Lauer2, S. W. Burchiel2 and J. D. McDonald1*
The potential health effects of inhaling carbon nanotubes are important because of possible exposures in occupational settings. Previously, we have shown mice that have inhaled multiwalled carbon nanotubes have suppressed systemic immune function. Here, we show the mechanisms for this immune suppression. Mice were exposed to 0, 0.3 or 1 mg m-3 multiwalled carbon nanotubes for 6 h per day for 14 consecutive days in whole-body inhalation chambers. Only those exposed to a dose of 1 mg m-3 presented suppressed immune function; this involved activation of cyclooxygenase enzymes in the spleen in response to a signal from the lungs. Spleen cells from exposed animals partially recovered their immune function when treated with ibuprofen, a drug that blocks the formation of cyclooxygenase enzymes. Knockout mice without cyclooxygenase enzymes were not affected when exposed to multiwalled carbon nanotubes, further confirming the importance of this enzyme in suppression. Proteins from the lungs of exposed mice suppressed the immune function of spleen cells from normal mice, but not those from knockout mice. Our findings suggest that signals from the lung can activate signals in the spleen to suppress the immune function of exposed mice.