Landmark study highlights a new avenue for air pollution research
An image showing an ultra low emission zone in London
Source: © Kristian Buus/In Pictures/Getty Images
Not all traffic-related air pollution originates from tail pipe exhaust fumes
European and American governments have no directives to monitor air pollution resulting from the wear and tear of cars. Yet, a study showing that the metallic components in brake dust disrupt airway immune cells on the same scale as diesel emissions could set the wheels in motion to change that.
Brake dust in urban areas currently makes up around 21% of traffic air particulates, but this is set to rise as regulations on diesel vehicles come into effect. With this information, a team led by Ian Mudway from King’s College London in the UK collected brake dust, produced under conditions representative of modern urban life, to investigate its effects on the immune function of human lungs. The team exposed lung cells to both diesel exhaust and brake dust particles to compare the toxicity of each. Their data showed that both pollution sources increased inflammation markers and impaired the ability of immune cells to ingest bacteria (called phagocytosis), which is the body’s primary mechanism for protecting the lungs against infection.
The study highlights how exposure to vehicle-derived pollution decreases the lungs ability to prevent infection and may be why such infections are more common in urban environments.
Furthermore, adding a metal chelator alongside the pollutants restored the cells ability to fight bacteria. This suggests that the high metallic component of brake dust causes the damage. The team detected raised levels of vanadium after exposure to both pollution sources. They say vanadium may disrupt cell function by interacting with isolated mitochondria, increasing the free radial production in the cell.
‘This is a motivation to also target and regulate non-tailpipe emissions from vehicles, to properly benefit public health,’ says Clare Heaviside from the Environmental Change Institute at the University of Oxford, UK.
Interestingly, cellular activity recovered after 24 hours in a pollution free environment. Wayne Carter, an expert in the health effects of environmental toxins from the University of Nottingham, UK, says ‘It is reassuring that challenged cells are able to regain their phagocytic function after a period free from particulate challenge, but this underscores the need for regular respite from potentially toxic roadside exposures.’
Davide Castelvecchi
@dcastelvecchi

References

Brake dust exposure exacerbates inflammation and transiently compromises phagocytosis in macrophages†
Liza Selley, ORCID logo ‡*a   Linda Schuster,‡bc   Helene Marbach,b   Theresa Forsthuber,b   Ben Forbes, ORCID logo b   Timothy W. Gant, ORCID logo de   Thomas Sandström,f   Nuria Camiña,g   Toby J. Athersuch, ORCID logo eh   Ian Mudway ORCID logo gi  and  Abhinav Kumarb  
 Author affiliations
Abstract
Studies have emphasised the importance of combustion-derived particles in eliciting adverse health effects, especially those produced by diesel vehicles. In contrast, few investigations have explored the potential toxicity of particles derived from tyre and brake wear, despite their significant contributions to total roadside particulate mass. The objective of this study was to compare the relative toxicity of compositionally distinct brake abrasion dust (BAD) and diesel exhaust particles (DEP) in a cellular model that is relevant to human airways. Although BAD contained considerably more metals/metalloids than DEP (as determined by inductively coupled plasma mass spectrometry) similar toxicological profiles were observed in U937 monocyte-derived macrophages following 24 h exposures to 4–25 μg ml−1 doses of either particle type. Responses to the particles were characterised by dose-dependent decreases in mitochondrial depolarisation (p ≤ 0.001), increased secretion of IL-8, IL-10 and TNF-α (p ≤ 0.05 to p ≤ 0.001) and decreased phagocytosis of S. aureus (p ≤ 0.001). This phagocytic deficit recovered, and the inflammatory response resolved when challenged cells were incubated for a further 24 h in particle-free media. These responses were abrogated by metal chelation using desferroxamine. At minimally cytotoxic doses both DEP and BAD perturbed bacterial clearance and promoted inflammatory responses in U937 cells with similar potency. These data emphasise the requirement to consider contributions of abrasion particles to traffic-related clinical health effects.

Graphical abstract: Brake dust exposure exacerbates inflammation and transiently compromises phagocytosis in macrophages
Significance to metallomics
Metal/metalloid content is consistently associated with the capacity of tailpipe-derived particulates to cause cellular stress and inflammation in the airways. Here, we demonstrate that exposure to brake dust (a richly metallic, non-tailpipe-derived, wear particle) also impairs the ability of immune cells to ingest respiratory pathogens and enhances inflammatory signalling in a transient but metal-dependent manner.

Introduction
Residential proximity to roads, particularly those carrying high proportions of diesel traffic, has been associated with increased mortality rates, exacerbations of respiratory disease, increased cardiovascular symptoms1 and adverse impacts on lung development and cognition in children.2,3 Traffic-derived pollution represents a heterogeneous mixture of gases and particulates, produced by fuel combustion and lubricant volatilisation, wear of mechanical components and abrasion of the road surface.4

To date, diesel exhaust has been the major focus of investigations into traffic-derived particulate toxicity. This focus has been supported by epidemiological studies which demonstrate significant associations between adverse health outcomes/endpoints and tracers of diesel tailpipe emissions.5,6 Additionally, concurrent in vitro and in vivo experimentation has demonstrated the capacity of diesel exhaust particles (DEP) to stimulate xenobiotic and antioxidant defences, redox-sensitive signalling pathways, inflammatory cascades and the activation of airway nerve fibres.7–10 In contrast, the toxic potential of non-tailpipe particulates, such as those produced by brake, tyre or clutch wear has received little attention.

Brake abrasion dust (BAD) is the most abundant non-tailpipe particulate measured in urban areas,11,12 contributing up to 55% of non-tailpipe PM1013 and 21% of total traffic PM10,14 with the contribution forecast to increase relative to tailpipe PM, as emission regulations for diesel vehicles tighten.4 Often portrayed as coarse mode PM, abrasion-derived particles also exist within the respirable fine and ultrafine fractions15 giving them potential to penetrate the distal lung, deposit in respiratory tract lining fluids (RTLFs) and interact with resident phagocytes and epithelial cells. Unlike tailpipe-derived particles, BAD is rich in metals,4 many of which (e.g. Fe and Cu) can catalyse the formation of reactive oxygen species (ROS) within the oxygen-rich conditions of the RTLFs, thus challenging the high concentrations of low molecular weight antioxidants (glutathione, urate and α-tocopherol), antioxidant enzymes (superoxide dismutase and catalase) and metal-binding proteins (transferrin and caeruloplasmin), which exist to protect the epithelial surface from oxidative attack.16 Indeed, within cell-free systems, BAD displays greater capacity to elicit damaging oxidation reactions than DEP.17

Despite associations existing between occupational metal fume exposures and inflammatory pulmonary pathophysiologies (including metal fume fever which has been recognised since the 1950's),18 very few publications have explored the direct adverse effects of BAD exposure in pulmonary models. Employing mechanistic understanding of how such occupational pathophysiologies develop,18 those that have, focus predominantly on the pro-inflammatory and oxidative stress-inducing potential of the particles. Mimicking exposure of the alveolar epithelium, Gasser et al. demonstrated that the metallic fraction of BAD induces pro-inflammatory signalling and disrupts tight junction integrity in A549 cells.19 A comparatively reduced inflammatory response was documented in Calu-3 bronchial epithelial cells following exposure to Fe and Cu-rich BAD, but increases in IL-6 secretion did associate with ROS production following exposure to concentrations greater than 50 μg cm−2.20 Using a mouse inhalation model, Gerlofs-Nijland et al. found that the fine fraction of BAD also induced pro-inflammatory responses in vivo following acute (1.5–6 h) exposures of up to 9 mg m−3. Based upon influx of circulating immune cells (polymorphonucleocytes) and concentrations of cytokines secreted into the bronchiolar lavage fluid (BALF) by these and by resident airway phagocytes and epithelial cells, the authors found that particles produced by low and semi-metallic brake pads induced pulmonary inflammation at a similar dose to DEP and tyre wear, but exhibited lower potency than particles produced through biomass combustion. BAD produced from non-asbestos organic (NAO) brake pads was found to be the most potent of all particles tested: a result that the authors attribute to its high copper content.21

Other studies have attributed toxicological endpoints to BAD and general abrasion-related particles by examining the toxicity of roadside PM, with and without the use of metal chelators to try and isolate the metal-dependent signature. Using this approach, the metal content of roadside PM in samples collected throughout Europe was shown to be related to the capacity of RAW264.7 monocyte-derived macrophages (MDM) to release arachidonic acid metabolites.22 Other within-city and pan-national studies have shown that the capacity of ambient PM to elicit damaging oxidation reactions is strongly related to their content of brake-wear related metals, such as Cu, Ba and Sb.23–25 Susceptibility to pulmonary infections remains unexamined in the context of BAD exposure, but exposure to metal-rich aerosols (especially those containing Fe, Cr, Al and Ti) has been associated with increased mortality from pneumonia in welders and foundry workers,26,27 enhance pneumococcal adhesion to airway epithelial cells,32 and impairment of bacterial phagocytosis in airway phagocytes.36,37 Given the metallic, Fe-rich composition of BAD, it is therefore possible that these and other metal-rich vehicle-derived particles could contribute to the heightened incidence of pulmonary infections that exists in urban environments.30,31

In the current study, we used the U937 monocyte-derived macrophage cell line to perform side-by-side characterisations of the toxicity that BAD and diesel exhaust particles exert on airway macrophages (phagocytes found resident to the alveolar lumen surface during homeostasis). The work explored endpoints additional to inflammatory potential that had previously been shown to be perturbed by diesel particulate challenge: mitochondrial dysfunction, cell morphometry as an indicator of apoptosis or necrosis and phagocytic competence. We hypothesised that both particle types would perturb cell function, but that there would be significant differences in response, reflecting the marked compositional difference between the particle species.