Characterisation of ambient particulate matter and its immunological effects on monocyte-derived dendritic cells

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

Airborne particulate matter has been defined as a heterogenous collection of solid or liquid droplets of varying size suspended in the air. Epidemiological studies have shown an association between an increase in the concentration of particulate matter (PM) and exacerbation of respiratory diseases such as asthma and chronic obstructive pulmonary disease (COPD). A central mechanism for adverse health outcomes has been attributed to primary combustion particles from vehicle exhausts (residing exclusively within the fine but not coarse mode) causing a pro-inflammatory cytokine milieu production by PM exposed dendritic cells (DC). Hence, I hypothesised that fine mode PM2.5 from high traffic sites in London would have a greater capacity to drive inflammatory DC activation than coarse mode PM10.

PM2.5 and PM10 filters collected over a whole year (2013) were sourced from the Marylebone Road (representing high traffic roadside) and North Kensington (representing urban background). Each individual filter was extracted, and the contents pooled to create a unique annual PM sample of PM2.5 and PM10 for each site. Standard reference material-1648a (SRM; urban particulate matter), SRM-2975 (diesel) and brake abrasion dust (BAD) were included since the SRMs are prevalent in air pollution studies. To investigate the compositional metal and metalloid differences between the PMs, an inductively coupled plasma mass spectrometry was used. This showed noticeable differences in metal contents (Fe and Ca) between the different sites. PM from RS had higher metal and metalloid content for both PM2.5 and PM10 than BG.

Noticeably, PM10 contained a higher degree of metal components, had a significant oxidative profile. As bacterial antigens can trigger an immune response, lipopolysaccharides (LPS) levels for all PMs in the study was measured. There was a higher concentration of LPS for SRM-1648a (30.54pg/ml), BG PM10 (9.91pg/ml) and RS PM10 (6.15pg/ml). However, all these values were noted as below the capacity to stimulate human DC maturation. Human monocyte-derived DC (MDDC) was used to review the immunogenicity of various PM samples by measuring their effects on cell surface maturation markers and cytokine secretion. However, contrary to the hypothesis, coarse mode PM10 but not fine mode PM2.5 caused a greater increase in the expression of maturation markers CD83, CD86 and MHC Class II. Among the SRMs, SRM-1648a was found to have the capacity to mature MDDC. The MDDCs exposed to PM10 (not PM2.5) had a significant increase in the secretion of cytokines TNFα, IL-6, IL-8, IL-10, and IL-23p40.

To elucidate the mechanistic relationship between PM and MDDC response, putative pathways including Fe, oxidative stress, and polycyclic aromatic hydrocarbons (PAH) were tested. There was a significant increase in transcripts related to Fe, oxidative stress and PAHs when measured by quantitative polymerase chain reaction (qPCR). Nevertheless, when measuring oxidative stress via (glutathione: glutathione disulphide ratio) on PM exposed MDDC, there was no significant difference, suggesting the PM exposed MDDC was able to tolerate the increased in oxidative stress. In the presence of the inhibitor for the aryl hydrocarbon receptor (PAH receptor), there was no significant change in the maturation response by PM exposed MDDCs. While the inhibitor was able to reduce the secretion of IL-23 by SRM-1648a, a similar response was not observable in ambient PM.

Results generated from this PhD project indicate the initial hypothesis was rejected- MDDC maturation and cytokine production were largely restricted to the coarse mode in PM10, with little evidence for an effect from PM2.5. The increased expression of antigen-presenting molecules and cytokine production is likely to have a potential impact on the activation of CD4 T cells. The cytokine profile (IL-6 and IL-23) secreted by PM10 stimulated MDDCs, suggests that CD4 T cells may be driven to polarise into pathogenic TH17 cells which are associated with steroid-refractory asthma as previously demonstrated by experiments with SRM-1648a. These findings corroborate with findings from the London low emission zone, which suggest PM10 may have a higher association with negative respiratory outcomes than originally perceived. Analysis of the transcripts results from PCR indicates Fe, oxidative and PAH pathways were affected by the presence of PM regardless of the size fraction. Although the components within the coarse mode that are responsible for the maturation of MDDCs remain to be determined, the results presented in this thesis have narrowed down the potential pathways for future studies.
Date of Award1 Aug 2020
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorCatherine Hawrylowicz (Supervisor) & Ian Mudway (Supervisor)

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