The early life effects of maternal exposure to particulate matter

Air pollution makes a significant contribution to global morbidity and mortality. Particulate matter (PM), which is the solid and liquid component of air pollution, has been linked to the development and exacerbation of cardiovascular and respiratory disease. More recently, epidemiological studies have highlighted the effect of early life exposure to PM. For example, there is evidence to suggest that in utero exposure to PM causes premature birth, lower birth weight, impairs lung function and increases the risk of developing respiratory disease later in life. However, no studies have addressed the effect of pregnancy on the response of the mother to PM, and the mechanism linking the maternal response to post-natal health is unclear. In addition, previous studies have focused exclusively on one source of PM. There is now evidence to suggest that the PM source has the potential to play a vital role in the magnitude of the health effect(s) of PM. Therefore, the aims of this Thesis were to 1) identify the effect of pregnancy on the response to PM; 2) determine the effect of in utero exposure to PM on post-natal immune development and the response to subsequent PM exposure and; 3) explore the effect of in utero exposure to PM on markers of placental function as a possible mechanism for the post-natal effects. In all aims, the effect of PM from different sources on the response was assessed. To identify the effect of pregnancy on the response to PM, pregnant and non-pregnant C57BL/6J mice were exposed to either 50µg of diesel exhaust particles (DEP), silica or iron oxide in 50µL of saline, or saline alone, on gestational days (E)7.5, E12.5 and E17.5. On E18.5, mice were sacrificed and lung inflammation as well as spleen and thymus T cell populations were examined. This study showed that pregnant mice had elevated levels of IL-4 and suppressed levels of IL-8 in the BAL and reduced neutrophil, lymphocyte, and eosinophil numbers in the lung in response to silica. Pregnancy did not alter the inflammatory response to DEP but did elevate the percentage of CD3+CD4+CD25+ T cells in the thymus in response to DEP. The elevation of these regulatory T cells may be a mechanism to protect the fetus from the harmful effects of PM. This study was able to highlight that pregnancy suppresses the inflammatory response induced by PM in the lung. While the pregnant dam may have been protected from the effects of PM inhalation, there is evidence to suggest that the developing fetus is susceptible. To explore this, pregnant C57BL/6J mice were exposed to either 50 µg of DEP or silica in 50 µL of saline, or 50 µL of saline alone, on E7.5, E12.5 and 17.5. Mice were allowed to give birth, and at four-weeks of age, 2 males and 2 females were randomly selected and exposed to either 50 µg of DEP in 50 µL of saline or saline alone. 6 hours later, mice were sacrificed, and lung inflammation (cells and cytokine production) was assessed. Lung inflammatory responses were assessed by quantifying inflammatory cells and cytokine production (MCP-1, MIP-2, IL-6). In separate groups of mice, the spleen was harvested to quantify B and T cell populations. Splenocytes were isolated and exposed to lipopolysaccharide or Poly I:C for assessment of cytokine production. Exposure to DEP in utero decreased B10 cells in female mice and IFN-γ production by splenocytes. Male mice had elevations in macrophage and lymphocyte numbers in response to DEP whereas female mice had elevated IL-6, MCP-1 and MIP-2 levels. In utero exposure to silica had no effect on these measures. B10 cells and IFN-γ are critical in responding to viral infections suggesting that in utero exposure to DEP may increase susceptibility to viral infection. To understand the link between maternal exposure to PM and altered post-natal immune responses in offspring, the effect of PM on markers of placental function was assessed. To achieve this, pregnant C57BL/6J mice were exposed to either 50µg of DEP, silica or iron oxide in 50µL of saline, or saline alone, on E7.5, E12.5 and E17.5. On E18.5 mice were sacrificed and placentas were extracted for analysis of gene expression by qPCR while lungs were fixed for stereological analysis of structure. In utero exposure to DEP increased the expression of placental growth factor (PlGF), 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) and angiopoietin-2 (Ang-2) and decreased the expression of angiopotien-1 (Ang-1). The elevation of PlGF and modulation of Ang-1/2 expression suggests an effect of PM exposure on placental angiogenesis. This may impact nutrient supply to the fetus however there was no effect on fetal or post-natal somatic lung development. Elevation of 11β-HSD1 is linked to cortisol regulation which may impact on immune development. This may explain the effect of in utero exposure to PM on post-natal immune responses described in the previous Aim. In summary, the data presented in this Thesis shows that pregnancy alters immune responses leading to reductions in the inflammatory response to PM suggesting that pregnant women are partly protected from the inflammatory effects of PM. However, it is clear that in utero exposure to PM impacted on fetal immune development leading to impaired production of antiviral cytokines which may increase susceptibility to infection later in life. This effect may be driven by alterations in placental function. However, further work is required to clarify this observation. The data from this Thesis have implications for understanding the health effects of exposure to PM. Particularly during pregnancy and in early life. The variability in the response to PM from different sources highlights that there cannot a “one-size fits all” approach to understanding the effect of PM on community health. Likewise, the identification of possible immune effects and the role of the placenta as the mechanism underlying the developmental effects of in utero exposure to PM may pave the way for identifying therapies to reduce the health effects of air pollution. However, further studies are needed to examine the long-term immunological effects of in utero exposure to PM and whether modulation of placental function can reduce these effects