Toxicological Effects of Air Pollutants on Human Airway Cell Models Using Air-liquid Interface Systems: A Systematic Review

Abstract

Purpose of review: Global air pollution has increased significantly in recent decades mainly due to anthropogenic emissions. This results in elevated concentrations of some airborne pollutants like nitrogen dioxide, ozone, volatile organic compounds (VOCs), and particulate matter (PM). In this review, we aim to provide an overview of the current state of knowledge on the toxicological effects of air pollution on airway epithelial cells, the first point of contact of the air pollutants with the body, using air-liquid interface (ALI) models.

Recent findings: Research on the health effects of air pollution has advanced through studies that take a multidisciplinary approach integrating toxicology, epidemiology, and molecular and cell biology. Submerged cell cultures have been used in most studies for the assessment of air pollution toxicity in vitro, but these show some important limitations. Thus, human airway cellular models based on ALI systems have emerged as very promising approaches in respiratory toxicology due to their closer resemblance to in vivo conditions. Results from 53 studies indicate that both, acute and prolonged exposures to air pollution induce oxidative, inflammatory, and genotoxic responses in airway epithelial cells. The changes in several biomarkers and genes related to the observed health effects were discussed through key molecular pathways, and particularly those related to the oxidative stress state. Lastly, we identified perspectives for future research in this field, such as the use of more complex test (e.g., photochemical ageing) atmospheres and exposure models that are reliable for long-term and repeated exposures. This review highlights the role of ALI cellular models as essential tools in respiratory toxicology and environmental health research, providing insights into the molecular mechanisms triggered by air pollution exposure.

Keywords: Aerosols; Air pollution; Air–liquid interface; Cellular inflammation; Human health; Oxidative stress; Toxicity.

Fig. 1

Traditional air pollutants and main emission sources of air pollution. Based on EAA Monograph of Assessment and Management of Urban Air Quality in Europe

Fig. 2

Flow diagram of study selection based on the PRISMA statement

Fig. 3

Schematic representation of the general experimental timeline for in vitro toxicology studies using ALI systems

Fig. 4

Examples of commercial ALI Systems: a) CULTEX ® exposure chamber [103] and b) Vitrocell ® exposure chamber. Extracted from https://www.vitrocell.com/inhalation-toxicology/exposure-systems/for-6-well-inserts/6-3-cf-stainless-steel/. Others (not pictured) include NaviCyte horizontal diffusion chamber [51], MINUCELL perfusion unit [90], XposeALI system [73], and the ExpoCube device [48]

Fig. 5

Schematic representation of several molecular pathways involved in the biologic responses induced by air pollution in human airway epithelial cells. AhR = aryl hydrocarbon receptor; ROS = reactive oxygen species; Keap1 = kelch-like ECH-associated protein 1; Nrf2 = nuclear factor erythroid 2–related factor 2; SOD = superoxide dismutase; CAT = catalase; GPx = glutathione peroxidase; TLRs = toll-like receptors; MAPKs = mitogen-activated protein kinases; ERKs = extracellular signal-regulated kinases; JNKs = c-Jun N-terminal kinases; p38s = p38 mitogen-activated protein kinases; AP-1 = activator protein 1; IKK = inhibitor of nuclear factor-κB (IκB) kinase; NF-κB = nuclear factor kappa B; MDA = malondialdehyde; 8-OHdG = 8-hydroxy-2'-deoxyguanosine; H2AX = H2A histone family member X

Fig. 6

Cellular damage induced by generation of reactive oxygen species and antioxidant defense system. Adapted from Rahman et al. 2006 [136]