Exposure to particulate matter 2.5 (PM2.5) induced macrophage-dependent inflammation, characterized by increased Th1/Th17 cytokine secretion and cytotoxicity

Abstract

Particulate matter PM2.5 is a class of airborne particles and droplets with sustained high levels in many developing countries. Epidemiological studies have shown the association between sustained high level of PM2.5 and the risk of many diseases in the respiratory system, including lung cancer. However, the precise mechanisms through which PM2.5 induces respiratory diseases are still unclear. In this study, we demonstrated that CD4⁺ and CD8⁺ T cells following PM2.5 treatment demonstrated significantly elevated mRNA and protein levels of interferon (IFN)-γ, interleukin (IL)-10, IL-17, and IL-21 production. This increase in cytokines required the presence of macrophages, such that CD4⁺ and CD8⁺ T cells treated with PM2.5 in the absence of macrophages did not present higher IFN-γ, IL-10, or IL-21 expression. In contrast, PM2.5-treated macrophages could significantly upregulate T cell cytokine secretion, even when excess PM2.5 was removed from cell culture. We also observed a macrophage-dependent upregulation of granzyme A and granzyme B expression by CD4⁺ and CD8⁺ T cells following PM2.5 treatment. These PM2.5-stimulated CD4⁺ and CD8⁺ T cells potently induced the death of human bronchial epithelial (HBE) cells. Interestingly, the CD4⁺ and CD8⁺ T cells presented synergistic effects at inducing HBE cytotoxicity, such that CD4⁺ T cells and CD8⁺ T cells combined resulted in higher HBE cell death than the sum of the separate effects of CD4⁺ T cells and CD8⁺ T cells. While blocking cytotoxic molecule release significantly compromised the T cell-mediated cytotoxicity against HBE cells, blocking IFN-γ, but not IL-10, could also slightly but significantly reduce T cell-mediated cytotoxicity. Together, these data demonstrated that PM2.5 could promote the inflammation of cytotoxicity of T cells in a macrophage-dependent manner. In addition, PM2.5-treated macrophages presented long-lasting proinflammatory effects on T cells.

Keywords: Macrophage; PM2.5; Th1; Th17.