Acute Silica Exposure Triggers Pulmonary Inflammation Through Macrophage Pyroptosis: An Experimental Simulation

Abstract

Silica is an essential substrate of various materials, and inhaling silica induces pulmonary diseases potentially associated with macrophage pyroptosis. Utilizing silica of micro- and nano- sizes, we explored the role of macrophage pyroptosis in silica-induced pulmonary inflammation. Under the transmission electron microscopy, we found that the internalization of silica nanoparticle induced membrane rupture and increased the number of intracellular vacuoles, and both sizes of silica could suppress cell viability and proliferation. Also, silica-exposed macrophages generated higher levels of ROS, together with the upregulated expression of NLRP3, ASC, Caspase-1, GSDMD, IL-1β, and IL-6. However, the expression of these proteins was suppressed after removing ROS or NLRP3. In addition, we found increased expression of TLR4 and NF-κB responsible for silica recognition and pyroptosis priming after silica exposure. For in vivo studies, we established animal model by intratracheally instilling 5 mg of silica into mice with/without NLRP3 inhibition. Four weeks later, we found diffused infiltration of inflammatory cells and enhanced collagen hyperplasia partially reversed by additional treatment with MCC950, so as the expression of pyroptotic molecules and proinflammatory cytokines. In particular, the dual immunofluorescent staining showed co-expression of macrophage-specific biomarker F4/80 and NLRP3 within the cells, and silica of nano-size showed more potent toxicity and pathogenicity than that of the micro-sized particles both in vitro and in vivo. To sum up, macrophage pyroptosis is an upstream event of silica-induced pulmonary inflammation promoted by ROS through the TLR4/NLRP3/NF-κB signaling axis.

Differential immunological effects of silica nanoparticles on peripheral blood mononuclear cells of silicosis patients and controls

“…It is necessary to underline those cytokines like TNF-a and IL-1b, are also highly secreted by macrophages in response to silica particles, and therefore can also participate in SiNP induced perpetuation of inflammation, in turn sustaining the increase of lymphocytes and interaction with macrophages observed. Although we did not conduct experiments to identify possible specific proteins responsible for macrophage maturation and migration to SiNPs, several studies that did so, support our findings with an added focus on viability and proliferation of macrophages when exposed to micro-and nano-silica (59) and provided evidences of macrophage secretome induction by Si NP exposure (60). We thus can speculate that both mechanisms may co-exist i.e., T-cell independent role of SiNPs and function of macrophages as APCs since we also observed phagocytosis of SiNPs by cells resembling macrophages/mature monocytes (Figure 5).…”

Pyroptosis in inflammation-related respiratory disease

“…Therefore, pyroptosis of AMs may play an important role in silicosis. Recently, it is shown that GSDMD-dependent pyroptosis is triggered in silica-treated AMs by activating the TLR4/NLRP3/caspase-1/GSDMD pathway, which results in aggravated pulmonary inflammation and diffuses silicon nodules [ 66 , 83 ]. Blocking pyroptosis of macrophages by inhibiting the NLRP3 inflammasome or caspase-1 reduces silica-mediated pulmonary inflammation and fibrosis [ 50 , 83 ].…”

NF‐κB mediates silica‐induced pulmonary inflammation by promoting the release of IL‐1β in macrophages

“…The size of silica particles in the air varies, which determines their toxic effects: the smaller the particle size, the stronger the toxicity 6,7 . As evidenced by previous studies, it is found that exposure to silica particles of nano‐size causes DNA damage to human bronchial epithelial cells (Beas‐2b) much severer than that of the microsized ones 8 .…”