Self-replicating murine ex vivo cultured alveolar macrophages as a model for toxicological studies of particle-induced inflammation

GSK3α/β Restrain IFN-γ-Inducible Costimulatory Molecule Expression in Alveolar Macrophages, Limiting CD4+ T Cell Activation

Macrophages play a crucial role in eliminating respiratory pathogens. Both pulmonary resident alveolar macrophages (AMs) and recruited macrophages contribute to detecting, responding to, and resolving infections in the lungs. Despite their distinct functions, it remains unclear how these macrophage subsets regulate their responses to infection, including how activation by the cytokine IFN-γ is regulated. This shortcoming prevents the development of therapeutics that effectively target distinct lung macrophage populations without exacerbating inflammation. We aimed to better understand the transcriptional regulation of resting and IFN-γ-activated cells using a new ex vivo model of AMs from mice, fetal liver-derived alveolar-like macrophages (FLAMs), and immortalized bone marrow-derived macrophages. Our findings reveal that IFN-γ robustly activates both macrophage types; however, the profile of activated IFN-γ-stimulated genes varies greatly between these cell types. Notably, FLAMs show limited expression of costimulatory markers essential for T cell activation upon stimulation with only IFN-γ. To understand cell type-specific differences, we examined how the inhibition of the regulatory kinases GSK3α/β alters the IFN-γ response. GSK3α/β controlled distinct IFN-γ responses, and in AM-like cells, we found that GSK3α/β restrained the induction of type I IFN and TNF, thus preventing the robust expression of costimulatory molecules and limiting CD4+ T cell activation. Together, these data suggest that the capacity of AMs to respond to IFN-γ is restricted in a GSK3α/β-dependent manner and that IFN-γ responses differ across distinct macrophage populations. These findings lay the groundwork to identify new therapeutic targets that activate protective pulmonary responses without driving deleterious inflammation.

Lysosomal BK channels facilitate silica-induced inflammation in macrophages

BACKGROUND

Lysosomal ion channels are proposed therapeutic targets for a number of diseases, including those driven by NLRP3 inflammasome-mediated inflammation. Here, the specific role of the lysosomal big conductance Ca2+-activated K+ (BK) channel was evaluated in a silica model of inflammation in murine macrophages. A specific-inhibitor of BK channel function, paxilline (PAX), and activators NS11021 and NS1619 were utilized to evaluate the role of lysosomal BK channel activity in silica-induced lysosomal membrane permeabilization (LMP) and NLRP3 inflammasome activation resulting in IL-1β release.

METHODS

Murine macrophages were exposed in vitro to crystalline silica following pretreatment with BK channel inhibitors or activators and LMP, cell death, and IL-1β release were assessed. In addition, the effect of PAX treatment on silica-induced cytosolic K+ decrease was measured. Finally, the effects of BK channel modifiers on lysosomal pH, proteolytic activity, and cholesterol transport were also evaluated.

RESULTS

PAX pretreatment significantly attenuated silica-induced cell death and IL-1β release. PAX caused an increase in lysosomal pH and decrease in lysosomal proteolytic activity. PAX also caused a significant accumulation of lysosomal cholesterol. BK channel activators NS11021 and NS1619 increased silica-induced cell death and IL-1β release. BK channel activation also caused a decrease in lysosomal pH and increase in lysosomal proteolytic function as well as a decrease in cholesterol accumulation.

CONCLUSION

Taken together, these results demonstrate that inhibiting lysosomal BK channel activity with PAX effectively reduced silica-induced cell death and IL-1β release. Blocking cytosolic K+ entry into the lysosome prevented LMP through the decrease of lysosomal acidification and proteolytic function and increase in lysosomal cholesterol.

Imipramine Treatment Alters Sphingomyelin, Cholesterol, and Glycerophospholipid Metabolism in Isolated Macrophage Lysosomes

Lysosomes are degradative organelles that facilitate the removal and recycling of potentially cytotoxic materials and mediate a variety of other cellular processes, such as nutrient sensing, intracellular signaling, and lipid metabolism. Due to these central roles, lysosome dysfunction can lead to deleterious outcomes, including the accumulation of cytotoxic material, inflammation, and cell death. We previously reported that cationic amphiphilic drugs, such as imipramine, alter pH and lipid metabolism within macrophage lysosomes. Therefore, the ability for imipramine to induce changes to the lipid content of isolated macrophage lysosomes was investigated, focusing on sphingomyelin, cholesterol, and glycerophospholipid metabolism as these lipid classes have important roles in inflammation and disease. The lysosomes were isolated from control and imipramine-treated macrophages using density gradient ultracentrifugation, and mass spectrometry was used to measure the changes in their lipid composition. An unsupervised hierarchical cluster analysis revealed a clear differentiation between the imipramine-treated and control lysosomes. There was a significant overall increase in the abundance of specific lipids mostly composed of cholesterol esters, sphingomyelins, and phosphatidylcholines, while lysophosphatidylcholines and ceramides were overall decreased. These results support the conclusion that imipramine's ability to change the lysosomal pH inhibits multiple pH-sensitive enzymes in macrophage lysosomes.

GSK3α/β restrains IFNγ-inducible costimulatory molecule expression in alveolar macrophages, limiting CD4+ T cell activation

Macrophages play a crucial role in eliminating respiratory pathogens. Both pulmonary resident alveolar macrophages (AMs) and recruited macrophages contribute to detecting, responding to, and resolving infections in the lungs. Despite their distinct functions, it remains unclear how these macrophage subsets regulate their responses to infection, including how activation by the cytokine IFNγ is regulated. This shortcoming prevents the development of therapeutics that effectively target distinct lung macrophage populations without exacerbating inflammation. We aimed to better understand the transcriptional regulation of resting and IFNγ-activated cells using a new ex vivo model of AMs from mice, fetal liver-derived alveolar-like macrophages (FLAMs), and immortalized bone marrow-derived macrophages (iBMDMs). Our findings reveal that IFNγ robustly activates both macrophage types; however, the profile of activated IFNγ-stimulated genes varies greatly between these cell types. Notably, FLAMs show limited expression of costimulatory markers essential for T cell activation upon stimulation with only IFNγ. To understand cell type-specific differences, we examined how the inhibition of the regulatory kinases GSK3α/β alters the IFNγ response. GSK3α/β controlled distinct IFNγ responses, and in AM-like cells, we found GSK3α/β restrained the induction of type I IFN and TNF, thus preventing the robust expression of costimulatory molecules and limiting CD4+ T cell activation. Together, these data suggest that the capacity of AMs to respond to IFNγ is restricted in a GSK3α/β-dependent manner and that IFNγ responses differ across distinct macrophage populations. These findings lay the groundwork to identify new therapeutic targets that activate protective pulmonary responses without driving deleterious inflammation.

Cholesterol-dependent molecular mechanisms contribute to cationic amphiphilic drugs' prevention of silica-induced inflammation

Silicosis is considered an irreversible chronic inflammatory disease caused by the inhalation of crystalline silica (cSiO2). The cycle of inflammation that drives silicosis and other particle-caused respiratory diseases is mediated by NLRP3 inflammasome activity in macrophages resulting in the release of IL-1β. Lysosomal membrane permeability (LMP) initiated by inhaled particles is the key regulatory step in leading to NLRP3 activity. In addition to its role in LMP, the lysosome is crucial to cellular cholesterol trafficking. Lysosomal cholesterol has been demonstrated to regulate LMP while cationic amphiphilic drugs (CADs) reduce cholesterol trafficking from lysosomes and promote endolysosomal cholesterol accumulation as seen in Niemann Pick disease. Using a bone marrow derived macrophage (BMdM) model, four CADs were examined for their potential to reduce cSiO2-induced inflammation. Here we found that FDA-approved CAD drugs imipramine, hydroxychloroquine, fluvoxamine, and fluoxetine contributed to reduced LMP and IL-1β release in cSiO2 treated BMdM. These drugs inhibited lysosomal enzymatic activity of acid sphingomyelinase, decreased lysosomal proteolytic function, and increased lysosomal pH. CADs also demonstrated a significant increase in lysosomal-associated free cholesterol. Increased lysosomal cholesterol was associated with a significant reduction in cSiO2 induced LMP and IL-1β release. In contrast, reduced lysosomal cholesterol significantly exacerbated cSiO2-induced IL-1β release and reduced the protective effect of CADs on IL-1β release following cSiO2 exposure. Taken together, these results suggest that CAD modification of lysosomal cholesterol may be used to reduce LMP and cSiO2-induced inflammation and could prove an effective therapeutic for silicosis and other particle-caused respiratory diseases.