Metabolic regulation has been implicated in the pathogenesis of inflammatory diseases. NADPH oxidase 4 (NOX4), a source of cellular superoxide anion, has multiple biological functions that may be of importance in inflammation, and in the pathogenesis of human metabolic diseases, including diabetes. However, the mechanisms by which NOX4-dependent metabolic regulation impacts the innate immune response remain unclear. Here, we show that deficiency of NOX4 resulted in reduced expression of carnitine palmitoyltransferase 1A (CPT1A), which is a key mitochondrial enzyme in the fatty acid oxidation (FAO) pathway. The reduced FAO resulted in less activation of the nucleotide binding domain, leucine-rich repeat-containing receptor (NLR), pyrin domain-containing-3 (NLRP3) inflammasome in human and mouse macrophages. In contrast, NOX4 deficiency did not inhibit the activation of the NLR family, CARD domain containing 4 (NLRC4), the NLRP1, or the absent in melanoma 2 (AIM2) inflammasomes. We also found that inhibition of FAO by etomoxir suppressed NLRP3 inflammasome activation. Furthermore, Nox4-deficient mice displayed significant reduction of caspase-1 activation and interleukin (IL)-1β and IL-18 production and improved survival in a mouse model of NLRP3-mediated Streptococcus pneumoniae (S. pneumoniae) infection. The pharmacologic inhibition of NOX4 by either GKT137831, currently in phase 2 clinical trials, or VAS-2870 attenuated NLRP3 inflammasome activation. Our results suggest that NOX4-mediated FAO promotes NLRP3 inflammasome activation.

People worldwide are living longer, and it is estimated that by 2050, the proportion of the world's population over 60 years of age will nearly double. Natural lung aging is associated with molecular and physiological changes that cause alterations in lung function, diminished pulmonary remodeling and regenerative capacity, and increased susceptibility to acute and chronic lung diseases. As the aging population rapidly grows, it is essential to examine how alterations in cellular function and cell-to-cell interactions of pulmonary resident cells and systemic immune cells contribute to a higher risk of increased susceptibility to infection and development of chronic diseases, such as chronic obstructive pulmonary disease and interstitial pulmonary fibrosis. This review provides an overview of physiological, structural, and cellular changes in the aging lung and immune system that facilitate the development and progression of disease.

Plasmacytoid dendritic cells (pDCs) are innate sensors that produce IFN-α in response to viral infections. Determining how aging alters the cellular and molecular function of these cells may provide an explanation of increased susceptibility of older people to viral infections. Hence, we examined whether aging critically impairs pDC function during infection with HSV-2, a viral pathogen that activates TLR9. We found that impaired IFN-α production by aged murine pDCs led to impaired viral clearance with aging. Upon TLR9 activation, aged pDCs displayed defective up-regulation of IFN-regulatory factor 7, a key adaptor in the type I IFN pathway, as compared with younger counterparts. Aged pDCs had more oxidative stress, and reducing oxidative stress in aged pDCs partly recovered the age-induced IFN-α defect during TLR9 activation. In sum, aging impairs the type I IFN pathway in pDCs, and this alteration may contribute to the increased susceptibility of older people to certain viral infections.

Summary Morbidity and mortality associated with viral infections increase with age, although the underlying mechanisms are unclear. Here, we investigated whether aging alters inflammatory responses during systemic viral infection and whether such age-related alterations contribute to viral-induced death. We found that infection of aged mice with systemic herpes viruses led to rapid increases in serum IL-17, neutrophil activation, and mortality due to hepatocyte necrosis. In contrast, all young mice survived infection, displaying weaker IL-17 induction and neutrophil activation. During viral activation, natural killer T cells isolated from the livers of aged mice exhibited greater RORγT gene expression and IL-17 production than young cells. Importantly, IL-17 neutralization or neutrophil depletion during viral infection reduced liver damage and prevented death of aged mice. These results demonstrate that, during systemic viral infection, aging alters the host-pathogen interaction to overproduce IL-17, inducing liver injury and death.

Aging has been implicated in the development of pulmonary fibrosis, which has seen a sharp increase in incidence in those older than 50 years. Recent studies demonstrate a role for the nucleotide-binding domain and leucine rich repeat containing family, pyrin domain containing 3 (NLRP3) inflammasome and its regulated cytokines in experimental lung fibrosis. In this study, we tested the hypothesis that age-related NLRP3 inflammasome activation is an important predisposing factor in the development of pulmonary fibrosis. Briefly, young and aged wild-type and NLRP3(-/-) mice were subjected to bleomycin-induced lung injury. Pulmonary fibrosis was determined by histology and hydroxyproline accumulation. Bone marrow and alveolar macrophages were isolated from these mice. NLRP3 inflammasome activation was assessed by co-immunoprecipitation experiments. IL-1β and IL-18 production was measured by ELISA. The current study demonstrated that aged wild-type mice developed more lung fibrosis and exhibited increased morbidity and mortality after bleomycin-induced lung injury, when compared with young mice. Bleomycin-exposed aged NLRP3(-/-) mice had reduced fibrosis compared with their wild-type age-matched counterparts. Bone marrow-derived and alveolar macrophages from aged mice displayed higher levels of NLRP3 inflammasome activation and caspase-1-dependent IL-1β and IL-18 production, which was associated with altered mitochondrial function and increased production of reactive oxygen species. Our study demonstrated that age-dependent increases in alveolar macrophage mitochondrial reactive oxygen species production and NLRP3 inflammasome activation contribute to the development of experimental fibrosis.

Aging is associated with metabolic diseases such as type 2 diabetes mellitus, cardiovascular disease, cancer, and neurodegeneration. Aging contributes to common processes including metabolic dysfunction, DNA damage, and reactive oxygen species generation. Although glycolysis has been linked to cell growth and proliferation, the mechanisms by which the activation of glycolysis by aging regulates fibrogenesis in the lung remain unclear. The objective of this study was to determine if glucose transporter 1 (GLUT1)-induced glycolysis regulates age-dependent fibrogenesis of the lung. Mouse and human lung tissues were analyzed for GLUT1 and glycolytic markers using immunoblotting. Glycolytic function was measured using a Seahorse apparatus. To study the effect of GLUT1, genetic inhibition of GLUT1 was performed by short hairpin RNA transduction, and phloretin was used for pharmacologic inhibition of GLUT1. GLUT1-dependent glycolysis is activated in aged lung. Genetic and pharmacologic inhibition of GLUT1 suppressed the protein expression of a-smooth muscle actin, a key cytoskeletal component of activated fibroblasts, in mouse primary lung fibroblast cells. Moreover, we demonstrated that the activation of AMPactivated protein kinase, which is regulated by GLUT1-dependent glycolysis, represents a critical metabolic pathway for fibroblast activation. Furthermore, we demonstrated that phloretin, a potent inhibitor of GLUT1, significantly inhibited bleomycin-induced lung fibrosis in vivo. These results suggest that GLUT1-dependent glycolysis regulates fibrogenesis in aged lung and that inhibition of GLUT1 provides a potential target of therapy of age-related lung fibrosis.

The NLRP3 inflammasome is activated in the lung during influenza viral infection; however the impact of aging on inflammasome function during influenza infection has not been examined. Here, we show that elderly mice infected with a mouse adapted strain of influenza produced lower levels of IL-1β during in vitro and in vivo infection. Dendritic cells from elderly mice exhibited decreased expression of ASC, NLRP3, and capase-1, but increased expression of pro-IL-1β, pro-IL-18, and pro-IL-33 when compared to young infected mice. Treatment with nigericin during influenza infection augmented IL-1β production, increased caspase-1 activity, and decreased morbidity and mortality in elderly mice. Our study demonstrates for the first time that during influenza viral infection, elderly mice have impaired NLRP3 inflammasome activity and that treatment with nigericin rescues NLRP3 activation in elderly hosts.

Conventional immunosuppressive drug delivery requires high systemic drug levels to provide therapeutic benefit, but frequently results in toxic side effects. Novel drug delivery methods, such as FDAapproved poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs), are promising drug delivery platforms to reduce drug doses and minimize toxicity. Using murine models of skin transplantation, we investigated whether PLGA NPs would effectively deliver mycophenolic acid (MPA), a common clinical immunosuppressant, and avoid the toxicity of conventional drug delivery. We found that intermittent treatment with NPs encapsulated with MPA (NP-MPA) resulted in a significant extension of allograft survival than intermittent conventional MPA treatment even though the concentration of MPA within NP-MPA was a 1000-fold lower than conventional drug. Importantly, recipients who were administered NP-MPA intermittently avoided drug toxicity, whereas those treated with daily conventional drug manifested cytopenias. Dendritic cells (DCs) endocytosed NP-MPA to upregulate programmed death ligand-1 (PD-L1) and displayed a decreased ability to prime alloreactive T cells. Importantly, the ability of NP-MPA to promote allograft survival was partly PD-L1 dependent. Collectively, this study indicates that NPs are potent drug delivery tools that extend allograft survival without drug toxicity.