(444) The Indigestible Burden of Microplastics Drives Bioenergetic Dysfunction in Macrophages

Introduction/Rationale: To date, over 7 billion tons of plastic waste has been generated, of which, 80% has accumulated in the environment. Macrophages, the first line of innate cell defense, phagocytose microplastics (MPs) but are incapable of breaking them down. Regrettably, the consequences of MP accumulation on innate immunity and human health remain unknown.

Methods: Monocyte-derived macrophages and Raw264.7 cells were cultured with polystyrene MPs of various sizes (.02-10µm), concentrations (1-50,000 MPs/cell), and durations and assessed for function and phenotype. Cell bioenergetics were measured by mitochondrial (mt) mass, mt polarization, and by Seahorse for cellular metabolism. 3x10^6 1µm or 4µm MPs were delivered intranasally to FVB mice and after 7 days, extrapulmonary dissemination measured and pulmonary macrophages (pMacs) analyzed via RNAseq.

Results: Macrophages readily phagocytose but fail to clear MPs which amass in Rab5a+ early phagosomes. In vitro, MPs induced IRF activation and increased ROS, TNFa, and CCL2, while inhibiting antigen presentation, T cell stimulation, and tumor phagocytosis. Tom20-stained mt were seen to wind around and coat intracellular MPs leading to decreased mt polarization, mass, and reduced OxPhos which could be partially reversed with an AMP kinase activator (AICAR). In vitro, MPs but not diesel exhaust particulate inhibit expression of the mtDNA genes ND3, TRNP, and ATP8 reducing ATP production. In vivo, MPs induce unique transcriptional profiles in pMacs, including chemokine upregulation (CXCL1 and CXCL3) and inhibition of autophagy (Ulk4), that were distinct from exposure to diesel exhaust and pesticides.

Conclusion: These findings suggest that progressive accumulation of MPs inhibits cellular bioenergetics resulting in macrophage dysfunction. Over time, reduced macrophage functions, such as efferocytosis, may result in chronic, smoldering inflammation underlying the pathobiology of chronic lung disease and lung cancer.