APOE Protects Against Severe Infection with Mycobacterium tuberculosis by Restraining Production of Neutrophil Extracellular Traps

Mice lacking apolipoprotein E (APOE, Apoe-/- mice) are highly susceptible to infection with Mycobacterium tuberculosis (Mtb) but the underlying immune dysregulation has been unclear. We found that Mtb infected Apoe-/- mice have significantly elevated levels of neutrophils in their lungs compared to controls and demonstrated that depleting neutrophils, depleting pDCs, blocking type1 interferon signaling, and blocking LTB4 receptor signaling all improved the outcome of Mtb-infected Apoe-/- mice. Neutrophils can undergo a specialized form of cell death termed NETosis in which activation of the enzyme PAD4 leads to extrusion of neutrophil extracellular traps (NETs). NETs have been associated with severe tuberculosis in other mouse models and in humans but a role of NETs in the pathology has not been directly established. We demonstrate that blocking PAD4 activation leads to a decrease in NETs in the lungs and, strikingly, completely reverses the hypersusceptiblity of Apoe-/- mice. To examine the role of Apoe in the resonses of pulmonary macrophages and neutrophils to infection with Mtb invivo, we generated 50:50 B6:Apoe-/- bone marrow chimeric mice on a B6 background and infected them with either~50 or ~4000 CFU of Mtb H37Rv expressing mCherry. Alveolar macrophages, monocyte-derived macrophages, and neutrophils of each genotype were isolated by flow sorting at either 10 days following infection (for ~4000 CFU infections) or 28 days following infection (for ~50 CFU infections). Expression profiling of each group was performed by RNA-seq.