Although mitochondria are well-known for their role in energy production, these organelles also have a number of specialized roles in immune responses. Emerging evidence suggests that mitochondrial dynamics (fusion/fission) also influence both cell metabolism and inflammation. Here we show that this process plays an essential role in macrophage responses against bacterial pathogens.
Using both pharmacological and genetic approaches, we show that Toll-like receptor (TLR)-mediated activation of macrophages results in increased mitochondrial fission, and that this response is functionally linked to both inflammatory outputs and host-protective antimicrobial responses in these cells. We also demonstrate key roles for Dynamin-1-like protein (Dnm1l/Drp1) and mitofusin 1 (Mfn1) in mitochondrial dynamics in the context of these responses in macrophages. Furthermore, we find that infection of mouse macrophages with the Gram-negative bacterial pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium) delivers a robust TLR response but does not result in increased mitochondrial fission, suggesting that this pathogen may antagonize this cellular response as a host evasion strategy. Accordingly, skewing towards mitochondrial fission by silencing Mfn1 enhances clearance of S. Typhimurium by mouse macrophages. Mfn1 is active in its deacetylated form, the generation of which is catalysed by the cytoplasmic enzyme histone deacetylase (HDAC) 6. We show that HDAC6 promotes mitochondrial fusion in macrophages and constrains the capacity of these cells to clear intracellular S. Typhimurium. Genetic (knock-out or knock-down) or pharmacological (tubastatin A) targeting of HDAC6 promotes mitochondrial fission in S. Typhimurium-infected macrophages and reduces bacterial loads within these cells. Furthermore, treating macrophages with mitochondrial fusion-promoting compounds (M1 and/or mdivi1), as well as silencing Drp1, antagonized the antimicrobial effect of tubastatin A, confirming the key role of mitochondrial fission in macrophage antimicrobial responses. Mechanistically, mitochondrial fission enhances several antimicrobial pathways in macrophages, including mitochondrial reactive oxygen species generation, and is efficient against a number of different bacterial pathogens including multi-drug-resistant S. Typhimurium, M. tuberculosis and uropathogenic Escherichia coli. Finally, the antimicrobial effect of mitochondrial fission was confirmed in vivo; Tubastatin A dramatically reduced bacterial dissemination to the spleen and liver after challenge of C57BL/6 mice with S. Typhimurium.
Collectively, our findings demonstrate a central role for mitochondrial dynamics in functional responses of macrophages to bacterial challenge. They also suggest that, through the modulation of mitochondrial dynamics and the reprogramming of innate immune host defence pathways, HDAC6 inhibitors may have applications for the treatment of acute and/or chronic bacterial infections.