Prototheca spp. induce an inflammatory response via mtROS-mediated activation of NF-κB and NLRP3 inflammasome pathways in bovine mammary epithelial cell cultures

Emergence of bovine mastitis caused by Prototheca algae is the impetus to better understand these infections. Both P. bovis and P. ciferrii belong to Prototheca algae, but they differ in their pathogenicity to induce inflammatory responses. The objective was to characterize and compare pathogenesis of inflammatory responses in bMECs induced by P. bovis versus P. ciferrii. Mitochondrial ultrastructure, activity and mtROS in bMECs were assessed with transmission electron microscopy and laser scanning confocal microscopy. Cytokines, including TNF-α, IL-1β and IL-18, were measured by ELISA and Real-Time PCR, whereas expressions of various proteins in the NF-κB and NLRP3 inflammasome pathways were detected with immunofluorescence or Western blot. Infection with P. bovis or P. ciferrii damaged mitochondria, including dissolution and vacuolation of cristae, and decreased mitochondrial activity, with P. bovis being more pathogenic and causing greater destruction. There were increases in NADPH production and mtROS accumulation in infected bMECs, with P. bovis causing greater increases and also inducing higher cytokine concentrations. Expressions of NF-κB-p65, p-NF-κB-p65, IκBα and p-IκBα proteins in the NF-κB pathway, as well as NLRP3, Pro Caspase1, Caspase1 p20, ASC, Pro IL-1β, and IL-1β proteins in the NLRP3 inflammasome pathway, were significantly higher in P. bovis-infected bMECs. However, mito-TEMPO significantly inhibited production of cytokines and decreased expression of proteins in NF-κB and NLRP3 inflammasome pathways in bMECs infected with either P. bovis or P. ciferrii. In conclusion, P. bovis or P. ciferrii infections induced inflammatory responses in bMECs, with increased mtROS in damaged mitochondria and activated NF-κB and NLRP3 inflammasome pathways, with P. bovis causing a more severe reaction.