Mildly pathogenic Rickettsia-macrophage interactions

The molecular details underlying differences in pathogenicity between Rickettsia species remain to be fully understood. Evidence points to macrophage permissiveness as a key mechanism in rickettsial virulence. Different studies have shown that several rickettsial species responsible for mild forms of rickettsioses can also escape macrophage-mediated killing mechanisms and establish a replicative niche within these cells. However, their manipulative capacity with respect to host cellular processes is far from being understood. A deeper understanding of the interplay between mildly pathogenic rickettsiae and macrophages and the commonalities and specificities of host responses to infection would illuminate differences in immune evasion mechanisms and pathogenicity. We have used quantitative proteomics by SWATH-MS/MS to profile alterations resulting from infection of THP-1 macrophages with three mildly pathogenic rickettsiae: R. parkeri, R. africae, and R. massiliae, all successfully proliferating in these cells. We show that all three species trigger different proteome signatures. Our results reveal a significant impact of infection in proteins categorized as type-I interferon responses, here included several components of the RIG-1-like signaling pathway, mRNA splicing, and protein translation. Moreover, significant differences in protein content between infection conditions anticipate species-specific induced alterations. Indeed, we confirm a distinct impact on host inflammatory responses between species during infection, demonstrating that these species trigger different levels of INF-β, differences in the bioavailability of the pro-inflammatory cytokine IL1-β, and in triggering pyroptotic events. This work reveals novel aspects and exciting nuances of macrophage-Rickettsia interactions, adding additional layers of complexity between Rickettsia and host cells constant arms race for survival.