Pathogenic, but not non-pathogenic,Rickettsiaevade inflammasome-dependent IL-1 responses to establish an intracytosolic replication niche

Abstract

Rickettsia species (spp.) are strict obligate intracellular bacteria, symbiotic in their arthropod vector, and some being pathogenic in their mammalian host, including humans. One critical feature of these stealthy group of pathogens is their ability to manipulate hostile cytosolic environments to their benefits. Although our understanding of Rickettsia cell biology and pathogenesis are evolving, the mechanisms of host innate immune defense evasion by pathogenic Rickettsia spp. remains to be elucidated. Here, we showed that disease severity in wild-type (WT) C57BL/6J mice infected with R. typhi- (etiologic agent of murine typhus) and R. rickettsii (etiologic agent of Rocky Mountain Spotted Fever), but not with non-pathogenic R. montanensis, correlated with levels of bacterial burden as detected in the spleens, as well as the serum concentrations of pro-inflammatory cytokine IL-1α and to a lesser extent IL-1β. Antibody-mediated neutralization of IL-1α confirmed a key role in controlling mortality rates and bacterial burdens of rickettsiae-infected WT mice. As macrophages are a primary source of both IL-1α and IL-1β cytokines, we determined the mechanism of the anti-rickettsial activities using bone-marrow-derived macrophages. We found that pathogenic R. typhi and R. rickettsii, but not non-pathogenic R. montanensis, induced autophagy, and avoided autophagolysosomal destruction, while simultaneously eluded pro-IL-1α induction and benefited from the dampening of IL-1α secretion, via Caspase-11-Gsdmd-dependent mechanism, to facilitate intracytosolic replication. Adoptative transfer experiments identified that IL-1α secretion by macrophages was critical for controlling rickettsiosis in WT mice. In sum, we identified a previously unappreciated pathway by which pathogenic, unlike non-pathogenic, rickettsiae preferentially target the Caspase-11-Gsdmd-IL-1α signaling axis in macrophages, possibly via an autophagy-dependent mechanism, to support their replication and dissemination within the host.

Pathogenic Rickettsia species evade autophagosomal maturation and reduce anti-microbicidal pro-inflammatory IL-1 responses to support their intracellular survival

“…Collectively, our data are in agreement with previous reports (21,22,29,31), indicating that, unlike nonpathogenic R. montanensis spp, both pathogenic R. typhi, and R. rickettsii spp, induce autophagy, but evade autophagolysosomal destruction, and reduce anti-rickettsial inflammasome-dependent IL-1 cytokine responses to establish an intracytosolic replication niche in macrophages.…”

“…Based on recent findings from other laboratories and ours (21)(22)(23)(24)29,(31)(32)(33)(34)39), it has become evident that Rickettsia spp exhibit variable pathogenicity, indicating species-specific strategies to respond to mechanism of host defense surveillance, including autophagy and inflammasome responses. In our efforts to address the role of autophagy in modulating the intracellular survival of Rickettsia spp, we first determined the colocalization status of R. montanensis, R. typhi, and R. rickettsii with LC3B during infection of ATG5 fl/fl or ATG5 fl/fl -LysM-Cre BMDMΦ by IFA.…”

“…Preceding findings suggest that intracellular pathogens, like Rickettsia, not only encounter inflammasomedependent defense mechanisms but are also confronted by another cytosolic defense pathway, autophagy (21)(22)(23)(24)29,(31)(32)(33)(34). Both responses not only are key to mount the appropriate host defense responses, but are also functionally interconnected (15,16).…”

“…Such strategies entail reprogramming host phosphoinositide (PI) metabolism, which can facilitate uptake into host cells, modify phagosomes, undermine apoptosis, and interfere with other cellular defense mechanisms, such as inflammasomes and autophagy. However, in the case of Rickettsia the mechanisms by which these pathogens modulate both inflammasome and autophagy responses to facilitate their replication in endothelial cells and immune cells, like macrophages, is only now emerging (21,22,40,23,24,29,(31)(32)(33)(34)39). In fact, recent findings from others and our laboratory have provided some insight on how highly pathogenic Rickettsia spp modulate inflammasome-mediated immune responses to establish an intracellular niche.…”

“…Our recent findings identified a previously unappreciated mechanism by which highly pathogenic Rickettsia spp (R. typhi, and R. rickettsii), unlike non-pathogenic rickettsiae, benefitted from a reduced IL-1 signaling response, specifically IL-1α, to support their replication within the host (31). Intriguingly, R. australis benefited from ATG5-dependent autophagy induction and reduction of pro-inflammatory cytokine responses (21,22), however, a successful host colonization of R. parkeri involved the evasion of autophagy and inflammasomemediated host cell death (23,24), leaving the precise mechanism inconclusive.…”