Activation of ASC Inflammasome Driven by Toll-Like Receptor 4 Contributes to Host Immunity against Rickettsial Infection

Abstract

Rickettsiae are cytosolically replicating, obligately intracellular bacteria causing human infections worldwide with potentially fatal outcomes. We previously showed that Rickettsia australis activates ASC inflammasome in macrophages. In the present study, host susceptibility of ASC inflammasome-deficient mice to R. australis was significantly greater than that of C57BL/6 (B6) controls and was accompanied by increased rickettsial loads in various organs. Impaired host control of R. australis in vivo in ASC−/− mice was associated with dramatically reduced levels of interleukin 1β (IL-1β), IL-18, and gamma interferon (IFN-γ) in sera. The intracellular concentrations of R. australis in bone marrow-derived macrophages (BMMs) of TLR4−/− and ASC−/− mice were significantly greater than those in BMMs of B6 controls, highlighting the important role of inflammasome and these molecules in controlling rickettsiae in macrophages. Compared to B6 BMMs, TLR4−/− BMMs failed to secrete a significant level of IL-1β and had reduced expression levels of pro-IL-1β in response to infection with R. australis, suggesting that rickettsiae activate ASC inflammasome via a Toll-like receptor 4 (TLR4)-dependent mechanism. Further mechanistic studies suggest that the lipopolysaccharide (LPS) purified from R. australis together with ATP stimulation led to cleavage of pro-caspase-1 and pro-IL-1β, resulting in TLR4-dependent secretion of IL-1β. Taken together, these observations indicate that activation of ASC inflammasome, most likely driven by interaction of TLR4 with rickettsial LPS, contributes to host protective immunity against R. australis. These findings provide key insights into defining the interactions of rickettsiae with the host innate immune system.

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

“…The copyright holder for this preprint this version posted September 8, 2021. ; https://doi.org/10.1101/2021.09.08.459437 doi: bioRxiv preprint and non-canonical inflammasome-dependent defense sensing, of which the former is in agreement with recent studies using R. australis (Rumfield et al, 2020). Thus, our data suggests that a dampening of both canonical and non-canonical inflammasomedependent signaling contributes to the enhanced survival and dissemination of pathogenic Rickettsia spp.…”

“…Recent findings further suggest that the intracytosolic survival of different Rickettsia spp. is either supported or suppressed by immune defense responses (e.g., IFN-I, TNF-α, or IL-1β) (Bechelli et al, 2018;Burke et al, 2020;Engström et al, 2019;Papp et al, 2016;Rumfield et al, 2020;Smalley et al, 2016), leaving the precise mechanism to be determined. Therefore, we first sought to evaluate the canonical and non-canonical inflammasome responses at the level of IL-1β and IL-1α cytokine secretion in the sera of R. typhi-, R. rickettsii-, and R. montanensis-infected animals.…”

“…manipulate immune defenses to replicate within the host cytosol not only relies almost exclusively on data from tick-transmitted rickettsiae [e.g., members of spotted fever group (SFG) or transition group (TRG)], but also shows that these pathogens likely employ speciesspecific strategies to evade host cytosolic defense mechanisms. For instance, R. australis, a pathogenic TRG member, benefited from ATG5-mediated autophagy induction and suppression of inflammasome-dependent IL-1b production to colonize both BMDMΦ (Bechelli et al, 2018) and mice (Rumfield et al, 2020). In contrast, R. parkeri, a mildly pathogenic member of SFG (see Figure 1A), utilizes its surface cell antigen Sca5 (OmpB) for protection against autophagic recognition, and consequently benefits from inflammasome-mediated host cell death that antagonizes the action of Type I IFN in BMDMΦ and mice (Burke et al, 2020;Engström et al, 2019).…”

Subversion of Host Innate Immunity by Rickettsia australis via a Modified Autophagic Response in Macrophages

“…These inflammatory cytokines upregulated by Atg5-dependent autophagy may contribute to reduced rickettsial loads in tissues of R. australisinfected Atg5-deficient macrophages (4). However, R. australis primarily targets microvascular endothelial cells in vivo while macrophages are the initial target cells at the cutaneous entry site of rickettsiae (3,59,60). It is not surprising to see a difference in cytokine profiles in R. australis-infected BMMs and sera of R. australis-infected Atg5-conditional knockout mice.…”

“…Although microvascular endothelial cells are the primary target of rickettsial infection (2), rickettsiae effectively invade macrophages and other types of cells, such as dendritic cells and hepatocytes. We, and others, have recently demonstrated that Rickettsia invades and survives in human macrophages, while macrophages play an important role in the pathogenesis of rickettsioses (3)(4)(5)(6)(7)(8)(9). However, it remains poorly understood how virulent rickettsiae utilize macrophages to develop a systemic infection in mammalian hosts.…”