Emerging Roles of Autophagy and Inflammasome in Ehrlichiosis

Abstract

Human monocytic ehrlichiosis (HME) is a potentially life-threatening tick-borne rickettsial disease (TBRD) caused by the obligate intracellular Gram-negative bacteria, Ehrlichia. Fatal HME presents with acute ailments of sepsis and toxic shock-like symptoms that can evolve to multi-organ failure and death. Early clinical and laboratory diagnosis of HME are problematic due to non-specific flu-like symptoms and limitations in the current diagnostic testing. Several studies in murine models showed that cell-mediated immunity acts as a "double-edged sword" in fatal ehrlichiosis. Protective components are mainly formed by CD4 Th1 and NKT cells, in contrast to deleterious effects originated from neutrophils and TNF-α-producing CD8 T cells. Recent research has highlighted the central role of the inflammasome and autophagy as part of innate immune responses also leading to protective or pathogenic scenarios. Recognition of pathogen-associated molecular patterns (PAMPS) or damage-associated molecular patterns (DAMPS) triggers the assembly of the inflammasome complex that leads to multiple outcomes. Recognition of PAMPs or DAMPs by such complexes can result in activation of caspase-1 and -11, secretion of the pro-inflammatory cytokines IL-1β and IL-18 culminating into dysregulated inflammation, and inflammatory cell death known as pyroptosis. The precise functions of inflammasomes and autophagy remain unexplored in infections with obligate intracellular rickettsial pathogens, such as Ehrlichia. In this review, we discuss the intracellular innate immune surveillance in ehrlichiosis involving the regulation of inflammasome and autophagy, and how this response influences the innate and adaptive immune responses against Ehrlichia. Understanding such mechanisms would pave the way in research for novel diagnostic, preventative and therapeutic approaches against Ehrlichia and other rickettsial diseases.

Keywords: autophagy; ehrlichiosis; inflammasome; innate immunity; mechanism; pathogenesis.

Figure 1

Cell-mediated pathogenic and protective responses in a model of fatal ehrlichiosis. Wild type C57BL/6 mice inoculated with virulent Ixodes Ovatus Ehrlichia (IOE) develop multi-organ failure and die between 8 and 10 days post-infection. Here we summarize protective and pathogenic mechanisms considering the cell-mediated immunity to Ehrlichia infection. Neutrophils migrate to infection site and induce strong pro-inflammatory response with hyperproduction of IL-1β, IL-6, IL-10, and chemokines. The role of neutrophils was also associated with induction of TNF-α-producing CD8 T cells that can either mediate organ damage or assist protective mechanisms. Another two important arms that mediate pathogenesis are the role of NK cells in eliminating infected cells in the target organs and the role of regulatory T cells (T regs), that end up shutting down protective mechanisms via IL-10. Protective mechanisms during infection by IOE include the induction of CD4 Th1 cells, that via IFN-γ activate on-site microbicidal functions of macrophages. NKT cells also play a role in stimulating such mechanism via IFN-γ.

Figure 2

Model of canonical inflammasome activation involving regulation of autophagy induced by Ehrlichia. The canonical iflammasome activation pathway proposed in Ehrlichia infection involves two steps. In the first step, Ehrlichia invade the target cell and induce TLR9/MyD88 downstream targets to activate NF-κB. Activated NF-κB function as a transcription factor to upregulate NRLP3 complexes, pro-IL1β and pro-IL-18. In the second step, NRLP3 inflammasomes would be activated after recognition of Ehrlichial PAMPS which are secreted to the cytosol via type I and IV secretion systems. Accumulation of mtDAMPS originated by inhibition of autophagy via MyD88/mTORC1 signaling would also induce inflammasome activation. Inflammasome activation results in activation of caspase-1 that subsequently induces cleavage of pro-IL1β and pro-IL-18 into their mature forms. Mature IL-1β and IL-18 are then secreted to the extracellular space to contribute to the establishment of a pro-inflammatory environment. Direct arrow: positive regulation; blocking arrow: negative regulation; question mark (?) means hypothetical, and not experimentally proven.

Figure 3

Model of regulation of inflammasome activation by type I IFNs in Ehrlichia infection. During Ehrlichia infection, type I IFNs are thought to regulate inflammasome activation under a non-canonical fashion. Initially, Ehrlichia invade the target cell and induce TLR9/MyD88 signaling to upregulate NRLP3 complexes, pro-IL1β and pro-IL-18 via NF-κB. Type I IFNs from autocrine or paracrine sources would signal through IFNAR to upregulate GBPs and caspase-11. GBPs would disrupt vesicles containing Ehrlichia allowing the escape of ehrlichial PAMPs to the cytosol. Ehrlichial PAMPs would bind caspase-11 and subsequently induce inflammasome activation followed by secretion of mature IL-1β and IL-18 (as in the canonical pathway) and cleavage of gasdermin D resulting in pyroptosis. Direct arrow: positive regulation; blocking arrow: negative regulation; question mark (?) means hypothetical, and not experimentally proven.