Critical role for the AIM2 inflammasome during acute CNS bacterial infection

Abstract

Interleukin-1β (IL-1β) is essential for eliciting protective immunity during the acute phase of Staphylococcus aureus (S. aureus) infection in the central nervous system (CNS). We previously demonstrated that microglial IL-1β production in response to live S. aureus is mediated through the Nod-like receptor protein 3 (NLRP3) inflammasome, including the adapter protein ASC (apoptosis-associated speck-like protein containing a caspase-1 recruitment domain), and pro-caspase 1. Here, we utilized NLRP3, ASC, and caspase 1/11 knockout (KO) mice to demonstrate the functional significance of inflammasome activity during CNS S. aureus infection. ASC and caspase 1/11 KO animals were exquisitely sensitive, with approximately 50% of mice succumbing to infection within 24 h. Unexpectedly, the survival of NLRP3 KO mice was similar to wild-type animals, suggesting the involvement of an alternative upstream sensor, which was later identified as absent in melanoma 2 (AIM2) based on the similar disease patterns between AIM2 and ASC KO mice. Besides IL-1β, other key inflammatory mediators, including IL-6, CXCL1, CXCL10, and CCL2 were significantly reduced in the CNS of AIM2 and ASC KO mice, implicating autocrine/paracrine actions of IL-1β, as these mediators do not require inflammasome processing for secretion. These studies demonstrate a novel role for the AIM2 inflammasome as a critical molecular platform for regulating IL-1β release and survival during acute CNS S. aureus infection.

Keywords: AIM2; ASC; Staphylococcus aureus; caspase 1; inflammasome; interleukin-1.

Figure 1. The inflammasome molecules ASC and caspase-1/11, but not NLRP3, are critical for survival during acute CNS S. aureus infection

C57BL/6 WT and NLRP3 KO (A and D), ASC KO (B and E), or caspase-1/11 KO (C and F) mice were injected intracranially with 10⁴ cfu of S. aureus USA300 and assessed for survival (A–C; n=7–16/group from 2–6 independent experiments). Bacterial burdens from NLRP3 KO mice (D) were assessed at days 3 and 7, whereas ASC and caspase-1/11 KO mice (E and F, respectively) were determined at 12 and 18 h after infection, to avoid potential confounds from survival bias (n=7–12/group). Significant differences in survival and bacterial burdens between groups were determined by a log-rank test and Student’s t-test, respectively (*, p < 0.05; **, p < 0.01).

Figure 2. ASC loss affects several inflammatory mediators besides IL-1β

C57BL/6 WT and ASC KO mice (7–8/group) were injected intracranially with 10⁴ cfu of S. aureus USA300, whereupon abscess homogenates were collected at the indicated time points after infection for quantiation of IL-1β (A), IL-6 (B), CCL2 (C), and CXCL10 (D) expression by MILLIPLEX multi-analyte bead arrays. Results were normalized to total protein content for each sample and significant differences in mediator expression were determined by Student’s t-test (*, p < 0.05; **, p < 0.01; and ***, p < 0.001).

Figure 3. AIM2 is critical for survival and inflammatory mediator induction during acute CNS infection

C57BL/6 WT and AIM2 KO mice (7–8/group) were injected intracranially with 10⁴ cfu of S. aureus USA300, whereupon survival (A), bacterial burdens (B), and IL-1β (C), IL-6 (D), CCL2 (E), and CXCL10 (F) levels in abscess homogenates were quantified by MILLIPLEX multi-analyte bead arrays. Results were normalized to total protein content for each sample and significant differences in mediator expression were determined by Student’s t-test (*, p < 0.05).

Figure 4. The AIM2 inflammasome is protective during acute CNS S. aureus infection

A disconnect in phenotypes between the inflammasome sensor NLRP3 and its adaptor ASC during acute CNS S. aureus infection led to the discovery of AIM2 as a critical inflammasome sensor. The AIM2 inflammasome is potentially triggered by dsDNA in cells harboring intracellular S. aureus, leading to ASC and caspase-1 recruitment, resulting in pro-IL-1β processing and cytokine secretion. This cascade, in turn, is protective to the host during acute infection that is independent of controlling bacterial burdens. The NLRP3 inflammasome is also activated in response to S. aureus challenge by α-hemolysin (hla); however, it is not critical for host survival. Furthermore, the cross-talk between AIM2 and NLRP3 inflammasomes is also a possibility as described in recent studies (Wu et al. 2010, Kim et al. 2010). ASC also regulates the production of other inflammatory mediators, presumably via indirect effects mediated by IL-1β action.