Inflammasome inhibition as a pathogenic stealth mechanism

Abstract

The activation of inflammasomes containing NBD-LRR (NLRs) or non-NLRs is critical for effective host defense against microbial pathogens. Recent discoveries have uncovered a plethora of pathogenic strategies to inhibit inflammasome-mediated processing of IL-1beta and IL-18. We review recent evidence for viral and bacterial manipulation of the inflammasome, ranging from perturbation of caspase-1 activation to targeting of specific inflammasome components.

Figure 1. Suppression of the two step regulation of IL-1β production by virus and bacteria

Host IL-1β release is prompted by a two step process. First, the activation of NF-κB and AP-1 leads the transcription of pro-IL-1β. A second signal sensed by an NLR triggers the assembly of the inflammasome and the subsequent activation of caspase-1. Active caspase-1 proteolytically processes pro-IL-1β into IL-1β for release. Viruses and bacteria have developed an array of strategies to interfere with inflammasome activation. Myxoma and Shope fibroma viruses utilize POP-like proteins M013 and gp013L to bind ASC/PYCARD and prevent the inflammasome assembly. Poxviruses also express the serpins crmA, Serp2, B13L, and SPI-2 which disrupt the proteolytic activity of caspase-1, potentially through direct binding to caspase-1. Influenza A virus and baculovirus express NS1 and p35, and Y. enterocolitica expresses Yop E and YopT that disrupt caspase-1 oligomerization.. The P. aeruginosa effector molecules ExoU and ExoS block the activation of the NLRC4 inflammasome, while the Mycobacterium tuberculosis gene zmp1 may target the NLRP3 inflammasome. Pneumolysin of Streptococcus pneumonia also blocks IL-1β release by diminishing caspase-1 activity. YopK of Yersinia pseudotuberculosis has a unique mechanisms in that it binds to T3SS and masks detection by the inflammasome Though each of these proteins inhibits caspase-1 activation, it is not known in most cases whether caspase-1 is directly targeted or whether effects are mediated indirectly by targeting of an upstream regulator.