title: Itaconate mediated NLRP3 inflammasome tolerance in the context of human monocyte Tenascin-C activation
Creators: name: Didov, Bogdan Stankov
Publisher: name: University of Glasgow
Year: 2025

description: Despite the extensive research conducted on innate immune cell memory, there are still many unknowns when it comes to the ability of endogenous danger molecules, also known as damage-associated molecular patterns (DAMPs), to influence the tolerisation of myeloid cells to repeated stimulation. This is partly due to the majority of tolerisation studies using exogenous pathogen-associated molecular patterns (PAMPs) such as Lipopolysaccharide (LPS). While understanding virulence factor-induced LPS tolerance is important for treating infectious diseases and sepsis patients, it is essential to examine cellular stimulations using disease-relevant DAMPs to fully comprehend the development of chronic inflammatory conditions such as rheumatoid arthritis. In my work, I have utilized the Toll-like receptor 4 (TLR4) activating fibrinogen-like globe domain (FBG-C) of the extracellular matrix protein tenascin-C to advance our scientific understanding of the ability of DAMPs to trigger and tolerise a critical aspect of TLR-mediated inflammation known as the NLRP3 inflammasome.

I discovered significant differences in the capacity of primary human monocytes to produce and secrete the non-inflammasome regulated cytokine tumour necrosis factor (TNF) compared to the inflammasome regulated cytokine interleukin-1 beta (IL-1β), as well as in their capacity to secrete IL-1β via classical NLRP3 mediated pyroptotic means versus alternative non-pyroptotic means. Additionally, I found that although the DAMP FBG-C and the PAMP LPS induce a similar cytokine response in primary human monocytes in the first 24 hours of activation, subsequent restimulation revealed fundamental differences in their capacity to induce tolerisation. Notably, this effect is specific to IL-1β, and hence the NLRP3 inflammasome, and not due to differences in the requirement for the TLR4 co-receptor cluster of differentiation 14 (CD14).

Furthermore, I found that the inability of monocytes to tolerise the NLRP3 inflammasome following FBG-C activation is due to a delayed upregulation response of the itaconate-producing enzyme aconitate decarboxylase 1 (ACOD1). I then uncovered that ACOD1 upregulation is crucial for inhibiting the processing of the pore-forming protein Gasdermin D and thus for inhibiting pyroptosis in tolerised monocytes. Importantly, I also showed that monocytes isolated from RA patient blood have a delayed ACOD1 upregulation response following both LPS and FBG-C TLR4 activation, which prevented the tolerisation of RA monocytes in a Gasdermin D and pyroptosis-dependent manner. Finally, I uncovered that a 2% oxygen (hypoxic) culture environment renders human monocytes unable to tolerise the NLRP3 inflammasome in an ACOD1-independent manner.

In summary, my research revealed significant differences in the way primary human monocytes interpret pathogen and damage signals, in addition to uncovering substantial changes in their ability to produce the pleiotropic proinflammatory cytokine IL-1β over a 48-hour period. This work demonstrated that environmental stressors, endogenous triggers and chronic pathology could all trigger loss of tolerance and a prolonged NLRP3 inflammasome priming phenotype.