Master of Science
Microbiology and Immunology
Kim, Sung O.
Macrophages are innate immune cells that change their cellular states in response to different stimuli. This process is referred to as “innate immune memory” which can render two polarizing states: hyporesponsive (tolerated) or hyperresponsive (trained) states. To date, epigenetic modifications were shown to play a key role in innate immune memory. However, the signalling that induces memory is poorly understood. In this study, I examined the effects of various inhibitors targeting key signalling molecules involved in the activation of macrophages by the Gram-negative cell wall component lipopolysaccharide. I found that prolonged inhibition of the MEK1/2-ERK-RSK signalling axis prevented tolerance and trained macrophages in expressing inflammatory cytokines. Through transcriptomic and biochemical analyses, I showed that inhibition of the signalling axis trained macrophages in part by relieving the gene repression mediated by histone 3 in lysine 9 methylation. This study unravelled a novel role of the signalling axis in innate immune memory.
Summary for Lay Audience
Macrophages are a type of innate immune cell that act as early responders to microbes, damaged cells and cellular components, and environmental substances. These cells can uptake foreign materials and cellular debris through phagocytosis and regulate inflammatory responses by activating signalling pathways and producing molecules involved in the inflammatory process. These cells can become more situation-specific by “polarizing” into specialized subsets based on stimuli. Upon stimulation, they can exhibit a phenomenon known as “innate immune memory”, where cells can become less responsive (hyporesponsive) or more responsive (hyperresponsive) upon further stimulation. Cells that become less responsive are referred to as “tolerated”, leading to less inflammation when restimulated. In contrast, cells that become more responsive become “trained” or “primed”, leading to more inflammation when restimulated. This phenomenon occurs partly through epigenetic modifications that can alter chromatin. Chromatin is DNA that wraps around proteins called histones to form nucleosomes that are either compact (heterochromatin) or open (euchromatin). Epigenetic modifications alter chromatin to adopt a compact or open state through repressive or active chromatin markers respectively. When macrophages become tolerated, they adopt repressive markers that make inflammatory genes inaccessible, preventing them from being expressed. In contrast, macrophages that are primed, adopt active markers that allow for increased production of inflammatory molecules. While these epigenetic modifications are important in innate immune memory, the signalling pathway that triggers these changes is still poorly understood. Using various chemical inhibitors targeting signalling pathways in macrophages, I found that inhibition of the MEK1/2-ERK-RSK signalling pathway caused macrophages to become primed and prevented tolerance. I further found that this inhibition triggered the change from a closed chromatin state to a more open state, allowing for greater expression of inflammatory genes. This study revealed a novel function of the signalling axis in inhibiting macrophage activation through an epigenetic mechanism.
Low, Rachel, "The Role of the MEK1/2-ERK-RSK Signalling Axis in Innate Immune Memory" (2023). Electronic Thesis and Dissertation Repository. 9498.
Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.