Inflammasomes

Question 1

Explain how a Mycobacterium tuberculosis infection can result in pyroptosis.

Answer 1

When Mtb escapes the phagosome, for example by using the type 7 secretion system ESX1 to damage the phagosomal membrane, it ends up in the cytoplasm. In the cytoplasm, ESX1 also forms pores in the plasma membrane, resulting in a K+ efflux that activates NLRP3. Subsequently, NLRP3 forms an inflammasome with pro-caspase-1 and ASC. Caspase-1 can then cleave itself, and form active caspase-1 which can cleave pro-IL-1B and pro-IL-18. Caspase-1 also cleaves pro-Gasdermin-D to form active Gasdermin-D, which can form pores that lead to pyroptosis. IL-1B and IL-18 use these pores to perform their function around neighbouring cells.

Question 2

How can a macrophage avoid pyroptosis?

Answer 2

A machinery called ESCRT can repair both the phagosomal membrane as the plasma membrane, resulting in Mtb not being able to escape the phagosome, or the macrophage not undergoing Mtb-induced pyroptosis due to avoiding a K+ efflux.

Question 3

What is the difference between the canonical and non-canonical activation of inflammasomes?

Answer 3

With the canonical activation of an inflammasome, the recognition of a PAMP results in the transcription of inflammasome components (e.g. NLRP3, ASC, pro-caspase-1, pro-IL-1B, and pro-IL-18). After this first signal, pathogens have to escape the phagosome and end up in the cytosol to activate the NLRP3 inflammasome by damaging the plasma membrane, resulting in a K+-efflux. After this second signal, the inflammasome can be activated, caspase-1 becomes activated, and cleaves pro-IL-1B, pro-IL-18, and pro-Gasdermin-D. Active Gasdermin-D forms pores in the membrane, which results in pyroptosis and the ability of the IL-1B and IL-18 to migrate through these pores and function at neighbouring cells. With non-canonical activation of the inflammasome, the PAMP LPS is directly sensed by the human pro-caspase-4/5, which become activated and can cleave pro-Gasdermin-D to form active Gasdermin-D, which forms pores in the membrane, which results in pyroptosis.