What is Pyroptosis and How It Proceeds
The term pyroptosis originates from two roots: pyros, meaning "fire" and symbolizing a prominent inflammatory response, and ptosis, meaning "falling" or death. Thus, it is a form of cell death that is always accompanied by inflammation.
The mechanism of this process follows a strict sequence of pathophysiological events:
- Cytokine activation. Initially, there is a hydrolytic activation of key pro-inflammatory mediators, primarily interleukins IL-1$\beta$ and IL-18.
- Membrane damage. Specific pores begin to form within the cytoplasmic membrane of the affected cell.
- Release of cellular contents. Through these formed pores, the internal contents of the cell are rapidly released outward. Notably, this release occurs directly into the interstitium (intercellular space), rather than into the cytosol, vascular lumen, or synovial fluid.
- Inflammatory response. The entry of intracellular components into the interstitium naturally triggers the development of a powerful perifocal inflammatory response around the lysed cell.
Biological Role of Pyroptosis
Despite the death of the cell, this process holds immense biological significance for host defense. Pyroptosis serves as a crucial tool of the innate immune system.
Its main objectives include:
- Pathogen destruction. The process ensures the reliable destruction of microbes that have managed to invade the intracellular environment.
- Stopping replication. By destroying the intracellular habitat, it effectively halts the intracellular proliferation of infectious agents. The cell literally sacrifices itself to prevent the further spread of infection.
Ferroptosis — An Iron-Dependent Alternative
Alongside pyroptosis, another variant of cell death exists — ferroptosis (from Latin ferrum — iron and ptosis — falling). This is a distinct, iron-dependent form of cell death whose mechanism is fundamentally different from inflammatory pyroptosis.
Stages of ferroptosis development:
- Initiation. The process is triggered by iron-dependent and excessive generation of reactive oxygen species (ROS).
- Progression. There is a sharp intensification of lipid peroxidation (LPO), driven by the direct participation of iron ($Fe$) ions.
- Biochemical basis. A critical link in the pathogenesis is the severe depletion of intracellular glutathione and the complete blockade of the protective enzyme glutathione peroxidase.
- Outcome. The process concludes with fatal lipid peroxidation, inevitably leading to the disruption of cellular structures.
General Patterns of Cell Injury
Any cell injury, regardless of whether it ends in pyroptosis, ferroptosis, or another outcome, induces two broad groups of changes. The first group comprises changes common to various damaging factors and different cell types. The second group consists of specific changes characteristic of a given pathogenic factor acting on different cells, or intrinsic to a specific (particular) cell type.
Examples of specific factor actions:
- Uncouplers of oxidation and phosphorylation. These include an excess of non-esterified fatty acids (NEFAs) and calcium ($Ca^{2+}$) ions. They reduce or completely block the coupling of oxidation and phosphorylation processes, leading to a drop in biological oxidation efficiency and critically reduced ATP levels.
- Aldosterone excess. Upregulation of this adrenal cortex hormone's effects leads to the pathological intracellular accumulation of excess sodium ($Na^+$) ions.
- Cellular specificity. The action of various agents on strictly defined cell types induces changes unique only to those specific cells.