Perforin/Granzyme Apoptosis Pathway

The perforin-granzyme pathway is a crucial immune mechanism used by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells to kill virus-infected cells and tumor cells. It involves the targeted delivery of cytotoxic proteins into infected or abnormal cells, leading to their death.

Key Components of the Perforin-Granzyme Pathway:

1. Perforin:

   Perforin is a pore-forming protein stored in cytotoxic granules. Perforin inserts into the target cell membrane, forming pores that allow entry of granzymes.

2. Granzymes:
   The Granzyme pathway induces apoptosis via either granzyme A or granzyme B. Granzymes are Serine proteases (especially granzyme B) that enter the target cell through perforin-formed pores. Granzymes trigger apoptosis by converting procaspase-10 into caspase-10. The caspase-10 convert procaspase-3 into caspase-3.  It also Cleave Bid, which promotes mitochondrial outer membrane permeabilization (MOMP) and the release of cytochrome c.

3. Apoptosis (Programmed Cell Death):

   Caspase-3 activates CAD. CAD proceeds for DNA fragmentation and associated with apoptosis. Granzyme A forms a SET complex which also induces DNA cleavage and apoptosis. The end result of the pathway is the activation of apoptosis. This process is non-inflammatory and prevents the spread of infection or malignant cells. This mechanism used to kill tumour cells and virus-infected cells. Cytotoxic T-cells release perforin and granzyme proteins. Perforin protein pore in the membrane of infected cells, allowing the entry of granzyme A and granzyme B.

Sequence of Events:

1. Recognition of virus infected cell by CTL/NK cell via MHC I (for CTLs) or stress ligands (for NK cells).

2. TCR leads the activation of the PI3K pathway for the fusion of vesicles which is containing perforin and granzyme..

3. Exocytosis of vesicle containing perforin and granzymes into the synaptic cleft.

4. Perforin forms pores in the target cell membrane.

5. Granzymes enter and initiate apoptosis.

Granzyme B has many substrates located in the nucleus. Granzyme B can cleave PARP (poly ADP ribose polymerase) and DNA PK (DNA protein kinase) to disrupt DNA repair and retroviral DNA integration. Granzyme B can also cleave nucleophosmin, topoisomerase 1 and nucleolin to prevent viral replication.

Biological Significance:

• Helps in immune surveillance.

• Crucial in clearing virus-infected cells and cancer cells.

• Defects in this pathway can lead to immune evasion by tumors or chronic viral infections.