How cytotoxic T cell induce apoptosis?

• When cell is infected by infectious agent; ex: virus, viral protein may be complexed with MHC-I and expressed on surface of cell.
• Cytotoxic t-cell comes and recognizes them with help of T-cell receptors (TCR) and with help of CD₈⁺ molecule, cytotoxic T-cell will confirm, if antigen presented with MHC-I molecule or not. MHC-I molecule interacts with CD₈⁺ cell.
• Once TCR and CD₈⁺ both becomes activated, they give signal to CD₃ molecule and cytotoxic T-cell gets activated.
• Within cytotoxic T-cell molecule, there is pre-formed peptide chain. These peptide chain are released and gets assembled on surface of target cell.
• Peptides get polymerized into pores called perforins. Intracellular constituents come out and extracellular constituents comes in; which hampers survivability of cell.
• Cytotoxic T-cell then release granzyme molecules into cell through porins.
• Granzyme digest inhibitory peptide of executionery caspases.
• Executioner caspases gets activated. They start to digest cytoskeleton of cytoplasm, nucleus and activate DNAases.
• DNA gets broken and cell membrane becomes altered and forms blebs around it. Blebs then gets separated into apoptotic bodies which are then phagocytosed.

Clearance of Apoptotic cells:

• In normal cell, phosphatidyl serine is present on inner leaflet of plasma membrane. But in apoptotic cells, this phospholipid flips to outer leaflet. It is then recognized by tissue macrophages and leads to phagocytosis of cell.
• Cells dying by apoptosis also secretes soluble factors that recruit phagocytes. This result in prompt clearance before they undergo secondary membrane damage.