Cytotoxic T cells (CTLs), often referred to as “killer T cells,” are specialized white blood cells in the body’s adaptive immune system. They continuously survey the body, acting as specific sentinels that identify and eliminate dangerous or abnormal cells. Their primary function is to target and destroy cells that pose a threat, protecting against various diseases. The precision with which these cells operate ensures that only compromised cells are removed while healthy tissues remain unharmed.
How Cytotoxic T Cells Identify Targets
Cytotoxic T cells identify targets through an intricate molecular recognition process involving Major Histocompatibility Complex (MHC) Class I molecules. Almost all nucleated cells express MHC Class I molecules on their surface, displaying small protein fragments, known as peptides, from inside the cell. Healthy cells typically present “self” peptides, which are normal fragments of the cell’s own proteins, signaling they are healthy and should not be attacked.
When a cell becomes infected by a virus or turns cancerous, it produces “non-self” or abnormal proteins. These abnormal proteins are processed within the cell, and their peptide fragments are loaded onto MHC Class I molecules. These MHC I-peptide complexes are then transported to the cell surface, presented for inspection by patrolling CTLs.
The T cell receptor (TCR) on the CTL’s surface specifically binds to these MHC I-peptide complexes. This specific binding acts as the primary mechanism by which CTLs differentiate between healthy and compromised cells. The CD8 coreceptor on the CTL also binds to the MHC class I molecule, stabilizing the interaction and ensuring effective recognition.
Specific Targets of Cytotoxic T Cells
Cytotoxic T cells target and destroy several types of compromised cells. A primary target includes virus-infected cells. When a virus invades a host cell, it hijacks the cell’s machinery to replicate, producing viral proteins. These viral proteins are processed and presented on the cell surface by MHC Class I molecules, signaling to CTLs that the cell is infected and must be eliminated to prevent further viral spread.
Another target for CTLs is cancer cells. Cancer cells often accumulate genetic mutations, leading to the production of abnormal proteins or “neoantigens” not found in healthy cells. These unique neoantigens are processed and presented on MHC Class I molecules on the cancer cell’s surface. This presentation allows CTLs to recognize and target the cancerous cell for destruction, contributing to the body’s immune surveillance. CTLs also target other abnormal cells, such as those with intracellular bacterial infections or cells undergoing significant stress, which present unusual peptides on their MHC Class I molecules.
How Cytotoxic T Cells Eliminate Targets
Once a cytotoxic T cell identifies and binds to a target cell, it initiates a precise process to eliminate the threat. The CTL achieves this by releasing specialized cytotoxic proteins stored in granules within its cytoplasm. This process, known as granule exocytosis, involves the CTL repositioning its internal machinery to direct these granules towards the target cell.
Two primary types of proteins are released: perforin and granzymes. Perforin forms pores in the target cell’s membrane, creating channels that disrupt its integrity. Through these pores, granzymes, a group of serine proteases, enter the target cell.
Once inside, granzymes initiate a cascade of enzymatic reactions that lead to programmed cell death, known as apoptosis. Apoptosis is a regulated form of cell death that causes the cell to shrink and break into small, membrane-bound fragments, which are then cleared away by other immune cells. This prevents the release of harmful cellular contents and minimizes inflammation. This controlled destruction differs from necrosis, an uncontrolled cell death that can release cellular components and trigger inflammation.