Cytotoxic T cells, often called killer T cells or CD8+ T cells, are specialized immune cells that detect and eliminate compromised cells within the body. They are a component of the adaptive immune system, meaning they learn to recognize specific threats. Their primary function involves targeting and destroying cells that have become infected with viruses or have turned cancerous. This precise elimination process helps prevent the spread of infections and the growth of tumors, maintaining overall health.
Identifying the Target
Cytotoxic T cells recognize their specific targets through a molecular interaction involving Major Histocompatibility Complex Class I (MHC-I) molecules. Almost all nucleated cells in the body display MHC-I molecules on their surface, serving as a display platform for internal cellular components. When a cell becomes infected with a virus or transforms into a cancerous cell, it begins to produce abnormal or foreign proteins.
These abnormal proteins are broken down into smaller fragments, called antigens, inside the cell. The MHC-I molecules then bind to these antigen fragments and transport them to the cell surface, effectively presenting them as “red flags”. A cytotoxic T cell possesses a unique T-cell receptor (TCR) on its surface that is highly specific for a particular MHC-I/antigen complex. This precise lock-and-key fit ensures that the cytotoxic T cell only binds to and targets cells displaying that specific abnormal antigen.
For full activation and to prevent accidental killing of healthy cells, a cytotoxic T cell requires not only the specific TCR-MHC-I/antigen binding but also additional co-stimulatory signals. These co-stimulatory signals, such as the interaction between CD28 on the T cell and CD80 or CD86 on an antigen-presenting cell, act as a second verification step, confirming that the target cell is indeed compromised. This multi-signal requirement ensures that the immune response is tightly controlled and directed only against true threats.
The Lethal Arsenal: How CTLs Kill
Once a cytotoxic T cell has successfully identified and bound to a compromised target cell, it employs mechanisms to induce programmed cell death, known as apoptosis. This controlled process of cell death is distinct from necrosis, which is a more chaotic and inflammatory form of cell demise. Apoptosis allows for the orderly dismantling of the target cell, preventing the release of harmful cellular contents and minimizing damage to surrounding healthy tissues.
One primary method involves the release of specialized cytotoxic granules containing perforin and granzymes. Upon binding to the target cell, the cytotoxic T cell repositions these granules towards the point of contact, ensuring targeted delivery. Perforin molecules are then released and insert themselves into the target cell’s membrane, polymerizing to form pores or channels. These pores allow granzymes, which are a family of serine proteases, to enter the target cell’s cytoplasm. Once inside, granzymes initiate a cascade of enzymatic reactions, activating proteins called caspases that systematically dismantle the cell’s internal structures and DNA, leading to apoptosis.
The second major pathway for inducing apoptosis is through the interaction of Fas ligand (FasL) on the cytotoxic T cell with the Fas receptor on the target cell. FasL is a protein expressed on the surface of activated cytotoxic T cells. When FasL binds to the Fas receptor on the target cell, it directly triggers a signaling pathway within the target cell. This signaling cascade also leads to the activation of caspases, ultimately resulting in the target cell undergoing apoptosis. Both the perforin/granzyme and Fas/FasL pathways efficiently lead to the target cell’s controlled demise.
Roles in Immune Defense
Cytotoxic T cells play an important role in maintaining the body’s health by targeting specific threats. They are important in defending against viral infections. When a virus infects a cell, it hijacks the cell’s machinery to replicate itself. Cytotoxic T cells recognize these infected cells by the viral protein fragments presented on their surface, then eliminate them before the virus can fully replicate and spread. This helps control the infection and prevent widespread viral dissemination.
Beyond viral defense, cytotoxic T cells are also active in cancer surveillance. Cancer cells often develop mutations that lead to the production of abnormal proteins. These abnormal proteins can be presented on the cell surface via MHC-I molecules, signaling to cytotoxic T cells that the cell is compromised. The cytotoxic T cells can then identify and destroy these nascent cancer cells, potentially preventing the formation or progression of tumors. This continuous monitoring acts as a natural defense against the development of malignancies.
Controlling the Kill: Preventing Self-Damage
Regulatory mechanisms dampen cytotoxic T cell responses once a threat has been cleared. Regulatory T cells, for instance, are a subset of T lymphocytes that help maintain immune tolerance and prevent autoimmunity by suppressing the activity of other immune cells, including cytotoxic T cells. If these control mechanisms are lost or become dysfunctional, cytotoxic T cells can mistakenly attack healthy cells, leading to autoimmune diseases where the body’s immune system turns against its own tissues. This balance is essential for the immune system to effectively protect the body without causing self-inflicted damage.