T cells are expanded, or proliferate rapidly, when a naïve T cell recognizes and binds to a specific antigen. This binding activates the T cell, initiating clonal expansion.
Here's a breakdown of the T cell expansion process:
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Antigen Recognition: The process begins with a T cell receptor (TCR) on the surface of a naïve T cell recognizing a specific antigen presented by an antigen-presenting cell (APC), such as a dendritic cell, macrophage, or B cell. The antigen is presented in the context of a major histocompatibility complex (MHC) molecule.
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Activation Signals: TCR binding to the antigen-MHC complex provides the first signal for T cell activation. However, a second co-stimulatory signal is also required for full activation and to prevent anergy (T cell inactivation). This second signal typically involves the interaction of co-stimulatory molecules on the APC (e.g., B7) with their receptors on the T cell (e.g., CD28).
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Clonal Expansion: Once the T cell is fully activated, it undergoes clonal expansion. This involves rapid cell division, leading to a large population of T cells with the same antigen specificity. These expanded T cells are essentially clones of the original activated T cell.
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Differentiation: After clonal expansion, T cells differentiate into effector T cells (e.g., cytotoxic T lymphocytes (CTLs) or helper T cells (Th cells)) that can carry out specific immune functions. Some T cells also differentiate into memory T cells, which provide long-lasting immunity.
In vitro (in the laboratory), T cell expansion can be achieved by:
- Stimulating T cells with specific antigens: This involves presenting the T cells with their cognate antigen, often in the form of peptide-loaded MHC molecules.
- Using anti-CD3 and anti-CD28 antibodies: These antibodies can mimic the TCR and co-stimulatory signals, respectively, to activate T cells and induce expansion.
- Adding cytokines: Cytokines like IL-2 are crucial for T cell proliferation and survival. They can be added to the culture medium to enhance T cell expansion.
Clonal T cell expansion is critical for effective adaptive immune responses against pathogens and other threats. It ensures that there are enough T cells specific to a particular antigen to eliminate the infection or target the threat effectively.
