T cell expansion is primarily caused by the activation of T cells through antigen recognition and co-stimulatory signals, leading to their proliferation.
Here's a breakdown of the process:
1. Antigen Recognition
- T Cell Receptor (TCR) Binding: T cells possess T cell receptors (TCRs) that recognize specific antigens presented on Major Histocompatibility Complex (MHC) molecules on the surface of antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells. This is the initial signal for T cell activation.
2. Co-Stimulatory Signals
- CD28 Signaling: While TCR binding is crucial, it's often insufficient to fully activate a T cell, especially a naive T cell. Co-stimulatory molecules on the APC, such as B7 molecules (CD80 and CD86), bind to CD28 on the T cell. This CD28 signaling increases the sensitivity of the T cell to antigen receptor engagement and is essential for initiating T cell expansion. Without proper co-stimulation, T cells may become anergic (unresponsive) or undergo apoptosis (programmed cell death).
3. Cytokine Production
- Interleukin-2 (IL-2): Upon activation, T cells, particularly CD4+ helper T cells, produce cytokines, most notably Interleukin-2 (IL-2). IL-2 acts as a growth factor for T cells, promoting their proliferation. Activated T cells also express the IL-2 receptor (CD25), further enhancing their responsiveness to IL-2.
4. Clonal Expansion
- Proliferation: The combination of TCR signaling, co-stimulation (like CD28), and IL-2 signaling drives T cell proliferation. A single activated T cell can undergo multiple rounds of division, resulting in a large population (clone) of T cells specific for the initiating antigen. This process is called clonal expansion.
5. Differentiation
- Effector and Memory Cells: As T cells expand, they also differentiate into effector cells (e.g., cytotoxic T lymphocytes (CTLs) and helper T cells) that carry out immune functions, and memory T cells, which provide long-lasting immunity.
In summary, T cell expansion is a complex process initiated by antigen recognition via the TCR, enhanced by co-stimulatory signals like CD28, and driven by cytokine production, primarily IL-2. This proliferation creates a large population of antigen-specific T cells to effectively combat the infection or threat.
