Cytokine control of lymphocyte expansion: Which cytokine is the principal growth factor that stimulates the activity and proliferation of T cells (and can promote B-cell responses indirectly)?

Introduction / Context:
Cytokines govern lymphocyte activation, clonal expansion, and differentiation. IL-2 is historically recognized as T-cell growth factor and plays a pivotal role after antigen recognition and costimulation. This question seeks the cytokine best known for directly stimulating T-cell activity and proliferation.

Given Data / Assumptions:

• T cells require signals beyond TCR engagement: costimulation and cytokines.
• IL-2 is produced mainly by activated CD4+ T cells and acts via the high-affinity IL-2 receptor.
• B-cell proliferation can be secondarily supported by IL-2-dependent T-cell help and cytokine milieu.

Concept / Approach:
IL-2 binding to IL-2R (alpha/beta/gamma chains) drives T-cell progression through the cell cycle, survival, and differentiation. While IL-1 is an innate cytokine that enhances T-cell activation, it is not the principal growth factor. Lymphotoxin (TNF-beta) mediates cytotoxicity and lymphoid organ development rather than broad mitogenic stimulation.

Step-by-Step Solution:

Verification / Alternative check:
In vitro T-cell culture requires IL-2; therapeutic IL-2 expands T cells (for example, in adoptive cell therapy).

Why Other Options Are Wrong:

Common Pitfalls:
Assuming any pro-inflammatory cytokine drives T-cell proliferation; in classic immunology, IL-2 is the defining T-cell growth factor.

Final Answer:
Interleukin-2 (IL-2).