Immunology checkpoint — Which signaling molecule specifically triggers clonal expansion (division) of activated T lymphocytes?

Introduction / Context:
Following antigen recognition and costimulation, T cells require cytokine signals to proliferate. Identifying the principal autocrine growth factor for T cells is a staple concept in basic immunology and vaccine science.

Given Data / Assumptions:

• Activated T cells upregulate the high-affinity IL-2 receptor (CD25).
• IL-2 promotes clonal expansion and survival of effector and regulatory T cells.
• Other growth factors primarily target non-lymphoid cells.

Concept / Approach:
Interleukin-2 (IL-2) acts in an autocrine and paracrine manner to drive T-cell proliferation. The IL-2/IL-2R axis is central to adaptive immune responses and is targeted therapeutically in transplantation and cancer immunotherapy.

Step-by-Step Solution:

Verification / Alternative check:
Classical experiments show exogenous IL-2 drives T-cell proliferation in culture; blocking IL-2/IL-2R signaling impairs clonal expansion in vivo.

Why Other Options Are Wrong:

• a/c: FGF and PDGF stimulate mesenchymal and vascular cells rather than T cells.
• d: EPO acts on erythroid progenitors in bone marrow.
• e: TNF-alpha is proinflammatory but not the primary mitogen for T-cell clonal expansion.

Common Pitfalls:
Confusing general growth factors with lymphocyte-specific cytokines; overlooking the requirement for IL-2 receptor upregulation (CD25) after activation.

Final Answer:
Interleukin-2 (IL-2)