**Core Concept**
The expansion of T cells in culture is a crucial aspect of immunology, particularly in the context of vaccine development and cancer immunotherapy. The process involves the activation and proliferation of T cells in response to specific antigens or mitogens.

**Why the Correct Answer is Right**
IL-2 (Interleukin-2) is a key cytokine that promotes the proliferation and differentiation of T cells. It acts by binding to its receptor on the surface of T cells, which triggers a signaling cascade that activates the transcription factor NF-κB. NF-κB then induces the expression of genes involved in cell cycle progression, including cyclin D3 and cyclin E. As a result, T cells undergo rapid proliferation, leading to an increase in their numbers.

**Why Each Wrong Option is Incorrect**
**Option A:** IL-1 (Interleukin-1) is a pro-inflammatory cytokine that plays a role in the activation of immune cells, but it does not directly enhance T cell proliferation.

**Option B:** TNF-α (Tumor Necrosis Factor-alpha) is a cytokine involved in systemic inflammation and one of the cytokines that makes up the acute phase reaction. While it has various effects on the immune system, it does not promote T cell proliferation.

**Option C:** TGF-β (Transforming Growth Factor-beta) is a cytokine that inhibits the proliferation of T cells and promotes their differentiation into regulatory T cells, which suppress the immune response.

**Option D:** There is no option D provided, so this section will be skipped.

**Clinical Pearl / High-Yield Fact**
IL-2 has been used as a therapeutic agent in the treatment of certain cancers, such as renal cell carcinoma, and has shown promise in the treatment of autoimmune diseases, such as type 1 diabetes.

**Correct Answer:** C. TGF-β is incorrect because it inhibits T cell proliferation. However, there is no option C. The question only has 3 options.