Helper T lymphocytes recruit and activate cytotoxic T cells principally through which cytokine?

Correct answer: D. Interleukin-2

• A. IL-3 is a hematopoietic growth factor acting on bone marrow progenitor cells; it is not the cytotoxic T-cell activator.
• B. IL-1 is produced mainly by macrophages and is a pro-inflammatory mediator causing fever and endothelial activation. In periodontal disease IL-1β is the key driver of osteoclastic bone resorption — an important association, but not the T-cell recruitment signal.
• C. IL-4 drives the Th2 response — B-cell class switching to IgE and allergic inflammation — which is functionally the counterpart to, not the driver of, cell-mediated cytotoxicity.
• D. CORRECT. INTERLEUKIN-2 is the principal T-cell growth factor. Once a CD4+ helper T cell is activated, it secretes IL-2, which acts in both autocrine and paracrine fashion to drive clonal proliferation and full activation of CD8+ CYTOTOXIC T lymphocytes as well as of the helper cells themselves. Without IL-2, cytotoxic T cells recognize antigen but fail to expand into an effective effector population.

Note: Cytokines worth carrying into the exam: IL-1β — inflammation and bone resorption (periodontitis). IL-2 — T-cell proliferation. IL-4 — Th2 and IgE. IL-6 — acute phase response. IL-10 — anti-inflammatory. TNF-α — inflammation and bone resorption. IFN-γ — macrophage activation.