Questions: T Cell Activation and Costimulatory Signals
3 questions to test your understanding
Score: 0 / 3
Question 1 Multiple Choice
A T cell's TCR engages an MHC-peptide complex on an antigen-presenting cell, but the APC lacks CD80 and CD86 expression. What is the most likely outcome for the T cell?
AFull activation with IL-2 secretion
BAnergy — functional inactivation without response
CImmediate apoptosis via Fas-FasL signaling
DDifferentiation into a regulatory T cell
Without CD80/CD86, CD28 cannot deliver signal 2. TCR engagement (signal 1) alone drives T cell anergy — the cell is rendered functionally unresponsive rather than activated. This is a deliberate safeguard against autoimmunity: self-reactive T cells that encounter antigen without proper APC context are silenced rather than expanded.
Question 2 True / False
The requirement for two signals (TCR engagement plus CD28 costimulation) before a T cell fully activates serves as a safeguard against inappropriate immune responses to self-tissue.
TTrue
FFalse
Answer: True
Normal self-tissues typically express MHC-peptide but lack the co-stimulatory molecules CD80/CD86, which are upregulated on APCs primarily during infection or inflammation. A T cell that recognizes self-antigen in the absence of costimulation becomes anergic rather than activated, preventing autoimmune attack on healthy tissue.
Question 3 Short Answer
What cytokine does costimulation primarily induce T cells to produce, and why is that cytokine's receptor also upregulated at the same time?
Think about your answer, then reveal below.
Model answer: Costimulation induces IL-2 production. The IL-2 receptor (IL-2R, specifically the high-affinity alpha chain CD25) is upregulated simultaneously so the activated T cell can respond to the IL-2 it secretes in an autocrine loop, driving its own proliferation and differentiation.
IL-2 is the primary T cell growth factor. Producing IL-2 without upregulating its receptor would be wasteful; co-upregulation creates an autocrine positive-feedback loop that amplifies the activated T cell clone rapidly. This two-step logic — produce the signal and the receptor simultaneously — is a recurring theme in lymphocyte activation.