Questions: T Cell Development and Thymic Selection

Question 1

A thymocyte produces a TCR that binds self-MHC class II molecules with moderate affinity but does not react strongly to the self-peptide presented. What is this thymocyte's most likely fate?

• A. Eliminated by negative selection because any binding to self-MHC is dangerous
• B. Eliminated by positive selection because it can't bind self-MHC class I
• C. Survives both selection steps and matures into a CD4+ T helper cell
• D. Survives both selection steps and matures into a CD8+ cytotoxic T cell

Answer: (C)

Positive selection requires moderate binding to self-MHC — this TCR passes. Negative selection only eliminates TCRs that bind self-MHC-peptide *too* strongly — this TCR does not. Since it binds MHC class II (not class I), lineage commitment produces a CD4+ helper T cell. A TCR that fails to bind self-MHC at all dies by neglect in positive selection; one that binds self-MHC-peptide too strongly is deleted in negative selection. This thymocyte threads the needle between both failure modes.

Question 2

A student proposes: 'The thymus should delete all T cells that bind self-MHC, to prevent autoimmunity — any self-MHC binding is a risk.' Why would implementing this policy be catastrophic?

• A. It would leave too few T cells because most thymocytes would survive negative selection anyway
• B. T cells require self-MHC binding to function at all — without MHC restriction, T cells could never recognize a pathogen-infected or antigen-presenting cell
• C. Without T cells that bind self-MHC, autoimmune T cells in the periphery would go unchecked
• D. This would deplete regulatory T cells, eliminating all suppression of B cell responses

Answer: (B)

This scenario targets the purpose of positive selection. T cells detect antigens only when presented on MHC molecules — that is how the adaptive immune system works. A T cell whose TCR cannot bind self-MHC would be useless: it could never recognize an infected cell, because all antigen presentation occurs through MHC. Positive selection ensures every mature T cell is MHC-restricted. The subtlety is that binding self-MHC (the function requirement) is distinct from reacting too strongly to self-MHC-peptide (the autoimmunity risk).

Question 3

Positive selection and negative selection apply opposite criteria: positive selection eliminates T cells that cannot bind self-MHC, while negative selection eliminates T cells that bind self-MHC-peptide too strongly.

• T. True
• F. False

Answer: True

This is the defining logic of thymic education. Positive selection in the cortex: fail to bind self-MHC → die by neglect. Negative selection in the medulla: bind self-MHC-peptide too strongly → die by apoptosis. The surviving thymocytes thread a narrow range — they recognize self-MHC well enough to function but do not react destructively to self-antigens. The two stages enforce logically opposite criteria applied sequentially, which is why both are required.

Question 4

AIRE (autoimmune regulator) allows the thymus to conduct negative selection against tissue-specific antigens like insulin, even though the thymus itself is not pancreatic tissue.

• T. True
• F. False

Answer: True

AIRE is a transcription factor expressed in thymic medullary epithelial cells that drives ectopic expression of tissue-specific proteins — including insulin, thyroid antigens, lens proteins, and many others — right in the thymus. Thymocytes whose TCRs react strongly to these self-antigens are deleted before they leave the thymus. Without AIRE (as in autoimmune polyendocrinopathy syndrome type 1), tissue-reactive T cells escape to the periphery and attack their target organs, demonstrating how critical AIRE-mediated negative selection is to self-tolerance.

Question 5

Why must thymic selection involve two sequential stages with opposite criteria, rather than a single selection round, and what failure mode does each stage prevent?

Model answer: The two stages address two distinct failure modes that a single stage cannot handle simultaneously. Positive selection (cortex) prevents useless T cells: without at least moderate affinity for self-MHC, a T cell can never recognize any antigen-presenting cell and is functionally inert — releasing it into circulation is wasteful and potentially risky. Negative selection (medulla) prevents autoimmunity: T cells that bind self-MHC + self-peptide too strongly would attack the body's own tissues if released. These criteria cannot be merged because they are opposite — the minimum (bind self-MHC) and the maximum (not too strongly) must be enforced in separate stages.

The two-stage logic is elegant: first build in the functional requirement (MHC restriction), then remove the hazardous subset (self-reactive). Only ~2–5% of thymocytes survive both checkpoints, which reflects how demanding the combined criteria are. The AIRE mechanism in negative selection adds a remarkable layer: the thymus can sample a broad inventory of tissue-specific antigens without actually containing those tissues, effectively testing T cell reactivity against the whole body. Failures at either stage have distinct consequences — loss of MHC restriction (positive selection failure) versus autoimmune disease (negative selection failure).