Questions: Lymphocyte Trafficking, Homing, and Adhesion Molecules

Question 1

A T cell activated in gut-associated lymphoid tissue (Peyer's patches) is now circulating in the blood. Which combination of homing receptors would you expect it to express, and to which tissue would it preferentially traffic?

• A. L-selectin and CCR7, directing it to lymph nodes for continued antigen surveillance
• B. α4β7 integrin and CCR9, directing it back to gut mucosa to combat the infection where it was first primed
• C. CLA and CCR4, directing it to inflamed skin tissue
• D. No specific homing receptors — memory cells circulate randomly and respond to any inflammatory signal

Answer: (B)

Dendritic cells in gut-associated lymphoid tissue produce retinoic acid and other imprinting signals during T cell activation. These induce expression of α4β7 integrin (which binds MAdCAM-1 on gut endothelium) and CCR9 (which responds to CCL25, a chemokine highly expressed in the small intestine). This ensures effector and memory T cells return to the gut — the tissue type where they first encountered antigen. L-selectin/CCR7 (option A) is the naive T cell phenotype for secondary lymphoid organ homing. CLA/CCR4 (option C) is the skin-homing phenotype imprinted by skin-draining lymph node dendritic cells.

Question 2

Which step in the multi-step adhesion cascade converts a slowly rolling lymphocyte into one that arrests firmly on the endothelium?

• A. L-selectin binding to GlyCAM-1, which directly triggers firm arrest by increasing adhesive friction
• B. Chemokine binding to chemokine receptors, triggering inside-out integrin activation from low- to high-affinity state, enabling firm ICAM-1 binding
• C. Integrin binding to ICAM-1, which initiates rolling by slowing the lymphocyte
• D. Transmigration through the endothelium, which anchors the lymphocyte and pulls it off the vessel wall

Answer: (B)

The cascade is strictly sequential: selectins mediate rolling (slowing), chemokine receptor signaling triggers inside-out activation of integrins (converting them to high-affinity state), and activated integrins then bind ICAM-1 for firm arrest — followed by transmigration. The chemokine-to-integrin step is the critical 'commit' decision: it converts transient selectin-mediated rolling into irreversible arrest. Inside-out signaling means the signal originates intracellularly (chemokine receptor activation) and modifies an outward-facing molecule (integrin conformation). Integrin-ICAM binding (option C) only happens after inside-out activation and causes arrest, not rolling.

Question 3

Naive T cells express L-selectin and CCR7 so that they can efficiently patrol peripheral tissues like the skin and gut, where most foreign antigens are first encountered.

• T. True
• F. False

Answer: False

Naive T cells express L-selectin and CCR7 to home to secondary lymphoid organs — lymph nodes and Peyer's patches — NOT to peripheral tissues. This is where dendritic cells present antigen and where the rare antigen-specific naive T cell is most likely to encounter its cognate antigen. L-selectin binds addressins on high endothelial venules (HEVs) in lymph nodes; CCR7 responds to CCL19/CCL21 expressed in lymphoid tissue. Homing to peripheral tissues is the phenotype of activated effector and memory cells, acquired after activation. Sending naive cells directly to peripheral tissue would be inefficient — they would almost never find their rare antigen.

Question 4

The tissue-homing pattern of a memory T cell reflects where it was originally activated — a T cell primed in gut-associated lymphoid tissue preferentially returns to gut tissue even during subsequent immune responses.

• T. True
• F. False

Answer: True

This is homing imprinting: the tissue environment during T cell activation durably alters homing receptor expression. Gut-draining dendritic cells imprint α4β7/CCR9 (gut-homing); skin-draining lymph node dendritic cells imprint CLA/CCR4/CCR10 (skin-homing). The memory T cell 'remembers' not just the antigen but also where it encountered it and is directed back there. This is physiologically sensible: re-exposure to the same pathogen most likely occurs at the same anatomical barrier site, so positioning memory cells at that site enables rapid local recall responses without requiring the whole adaptive immune system to mobilize from scratch.

Question 5

The lymphocyte trafficking system is often described as a 'molecular postal service with zip codes.' Explain what the zip code analogy captures and identify which molecular players serve as the address, the grip, and the commitment step.

Model answer: The analogy captures tissue-specificity: each tissue expresses a distinctive combination of chemokines and adhesion molecule ligands, and each lymphocyte subset expresses matching receptors that allow it to read specific tissue 'addresses.' The address (zip code) is encoded by the combination of chemokines and addressins displayed on the endothelium of a specific tissue — gut expresses MAdCAM-1 and CCL25; skin-draining nodes express peripheral node addressin and CCL19/21. The commitment step — reading and acting on the address — is chemokine binding triggering inside-out integrin activation. The grip that physically stops the lymphocyte is the high-affinity integrin-ICAM interaction following inside-out activation. Selectins mediate the initial slowing (rolling), analogous to the delivery truck scanning for the right address before slowing to stop.

The elegance of this molecular postal system is that it solves a combinatorial targeting problem in the bloodstream. A single circulation system serves dozens of different tissue destinations. Rather than separate delivery routes, different tissues display unique molecular addresses and different lymphocyte subsets display matching receptors — activated by different tissue environments during priming. The system is dynamic: naive cells read one set of addresses, effector and memory cells read different tissue-specific addresses imprinted during their activation.