Question 15 of 20 Intermediate Concept
What do regulatory T cells do, and how are they identified in human blood?
Short answer
Regulatory T cells are CD4 T cells that suppress other immune responses and maintain tolerance to self. In human blood they are identified as CD4 positive, CD25 high and CD127 low, with the transcription factor FOXP3 as the most specific marker, though activated conventional T cells can express FOXP3 and CD25 transiently.
Why
Without regulatory T cells, people develop severe autoimmunity, as in the FOXP3 deficiency IPEX syndrome. They suppress through IL-10 and TGF-beta, by consuming IL-2, and through CTLA-4, which strips costimulatory molecules from antigen-presenting cells. Identifying them is harder than it sounds.
CD25 alone overlaps with recently activated T cells. FOXP3 is intracellular, so it cannot be used to sort living cells. Liu and colleagues showed that CD127, the IL-7 receptor, is low on regulatory T cells, inversely tracks FOXP3 and suppressive function, and allows live sorting as CD4 positive, CD25 positive, CD127 low.
They also found FOXP3 in some CD25 low cells, so a strict CD25 high gate misses some. Human activated conventional T cells can express FOXP3 transiently without suppressing, which complicates analysis of stimulated samples. Methylation of the FOXP3 TSDR region, a stable epigenetic mark, separates committed regulatory cells from activated ones.
For example, an apparent rise in regulatory cells after stimulation may be activated effector cells. Function, measured in a suppression assay, is the final test.
A strong answer covers
- Suppress responses and maintain self-tolerance
- CD4, CD25 high, CD127 low surface phenotype
- FOXP3 is specific but intracellular
- Activated conventional T cells can express FOXP3 transiently
- Suppression assay or TSDR methylation confirms identity
Common mistakes
- Gating regulatory T cells on CD25 alone
- Counting FOXP3 positive cells in stimulated cultures as regulatory without further markers
- Assuming a higher frequency means more suppression without a functional test
The interviewer might ask next
How would you show that expanded regulatory T cells in a cell therapy product are actually suppressive?