Table 1.

Glossary of key terms.

CD4	CD4 is a co-receptor expressed by CD4+ T cells. Following engagement of the T cell receptor/CD4 complex, naïve CD4+ T cells differentiate into various CD4 helper T cells, including regulatory T cells involved in suppressing immune responses and inflammation. Other helper T cells, such as Th17 cells, typically mediate proinflammatory immune response.
CD28	CD28 is a co-stimulatory receptor expressed by T cells. Activation of CD28 by its ligand CD80 or CD86 is necessary to provide the co-stimulation required for effective T cell receptor signaling and activation of T cells.
CD80/86	Both CD80 and CD86 are ligands for CD28 and is expressed mostly by antigen-presenting cells. Both CD80 and CD86 can also bind to CTLA-4, which instead attenuates the T cell receptor signaling response.
CD25	High-affinity receptor for IL-2. CD25 is highly expressed by Treg and its activation by IL-2 is crucial for the maintenance and survival of Treg.
CD103	Also known as Integrin alpha-E, CD103 is a receptor involved in cell homing and adhesion via its binding to its ligand E-cadherin. CD103 is expressed by specialized subset of mucosal dendritic cells (known as CD103+ dendritic cells) that promotes the differentiation of regulatory T cells.
CD69	CD69 expression is upregulated following the activation of T cells. CD69 expression promotes Treg differentiation and also enhances their suppressive by promoting IL-2 and TGF-β production (10)
CD14	CD14 is expressed mostly by macrophages and act as a co-receptor for the detection of bacterial lipopolysaccharide (LPS) alongside Toll-like receptor 4 (TLR4) and MD-2.
T cell receptor (TCR)	T cell receptor are expressed by all T cells and recognize specific antigen (typically peptides) presented by antigen-presenting cells on major histocompatibility complex (MHC) molecules. Engagement of TCR with peptide-MHC molecule leads to the activation of the T cell.
Thymocytes	T cell lineage committed progenitors that develops into mature naïve CD4+ or CD8+ T cells following negative and positive selection in the thymus.
Antigen-presenting cells (APC)	Antigen-presenting cells present antigens loaded on MHC molecules for presentation to naïve T cells and are thus involved in the initiation of an adaptive immune response.
Dendritic cells (DC)	Dendritic cells are a major subset of professional antigen-presenting cells
Medullary thymic epithelial cells (mTEC)	Medullary thymic epithelial cells are the major subset of antigen presenting cell in the thymus. mTEC play a key role in the negative selection of thymocytes, which ensures that thymocyte expressing TCR against self-antigens are removed.
TGF-β	A key cytokine involved in promoting Treg development, by promoting the expression of the transcription factor FoxP3. FoxP3 is a master regulator of Treg differentiation and function.
IL-10	A key cytokine produced by regulatory T cell involved in immune suppression. IL-10 can also be produced by other cell types to promote Treg differentiation.
Pathogen associated molecular pattern (PAMP)	Pathogen associated molecular pattern are conserved microbial motifs that are recognized by pattern recognition receptors such as toll-like receptors (TLR). A common PAMP is lipopolysaccharide (LPS), which is expressed by gram-negative bacteria.
Extracellular vesicles (EV)	Extracellular vesicles are nano-sized particles released by all cell types via the budding of the plasma membrane. They can cargo nucleic acid, proteins and metabolites. Bacterial-derived EV has been shown to interact with host cells, activating TLR to promote Treg differentiation.
Short-chain fatty acids (SCFA)	Short-chain fatty acids are the major metabolite produced by gut bacteria during the fermentation of dietary fiber. SCFA are sensed by the host, and they can directly promote Treg differentiation.