Figure 1.

The classification and location of intraepithelial lymphocytes (IELs). Gut epithelium is composed of a single layer of enteroendocrine cells (intestinal epithelial cells). IELs are a group of heterogenous cells embedded within intestinal epithelium. Dependeing on the expression of TCR, they can be divided into TCR⁺ and TCR^- IELs. TCR⁺ IELs include αβ and γδ T cells. The former includes TCRαβ⁺ CD4⁺, TCRαβ⁺ CD8αβ⁺, as well as induced TCRαβ⁺ CD4⁺ CD8αα⁺ and TCRαβ⁺ CD8αβ⁺ CD8αα⁺ cells. TCRαβ⁺, TCRαβ⁺ CD8αα⁺, γδ IELs and TCR^- IELs cells are natural cellular subsets. γδ IELs consists of TCRγδ⁺ and TCRγδ⁺ CD8αα⁺ cells, while TCR^- IELs comprises ILC1-like, ILC3-like, and iCD3⁺ cells including its special subset, iCD8α.