Intraepithelial Lymphocytes (IELS)

Intraepithelial Lymphocytes (IELS) are a unique group of T cells that are located within the epithelial layer of the intestine. They play an essential role in maintaining the immune balance of the intestine and in defending against harmful pathogens. The development and functionality of CD4+ intraepithelial lymphocytes are shaped by a variety of factors. These include dietary elements, interactions with microbes, and specific signaling pathways. For example, the expression of epithelial MHC-II induced by segmented filamentous bacteria (SFB) is vital for regulating cognate CD4+ intraepithelial lymphocytes and for the turnover of epithelial cells.
Brabec et al. 2024 J Exp Med. 221(1): e20230194

Intestinal intraepithelial lymphocytes are a distinct group of cells located within the intestinal mucosal epithelium. These cells originate from two different sources. Notably, around 40 percent of iIELs are thymus-dependent TCRalfa/beta+ IELs, and their characteristics closely resemble those of peripheral T cells.
Zhang et al. 2019l

About 60 percent of intestinal Intraepithelial Lymphocytes (IELS) are thymus-independent gamma/delta T cells.
Zhang et al. 2019l

Non-classical Lymphoid cells / Lymphocytes, broadly referred to as intraepithelial lymphocytes (IELs), intercalate the intestinal epithelium
Cheroutre et al. 2011 Nat. Rev. Immunol. 11: 445–456
McDonald et al. 2018a Nat. Rev. Immunol. 18: 514–525

This anatomical position has implicated them as first-line defenders in resistance to infections
McDonald et al. 2018a Nat. Rev. Immunol. 18: 514–525

Intraepithelial Lymphocytes (IELS) are divided into two main subtypes: Natural Intraepithelial Lymphocytes (IELS) and induced Intraepithelial Lymphocytes (iIELS). Natural IELs arise in the thymus and migrate to the effector tissue. iIELs arise from activated, antigen-experienced conventional Alfa/beta T-Cell Receptor (TCR) T helper cell
Cheroutre et al. 2011 Nat. Rev. Immunol. 11: 445–456

Intraepithelial Lymphocytes (IELS) express Alfa/beta T-Cell Receptor (TCR) or gamma/delta T-Cell Receptor (TCR) or CD8aa homodimers, respectively.
Leishman et al. 2001 Science 294: 1936-1939

see also:
Biological effects / Functions & Candidatus Savagella (Segmented filamentous bacteria/SFB)
TCRgamma/delta+ IELs