Immunology. A molecular gut reaction

Intestinal intraepithelial lymphocytes: Maintainers of intestinal immune tolerance and regulators of intestinal immunity

Intestinal immune tolerance is essential for the immune system, as it prevents abnormal immune responses to large quantities of antigens from the intestinal lumen, such as antigens from commensal microorganisms, and avoids self‐injury. Intestinal intraepithelial lymphocytes (IELs), a special group of mucosal T lymphocytes, play a significant role in intestinal immune tolerance. To accomplish this, IELs exhibit a high threshold of activation and low reactivity to most antigens from the intestinal lumen. In particular, CD8αα⁺TCRαβ⁺ IELs, TCRγδ⁺ IELs, and CD4⁺CD8αα⁺ IELs show great potential for maintaining intestinal immune tolerance and regulating intestinal immunity. However, if the intestinal microenvironment becomes abnormal or intestinal tolerance is broken, IELs may be activated abnormally and become pathogenic.

TL and CD8αα: Enigmatic partners in mucosal immunity

The intestinal mucosa represents a large surface area that is in contact with an immense antigenic load. The immune system associated with the intestinal mucosa needs to distinguish between innocuous food antigens, commensal microorganisms, and pathogenic microorganisms, without triggering an exaggerated immune response that may lead to excessive inflammation and/or development of inflammatory bowel disease. The thymus leukemia (TL) antigen and CD8αα are interacting surface molecules that are expressed at the frontline of the mucosal immune system: TL is expressed in intestinal epithelial cells (IEC) whereas CD8αα is expressed in lymphocytes, known as intraepithelial lymphocytes, that reside in between the IEC. In this review we discuss the significance of the interaction between TL and CD8αα in mucosal immunity during health and disease.

The CD8 isoform CD8αα is not a functional homologue of the TCR co-receptor CD8αβ

Although structurally similar, CD8alphabeta and CD8alphaalpha have notably diverted with regard to function. Whereas CD8alphabeta functions as a T-cell receptor (TCR) co-receptor on MHC-class-I-restricted thymocytes and mature T cells, CD8alphaalpha is unable to support conventional positive selection, and can be expressed on T cells independent of the MHC restriction of their TCR. CD8alphaalpha induction is consistent with antigenic stimulation through the TCR, and recent developments have now shown that CD8alphaalpha induced on agonist-triggered immature thymocytes, antigenic-stimulated conventional CD8alphabeta T cells and mucosal T cells mediates the specific modulation of TCR activation signals to facilitate their survival and differentiation into various specialized T-cell subsets.