Human intraepithelial lymphocytes

Abstract

The location of intraepithelial lymphocytes (IEL) between epithelial cells, their effector memory, cytolytic and inflammatory phenotype positions them to kill infected epithelial cells and protect the intestine against pathogens. Human TCRαβ⁺CD8αβ⁺ IEL have the dual capacity to recognize modified self via natural killer (NK) receptors (autoreactivity) as well as foreign antigen via the T cell receptor (TCR), which is accomplished in mouse by two cell subsets, the naturally occurring TCRαβ⁺CD8αα⁺ and adaptively induced TCRαβ⁺CD8αβ⁺ IEL subsets, respectively. The private/oligoclonal nature of the TCR repertoire of both human and mouse IEL suggests local environmental factors dictate the specificity of IEL responses. The line between sensing of foreign antigens and autoreactivity is blurred for IEL in celiac disease, where recognition of stress ligands by induced activating NK receptors in conjunction with inflammatory signals such as IL-15 can result in low-affinity TCR/non-cognate antigen and NK receptor/stress ligand interactions triggering destruction of intestinal epithelial cells.

Fig. 1

A summary of the regional distribution of intraepithelial lymphocyte (IEL) subsets in human and mouse. We compare here the proportion of T cell subsets between the small intestine (duodenum) and large intestine (colon; right colon in human) while highlighting key differences between human and mouse. Human data is from healthy adults and mouse data is from 8-week-old C57BL/6 specific pathogen-free (SPF) mice from our colony at the University of Chicago. a The proportion of TCRγδ⁺ IEL among CD3⁺ IEL is summarized for human and mouse in duodenum and colon. The proportion of TCRαβ⁺CD8αβ⁺ and TCRαβ⁺CD8αα⁺ among TCRαβ⁺CD4⁻ IEL and TCRαβ⁺CD4⁺ IEL among TCRαβ⁺ IEL is summarized for human and mouse in duodenum and colon. Ranges are based on published data in addition to our own unpublished data. b Freshly isolated IEL were stained with fluorescently labeled antibodies against CD45, CD3, TCRγδ, TCRαβ, CD4, CD8α, CD8β, and CD103 (human only). Top: representative flow cytometry contour plots with outliers and large dots are shown for the indicated populations (pre-gate indicated above plot) for human (left) or mouse (right) duodenum and colon. Bottom: the proportion of a given cell subset is summarized between the duodenum and colon for both human and mouse. For example, the proportion of TCRγδ⁺ IEL among CD3⁺ IEL in human is lower in the duodenum relative to the colon. The comparisons are made directly between the duodenum and colon, therefore the linear depiction of increases/decreases in proportions between the two segments are meant for simplicity and not to illustrate the progression across the other segments of the gut

Fig. 2

Here we compare the human TCRαβ⁺CD8αβ⁺ IEL subset with the TCRαβ⁺CD8αβ⁺ and TCRαβ⁺CD8αα⁺ IEL subsets found in mouse for NK receptor expression and cytokine producing capacity. a The human TCRαβ⁺CD8αβ⁺ IEL expresses NK receptors such as the activating NKG2D and inhibitory NKG2A/CD94 in the steady state, whereas the TCRαβ⁺CD8αα⁺ IEL and not the TCRαβ⁺CD8αβ⁺ IEL subset is enriched for NK receptor expression in mouse. These TCRαβ⁺CD8αα⁺ IEL express NK receptors that can be found in steady-state human IEL such as NKG2D in addition to receptors such as those of the Ly49 family and NKG2C/CD94 which can associate with the ITAM adapter molecule DAP12. Interestingly, human IEL can also gain expression of ITAM bearing NK receptors such as NKG2C/CD94 in patients with CeD. The propensity for autoreactivity of IEL TCRs is shown with more autoreactive TCRs, such as those attributed to the development of TCRαβ⁺CD8αα⁺ IEL, illustrated in red and less autoreactive TCRs illustrated in green. The threshold for TCR/non-cognate antigen interactions to result in activation can be met in the case of human TCRαβ⁺CD8αβ⁺ IEL in CeD via the costimulatory impact of inflammatory signals such as IL-15 and the engagement of activating NK receptors. b Freshly isolated IEL were treated with 50 ng/mL of PMA and 500 ng/mL of Ionomycin for 3 h to assess the expression of Granzyme B and IFN-γ at steady state for human TCRαβ⁺CD8αβ⁺ IEL from a 30-year-old individual and TCRαβ⁺CD8αβ⁺ and TCRαβ⁺CD8αα⁺ IEL from 4-week and 11-week-old C57BL/6 SPF mice, revealing human TCRαβ⁺CD8αβ⁺ IEL are potent cytokine producers and simultaneously express Granzyme B, while cytokine production is age dependent and exclusive to the mouse TCRαβ⁺CD8αβ⁺ IEL with TCRαβ⁺CD8αα⁺ IEL being more potent expressers of Granzyme B. Representative flow cytometry contour plots with outliers and large dots are shown for intracellular staining with fluorescently labeled antibody against IFN-γ and Granzyme B for human (left) and mouse (right)