<unk> s can be divided into different subpopulations based on molecular markers expression, mainly by expression of TCR and CD8αα, and by origin. Induced TCR+<unk> s Populations of induced<unk> s emerge from conventional peripheral CD4+ T-cells. They have in common the expression of TCR and can also express CD8αα that is induced after migration into the intestinal epithelium. TCRαβ+CD4+<unk> s TCRαβ+CD4+<unk> s arise from conventional peripheral CD4+ T-cells. These cells migrate into the intestinal epithelium as effector or tissue-resident memory T cells. In mice, up to 50% of these<unk> s can express CD8αα homodimer, which they acquire in the intestinal epithelium after external stimuli such as TGF-β, IFN-γ, IL-27 and retinoic acid. Function of TCRαβ+ CD4+ CD8αα+<unk> s is unclear. Even though they express granzymes and have cytolytic properties, it has been suggested that they can also have regulatory properties in the context of chronic intestinal inflammation. TCRαβ+CD8αβ+<unk> s These<unk> s emerge from peripherally activated conventional CD8+ T-cells and home to the intestinal epithelium, where they function as effector or memory cells. They continuously express integrin β7, granzyme B, CD103 and CD69 and produce lower amounts of TNF-α and IFN-γ as opposed to the conventional CD8+ T-cells. Some of these cells also express CD8αα homodimer and can be pathogenic during coeliac disease in humans. Natural TCR+<unk> s Natural<unk> s also express TCR and right after their development home to the epithelium, where they acquire expression of CD8αα. TCRαβ+<unk> s In mice, these<unk> s are the most abundant at birth and with age their numbers decrease. In humans, these cells are present during gestation but are very rare in adulthood. TCRαβ+<unk> s develop in thymus where they undergo agonist positive selection and thereby are self-reactive. Nevertheless, they have regulatory properties and protect against colitis in animal experiments. These cells are influenced by normal intestinal microbiota and vitamin D. NOD2 receptor expressed by antigen presenting cells and epithelial cells in the intestine recognizes microbes and triggers the production of IL-15 cytokine, which promotes TCRαβ+CD8αα+<unk> s. TCRγδ+<unk> s TCRγδ+<unk> s develop outside of thymus and their maintenance and function in the intestinal epithelium is influenced by a cross-talk with enterocytes. Moreover, they can migrate through the epithelium with the help of interactions with epithelial cells. Most of these cells express Vγ7 in mice and Vγ4 in humans. Their function resides in the protection of the intestinal barrier against pathogens early in the infection and later they quench the inflammation and protect the barrier from tissue damage. The mechanism is not clear, but TCRγδ+<unk> s have cytotoxic properties and can produce cytokines TGF-β, TNF-α, IFN-γ, IL-13 and IL-10 and antimicrobial peptides, all of which can contribute to the diverse functions. Similar functions have been found in the context of colitis, where these cells seem to have pathogenic role at the beginning, whereas later they protect the epithelium against tissue damage. TCR−<unk> s<unk> s that do not express TCR. ILC-like<unk> s These cells show properties of NK cells. In humans, they are elevated during Crohn´s disease and in mice, they are pathogenic during colitis. iCD8α These innate lymphocytes express homodimer CD8αα and CD3 and develop outside of thymus. They have cytotoxic and phagocytic properties, express MHC II and thereby can present antigens to conventional CD4+ T-cells. iCD8α protect against bacterial infections and promotes experimental colitis. TCR−iCD3+CD8αα−<unk> s These cells are very similar to iCD8α population and it is unclear if this is a different subset of cells or only precursors of iCD8α.
Intraepithelial lymphocyte
Tingible body macrophage
Tumor-infiltrating lymphocytes
Lutzner cells
Lipid-laden alveolar macrophage
Touton giant cell
Langhans giant cell
Hypersegmented neutrophil