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CD8+ resident memory T (TRM) cells are present in various tissues thus forming a heterogeneous population. Transforming growth factor (TGF)-β-independent CD103– TRM cells in the liver retain the ability of movement towards the barrier tissues or coming back to secondary lymphoid organs.
TRM cells, since located within the tissue, are the first subset to encounter the pathogen thus providing superior protection against them. They have been identified in barrier tissues, like the gut, skin, lung and reproductive tract, as well as in the visceral organs, like the liver. They spread in various organs but are united by a shared transcriptional program, driven by a core group of transcription factors—particularly, by upregulation of Hobit, Blimp-1 and Runx3 and downregulation of Eomes and Tbet.
Parabiotic mice, which get the blood supplies of two congenically distinct mice joined together, have helped in establishing the residency of TRM cells. These experiments have predominantly identified tissue-resident populations within the original animal, demonstrating a very lesser capacity to recirculate between animals, contrasting their blood-borne counterparts. Tissue transplantation has also been utilized in the Analogous methods to demonstrate the restricted recirculation of cells from donor tissue into the host circulation.
Source: McNamara HA, Cockburn IA. Resident T cells seek the perfect place to work from home. Nat Immunol, 2021; 22:1076–1078.