positive selection of t cells occurs in the thymus.
positive selection of t cells occurs in the thymus.
where does positive selection of t cells occur?
the thymus
after the tcrα locus has been re-arranged, the cell will express the full tcr on the surface and upregulate the co-receptor genes cd4 and cd8 and be termed "double-positive" (dp) cells. these cells will then undergo positive selection, where t cells are selected to be capable of interacting with mhc. positive selection involves the production of a signal by double-positive precursors that express either mhc class i or ii restricted receptors. dp cells selected on mhc class i will become cd8+ cells; dp cells selected on mhc class ii will become cd4+ t cells. the cells then downregulate the non-selected co-receptor and become single-positive either cd4+ or cd8+ t cells (sp cells). the signal produced by these thymocytes result in rag gene repression, long-term survival and migration into the medulla. the majority of cells in the thymus are dp cells, and the process of positive selection takes a number of days.
t cell
positive selection is another mechanism that has allowed for the selection of genes contributing to the presence of schizophrenia. positive selection is a mechanism of natural selection in which beneficial traits are selected for and become prevalent over time in a population. in a study conducted using phylogeny-based maximum-likelihood (paml), a method that was used to test for positive selection, significant evidence of positive selection was found in the genes associated with schizophrenia. an example of a beneficial trait that has been selected for through positive selection is creativity. three allelic variants of creativity genes that are also associated with schizophrenia include slc6a4, tph1 and drd2. the high inheritance of creative and cognitive characteristics by these allelic variants in individuals expressing schizophrenia confirms evidence of positive selection within some schizophrenia genes. additional studies conducted using snp analysis on the slc39a8 gene, a gene associated with schizophrenia, found that the t-allele on the gene was associated with reduced blood pressure and a decreased risk of hypertension. these beneficial traits associated with schizophrenia genes provide a reason for why these genes have been selected for in human development. while promising evidence persists, additional evidence claims that the effect of positive selection may not play a significant role in the presence of schizophrenia. studies conducted through the use of fst and methods based on sfs failed to find convincing signals of positive selection on the cgc-type of the st8sia2 gene, another gene that is associated with schizophrenia.
evolution of schizophrenia