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The role of cell cycle progression and cyclin-dependent kinase 2 in thymocyte negative selection
Autoreactive, immature T cells (thymocytes) are deleted from the thymus during development by the process of negative selection. This mechanism occurs when the thymocyte, T cell receptor (TCR) recognizes self-antigen, causing the cell to die by an apoptotic pathway. This mechanism results in the deletion of autoreactive T cells. Thymocyte development proceeds through several stages, determined by the differential expression of the T cell co-receptor molecules CD4 and CD8. The developmental stage where negative selection occurs is one in which thymocytes are expressing a functional TCR on the cell surface along with both CD4 and CD8, termed the double positive stage. These thymocytes are in a quiescent, G0 state and make up greater than 80% of the total population. ^ The demonstration that cell cycle progression plays a role in the apoptotic process of several quiescent cell types, as well as the requirement for mature T cells to be in the late G1 phase during activation induced cell death, suggested that thymocytes may also advance to the G1 stage of the cell cycle prior to apoptosis. It has been established that the early cell cycle genes c-fos, c-jun, and c-myc are induced in thymocytes after stimulation, indicating possible entry into the cell cycle. The hypothesis that thymocytes enter the cell cycle before undergoing apoptosis was tested by examining expression levels of the various G1 cyclins and cyclin dependent kinase inhibitors at the mRNA and protein levels. Several indications of an early G1 cell cycle transition occurring during thymocyte apoptosis were observed, such as the downregulation of p27KIP1 and p130, the upregulation of cyclin D3, and the phosphorylation of the retinoblastoma protein. ^ Finally, the requirement for the activation of the cyclin-dependent kinase 2 (CDK2) in negative selection was examined. It was shown that the phosphorylation and expression of the TCR-mediated apoptosis-related transcription factors Nur77 and Egr1 are downstream of CDK2 activation. In addition, a protein associated with the transcription factor Egr-1 was identified as a possible target of CDK2 kinase activity. ^
Biology, Molecular|Biology, Cell
Jennifer Lynn Trimble,
"The role of cell cycle progression and cyclin-dependent kinase 2 in thymocyte negative selection"
(January 1, 2000).
Electronic Doctoral Dissertations for UMass Amherst.