Protein phosphatase 2A has an essential role in promoting thymocyte survival during selection

Abstract

The development of thymocytes to mature T cells in the thymus is tightly controlled by cellular selection, in which only a small fraction of thymocytes equipped with proper quality of TCRs progress to maturation. It is pivotal to protect the survival of the few T cells, which pass the selection. However, the signaling events, which safeguard the cell survival in thymus, are not totally understood. In this study, protein Ser/Thr phosphorylation in thymocytes undergoing positive selection is profiled by mass spectrometry. The results revealed large numbers of dephosphorylation changes upon T cell receptor (TCR) activation during positive selection. Subsequent substrate analysis pinpointed protein phosphatase 2A (PP2A) as the enzyme responsible for the dephosphorylation changes in developing thymocytes. PP2A catalytic subunit α (Ppp2ca) deletion in the T cell lineage in Ppp2ca flox/flox -Lck-Cre mice (PP2A cKO) displayed dysregulated dephosphorylation of apoptosis-related proteins in double-positive (DP) cells and caused substantially decreased numbers of DP CD4 + CD8 + cells. Increased levels of apoptosis in PP2A cKO DP cells were found to underlie aberrant thymocyte development. Finally, the defective thymocyte development in PP2A cKO mice could be rescued by either Bcl2 transgene expression or by p53 knockout. In summary, our work reveals an essential role of PP2A in promoting thymocyte development through the regulation of cell survival. thymocyte selection | PP2A | apoptosis T lymphocytes are the major form of adaptive immune cells that target pathogens (1, 2). T cell precursors originate in the bone marrow and then migrate to the thymus to initiate differentiation into mature T cells. In the thymus, CD4 -CD8doublenegative (DN) thymocytes (which can be subcategorized as stages DN1-DN4) acquire T cell receptor (TCR) expression through VDJ recombination and develop into CD4 + CD8 + double-positive (DP) thymocytes, which subsequently give rise to CD4 + or CD8 + single-positive (SP) T cells. Two pivotal selection processes occur, namely positive selection and negative selection, and both serve as gatekeepers in the progress of DP T cells to the SP stage. Notably, only a small percentage of DP thymocytes survive through the selection process to become mature T cells bearing TCRs with suitable reactivity. Secure survival of the T cells which do pass selection is then of pivotal importance during thymic development.It has been shown that TCRs on the DP cell surface can bind to self-peptide-major histocompatibility complex (MHC) complexes on thymic epithelial and dendritic cells, which provide signals for thymocyte survival (3-5). Up-regulation of expression of survival-related proteins is one of the known mechanisms to promote thymocyte survival in this context. Well-studied examples include the Bcl-2 family prosurvival proteins Bcl-xL, whose stage-specific enrichment promotes the survival of DP thymocytes (6). In addition to regulations at the level of gene expression, further studies revealed that posttranslational ...

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