Defects in pro-apoptotic genes, Bim or Bax/Bak, cause accumulation of TCRβhi DN4 cells in thymus (HEM4P.241)

Abstract

Abstract Regulated induction of apoptosis during thymocyte development is critical to prevent the generation of self-reactive T cells whilst maintaining a repertoire of T cells that can promote protective immunity. Although the pro-apoptotic Bcl-2 family member Bim has been shown to promote apoptotic negative selection of self-reactive thymocytes, the role of Bim in this process is controversial. Here, we have investigated the role of Bim and downstream mediators Bax and Bak on thymocyte selection. Surprisingly, the absence of Bim results in a striking accumulation of DN4 cells with surface TCRβhi feature in the thymus. Similar results were observed in mice with a T cell specific loss of Bax in a Bak knock out background. DN4 cells from Bim-/- or LckCreBaxfl/flBak-/- mice cultured with OP9-DL1 cells largely failed to generate DP cells relative to WT DN4 cells. Further, using a dLckCre system, in which Cre is expressed late in the DP stage, revealed that conditional knock out of Bim or Bax did not result in an accumulation of DN4-TCRβhi cells. Combined, these data suggest that accumulation of DN4-TCRβhi cells in either Bim-/- or LckCreBaxfl/flBak-/- are the result of cells having failed negative selection that downregulate CD4 and CD8 coreceptors. Such a strategy would inherently limit their TCR avidity and reduce their potential for autoreactivity. Our current studies are examining the persistence and location of DN-TCRβhi cells in the periphery of Bim-/- or LckCreBaxfl/flBak-/- mice.