Fas-mediated apoptosis regulates the composition of peripheral αβ T cell repertoire by constitutively purging out double negative T cells

Background: The Fas pathway is a major regulator of T cell homeostasis, however, the T cell population that is controlled by the Fas pathway in vivo is poorly defined. Although CD4 and CD8 single positive (SP) T cells are the two major T cell subsets in the periphery of wild type mice, the repertoire of mice bearing loss-of-function mutation in either Fas (lpr mice) or Fas ligand (gld mice) is predominated by CD4^-CD8^- double negative αβ T cells that also express B220 and generally referred to as B220⁺DN T cells. Despite extensive analysis, the basis of B220⁺DN T cell lymphoproliferation remains poorly understood. In this study we re-examined the issue of why T cell lymphoproliferation caused by gld mutation is predominated by B220⁺DN T cells. Methodology and Principal Findings: We combined the following approaches to study this question: Gene transcript profiling, BrdU labeling, and apoptosis assays. Our results show that B220⁺DN T cells are proliferating and dying at exceptionally high rates than SP T cells in the steady state. The high proliferation rate is restricted to B220+DN T cells found in the gut epithelium whereas the high apoptosis rate occurred both in the gut epithelium and periphery. However, only in the periphery, apoptosis of B220⁺DN T cell is Fas-dependent. When the Fas pathway is genetically impaired, apoptosis of peripheral B220⁺DN T cells was reduced to a baseline level similar to that of SP T cells. Under these conditions of normalized apoptosis, B220⁺DN T cells progressively accumulate in the periphery, eventually resulting in B220⁺DN T cell lymphoproliferation. Conclusions/Significance:The Fas pathway plays a critical role in regulating the tissue distribution of DN T cells through targeting and elimination of DN T cells from the periphery in the steady state. The results provide new insight into pathogenesis of DN T cell lymphoproliferation.