Deranged Early T Cell Development in Immunodeficient Strains of Nonobese Diabetic Mice

Abstract

NOD mice exhibit defects in T cell functions that have been postulated to contribute to diabetes susceptibility in this strain. However, early T cell development in NOD mice has been largely unexplored. NOD mice with the scid mutation and Rag1 deficiency were analyzed for pre-T cell development in the NOD genetic background. These strains reveal an age-dependent, programmed breakdown in β selection checkpoint enforcement. At 5–8 wk of age, even in the absence of TCRβ expression, CD4^+ and CD4^+CD8^+ blasts appear spontaneously. However, these breakthrough cells fail to restore normal thymic cellularity. The breakthrough phenotype is recessive in hybrid (NOD×B6)F_1-scid and -Rag1^(null) mice. The breakthrough cells show a mosaic phenotype with respect to components of the β selection program. They mimic normal β selection by up-regulating germline TCR-Cα transcripts, CD2, and Bcl-x_L and down-regulating Bcl-2. However, they fail to down-regulate transcription factors HEB-alt and Hes1 and initially express aberrantly high levels of Spi-B, c-kit (CD117), and IL-7Rα. Other genes examined distinguish this form of breakthrough from previously reported models. Some of the abnormalities appear first in a cohort of postnatal thymocytes as early as the double-negative 2/double-negative 3 transitional stage. Thus, our results reveal an NOD genetic defect in T cell developmental programming and checkpoint control that permits a subset of the normal outcomes of pre-TCR signaling to proceed even in the absence of TCRβ rearrangement. Furthermore, this breakthrough may initiate thymic lymphomagenesis that occurs with high frequency in both NOD-scid and -Rag1^(null) mice.