Massive Apoptosis of Thymocytes in T-Cell-Deficient Id1 Transgenic Mice

Abstract

Id1 is an inhibitor of a group of basic helix-loop-helix transcription factors, collectively called E proteins, which includes E12, E47, E2-2, and HEB. We have generated transgenic mice in which Id1 is specifically expressed in T cells. The total number of thymocytes in these mice is less than 4% of that in wild-type mice. The majority of the transgenic thymocytes are CD4 and CD8 double negative and bear the cell surface markers of multipotent progenitor cells. A small number of thymocytes, however, differentiate into CD4 or CD8 single-positive T cells, which also display different characteristics from their wild-type counterparts. More importantly, apoptotic cells constitute about 50% of the total thymocytes. These apoptotic thymocytes have rearranged their T-cell receptor genes, suggesting that they are differentiating T cells. This finding has raised the possibility that the T-cell deficiency in Id1 transgenic mice is the result of a massive apoptosis of differentiating T cells triggered by Id1 expression as opposed to a developmental block at the earliest progenitor stage. The progenitor cells accumulated in the transgenic mice might have survived because they are not susceptible to the apoptotic signals. Despite the massive cell death of the thymocytes at young ages, Id1 transgenic mice frequently develop T-cell lymphoma later in their life span, and lymphomagenesis appears to occur at different stages of T-cell development. Taken together, our data suggest that E proteins, being the targets of Id1, are essential regulators for normal T-cell differentiation and tumor suppression.

ACKNOWLEDGMENTS

We are grateful to Sankar Ghosh for discussion and critical reading of the manuscript. We thank Yang Liu, Stanislav Vukmanovic, and Alan Frey for advice and John Hirst of the cell-sorting facility of Kaplan Cancer Center for excellent assistance in cell-sorting analyses. The Id1 transgenic mice were produced by the Transgenic Facility of Rockefeller University and the facility of the Kaplan Cancer Center at NYU School of Medicine.

This work was supported by grants from NIH (1R01AI33597 and 1R01CA77553), the American Cancer Society (RPG-98-247-01-LBC), and the Life and Health Insurance Fund. X.H.S. is an Irma T. Hirschl Trust Scholar.