The role of Ikaros family proteins in T cell development

The highly-conserved Ikaros family of transcription factors plays a critical role in the development and maintenance of the lymphoid system. Ikaros family proteins regulate T cell development, in part, by associating with chromatin-remodeling complexes. I have studied the role of Ikaros family members in the transition from the pre-T cell to the CD4+ CD8 + thymocyte using an Ikaros null-/- CD4-CD8 - thymoma cell line. I demonstrate that this cell line carries a single, functional Tcrbeta gene rearrangement and expresses a surface pre-TCR. The vast majority of the cells also carry non-functional primary rearrangements on both alleles of the Tcralpha locus. I show that multiple Ikaros family members can drive the expression of the developmental markers CD4, CD8, and CD25. I also show that expression of exogenous Ikaros, and, to a lesser extent, Helios or Aiolos, dramatically increased the rate of secondary Tcralpha gene rearrangement and the appearance of cells bearing alphabetaTCRs. This process was dependent on SWI/SNF chromatin-remodeling complexes. Furthermore, knock-down of Mi2beta/NuRD complexes increased the frequency of Tcralpha rearrangements. Lastly, I show that Ikaros does not induce Tcralpha rearrangement via induction of Rag1/2 gene expression, suggesting that Ikaros acts directly on the Tcralpha locus. Together, my data are consistent with the idea that Ikaros family transcription factors regulate T cell development, in part, by controlling access of the recombination machinery to the Tcralpha loci.