The Transcription Factor Pbx1 Is Required for the Development of Double Positive Thymic T Cells.

It has become increasingly apparent that transcription factors originally discovered at the sites of chromosomal translocations in acute leukemias serve critical roles for hematopoietic cell development and function, which require fine orchestration of gene expression. Pbx1 is one such transcription factor, which forms a chimeric oncoprotein, E2a-Pbx1, resulting from t(1;19) chromosomal translocations between the E2a and Pbx1 loci in a subset of the pediatric pre-B cell acute lymphoblastic leukemias. Previously, our laboratory has shown that mice nullizygous for Pbx1 die in utero with substantial myelo-erythroid defects associated with profound fetal anemia. To determine whether Pbx1 is also required for normal lymphoid development, we performed Rag1-deficient blastocyst complementation assays using Pbx1 homozygous null embryonic stem (ES) cells. Interestingly, analysis of resulting chimeric mice displayed a partial rescue of the Rag1 phenotype as evidenced by T cell-restricted lymphocyte development. Pbx1 null ES cells produced circulating CD4 and CD8 single positive T cells, however in contrast to the typical naive T cell phenotype (CD62L^highCD44^loCD45RB^high) in control mice they exclusively displayed a characteristic memory T cell phenotype (CD62L^lowCD44^highCD45RB^low). Analysis of the thymi of the chimeric mice for T cell developmental precursors showed the complete absence of CD4/CD8 double-positive intermediates. Rather, T cells in the thymus, and all secondary lymphoid organs, were single positive CD4 or CD8 cells that exclusively displayed a similar memory T cell phenotype typical of T cells derived from bone marrow resident committed T cell progenitors (CTP). The striking absence of classical CD4/CD8 double-positive cells in the thymus indicates that Pbx1 deficiency interrupts the intrathymic T cell developmental pathway. Furthermore, the phenotypic resemblance of Pbx1 null T cells with T cells derived from bone marrow resident CTP strongly suggests that T cell development in the absence of Pbx1 may occur via an extrathymic pathway.