Involvement of Prep1 in the αβ T-cell receptor T-lymphocytic potential of hematopoietic precursors

Prep1 is a homeodomain transcription factor that acts by dimerizing with Pbx. Since Prep1 null embryos die at gastrulation, we studied Prep1(i/i) hypomorphic mice to study the physiological role of Prep1. A low percentage of homozygous Prep1(i/i) mice survived at birth, and their postnatal functions could be investigated. Reduced Prep1 expression caused an abnormal thymic T-cell development: increased CD4(-) CD8(-) double-negative thymocytes, decrease in alpha beta PTCRhigh cells (cells with high levels of the alpha beta T-cell receptor [alpha beta BTCR]) and CD4(+) and CD8(+) single-positive (SP) thymocytes, and increase in gamma delta TCR cells. Peripheral lymphoid organs of Prep1(i/i) mice contained fewer alpha beta TCR mature T cells and more gamma delta TCR T cells than wild-type littermates. Moreover, Prep1(i/i) CD4(+) CD8(+) double-positive thymocytes underwent more apoptosis, and SP thymocytes proliferated less than control littermates. Mice that were lethally irradiated and then had Prep1(i/i) fetal liver cells transplanted showed the same defects as the Prep1(i/i) mice did. Among PBC family members, Pbx2 and very low levels of Pbx3 were observed in the thymi of wild-type mice. In Prep1(i/i) mice, the level of Pbx2 protein was profoundly decreased, while for Pbx3 no definitive conclusion could be reached. Therefore, the deficient postnatal Tlymphocytic potential of the Prep1 hematopoietic progenitors depends on the combined, not compensated, absence of Prep1 and at least Pbx2.