Pax5 is not a master regulator of B-cell development. However, it is important for controlling lymphoid cell-fate decisions, as ectopic expression of Pax5 interferes with early Notch1-mediated T-cell lymphopoiesis and diverts lymphoid progenitors into the B-cell pathway.
Gene-knockout studies have shown that the B-cell-specific transcription factor Pax5 is essential for B-cell commitment and development, thereby identifying Pax5 as a key regulator of B-cell lymphopoiesis. But, does Pax5 function as a master regulator to promote B-cell development at the expense of other haematopoietic lineages when it is expressed prematurely in haematopoietic progenitors?
To answer this, Meinrad Busslinger's lab generated Pax5-knock-in mice, in which a Pax5 minigene, together with a 'floxed' neomycin resistance gene, is expressed under the control of the Ikaros locus. As Ikaros is expressed by all types of haematopoietic cell, including haematopoietic stem cells (HSCs), the knock-in alleles are expressed by all lineages, so allowing the effects of ectopic expression of Pax5 to be assessed.
Three approaches were used — conditional activation of the IkPax5 allele, constitutive expression of Pax5 (IkPax5/+ mice) and competitive bone-marrow reconstitution experiments — which together showed that the expression of Pax5 by early haematopoietic progenitors did not interfere with myeloid development and affected erythropoiesis only slightly. However, the effects on lymphoid development were marked, with B-cell lymphopoiesis being promoted at the expense of T-cell development.
The authors further investigated the effects of Pax5 expression on T-cell development by analysing the thymi of IkPax5/+ mice (which express Pax5 constitutively). Thymic cellularity was reduced in comparison with wild-type mice, a phenotype that worsened with age, and thymocyte development was partially blocked at an early (double-negative (DN)3, pro-T-cell) stage. As Notch1 is required for early T-cell development, Souabni and colleagues asked whether Pax5 affects the expression of Notch1. By three different approaches, they showed that Pax5 can repress the transcription of Notch1 in lymphoid progenitors, and that this might provide a molecular mechanism to explain the defective T-cell development of IkPax5/+ mice.
These experiments show that Pax5 is not a master regulator of B-cell development, as its ectopic expression does not direct HSCs, erythroid progenitors or myeloid progenitors towards the B-cell lineage. However, Pax5 is important for controlling lymphoid cell-fate decisions as ectopic expression of Pax5 interfered with early Notch1-mediated T-cell lymphopoiesis and diverted lymphoid progenitors into the B-cell pathway.
Jenny Buckland
References
Souabni, A. et al. Pax5 promotes B lymphopoiesis and blocks T-cell development by repressing Notch1. Immunity17, 781–793 (2002) | PubMed |
Schebesta, M. et al. Transcriptional control of B-cell development. Curr. Opin. Immunol.14, 216–223 (2002) | Article | PubMed |
