Developmental stage-specific effects of Pim-1 dysregulation on murine bone marrow B cell development

Background: The serine threonine kinase Pim-1 has documented roles in hematopoietic progenitor and B cell precursor proliferation and survival. Pim-1 is a molecular target of the transcription factor Hoxa9. Previous studies showed that Pim-1 deficiency phenocopied the hematopoietic progenitor defect in hoxa9-/- mice and forced expression of Pim-1 normalized the in vitro proliferation defect inherent to hoxa9-/- hematopoietic progenitors. Pim-1 is induced by cytokine signaling, including the early lymphoid/B lineage regulators Flt3 and IL-7, and expression levels were shown to influence the size of the B cell compartment in bone marrow (BM). Results: In this study, we sought to determine if transgenic expression of Pim-1, driven by the immunoglobulin enhancer, Eμ, was sufficient to rescue the lymphoid/B cell precursor defect in hoxa9 or flt3-ligand (flt3l) deficient mice. Unexpectedly, expression of Eμ - Pim1 exacerbated lymphoid progenitor deficiencies in flt3l-/-, and to a lesser extent, hoxa9-/- mice. Furthermore, Eμ - Pim1 expression alone reduced early myeloid and lymphoid, but not erythroid, progenitors. In contrast, Pim-1 deficiency had no significant effect on early lymphoid/B cell development through the Pre-Pro-B cell stage, but caused a significant reduction in IgM^- B cell precursors. Importantly, loss of Pim-1 did not phenocopy hoxa9- or flt3l-deficiency on the lymphoid/early B cell progenitor pools. Conclusions: These experimental findings demonstrate that Pim-1 overexpression has developmental-stage-specific effects on B lymphopoiesis and myelopoiesis. Importantly, these suggest that Pim-1 deficiency does not contribute significantly to the early lymphoid/B cell developmental deficiency in hoxa9-/- or flt3l-/- mice.