Pim1 Reconstitutes Thymus Cellularity in Interleukin 7–And Common γ Chain–Mutant Mice and Permits Thymocyte Maturation in Rag- but Not Cd3γ-Deficient Mice

The majority of lymphomas induced in Rag-deficient mice by Moloney murine leukemia virus (MoMuLV) infection express the CD4 and/or CD8 markers, indicating that proviral insertions cause activation of genes affecting the development from CD4⁻8⁻ pro-T cells into CD4⁺8⁺ pre-T cells. Similar to MoMuLV wild-type tumors, 50% of CD4⁺8⁺ Rag-deficient tumors carry a provirus near the Pim1 protooncogene. To study the function of PIM proteins in T cell development in a more controlled setting, a Pim1 transgene was crossed into mice deficient in either cytokine or T cell receptor (TCR) signal transduction pathways. Pim1 reconstitutes thymic cellularity in interleukin (IL)-7– and common γ chain–deficient mice. In Pim1-transgenic Rag-deficient mice but notably not in CD3γ-deficient mice, we observed slow expansion of the CD4⁺8⁺ thymic compartment to almost normal size. Based on these results, we propose that PIM1 functions as an efficient effector of the IL-7 pathway, thereby enabling Rag-deficient pro-T cells to bypass the pre-TCR–controlled checkpoint in T cell development.