The Earliest Thymic T Cell Progenitors Sustain B Cell and Myeloid Lineage Potentials

Abstract 2335

The stepwise lineage-commitment from hematopoietic stem cells (HSCs) in the bone marrow (BM) to T-lymphocyte-restricted progenitors in the thymus represents a paradigm for how distinct stages of lineage restriction from a multipotent to a lineage-restricted progenitor require different extrinsic cues. However, the commitment stage at which progenitors migrate from the BM to the thymus remains unclear. Previous studies demonstrated the existence of adult early thymic progenitors (ETPs) restricted to T lymphocyte and granulocyte-monocyte (GM) fates (Bell and Bhandoola, Nature 2008; Wada et al., Nature 2008). The challenge remains to couple this thymic T–GM restricted progenitor to candidate thymus seeding progenitors (TSPs) identified in the BM, such as common lymphoid progenitors, lymphoid-primed multipotent progenitors (LMPPs) and HSCs, which all also possess B cell potential.

Using high resolution FACS and sensitive clonal assays, we have identified the most primitive ETPs in the neonatal thymus and demonstrate at the single cell level that rare Lin⁻CD4⁻CD8a⁻CD25⁻KIT^hiFLT3^hi ETPs possess combined granulocyte-monocyte (GM), T and B lymphocyte but not megakaryocyte-erythroid (MkE) lineage potentials, identical to closely molecularly related thymus-seeding progenitors in the BM.

Moreover, global molecular profiling demonstrates that neonatal Lin⁻CD4⁻CD8a⁻CD25⁻KIT^hiFLT3^hi ETPs cluster closest to multipotent progenitors in the BM, rather than subsequent (DN1 and DN2) progenitor stages in the thymus. Finally, in support of Lin⁻CD4⁻CD8a⁻CD25⁻KIT^hiFLT3^hi ETPs with combined T, B and GM lineage potentials, being the most multipotent progenitors in the thymus, phenotypic and functional studies demonstrate that the neonatal thymus is not seeded by HSCs or MPPs with MkE potential. These findings establish for the first time a distinct lineage commitment stage for the transition of T-lineage commitment from the BM to the remote thymus.