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August 5, 2004

Fundamental Change to Immunology 101
Penn Researchers Pinpoint Identity of
Early-Stage T-cells Circulating in Blood



Current and revised models of hematopoeisis.
(A) Current Model. Hematopoietic stem cells (HSCs) in the bone marrow (yellow) give rise to all blood lineages and have the ability to self-renew. Their direct progeny are multipotent progenitors (MPPs) that give rise to all blood lineages but have lost the ability to self-renew. The originally proposed model predicted a split between the lymphoid lineages, arising from a common lymphoid progenitor (CLP), and the myeloid and erythroid lineages, both arising from a common myeloid progenitor (CMP). T cells developing in the thymus (grey) were proposed to be derived from CLPs, which travel from the bone marrow to the thymus through the blood (red).
(B) Revised model. Direct examination of blood revealed that the only circulating cells with T potential were HSCs and MPPs, not CLPs. The so-called CLP, therefore, does not give rise to T cells, but instead remains in the bone marrow to make B and NK cells. Instead HSCs or MPPs must enter the thymus and give rise to early T lineage progenitors (ETPs) and thus T cells.

Diagram courtesy of Benjamin Schwarz.

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