Maturation of bone marrow-derived T cell progenitors in euthymic and athymic mice

The generation of T lymphocytes from hematopoietic stem cells (HSC) is well characterized in mice. Multipotent HSC give rise to the committed lymphocyte progenitor (CLP) which can then give rise to T cells via a thymic maturation pathway. Conventional views of T cell development maintain that the sojourn through the thymus is critical to T lymphopoiesis and commitment of a multipotent lymphocyte progenitor to the T lineage occurs only within the thymic microenvironment. Here we characterized, using 5-color flow cytometry, a population of Thy1.2 hiLin−CD2− cells in the bone marrow of C57BL/6 mice which were capable of giving rise to TCRαβ + CD4 and CD8 single positive T cells in vivo in both euthymic and congenitally athymic nu/nu adoptive hosts. Reverse-transcription PCR analysis revealed that bone marrow precursors expressed early T cell developmental genes including RAG-1, RAG-2 and pTα. They underwent maturation via a Thy1.2+TCRαβ −CD2+ intermediate, which had a rearranged TCRβ gene while the progenitors themselves had an unrearranged TCRβ locus as judged by genomic PCR analysis. Thy1.2hiLin− CD2− cells in the marrow of athymic nu/nu mice failed to express pTα and did not progress to the CD2+ intermediate stage. Nu/ nu-derived precursors were unable to generate TCRαβ + CD4 or CD8 cells in vivo in adoptive congenic hosts indicating that the whn mutation in the nu/ nu animal affected the bone marrow microenvironment such that these early T cell precursors failed to develop properly. These committed T cell precursors (CTP) developed from HSC in both euthymic and thymectomized hosts very rapidly. HSC-derived CTP from both euthymic and thymectomized hosts are able to give rise to T cells in adoptive hosts showing that the thymus was not required in the maturation of the HSC into functional bone marrow CTP. Detailed flow cytometric analysis showed that extrathymically derived naïve T cells from CTP in either athymic or euthymic hosts or from HSC in thymectomized hosts had a CD44hiCD54hi surface phenotype not seen in thymus-derived naïve T cells.