CD135 (Flk2/Flt3) expression by human thymocytes delineates a possible role of FLT3-ligand in T-cell precursor proliferation and differentiation

The expression of the Flk2/Flt3 molecule (CD135), the receptor for Flt3 lig and (FL), was investigated in the human thymus. Results showed that there i s a high level of expression of CD135 by thymocyte populations, especially by intrathymic T-cell precursor populations. As these results suggested a r ole for FL in the regulation of thymic T-cell precursor differentiation and /or proliferation, we used an in vitro model of thymic stromal cell culture s in order to delineate the activity of FL on human CD7(high)CD3(-)CD4(-)CD 8(-) triple negative intrathymic T-cell precursors. Results showed that FL, either alone or in combination with stem cell factor (SCF) induced the pro liferation of CD7(high) precursors, but to a lower extent than interleukin- 7 (IL-7) or IL-7 + SCF, used as positive controls. In the presence of FL SCF, CD7(high) cells developed mainly towards a CD11b(+) phenotype whereas IL-7 + SCF preferentially induce a CD3(+)TcR gamma delta(+)CD8(+) phenotype . Taken together, these data suggest that FL may play a role in inducing th e proliferation of CD7(high) human intrathymic T-cell precursors, but also in the induction of a myeloid differentiation pathway within the human thym us.