MHC class II β-chain and αIIbβ3 integrin are expressed on T-cell progenitors in embryonic bone marrow

The RR5 monoclonal antibody (mAb) was obtained after immunization of mice with hemopoietic cells from chicken embryos. The cDNA encoding the protein recognized by RR5 was cloned using COS-7 cells transfected with an embryonic bone marrow (BM) cDNA library. The epitope recognized by the RR5 mAb was located on the non-polymorphic MHC class II beta-chain molecule. In the embryonic BM, RR5 labeled 50% of the c-kit expressing cells. Previous experiments have shown that the T-cell progenitors are present in the MHC class II(+)/c-kit(+) BM population along with myeloid progenitors and that T-cell and myeloid progenitors also express the integrin alphaIIbbeta3. In this study, using intrathymic cell transfer experiments in chicks, we have tested the T-cell differentiation potential of MHC class II/alphaIIbbeta3 double positive cells. It proved to be similar to that of the c-kit/MHC class II positive cells. However, injection of triple positive cells resulted in a selection of cells with an increased T-cell potential. Most of the MHC class II positive cells which do not express c-kit are prone to apoptosis, indicating that these progenitors might need a survival signal via c-kit. Interestingly, the MHC class II positive progenitors lose this expression after intrathymic transfer. Taken together our data suggest that the presence of the MHC class II beta-chain molecule on the surface of BM progenitor cells could be implicated in differentiation toward myeloid and lymphoid lineages.