MHC class II beta-chain and alpha IIb beta 3 integrin are expressed on T-cell progenitors in embryonic bone marrow

The RR5 monoclonal antibody (mAb) was obtained after immunization of mice w ith hemopoietic cells from chicken embryos. The cDNA encoding the protein r ecognized 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 l ocated on the non-polymorphic MHC class II P-chain molecule. In the embryon ic 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-k it(+) BM population along with myeloid progenitors and that T-cell and myel oid progenitors also express the integrin alpha IIb beta3. In this study, u sing intrathymic cell transfer experiments in chicks, we have tested the T- cell differentiation potential of MHC class II/alpha IIb beta3 double posit ive cells. It proved to be similar to that of the c-kit/MHC class II positi ve cells. However, injection of triple positive cells resulted in a selecti on of cells with an increased T-cell potential. Most of the MHC class Il po sitive cells which do not express c-kit are prone to apoptosis, indicating that these progenitors might need a survival signal via c-kit. Interestingl y, the MHC class II positive progenitors lose this expression after intrath ymic 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. (C) 2001 Elsevier Science Ltd. All rights reserved.