A role for BP-3/BST-1 antigen in early T cell development.

In the mouse thymus, pre-T cells are defined by their CD3(-)CD4(-)CD8(-) triple-negative, CD44lo/- CD25+ phenotype. We made a rat mAb IF-7, that, among all T cell subsets analyzed, reacted exclusively with pre-T cells. Molecular cloning revealed that the antigen recognized by IF-7 was identical to BP-3/BST-1, a glycosyl-phosphatidylinositol-linked, CD38-related molecule previously described as a possible co-activation molecule of pre-B cells. We found that IF-7 cross-linking enhances the proliferative response of sorted pre-T cells to anti-CD3 stimulation. In addition, IF-7 enhances and accelerates the development of fetal thymic organ culture (FTOC), although the gamma delta lineage is unaffected by the treatment. In addition, sorted IF-7+ pre-T cells give preferentially rise to alpha beta TCR+ thymocytes in FTOC. Our observations strongly suggest that BP-3/BST-1 is implicated in both early B and T cell growth and development, and is an early marker for the alpha beta lineage.