8B4/20, a private CD43 epitope on developing human thymocytes, is involvedin thymocyte maturation

The 8B4/20 Ag is a 120-kDa molecule whose expression on human thymocytes va ries according to the differentiation stage: high density on immature CD3(- )/(low) thymocytes, reduced density on CD3(medium) and double-positive thym ocytes, and absent on CD3(high) and single-positive thymocytes and on circu lating T cells, In this paper are present immunological and biochemical evi dence demonstrating that 8B4/20 Ag is a variant of CD43, We show that 8B4/2 0-expressing molecules, which are a subset of the CD43 molecules on thymocy tes, are heterogeneous in charge, suggesting varying sialylation levels. Th e 8B4/20 epitope was mapped to the peripherally exposed N-terminal region o f CD43, and the 8B4/20 antigenic determinant was characterized by requireme nt for the sialic acid exocyclic polyhydroxyl side chain, a feature shared with ligands of CD22, Altogether, 8B4/20-CD43 expression pattern and bioche mical characteristics suggest its participation in carbohydrate-based inter actions in the thymus, We therefore used specific Ab to mimic putative 8B4/ 20 interactions with natural Ligand and examined the effect on isolated thy mocytes, Treatment with 8B4/20 had no effect on in vitro apoptosis of isola ted thymocytes, In contrast, 8B4/20 ligation enhanced the conversion of iso lated thymocytes to differentiated phenotypes, Increased numbers were found in 8B4/20-treated cultures of CD3(high) and single-positive thymocytes and decreased numbers of CD3(-/low) and double-positive thymocytes, strongly s uggesting that engagement of 8B4/20 delivers a positive signal that favors completion of the thymocyte maturation program. The ability of 8B4/20 mAb t o drive thymocyte maturation in vitro suggests that CD43 molecules bearing the 8B4/20 epitope participate in early events of thymic selection.