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

The 8B4/20 Ag is a 120-kDa molecule whose expression on human thymocytes varies according to the differentiation stage: high density on immature CD3(-)/(low) thymocytes, reduced density on CD3(medium) and double-positive thymocytes, and absent on CD3(high) and single-positive thymocytes and on circulating T cells, In this paper are present immunological and biochemical evidence demonstrating that 8B4/20 Ag is a variant of CD43, We show that 8B4/20-expressing molecules, which are a subset of the CD43 molecules on thymocytes, are heterogeneous in charge, suggesting varying sialylation levels. The 8B4/20 epitope was mapped to the peripherally exposed N-terminal region of CD43, and the 8B4/20 antigenic determinant was characterized by requirement for the sialic acid exocyclic polyhydroxyl side chain, a feature shared with ligands of CD22, Altogether, 8B4/20-CD43 expression pattern and biochemical characteristics suggest its participation in carbohydrate-based interactions in the thymus, We therefore used specific Ab to mimic putative 8B4/20 interactions with natural Ligand and examined the effect on isolated thymocytes, Treatment with 8B4/20 had no effect on in vitro apoptosis of isolated thymocytes, In contrast, 8B4/20 ligation enhanced the conversion of isolated 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 suggesting that engagement of 8B4/20 delivers a positive signal that favors completion of the thymocyte maturation program. The ability of 8B4/20 mAb to drive thymocyte maturation in vitro suggests that CD43 molecules bearing the 8B4/20 epitope participate in early events of thymic selection.