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8B4/20, A Private CD43 Epitope on Developing Human Thymocytes, Is Involved in Thymocyte Maturation¹

Marina Fabbi^2,*, Jens Geginat, Micaela Tiso, Dunia Ramarli^§, David Parent^¶, Antonio Bargellesi and Eileen Remold-O’Donnell^¶

^* Istituto Nazionale per la Ricerca sul Cancro, Genova, Italy; FB Biologie Chemie Pharmatie, FU Berlin, Berlin, Germany; Dipartimento Medicinia Sperimentale, Sezione Biochimica, Università di Genova, Genova, Italy; ^§ Azienda Ospedaliera e Università di Verona, Policlinico Borgo Roma, Verona, Italy; and ^¶ The Center for Blood Research, Harvard Medical School, Boston, MA 02115

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 we 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.

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