CHARACTERIZATION OF TERMINAL SIALIC-ACID LINKAGES ON HUMAN THYMOCYTES- CORRELATION BETWEEN LECTIN-BINDING PHENOTYPE AND SIALYLTRANSFERASE EXPRESSION

T cell surface sialylation changes during maturation in the thymus, We have previously demonstrated increased expression of mRNA encoding th e Gal beta 1, 3GalNAc alpha 2,3-sialyltransferase in mature medullary human thymocytes, compared with immature cortical thymocytes, For this enzyme, increased expression of transferase mRNA correlated with incr eased sialylation of O-glycans, We have now examined the pattern of ex pression in the human thymus of two additional sialyltransferases, the Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase (ST6N) and the Gal bet a 1,3/4GlcNAc alpha 2,3-sialyltransferase (ST3N), The patterns of mRNA expression were compared with the pattern of binding of two sialic ac id-specific plant lectins, Sambucus nigra agglutinin and Maackia amure nsis agglutinin, which preferentially recognize alpha 2,6- and alpha 2 ,3-linked sialic acids, respectively, on N-glycans, By in situ hybridi zation, mRNA encoding ST3N was detected uniformly throughout the thymu s, All thymocytes bound M, amurensis agglutinin, demonstrating a direc t correlation between the level of ST3N mRNA expression and cell-surfa ce glycosylation, In contrast, mRNA encoding ST6N was also expressed u niformly throughout the thymus; however, only mature (CD3(hi)) medulla ry thymocytes bound S. nigra agglutinin, On mature thymocytes, S. nigr a agglutinin appeared to bind primarily to the cell-surface glycoprote in CD45; since only the mature thymocytes expressed the CD45RA isoform , while both mature and immature populations expressed the CD45R0 isof orm, CD45RA may be a preferred substrate for ST6N, These results demon strate that glycoprotein sialylation is tightly regulated during T cel l development and that the developmentally regulated expression of spe cific oligosaccharide structures on the cell surface may be influenced by expression of both the relevant glycosyltransferase and specific a cceptor substrate.