Relevance of sialoglycoconjugates in murine thymocytes during maturation and selection in the thymus

Differentiation of most T lymphocytes is characterized not only by the vari able expression of CD4/CD8 coreceptor molecules and increased surface densi ty of the T cell antigen receptor, but also by changes in the glycosylation pattern of cell surface glycolipids or glycoproteins, In this work we eval uated the changes in the sialylation pattern in thymus sections from normal and dexamethasone treated mice. We used sialic acid specific lectins, such as Sambucus nigra agglutinin (SNA, NeuAc alpha 2,6-Gal specific) and Maack ia amurensis agglutinin (MAA, NeuAc alpha 2,3-Gal specific). Our results in dicate that the sialylation pattern was modified during the maturation proc ess of thymic cells. The immature CD4-CD8- and CD4+CD8+ cortical thymocytes were recognized by SNA, whereas the mature single positive (CD4+ or CD8+) medullary cells, preferentially bound MAA lectin. However, in the cortico-m edullary region we found not only SNA+ cells, but also MAA+ cells. In the t hymus of dexamethasone treated mice, the clusters of thymocytes undergoing apoptosis in the cortex were characteristically stained by SNA. These resul ts suggest that in the initial stages of the differentiation pathway, a gre at number of thymocytes express an alpha 2,6 linked sialic acid on their su rface and as they progress to more mature stages there is a change in the s ialylation pattern to alpha 2,3 linked sialic acids probably due to a regul ated expression of different sialyltransferases, which could be modulated b y the thymic microenviroment.