Sialyl LewisX carbohydrate is expressed differentially during avian lymphoid cell development

Sialyl LewisX is a carbohydrate moiety involved in the regulation of white blood cell adhesion to endothelial cells. In this work, we have studied the expression, localization and function of sialyl LewisX carbohydrate on maturing B and T cells and the stroma of avian bursa, thymus and spleen as well as the role of sialyl LewisX in the generation of immune response and formation of germinal centers in the spleen. The expression of sialyl LewisX on bursal B cells decreases during the embryonic period, while on T cells it remains at a stable low level throughout their thymic development. Immediately after hatching, by which time the expression of sialyl LewisX on bursal B cells has already decreased, the stromal microenvironment of the bursa starts to express it. After hatching, sialyl LewisX is localized exclusively in the medullae of the follicles and the number of sialyl LewisX-positive follicles declines gradually during the following weeks. Thymic stroma does not express sialyl LewisX at any stage of development. In spleen, sialyl LewisX is expressed on a number of B and T cells as well as on other cell types in a way that seems not to be restricted to any structurally defined area. The cells or stroma of the germinal centers, however, do not express sialyl LewisX. When given simultaneously with an immunizing agent, anti-sialyl LewisX monoclonal antibody reduced the formation of germinal centers and suppressed antibody response in young birds. These results demonstrate that sialyl LewisX-bearing cell surface molecules are required during the generation of immune responses. They also suggest a role for sialyl LewisX in B cell maturation.