CD43 is a leukocyte membrane glycoprotein rich in negatively charged sialic acid residues that may act as an anti-adhesion molecule. In this study, we analyzed the expression and function of CD43 on epidermal Langerhans cells (LC) and dendritic cells derived from adherent peripheral blood monocytes treated with granulocyte-macrophage CSF and IL-4 (Mo-DC). Both freshly isolated LC (fLC) and Mo-DC expressed uniformly high amounts of membrane CD43, with levels unchanged in 48-h cultured LC or in TNF-alpha-treated Mo-DC. When fLC or Mo-DC were transiently preincubated with anti-CD43 mAb or neuraminidase, they formed much larger clusters with T cells as compared with cells incubated with control mAb. Moreover, a significantly increased proliferation of naive allogeneic T cells in a primary MLR assay was induced. Anti-CD43-treated Mo-DC also presented tetanus toxoid to primed T cells more efficiently. Immunofluorescence and immunogold electron microscopy showed that following Ab cross-linking and warming to 37 degrees C, CD43 is progressively removed from the cell surface and is internalized within endosomes and vacuoles. In addition, fLC and Mo-DC incubated with anti-CD43 mAb increased expression of HLA-DR, ICAM-1, B7-1, B7-2, CD40, and CD83 after 24 h, and this was partially prevented by culturing cells in the presence of cycloheximide. We thus postulate that membrane CD43, other than serving as anti-adhesion molecule, can mediate activation signals in immature DC, leading to augmented expression of adhesion/costimulatory molecules and Ag-presenting function.