Internalization by receptor-mediated endocytosis of T6 (CD1 "NA1/34") surface antigen in T6 positive human cord blood cells (Langerhans cell precursors?).

A subset of T6 positive cells was recently separated from normal human cord blood mononuclear cells. It was shown to coexpress HLA-DR and myeloid differentiation antigens (Mo1, MY4). The phenotype and ultrastructure of the cells suggested that these T6 positive cells might be the precursors of the Langerhans cells of the skin. We have previously demonstrated by immunogold labeling techniques that the T6 surface antigen of human Langerhans cells of the skin is internalized in unfixed Langerhans cells or indeterminate cells by a process of receptor-mediated endocytosis. This process involved the formation of coated pits, coated vesicles, endosomes and lysosomes. Following this process, in Langerhans cells, gold labeled Birbeck granules appeared in the cell center often in continuity with endosomes. In the present study, we used an indirect immunogold labeling technique to reveal the T6 antigen present on the surface of living T6 positive cord blood mononuclear cells. We observed the internalization of the T6 surface antigen by a process of receptor-mediated endocytosis similar to that described in Langerhans cells of the skin. This process, however, was not followed by the appearance of intracytoplasmic Birbeck granules.