HUMAN DENDRITIC CELLS OF THE THYMUS

Human thymic dendritic cells (DC) represent a member of the family of bone marrow-derived dendritic cells. They have a dendritic shape and a re found in small number mainly at the corticomedullary border and in medullary regions of the thymus. Human thymic DC were isolated by dens ity gradient separation followed by treatment with CD2, CD7 and CD11b mAb and antibody-coated immunobeads. The resulting population contains 60 to 75 % of brightly HLA-DR+ cells which present morphological char acteristics of DC observed in situ. An extensive phenotypic analysis h as confirmed that they are of mesenchymal origin and revealed that som e express CD11a and CD54 molecules. Freshly isolated DC do not stain w ith a wide variety of anti-T-B and -monocyte or -macrophages mAb, howe ver they acquire the CD1 molecule after a few days culture. It is now possible to obtain 90 to 95 % purified human thymic DC by cell sorting . Functional studies have shown that human thymic DC are potent activa tors of mixed lymphocyte reaction, act as accessory cells in mitogenic thymocyte proliferation, increase the thymocyte proliferative respons e to toxin signal and produce IL-1. They also form spontaneous associa tions with thymocytes : this raises questions about the implication of DC in differentiation and/or maturation of thymocytes.