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Human thymus contains 2 distinct dendritic cell populations.

In this study, 2 distinct populations of mature dendritic cells (DCs) were identified in the human thymus. The major population is CD11b-, CD11c+, and CD45RO(low) and does not express myeloid-related markers. It displays all the characteristics of mature DCs with a typical dendritic morphology, high surface levels of HLA-DR, CD40, CD83, and CD86, and expression of DC-lysosome-associated membrane glycoprotein messenger RNA (mRNA). In addition, CD11b- thymic DCs do not express macrophage inflammatory protein-1alpha (MIP-1alpha) mRNA, but express thymus-expressed chemokine (TECK) mRNA and are able to secrete bioactive interleukin 12 (IL-12) upon stimulation. In contrast, the minor and variable thymic DC population is CD11b+, CD11c(high), and CD45RO(high) and comprises CD83+CD14- mature and CD83- CD14+ immature DCs. It expresses macrophage-colony stimulating factor receptor, MIP-1alpha mRNA and high amounts of decysin mRNA after CD40 activation, but does not express TECK and is a weak bioactive IL-12 producer. Also identified were the IL-3Ralpha(high) plasmacytoid cells, which are present in the thymic cortex and medulla. Upon culture with IL-3, granulocyte/macrophage-colony stimulating factor, and CD40 ligand, the plasmacytoid cells can adopt a phenotype resembling that of freshly isolated CD11b- thymic DCs. However, these plasmacytoid-derived DCs fail to secrete bioactive IL-12; therefore, conclusions cannot be made about a direct relation between thymic plasmacytoid cells and CD11b- DCs. Whereas CD11b+ thymic DCs appear to be related to tonsillar germinal-center DCs, the major CD11b- IL-12-secreting human thymus DC population has similarities to mouse CD11b- CD8+ DCs.