Functional and phenotypic analysis of thymic CD34CD1aprogenitor- derived dendritic cells: Predominance of CD1a differentiation pathway

Whether thymic dendritic cells (DC) are phenotypically and functionally distinct from the monocyte lineage DC is an important question. Human thymic progenitors differentiate into T, NK, and DC. The latter induce clonal deletion of autoreactive thymocytes and therefore might be different from their monocyte-derived counterparts. The cytokines needed for the differentiation of DC from thymic progenitors were also questioned, particularly the need for GM-CSF. We show that various cytokine combinations with or without GM-CSF generated DC from CD34CD1a but not from CD34CD1a thymocytes. CD34 thymic cells generated far fewer DC than their counterparts from the cord blood. The requirement for IL-7 was strict whereas GM-CSF was dispensable but nonetheless improved the yield of DC. CD14 monocytic intermediates were not detected in these cultures unless macrophage-CSF (M-CSF) was added. Cultures in M-CSF generated CD14CD1a DC precursors but also CD14CD1a cells. When sorted and recultured in GM- CSF, CD14 cells down-regulated CD14 and up-regulated CD1a. TNF-α accelerated the differentiation of progenitors into DC and augmented MHC class II transport to the membrane, resulting in improved capacity to induce MLR. The trafficking of MHC class II molecules was studied by metabolic labeling and immunoprecipitation. MHC class H molecules were transported to the membrane in association with invariant chain isoforms in CD14 (monocyte)-derived and in CD1a thymic-derived DC but not in monocytes. Thus, thymic progenitors can differentiate into DC along a preferential CD1a pathway but have conserved a CD14 maturation capacity under M-CSF. Finally, CD1a-derived thymic DC and monocyte-derived DC share very close Ag-processing machinery.