Identification of mature and immature human thymic dendritic cells that differentially express HLA-DR and interleukin-3 receptor in vivo

We have previously shown that thymic CD34(+) cells have a very limited myel oid differentiation capacity and differentiate in vitro mostly into CD1a(+) -derived but not CD14(+)-derived dendritic cells (DC). Herein we characteri zed the human neonatal thymic DC extracted from the organ in relationship,w ith the DC generated from CD34(+) cells in situ. We show that in vivo thymi c DC express E cadherin, CLA, CD4, CD38, CD40, CD44, and granulocyte-macrop hage colony-stimulating factor-R (GM-CSF-R; CD116) but no CD1a. According t o their morphology, functions, and surface staining they could be separated into two distinct subpopulations: mature HLA-DRhi, mostly interleukin-3-R (CD123) negative cells, associated with thymocytes, some apoptotic, and exp ressed myeloid and activation markers but no lymphoid markers. In contrast, immature HLA-DR+ CD123(hi) CD36(+) cells with monocytoid morphology lacked activation and myeloid antigens but expressed lymphoid antigens, The latte r express pT alpha mRNA, which is also found in CD34(+) thymocytes and in b lood CD123(hi) DC further linking this subset to lymphoid DC. However, the DC generated from. CD34(+) thymic progenitors under standard conditions wer e pT alpha -negative, Thymic lymphoid DC showed similar phenotype and cytok ine production profile as blood/tonsillar lymphoid DC but responded to GM-C SF, and at variance with them produced no or little type I interferon upon infection with viruses and did not induce a strict polarization of naive T cells into TH2 cells. Their function in the thymus remains therefore to be elucidated.