Murine dendritic cells derived from myeloid progenitors of the thymus are unable to produce bioactive IL-12p70

Cryptic O2- -generating NADPH oxidase in dendritic cells

All the components of the O(2)(-)-generating NADPH oxidase typically found in neutrophils, namely a membrane-bound low potential flavocytochrome b and oxidase activation factors of cytosolic origin, are immunodetectable in murine dendritic cells (DCs). However, in contrast to neutrophils, DCs challenged with phorbol myristate acetate (PMA) can barely mount a significant respiratory burst. Nevertheless, DCs generate a substantial amount of O(2)(-) in the presence of PMA following preincubation with pro-inflammatory ligands such as lipopolysaccharide and pansorbin, and to a lesser extent with anti-CD40 or polyinosinic polycytidylic acid. We found that the virtual lack of the oxidase response to PMA alone is specifically controlled in DCs. Through the use of homologous and heterologous cell-free systems of oxidase activation, we showed the following: (1) a NADPH oxidase inhibitory factor is located in DC membranes; it exerts its effect on oxidase activation and not on the activated oxidase. (2) The inhibition is relieved by pretreatment of DC membranes with beta-octylglucoside (beta-OG). (3) The beta-OG-extracted inhibitory factor prevents the activation of neutrophil oxidase. (4) The inhibitory activity is lost after treatment of DC membranes with proteinase K or heating, which points to the protein nature of the inhibitory factor. Overall, these data indicate that the O(2)(-)-generating oxidase in DCs is cryptic, owing to the presence of a membrane-bound inhibitor of protein nature that prevents oxidase activation. The inhibition is relieved under specific conditions, including a prolonged contact of DCs with pro-inflammatory ligands from microbial origin, allowing a substantial production of O(2)(-), which may contribute to the response of DCs to a microbial exposure.

Fibroblasts inhibit the production of interleukin-12p70 by murine dendritic cells

Interleukin-12 p70 (IL-12p70) is a key cytokine produced by dendritic cells (DC) able to drive the development of T helper type 1 (Th1) lymphocytes. We showed that thymic and other fibroblasts strongly inhibit IL-12p70 production by splenic DC stimulated by lipopolysaccharide plus either anti-CD40 or interferon-gamma (IFN-gamma) and by purified splenic DC stimulated by Pansorbin plus IFN-gamma. This IL-12p70 inhibitory activity is secreted in the conditioned medium of primary fibroblasts and fibroblast cell lines but not by haematopoietic cell lines. As IL-10 was the unique factor able to inhibit IL-12p70 produced by cultured splenic DC, we showed that a neutralizing antibody to IL-10 did not suppress the IL-12p70 inhibitory activity of thymic fibroblast-conditioned medium (FCM). This FCM potently inhibits the maturation and expression of major histocompatibility complex class II and co-stimulatory molecules induced by stimulation of spleen-derived DC. While thymic FCM suppressed the IL-12p70 expression by stimulated spleen-derived DC, tumour necrosis factor-alpha production is not affected. This inhibitory activity is able to down-regulate the IL-12p35 subunit transcription and expression, resulting in the impaired assembly of IL-12p70 heterodimer. As fibroblasts are present in the tissue microenvironment and are active players in the establishment of an immune response, the nature and role of the fibroblastic inhibitory activity remain to be established.

T lymphocytes potentiate murine dendritic cells to produce IL-12

IL-12 is mainly produced by CD8alpha(+) dendritic cells (DCs) and induces Th1 polarization of the immune response. We investigated the influence of lymphocytes on splenic DC (SDC) and thymic DC (TDC) development and on their IL-12 production capacity. First, CD3epsilon(-/-) mice, lacking T cells, and RAG-2(-/-) mice, lacking T and B cells, possess numbers of SDCs, TDCs, and CD8alpha(+) SDCs similar to wild-type (WT) mice. Second, SDCs and TDCs from CD3epsilon(-/-) mice do not secrete IL-12 in vitro after different stimulations, whereas DCs from pTalpha(-/-) mice, possessing reduced T cell number, and RAG-2(-/-) mice, produce an IL-12 level similar to that of WT DCs. We show that T lymphocytes restore the capacity of DCs to produce IL-12 after stimulation in vivo by reconstitution of CD3epsilon(-/-) mice with WT T cells and in vitro by coculture of CD3epsilon(-/-) DCs with WT T cells. The regulation of IL-12 production occurred at the transcriptional level, with an increase of IL-12p35 transcripts and a decrease of IL-12p40 transcripts. Although IL-4 restores IL-12 production by CD3epsilon(-/-) SDCs, anti-IL-4 Abs inhibited only partially the IL-12 production in coculture of CD3epsilon(-/-) DCs and WT T cells. Taken together, these data show that T lymphocytes potentiate IL-12 production by DCs and that IL-4 is not solely involved in this regulation. In conclusion, B and T cells exert balanced actions on DCs by respectively inhibiting or promoting IL-12 production.

Identification of a myeloid intrathymic pathway of dendritic cell development marked by expression of the granulocyte macrophage-colony-stimulating factor receptor

In this study, the finding that a significant proportion of all dendritic cells (DCs) resident in vivo in the human postnatal thymus displayed a myeloid-related phenotype prompted us to re-examine the developmental origin of thymic DCs, a cell type hitherto considered to represent a homogeneous lymphoid-derived population. We show here that these novel intrathymic DCs are truly myeloid, as they arise from CD34(+) early thymic progenitors through CD34(lo) intermediates which have lost the capacity to generate T cells, but display myelomonocytic differentiation potential. We also demonstrate that phenotypically and functionally equivalent myeloid precursors devoid of T-cell potential do exist in vivo in the postnatal thymus. Moreover, although interleukin 7 (IL-7) supports the generation of such myeloid intermediates, we show that their developmental branching from the main intrathymic T-cell pathway is linked to the up-regulation of the myelomonocytic granulocyte macrophage-colony-stimulating factor (GM-CSF) receptor, to the down-regulation of the IL-7 receptor and to the lack of pre-T-cell receptor alpha (pTalpha) gene transcriptional activation. Taken together, these data challenge the current view that the thymus is colonized by a lymphoid-restricted progenitor and provide evidence that a more immature precursor population with lymphoid and myelomonocytic potential is actually seeding the human postnatal thymus.