Thymic Dendritic Cells Express Inducible Nitric Oxide Synthase and Generate Nitric Oxide in Response to Self- and Alloantigens

Abstract

Thymocytes maturing in the thymus undergo clonal deletion/apoptosis when they encounter self- or allo-Ags presented by dendritic cells (DCs). How this occurs is a matter of debate, but NO may play a role given its ability of inducing apoptosis of these cells. APC (a mixed population of macrophages (Mφ) and DCs) from rat thymus expressed high levels of inducible NO synthase (iNOS) and produced large amounts of NO in basal conditions whereas iNOS expression and NO production were very low in thymocytes. Analysis by FACS and by double labeling of cytocentrifuged preparations showed that DCs and MΦ both express iNOS within APC. Analysis of a purified preparation of DCs confirmed that these cells express high levels of iNOS and produce large amounts of NO in basal conditions. The capacity of DCs to generate NO was enhanced by exposure to rat albumin, a self-protein, and required a fully expressed process of Ag internalization, processing, and presentation. Peptides derived from portions of class II MHC molecules up-regulate iNOS expression and NO production by DCs as well, both in self and allogeneic combinations, suggesting a role of NO in both self and acquired tolerance. We also found that NO induced apoptosis of rat double-positive thymocytes, the effect being more evident in anti-CD3-stimulated cells. Altogether, the present findings might suggest that DC-derived NO is at least one of the soluble factors regulating events, in the thymus, that follow recognition of self- and allo-Ags.

Suppression of thymic lymphomas and increased nonthymic lymphomagenesis in Trp53‐deficient mice lacking inducible nitric oxide synthase gene

“…19,20 There was no significant difference in the distribution of lymphocyte subtypes: CD4 and CD8 in thymus or spleen between iNOS -/-and iNOS ϩ/ϩ mice aged 8 weeks (Fig. 2).…”

“…19 NO produced by iNOS activity in dendritic cells could apply apoptotic pressure on thymocytes having TCR-␤ when they make contact with MCH-I expressed on the membrane of dendritic cells. 20 NO induces apoptosis in thymocytes via a p53-dependent mechanism. 21 These previous findings led us to hypothesize that Trp53-deficient thymocytes are rescued from apoptotic signals by NO produced by iNOS, resulting in prolonged survival and accumulation of DNA damage in the presence of NO.…”

The brain as an immune privileged site: dendritic cells of the central nervous system inhibit T cell activation

“…4b). Our attempts to identify the molecules by which CNS-DC hinder T cell proliferation was guided by numerous reports on the ability of IL-4, IL-10, TGF g , CTLA-4, NO, FasL and TRAIL to inhibit the course of disease in EAE [29][30][31][32][33] and to lead to T cell tolerance [20][21][22][23][24]. We identified TGF g , IL-10 and TRAIL to contribute to the inhibitory phenotype of the CNS-DC.…”

“…Indeed, DC have been described to produce immuno-modulatory factors or to be rendered anergy/ tolerance-inducing by certain molecules [20][21][22][23][24]. We tested a wide array of blocking molecules either alone or in combination and obtained two types of effect: NOproduction blocker (L-NMMA), neutralizing anti-IL-4 or blocking anti-CTLA-4 antibodies as well as neutralizing Fas:Fc had no effect on the inhibition (not shown).…”

The Gli3 Transcription Factor Expressed in the Thymus Stroma Controls Thymocyte Negative Selection Via Hedgehog-Dependent and -Independent Mechanisms

“…Nos2 was also significantly down-regulated (more than 2-fold) both in Gli3 ϩ/Ϫ and Gli3 Ϫ/Ϫ stroma. In the thymus, Nos2 is expressed exclusively in the stroma in thymic epithelial cells and dendritic cells, but not in the thymocytes (44,45), where it induces apoptosis in DP thymocytes undergoing negative selection (44 -49).…”