EXPRESSION OF AN INDUCIBLE TYPE OF NITRIC-OXIDE (NO) SYNTHASE IN THE THYMUS AND INVOLVEMENT OF NO IN DELETION OF TCR-STIMULATED DOUBLE-POSITIVE THYMOCYTES

The present study investigates the role of nitric oxide (NO) in the de letion of TCR-stimulated double-positive (DP) thymocytes. Fetal thymi expressed mRNA for an inducible type of NO synthase (iNOS), The levels of iNOS mRNA became maximal around gestation day 18 with a decline af ter birth, Administration of anti-CDS mAb to fetal thymus organ cultur e (FTOC) or young mice resulted in enhanced expression of mRNAs for iN OS as well as IFN-gamma. Immunohistochemical analyses revealed that iN OS was produced in the corticomedullary junction and medulla, The effe cts of iNOS-induced NO on anti-CD3-unstimulated or anti-CD3-stimulated thymocytes were examined by culturing them in the presence or absence of a NO-generating compound, Stimulation of DP thymocytes with anti-C D3 alone induced the generation of CD4(low)CD8(low) thymocytes, The su bsequent exposure of these anti-CD3-stimulated thymocytes to NO promot ed down-regulation of CD4 and CD8 expression, The recovery of viable D P cells was considerably reduced compared with stimulation with anti-C DS or NO alone, Even in a viable DP population, high incidences of DNA strand breaks were detected in the CD4(low)CD8(low) compartment, In c ontrast to DP cells, the recovery of viable single-positive cells was not decreased but rather slightly enhanced by treatment with anti-CDS and/or NO, The recovery of anti-CD3-stimulated thymocytes were also re duced when cultured on the thymic stromal monolayer with the capacity to produce NO upon IFN-gamma stimulation, These results indicate that NO, which is generated in association with TCR stimulation in the thym us, functions to induce deletion of DP thymocytes, especially when the ir TCR is stimulated.