NO, Immunosuppression and Tumor Immmunotherapy

Abstract: This study aims at clarifying the role of NO in the immunosuppression induced by in vivo tumor growth and by tumor immunotherapy, and determining whether the inhibition of NO production can be used as an adjuvant in tumor immunotherapy. We have shown previously that tumor cells, glioma (N32) and colon carcinoma (H1D2), when genetically engineered to express such immune stimulatory cytokines as IFN-g and IL-18, induce strong anti-tumor immune response, in immunized tumor-free rats, whereas only a limited therapeutic effect is achieved in rats in which a tumor has already become established. This led us to our studying whether immmunosuppression induced in rats by a subcutaneously growing malignant glioma and in rats with an intrahepatic growing colon carcinaoma would attenuate anti-tumor immune responses. Anti-tumor cytolytic responses, proliferative responses and cytokine production were found to all be strongly suppressed in the spleen cells of these tumor-bearing rats. The suppression was also shown to be partially dependent on the production of nitric oxide (NO) by the suppressor cells found in the plastic adherent fraction of the spleen cells. Since during inflammatory responses the major part of NO is derived from the IFN-g induced expression of inducible nitric oxide synthase (iNOS), it was of interest to investigate whether the expression of iNOS is induced after immunization by IFN-g-secreting glioma cells (N32 IFN-g) and whether the inhibition of NO generated by iNOS in immunized rats would enhance anti-tumor immune responses. The expression of iNOS was found to be elevated, both at the immunization site and in the brain tumors. The selective inhibition of iNOS was shown to enhance anti-tumor immune responses. In conclusion, our results demonstrate that in immunized rats both growing tumors and immunotherapeutic intervention by use of IFN-g secreting tumor cells tends to induce NO-dependent suppressor cell activity that inhibits anti-tumor immune responses. It appear then that the selective inhibition of iNOS can be used as an adjuvant for enhancing type 1 anti tumor T-cell responses during anti-tumor immunotherapy.