Large suppressive macrophages (LSM) were induced by restimulating spleen cells from rats with experimental autoimmune myasthenia gravis (EAMG) in vitro, with the autoantigen acetylcholine receptor (AChR) in the presence of cyclosporine A. LSM, purified from these cultures, are extremely potent suppressors of AChR-stimulated lymphoproliferative responses and antibody responses in vitro. In the present study, we have analyzed the factors that determine susceptibility of primed lymph node cells (pLNC) to suppression by LSM and examined the fate of these cells. We found three characteristics of pLNC that influenced their susceptibility to suppression. First, pLNC were required to be activated (by antigen in these experiments) in order for suppression to occur. Resting lymphocytes were not affected, even when they were present in cultures where antigen-activated lymphoblasts were being actively suppressed. Second, antigen specificity of the responder cells influenced their susceptibility to suppression by LSM. AChR-specific cells were relatively more susceptible to suppression by AChR-induced LSM than pLNC primed to an unrelated antigen, keyhole limpet hemocyanin. Third, T cell proliferation was suppressed by LSM to a far greater extent than antibody production by B cells. Using enriched T cell blasts generated from AChR- stimulated T cell lines, we found that LSM rapidly suppressed [3H]TdR uptake and induced DNA fragmentation assessed by the TUNEL assay (within 8 h of coculture) and induced morphological signs of apoptosis of T cells (within 24 h). Few, if any, blasts remained by 48 h of coculture. The ability to suppress an activated immune response permanently, without affecting nonactivated, bystander lymphocytes, holds promise that LSM, or their cellular products, could be used for immunotherapy of autoimmune diseases such as myasthenia gravis.
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