CD4(+ )CD25(+ )FoxP3(+ )regulatory T cells suppress cytotoxicity of CD8(+ )effector T cells: implications for their capacity to limit inflammatory central nervous system damage at the parenchymal level

BACKGROUND: CD4(+ )CD25(+ )forkhead box P3 (FoxP3)(+ )regulatory T cells (T reg cells) are known to suppress adaptive immune responses, key control tolerance and autoimmunity. METHODS: We challenged the role of CD4(+ )T reg cells in suppressing established CD8(+ )T effector cell responses by using the OT-I/II system in vitro and an OT-I-mediated, oligodendrocyte directed ex vivo model (ODC-OVA model). RESULTS: CD4(+ )T reg cells dampened cytotoxicity of an ongoing CD8(+ )T effector cell attack in vitro and within intact central nervous system tissue ex vivo. However, their suppressive effect was limited by the strength of the antigen signal delivered to the CD8(+ )T effector cells and the ratio of regulatory to effector T cells. CD8(+ )T effector cell suppression required T cell receptor-mediated activation together with costimulation of CD4(+ )T reg cells, but following activation, suppression did not require restimulation and was antigen non-specific. CONCLUSIONS: Our results suggest that CD4(+ )T reg cells are capable of suppressing CD8(+ )T effector cell responses at the parenchymal site, that is, limiting parenchymal damage in autoimmune central nervous system inflammation.