Naturally occurring CD4+CD25+ regulatory T cells prevent but do not improve experimental myasthenia gravis

In the current study, we investigated whether naturally occurring CD4 ⁺CD25⁺ T cells, separated by immunomagnetic anti-CD4 and anti-CD25 Abs from naive animals, are able to protect from experimental autoimmune myasthenia gravis (EAMG) and modify the progression of ongoing disease when administered to Torpedo californica acetylcholine receptor (AChR)-immunized Lewis rats. Even though CD4⁺CD25⁺ and CD4⁺CD25^high T cell frequencies were similar in the spleens and lymph nodes of EAMG and healthy rats, we observed that CD4 ⁺CD25⁺ T cells isolated from the spleens of naive animals inhibited in vitro the Ag-induced proliferation of T cell lines specific to the self-peptide 97-116 of the anti-AChR subunit (R97-116), an immunodominant and myasthenogenic T cell epitope, whereas CD4⁺CD25⁺ T cells purified from the spleens of EAMG rats were less effective. CD4⁺ CD25⁺ T cells from EAMG rats expressed less forkhead box transcription factor P3 but more CTLA-4 mRNA than healthy rats. Naive CD4 ⁺CD25⁺ T cells, obtained from naive rats and administered to T. californica AChR-immunized animals according to a preventive schedule of treatment, reduced the severity of EAMG, whereas their administration 4 wk postinduction of the disease, corresponding to the onset of clinical symptoms (therapeutic treatment), was not effective. We think that the exogenous administration of CD4⁺CD25⁺ naive T cells prevents the early events underlying the induction of EAMG, events linked to the T cell compartment (Ag recognition, epitope spreading, and T cell expansion), but fails to ameliorate ongoing EAMG, when the IgG-mediated complement attack to the AChR at the neuromuscular junction has already taken place.