Central Nervous System-Infiltrating T Lymphocytes in Stroke Are Activated via Their TCR (T-Cell Receptor) but Lack CD25 Expression

T lymphocytes contribute to secondary brain damage after stroke. It has not been fully investigated whether this contribution is caused by antigen-specific or antigen-nonspecific activation of T lymphocytes. Lymphocytes from Nur77GFP transgenic mice express a fluorescent protein upon activation via the TCR (T-cell receptor), allowing the differentiation of activation mode in a natural repertoire of immune cells and antigens.Middle cerebral artery occlusion or sham surgery was performed, and T-lymphocyte activation was analyzed by flow cytometry in the brain, spleen, and blood 16 hours, 2 days, 3 days, 4 days, and 7 days after surgery.Ipsilateral hemispheric T-lymphocyte invasion peaked on day 4 poststroke. Here, we observed PD-1 (programmed cell death protein 1) expression on almost all invading T lymphocytes, while CD25 expression was low. CD25+, CD69+, or PD-1+ T lymphocytes predominantly displayed antigen-specific activation; the opposite was observed for T lymphocytes isolated from the blood. A mixed activation that favored antigen-specific activation was observed in the spleen. PD-1 was upregulated within the brain, whereas CD25 was not. Antigen-specific T lymphocytes home to the brain, while antigen-nonspecifically activated cells remain within the blood.Our data clearly demonstrate antigen-specific activation of T lymphocytes infiltrating ischemic brain lesions in stroke. The high expression of inhibitory PD-1 and low expression of CD25 on activated T lymphocytes in the brain most likely reflect immunosuppressive mechanisms.