(A) Quantification of TH⁺ DNs in the SNpc at day 7 after MPTP (4 × 18 mg/kg) or saline treatment in WT, Tcrb^–/–, Cd8a^–/–, and Cd4^–/– mice. Bars represent the mean number of total nigral TH⁺ DNs. Open symbols indicate saline-treated animals and filled symbols indicate MPTP-treated animals. Each symbol represents 1 individual animal. A significant protection against MPTP-induced DN cell loss is observed in Tcrb^–/– and Cd4^–/– mice but not in Cd8a^–/– animals as compared with their WT littermates. *P < 0.05 compared with MPTP-treated WT and Cd8a^–/– mice; ^†P = 0.956 compared with MPTP-treated WT littermates (Tukey post-hoc analysis). Nissl⁺ cell counts confirmed that TH⁺ cell loss was not a consequence of downregulated expression of TH (data not shown). (B) Representative photomicrographs of mesencephalic sections immunostained for TH with Nissl counterstain from saline- or MPTP-treated WT and lymphocytic deficient mice. Insets show Mac-1⁺ microglial cells (arrows) in the SNpc from the same animals. Scale bar: 300 μm; 100 μm (insets). (C) Quantification of the total number of infiltrated CD4⁺ and CD8⁺ T cells in the SNpc from MPTP-treated WT and lymphocytic deficient mice. MPTP-treated Cd8a^–/– mice display as many CD4⁺ T cells as MPTP-treated WT littermates. **P < 0.05 compared with saline-injected WT mice (Mann-Whitney U test). ^††P < 0.001 compared with MPTP-treated WT and Cd8a^–/– mice; ^#P < 0.001 compared with MPTP-treated WT mice (Dunn test). ND, not detected.