Group assignment and intervention

This study used a completely randomized method for group assignment. The mice were numbered from light to heavy in weight, and were then divided into several (three or four, according to the experiment design) similar-weight groups in sequence. The experimental groups were formed by mice that were randomly picked up from each similar-weight group.

To induce the PD mouse model (Jackson-Lewis and Przedborski, 2007), 36 mice were randomly divided into two groups (n = 18 per group). The first group of 18 mice was randomly divided into two groups of MPTP treatment (1 day or 7 days) and one group of saline treatment (n = 6 per group). Mice in the MPTP treatment group were treated with four intraperitoneal injections of MPTP HCl (18 mg/kg; M103-10MG; Sigma-Aldrich) over 1 day, at 2-hour intervals. In the control group, mice were injected with an equivalent amount of saline. The other 18 mice were divided into groups and intervened in the same way.

To induce the MHC-I knockdown model (Ye et al., 2018), we used a mouse MHC-I knockdown adenovirus designed by GenePharma Co. (Shanghai, China). Its full length is provided in Additional Table 1. Immunofluorescence and western blot assay were used to measure MHC-I expression, to confirm the knockdown efficiency (Additional Figure 1). In total, 24 mice were randomly assigned to four groups (n = 6 per group). The mice were then treated with either the adenovirus or the negative control (NC). In the control group, there was no administration of adenovirus to the SN. In the Ad-NC group, both sides of the SN were injected with empty adenoviruses as the NC. In the MPTP + Ad-NC group, both sides of the SN were injected with empty adenoviruses, and after 7 days, the mice were treated with MPTP. In the MPTP + Ad-MHC-I shRNA group, the left side of the SN was stereotaxically injected with MHC-I short hairpin RNA (shRNA) adenoviruses and the right side was stereotaxically injected with empty adenoviruses, and after 7 days, the mice were treated with MPTP.

The sequence of adenovirus

Highlight: the Major histocompatibility complex class I (MHC-I) suppression sequence.

The stereotaxic injection process was as follows. First, the mice were anesthetized and fixed into a stereotaxic skull-frame (Stoelting Co., Wood Dale, IL, USA). They were then injected with either the adenovirus (the MHC-I knockdown adenovirus, designed by GenePharma Co.; 3 μL; 1 × 10¹⁰ infection forming units/μL per construct) or the empty adenovirus (NC) into the SN region (the stereotactic target was anteroposterior: –3.2 mm, mediolateral: 1.0 mm, and dorsoventral: –5.0 mm) (Burguillos et al., 2011). After 1 week, the mice were intraperitoneally injected with either MPTP or saline.