Pe anti mouse cd19

100 μL of peripheral blood sample was lysed by RBC Lysis Buffer, cells were collected and suspended in Cell Staining Buffer (BioLegend, Cat# 420201). Flow cytometry was performed performed on FACS Aria III (Becton Dickinson) using standard methods. For detection of surface antigens, cells were washed and stained with saturating amounts of antibodies conjugated with fluorescein in the presence of FcR-specific blocking antibody for 20 min on ice. Antibodies used were as follows: Alexa Fluor-488 anti-mouse Gr-1 (Biolegend, Cat# 108417, clone: RB6-8C5, 1:100), PE anti-mouse CD19 (Biolegend, Cat# 152408, clone: 1D3/CD19, 1:100), Alexa Fluor-647 anti-mouse CD3ε (Biolegend, Cat# 100322, clone: 145-2C11, 1:100), PE anti-mouse CD8a (Biolegend, Cat# 100708, clone: 53-6.7, 1:100), Alexa Fluor-488 anti-mouse CD4 (Biolegend, Cat# 100423, clone: GK1.5, 1:100), Alexa Fluor-647 anti-mouse c-Kit (Biolegend, Cat# 105818, clone: 2B8, 1:100), APC anti-mouse CD25 (Biolegend, Cat# 102012, clone: PC61, 1:100), PE/Cy5 anti-mouse/human CD44 (Biolegend, Cat# 103010, clone: PC61, 1:100), Alexa Fluor-647 anti-mouse/human Foxp3 (Biolegend, Cat#320013, clone: 150D, 1:100), PE anti-mouse/human CCR4 (Biolegend, Cat# 131203, clone: 2G12, 1:100). The data were analyzed by FlowJo_V10 software. The gating strategy are shown in Supplementary Fig. 6.

Spleen and mesenteric lymph nodes were isolated from naïve SPF mice and single-cell suspensions were prepared. After counting, 1 × 10⁶ single cells were resuspended in PBS and stain with Live/dead Zombie Aqua Fixable viability Dye (Cat#423101, Biolegend, USA) for 15 min at room temperature. After incubation, the cells were washed 500 µl R10 (RPMI1640 containing 10% fatal bovine serum) and then incubated with biotinylated S1 protein (Cat# 40591-V08H-B, Sino Biological, China) or biotinylated S2 protein (Cat# 40590-V08B-B, Sino Biological, China) for 30 minutes at 4°C. After incubation, the cells were washed twice with 500 µl R10. Then, the cells were incubated with the mixture of PE-anti-mouse CD19 (Cat# 152408, Biolegend, USA, 1 µl/test), BV785-anti-mouse CD45 (Cat# 103149, Biolegend, USA, 1 µl/test) and Streptavidin-IF647 (Cat# 46006, AAT Bioquest, USA, 0.2 µl/test) at 4°C for 30 minutes. After washing, the stained cells were resuspended in 200 µl 1× PBS and analyzed using a BD LSRFortessa™ Flow Cytometer. The data were analyzed using the FlowJo software (BD Biosciences, USA).