Tissue harvest and cell isolation

At the indicated time points, all MLNs or PPs were harvested and ground between glass slides, followed by filtration with a 70-μm cell strainer. BM cells were flushed out from femurs and tibia of mice, followed by red blood cell lysis. Single-cell suspensions from PP were prepared by grinding PP between glass slides, followed by filtration with a 70-μm cell strainer. For AID-GFP fluorescence-activated cell sorting (FACS) and RV-ELISpot with SILP cells, PPs were first removed from the small intestine. The segmented small intestine was washed with CMF buffer (Hanks’ balanced salt solution, 2% fetal bovine serum, and 15 mM Hepes) and then vigorously shaken in CMF/EDTA buffer (5 mM EDTA), followed by digestion with freshly made enzyme mix composed of RPMI 1640 containing Collagenase IV (0.25 mg/ml; Sigma-Aldrich) and deoxyribonuclease I (0.025 mg/ml; Roche) in a 37°C water bath.