Cell culture CH12 cells were maintained in RPMI medium with 10% fetal bovine serum (FBS; Invitrogen), glutamine (Invitrogen), sodium pyruvate (Invitrogen), Hepes (made in-house) and antibiotic/antimycotic (Invitrogen). LentiX and PlatE packaging cells were maintained in DMEM medium with 10% FBS (Invitrogen) and Penicillin/Streptomycin (Invitrogen). Mice Mature, naïve B cells were isolated from spleens of 8-16 week old WT C57BL/6J mice, as per established protocols (61) . B cells were cultured in complete RPMI medium supplemented with 10% fetal bovine serum and antibiotics, Interleukin 4 (IL4; made in-house by the Molecular Biology Service, IMP) and 25 g/ml Lipopolysaccharide (Sigma). All animal experiments were carried out with a valid breeding license (No.: GZ: MA58-320337-2019-9) obtained from the Austrian Veterinary Authorities and in compliance with IMP-IMBA animal house regulations.
CH12 cell activation and class switch recombination (CSR) assay CH12 cells were activated with 5 ng/mL interleukin-4 (IL-4; home-made), anti-CD40 (home-made), and 1 ng/mL transforming growth factor β (TGF-β; R&D Systems), as described (43) . For CSR, cells were analyzed 48-72 h later by flow cytometry (BD Fortessa) following staining with anti-IgA coupled to phycoerythrin (PE) (Southern Biotech) (Table S2).
CH12 cell activation and class switch recombination (CSR) assay CH12 cells were activated with 5 ng/mL interleukin-4 (IL-4; home-made), anti-CD40 (home-made), and 1 ng/mL transforming growth factor β (TGF-β; R&D Systems), as described (43) . For CSR, cells were analyzed 48-72 h later by flow cytometry (BD Fortessa) following staining with anti-IgA coupled to phycoerythrin (PE) (Southern Biotech) (Table S2).