EndoU is a novel regulator of AICD during peripheral B cell selection

Poe, Jonathan C.; Kountikov, Evgueni I.; Lykken, Jacquelyn M.; Natarajan, Abirami; Marchuk, Douglas A.; Tedder, Thomas F.

EndoU was identified in a gene modifier screen as a unique and potent regulator of B cell AICD. In WT [B6] mice, EndoU transcription and protein expression were suppressed by the balance between tonic BCR signals and their modulation by CD22, permitting normal BCR-mediated responses without blatant AICD after BCR ligation. In contrast, heightened BCR signals in peripheral B cells of CD22 /[B6] mice drove EndoU overexpression and the CD5 high HSA high phenotype with predominant AICD after BCR ligation. Genetic deletion of EndoU expression reversed the CD5 high HSA high phenotype of B cells from CD22 /[B6] mice and normalized AICD, confirming its important regulatory function in these processes. Bona fide EndoU substrates are likely to include c-Myc and other regulatory transcripts in vivo because c-Myc EndoU / Ig Tg sHEL mice expressed relatively high surface IgM a levels and bound fluorescently labeled HEL protein at similar high levels ( Fig. 7 A) , suggesting that their BCRs were not occupied by sHEL. In contrast, B cells from both Ig Tg sHEL and augmented-responder EndoU / Ig Tg sHEL mice maintained an IgM a-low phenotype and did not bind labeled HEL protein, indicating BCR occupancy by sHEL. Dye-labeled spleen B cells from high-responder EndoU / Ig Tg sHEL mice were also adoptively transferred into WT or sHEL transgenic mice (Fig. 7 B) . There was a dramatic reduction in IgM levels and HEL-binding capacity for high-responder B cells recovered from sHEL recipients in comparison with WT recipients (Fig. 7, B and C) . Thus, the absence of AICD in high-responder EndoU / Ig Tg sHEL mice promoted auto-Ab production and endogenous sHEL clearance in vivo.