An AICDA-independent mechanism of secondary VH gene rearrangement in preimmune human B cells Running title: Secondary rearrangement in human B cells

摘要

VH replacement is a form of IgH chain receptor editing that is believed to be mediated by recombinase cleavage at cryptic recombination signals (cRSS) embedded in IGHV genes. Whereas there are several reports of IGHV replacement in primary and transformed human B cells and murine models, it remains unclear whether IGHV replacement contributes to the normal human B cell repertoire. We identified VH → VHDJH compound rearrangements from fetal liver, fetal bone marrow and naive peripheral blood, all of which involved invading and recipient IGHV4 genes that contain a cryptic heptamer, 13 base pair (bp) spacer and nonamer in the 5' portion of framework region (FR) 3. Surprisingly, all pseudohybrid joins lacked molecular processing associated with typical VHDJH recombination or nonhomologous end joining. Although inefficient compared to a canonical RSS, the IGHV4 cRSS was a significantly better substrate for in vitro RAG-mediated cleavage than the IGHV3 cRSS. It has been suggested that activation induced cytidine deamination (AICDA) may contribute to VH replacement. However, we found similar secondary rearrangements utilizing IGHV4 genes in AICDA-deficient human B cells. The data suggest that IGHV4 replacement in preimmune human B cells is mediated by an AICDA-independent mechanism resulting from inefficient but selective RAG activity.