Autoreactivity in naive human fetal B cells is associated with commensal bacteria recognition

Restricted V(D)J recombination during fetal development was postulated to limit antibody repertoire breadth and prevent autoimmunity. However, newborn serum contains abundant autoantibodies, suggesting that B cell tolerance during gestation is not yet fully established. To investigate this apparent paradox, we evaluated the reactivities of more than 450 antibodies cloned from single B cells from human fetal liver, bone marrow, and spleen. We found that incomplete B cell tolerance in early human fetal life favored the accumulation of polyreactive B cells that bound both apoptotic cells and commensal bacteria from healthy adults. Thus, the restricted fetal preimmune repertoire contains potentially beneficial self-reactive innate-like B cell specificities that may facilitate the removal of apoptotic cells during development and shape gut microbiota assembly after birth.