Absence of surface IgD does not impair naive B cell homeostasis or memory B cell formation in IGHD haploinsufficient humans

Surface IgD is coexpressed with IgM on naive mature B cells. Still, the role of surface IgD remains enigmatic even 50 y after its initial discovery. In this study, we examined the in vivo role of surface IgD in human B cell homeostasis and Ab responses in four individuals with heterozygous nonsense mutations in IGHD. All IGHD heterozygous individuals had normal numbers of B cells and serum Igs and did not show signs of immunodeficiency or immune dysregulation. IgD⁺ and IgD² naive mature B cells were present in equal numbers and showed similar immunophenotypes, except for decreased expression of CD79b in the IgD² subset. Furthermore, both IgD⁺ and IgD² naive mature B cells had normal replication histories and similar capacities to differentiate into plasma cells upon in vitro stimulation, and Ig class-switched memory B cells showed similar levels of somatic hypermutations. Thus, human B cells lacking IgD expression develop normally and generate immunological memory in vivo, suggesting that surface IgD might function more restrictedly in regulating of B cell activation to specific antigenic structures.