Characterization of human FCRL4-positive B cells

FCRL4 is an immunoregulatory receptor that belongs to the Fc receptor-like (FCRL) family. In healthy individuals, FCRL4 is specifically expressed by memory B cells (MBCs) localized in sub-epithelial regions of lymphoid tissues. Expansion of FCRL4(+) B cells has been observed in blood and other tissues in various infectious and autoimmune disorders. Currently, the mechanisms involved in pathological FCRL4(+) B cell generation are actively studied, but they remain elusive. As in vivo FCRL4(+) cells are difficult to access and to isolate, here we developed a culture system to generate in vitro FCRL4(+) B cells from purified MBCs upon stimulation with soluble CD40 ligand and/or CpG DNA to mimic T-cell dependent and/or T-cell independent activation, respectively. After 4 days of stimulation, FCRL4(+) B cells represented 17% of all generated cells. Transcriptomic and phenotypic analyses of in vitro generated FCRL4(+) cells demonstrated that they were closely related to FCRL4(+) tonsillar MBCs. They strongly expressed inhibitory receptor genes, as observed in exhausted FCRL4(+) MBCs from blood samples of HIV-infected individuals with high viremia. In agreement, cell cycle genes were significantly downregulated and the number of cell divisions was two-fold lower in in vitro generated FCRL4(+) than FCRL4(-) cells. Finally, due to their reduced proliferation and differentiation potential, FCRL4(+) cells were less prone to differentiate into plasma cells, differently from FCRL4(-) cells. Our in vitro model could be of major interest for studying the biology of normal and pathological FCRL4(+) cells.