B cells and beyond Regulation of human B cell differentiation and antibody formation

Differentiation of naive B cells into IgG- and IgE-secreting plasmablasts and plasma cells, collectively termed antibody-secreting cells (ASCs), depends on antigen internalization and processing and T follicular helper (T_FH) cell signals in secondary lymphoid organs. In this thesis we addressed various aspects involved in the generation of human T cell dependent antibody responses.
We showed that B cell receptor-activated NCK-PI3K-RAC1 pathway, but not the CD19 co-receptor complex, is required for actin-mediated internalization of large particles by B cells. In addition, we demonstrated that strong and repetitive CD40 co-stimulation in the presence of IL-21 is a minimal requirement for human naive B cell differentiation into ASCs. Furthermore, single-cell RNA sequence analysis revealed transiting B cell subsets and potential novel ASC differentiation factors. In addition, a potential bottleneck in the naive B cell priming phase was identified for the induction of the IgG4 antibody response. This was mediated by IL-4-secreting T_FH cells resulting in reduced chemokine receptor expression and improper GC reactions and inducing mainly short-lived plasma cells. Similar differentiation kinetics could be present for the formation of IgE-secreting plasma cells, as we observed a transient rise of tetanus-specific IgE antibodies, indicative of short-lived plasma cells, in hyperimmunized individuals boosted with tetanus toxoid. Finally, we revealed that CpG-stimulated B cells in vitro do not represent a bona fide regulatory B cell subset, as it co-produced IL-6 and TNFα and did not have a unique discriminative phenotype. These insights may be beneficial for immunomonitoring and therapies targeting B cell/antibody-mediated (auto-) immune diseases.