The CD21^low B Cell Population Is Composed of an Anergic and an Immunostimulatory Germinal Center-like B Cell Subset

In recent years we have gained an increased understanding of the complexity of B cell biology and function. It has become increasingly recognized that apart from antibody production B cells exert many more function. B cells serve as antigen-presenting cells (APC), they contribute to immunoregulation and represent an important source of cytokines and chemokines. A deeper understanding of the role of B cells in the pathophysiology of human diseases has been hampered by the lack of well-defined functional B cell subsets. We therefore aimed to identify novel human B cell subsets which could serve as biomarkers or targets of therapeutic intervention. Using a transcriptomic approach combined with flowcytometric immune assessment of healthy human subjects and patients we were able to identify several functional B cell subpopulations with relevance to human disease. We were able to define a CD21^low CD86^pos human B cell subset with strong antigen-presenting capacity which gradually develops from conventional resting B cells under the continuous stimulation via CD40. These cells were phenotypically and functionally distinct from CD21^low CD86^neg B lymphocytes, which represent anergic B cells. Using calcium flux assays and phospho-specific flow cytometry we were able to show that the CD21^low B cell subsets displayed distinct signaling states. Both CD21^low B cell subpopulations had an impaired response to B cell receptor stimulation. However, CD21^low CD86^pos B cells had higher basal calcium levels and basal phosphorylation of BCR-associated signaling molecules such as Syk and Erk. Contrary to CD21^low CD86^neg B cells, which demonstrated poor antigen-presenting capacity, CD21^low CD86^pos B cells were potent immunostimulatory antigen-presenting cells. CD21^low CD86^pos B cells were increased in acute inflammation and autoimmune diseases such as rheumatoid arthritis. CD21^low CD86^neg B cells, on the other hand, were increased in chronic inflammatory conditions such as chronic HIV infection. The balance between the CD21^low B cell subsets varied with the functional state of the B cell compartment in inflammatory conditions and could be used to classify the functional state of the B cell compartment. In summary, we have identified several novel human B cell subsets with distinct functions. Given the large number of B cell-directed drugs which are in clinical development or already approved it seems likely that an increased knowledge of the human B cell subsets will not only provide important insights into the pathology of immune-mediated diseases but will also result in novel therapeutic strategies.