Human CD21low CD86pos B Cells Are Potent Antigen Presenting Cells, Which Are Expanded in Inflammatory Conditions

Accumulating evidence from animal models demonstrates that antigen presentation by B cells plays a crucial role during the natural immune response as well as in several important diseases. While resting B cells are poor antigen-presenting cells they can acquire strong immunostimulatory properties after activation via receptors such as CD40. Studies in animal models suggest that ex vivo generated CD40-actived B cells could serve a promising platform of cellular cancer immunotherapy. However, our understanding of the function of antigen presentation by B cells in human remains limited. This is partly due to the lack of well-defined cell surface markers that can be used to identify distinct immunostimulatory B cell subsets. We hypothesized that antigen-presenting B cells in humans would be characterized by a similar phenotype than in vitro generated CD40-activated B cells. Using a transcriptomic approach combined with flow cytometric phenotypic screening, we were able to show that CD40-activated human B cells can be distinguished from resting B cells by high expression of costimulatory receptors and low expression of BCR-associated coreceptors such as CD21 and FcγRIIB. Here, we report that CD21^low CD86^pos B cells represent a distinct B lymphocyte subpopulation with potent immunostimulatory capacity. This antigen-presenting B-cell subset possesses phenotypic and functional features of strong antigen-presenting cells, i.e. they express high levels of costimulatory molecules and inflammatory adhesion molecules and low levels of ITIM-bearing inhibitory receptors.  In vitro, purified human CD21^low CD86^pos B cells induced stronger T cell proliferation than the other three subsets, which are defined by the markers CD21 and CD86. Since Epstein Barr Virus (EBV) transforms human B cells with the help of the viral protein LMP1, which mimics CD40-mediated activation of B cells, we first studied patients with replicative EBV infection. We found that CD21^low CD86^pos B cells were expanded in severely immunosuppressed patients with EBV-reactivation. Analysis of CD21^low CD86^pos B lymphocytes in blood samples of healthy volunteers and patients with rheumatoid arthritis revealed that this subpopulation was increased in inflammatory conditions. One week following vaccination there was an increase in the frequency of this B-cell subset. Furthermore, compared to healthy controls CD21^low CD86^pos B cells were significantly elevated in the blood of patients with rheumatoid arthritis (RA). Interestingly, the CD21^low CD86^pos B-cell subset made up the majority of the B lymphocytes in synovial fluid of RA patients with inflammatory knee effusion. In summary, we demonstrate that CD21^low CD86^pos B cells are a novel antigen-presenting B-cell subset, which is expanded in inflammatory conditions. Our findings suggest that this subset might represent a promising therapeutic target for the treatment of inflammatory diseases, such as rheumatoid arthritis, were antigen presentation by B cells plays a pathogenetic role.