Complex interactions between B cells and dendritic cells

To the editor:

In the last decade, we have gained increasing insights into the antibody-independent functions of B cells. It has now become clear that apart from producing antibodies, B cells also contribute to immunity by antigen presentation, cytokine production, and a wealth of cellular interactions with other components of the immune system. In their 2012 paper, Morva et al¹  report that human B cells that were activated via CD40 and Toll-like receptor 9 (TLR9) are able to regulate dendritic cells (DCs) in a cell contact–dependent fashion. In response to this article, Maddur et al²  furthermore show that the nature of the regulatory activity of B cells on DC function depends on the activation stimulus used to activate the B lymphocytes. These two papers provide important insights into the intricate mutual interactions between B lymphocytes and DCs.

The effect of CD40 on B-cell function has been controversial. While signaling through CD40 is generally believed to have pro-inflammatory effects on B lymphocytes, leading to enhanced antigen presentation and cytokine production, some groups have found that CD40-activation can induce regulatory features in B cells.^3-5  As exemplified by the results of Maddur et al,²  the type, strength, and duration of the activation stimulus has a crucial impact on the function of human B lymphocytes.^2,6  Similarly, we found striking functional differences between B cells that were stimulated by CD40L compared with B cells that were stimulated by CD40L and cytosine guanine dinucleotide (CpG). The CD80^lowCD86^lowHLA-DR^low DC population described by Morva et al¹  was increased in the cocultures with CD40L/CpG-stimulated B lymphocytes but not with CD40L-stimulated B lymphocytes (Figure 1A). Collectively, the experiments suggest that TLR9-mediated signals seem to induce a regulatory capacity in B cells and counter the pro-inflammatory effects of CD40-stimulation on B cells.

Importantly, even though we could confirm the findings by Morva et al¹  and Maddur et al,²  there is one major caveat in the interpretation of these data. As far as one can judge from the descriptions of the methods, both research groups cocultured DCs with B cells and subsequently performed a mixed lymphocyte reaction without prior purification of the DCs. Since signaling through CD40 has anti-apoptotic effects, it could favor the survival of the CD40-activated B cells in the cocultures. Using an identical experimental setup, we found significant differences in the composition of the cell mixture after the end of the 48-hour coculture period. B cells activated by CD40L or CD40L and CpG seemed to survive longer, and they made up a higher percentage of the cells after coculture (Figure 1B). Thus, the DC/B-cell ratio in the cell mixture was 2:1 and 7:1 in the cocultures with CD40L/CpG-stimulated and unstimulated B cells, respectively. As a consequence, the mixed lymphocyte reactions with DCs exposed to CD40L/CpG-activated B cells contained a relatively higher number of B cells and a lower number of DCs. It should therefore be taken into consideration that the observed effects on T-cell activation and proliferation could be, at least in part, attributable to the “contaminating” B cells.