Simultaneous Stimulation of Peripheral Blood Mononuclear Cells with CpG ODN2006 and α-IgM Antibodies Leads to Strong Immune Responses in Monocytes Independent of B Cell Activation

CpG ODN2006 is widely used both in vitro and in vivo to achieve B cell activation and has been previously applied in clinical trials as an adjuvant and anti-cancer agent. Recent studies have demonstrated the benefit of combining CpG ODN2006 with α-IgM antibodies to obtain optimal B cell activation in vitro. In this study, we expanded the knowledge of how both agents affect other types of peripheral blood mononuclear cells (PBMCs), thereby highlighting beneficial and potentially unfavorable properties of the combination of CpG ODN2006 and α-IgM when applied beyond isolated B cells. We elucidated the effects of both compounds on mixed PBMCs, as well as on B cell- and monocyte-depleted PBMCs, allowing us to distinguish between direct effects and indirect influences mediated by other interacting immune cells. Flow cytometry was used to measure the expression of surface markers and intracellular cytokines, while ELISA and multiplex assays were performed to determine cytokine secretion. Our results revealed that stimulation of mixed PBMCs with CpG ODN2006 and α-IgM strongly increased cytokine secretion, primarily originating from α-IgM-stimulated monocytes. Monocyte activation was confirmed by increased CD86 and HLA-DR expression and occurred independently of B cells. The high level of monocyte-derived cytokines after α-IgM exposure did not affect B cell activation. However, it represents a rather unfavorable property for clinical applications. In conclusion, α-IgM is a potent inducer of cytokine production in monocytes. Based on our findings we hypothesize that significant side effects on monocytes can occur when using α-IgM to enhance CpG ODN2006's efficacy on B cells, particularly in clinical settings.