Human bone marrow Mesenchymal Stromal Cells (MSC) are potent modulators of T cell activation and proliferation, mainly through the production of partially defined soluble factors, including the IFN-induced tryptophan-degrading enzyme IDO, a key immunosuppressive effector pathway. Actually, MSC may affect the functions of virtually all immune effector cells, including B cells. However, current literature con- cerning MSC immunomodulatory activity on B cells is still controver- sial, due to both biological peculiarities of B cells, which do not produce IFN- , a key MSC-triggering cytokine, and to different and poorly com- parable experimental approaches. Human purified B cells, either resting or activated for 4 days with a stimulation cocktail (CD40 ligand + enhancer polyhistidine mAb MAB050+rhIL-2+mouse F(ab’)2 anti- human IgM/IgA/IgG+CpG oligodeoxynucleotide 2006) were co-cultured with MSC, either at resting conditions or following inflammatory priming (MSC pre-incubation with IFN- +TNF- for 48 hours), or with MSC supernatants. CD27-positive (memory) and CD27-negative (naïve) B cell survival, proliferation, and intracellular activation status (through signaling network analysis by Phosphoflow) were assessed. Our results showed that MSC are normally supportive cells, not intrinsically capable of suppressing B cell proliferation, and require inflammatory priming to acquire B cell inhibitory potential. Inflammatory-primed MSC impair significantly activated B cell growth in a cell contact-independent manner, as supernatant is sufficient to provide maximal inhibition of B cell proliferation. B cell inhibition by MSC is not related to either induction of B cell apoptosis or early signaling events necessary for B cell activation. In addition, IDO pathway triggered in IFN-primed MSC seems to have a role also in B cell inhibition by interfering with the tryptophan metabolism. Overall, B cell behavior following the interaction with MSC depends on the functional state of both B cells and MSC. The role of IDO in B cell regulation needs further investigation, as it may be relevant to develop new therapeutic approaches in pathological conditions related to B cell hyper-activation.
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