Proteasome Inhibitors Affect the Function of Human Dendritic Cells and Induce Caspase-Mediated Apoptosis.

Proteasome inhibitors (PI) show antitumor activity against a broad spectrum of human malignancies both in vitro and in vivo. Yet, the consequences of exposure to these compounds on the immune system still have to be clearly determined. In the present study, we have investigated the effect of the proteasome inhibitors on human monocyte-derived dendritic cells (DCs). Exposure to PI results in a reduced activation induced DC maturation and cytokine production, inhibition of their migratory capacity and impaired ability to stimulate T-cell responses. These functional and phenotypic alterations were paralleled by a decreased phosphorylation of the MAP kinase member ERK1/2 while not affecting p38. Furthermore, incubation of DC with bortezomib inhibited the expression of nuclear localized transcription factors that were shown to be important for DC differentiation and function like IRF3, Rel-b and c-rel. Addition of PI to culture medium induced apoptosis of differentiated DCs and strongly reduced the yield of viable DCs when given to monocytes before differentiation to DCs was induced. DC apoptosis was accompanied by caspase activation as detected by caspase-3 and PARP cleavage. Cytochrome c cytosolic relocalization was detectable following exposure to bortezomib and was not prevented by caspase inhibition. This points to the mitochondrial damage as to an upstream event in DC apoptosis via proteasome inhibition. While not affecting Bcl-2 levels, bortezomib was found to promote Bax upregulation in DCs, thus providing a possible explanation for mitochondria dysfunction in response to this compound. In conclusion, this study shows that besides the inhibition of Nf-kB bortezomib is affecting several other pivotal signal transduction pathways in human cells and suggests that inhibition of DC function and induction of apoptosis in DCs may represent a mechanism by which bortezomib can affect the immune responses in patients treated with this compound.