Dendritic cells (DCs), as potent phagocytes engulf dead cells and present peptide
fragments of tumor antigens or pathogens derived from infected cells to naïve CD8+
T-lymphocytes. Dendritic cells can also induce apoptosis in target cells, thus getting
an opportunity to sample their microenvironment. Here, we present that the supernatants
of LPS- or CL075-activated DCs induced cell death in different cell lines, but during
the differentiation to mature DCs, they lost their cytotoxic potential. Dexamethasone-pre-treated
tolerogenic DCs induced less intensive death indicating that the tissue microenvironment
can downregulate DC-mediated killing. Exploring the signaling of DC-induced cell death,
we observed that the supernatant of activated DCs induced TNF-dependent cell death,
since TNF antagonist blocked the cytotoxic activity of DCs, contrary to inhibitors
of Fas and TRAIL receptors. We identified that the DC-induced killing is at least
partially a RIPK1-dependent process, as RIPK1 positive target cells were more susceptible
to DC-induced cell death than their RIPK1 deficient counterparts. Moreover, both the
elevated phosphorylation of RIPK1 and the increase in RIPK1-caspase-8 interaction
in target cells suggest that RIPK1-mediated signals contribute to DC supernatant-induced
cell death. We also proved that the cytotoxic activity of DC-derived supernatant induced
apoptosis in the target cells and not necroptosis, as it was completely abrogated
with the pan caspase inhibitor (Z-VAD), while the necroptosis inhibitor (Nec-1) had
no effect. Our work revealed that the supernatant of activated DCs induces the apoptosis
of target cells in a RIPK1-dependent manner. This phenomenon could be relevant for
the initiation of cross-presentation and may broaden the plethora of cytotoxic mechanisms
acting against tumor cells.
