Apoptotic pancreatic tumor cells are superior to cell lysates in promoting cross-priming of cytotoxic T cells and activate NK and gammadelta T cells.

Tumor vaccines using dendritic cells (DCs) have been shown to induce antitumor CTL responses. The choice of the tumor antigen preparation used for DC loading is still an unresolved issue. We compared DCs pulsed with cell lysates, whole apoptotic tumor cells or their supernatants of the HLA-A2(+) human pancreatic carcinoma cell line Panc-1 for their capacity to activate T cells. Monocyte-derived DCs from HLA-A2(+) donors were pulsed with tumor antigen, matured subsequently, and cocultured with autologeous peripheral blood mononuclear cells. After three weekly restimulations with DCs, T-cell activation was assessed by intracellular IFN-gamma staining and cytotoxicity assays. Compared with lysate, pulsing DCs with the supernatant of apoptotic tumor cells induced a higher frequency of activated CTLs and T-helper cells, as well as an enhanced MHC class I-restricted tumor cell lysis. No activation of natural killer (NK) or gammadelta T cells was detected. Pulsing DCs with whole apoptotic tumor cells induced an even more pronounced lytic effect. However, in this case, MHC class-I blocking was only partially effective, and unrelated cell lines were also killed. IFN-gamma staining revealed activation of CTLs and T-helper cells, as well as NK and gammadelta T cells. Trans-well cultures of NK cells, apoptotic tumor cells, and DCs showed that NK cell activation was dependent on direct cell-to-cell contact with tumor cells and the presence of interleukin-12 produced by DCs. These results indicate that the choice of antigen preparation is a critical determinant in the induction of antitumor immunity. Tumor vaccines consisting of DCs and apoptotic tumor cells may be able to activate CTLs, as well as effector cells of the innate immune system.

Interferon-alpha disables dendritic cell precursors: dendritic cells derived from interferon-alpha-treated monocytes are defective in maturation and T-cell stimulation.

Dendritic cells (DC) can be derived from monocytes in vitro by culture with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). It is unknown whether this regimen reflects DC differentiation from blood precursors under physiological conditions. Induction of DC development from monocytes by interferon-alpha (IFN-alpha) may occur in vivo during infection or inflammation and thus may represent a more physiological approach to DC differentiation in vitro. Here, we show that incubation of GM-CSF-cultured monocytes with IFN-alpha does not induce DC differentiation: cells maintain their original phenotype and cytokine secretion pattern. Even after stimulation with pro-inflammatory or T-cell-derived activation signals, IFN-alpha-treated monocytes do not develop DC characteristics. Addition of IL-4 during stimulation of IFN-alpha-treated monocytes results in the rapid development of DC-like cells expressing co-stimulatory molecules, CD83 and chemokine receptor CCR7, indicating that some degree of developmental plasticity is preserved. However, DC pre-activated with IFN-alpha are less effective in inducing allogeneic or antigen-specific autologous T-cell proliferation, produce less IL-12 and express lower levels of CCR7 compared to DC generated by culture with GM-CSF and IL-4. Incubating GM-CSF-cultured monocytes simultaneously with IFN-alpha and IL-4 does not affect phenotypic maturation of DC, but reduces IL-12 production upon pro-inflammatory activation. We conclude that: (1) IFN-alpha fails to induce DC differentiation and thus cannot replace IL-4 in generating DC from monocytes in vitro; and (2) the presence of IFN-alpha prior to or during differentiation of DC from monocyte precursors alters their response to maturation stimuli and may affect their capacity to stimulate T helper type 1 immune responses in vivo.