Residual Lymphocytes in GM-CSF and IL-15 Differentiated Monocyte-Derived Dendritic Cells Enables Cytotoxic T Lymphocyte Responses.

We investigated the capacity of IL-15 to differentiate human dendritic cells (DC) from monocytes in the presence of GM-CSF (IL-15 MoDC) and compared them with MoDC differentiated in IL-4 and GM-CSF (IL-4 MoDC) as used in many immunotherapy protocols. IL-15 MoDC expressed higher levels of CD40 and HLA-DR and importantly, induced MART-1 specific cytotoxic T lymphocyte (CTL) responses with superior lytic capacity, when compared to IL-4 MoDC. In response to activation, IL-15 MoDC secreted high levels of IFN-γbut low or no IL-12, whereas IL-4 MoDC secreted high IL-12 but low or no IFN-γ. Using an IFN-γ blocking antibody, we demonstrated that IFN-γ production by the IL-15 MoDC did not account for the superior CTL responses induced. Despite immunoselecting monocytes to greater than 97% purity prior to DC differentiation, we noticed a small population (1–2%) of CD56⁺ and CD3⁺ lymphocytes in the IL-15 MoDC preparations that were less prominent in IL-4 MoDC differentiated from the same monocytes. Removal of the residual lymphocytes from monocytes prior to differentiation into IL-15 MoDC diminished their capacity to induce CTL but did not affect the expression of HLA-DR or CD40. These data suggest that IL-15-dependent cross-talk between the small lymphoid populations present and DC, during DC differentiation from monocytes results in superior CTL priming that is independent of IL-12 and IFN-γ. Based on these results, appropriately manufactured IL-15 MoDC preparations containing defined numbers of lymphoid cells should be considered for immunotherapy.