Direct stimulation of macrophages by IL-12 and IL-18--a bridge too far?

A novel pathway of autocrine macrophage activation based on a positive feedback loop involving interleukin (IL)-12, IL-18 and IFN-gamma has recently been suggested. However, the macrophage isolation technique employed to describe the above phenomenon does not allow obtaining a pure population of macrophages casting some doubt to its existence. In the present study, we show that even minor contamination with lymphoid cells of a pure population of macrophage-like cells (Raw 264.7) results in a marked production of nitric oxide after stimulation with both IL-12 and IL-18. Neither macrophage-like cells nor lymphoid cells were capable of secreting high amounts of nitric oxide after stimulation with IL-12 and/or IL-18. Based on these observations we hypothesize that proposed autocrine feedback loop of macrophage activation is rather paracrine in nature and involves direct stimulation of residual lymphoid cells to secrete IFN-gamma that is then capable of activating macrophages.