Minute numbers of contaminant CD8 T cells or CD11b CD11c NK cells are the source of IFN- in IL-12/IL-18–stimulated mouse macrophage populations

Macrophages were reported to be strong producers of interferon (IFN) after stimulation by interleukin 12 (IL-12) plus IL-18, which gave rise to a novel concept of autocrine macrophage activation. Here, we show that peritoneal exudate and bone marrowderived mouse macrophages generated by conventional techniquescontainsmallquantities of CD11b CD11c CD31 DX5 NK1.1 natural killer (NK) cells or CD3 CD8 TCR T cells, respectively. Intracellular cytokine staining, purification of macrophages by sorting, and the analysis of macrophages from alymphoid RAG2 / -chain / mice revealed that the high amount of IFNprotein in the supernatants of unseparated IL-12/IL18–stimulated macrophage populations originates exclusively from the contaminating lymphoid cells. Notably, IL-12/IL-18 still induced IFNmRNA in highly purified macrophages from wild-type mice and in macrophages from RAG2 / -chain / mice, whereas nuclear translocation of signal transducer and activator of transcription 4 (STAT4) and production of IFNprotein were no longer detectable. These results question the concept of autocrine macrophage activation by secreted IFN, suggest differences in the expression of IFNmRNA and protein between macrophages and lymphoid cells, and illustrate that the limited purity of most myeloid cell populations (< 98%) might lead to false conclusions. (Blood. 2005;105:1319-1328)