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

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