Microglia and macrophages as innate producers of interferon-gamma in the brain following infection with Toxoplasma gondii

We previously reported the requirement of interferon-gamma (IFN-γ) expression by cells other than T and natural killer (NK) cells in the brain, in addition to T cells, for prevention of toxoplasmic encephalitis following infection with Toxoplasma gondii. In the present study, we analysed the identity of the IFN-γ-producing non-T, non-NK cells in the brain using infected athymic nude and SCID mice that lack T cells but express IFN-γ in their brains. Intracellular staining for IFN-γ followed by flow cytometry revealed that approximately 45-60% of the cells expressing IFN-γ in their brains were positive for CD11b or F4/80 on their surfaces. Smaller portions of the cells were positive for pan-NK marker. Further smaller portions were positive for CD11c, and these cells were less than 5% of the IFN-γ-expressing cells in brains of infected SCID mice. In addition to IFN-γ proteins, large amounts of mRNA for IFN-γ were detected in CD11b⁺ cells purified from brains of infected mice, but it was not the case in the cells obtained from uninfected animals. In infected SCID mice depleted of NK cells by treatment with anti-asialo-GM1 antibody, cells expressing IFN-γ in their brains were all positive for CD11b, and the IFN-γ-producing cells were detected in both CD45^low and CD45^high populations. These results suggest that CD11b⁺ CD45^low microglia and CD11b⁺ CD45^high blood-derived macrophages are the major non-T, non-NK cells which express IFN-γ in the brain of mice infected with T. gondii.