CD169-positive macrophages dominate antitumor immunity by crosspresenting dead cell-associated antigens.

The generation of tumor-directed cytotoxic T lymphocytes is considered crucial for the induction of antitumor immunity. To activate these CD8(+) T cells, antigen-presenting cells (APCs) must initially acquire tumor cell-associated antigens. The major source of tumor antigens is dead tumor cells, but little is known about how APCs in draining lymph nodes acquire and crosspresent these antigens. Here we show that CD169(+) macrophages phagocytose dead tumor cells transported via lymphatic flow and subsequently crosspresent tumor antigens to CD8(+) T cells. Subcutaneous immunization with irradiated tumor cells protects mice from syngenic tumor. However, tumor antigen-specific CD8(+) T cell activation and subsequent antitumor immunity are severely impaired in mice depleted with CD169(+) macrophages. Neither migratory dendritic cells (DCs) nor lymph node-resident conventional DCs are essential for the crosspresentation of tumor antigens. Thus, we have identified CD169(+) macrophages as lymph node-resident APCs dominating early activation of tumor antigen-specific CD8(+) T cells.

xCT deficiency accelerates chemically induced tumorigenesis.

During the course of inflammation and its resolution, macrophages are exposed to various cytotoxic materials, including reactive oxygen species. Thus, macrophages require a protective machinery against oxidative stress to survive at the inflammatory site. Here, we showed that xCT, a component of transport system x(c)(-), was significantly up-regulated in activated infiltrating cells, including macrophages and neutrophils at the inflammatory site. System x(c)(-) mediates the uptake of extracellular L-cystine and is consequently responsible for maintenance of intracellular glutathione levels. We established a loss-of-function mouse mutant line of xCT by N-ethyl-N-nitrosourea mutagenesis. Macrophages from xCT(mu/mu) mice showed cell death in association with the excessive release of high mobility group box chromosomal protein 1 upon stimulation with LPS, suggesting that xCT deficiency causes unremitting inflammation because of the impaired survival of activated macrophages at the inflammatory site. Subcutaneous injection of 3-methylcholanthrene (3-MCA) induced the generation of fibrosarcoma in association with inflammation. When 3-MCA was injected s.c. into mice, xCT mRNA was up-regulated in situ. In xCT(mu/mu) mice, inflammatory cytokines (such as IL-1beta and TNFalpha) were overexpressed, and the generation of 3-MCA-induced fibrosarcoma was accelerated. These results clearly indicate that the defect of the protective system against oxidative stress impaired survival of activated macrophages and subsequently enhanced tumorigenecity.

Ascorbate-mediated iron release from ferritin in the presence of alloxan.

Release of iron from ferritin requires reduction of ferric to ferrous iron. The iron can participate in the diabetogenic action of alloxan. We investigated the ability of ascorbate to catalyze the release of iron from ferritin in the presence of alloxan. Incubation of ferritin with ascorbate alone elicited iron release (33 nmol/10 min) and the generation of ascorbate free radical, suggesting a direct role for ascorbate in iron reduction. Iron release by ascorbate significantly increased in the presence of alloxan, but alloxan alone was unable to release measurable amounts of iron from ferritin. Superoxide dismutase significantly inhibited ascorbate-mediated iron release in the presence of alloxan, whereas catalase did not. The amount of alloxan radical (A.(-)) generated in reaction systems containing both ascorbate and alloxan decreased significantly upon addition of ferritin, suggesting that A.(-) is directly involved in iron reduction. Although release of iron from ferritin and generation of A.(-) were also observed in reactions containing GSH and alloxan, the amount of iron released in these reactions was not totally dependent on the amount of A.(-) present, suggesting that other reductants in addition to A.(-) (such as dialuric acid) may be involved in iron release mediated by GSH and alloxan. These results suggest that A.(-) is the main reductant involved in ascorbate-mediated iron release from ferritin in the presence of alloxan and that both dialuric acid and A.(-) contribute to GSH/alloxan-mediated iron release.