Tatsuhiko Suwa

Tumor-infiltrating myeloid-derived suppressor cells are pleiotropic-inflamed monocytes/macrophages that bear M1- and M2-type characteristics

Here, tumor‐infiltrating CD11b+ myelomonocytoid cells in murine colon adenocarcinoma‐38 and GL261 murine glioma were phenotypically characterized. Over 90% were of the CD11b+F4/80+ monocyte/macrophage lineage. They also had a myeloid‐derived suppressor cell (MDSC) phenotype, as they suppressed the proliferation of activated splenic CD8+ T cells and had a CD11b+CD11c+Gr‐1lowIL‐4Rα+ phenotype. In addition, the cells expressed CX3CR1 and CCR2 simultaneously, which are the markers of an inflammatory monocyte. The MDSCs expressed CD206, CXCL10, IL‐1β, and TNF‐α mRNAs. They also simultaneously expressed CXCL10 and CD206 proteins, which are typical, classical (M1) and alternative (M2) macrophage activation markers, respectively. Peritoneal exudate cells (PECs) strongly expressed CD36, CD206, and TGF‐β mRNA, which is characteristic of deactivated monocytes. The MDSCs also secreted TGF‐β, and in vitro culture of MDSCs and PECs with anti‐TGF‐β antibody recovered their ability to secrete NO. However, as a result of secretion of proinflammatory cytokines, MDSCs could not be categorized into deactivated monocyte/macrophages. Thus, tumor‐infiltrating MDSCs bear pleiotropic characteristics of M1 and M2 monocytes/macrophages. Furthermore, CD206 expression by tumor‐infiltrating MDSCs appears to be regulated by an autocrine mechanism that involves TGF‐β.

Depletion of CD4+CD25+ regulatory T cells enhances interleukin-2-induced antitumor immunity in a mouse model of colon adenocarcinoma

Interleukin 2 (IL)‐2 induces antitumor immunity and clinical responses in melanoma and renal cell carcinoma. However, IL‐2 also increases the number of CD4+CD25+ regulatory T (Treg) cells that suppress antitumor immune responses. The aim of the present study was to elucidate the effect of depletion of Treg cells on IL‐2‐induced antitumor immunity. IL‐2‐transfected mouse colon adenocarcinoma (MC38/IL‐2) cells were implanted subcutaneously or intrahepatically into male C57BL/6 mice, and tumor growth and the proportion of tumor‐infiltrating lymphocytes with Treg‐cell depletion in response to treatment with anti‐CD25 monoclonal antibody (PC61) were determined. In mice treated with phosphate‐buffered saline, 40–60% of MC38/IL‐2 tumors were rejected. In contrast, all MC38/IL‐2 tumors were rejected in mice treated with PC61. The number of tumor‐infiltrating CD8+ T cells in mice treated with PC61 was approximately twice that in mice treated with PBS. The numbers of tumor‐infiltrating CD4+ and natural killer cells were also increased significantly. To test the antimetastatic effects of IL‐2 treatment in combination with Treg‐cell depletion, human recombinant IL‐2 (rIL‐2) and PC61 were administered to mice implanted with MC38/mock cells in the spleen, and hepatic metastasis was investigated. The average liver weight in mice treated with rIL‐2 plus PC61 was 1.04 ± 0.03 g, less than that in mice treated with rIL‐2 (2.04 ± 0.51 g) or PC61 alone (1.81 ± 0.38 g). We conclude that IL‐2‐induced antitumor immunity is enhanced by Treg‐cell depletion and is due to expansion of the tumor‐infiltrating cytotoxic CD8+ T‐cell population. (Cancer Sci 2007; 98: 416–423)

Skewing the Th cell phenotype toward Th1 alters the maturation of tumor-infiltrating mononuclear phagocytes

Mononuclear phagocytes (MPCs) at the tumor site can be divided into subclasses, including monocyte‐lineage myeloid‐derived suppressor cells (MDSCs) and the immunosuppressive tumor‐infiltrating macrophages (TIMs). Cancer growth coincides with the expansion of MDSCs found in the blood, secondary lymphoid organs, and tumor tissue. These MDSCs are thought to mature into macrophages and to promote tumor development by a combination of growth‐enhancing properties and suppression of local antitumor immunoresponses. As little is known about either subset of MPCs, we investigated MPCs infiltrating into murine adenocarcinoma MCA38 tumors. We found that these MPCs displayed immunosuppressive characteristics and a MDSC cell‐surface phenotype. Over 70% of the MPCs were mature (F4/80+Ly6C−) macrophages, and the rest were immature (F480+ Ly6C+) monocytes. MPC maturation was inhibited when the cells infiltrated a tumor variant expressing IL‐2 and soluble TNF type II receptor (sTNFRII). In addition, the IL‐2/sTNFRII MCA38 tumor microenvironment altered the MPC phenotype; these cells did not survive culturing in vitro as a result of Fas‐mediated apoptosis and negligible M‐CSFR expression. Furthermore, CD4+ tumor‐infiltrating lymphocytes (TILs) in wild‐type tumors robustly expressed IL‐13, IFN‐γ, and GM‐CSF, and CD4+ TILs in IL‐2/sTNFRII‐expressing tumors expressed little IL‐13. These data suggest that immunotherapy‐altered Th cell balance in the tumor microenvironment can affect the differentiation and maturation of MPCs in vivo. Furthermore, as neither the designation MDSC nor TIM can sufficiently describe the status of monocytes/macrophages in this tumor microenvironment, we believe these cells are best designated as MPCs.