T cell-deficient mice such as nude mice are often used to generate tumor xenograft for the development of anticancer agents. However, the functionality of the other immune cells including macrophages, dendritic cells (DCs), and myeloid-derived suppressor cells (MDSCs) in the xenograft are largely unknown. Macrophages and dendritic cells (DCs) acquire functionally distinct properties in response to various environmental stimuli; the interaction of these cells with MDSCs in tumor microenvironments regulates cancer progression. Nude mice are less likely to reject human cancer cells because of major histocompatibility complex (MHC) mismatches. The tumor microenvironment in a xenograft, comprising human and mouse cells, exhibits more complex bidirectional signaling and function than that of allograft. Here, we evaluated the differences of myeloid cells between them. Plasma interferon-γ and interleukin-18 concentrations in the xenograft tumor model after lipopolysaccharide (LPS) administration were significantly higher than those in the allograft tumor model. MHC class I, II, and CD80 expression levels were increased in CD11b+ and MDSC populations after LPS administration in the spleen of a xenograft tumor model but not in that of an allograft tumor model. Additionally, the number of CD80- and mannose receptor C type 1 (MRC1)-expressing cells was decreased upon LPS administration in the tumor of the xenograft tumor. These results suggest that functions of macrophages and DCs are sustained in the xenograft, whereas their functions in response to LPS were suppressed in the allograft. The findings will encourage the consideration of the effects of myeloid cells in the xenograft for drug development.

Macrophages and dendritic cells (DCs) acquire functionally distinct properties in response to various environmental stimuli; the interaction of these cells with myeloid-derived suppressor cells (MDSCs) in tumor microenvironments regulates cancer progression. Immunodeficient mice lacking T cells are less likely to reject human cancer cells because of major histocompatibility complex (MHC) mismatches. The xenograft tumor microenvironment, comprising human cancer and mouse host cells, exhibits more complex bidirectional signaling and function than a syngeneic tumor microenvironment. Here human and mouse colorectal cancer cells were transplanted into nude mice to elucidate differences in macrophage, DC, and MDSC functions in human xenograft and mouse allograft tumor models. Plasma interferon-γ and interleukin-18 concentrations in the former model after intraperitoneal lipopolysaccharide (LPS) administration were significantly higher than those in the latter model and non-transplanted control group. Splenic MHC class I, II, and CD80 expression increased in CD11b+ and MDSC populations after LPS administration in only the xenograft tumor model. The number of CD80- and MRC1-expressing cells decreased upon LPS administration in only the xenograft tumor. These results suggxest that macrophages and DCs function normally in xenograft tumor models, whereas their functions in response to LPS administration vary in allograft tumor models.