Myeloid suppressor cells regulate the lung environment--letter.

4T1 murine mammary carcinoma cells implanted in syngeneic Balb/c mice are increasingly being used in metastasis research, with some groups using this model to study tumor-induced accumulation of bone marrow-derived cells in metastatic target organs. Bone marrow-derived cells (including CD11b(+)Gr-1(+) myelomonocytic cells) are thought to modify the local lung microenvironment to facilitate subsequent colonization by metastatic tumor cells. While quantification of metastatic 4T1 tumor cells in various tissues can be done using ex vivo colony-forming assays, detection of metastatic 4T1 cells is often facilitated by expressing fluorescent proteins in the tumor cells prior to implantation. We found that Balb/c mice mount a potent immune response against 4T1 cells expressing green fluorescent protein (GFP) that includes the generation of anti-GFP antibodies in the circulation. Importantly, the number of bone marrow-derived CD11b(+)Gr-1(+) cells and metastatic tumor cells that accumulate in the lungs is significantly decreased in mice implanted with 4T1 cells expressing GFP compared with mice bearing wild-type 4T1 tumors. Taken together, our data caution against the use of GFP-expressing tumor cells in the Balb/c mouse strain, particularly for studying the influence of immunomodulatory cells on tumor cell metastasis.