A gelatin sponge model for studying tumor growth: flow cytometric analysis and quantitation of leukocytes and tumor cells in the EMT6 mouse tumor.

This study examined the recruitment of host cells into a progressing EMT6 tumor following the inoculation of tumorigenic cells into a preimplanted gelatin sponge. Tumor cell proliferation occurred to a greater extent in sponge tumors than in tumors obtained by direct subcutaneous injection of tumor cells. Blank sponges, lacking tumorigenic cells and used as controls, elicited an inflammatory response characterized by a modest infiltration of granulocytes and macrophages whose numbers, after Day 7 postimplantation, remained unchanged for 21 days of sponge residence in the animal. In contrast, when the sponge contained tumor cells, there was a continuous increase in host cell infiltration that paralleled the increase in tumor cells. Whether in a sponge or not, tumor cells represented more than half of the recovered cells by Day 21 after tumor cell inoculation. Macrophages comprised the greatest fraction of host cells infiltrating the tumors. Flow cytometric analysis and morphological examination of disaggregated tumors indicated that macrophages (19-51%) made up the largest proportion of host cells followed in order by granulocytes (6-18%) and lymphocytes (2-9%). Sorting of marker-labeled cells revealed that 94% of the surface immunoglobulin-bearing cells were macrophages. Twenty-two % of the cells bearing the Thy 1.2 marker were lymphocytes, and 68% were macrophages. The data confirm the occurrence of a cellular host response in the tumor-bearing animal which is distinct from the foreign body reaction elicited by a blank sponge. Additionally, the sponge system described here represents a recoverable environment that would facilitate the study of in vivo host-tumor cell interactions that occur during early tumor development or later during therapy-induced tumor rejection when a palpable tumor is not present.