Effect of intraperitoneally administered recombinant murine granulocyte-macrophage colony-stimulating factor (rmGM CSF) on the cytotoxic potential ofmurine peritoneal cells

We studied the effect of recombinant murine granulocyte-macrophage colony-s timulating factor (rmGM-CSF) on the cytotoxic potential of murine peritonea l cells. Mice received rmGM-CSF intraperitoneally using different dosages a nd injection schemes. At different time points after the last injection, mi ce were sacrificed, peritoneal cells isolated and their tumour cytotoxicity was determined by a cytotoxicity assay using syngeneic [methyl-H-3]thymidi ne-labelled colon carcinoma cells. Also, the cytotoxic response to a subseq uent in vitro stimulation with lipopolysaccharide was determined. Upon dail y injection of 6000-54 000 U rmGM-CSF over a B-day period, the number of pe ritoneal cells increased over ten fold with the highest rmGM-CSF dose. Incr eases in cell numbers was mainly due to increases in macrophage numbers. Up on injection of three doses of 3000 U rmGM-CSF per day for 3 consecutive da ys, the number of macrophages remained elevated for minimally 6 days. Altho ugh the peritoneal cells from rmGM-CSF-treated mice were not activated to a tumoricidal state, they could be activated to high levels of cytotoxicity with an additional in vitro stimulation of lipopolysaccharide. Resident cel ls isolated from control mice could be activated only to low levels of tumo ur cytotoxicity with lipopolysaccharide. Tumour cytotoxicity strongly corre lated with nitric oxide secretion. When inhibiting nitric oxide synthase, t umour cell lysis decreased. Thus, the expanded peritoneal cell population i nduced by multiple injections of rmGM-CSF has a strong tumour cytotoxic pot ential and might provide a favourable condition for immunotherapeutic treat ment of peritoneal neoplasms.