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Mononuclear lymphocytes comprise a large portion of the lymphoreticular infiltrate of malignant tumors, which suggest that they may play a role in tumor development and progression. Previous studies provide evidence that neoplastic cells secrete factors into the tumor microenvironment that functionally polarize tumor-associated macrophages (TAMs) into either tumor-killing (M1) or tumor-promoting (M2) phenotypes. It has been suggested that macrophage clearance of apoptotic bodies fails to elicit secretions of pro-inflammatory mediators whereas clearance of necrotic cell debris induces a strong inflammatory response. Cancer therapeutic drugs induce cell death via apoptosis and necrosis; the former may play a vital role in the conversion of TAMs to the passive, tissue-healing phenotype (M2). The present study indicates that macrophage activation profiles change upon exposure to apoptotic and necrotic samples in human and murine models. U937 (histiocytic monocyte lymphoma) cell line was differentiated into macrophages using phorbol 12-myristate-13-acetate (PMA). Apoptosis and necrosis of Jurkat cells (T-lymphocyte acute leukemia) was induced through heat shock. Successful apoptosis was determined by examining phosphatidylinositol flip using flow cytometry. Macrophages were incubated with apoptotic bodies or necrotic cell debris from Jurkat cells for 48 hours, after which they were resuspended with E. coli expressing green fluorescent protein (GFP+ E. coli). The ability of the macrophage to engulf GFP+ E.coli was measured by flow cytometry. Unexposed macrophages showed an expected 60.7% increase in fluorescence, indicating augmented phagocytic activity. Macrophages exposed to apoptotic cells showed a 20.75% increase in fluorescence, whereas those exposed to necrotic cells induced a 43.48% increase in fluorescence. Moreover, macrophages demonstrated similar trends in engulfment when exposed solely to the media taken from treated macrophage samples. These results suggest that the presence of apoptotic bodies suppresses macrophage engulfment, polarizing macrophages towards tumor-friendly M2 type activity. Further research is ongoing using cytokine array analysis.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]