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BACKGROUND The functional significance of cytokines expressed in situ by tumor cells and tumor infiltrating lymphocytes (TIL) in human colon carcinomas is largely unknown. METHODS We assessed TIL expression of interleukin-2 (IL-2), IL-4, tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), granulocyte-macrophage colony stimulating factor (GM-CSF), IL-10, and transforming growth factor-beta (TGF-beta), and tumor cell expression of IL-10 and TGF-beta in situ in 49 primary colon carcinomas and 20 metastases using immunohistochemistry. RESULTS The percentage of primary colon carcinoma samples in which > 20% of TIL expressed each cytokine was as follows: IL-4: 47%; TNF-alpha: 22%; TGF-beta: 10%; IFN-gamma: 6%; IL-2:2%; IL-10: 0%; and GM-CSF: 0%. Lymphocytes more commonly infiltrated colon carcinoma primaries than metastases, and TIL expression of IL-4 and TNF-alpha was more common in primary than metastastatic carcinomas. Expression of TNF-alpha by even a small proportion (> or = 3%) of the TIL in a colon carcinoma specimen was associated with better overall survival (P = 0.01) when compared with patients with little or no TIL TNF-alpha expression (5-year survival 82% vs. 47%). Expression of IL-4 by > or = 20% of colon carcinoma TIL was also associated with improved survival (P = 0.01; 5-year survival 87% vs. 50%). The expression of IL-10 or TGF-beta by colon carcinoma TIL or colon tumor cells themselves was not associated with impaired survival. Benign epithelial cells stained positively for IL-10 and TGF-beta more frequently than tumor cells (P < 0.001). CONCLUSIONS There are differences between the immune microenvironment of primary tumors and metastases. Although IL-10 is expressed by colon carcinoma cells and TIL, it is unlikely that it plays an important immunosuppressive role. TNF-alpha and IL-4 are commonly expressed by colon carcinoma TIL and both are associated with improved survival.