The immune stimulatory protein 90K increases major histocompatibility complex class I expression in a human breast cancer cell line.

90K is a widely expressed, secreted 90 kDa human serum protein found both in normal individuals and at elevated levels in the serum of cancer patients. Functional characterization revealed stimulatory effects of 90K on immune defense systems, such as natural killer and lymphokine-activated killer cell activity. Recently, experiments have shown that 90K expression in several tumor cell lines inversely correlates with tumor formation in athymic mice. The mechanism of this tumor suppressive effect is unknown. In the present study, we evaluated the ability of 90K to affect the expression of MHC class I molecules in the human breast cancer cell line EVSA-T. Treatment with 90K (1-50 micrograms/ml) caused the levels of MHC class I expression to increase approximately sixfold above control levels, as measured by flow cytometry. IFN-gamma was used as a positive control and yielded increased expression of MHC class I molecules approximately 8 times over control levels. These data demonstrate that 90K can directly affect the expression of molecules that are engaged in protective antitumor response.