Objective: The overall goal of this project is to determine whether conditions of fever-range whole body hyperthermia (WBH) affect the number of macrophages present in tumors compared to that seen normothermic mice and whether a prolonged (6 hr) fever-like state can alter the phenotype or function of these macrophages. Tumors secrete chemoattractants which are thought to recruit macrophages. Some macrophages present in the tumors can be activated to become tumor associated macrophages (TAMs). Recent evidence suggests that these TAMs may actually promote rather than inhibit tumor growth; therefore it is important to determine precisely how WBH affects the number and type of macrophages present in the tumor.

Methods: To determine the number of tumor infiltrating macrophages after hyperthermia, Balb/c mice were injected with 1x10⁶ 4T1 or CT26 tumor cells and after 1-2 weeks, a 6 hour WBH (39.5 to 40°C) was administered. At various time points (3 hrs, 6hrs, 1day, 2 days, 3 days, and 4 days post-WBH) mice were sacrificed and tumors were harvested and embedded in paraffin. The paraffin embedded tumors were sectioned and phenotype analyzed by immunohistochemistry (IHC) using the macrophage marker F4/80.

Results: We observed a significant increase in the number of macrophages present in tumors following WBH compared to normothermic controls. These F4/80+ macrophages appear to be actively phagocytic. We are currently assessing the presence of TAMs within these tumors.

Conclusions: The number of infiltrating macrophages present in the tumor microenvironment increases significantly in response to WBH. These macrophages may contribute to the observed ability of WBH to inhibit tumor growth. Currently we are investigating whether blocking macrophage function will interfere with the effects of WBH on tumor growth. The outcome of those experiments will further our understanding of the effects of WBH on immune cell subsets, and improve our understanding of how to use hyperthermia to enhance anti-tumor immunity in cancer patients.