Accessory cell activity of murine tumor-associated macrophages

The accessory cell activity of macrophages associated with the murine 3-methylcholanthrene-induced fibrosarcoma FSa was investigated. On the basis of Fc receptor expression and phagocytic activity, 20-25% of cells present within enzymatically disaggregated tumor cell suspensions could be classified as macrophages. These cells were approximately 50% I-Ak positive but did not express the Mac-1 antigen. T-cells played an important role in regulating I-Ak expression, and macrophages obtained from tumors grown in nu/nu mice were I-Ak negative. Tumor-associated macrophages were shown to possess potent accessory cell activity and were fully capable of reconstituting the primary anti-calf red blood cell plaque-forming cell (PFC) response of Sephadex G-10-passed spleen cells. This function required the presence of the I-Ak-positive subpopulation, and macrophages treated with anti-Ia serum and complement or obtained from tumors grown in nu/nu hosts lacked accessory cell activity. Tumor-associated macrophages were also able to provide the essential accessory cell function required for cooperation between tumor-specific TH cells and normal B-cells in the generation of an anti-trinitrophenyl (TNP) PFC response in the presence of TNP-coupled FSa antigen. These results suggest that progressive growth of the FSa tumor in vivo cannot be readily attributed to a defect in the accessory cell function of tumor-associated macrophages.