Production of a macrophage chemotactic factor by cultured jaagsiekte tumour cells

The increase of alveolar macrophages in jaagsiekte sheep lungs is not caused by excessive surfactant production but is due to a chemotactic factor secreted by the tumor cells. This factor has a molecular mass in the region of 13 kilodaltons, is stable at 56 degrees C but labile at 100 degrees C and, being sensitive to proteases, indicates that it is a small protein molecule.