TNF and cancer: IFNγ is a cytokine that sensitizes many cancer cell lines, including HeLa cells, to TNF-induced apoptosis, partly via increased initiator caspase-8 expression35, 36, 37, 38, 39 and, in vivo, TNF and IFNγ often co-occur as IFNγ serves to activate macrophages which are a major source of TNF40.