Mechanisms mediating the effects of IL-3 gene expression on tumor growth.

IL-3 gene expression within tumors leads to host-cell infiltration, particularly by macrophages, slower tumor growth, and enhanced immunogenicity. Surprisingly, tumor-associated macrophages (TAMs) from within FSAN-JmIL3 tumors had decreased expression of TNF-alpha and iNOS. On short-term culture, TAMs from FSAN-JmIL3 tumors regained their capacity to produce TNF-alpha and NO, indicating that they were primed in vivo. In vitro experiments were unable to demonstrate differences between FSAN-JmIL3 and FSAN tumor cells in their ability to stimulate TNF-alpha production by TAMs. In the absence of evidence that TAM activation was responsible for the slower growth of FSAN-JmIL3 tumors, the response of tumor cells to these effector molecules was studied. TNF-alpha and NO were cytotoxic for FSAN-JmIL3 cells but growth stimulatory for FSAN. These tumor-related phenotypic changes may contribute as much if not more than functional changes in host infiltrating cells to the slower growth of FSAN-JmIL3 tumors in vivo.