Role of natural killer cells in psychosocial stressor-induced changes in mouse mammary tumor growth.

We have shown that social housing conditions affect the growth rate of the androgen-responsive Shionogi mouse mammary tumor (SC115) and differentially stimulate splenic natural killer (NK) cell activity. Mice reared in a social group and then singly housed (GI) following tumor cell injection have increased tumor growth rates and increased NK cell activity, whereas mice reared individually and then group housed following tumor cell injection have decreased tumor growth rates and decreased NK cell activity compared to that in mice remaining in their rearing group. The present study was undertaken to determine whether NK cells are involved in mediating the effects of social housing conditions on SC115 tumor growth rates. We demonstrate that the presence of the SC115 tumor significantly stimulates the activity of NK cells at the tumor site in the first 7 days after tumor cell injection, and that, consistent with the data on splenic NK cells, mice of the GI group (largest tumors) have significantly greater levels of NK cell activity than mice reared individually and then group housed (smallest tumors). We further demonstrate that in mice of the GI group, in vivo stimulation of NK cell activity by polyinosinic:polyCMP correlates with a corresponding increase in tumor growth rate. These findings suggest that NK cell activity may play a role in mediating the increased tumor growth rate observed in mice of the GI housing condition.