Splenocyte natural killer cell activity and metastatic potential are inversely dependent on estrous stage

We have previously shown that the timing of surgical removal of an estrogen-receptor-bearing mammary adenocarcinoma within the estrous cycle of the female C3HeB/FeJ mouse profoundly influences the frequency of subsequent tumor cell metastasis. In order to investigate the role of the immune response in this phenomenon, we measured splenic natural killer (NK) cell activity and interleukin-2 (IL-2) production in 80 female cycling mice, 16 to 18 weeks old, assigned to one of four estrous stages as determined by relative quantity of vaginal cellularity; proestrus, estrus, metestrus, and diestrus. After prolonged synchronization on 12-hours-on, 12-hours-off light-dark circadian schedules, daily vaginal smears were obtained for 2 weeks to characterize estrous cycling. On the day the animals were killed, vaginal smears were performed and single-cell suspensions were prepared from the harvested spleens. Direct cytotoxicity of spleen cells against the YAC tumor target was assessed immediately in a 3 1/2 hour 51Cr release assay and expressed as NK activity in lytic units (LU 20%). IL-2 production was determined in a bioassay with the IL-2-dependent CTLL-2 cell line. Significant differences in NK activity among estrous stages mimicking the variation found in frequency of surgical cure from mammary adenocarcinoma were observed (p = 0.035; one-way analysis of variance), with the time of lowest metastatic potential corresponding precisely with the time of highest splenocyte NK activity. These both occurred during the proestrus and estrus stages, characterized by high fertility, ovulation, and peak FSH, LH, and estrogen concentrations. In addition, NK activity was found to correlate significantly with IL-2 production (r = 0.4, p less than 0.0005). These results indicate that important components of the cellular immune response to cancer vary rhythmically with hormonal changes in the host and may represent one of the factors affecting the delicate balance between host and tumor that alters the frequency of postsurgical metastatic dissemination.