COMPLEX EFFECTS OF LONG-TERM 50 HZ MAGNETIC-FIELD EXPOSURE IN-VIVO ONIMMUNE FUNCTIONS IN FEMALE SPRAGUE-DAWLEY RATS DEPEND ON DURATION OF EXPOSURE

In previous studies we have demonstrated that 50 Hz, 100 mu T magnetic field (MF) exposure of female Sprague-Dawley rats for 13 weeks signif icantly enhances the development and growth of mammary tumors in a bre ast cancer model. The present study was designed to test the hypothesi s that, at least in part, the tumor (co)promoting effect of MF exposur e is due to MF effects on the immune surveillance system, which is of critical importance in protecting an organism against the development and growth of tumors. For this purpose, female Sprague-Dawley rats of the same age as in the mammary tumor experiments were continuously exp osed for different periods (2, 4, 8, and 13 weeks) to a 50 Hz, 100 mu T MF. Control groups were sham-exposed simultaneously. Following the d ifferent exposure periods, splenic lymphocytes were cultured and the p roliferative responses to the T-cell-selective mitogen concanavalin A (Con A) and the B-cell-selective pokeweed mitogen (PWM) were determine d. Furthermore, the production of interleukin-1 (IL-1) was determined in the splenocyte cultures. The mitogenic responsiveness of T cells wa s markedly enhanced after 2 weeks of MF exposure, suggesting a co-mito genic action of MF. A significant, but less marked increase in T-cell mitogenesis was seen after 4 weeks of MF exposure, whereas no differen ce from sham controls was determined after 8 weeks, indicating adaptat ion or tolerance to this effect of MF exposure. Following 13 weeks of MF exposure, a significant decrease in the mitogenic responsiveness of lymphocytes to Con A was obtained. This triphasic alteration in T-cel l function (i.e., activation, tolerance, and suppression) during prolo nged MF exposure resembles alterations observed during chronic adminis tration of mild stressors, substantiating the hypothesis that cells re spond to MF in the same way as they do to other environmental stresses . In contrast to T cells, the mitogenic responsiveness of B cells and IL-1 production of PWM-stimulated cells were not altered during MF exp osure. The data demonstrate that MF in vivo exposure of female rats in duces complex effects on the mitogenic responsiveness of T cells, whic h may lead to impaired immune surveillance after long-term exposure. ( C) 1998 Wiley-Liss, Inc.