Primary responsibility for the induction of various acute phase reactions has been ascribed to interleukin 1 (IL-1), tumor necrosis factor (TNF), or IL-6, suggesting that these cytokines may have many overlapping activities. Thus, it is difficult to identify the cytokine primarily responsible for a particular biologic effect, since IL-1 and TNF stimulate one another, and both IL-1 and TNF stimulate IL-6. In this work, the contribution of IL-6 in radioprotection, induction of adrenocorticotropic hormone (ACTH), and induction of hypoglycemia was assessed by blocking IL-6 activity. Administration of anti-IL-6 antibody to otherwise untreated mice greatly enhanced the incidence of radiation-induced mortality, indicating that like IL-1 and TNF, IL-6 also contributes to innate resistance to radiation. Anti-IL-6 antibody given to IL-1-treated or TNF-treated mice reduced survival from lethal irradiation, demonstrating that IL-6 is also an important mediator of both IL-1- and TNF-induced hemopoietic recovery. A similar IL-1/IL-6 interaction was observed in the case of ACTH induction. Anti-IL-6 antibody blocked the IL-1-induced increase in plasma ACTH, whereas recombinant IL-6 by itself did not induce such an increase. Anti-IL-6 antibody also mitigated TNF-induced hypoglycemia, but did not reverse IL-1-induced hypoglycemia. It is, therefore, likely that TNF and IL-1 differ in their mode of induction of hypoglycemia. Our results suggest that an interaction of IL-6 with IL-1 and TNF is a prerequisite for protection from radiation lethality, and its interaction with IL-1 for induction of ACTH.

Interleukin-1 (IL-1) is radioprotective and induces both circulating colony-stimulating factor(s) (CSF) and IL-6 in mice. We evaluated the relationship among these three responses to IL-1 using anti-IL-1 receptor antibody 35F5. This antibody in vitro blocks responses of T cells and fibroblasts, but not of B cells or myeloid cell lines, to IL- 1. Administration of 35F5 alone before irradiation reduced the number of surviving mice compared with those not treated with 35F5, demonstrating that endogenous IL-1 participates in the natural resistance to radiation. Thirty micrograms of 35F5 per mouse also reduced by 92% the survival of irradiated mice pretreated with 0.3 micrograms of IL-1. Similarly, 30 micrograms of 35F5 reduced by 96% to 98% the induction of IL-6 by IL-1. In contrast, 30 micrograms of 35F5 resulted in only moderate reduction of circulating CSF. Consequently, the level of circulating CSF after 35F5 treatment was still equivalent to levels of CSF that were induced by doses of IL-1 in the radioprotective range. Because treatment with 35F5 antibody resulted in the blocking of IL-1-reduced radioprotection, the above results suggest that circulating CSF, by itself, may not be sufficient for radioprotection. This conclusion supports our previous results which showed that granulocyte-macrophage CSF (GM-CSF) and G-CSF were radioprotective only when administered with suboptimal doses of IL-1.

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