Induction of the activity of the histamine-forming enzyme, histidine decarboxylase, in mice by IL-18 and by IL-18 plus IL-12

Objective and Design: IL-18 shares functional properties with IL-12, which induces an elevation of histidine decarboxylase (HDC) activity in mouse tissues. Therefore, we examined the effects of IL-18 and IL-18+IL-12 on HDC activity.¶Methods: IL-18, IL-12 or IL-18+IL-12 was intraperitoneally injected into BALB/c and C3H/HeN mice and their HDC activities were measured.¶Results: IL-18, at effective antitumour doses, induced HDC activity in lung, liver, spleen and bone. The IL-18-induced HDC elevation was more rapid than that induced by IL-12. As with IL-12, repeated injections of IL-18 produced more HDC activity than a single injection. Repeated injections of IL-18+IL-12 induced serious illness and produced marked HDC activity. However, an inhibitor of HDC did not prevent the illness.¶Conclusions: IL-18 may be involved in the stimulation of histamine synthesis during the course of immune responses, though an elevation of HDC activity is not itself the direct cause of the serious illness induced by IL-18+IL-12.¶