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Immunosuppressive activity of the pyrrolizidine alkaloid metabolite dehydroheliotridine.

Immunosuppression was observed with dehydroheliotridine (DHH), a recently isolated and identified metabolite of certain pyrrolizidine alkaloids. The primary response to sheep red blood cells (SRBC) and to influenza virus vaccine (IVV) was significantly depressed. The secondary response to SRBC was suppressed only if the DHH was given at the time of the secondary stimulus. DHH is active when given before or with the antigen but not when given 6 or 24 hours after the antigen. There was some evidence that the withdrawal of blood for haemagglutination inhibition tests derepressed the immunological response of DHH treated mice.