The effect of actinomycin-D on the haemopoietic and immune response to tetanus toxoid.

A single dose of 15 μg (0·6 mg/kg) Actinomycin-D administered to female BDF mice after intraperitoneal injections of pertussis vaccine and tetanus toxoid caused: (1) a prolonged depression in thymic and splenic weights; (2) an inhibition of the bone marrow leucocytosis which normally occurs between the 2nd and 14th days after antigen injection; (3) a blood leucopenia and marked decrease in the number of mononuclear cells in the inflammatory exudate during both the primary response and a secondary response initiated 4 weeks later; and (4) an inhibition of primary immunization, as evidenced by low antitoxin titres 4 weeks after priming, and by the failure to achieve a normal secondary response to a challenging injection of antigen given 4 weeks later. It was suggested that the sensitivity of mononuclear cells to Actinomycin-D may be responsible for the suppression of priming, possibly by preventing RNA synthesis and differentiation induced by antigen.