Potent immunosuppression by secretory/excretory products of larvae from the sheep blowfly Lucilia cuprina.

Secretory/excretory products (sec/ex) of parasitic larvae of the sheep blowfly Lucilia cuprina potently inhibited proliferation of peripheral blood leucocytes stimulated by mitogens in vitro. Suppression of proliferation was not due to irreversible damage because cells cultured for 24 h in high concentrations of sec/ex appeared viable (assessed by Trypan blue exclusion) and did not show impaired proliferation after washing. Furthermore, suppression induced by sec/ex could be overcome by increasing concentrations of mitogen. The inhibitory activity could be demonstrated in cultures where sec/ex was added at different times during the culture period. Inhibitory activities in sec/ex were heat-labile and sensitive to treatment with trypsin. In addition to effects in vitro, sec/ex was strongly immunosuppressive in vivo. Sheep given combined injections of myoglobin and sec/ex had markedly lower anti-myoglobin antibody levels in sera than sheep that received injections of myoglobin alone. There was no significant antibody response to sec/ex itself. Immunosuppressive moieties in sec/ex produced by blowfly larvae may promote parasite survival by inhibiting the immune response of host sheep.