Antibody-Independent, Natural Resistance of Birds to Trypanosoma cruzi Infection

Complement (C) activity present in normal human serum has been reported to lyse circulating forms of Trypanosoma cruzi following activation by specific host antibodies bound to the surface of the parasites. In view of this observation, we examined the possibility that a similar phenomenon may cause lysis of T. cruzi by avian complement, a mechanism postulated to be responsible for the natural resistance of birds to T. cruzi infection and previously described as being antibody independent. Trypomastigote forms of T. cruzi grown either in lethally irradiated mice or in cell cultures were lysed readily by the sera of agammaglobulinemic chickens. Lytic activity and titers of normal and agammaglobulinemic sera were comparable. The lytic reaction was inhibited by heat inactivation of the sera, or by addition of EDTA or cobra venom factor to the assay mixtures. Lysis of T. cruzi was observed when calcium, but not magnesium ions, were chelated with EGTA. Furthermore, a significant loss of lytic activity was observed when sera from C-depleted chickens were tested. Normal and agammaglobulinemic chickens cleared intravenously injected parasites (from either lethally irradiated mice or cell cultures) from their circulation in 7 min or less whereas C-depleted animals required 1,740 min or longer. Routine examination of the parasites from these two sources by immunofluorescence confirmed the absence of immunoglobulins on their surface. These results emphasize the lack of requirement of antibodies for, and the important role of complement in both the natural resistance that birds exhibit against T. cruzi infection and the lytic activity displayed by avian serum on virulent forms of T. cruzi.