Members of the Trypanosomatidae were studied for their ability to acquire host IgG through a possible Fc receptor. A simple rosette test was devised in which the different species and forms of protozoa were mixed with SRBC sensitized with subagglutinating does of IgG, IgM, and F (ab') 2 anti-SRBC, and the pelleted mixture was observed for the number of clumps (rosettes) formed between the parasites and SRBC. Rosettes were formed between parasites and SRBC sensitized with IgG but not with IgM or F(ab')2, indicating the presence of a receptor for IgG Fc. The specificity of this receptor for Fc was confirmed by inhibition experiments with normal rabbit aggregated gammaglobulins or with purified normal rabbit Fc. The receptor is sensitive to treatment with trypsin but regenerates after a short period of incubation (1 h), which indicates that it is synthesized by the parasite itself. Interesting was the observation that only pathogenic members of the Trypanosomatidae formed rosettes with sensitized SRBC. In none of the nonpathogenic forms studied could we demonstrate the Fc receptor. Also important was the finding that freshly isolated blood stream forms of Trypanosoma cruzi from infected mice did not form rosettes. However, after trypsinization, these forms clearly displayed the ability to do so, possibly indicating a previous acquisition of the host IgG by the parasites in the mouse blood stream. These findings point to a possible and important means of parasite evasion of the host immune response by masking their surface with host IgG.

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