Trypanosoma musculi co-express several receptors binding rodent IgM, IgE, and IgG subclasses

The present work demonstrates the expression of receptors for the Fc portion of rodent Ig by the murine parasite Trypanosoma musculi. By using a rosette assay adapted to the parasite morphology and by flow cytometry analysis, three distinct receptors were identified. A receptor binding rabbit or rat polyclonal IgG and mouse monoclonal IgG1, IgG2a, and IgG2b was found on parasites purified from the blood and the peritoneal cavity of infected mice and on parasites maintained in culture conditions. This IgG receptor was degraded by pepsin. A separate receptor, binding only mouse monoclonal IgG3 was observed on cultured parasites. A receptor binding rabbit, rat, and mouse IgM was found on cultured and peritoneal parasites, but not on blood parasites. This receptor did not bind IgG or IgA but it bound mouse and rat IgE as well as IgM. It was degraded by trypsin. IgG and IgM/IgE receptors were co-expressed on single parasites. They were not of host origin but synthesized by trypanosomes as shown by reexpression in vitro after proteolytic degradation. Their expression was variable with the development of trypanosomes both in vitro and in vivo.