Comparative immunological analysis of host plasma proteins bound to bloodstream forms of Trypanosoma brucei subspecies.

The presence, location, host specificity, identity, and quantity of rat plasma proteins bound to bloodstream forms of Trypanosoma brucei subsp. brucei, T. brucei subsp. rhodesiense, and T. brucei subsp. gambiense were determined by a quantitative indirect fluorescent-antibody method and gel immunoassays. Fluorescence differences between trypanosomes obtained from rats and mice and treated with antiserum to normal rat plasma indicated that most, if not all, of the bound plasma proteins were host specific. Removal of plasma proteins by trypsinization of parasites provided evidence for their attachment to the surface of the parasite. The accreted proteins were found to be host albumin, immunoglobulin G (IgG), and complement (C3). The same quantities of these three plasma proteins were present on T. brucei subspecies collected from normal rats at 2 days postinfection, during low or peak parasitemias, or from cortisone-treated rats. IgM could only be detected on parasites collected from normal rats at peak parasitemia. With the aid of rocket immunoelectrophoresis, host albumin and IgG were found to account for 0.2 and 0.05%, respectively, of the total soluble proteins of the bloodstream forms. It was concluded from this study that: (i) host plasma proteins were bound to parasites early in the infection, suggesting a mechanism of adaptation to the mammalian host; (ii) the surface-bound IgG was not the result of a specific immune response against the parasites but might be the cause of C3 attachment; (iii) among the bloodstream forms of the three T. brucei subspecies, there were no differences in amounts of surface-bound albumin, IgG, or C3. A comparison between the present data dealing with T. brucei subspecies, on the one hand, and the previously published results concerning T. congolense, on the other, revealed significant differences in the amounts of the host plasma proteins attached to these hemoflagellates.