Immunologic Analysis of Host Plasma Proteins on Bloodstream Forms of African Pathogenic Trypanosomes. II. Identification and Quantitation of Surface-Bound Albumin, Nonspecific IgG, and Complement on Trypanosoma congolense

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Abstract
The presence of host albumin, nonspecific Ig[immunoglobulin]G, and complement (C3 [3rd component]) bound to the surface of bloodstream forms of T. congolense was established. The quantitative indirect fluorescent antibody method showed that the same quantities of these plasma proteins were present on the surfaces of parasites collected from: normal rats by the 3rd postinfection day; normal rats at peak parasitemia; and cortisone-suppressed rats. The same situation was noted with trypanosomes collected from mice and incubated for 3-5 h in the presence of normal rat plasma. The presence of albumin, IgG, and C3 bound to the parasites was confirmed by the results of gel-diffusion reactions between the antisera to specific rat plasma proteins and a soluble extract of T. congolense. Rat IgM and IgE were not detected on the trypanosomes by either the quantitative indirect fluorescent antibody method or by gel diffusion. The localization of the host plasma proteins on the surface of the bloodstream forms was demonstrated by the use of trypsinized trypanosomes. No statistical difference was found between the fluorescence means of the trypsinized parasites treated with anti-rat plasma protein sera, and with normal rabbit serum. A quantitative immunoelectrophoretic method showed that rat albumin and IgG comprised 0.4% and 0.1%, respectively, of the total soluble proteins of T. congolense. Complement was consumed when bloodstream forms collected from mice or rats Were incubated in normal rat serum for 1 h at 37.degree. C. Complement was fixed when parasites were incubated in serum treated with ethyleneglycol-bis-(.beta.-amino ethyl ether)-N,N'' tetraacetic acid (EGTA), which indicated activation of the alternate pathway. Functions of the surface-bound plasma proteins on T. congolense and their possible implications for pathogenesis and chemotherapy are discussed.