Trypanosoma brucei: analysis of relapsing populations in sensitive and resistant breeds of cattle.

The clone DiTat 1.1 of Trypanosoma brucei brucei was injected into four bovids, and clones obtained from successive waves of parasitemia were used to study the expressed variant-specific surface glycoprotein repertoire. Twenty-four clones were obtained which could be classified into 12 different variable antigen types, in addition to the clone injected, using agglutination or immunofluorescence with monospecific antisera. The variable surface glycoproteins of the 25 clones were extracted using the detergent octyl-beta-D-glucopyranoside in the presence of the protease inhibitor, N-cbz-L-phenylalaninechloromethylketone. The molecular weights varied from 52,000 to 69,000 and the pI from 5.0 to 8.8. The virulence of 14 clones representing 13 variable antigen types was ascertained in mice. The mean survival time ranged from 20.5 to 43.0 days. Clones isolated from early peaks of parasitemia in the bovid were the most virulent while clones derived from later peaks were less virulent. It seems that organisms of diminishing virulence appear in bovids, leading to self-cure of the disease. All clones were sensitive to human serum in a blood infectivity inhibition test. Antibody against all virulent clones appeared in 20 cattle (10 Zebus, 10 Baoulés) which had been injected with T. brucei DiTat 1.1. There was no evidence for parasites of high or low virulence being preferentially expressed in resistant or sensitive hosts.

Alpha-helical coiled-coil structures of Trypanosoma brucei variable surface glycoproteins.

We have used electron microscopy to examine purified intact variable surface glycoproteins (VSGs) from clones derived from two distinct stocks of Trypanosoma brucei. The VSG molecule from MITat 1.2 has a large elongated domain consistent with the shape of the dimeric N-terminal domain determined by X-ray analysis (see preceding paper), and a heretofore unseen short, thin fibrous tail presumed to be the C-terminal domain. Electron microscopy on DiTat 1.3, however, indicates a morphology quite distinct from that of MITat 1.2. Analysis of four VSG amino acid sequences reveals 7-fold periodicities (heptad repeats) which indicate that alpha-helical coiled-coil secondary structure elements occur in all of these VSGs, consistent with the observation of helical bundles in one VSG. These results suggest the possibility that VSG antigenic diversity may be related to a diversity in length and disposition of alpha-helical bundles and coiled-coil domains.