SUMMARY: Column-separated, clean trypanosomes were subjected to nitrogen cavitation and ultracentrifugation (90 min, 99000 ); the supernatant contained most of the immunogens but a few remained in the well washed residue. Column chromatography yielded a strongly immunogenic protein fraction (SAF1), small doses (0·035 mg protein/dose × 4) of which protected mice against a maximum challenge of 5 × 10 of the homologous variant organisms for a minimum of 17 weeks. No heterologous (variant or strain) protection was obtained with SAF1 either with increased doses or by homologous challenge followed by heterologous challenge 3 weeks later. Disc electrophoresis of SAF1 showed four anodic components intermediate in mobility between α-macroglobulin and transferrin of normal rat serum. Analytical ultracentrifugation indicated a substantial proportion of 6·5S protein together with 3·0S and 1–1·5S proteins. A smaller soluble antigen fraction (SAF8) consisted of 2·0S protein but preliminary experiment has not shown it to be immunogenic. Infected rat plasma separated after 6 h was as effective as SAF1 in protecting mice against homologous challenge whereas rapidly separated infected rat plasma gave only slight protection. Two identical precipitinogens were detected in infected rat plasma and SAF1 by immunodiffusion. Three additional precipitinogens were present in SAF1 and one other in the infected rat plasma. Antisera raised against various trypanosome fractions, including SAF1, agglutinated the homologous trypanosomes but to a lesser extent than antisera to living trypanosomes.

Incubation of trypanosomes in either SAF1, or infected rat serum or plasma decreased their infectivity which was not altered by 8 h of incubation in normal rat plasma.


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