Scientific Proceedings. 



43 



come attenuated by such prolonged consecutive passage but readily 

 infect susceptible animals. 



We have shown, however, that cultures of Tr. brucei can be 

 attenuated by exposure for about two days at 34 0 C. By repeated 

 injections of cultures thus treated, attempts have been made to 

 immunize rats and guinea-pigs against Tr. brucei but thus far 

 these have been but partially successful. That is to say, there has 

 been at most a survival for a few days of the treated as compared 

 with the untreated animals. The failure to immunize with such 

 cultures is attributable in part to the excessive susceptibility, of the 

 animals employed, to infection with Tr. brucei, and in part to the 

 existence of a negative phase following the injections. It is desir- 

 able to repeat these experiments with less susceptible animals. 



In view of the fact that rats invariably recover, some soon, 

 others late, from infection with Tr. Lewisi, and the further fact that 

 rich cultures of this organism are readily obtainable, it is evident 

 that this species is well adapted for studies on immunity. Up to 

 the present time it has not been satisfactorily shown that trypano- 

 somes elaborate toxins or that they confer immunity by means of 

 soluble or intracellular products. The latter problem was ap- 

 proached by means of plasmolyzed cultures. To effect solution of 

 the trypanosomal cells the cultures were taken up in distilled water 

 and dialyzed in collodium sacs. Usually after one or two hours of 

 such dialysis in distilled water the trypanosomes completely dis- 

 appear and the intracellular matter apparently passes into solution. 

 By means of such cultures it has been shown that rats which 

 receive three or more injections on alternate days, on subsequent 

 inoculation with a minimal infective dose of fresh trypanosomal 

 blood from a rat, do not become infected, whereas controls are 

 positive. With such solutions it is possible to hyperimmunize 

 rats so that 0.5 c.c. of the immune rat blood protects against a 

 simultaneous and separate injection of the infective blood. 



Protection is seemingly obtained against Tr. Lewisi by simul- 

 taneous and separate injection of the infective blood and plasmo- 

 lyzed culture, followed 24 hours later by a second injection of the 

 latter. Repeated injections of too large a quantity of the plasmo- 

 lyzed culture and at too short an interval leads to a negative phase, 

 the presence of which is indicated by the unusually early appear- 

 ance of trypanosomes in the blood after inoculation with the virus. 



