Analysis of antigen switching rates inTrypanosoma brucei

Abstract

SUMMARYPreviously quoted figures for the frequency of antigen switching inTrypanosoma bruceiare based on incorrect assumptions. In order to determine the correct switching frequency, an equation was derived that takes the growth rates of the newly expressed antigen types into consideration as well as the proportion of switched trypanosomes and the number of generations since the population was antigenically homogeneous. When this equation was applied to publishedin vitrodata, variable values were obtained for the switching frequency in clonal populations originally expressing one antigen type. The calculated most likely switching frequencies ranged from 1·4×10−7to 3·5×10−6. This variation was probably caused by differences in the growth rates of the new antigen types in the population and failure to detect slow growing variants. To overcome these problems, an experimental procedure was developed to analyse the switching frequencyin vitro. Trypanosomes were cloned and grown in parallel cultures. After an appropriate number of generations, cells expressing the original antigen type were destroyed and, from the proportion of cultures that contained new antigen types, the switching frequency was calculated. The technique minimized subculturing or other procedures that could distort the results. Although the method was optimized for analysing switching frequency, the values differed between experiments, ranging from 2·2×10−7to 2·6×10−6for one variant. Possible causes for the variations in switching frequency are discussed.