Babesia bovis: Proteins of virulent and avirulent parasites passaged through ticks and splenectomized or intact calves

Passage of the avirulent vaccine (K) strain of Babesia bovis (K_A) through either Boophilus microplus ticks, intact calves, or intact calves and then ticks, resulted in two distinct protein and protein antigen profiles as analyzed by two-dimensional gel electrophoresis of biosynthetically labeled proteins and immunoprecipitates. Different degrees of expression of two major acidic antigens of K_A designated Ka₁ (M_r 47,500) and Ka₂ (M_r 43,000) were observed. Ka₁ was apparently lost following passage of K_AB. bovis through intact calves but was strongly represented in the parasite population following a single tick passage. In contrast, passage through ticks of the virulent K_VB. bovis (from which K_A was derived by passage in splenectomized calves) did not lead to strong representation of the Ka₁ protein although there was increased representation of another major acidic protein antigen, designated Kv (M_r 35,000). These data suggest that the previously recognized reversion to a strain-dependent basal antigenic type in the tick vector depends also on intrastrain characteristics such as virulence and strain heterogeneity. The data suggest that K_A is a more heterogeneous population than K_V although cloned isolates are required to establish this point. Comparable syringe passage of another strain of B. bovis, designated C strain, through splenectomized calves resulted in less marked differences between the putative C_A and C_VB. bovis. This may explain the less stable avirulence of C_A compared to K_AB. bovis. Various selection pressures must act, in either the tick or the vertebrate host, on subpopulations in heterogeneous isolates to produce the changes described in protein antigen profiles of B. bovis. The possible relevance of changes in representation of proteins to biological characteristics of B. bovis (such as virulence and tick transmissibility) is discussed.