Molecular characterization of Babesia bovis merozoite surface proteins bearing epitopes immunodominant in protected cattle

Eight surface-radioiodinated merozoite proteins from a cloned, pathogenic isolate of Babesia bovis can be immunoprecipitated by antibody from cattle that are completely protected against clinical babesiosis. Among these eight surface proteins, the 55- and 42-kDa molecules are biosynthetically labeled with [3H]glucosamine. The 42-kDa glycoprotein can also be labeled with [3H]myristic acid and partitions exclusively into the detergent phase in Triton X-114 extracts, indicating that it is an integral membrane protein and suggesting that it is anchored by a glycosylphosphatidylinositol moiety. Antibody-mediated protection against B. bovis merozoites most probably requires a high level of circulating antibody to ensure antibody-merozoite binding during the parasite's brief extra-erythrocytic phase. Antibodies in diluted sera selectively recognize the 120-, 85-, 55- and 42-kDa surface proteins. Only the 42-kDa integral membrane protein is reactive with serum antibodies diluted greater than or equal to 1:16,000. Thus, we hypothesize that these immunodominant proteins, especially the transmembrane 42-kDa glycoprotein, are important to the induction of the protective immune response and are candidates for an improved vaccine against babesiosis.

A strain of Babesia divergens, attenuated after long term culture

The Weybridge strain of Babesia divergens became less virulent after 18 months in culture and was believed to be attenuated. Inoculations with the attenuated line using as many as 5 x 10(8) infected erythrocytes failed to raise a normal infection in Mongolian gerbils (Meriones unguiculatus) whereas, with the line that had been passaged from culture through gerbils at bimonthly intervals, inoculations of 10(7) infected erythrocytes gave rise to fatal infections. Gerbils were immunised with the attenuated line using doses ranging from 5 x 10(5) to 5 x 10(7) infected erythrocytes. Parasitaemias in the infected gerbils did not exceed 0.7 per cent after immunisation. All animals were protected when challenged with the virulent line four weeks later. Control animals died four days after challenge infection.

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 (KA) 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 KA designated Ka1 (Mr 47,500) and Ka2 (Mr 43,000) were observed. Ka1 was apparently lost following passage of KA B. 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 KV B. bovis (from which KA 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 (Mr 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 KA is a more heterogeneous population than KV 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 CA and CV B. bovis. This may explain the less stable avirulence of CA compared to KA B. 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.