A Culture-Adapted Strain of Babesia bovis Has Reduced Subpopulation Complexity and Is Unable to Complete Its Natural Life Cycle in Ticks.

Heba F. Alzan,Massaro W. Ueti,Glen A. Scoles,Reginaldo G. Bastos,Carlos E. Suarez,Wendell C. Johnson,Jacob M. Laughery

doi:10.3389/fcimb.2022.827347

Abstract

Babesia bovis natural field strains are composed of several geno-phenotypically distinct subpopulations. This feature, together with possible epigenetic modifications, may facilitate adaptation to variable environmental conditions. In this study we compare geno-phenotypical features among long-term (more than 12 years) (LTCP) and short-term cultured B. bovis parasites (STCP) derived from the B. bovis S74-T3Bo strain. LTCPs intraerythrocytic forms are smaller in size than STCPs and have faster in vitro growth rate. In contrast to its parental strain, the LTCP lack expression of the sexual stage specific 6cysA and 6cysB proteins and are unable to develop sexual forms upon in vitro sexual stage induction. Consistently, in contrast to its parental strain, LTCPs have reduced virulence and are not transmissible to cattle by vector competent Rhipicephalus microplus (R. microplus). Similar to previous comparisons among attenuated and virulent B. bovis strains, the LTCP line has decreased genomic diversity compared to the STCP line. Thus, LTCP may contribute to our understanding of adaptive mechanisms used by the parasites in response to environmental changes, protective immunity, virulence, and transmission by ticks. In addition, LTCPs may be considered as candidates for a non-tick transmissible vaccine against bovine babesiosis.

Highlights

The tick borne apicomplexan Babesia bovis (B. bovis) is responsible for acute and persistent disease among cattle in tropical and subtropical areas and improved control is urgently needed
Under the conditions used in this experiment the Long Term Culture Parasites” (LTCPs) line had a significantly distinct, faster growth rate, in in vitro cultures, compared to the shortterm cultured B. bovis parasites (STCP) line, suggesting an increased fitness in terms of parasite expansion
The findings in this study suggest the occurrence of effective selection and adaptive mechanisms operating during the process of in vitro culturing of B. bovis, resulting in important phenotypic and genotypic changes among the LTCP and parental parasites, as represented by the STCP

Summary

Introduction

The tick borne apicomplexan Babesia bovis (B. bovis) is responsible for acute and persistent disease among cattle in tropical and subtropical areas and improved control is urgently needed. Some of the parasite adaptive responses to selective pressures encountered during their life cycle use strategies based on genetic, phenotypic, antigenic, and strain population diversity Such strategies are reflected in the ability of individual B. bovis parasites to express VES1/2 antigenic variants in order to escape the immune system pressure of the vertebrate host (Allred et al, 2000; Al-Khedery and Allred, 2006). Diversity is displayed as the existence of parasite strains containing a mix of genetically and phenotypically distinct subpopulations (Timms et al, 1990; Mazuz et al, 2012; Combrink et al, 2015; Mans et al, 2019) Likewise, this diverse strain composition may facilitate the selection of more apt parasite subpopulations as the parasites encounter distinct environments such as those in tick and vertebrate hosts, or in vitro cultures. Better understanding of B. bovis strain composition and the ability of the parasite to adapt to changing environments are key to design improved methods of control

Methods

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Conclusion