Structural and functional characterization of Bc28.1, major erythrocyte-binding protein from Babesia canis merozoite surface

Animal models of the immunology and pathogenesis of human babesiosis

Animal models of human babesiosis have provided a basic understanding of the immunological mechanisms that clear, or occasionally exacerbate, Babesia infection and those pathological processes that cause disease complications. Human Babesia infection can cause asymptomatic infection, mild to moderate disease, or severe disease resulting in organ dysfunction and death. More than 100 Babesia species infect a wide array of wild and domestic animals, and many of the immunologic and pathologic responses to Babesia infection are similar in animals and humans. In this review, we summarize the knowledge gained from animal studies, their limitations, and how animal models or alternative approaches can be further leveraged to improve our understanding of human babesiosis.

Genetic Diversity of Babesia canis Strains in Dogs in Lithuania

Canine babesiosis is an emerging and rapidly expanding tick-borne disease in central and northeast Europe. In the last two decades, the endemic area of Babesia canis has expanded from central Europe to the Baltic region. This study aimed to investigate the genetic diversity of B. canis strains isolated from naturally infected dogs in different regions of Lithuania using PCR-RFLP and sequence analyses based on a partial region of 18S rRNA and Bc28.1 genes. Blood samples from 149 dogs suspected of having babesiosis were collected in Lithuania during 2016–2017. Based on PCR-RFLP profiles and two nucleotide substitutions observed in 18S rRNA gene sequences, three B. canis genotypes were identified in Lithuania—18S rRNA-A, 18S rRNA-B and 18S rRNA-A/B—with the A/B genotype predominating (83.9%). Based on the obtained PCR-RFLP profiles of the Bc28.1 gene, four B. canis genotypes were identified: Bc28.1-B (53.8%), Bc28.1-34 (20.8%), Bc28.1-A (17.9%), and Bc28.1-34/A or B (7.5%). Sequence analysis of the partial Bc28.1 gene revealed eighteen polymorphic sites and thirteen sequence variants among the Lithuanian samples. The B. canis genotypes obtained were detected with varying prevalences in different regions of Lithuania.

Comparative genomic analysis of Babesia duncani responsible for human babesiosis

Background

Human babesiosis, caused by parasites of the genus Babesia, is an emerging and re-emerging tick-borne disease that is mainly transmitted by tick bites and infected blood transfusion. Babesia duncani has caused majority of human babesiosis in Canada; however, limited data are available to correlate its genomic information and biological features.

Results

We generated a B. duncani reference genome using Oxford Nanopore Technology (ONT) and Illumina sequencing technology and uncovered its biological features and phylogenetic relationship with other Apicomplexa parasites. Phylogenetic analyses revealed that B. duncani form a clade distinct from B. microti, Babesia spp. infective to bovine and ovine species, and Theileria spp. infective to bovines. We identified the largest species-specific gene family that could be applied as diagnostic markers for this pathogen. In addition, two gene families show signals of significant expansion and several genes that present signatures of positive selection in B. duncani, suggesting their possible roles in the capability of this parasite to infect humans or tick vectors.

Conclusions

Using ONT sequencing and Illumina sequencing technologies, we provide the first B. duncani reference genome and confirm that B. duncani forms a phylogenetically distinct clade from other Piroplasm parasites. Comparative genomic analyses show that two gene families are significantly expanded in B. duncani and may play important roles in host cell invasion and virulence of B. duncani. Our study provides basic information for further exploring B. duncani features, such as host-parasite and tick-parasite interactions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01361-9.

High genetic diversity of Babesia canis (Piana & Galli-Valerio, 1895) in a recent local outbreak in Berlin/ Brandenburg, Germany

Canine babesiosis caused by Babesia canis (Piana & Galli-Valerio, 1895) is emerging in new regions in Europe since its vector Dermacentor reticulatus (Fabricius, 1794) is expanding its geographic range. In the Berlin/Brandenburg area in northeast Germany, D. reticulatus is highly abundant but in the past only one autochthonous B. canis infection was reported. Since 2015, autochthonous cases were occasionally diagnosed but numbers increased since autumn 2019. The aim of the study was to genotype autochthonous canine Babesia spp. infections from Berlin/Brandenburg. Between 04/2015 and 01/2022, 46 dogs with acute babesiosis were presented to the small animal clinic (one dog was infected twice resulting in 47 samples). There were 32 dogs that had never left Berlin/Brandenburg and 14 others that had not left the region in the 6 weeks prior to disease onset. PCRs targeting the 18S rRNA and the Bc28.1 merozoite surface antigen were positive in 47 and 42 samples, respectively. Sequencing of cloned PCR products identified all samples as B. canis with 17 18S rRNA and 12 Bc28.1 haplotypes. Based on network analysis for 18S rRNA sequences and a previously described polymorphic dinucleotide, samples were assigned to two distinct clusters. One contained 31 and the other 16 samples. Using network analysis, the Bc28.1 haplotypes could also be separated into two clusters differing by at least five polymorphisms. Analyses of sequences from multiple clones indicated the presence of up to five 18S rRNA and eight Bc28.1 haplotypes and thus high parasite variability in an individual host. The genetic diversity could suggest that the parasites in the region have multiple origins, but diversity in individual dogs and dog populations from endemic regions is unknown. The suitability of both markers for genotyping is questionable due to potential intragenomic diversity for the rRNA and high intergenomic variability for the Bc28.1 marker.

Major Surface Antigens in Zoonotic Babesia

Human babesiosis results from a combination of tick tropism for humans, susceptibility of a host to sustain Babesia development, and contact with infected ticks. Climate modifications and increasing diagnostics have led to an expanded number of Babesia species responsible for human babesiosis, although, to date, most cases have been attributed to B. microti and B. divergens. These two species have been extensively studied, and in this review, we mostly focus on the antigens involved in host–parasite interactions. We present features of the major antigens, so-called Bd37 in B. divergens and BmSA1/GPI12 in B. microti, and highlight the roles of these antigens in both host cell invasion and immune response. A comparison of these antigens with the major antigens found in some other Apicomplexa species emphasizes the importance of glycosylphosphatidylinositol-anchored proteins in host–parasite relationships. GPI-anchor cleavage, which is a property of such antigens, leads to soluble and membrane-bound forms of these proteins, with potentially differential recognition by the host immune system. This mechanism is discussed as the structural basis for the protein-embedded immune escape mechanism. In conclusion, the potential consequences of such a mechanism on the management of both human and animal babesiosis is examined.

Immune Response of Mice Against Babesia canis Antigens is Enhanced When Antigen is Coupled to Gold Nanoparticles

PURPOSE

The aim of this study was to isolate Babesia canis soluble antigens and to investigate the effect of their conjugates with gold nanoparticles on the immunogenicity in laboratory animals.

METHODS

A procedure was developed for isolating and purifying B. canis antigens. The isolated culture antigen of B. canis 495 was coupled to gold nanoparticles, and the conjugate was used to immunize laboratory mice.

RESULTS

Western blotting showed that the resultant antiserum specifically recognized the proteins of the B. canis strains isolated from naturally infected dogs. The antibody titer, the respiratory activity of peritoneal macrophages, the proliferative activity of splenocytes, and the production of cytokines were maximal when the animals were immunized with the antigen-nanoparticle conjugate emulsified in complete Freund's adjuvant. Without adjuvant, the babesial antigen was weakly immunogenic.

CONCLUSION

Therefore, the use of gold nanoparticles as an antigen carrier induced a broad immune response involving both cellular and humoral responses. The antibodies raised by the proposed procedure are potentially effective at immunodetection of Babesia canis infections in dogs.

Development of a real-time PCR method for rapid diagnosis of canine babesiosis and anaplasmosis

Background

Canine babesiosis and anaplasmosis, caused by Babesia canis and Anaplasma phagocytophilum, respectively, are significant tick-borne diseases in Baltic countries. Both diseases can be diagnosed on the basis of clinicopathological findings, by direct pathogen detection in blood smears or by indirect pathogen detection; however, because of high selectivity and specificity, molecular methods may be advantageous. The goal of this study was to develop a duplex real-time polymerase chain reaction (RT-PCR) method for the detection of B. canis and A. phagocytophilum in canine clinical samples.

Methods

Sequence-based polymorphism analysis of genes encoding B. canis-specific merozoite surface protein Bc28.1 (Bc28.1) and A. phagocytophilum malate dehydrogenase (mdh) was performed on pathogen isolates present in Latvian domestic dogs. The obtained results were used to design a species-specific duplex RT-PCR assay.

Results

The presence of three B. canis Bc28.1 gene sequence types was revealed in canine samples with a nonuniform geographical distribution, and two types of A. phagocytophilum mdh genes were detected. The novel duplex RT-PCR assay provided correct classification of samples positive and negative for B. canis and A. phagocytophilum. The analytical sensitivity of this assay was ten gene copies/ reaction for both pathogens.

Conclusions

A novel duplex RT-PCR molecular method was developed for the detection of B. canis and A. phagocytophilum in canine clinical samples. Sequence variability of Bc28.1 and mdh genes indicated the genetic variability of B. canis and A. phagocytophilum isolates occurring in Latvian domestic dogs.

Graphic Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04756-9.

Erythrocyte Adhesion of Merozoite Surface Antigen 2c1 Expressed During Extracellular Stages of Babesia orientalis

Babesia orientalis, a major infectious agent of water buffalo hemolytic babesiosis, is transmitted by Rhipicephalus haemaphysaloides. However, no effective vaccine is available. Essential antigens that are involved in parasite invasion of host red blood cells (RBCs) are potential vaccine candidates. Therefore, the identification and the conduction of functional studies of essential antigens are highly desirable. Here, we evaluated the function of B. orientalis merozoite surface antigen 2c1 (BoMSA-2c1), which belongs to the variable merozoite surface antigen (VMSA) family in B. orientalis. We developed a polyclonal antiserum against the purified recombinant (r)BoMSA-2c1 protein. Immunofluorescence staining results showed that BoMSA-2c1 was expressed only on extracellular merozoites, whereas the antigen was undetectable in intracellular parasites. RBC binding assays suggested that BoMSA-2c1 specifically bound to buffalo erythrocytes. Cytoadherence assays using a eukaryotic expression system in vitro further verified the binding and inhibitory ability of BoMSA-2c1. We found that BoMSA-2c1 with a GPI domain was expressed on the surface of HEK293T cells that bound to water buffalo RBCs, and that the anti-rBoMSA2c1 antibody inhibited this binding. These results indicated that BoMSA-2c1 was involved in mediating initial binding to host erythrocytes of B. orientalis. Identification of the occurrence of binding early in the invasion process may facilitate understanding of the growth characteristics, and may help in formulating strategies for the prevention and control of this parasite.

Characterization of the variable merozoite surface antigen (VMSA) gene family of Babesia orientalis

Due to its wide presence in apicomplexan parasites as well as high polymorphism and antigenic diversity, the variable merozoite surface antigen (VMSA) family in Babesia sp. has attracted increasing attention of researchers. Here, all the reported VMSA genes of Babesia spp. were obtained from GenBank, and multiple alignments were performed by using conserved regions to blast the Babesia orientalis genome database (unpublished data). Five MSA genes (named MSA-2a1, MSA-2a2, MSA-2c1, MSA-1, and MSA-2c2, respectively) were identified, sequenced, and cloned from B. orientalis, which were shown to encode proteins with open reading frames ranging in size from 266 (MSA-2c1) to 317 (MSA-1) amino acids. All the five proteins contain an MSA-2c superfamily conserved domain, with an identical signal peptide and glycosyl phosphatidyl inositol (GPI)-anchor for each of them. The five proteins were also predicted to contain B cell epitopes, with only three for BoMSA-2c1, the smallest protein in the BoVMSA family, while at least six for each of the others. Notably, BoMSA-2a1 has 2 identical copies, a specific phenomenon only present in B. orientalis. This research has determined the MSA genes of B. orientalis and provides a genetic basis for further research of functional genes in B. orientalis.

Evaluation of Babesia gibsoni GPI-anchored Protein 47 (BgGPI47-WH) as a Potential Diagnostic Antigen by Enzyme-Linked Immunosorbent Assay

Babesia gibsoni is one of the important pathogens causing severe incurable canine babesiosis, suggesting the necessity to develop a sensitive, specific, and highly automated diagnostic method for clinical application. Surface proteins are ideal candidates for diagnostic targets because they are the primary targets for host immune responses during host-parasite interactions. Glycosylphosphatidylinositol (GPI)-anchored proteins are abundant on the surface of parasites and play an important role in parasite diagnosis. In this study, a GPI-anchored protein named BgGPI47-WH was obtained and mouse anti-rBgGPI47-WH polyclonal antibody was produced by immunizing mice with the purified protein and Freund's adjuvant. Western blot was used to identify the native form and immunogenicity of BgGPI47-WH. An ELISA method was established by using recombinant BgGPI47-WH protein to evaluate its potential as a diagnostic antigen and the established method exhibited high specificity. The antibody response was evaluated by using the B. gibsoni-infected sera collected from different experimental dogs and the established ELISA could recognize antibodies at day 6 until day 101 post infection, indicating the potential use of BgGPI47-WH for early stage diagnosis. The specificity of the established ELISA was further evaluated by using 147 clinical samples collected from animal hospitals and 17.0% (25/147) of the samples were tested positive, with an overall proportion agreement of 86.39% between the results from BgGPI47-WH and BgSA1. Our results indicated that BgGPI47-WH could be used as a reliable diagnostic antigen and this study has proposed a practical method for early diagnosis of B. gibsoni.

An ELISA for the early diagnosis of acute canine babesiosis detecting circulating antigen of large Babesia spp

Babesia canis is the predominant Babesia species in dogs in Europe and is responsible for a severe and fatal disease. An increase in global pet tourism and a widening of the geographic distribution of the tick vector has led to the emergence of infections in areas where previously only imported cases have been reported. Due to the potential for rapid and serious disease progression, direct parasite detection by stained blood smears and light microscopy or DNA-based methods have traditionally been used for the diagnosis of acute infections. This study describes the production of a murine monoclonal antibody ('mAb BcFIII 7/1/2') that reacts to a 65kDa corpuscular epitope present in B. canis-infected erythrocytes and can be used in an ELISA to detect circulating Babesia antigen during acute infections. The sensitivity of the ELISA was 100% (95%CI: 84.5-100) as determined using blood lysate samples from 27 dogs with acute B. canis infections. Sensitivity was reduced to 53.8% in 13 patent Babesia vogeli infections (95%CI: 26.1-79.6) based on the current test design using convalescent serum from a B. canis-infected dog. The specificity was determined to be 86.4% (95%CI: 64-96.4) using 22 samples from healthy canine blood donors. In the course of acute B. canis infections, the ELISA showed a positive result at the same time as a positive PCR result was recorded. This was 24-48h before parasites could be detected by light microscopy. Convalescent samples collected from 6 B. canis-infected dogs at least 14days post treatment resulted in negative ELISA reactions. The hyper-acute to acute phase of a B. canis infection represents an emergency situation with high mortality. To increase the chances of survival, a fast and accurate diagnosis and immediate treatment is required. The current study demonstrates the opportunity of an early and specific detection of acute infections by an AgELISA that is potentially translatable to a rapid diagnostic test design.

Genome-wide analysis of gene expression and protein secretion of Babesia canis during virulent infection identifies potential pathogenicity factors

Infections of dogs with virulent strains of Babesia canis are characterized by rapid onset and high mortality, comparable to complicated human malaria. As in other apicomplexan parasites, most Babesia virulence factors responsible for survival and pathogenicity are secreted to the host cell surface and beyond where they remodel and biochemically modify the infected cell interacting with host proteins in a very specific manner. Here, we investigated factors secreted by B. canis during acute infections in dogs and report on in silico predictions and experimental analysis of the parasite’s exportome. As a backdrop, we generated a fully annotated B. canis genome sequence of a virulent Hungarian field isolate (strain BcH-CHIPZ) underpinned by extensive genome-wide RNA-seq analysis. We find evidence for conserved factors in apicomplexan hemoparasites involved in immune-evasion (e.g. VESA-protein family), proteins secreted across the iRBC membrane into the host bloodstream (e.g. SA- and Bc28 protein families), potential moonlighting proteins (e.g. profilin and histones), and uncharacterized antigens present during acute crisis in dogs. The combined data provides a first predicted and partially validated set of potential virulence factors exported during fatal infections, which can be exploited for urgently needed innovative intervention strategies aimed at facilitating diagnosis and management of canine babesiosis.

Classification of Babesia canis strains in Europe based on polymorphism of the Bc28.1-gene from the Babesia canis Bc28 multigene family

The vast majority of clinical babesiosis cases in dogs in Europe is caused by Babesia canis. Although dogs can be vaccinated, the level of protection is highly variable, which might be due to genetic diversity of B. canis strains. One of the major merozoite surface antigens of B. canis is a protein with a Mr of 28 kDa that belongs to the Bc28 multigene family, that comprises at least two genes, Bc28.1 and a homologous Bc28.2 gene. The two genes are relatively conserved but they are very distinct in their 3' ends, enabling the design of specific primers. Sequencing of the Bc28.1 genes from 4 genetically distinct B. canis laboratory strains (A8, B, 34.01 and G) revealed 20 mutations at conserved positions of which three allowed the classification of B. canis strains into three main groups (A, B and 34.01/G) by RFLP. This assay was subsequently used to analyze blood samples of 394 dogs suspected of clinical babesiosis from nine countries in Europe. All blood samples were first analyzed with a previously described assay that allowed detection of the different Babesia species that infect dogs. Sixty one percent of the samples contained detectable levels of Babesia DNA. Of these, 98.3% were positive for B. canis, the remaining cases were positive for B. vogeli. Analysis of the Bc28.1 gene, performed on 178 of the B. canis samples, revealed an overall dominance of genotype B (62.4%), followed by genotypes A (37.1%) and 34 (11.8%). Interestingly, a great variation in the geographical distribution and prevalence of the three B. canis genotypes was observed; in the North-East genotype A predominated (72.1% A against 27.9% B), in contrast to the South-West where genotype B predominated (10.3% A against 89.7% B). In the central part of Europe intermediate levels were found (26.0-42.9% A against 74.0-57.1% B, from West to East). Genotype 34 was only identified in France (26.9% among 78 samples) and mostly as co-infection with genotypes A or B (61.9%). A comparative analysis of the classification of 35 B. canis strains in genotypes A and B using a previously described 18SrDNA-derived PCR-RFLP test revealed a partial but no direct correlation with the classification based on polymorphism of the Bc28.1-gene described here.